Clinical improvement after surgery for degenerative cervical myelopathy; A comparison of Patient-Reported Outcome Measures during 12-month follow-up.

OBJECT: Although many patients report clinical improvement after surgery due to degenerative cervical myelopathy, the aim of intervention is to stop progression of spinal cord dysfunction. We wanted to provide estimates and assess achievement rates of Minimal Clinically Important Difference (MCID) at 3- and 12-month follow-up for Neck Disability Index (NDI), Numeric Rating Scale for arm pain (NRS-AP) and neck pain (NRS-NP), Euro-Qol (EQ-5D-3L), and European Myelopathy Score (EMS).
METHODS: 614 degenerative cervical myelopathy patients undergoing surgery responded to Patient-Reported Outcome Measures (PROMs) prior to, 3 and 12 months after surgery. External criterion was the Global Perceived Effect Scale (1-7), defining MCID as "slightly better", "much better" and "completely recovered". MCID estimates with highest sensitivity and specificity were calculated by Receiver Operating Curves for change and percentage change scores in the whole sample and in anterior and posterior procedural groups.
RESULTS: The NDI and NRS-NP percentage change scores were the most accurate PROMs with a MCID of 16%. The change score for NDI and percentage change scores for NDI, NRS-AP and NRS-NP were slightly higher in the anterior procedure group compared to the posterior procedure group, while remaining PROM estimates were similar across procedure type. The MCID achievement rates at 12-month follow-up ranged from 51% in EMS to 62% in NRS-NP.
CONCLUSION: The NDI and NRS-NP percentage change scores were the most accurate PROMs to measure clinical improvement after surgery for degenerative cervical myelopathy. We recommend using different cut-off estimates for anterior and posterior approach procedures. A MCID achievement rate of 60% or less must be interpreted in the perspective that the main goal of surgery for degenerative cervical myelopathy is to prevent worsening of the condition.

Introduction

Degenerative cervical myelopathy (DCM) describes a range of conditions in the cervical spine causing cord compression and neurological dysfunction [1]. There is current lack of evidence for nonoperative management in terms of preventing neurological deterioration, although physical rehabilitation and close observation can be considered in mild to asymptomatic cases. For moderate to severe cases, individualized surgical treatment is recommended [2-4]. Anterior Cervical Discectomy and Fusion (ACDF) and Anterior Cervical Disc Arthroplasty (ACDA) are frequently used in patients with disc herniation, while posterior approach procedures are well-established treatments options for patients with posterior and/or multi-level spinal cord compression [5]. In cases where symptoms are caused by spinal cord compression due to cervical ossification of the posterior longitudinal ligament, no treatment consensus is obtained and various anterior and posterior approach procedures are currently applied [6, 7].

The aim of surgery has traditionally been to stop progression of spinal cord dysfunction symptoms. However, recent studies have shown that many patients report improvement post intervention both regarding functionality and disability, as well as quality-of-life outcomes [2, 8]. Depending on PROMs used, severity of preoperative disease and length of follow-up, improvement rates range from around 20 to 80% [9, 10].

Patient-Reported Outcome Measures (PROMs) are commonly used to measure clinical improvement or worsening in spine literature. In combination with the concept of Minimal Clinically Important Difference (MCID), defined as the smallest change in an outcome score that is clinically beneficial within a patient group [11], optimal cut-off estimates for an individual PROM can be assessed [12, 13]. The traditional method is to assess the MCID change score, or the delta value. However, since the interpretation of a change score is dependent on the baseline score, the percentage change score can provide a more representative result at group level [14]. To date, MCID estimates for PROM percentage change scores have not been reported for DCM patients undergoing surgery. Further, there is current lack of evidence in terms of which PROMs are the more accurate in capturing changes in health status among these patients and whether results differ across surgical approach.

The purpose of this study was to estimate MCID for frequently used PROMs 3 and 12 months after surgery for DCM; NDI, Numeric Rating Scale for arm pain (NRS-AP) and neck pain (NRS-NP), Euro-Qol (EQ-5D-3L), and European Myelopathy Score (EMS). A secondary aim was to report achievement rates of MCID through 12 months of follow-up. The MCID estimates are reported for change scores and percentage changes scores for the whole sample, as well as for anterior and posterior approach procedural groups.

Materials and methods

Data collection

All data were collected through the Norwegian Registry for Spine Surgery (NORspine) which is a government funded comprehensive clinical registry. Participation in NORspine is not required for a patient to gain access to the health care, or for payment/reimbursement to a provider. All Norwegian health care providers offering cervical spine surgery (six public hospitals and three private clinics) report to NORspine. The proportion of operated patients reported to the registry was 75–78% over the study period [15].

Our research protocol was approved by the Norwegian Committee for Medical and Health Research Ethics Midt (2014/344). Informed consent was obtained from all patients before entering the registry.

Design

This is a multicenter observational study with follow-up at 3 and 12 months. Results are reported consistent with the Strengthening The Reporting of Observational Studies in Epidemiology (STROBE) statement [16], and methods are in accordance with the COnsensus-based Standards for the selection of health Measurement INstruments (COSMIN) recommendations [12].

Eligibility criteria

A cohort of 614 patients undergoing surgery for DCM between January 2011 and August 2016 were found to be eligible (Fig 1). Exclusion criteria were: 1) prior surgery the index level; and 2) patients undergoing combined anterior and posterior approach, since these patients commonly are selected on a case-by-case basis [17]. Of the 614 patients, 371 underwent either ACDF (363, 98%) or ACDA (8, 2%), and 243 patients underwent posterior approach procedures, such as laminectomy with or without fusion, hemilaminectomy or laminoplasty.

Fig 1

Project flow chart.

Exclusion criteria for patients included in the study with follow-up rates.

Measurements

At admission for surgery (baseline), patients complete the NORspine questionnaire which cover demographics, location and extent of pain and PROMs. During the hospital stay, the surgeon records data concerning diagnosis, American Society of Anesthesiologists physical status (ASA), surgical treatment and comorbidity on a separate form. Under ‘indication for operation’ the surgeon can checkmark if he/she considers the patient to have myelopathy based on clinical assessment and radiological findings. To avoid selective reporting, the 3- and 12-month follow-up is conducted by the NORspine central registry unit without involvement from treating hospitals. After surgery, a questionnaire identical to that used at baseline is distributed by mail to every registered patient. One reminder questionnaire is sent to those who do not respond. The following PROMs are collected:

Neck Disability Index (NDI): a patient-completed questionnaire focusing on the patient’s functional status and scores ranging from 0 (no disability) to 100 (greatest disability) [18].

Numeric Rating Scale for arm (NRS-AP) and neck pain (NRS-NP): a scale that assesses pain level ranging from 0 (no pain) to 10 (worst conceivable pain) [19].

EuroQoL (EQ-5D-3L): a generic measure assessing health-related quality of life with scores ranging from -0.59 (worse than death) to 1 (perfect health) [20].

European Myelopathy Score (EMS): a patient-based questionnaire derived for assessing spinal cord function. Scoring is between 5 (severe deficit) and 18 (no symptoms) [21].

