Minimally Invasive Unilateral vs. Bilateral Pedicle Screw Fixation and Lumbar Interbody Fusion in Treatment of Multi-Segment Lumbar Degenerative Disorders.

BACKGROUND The choice for instrumentation with minimally invasive transforaminal lumbar interbody fusion (MIS-TLIF) in treatment of degenerative lumbar disorders (DLD) remains controversial. The goal of this study was to investigate clinical outcomes in consecutive patients with multi-segment DLD treated with unilateral pedicle screw (UPS) vs. bilateral pedicle screw (BPS) instrumented TLIF. MATERIAL AND METHODS Eighty-four consecutive patients who had multi-level MIS-TLIF were retrospectively reviewed. All data were collected to compare the clinical outcomes between the 2 groups. RESULTS Both groups showed similar clinical function scores in VAS and ODI. The two groups differed significantly in operative time (P<0.001), blood loss (P<0.001), and fusion rate (P=0.043), respectively. CONCLUSIONS This study demonstrated similar clinical outcomes between UPS fixation and BPS procedure after MIS-TLIF for multi-level DLD. Moreover, UPS technique was superior in operative time and blood loss, but represented lower fusion rate than the BPS construct did.

Background

Minimally invasive transforaminal lumbar interbody fusion (MIS-TLIF) is a popular and effective surgical technique for the treatment of degenerative lumbar disorders (DLD), including spondylolisthesis, lumbar spinal canal stenosis associated with deformities, and discogenic pain identified by provocative discography [1-4]. Compared with the traditional open surgery, MIS-TLIF has multiple advantages, such as the decreased approach-related muscle damage, lesser blood loss, lower postoperative pain, shorter length of hospital stay, and minor postoperative narcotic usage allowing for early activity [1,5]. Many previous studies have demonstrated that MIS-TLIF could achieve excellent clinical outcomes [6-9].

Although MIS-TLIF is widely performed with the treatment of DLD, the choice for instrumentation with spinal fusion procedures remains controversial. In general, bilateral pedicle screw (BPS) fixation for MIS-TLIF is preferred as a standard procedure due to its rigid fixation, great biomechanical stability and good clinical results [9,10]. However, some studies have indicated that increased number of implants and excessive rigidity can lead to more adverse clinical effects, such as reducing fusion rate and adjacent segment degeneration [11,12]. Recently, unilateral pedicle screw (UPS) fixation has been recommended by an increasing number of surgeons [13-16]. UPS fixation for the MIS-TLIF has multiple advantages in reduced soft tissue disruption of the contralateral side, shorter surgical time, and lower implant costs [17-19], but relatively provides less rotational stability and stiffness based on many biomechanical studies[20].

As far as we know, few previous clinical trials comparing UPS versus BPS fixation for open or mini-open TLIF in multi-segment DLD have been reported. Based on the previous studies which have shown similar clinical and fusion results of UPS as those of bilateral fixation, we conducted this retrospective study to compare clinical outcomes in consecutive patients with multi-segment DLD treated with UPS or BPS instrumented TLIF.

Material and Methods

This study was approved by the Committee of Medical Ethics and the institutional review boards of Yuhuangding Hospital. The study period was from January 2010 to April 2013. Informed consent was obtained from patients or their family members if the patient was unable to provide consent. A total of 84 consecutive patients who had undergone a multi-level MIS TLIF by the senior surgeon were enrolled. Patients treated with BPS fixation for MIS-TLIF were compared with those with UPS construct, based on age, sex, and body mass index (BMI). Indications for surgery were: 1) spinal stenosis, 2) lumbar disk herniation, and 3) spondylolisthesis. Only those subjects aged 18 to 70 years could be included. Patients enrolled in our study were excluded if they had the following: 1) active infection, 2) metabolic disease, 3) severe osteoporosis, 4) severe chronic disease, 5) symptomatic vascular disease, or 6) previous lumbar surgery. The merits and drawbacks of each procedure were thoroughly discussed with the patients and their family. All data, including patient demographics, examination results, and operative data, were obtained from hospital records. All patients received 2-year follow-up postoperatively. The radiographic data were assessed individually by 2 senior specialists.

Surgical techniques

The patients were placed in the prone position under general anesthesia. A C-arm image intensifier was used to determine the location of the interbody level. We used the local autograft and Capstone cages (Medtronic Sofamor Danek, Memphis, Tennessee) and pedicle screws (Legacy; Medtronic Sofamor Danek) in the surgery. UPS fixation placed at the time of MIS-TLIF applied in this study was previously described by Lee et al. [21], and BPS was introduced as by Choi et al. [22]. All operations were performed by the same surgeon (Figure 1).