The Global Perceived Effect scale (GPE) was in the present study used as an external criterion for defining MCID. The GPE measures patient-reported treatment outcome through one single question and seven answer choices; “completely recovered”, “much improved”, “slightly improved”, “unchanged, “slightly worse”, “much worse” and “worse than ever” [22]. Patients reporting to be “completely recovered”, “much improved” or “slightly improved” (1–3) were classified as having achieved a MCID. Those who considered themselves to be “unchanged” or worse (4–7) were classified as not improved.

Statistics

All statistical analyses were performed with the Statistical Package for the Social Sciences (SPSS, version 26). Continuous variables were reported as means and standard deviations and categorical variables as numbers and percentages. Differences were evaluated by Chi-square test for categorical variables and by t-tests for continuous variables. PROM change scores were obtained by subtracting the follow-up score from the baseline score. The percentage change score was calculated by dividing the change score with the baseline score and multiplying by 100. To be able to calculate the EQ-5D-3L percentage change score we converted the value range from -0.6 to 1.0 into a relative score from 0 to 100.

The correlations between the GPE scale and the different PROMs were analyzed using the Spearman correlation coefficient. Receiver Operating Curves (ROCs) were used to assess discriminative ability of each PROM and to define the optimal cut-off with the highest sensitivity and specificity. ROCs were made by plotting the sensitivity against (1 –specificity) for each possible MCID cut-off estimate. The sensitivity refers to the probability of correctly classifying an individual replying “slightly improved” or better (1–3) according to the PROM score. Correspondingly, the specificity refers to the probability of correctly classifying a patient reporting to be “unchanged” or worse as having “not improved” after surgery (4–7). The area under the ROC (AUC) with 95% confidence interval (CI) describes the test’s accuracy of correctly classifying a case according to the anchor. The AUC is classified as “acceptable” from 0.7 to 0.8, “excellent” from 0.8 to 0.9 and “outstanding” from 0.9 to 1.0 [23]. To determine MCID cut-off estimates with highest sensitivity and specificity, the closest point to the upper left corner of the ROC-curve was calculated from the coordinates of the curve. Cut-off estimates were assessed for the whole DCM group and for both procedural groups. Lastly, proportions of patients achieving MCID for the whole group and both procedural groups were calculated using the cut-off estimates for each PROM.

Results

Respondents and baseline characteristics

Of 4229 consecutive patients undergoing surgery for degenerative disorders in the cervical spine between January 2011 and August 2016, 614 patients were included. Of these patients, 371 underwent an anterior approach procedure, while 243 underwent a posterior approach procedure. A total of 67.9% and 70.1% of patients responded to the 3- and 12-month follow-up questionnaire, respectively (Fig 1). The non-responding patients were slightly younger (p<0.001), less likely to be retired (p<0.001), and more likely to smoke (p<0.001) (Table 1). There were no statistically significant differences in PROM scores, except for the EQ-5D-3L mean, which was lower (poorer health-related quality-of-life) among non-responders (p<0.008) (Table 1).

Table 1

Baseline characteristics of respondents and non-respondents to 12-month follow-up.

	Respondents N = 430		Non-respondents N = 184		Sig. (2-tailed)/ chi-square
	N		N
Age (years); Mean (SD)	430	59.1 (11.9)	184	53.5 (12.2)	<0.001
Female, no (%)	430	167 (38.8)	184	66 (35.9)	0.488
ASA level (1–4); Mean (SD)	430	2.4 (1.7)	184	2.3 (1.5)	0.414
Body Mass Index; Mean (SD)	417	27.0 (4.5)	179	27.5 (5.2)	0.220
Smokers, no (%)	428	106 (24.8)	184	84 (45.7)	<0.001
University/College education	402	137 (31.6)	173	56 (32.4)	0.823
Retired, no (%)	430	121 (28.1)	183	23 (12.6)	<0.001
Comorbidity, no (%)	422	227 (53.8)	183	109 (59.6)	0.189
Levels operated, Mean (SD)	418	1.9 (1.1)	184	1.85 (1.1)	0.376
NDI; Mean (SD)	428	33.7 (17.3)	178	36.6 (17.4)	0.060
NRS-AP; Mean (SD)	399	5.0 (2.9)	164	5.1 (3.0)	0.794
NRS-NP; Mean (SD)	401	4.7 (3.0)	162	5.1 (2.9)	0.134
EQ-5D-3L; Mean (SD)	392	0.47 (0.32)	171	0.39 (0.34)	0.008
EMS; Mean (SD)	384	14.5 (2.3)	165	14.4 (2.5)	0.750

SD, Standard Deviation; NDI, Neck Disability Index (0–100); NRS-AP Numeric Rating Scale for arm pain (0–10); NRS-NP, Numeric Rating Scale for neck pain (0–10); EQ-5D-3L, Health-Related Quality-of-Life by EuroQol (-0.4–1.0); EMS, European Myelopathy Score (5–18).

Baseline characteristics of the whole myelopathy group and the two procedural groups are presented in Table 2. Compared to the anterior approach procedure group, patients in the posterior approach group were more likely to be male, not working, and to be operated at a higher number of levels. Also, they had significantly higher mean age, higher mean ASA level, more comorbidity, and more severe myelopathy symptoms according to EMS.

Table 2

Baseline characteristics of the whole myelopathy group and of the two procedural groups.

	Whole myelopathy group N = 430		Anterior approach group N = 245		Posterior approach group N = 185		Sig. (2-tailed)/ chi-square
	N		N		N
Age (years); Mean (SD)	430	59.1 (11.9)	245	53.7 (11.0)	185	66.1 (8.9)	<0.001
Female; no (%)	430	167 (38.8)	245	108 (44.1)	185	59 (31.9)	0.01
ASA level (1–4); Mean (SD)	430	2.4 (1.7)	245	2.0 (1.4)	185	2.9 (1.9)	<0.001
Body Mass Index; Mean (SD)	417	27.0 (5.0)	363	27.3 (4.4)	178	26.8 (5.1)	0.260
Smokers; no (%)	425	106 (24.9)	243	62 (25.5)	182	44 (24.2)	0.752
No of levels operated; Mean (SD)	418	1.9 (1.1)	241	1.4 (0.6)	177	2.7 (1.2)	<0.001
Comorbidity; no (%)	422	227 (53.8)	238	110 (46.2)	184	117 (63.6)	<0.001
Currently working; no (%)	430	110 (25.9)	240	85 (35.4)	184	25 (13.6)	<0.001
Retired; no (%)	430	121 (28.1)	245	34 (13.9)	185	87 (47.0)	<0.001
NDI; Mean (SD)	428	33.7 (17.3)	244	33.9 (16.9)	184	33.4 (18.0)	0.753
NRS-AP; Mean (SD)	399	5.0 (2.9)	232	5.1 (3.0)	167	4.9 (2.9)	0.442
NRS-NP; Mean (SD)	401	4.7 (3.0)	234	4.9 (2.9)	167	4.4 (3.1)	0.062
EQ-5D-3L; Mean (SD)	392	0.47 (0.32)	225	0.49 (0.30)	167	0.44 (0.33)	0.084
EMS; Mean (SD)	427	14.5 (2.4)	243	14.9 (2.2)	184	13.9 (2.5)	<0.001

SD, Standard Deviation; NDI, Neck Disability Index (0–100); NRS-AP Numeric Rating Scale for arm pain (0–10); NRS-NP, Numeric Rating Scale for neck pain (0–10); EQ-5D-3L, Health-Related Quality-of-Life by EuroQol (-0.4–1.0); EMS, European Myelopathy Score (5–18).

Correlation between the PROMs and the external criterion

For all PROMs, there was a stepwise decrease in mean change scores and mean percentage change scores at 12 months for patients who reported themselves to be completely recovered, much better and slightly better compared to those reporting no change or some degree of worsening (S1 Table). A similar pattern was found for results at 3 months (obtained on request). For the whole group, the Spearman correlation coefficients ranged from 0.30 to 0.59. The NDI showed the strongest correlation with the external anchor.

AUC and MCID

We found minor differences in AUC and MCID cut-off estimates at 3 and 12 months. Therefore, further analysis of the data is presented only for the PROMs at 12-month follow-up. 3-month scores are presented in S2 Table.

The change scores of NDI, NRS-NP and the EQ-5D-3L showed acceptable AUC values (>0.70), whereas AUC values of the NRS-AP change score and EMS percentage change score were slightly lower than acceptable (0.69 and 0.68, respectively) (Table 3). Most of the AUC change score values (0.64–0.74) were similar to or lower than the corresponding AUC percentage change score value (0.68–0.77). Only for EMS, the change score AUC (0.69) was higher than the percentage change score AUC (0.68) (Table 3). The percentage change scores of the NDI and NRS-NP had the highest sensitivity and specificity.

Table 3

Area under the curve and cut-off estimates for Minimal Clinically Important Difference for all Patient-Reported Outcome Measures at 12 months.

		Change score (points)	Percentage change score (%)
NDI	AUC (95% CI)	0.74 (0.69, 0.79)	0.77 (0.72, 0.81)
	Cut-off (% sensitivity, % specificity)	4.3 (0.68, 0.68)	15.7 (0.71, 0.71)
NRS-AP	AUC (95% CI)	0.64 (0.58, 0.70)	0.69 (0.63, 0.75)
	Cut-off (% sensitivity, % specificity)	0.5 (0.66, 0.53)	23.6 (0.63, 0.61)
NRS-NP	AUC (95% CI)	0.73 (0.67, 0.78)	0.76 (0.70, 0.81)
	Cut-off (% sensitivity, % specificity)	0.5 (0.71, 0.64)	15.5 (0.72, 0.71)
EQ-5D-3L	AUC (95% CI)	0.70 (0.64, 0.77)	0.70 (0.64, 0.77)
	Cut-off (% sensitivity, % specificity)	0.02 (0.70, 0.66)	2.2 (0.68, 0.66)
EMS	AUC (95% CI)	0.69 (0.63, 0.75)	0.68 (0.61, 0.74)
	Cut-off (% sensitivity, % specificity)	0.5 (0.58, 0.69)	4.2 (0.58, 0.69)

NDI, Neck Disability Index (0–100); AUC, Area Under the Curve, NRS-AP, Numeric Rating Scale for arm pain (0–10); NRS-NP, Numeric Rating Scale for neck pain (0–10); EQ-5D-3L, Health-Related Quality-of-Life by EuroQol (-0.4–1.0); EMS, European Myelopathy Score (5–18).

Similar results were found for AUCs analyzed for the anterior and posterior approach groups. However, there was a tendency to lower discriminative ability for all PROMs in the posterior approach group except for EMS in which case the AUCs were higher in this group (Table 4).

Table 4

Minimal Clinically Important Difference cut-off estimates for all Patient-Reported Outcome Measures in the two procedural subgroups at 12 months.

		Anterior approach (% sensitivity, % specificity)	AUC (95% Confidence Interval)	Posterior approach (% sensitivity, % specificity)	AUC (95% Confidence Interval)
NDI	Change score (points)	5.9 (0.70, 0.70)	0.74 (0.67, 0.81)	2.4 (0.68, 0.68)	0.73 (0.66, 0.81)
	Percentage change score (%)	16.2 (0.72, 0.71)	0.77 (0.71, 0.84)	14.4 (0.71, 0.71)	0.76 (0.68, 0.83)
NRS-AP	Change score (points)	0.5 (0.66, 0.52)	0.66 (0.58, 0.74)	0.5 (0.65, 0.54)	0.62 (0.52, 0.72)
	Percentage change score (%)	23.6 (0.64, 0.59)	0.69 (0.62, 0.77)	21.1 (0.62, 0.61)	0.69 (0.60, 0.77)
NRS-NP	Change score (points)	0.5 (0.76, 0.62)	0.77 (0.69, 0.84)	0.5 (0.63, 0.66)	0.66 (0.58, 0.75)
	Percentage change score (%)	18.3 (0.73, 0.73)	0.77 (0.69, 0.85)	11.8 (0.69, 0.69)	0.73 (0.65, 0.81)
EQ-5D-3L	Change score (points)	0.02 (0.72, 0.71)	0.74 (0.66, 0.82)	0.02 (0.67, 0.61)	0.66 (0.57, 0.76)
	Percentage change score (%)	2.2 (0.70, 0.71)	0.74 (0.66, 0.82)	2.3 (0.63, 0.61)	0.66 (0.57, 0.76)
EMS	Change score (points)	0.5 (0.58, 0.66)	0.67 (0.58, 0.76)	0.5 (0.59, 0.72)	0.72 (0.63, 0.80)
	Percentage change score (%)	4.2 (0.58, 0.66)	0.65 (0.55, 0.74)	4.2 (0.59, 0.72)	0.71 (0.62, 0.81)

AUC, Area Under the Curve; NDI, Neck Disability Index (0–100); NRS-AP, Numeric Rating Scale for arm pain (0–10); NRS-NP, Numeric Rating Scale for neck pain (0–10); EQ-5D-3L, Health-Related Quality-of-Life by EuroQol (-0.4–1.0); EMS, European Myelopathy Score (5–18).

Proportions of patients with clinical improvement at 12-month follow-up

In Fig 2, we present the proportions of patients that achieved a clinical improvement according to MCID estimates for percentage change scores at 12-month follow-up. Overall, NDI (59%), NRS-NP (61%) and EQ-5D-3L (59%) showed similar proportions of achieving a MCID, whereas NRS-NP (56%) and, in particular, EMS (51%) showed lower proportions of improvement. The rates were slightly higher for the anterior approach group compared to the posterior approach group for both change score and percentage change score (S3 Table).

Fig 2

Clinical improvement rates.

Percentage of patients achieving improvement larger than the Minimal Clinically Important Difference (MCID) according to the Neck Disability Index (NDI), Numeric Rating Scale for arm pain (NRS-AP) and neck pain (NRS-NP), Euro-Quol-5D-3L (EQ-5D-3L) and European Myelopathy Score (EMS). Results are provided by the percentage change score from baseline to 12-month follow-up.