Figure 1

X-ray films showed MIS-TLIF with pedicle screw fixation. (A) MIS TLIF with BPS fixation. (B) MIS TLIF with UPS fixation.

A standard postoperative protocol was used for all patients. Drainage was placed for 48 hours postoperatively and intravenously prophylactic antibiotics were given for 24 hours postoperatively. Waist muscle function exercises with the legs straight were required. Patients in the unilateral group were mobilized early out of bed 24 hours postoperatively if no contraindications existed.

Outcomes assessment

All parameters, including blood loss, operative time, duration of hospital stay, complication rate, visual analog scale (VAS), and Oswestry Disability Index (ODI) scores, were obtained and compared to evaluate efficacy between the 2 groups. All patients were asked to return for follow-up at 1 week and 3, 6, 12, and 24 months postoperatively. Preoperative and postoperative radiographs, including anteroposterior and lateral flexion-extension, were used to evaluate fusion status, screw failure, and other complications. Fusion rate was measured according to the method of Schulte et al. [23].

All statistical analyses were performed with SPSS version 17.0 software. Categorical variables were compared with the chi-square test. The outcomes between 2 groups were tested with a paired t test. The comparisons of continuous data presented as mean±standard deviation (SD) were analyzed with an independent-samples t test. A P value less than 0.05 was considered significant.

Results

In this study, a total of 84 patients were followed up for an average of 26.2 months (range, 23–36 months). Mean length of follow-up was 26.7 months for the UPS group and 23.6 months for the BPS group. In the UPS group, 7 patients (16.7%) were diagnosed with spinal stenosis, 25 (59.5%) were diagnosed with lumbar disk herniation, and 10 patients (23.8%) were diagnosed with spondylolisthesis. In the BPS group, 9 (21.4%) were diagnosed with spinal stenosis, 22 (52.4%) with lumbar disk herniation, and 11 (26.2%) with spondylolisthesis. No significant differences were found between the 2 groups in patient demographics (Table 1).

Table 1

Patient demographics and preoperative data.

Parameter	UPS group	BPS group	P value
Sample size (n)	42	42	–
Age (mean ±SD, years)	61.4±11.8	62.1±10.2	0.613
Gender (M/F)	26/16	25/17	0.823
BMI (kg/m2)	24.3±3.2	24.5±3.1	0.437
Diagnosis			0.659
Spinal stenosis	7 (16.7%)	9 (21.4%)	–
Lumbar disk herniation	25 (59.5%)	22 (52.4%)	–
Spondylolisthesis	10 (23.8%)	11 (26.2%)	–
Symptom duration (mo)	11.2±4.3	12.4±4.1	0.776
Follow-up (mo)	25.8±16.4	26.4±17.2	0.898
Preoperative VAS (mean ±SD)	7.7±1.2	7.8±1.5	0.813
Preoperative ODI (mean ±SD)	44.2±12.1	43.8±11.9	0.689

UPS – unilateral pedicle screw; BPS – bilateral pedicle screw; SD – standard deviation; BMI – body mass index; VAS – visual analog scale; ODI – Oswestry Disability Index.

Table 2 shows the clinical and functional outcomes for the 2 groups. Mean length of hospital stay was 12.6 days in the UPS group and 13.4 days in the BPS group. No significant difference was found in hospital stay between the 2 groups. However, the UPS group required a shorter operative time and had less blood loss than the BPS group (P<0.01).

Table 2

Clinical and functional outcomes for the two groups.

	UPS group (n=42)	BPS group (n=42)	P value
Hospital stay (d)	12.6±2.6	13.4±2.1	0.122
Operative time (min)	92.1±21.6	112.3±25.6	<0.001*
Blood loss (mL)	254±48.2	467±43.3	<0.001*
Complication rate (%)	1 (2.4%)	3 (7.1%)	0.306
Fusion rate (%)	34 (81.0%)	40 (95.2%)	0.043*
VAS (mean ±SD)	1.8±1.2	2.2±1.4	0.162
ODI (mean ±SD)	17.4±4.7	16.6±7.5	0.558

P value was significant.