Discussion

This study showed that NDI and NRS-NP were the most accurate PROMs to measure MCID among patients undergoing surgery due to DCM. EQ-5D-3L also showed acceptable accuracy for both change and percentage change score. Further, achievement of clinical improvement according to the optimal MCID estimates of the investigated PROMs ranged from 51% to 62%, depending on type of PROM, type of MCID and surgical approach.

Although there are several studies investigating MCID for DCM patients undergoing surgery [24-28], there are no reports of percentage change scores for this patient group. In our study, the majority of the percentage change scores were more accurate than the change scores. As shown in Table 3, percentage change scores for NDI, NRS-AP and NRS-NP showed higher AUC, including higher sensitivity and specificity, compared to the change scores. For EQ-5D, the AUCs were identical, while the EMS AUC was slightly higher for the change score than for the percentage change score (0.69 vs. 0.68). Since the use of change scores for benchmarking has been criticized for not taking into account the baseline score [29-31], we recommend using percentage change scores in future research.

The observed MCID estimate of 4.3 points for the NDI 12-month change score is similar to a previous study of Kato et al., who found a cut-off estimate of 4.2 in 101 myelopathy patients undergoing cervical laminoplasty [32]. Chien et al. report a slightly higher cut-off of 6 for NDI which might be due to a very small patient sample (n = 45) [26]. Similarly, in a study of 30 DCM patients by Auffinger et al., five statistical methods used for calculation of cut-off estimates showed similar or substantially higher findings for both NDI (4.8–13.4) and NRS-NP (0.36–3.11) [25].

The accuracy of EQ-5D-3L has also been assessed in a previous study. Kato et al. reported a MCID estimate of 0.05 for EQ-5D-3L with an AUC of 0.704 [32], which is in accordance with the results in the present study. Since the accuracy of EQ-5D-3L has been found to be acceptable (>0.70) in both these studies, we recommend further use of this PROM for DCM patients.

Several studies have reported MCID estimates for degenerative neck surgery patients. However, in many of the investigated cohorts there have been a mix of radiculopathy and myelopathy patients [33-35]. We argue that it is necessary to distinguish between myelopathy and radiculopathy patient cohorts considering the smaller amount of expected improvement among DCM patients. For example, Carreon et al., who analyzed a mixed sample of 505 patients, reported higher MCID estimates than our study for both NDI (7.5 vs. 4.3), NRS-AP (2.5 vs. 0.5) and NRS-NP (2.5 vs. 0.5) [34].

As far as we know, no previous study has presented MCID estimates for EMS and NRS-AP in a DCM cohort.

Surgical approach

We found minor differences in accuracy of NDI and NRS-NP across patients undergoing anterior versus posterior surgical procedures. However, there was a tendency to lower discriminative ability for NDI, the two NRS scores and EQ-5D-3L in the posterior approach group (Table 4). In each group, NDI and NRS-NP showed the best discriminative ability.

The MCID estimates for NDI, NRS-AP and NRS-NP were lower in the posterior approach group compared to the anterior approach group. This may indicate that posterior approach patients, which were older and had multilevel degenerative disease, were satisfied with less improvement compared to the younger and healthier patients in the anterior approach group. These results confirm that it is reasonable to analyze these two surgical groups separately. They also suggest that the interpretation of change and percentage change scores of PROMs should be different across anterior and posterior procedures.

Proportion of patients achieving MCID

The proportion of DCM patients that achieved MCID varied between 51% and 61% for the percentage change score. This is in line with a previous study by Stull et al. which reported that 40 to 61% achieve MCID in a sample of 53 DCM patients [9]. Although Stull et al. found little or no difference in achievement rates between radiculopathy and myelopathy patients, others have shown that the proportion of patients achieving a MCID is substantially higher among radiculopathy patients. Applying a cut-off estimate of 15 for NDI, two recent studies found NDI success rates of 80–92% for patients undergoing ACDF or ACDA [36, 37].

Limitations and strengths

GPE is a self-reported scale and not an objective anchor. This represents the main limitation of our study as global scale ratings tend to be influenced by the current health status of the patient [22]. However, no alternative gold standard currently exists, and the GPE is still the most frequently used anchor in scientific literature [38-42].

The main inclusion criterion for all patients was that the operating surgeon had made a checkmark for myelopathy (yes/no) in the post-operative questionnaire under “indication for operation”. This response represents a subjective judgement based on patient history, clinical features, and radiological findings. Since we have no objective reference for evaluating the accuracy of the surgeons’ judgment, misclassifications could exist.

The non-respondent rate of approximately 30% is usually regarded as acceptable for a spine registry [43]. As some of the baseline characteristics of the non-respondents have been associated with poorer outcomes [44], this might still be considered a selection bias especially since we are estimating the proportion of patients achieving MCID. However, this should be of less importance when assessing actual cut-off estimates across a wide range of outcomes. Two previous studies found no differences in outcome when comparing respondents and non-respondents at follow-up, though both had slightly lower non-respondent rates [45, 46].

A major strength of this study is the large sample size of surgical patients in daily clinical practice and the high coverage rate [15] indicating a high external validity of our results.

Conclusion

NDI and NRS-NP were the most accurate PROMs to measure a clinical improvement according to MCID estimates 12 months after surgery for DCM. Also, EQ-5D-3L showed acceptable discriminative ability.

Percentage change scores were more accurate than change scores, hence, we recommend using percentage change cut-off estimates in future studies. The cut-off estimates and MCID achievement rates were also slightly higher for the anterior approach group compared to the posterior approach group indicating that separate cut-off estimates should be used for each surgical approach.

An achievement of a MCID of 60% or less among DCM patients must be interpreted in the perspective that the main goal of surgery is to prevent worsening of the condition.

Mean scores with standard deviation of the Patient-Reported Outcome Measures at 12 months for the whole myelopathy group according to the Global Perceived Effect Scale.

Spearman, Spearman’s rank correlation coefficient; Neck Disability Index (0–100); SD, Standard Deviation; NRS-AP, Numeric Rating Scale for arm pain (0–10); NRS-NP, Numeric Rating Scale for neck pain (0–10), EQ-5D-3L, Health-Related Quality-of-Life by EuroQol (-0.4–1.0), EMS, European Myelopathy Score (5–18).

(DOCX)

Click here for additional data file.

Area under the curve and cut-off estimates for “Minimal Clinically Important Difference” for all Patient-Reported Outcome Measures at 3 months.

(DOCX)

Click here for additional data file.

Proportion of patients with an improvement larger than “Minimal Clinically Important Difference” at 12-months follow-up according to Patient-Reported Outcome Measures.

NDI, Neck Disability Index (0–100), NRS-AP, Numeric Rating Scale for arm pain (0–10), NRS-NP, Numeric Rating Scale for neck pain (0–10), EQ-5D-3L, Health-Related Quality-of-Life by EuroQol (-0.4–1.0), EMS, European Myelopathy Score (5–18).

(DOCX)

Click here for additional data file.

28 Sep 2021

A rebuttal letter that responds to each point raised by the academic editor and reviewer(s). You should upload this letter as a separate file labeled 'Response to Reviewers'.

A marked-up copy of your manuscript that highlights changes made to the original version. You should upload this as a separate file labeled 'Revised Manuscript with Track Changes'.

An unmarked version of your revised paper without tracked changes. You should upload this as a separate file labeled 'Manuscript'.

If you would like to make changes to your financial disclosure, please include your updated statement in your cover letter. Guidelines for resubmitting your figure files are available below the reviewer comments at the end of this letter.

If applicable, we recommend that you deposit your laboratory protocols in protocols.io to enhance the reproducibility of your results. Protocols.io assigns your protocol its own identifier (DOI) so that it can be cited independently in the future. For instructions see: https://journals.plos.org/plosone/s/submission-guidelines#loc-laboratory-protocols. Additionally, PLOS ONE offers an option for publishing peer-reviewed Lab Protocol articles, which describe protocols hosted on protocols.io. Read more information on sharing protocols at https://plos.org/protocols?utm_medium=editorial-email&utm_source=authorletters&utm_campaign=protocols.

We look forward to receiving your revised manuscript.

Kind regards,

Michael G. Fehlings

Academic Editor

PLOS ONE

Journal requirements:

When submitting your revision, we need you to address these additional requirements.

1. Please ensure that your manuscript meets PLOS ONE's style requirements, including those for file naming. The PLOS ONE style templates can be found at

https://journals.plos.org/plosone/s/file?id=wjVg/PLOSOne_formatting_sample_main_body.pdf and

https://journals.plos.org/plosone/s/file?id=ba62/PLOSOne_formatting_sample_title_authors_affiliations.pdf

[Note: HTML markup is below. Please do not edit.]

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: No

Reviewer #2: Yes

**********

2. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: No

Reviewer #2: No

**********

3. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: No

Reviewer #2: Yes

**********

4. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: Yes

Reviewer #2: Yes

**********

5. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: The authors present a retrospective review of prospectively-collected data in an attempt to determine the ability of the listed patient reported outcomes to produce a significant clinical improvement (termed “Minimal Important Change”) after surgery for degenerative cervical myelopathy (DCM). These results are based on national registry data, and the outcomes of NDI, NRS for arm and neck, the EQ-5D and the European Myelopathy Score. The data collection methods are clearly presented, and the response to follow up at 12 months is acceptable (70%).

Unfortunately, this article suffers from a number of major flaws, including (but not limited to) the interpretation of the currently available literature on the topic, the selected methodology, the inclusion criteria, as well as the interpretation and representation of the results. These are explained in detail below, however due to the combination of these flaws and lack of novelty, I am unable to support publication of this article.

1 – Introduction – I agree DCM is a progressive disorder, however surgery should not be presented as the ‘standard treatment’. The surgical treatment recommendations varies depending on the severity of myelopathy, judged on a functional outcome assessment such as the Modified Japanese Orthopedic Association score (MJOA). This is a recommendation from the international clinical practice guidelines published in 2017 (Fehlings et al). The references 4 & 5 presented in the article refer to a mixture of quality of life as well as functional outcome measures after surgery specifically after ACDF surgery, in small numbers – there are more appropriate articles to cite regarding outcomes after surgery for DCM (i.e. the CSM-North America and CSM-International studies).

2- Throughout the article the authors refer to the “Minimal Important Change” which I suggest is a misnomer for “Minimum Clinically Important Difference” or “MCID”. The MCID is often cited in literature when discussing patient reported outcome measures, particularly with DCM.

3- The authors state repeatedly that the MCID has not been reported for DCM patients – when there are multiple works that have explored this exact fact- Tetreault et al 2015 for the MJOA, Badhiwala et al 2018 for the SF-36 to name a couple.

4- The authors refer to ‘percentage change score’ and present this as a novel finding, when in fact the ‘Delta’ value (change from baseline) is a more common modality when discussing PROMs in spine surgery and should therefore be used.

5- The authors excluded combined anterior/posterior surgery – with no explanation

6- The authors present a table of demographics for patients initially enrolled (n=614) but not for the patients who were actually followed up at 12 months (n=430).

7- The authors report a high number of Tobacco smokers in both anterior and posterior groups – and at no point is this fact addressed, adjusted for, or even commented on when it comes to the results and discussion.

8- It is not stated the proportion of patients undergoing disc arthroplasty vs ACDF – I would argue the use of arthroplasty is not internationally substantiated in the treatment of DCM.

9-The statistical analysis is incorrect – the use of T tests when attempting to report the potential outcomes of PROMs standardized to a binary outcome is not going to produce reliable results.

Odds ratios and, at the very least, linear regression modelling is the best initial modality to interrogate the data and represents the most suitable first approach. Multiple regression analysis should then be employed to adjust for confounders such as smoking, gender, number of operated levels, age – all of which have been consistently demonstrated to affect the outcomes after surgery for DCM.

All of this should be performed and validated before any attempt at AUC analysis.

10- Given the above – none of the AUC analysis is accurate.

11- No attempt was made to internally validate the data presented.

12- The authors state the sample size (417) is large. ROC and AUC analysis require significantly larger sample sizes to produce reliable results. See this recent article:

Wilson et al 2020 Frailty Is a Better Predictor than Age of Mortality and Perioperative Complications after Surgery for Degenerative Cervical Myelopathy: An Analysis of 41,369 Patients from the NSQIP Database 2010–2018

Reviewer #2: Page 3 Line 4 Clarify this sentence please- surgery is recommended in moderate-severe at the moment by the AO, however evidence suggests that mild cases benefit too.

Line 9: Comment on OPLL please

Line 12: What incidence of patient report an improvement.

Line 17: Its not more correct, perhaps could be more representative or easier to interpret.

Page 4 line 2: Why was mJOA not evaluated?

Line 9: what was the distribution ACDF/ ACDA?

Page 7 Line 1 How did the authors check for normality?

Page 15 Line 5: More accurate needs to be qualified more comprehensively please

Page 16 Line 15: How do you discriminate between the effects of ant/post approach and age? Summarise here please

**********

6. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #1: Yes: Jamie R F Wilson

Reviewer #2: No

[NOTE: If reviewer comments were submitted as an attachment file, they will be attached to this email and accessible via the submission site. Please log into your account, locate the manuscript record, and check for the action link "View Attachments". If this link does not appear, there are no attachment files.]

While revising your submission, please upload your figure files to the Preflight Analysis and Conversion Engine (PACE) digital diagnostic tool, https://pacev2.apexcovantage.com/. PACE helps ensure that figures meet PLOS requirements. To use PACE, you must first register as a user. Registration is free. Then, login and navigate to the UPLOAD tab, where you will find detailed instructions on how to use the tool. If you encounter any issues or have any questions when using PACE, please email PLOS at figures@plos.org. Please note that Supporting Information files do not need this step.