Mean VAS score was 1.8±1.2 in the UPS group postoperatively and 2.2±1.4 in the BPS group. Patients in the UPS group postoperatively had an average ODI score of 17.4±4.7 and patients in the BPS group with16.6±7.5. Both mean postoperative VAS and ODI improved significantly in each group, compared with preoperative VAS and ODI; however, no statistically significant differences were obtained between the 2 groups (Table 2). With respect to fusion rate, 81.0% of patients in the UPS group and 95.2% of patients in the BPS group achieved successful fusion, which showed a significant difference (P=0.043). Neither group showed device-related complications, such as screw loosening or breakage, or fusion cage migration. With regard to general complications, there was no difference between the BPS and the UPS group (P>0.05). One patient in the UPS group and 3 patients in the BPS group developed superficial wound infections. All infections were completely controlled by intravenous antibiotics and daily dressing.

Discussion

Spinal fusion surgery is an effective method in the treatment of painful DLD. With the development of advanced systems that can provide adequate access for decompression and instrumentation placement and reduce tissue disruption, the MIS-TLIF has become more popular in the treatment of DLD. Recent biomechanical studies have suggested the equivalence between UPS fixation and standard BPS constructs [24-26], and some clinical data [14,15,17] have demonstrated acceptably reliable fusion rates in patients with UPS fixation, which requires fewer pedicle screws. However, Slucky et al. [24] reported that UPS after MIS-TLIF led to less rotational stability and stiffness than BPS fixation. Thus, the choice between BPS or UPS fixation after lumbar fusion for the treatment of DLD remains controversial.

The main aim of the pedicle screw is to stabilize the spine to promote fusion; therefore, the fusion rate is considered as the most important outcome. Several studies have revealed that the unilateral fixation for TLIF achieves good outcomes [27,28]. Deutsch et al. [28], in their studies in 2006, reported good outcomes of UPS fixation after TLIF with a greater than 20-point reduction in the Oswestry Disability Index (ODI) score, but they did not report the exact fusion rate and only evaluated the outcomes of short-term follow-up of less than 1 year. Both Suk et al. [14] and Xue et al. [17] compared the efficacy of UPS fixation vs. BPS fixation after MIS-TLIF and reported lower fusion rates for the former; however, all patients in their studies received single-level fusion procedure. Some studies have shown that unilateral instrumentation might be not be suitable for multilevel fusion due to its inadequate fixation strength. Zhang et al. [29], in their prospective randomized study, presented similar failed fusion rates of 2-level fusion in the UPS group (3/16) and the BPS group (2/11). They concluded that UPS fixation in multi-level DLD is similarly effective and safe. However, our study showed a different result – that patients with UPS had a lower fusion rate (81.0% vs. 95.2%) and that BPS fixation was significantly safer. As many biomechanical studies have reported, the negative impact of the fusion in unilateral instrumentation might be due to less biomechanical stability. Moreover, we found less blood loss and a shorter operative time in patients with UPS fixation after MIS TLIF, compared with BPS fixation. These findings are consistent with the results of previous studies [17,28,30].

With respect to functional scores, there were no differences between UPS and BPS fixation procedure in VAS and ODI. This finding is consistent with the results from many previous studies [31-33], although the outcomes of patients in some of these studies were assessed using other assessment systems, including the Japanese Orthopaedic Association (JOA), mProlo, and 36-Item Short Form Healthy Survey version 2 (SF-36v2) scores. Patients with UPS fixation procedure had significantly had less blood loss and experienced shorter operation time as compared with those the BPS fixation in our study. It is mainly due to dissection of soft tissue and insertion of pedicle screws only on 1 side for UPS fixation, which takes less time and decreases blood loss.

Hardware-related complications often cause serious adverse effects in fusion surgery. In our study, some patients had infections. One patient in the UPS group and 3 patients in the BPS group developed superficial wound infections. Similar to results in previous studies [14-16,34,35], we also found there was no difference in terms of complication rate between the 2 procedures (UPS vs. BPS, 2.4% vs. 7.1%). Several meta-analyses also demonstrated that patients with UPS procedure experienced similar complication rates as those with BPS procedure.

Several important limitations in this study should be considered. First of all, the relatively small sample size might limit the comparability and outcomes. Secondly, the follow-up durations in our study were not long enough to determine the results. Finally, the design of this study was not random, which could not adequately assess the outcomes of the 2 surgical methods. Further studies are required to compare the efficiency and safety of UPS fixation in multi-level DLD.

Conclusions

In summary, the present study demonstrated that MIS-TLIF with UPS fixation leads to similar clinical outcomes, compared with BPS procedure for multi-level DLD. Despite an association with decreased operative time and less blood loss, the UPS technique had a lower fusion rate than the BPS construct did. Due to the limitations of this study, multi-center studies with more patients and longer follow-up period are required to further evaluate the outcomes of the 2 systems.