4 Dec 2021

Reviewer #1:

General comment:

The authors present a retrospective review of prospectively-collected data in an attempt to determine the ability of the listed patient reported outcomes to produce a significant clinical improvement (termed “Minimal Important Change”) after surgery for degenerative cervical myelopathy (DCM). These results are based on national registry data, and the outcomes of NDI, NRS for arm and neck, the EQ-5D and the European Myelopathy Score. The data collection methods are clearly presented, and the response to follow up at 12 months is acceptable (70%).

Unfortunately, this article suffers from a number of major flaws, including (but not limited to) the interpretation of the currently available literature on the topic, the selected methodology, the inclusion criteria, as well as the interpretation and representation of the results. These are explained in detail below, however due to the combination of these flaws and lack of novelty, I am unable to support publication of this article.

Answer to general comment:

In our article, we have found cut-off estimates for PROMs for cervical degenerative myelopathy patients undergoing surgery. This is not an attempt to produce a predictive model for DCM patients. Although we believe such a model is needed, this is beyond the scope of this article.

Since reviewer #1 bring up multiple discussions in spine literature, we have chosen to reference our answers and added a reference list at the very end of the document.

Comment #1:

Introduction – I agree DCM is a progressive disorder, however surgery should not be presented as the ‘standard treatment’. The surgical treatment recommendations vary depending on the severity of myelopathy, judged on a functional outcome assessment such as the Modified Japanese Orthopedic Association score (MJOA). This is a recommendation from the international clinical practice guidelines published in 2017 (Fehlings et al). The references 4 & 5 presented in the article refer to a mixture of quality of life as well as functional outcome measures after surgery specifically after ACDF surgery, in small numbers – there are more appropriate articles to cite regarding outcomes after surgery for DCM (i.e. the CSM-North America and CSM-International studies).

Answer:

We agree that more appropriate references are needed. The articles mentioned have been referenced. The reason for referencing the two original articles was that they both report MCID attainment rates for DCM patients undergoing surgery [1, 2]. Since Reivewer #2 asks for these figures, we have chosen to split the information in two sentences in the revised manuscript (page 3, line 22 – page 4, line 2).

Comment #2:

Throughout the article the authors refer to the “Minimal Important Change” which I suggest is a misnomer for “Minimum Clinically Important Difference” or “MCID”. The MCID is often cited in literature when discussing patient reported outcome measures, particularly with DCM.

The term Minimal Important Change/”MIC” is described in depth in the article by Mokkink et al from 2010 and is based on the work made by the COSMIN group (COnsensus-based Standards for the selection of health status Measurement INstruments) that aimed to recommend methodological measurement properties for studies of health-related patient reported outcomes [3]. As stated in this article and by de Vet et al [4], literature often interchanges the terms MIC and MCID. However, it has been proposed that MCID is to be used as a term for cross-sectional between-person differences, while MIC is the minimal change in the scoring measure/PROM for a group of patients that is perceived as beneficial. This has been nicely illustrated in Figure 1 in the 2017 article by Clement et al [5].

In spine literature, MCID is the preferred term [6]. Therefore, we have decided to follow the advice of the reviewer and apply the term MCID in our article instead of MIC.

Comment #3

The authors state repeatedly that the MCID has not been reported for DCM patients – when there are multiple works that have explored this exact fact- Tetreault et al 2015 for the MJOA, Badhiwala et al 2018 for the SF-36 to name a couple.

Answer:

The authors acknowledge that there are multiple studies reporting MCID for DCM patients, and we would like to challenge the reviewer’s comment:

• We are referring to several articles that have reported MCID for neck surgery patients in the discussion section (page 16-17 in the revised manuscript). We have particularly focused on the studies reporting cut-off estimates for the PROMs in our study (NDI, arm and neck pain, EQ-5D and EMS).

• On page 3, line 18 in the original manuscript, we state: “To date, MIC estimates for PROM percentage change scores have not been reported for DCM patients”. Also, on page 15, line 3, we state again: “To our knowledge, the percentage change scores for patients undergoing surgery for DCM have not previously been reported”. We believe this is true. We have found no percentage change scores in literature, and this article is the first to report such scores. We are not claiming that MCID estimates in general have not previously been reported. In fact, we state on page 15, line 19 in the original manuscript: “Several studies have reported MIC estimates for degenerative neck surgery patients”.

To clarify this to the reader, we have added a sentence in the discussion section (page 16, line 3-4 in the revised manuscript) stating that there are multiple studies investigating MCID for DCM patients, but that we find no other study reporting “percentage change scores”.

Comment #4

The authors refer to ‘percentage change score’ and present this as a novel finding, when in fact the ‘Delta’ value (change from baseline) is a more common modality when discussing PROMs in spine surgery and should therefore be used.

Answer:

We agree that the change score or the delta value is a more common modality. In fact, in our article, we are reporting both the change score/the delta value, as well as the percentage change score for each PROM. Since the percentage change score take into account the baseline score, we find that the results are more accurate. This is in line with a previous study where we investigated cut-off scores for a radiculopathy patient group [7]. Similar findings have also been reported for lumbar spine patients [8, 9]. In the discussion section, we, therefore, argue that the percentage change score should be more frequently used in the future (page 16, line 9-11 in the revised manuscript).

Comment #5

The authors excluded combined anterior/posterior surgery – with no explanation

Answer:

We agree that the exclusion of the anterior-posterior surgery patients needs an explanation.

A combined anterior-posterior approach is not a common treatment of DCM. According to Kim et al [10], patients should be selected for this kind of procedure on a case-by-case basis. Indications include acute spinal trauma, post-laminectomy kyphosis, kyphotic deformity with intact posterior tension band, multilevel spondylosis and OPLL, and pre-existing risk factors for pseudarthrosis. We believe this cohort is substantially different from the general myelopathy cohort and should be investigated separately.

A comment regarding this fact has been added to the article on page 6, line 5-6.

Comment #6:

The authors present a table of demographics for patients initially enrolled (n=614) but not for the patients who were actually followed up at 12 months (n=430).

Answer:

We agree that this information will be useful and have made edits to the article accordingly. Please see edits in the manuscript on page 10, line 9, and in Table 2 on page 10-11.

Comment #7:

The authors report a high number of Tobacco smokers in both anterior and posterior groups – and at no point is this fact addressed, adjusted for, or even commented on when it comes to the results and discussion.

Answer:

Again, as mentioned in the introduction, we are not attempting to develop a prognostic model for DCM patients in this study. Therefore, we believe that the characteristics of the patients in the cohort is of less importance than the actual results. Also, a comparable study by Fehlings et al. report similar findings regarding tobacco use in their cohort (27.35% vs. 24.9%) [11]. Therefore, we have not made any edits in the article related to this comment..

Comment #8:

It is not stated the proportion of patients undergoing disc arthroplasty vs ACDF – I would argue the use of arthroplasty is not internationally substantiated in the treatment of DCM.

Answer:

In our study, 8 patients underwent disc arthroplasty. All of them were younger than the mean and had surgery in only one and two levels. The use of arthroplasty has been shown to have similar results as ACDF procedures in several studies of patients with symptoms of radiculopathy or/and myelopathy, e.g., the articles of Gornet et al [12, 13]. We, therefore, believe the inclusion of these few procedures are justified.

Comment #9:

The statistical analysis is incorrect – the use of T tests when attempting to report the potential outcomes of PROMs standardized to a binary outcome is not going to produce reliable results. Odds ratios and, at the very least, linear regression modelling is the best initial modality to interrogate the data and represents the most suitable first approach. Multiple regression analysis should then be employed to adjust for confounders such as smoking, gender, number of operated levels, age – all of which have been consistently demonstrated to affect the outcomes after surgery for DCM.

All of this should be performed and validated before any attempt at AUC analysis.

Answer:

In this study, we are reporting PROM cut-off estimates for patients undergoing surgery for degenerative cervical myelopathy. Also, we have investigated the accuracy of each PROM in the Norwegian Spine Registry for the DCM group. This method is quite standardized. Several studies have performed similar investigations for other types of spine patients undergoing surgery [7, 14, 15]. We have not attempted to do a multiple regression analysis. However, based on our cut-off estimates such an analysis will be possible to do in the future.

Comment #10:

Given the above – none of the AUC analysis is accurate.

Answer:

Based on the argument above, we believe the AUC analysis is accurate.

Comment #11:

No attempt was made to internally validate the data presented.

Answer:

In a prediction analysis, validation of the initial result is necessary. However, in our case, we are not attempting to do such an analysis and only trying to produce cut-off estimates for a designated cohort. Our findings need to be validated if one is to use the cut-off estimates for a different cohort. However, that is beyond the scope of this paper.

Comment #12

The authors state the sample size (417) is large. ROC and AUC analysis require significantly larger sample sizes to produce reliable results. See this recent article:

Wilson et al 2020 Frailty Is a Better Predictor than Age of Mortality and Perioperative Complications after Surgery for Degenerative Cervical Myelopathy: An Analysis of 41,369 Patients from the NSQIP Database 2010–2018

Answer:

Wilson et al has done a predictive analysis of a DCM cohort. In the Wilson et al article, the objectives are as follows: «(1) define the effect of age on the perioperative outcomes of mortality, unplanned readmission/reoperation, major complication, length of stay and discharge to non-home destination for patients undergoing surgery for DCM, (2) directly compare measures of frailty in the same cohort to determine which factor exhibits a greater influence on the observed outcomes, and (3) define the potential correlation between MFI-5 and MFI-11 in DCM patients» [16]. We do not attempt to develop a predictive model in our article, only produce cut-off estimates for patients.

Also, other recent studies investigating cut-off estimates for similar DCM cohorts and reviewed in the discussion section on page 16-17 (in the revised manuscript), have similar or smaller sample sizes than ours. We therefore believe it is justified to rate our sample size (614) as «large».

Reviewer #2:

Comment #1:

Page 3 Line 4 Clarify this sentence please - surgery is recommended in moderate-severe at the moment by the AO, however evidence suggests that mild cases benefit too.

Answer:

We agree that this sentence needs clarifications and have made edits accordingly with proper references in the introduction on page 3, line 10-13.

Comment #2

Line 9: Comment on OPLL please

Answer:

Information and relevant references have been added regarding the treatment of OPLL in the introduction on page 3, line 17-19.

Comment #3

Line 12: What incidence of patient report an improvement.

Answer:

We believe what the reviewer is asking for is the MCID attainment rate for degenerative cervical myelopathy patients undergoing surgery. The figures reported by Stull et al. and Goh et al. are now inserted in the text and referenced accordingly (page 4, line 1-2).

Comment #4

Line 17: It’s not more correct, perhaps could be more representative or easier to interpret.

Answer:

We agree that this sentence can be misleading and have changed the wording to: “… the percentage change score can provide a more representative result at group level” (page 4, line 9).

Comment #5

Page 4 line 2: Why was mJOA not evaluated?

Answer:

Although mJOA is a widely used PROM, the use is not frequent in Scandinavia. Also, mJOA is not included in the patient questionnaire of the Norwegain Spine Registry and could followingly not be included in the study.

Comment #6:

Line 9: what was the distribution ACDF/ ACDA?

Answer:

As mentioned in the aswers to Reviewer #1, the majority of the anterior approach procedures were ACDF (363, 98%) and only 8 (2%) ACDAs were performed. The information has been added to the text on page 6, line 7.

Comment #7:

Page 7 Line 1 How did the authors check for normality?

Answer:

We examined all continuous variables with respect to normality by using histograms and by analyzing skewness and kurtosis. All variables were normally distributed.

Comment #8:

Page 15 Line 5: More accurate needs to be qualified more comprehensively please

Answer:

If we understand the reviewer correctly, there is need for a quantification of the term “accuracy”.

In general, the percentage change scores were more accurate than the change scores. Table 3 shows that the majority of percentage change scores in general had higher AUC values, including higher sensitivity and specificity values, than change scores of the majority of PROMs.

Please see edits on page 16, line 4-9. If we have misinterpreted the question, please clarify again.

Comment #9

Page 16 Line 15: How do you discriminate between the effects of ant/post approach and age? Summarize here please.

Answer:

We are not sure how to interpret this comment. However, if the question is whether surgery OR age influence the proportion of improvement according to MCIDs, our study results do not reflect this issue directly. We would need a RCT in order to respond properly to this question. We have, however, conducted a logistic regression analysis in which we investigated to what extent surgery approach, age and the interaction between these two factors predicted the proportion of improvements when using the MCID for NDI (4.30 cut-off value). We found that neither surgical approach (p=0,17), age (p=0,22) or the interaction between these two factors (p=0,26) predicted this outcome.

If this is not the right interpretation of the reviewer’s question, please specify the comment.

References:

1. Goh GS, Liow MHL, Ling ZM, Soh RCC, Guo CM, Yue WM, Tan SB, Chen JL (2020) Severity of Preoperative Myelopathy Symptoms Affects Patient-reported Outcomes, Satisfaction, and Return to Work After Anterior Cervical Discectomy and Fusion for Degenerative Cervical Myelopathy. Spine (Phila Pa 1976) 45:649-656. doi: 10.1097/brs.0000000000003354

2. Stull JD, Goyal DKC, Mangan JJ, Divi SN, McKenzie JC, Casper DS, Okroj K, Kepler CK, Vaccaro AR, Schroeder GD, Hilibrand AS (2020) The Outcomes of Patients With Neck Pain Following ACDF: A Comparison of Patients With Radiculopathy, Myelopathy, or Mixed Symptomatology. Spine (Phila Pa 1976) 45:1485-1490. doi: 10.1097/brs.0000000000003613

3. Mokkink LB, Terwee CB, Knol DL, Stratford PW, Alonso J, Patrick DL, Bouter LM, de Vet HC (2010) The COSMIN checklist for evaluating the methodological quality of studies on measurement properties: a clarification of its content. BMC Med Res Methodol 10:22. doi: 10.1186/1471-2288-10-22

4. de Vet HC, Ostelo RW, Terwee CB, van der Roer N, Knol DL, Beckerman H, Boers M, Bouter LM (2007) Minimally important change determined by a visual method integrating an anchor-based and a distribution-based approach. Qual Life Res 16:131-142. doi: 10.1007/s11136-006-9109-9

5. Clement ND, Weir D, Holland J, Gerrand C, Deehan DJ (2019) Meaningful changes in the Short Form 12 physical and mental summary scores after total knee arthroplasty. Knee 26:861-868. doi: 10.1016/j.knee.2019.04.018

6. Chung AS, Copay AG, Olmscheid N, Campbell D, Walker JB, Chutkan N (2017) Minimum Clinically Important Difference: Current Trends in the Spine Literature. Spine (Phila Pa 1976) 42:1096-1105. doi: 10.1097/BRS.0000000000001990

7. Mjåset C, Zwart J-A, Goedmakers CMW, Smith TR, Solberg TK, Grotle M Criteria for Success after Surgery for Cervical Radiculopathy; Estimates for a Substantial Amount of Improvement in Core Outcome Measures. The Spine Journal. doi: 10.1016/j.spinee.2020.05.549

8. Werner DAT, Grotle M, Gulati S, Austevoll IM, Lonne G, Nygaard OP, Solberg TK (2017) Criteria for failure and worsening after surgery for lumbar disc herniation: a multicenter observational study based on data from the Norwegian Registry for Spine Surgery. Eur Spine J 26:2650-2659. doi: 10.1007/s00586-017-5185-5

9. Austevoll IM, Gjestad R, Grotle M, Solberg T, Brox JI, Hermansen E, Rekeland F, Indrekvam K, Storheim K, Hellum C (2019) Follow-up score, change score or percentage change score for determining clinical important outcome following surgery? An observational study from the Norwegian registry for Spine surgery evaluating patient reported outcome measures in lumbar spinal stenosis and lumbar degenerative spondylolisthesis. BMC Musculoskelet Disord 20:31. doi: 10.1186/s12891-018-2386-y

10. Kim PK, Alexander JT (2006) Indications for circumferential surgery for cervical spondylotic myelopathy. Spine J 6:299s-307s. doi: 10.1016/j.spinee.2006.04.025

11. Fehlings MG, Ibrahim A, Tetreault L, Albanese V, Alvarado M, Arnold P, Barbagallo G, Bartels R, Bolger C, Defino H, Kale S, Massicotte E, Moraes O, Scerrati M, Tan G, Tanaka M, Toyone T, Yukawa Y, Zhou Q, Zileli M, Kopjar B (2015) A global perspective on the outcomes of surgical decompression in patients with cervical spondylotic myelopathy: results from the prospective multicenter AOSpine international study on 479 patients. Spine (Phila Pa 1976) 40:1322-1328. doi: 10.1097/brs.0000000000000988

12. Gornet MF, Burkus JK, Shaffrey ME, Schranck FW, Copay AG (2019) Cervical disc arthroplasty: 10-year outcomes of the Prestige LP cervical disc at a single level. J Neurosurg Spine:1-9. doi: 10.3171/2019.2.Spine1956

13. Gornet MF, Lanman TH, Burkus JK, Hodges SD, McConnell JR, Dryer RF, Copay AG, Nian H, Harrell FE (2017) Cervical disc arthroplasty with the Prestige LP disc versus anterior cervical discectomy and fusion, at 2 levels: results of a prospective, multicenter randomized controlled clinical trial at 24 months. Journal of Neurosurgery: Spine SPI 26:653-667. doi: 10.3171/2016.10.SPINE16264

14. Hermansen E, Myklebust TA, Austevoll IM, Rekeland F, Solberg T, Storheim K, Grundnes O, Aaen J, Brox JI, Hellum C, Indrekvam K (2019) Clinical outcome after surgery for lumbar spinal stenosis in patients with insignificant lower extremity pain. A prospective cohort study from the Norwegian registry for spine surgery. BMC Musculoskelet Disord 20:36. doi: 10.1186/s12891-019-2407-5

15. Solberg T, Johnsen LG, Nygaard OP, Grotle M (2013) Can we define success criteria for lumbar disc surgery? : estimates for a substantial amount of improvement in core outcome measures. Acta Orthop 84:196-201. doi: 10.3109/17453674.2013.786634

16. Wilson JRF, Badhiwala JH, Moghaddamjou A, Yee A, Wilson JR, Fehlings MG (2020) Frailty Is a Better Predictor than Age of Mortality and Perioperative Complications after Surgery for Degenerative Cervical Myelopathy: An Analysis of 41,369 Patients from the NSQIP Database 2010-2018. J Clin Med 9:3491. doi: 10.3390/jcm9113491

Submitted filename: Response to Reviewers.docx

Click here for additional data file.

21 Feb 2022

Clinical Improvement after Surgery for Degenerative Cervical Myelopathy; a Comparison of Patient-Reported Outcome Measures during 12-Month Follow-Up

PONE-D-21-25531R1

Dear Dr. Mjåset,

We’re pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it meets all outstanding technical requirements.

Within one week, you’ll receive an e-mail detailing the required amendments. When these have been addressed, you’ll receive a formal acceptance letter and your manuscript will be scheduled for publication.

An invoice for payment will follow shortly after the formal acceptance. To ensure an efficient process, please log into Editorial Manager at http://www.editorialmanager.com/pone/, click the 'Update My Information' link at the top of the page, and double check that your user information is up-to-date. If you have any billing related questions, please contact our Author Billing department directly at authorbilling@plos.org.

If your institution or institutions have a press office, please notify them about your upcoming paper to help maximize its impact. If they’ll be preparing press materials, please inform our press team as soon as possible -- no later than 48 hours after receiving the formal acceptance. Your manuscript will remain under strict press embargo until 2 pm Eastern Time on the date of publication. For more information, please contact onepress@plos.org.

Kind regards,

Michael G. Fehlings

Academic Editor

PLOS ONE

Additional Editor Comments (optional):

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #2: All comments have been addressed

**********

2. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #2: Yes

**********

3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #2: Yes

**********

4. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #2: Yes

**********

5. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #2: No

**********

6. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #2: A much improved manuscript which is clearer in its aims and in its conclusions. The authors defend their manuscript in the response to reviewers. There remains several typographical and grammatical errors, otherwise the report is satisfactory at this stage.

**********

7. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #2: No

28 Feb 2022

PONE-D-21-25531R1

Clinical Improvement after Surgery for Degenerative Cervical Myelopathy; a Comparison of Patient-Reported Outcome Measures during 12-Month Follow-Up

Dear Dr. Mjåset:

I'm pleased to inform you that your manuscript has been deemed suitable for publication in PLOS ONE. Congratulations! Your manuscript is now with our production department.

If your institution or institutions have a press office, please let them know about your upcoming paper now to help maximize its impact. If they'll be preparing press materials, please inform our press team within the next 48 hours. Your manuscript will remain under strict press embargo until 2 pm Eastern Time on the date of publication. For more information please contact onepress@plos.org.

If we can help with anything else, please email us at plosone@plos.org.

Thank you for submitting your work to PLOS ONE and supporting open access.

Kind regards,

PLOS ONE Editorial Office Staff

on behalf of

Dr. Michael G. Fehlings

Academic Editor

PLOS ONE