Anterior bone loss after cervical disc replacement: A systematic review.

BACKGROUND: Anterior bone loss (ABL) is a relatively easily neglected condition after cervical disc replacement (CDR). Whether this phenomenon is a radiological anomaly or a complication remains controversial. Several studies have reported the clinical characteristics of ABL and speculated on the pathogenic mechanism based on a certain type of artificial disc, while the overall understanding of ABL is lacking. AIM: To describe the prevalence, impacts, and risk factors of ABL after CDR.
METHODS: We searched the PubMed, Cochrane Library, and Excerpta Medica databases using the terms "bone loss" or "bone remodeling" or "bone absorption" or "osteolysis" or "implant loosening" or "implant migration" or "hypersensitivity" or "hyperreactivity", "cervical disc replacement" or "cervical disc arthroplasty" or "total disc replacement". Eligible manuscripts on the prevalence and impacts of ABL were reviewed by the authors. Data extraction was performed using an established extraction form. The results of the included studies were described narratively.
RESULTS: Six studies met the inclusion and exclusion criteria. One was a prospective study and the others were retrospective studies. A total of 440 patients with 536 segments were included. The artificial cervical discs included Bryan, Baguera-C, Discocerv, and Mobi-C. The prevalence of ABL ranged from 3.13% to 91.89%, with a combined overall prevalence of 41.84%. ABL occurred within 6 mo and stopped 12 mo after surgery. Several cases were noted to have a self-healing process. Severe ABL resulted in segmental kyphosis, implant subsidence, and persistent neck pain. ABL may be related to heterotopic ossification. Multilevel surgery may be one of the risk factors for ABL.
CONCLUSION: ABL is a common condition after CDR. The underlying mechanisms of ABL may include stress concentration and injury to nutrient vessels. ABL should be considered a complication after CDR as it was associated with neck pain, implant subsidence, and heterotopic ossification. ©The Author(s) 2020. Published by Baishideng Publishing Group Inc. All rights reserved.

Core Tip: Anterior bone loss (ABL) is a common condition after cervical disc replacement. Several studies have reported the clinical characteristics of ABL; however, it remains unclear whether this phenomenon is a radiological anomaly or a complication. In this review, we found that the prevalence of ABL after cervical disc replacement was related to the type of implant. ABL should be considered a complication after cervical disc replacement as it was associated with neck pain, implant subsidence, and heterotopic ossification. Fortunately, ABL did not progress after 12 mo postoperatively, and some cases showed a self-healing phenomenon. The underlying mechanisms of ABL may include stress concentration and injury to nutrient vessels.

INTRODUCTION

Anterior cervical discectomy and fusion (ACDF) is recognized as the standard procedure for cervical degenerative disc disease. However, concerns such as non-union, pseudarthrosis, and acceleration of adjacent segment degeneration have been raised[1-4]. Compared with ACDF, cervical disc replacement (CDR), a motion preservation technique, can reduce intervertebral disc pressure in adjacent levels and better simulate the physiological condition of the cervical spine[5,6]. Studies have shown that CDR and ACDF have equivalent efficiency in clinical outcomes such as symptom relief[7,8]. However, CDR is better in some respects, such as faster recovery, higher patient satisfaction, and is more cost-effective[9,10].

Although CDR is widely performed and patients have benefited due to its satisfying results, it has some implant-related complications, such as heterotopic ossification (HO), implant migration, and implant subsidence[11-16]. With regard to these complications, in-depth studies have been performed to determine the potential risk factors and clinical impacts[17-20]. In contrast to excessive bone formation (which is HO), anterior bone loss (ABL) is another implant-related condition after CDR and has gained more attention in recent years[21]. ABL is the process of bone rebuilding in the ventral part of vertebral bodies, and can usually be recognized from lateral radiographs. Peri-prosthetic bone loss has been extensively described in large joint arthroplasty. However, ABL is easily ignored in cervical disc arthroplasty.

The prevalence of ABL after CDR varies from 3.13% to 91.89% and differs greatly among different types of artificial cervical discs[22-27]. Several studies have reported the clinical characteristics of ABL and speculated the pathogenic mechanism based on a certain type of artificial disc, while the overall understanding of ABL after CDR is lacking. In addition, it remains controversial whether ABL is a radiological anomaly or a complication. Therefore, we performed this systematic review of currently available clinical data to comprehensively describe the prevalence, impacts, and risk factors of ABL after CDR.

MATERIALS AND METHODS

A comprehensive literature search was carried out in PubMed, Cochrane Library, and the Excerpta Medica databases. Studies before May 2019 were included. The following keywords were used in the search: “bone loss” or “bone remodeling” or “bone absorption” or “osteolysis” or “implant loosening” or “implant migration” or “hypersensitivity” or “hyperreactivity”, “cervical disc replacement” or “cervical disc arthroplasty” or “total disc replacement”. Articles published in English that involved peri-prosthetic bone absorption after CDR were specifically identified. The references of all identified papers were manually reviewed to identify further potentially relevant studies.

The search produced 1460 published articles. We then systematically assessed the selected articles. The inclusion criteria were as follows: (1) peri-prosthetic bone loss occurred in the surgical segment after CDR; and (2) bone remodeling occurred in the anterior/ventral part of vertebral bodies. The exclusion criteria were as follows: (1) Peri-prosthetic bone loss occurred in another part of the vertebral bodies, (2) Case reports, (3) Reviews, (4) Commentaries, and (5) Cadaveric or experimental studies. The results of the literature search are shown in Figure 1.

Figure 1

Preferred reporting items for systematic reviews. Preferred reporting items for systematic reviews flow diagram for selection of studies based on inclusion criteria during systematic review.

The screening of titles and abstracts was performed by two reviewers (Wang XF and Meng Y) independently. Articles that met the eligibility criteria on the first screening were further assessed by reading the full text. After screening, six clinical studies were included in the systematic review. The two reviewers (Wang XF and Meng Y) then performed data extraction and rated the level of evidence of each article independently using the published guideline[28]. The results of this study were described narratively.

RESULTS

One randomized controlled study and five retrospective studies were included in this review[22-27]. A total of 440 patients with 536 segments were included. The mean age of these patients ranged from 45.3 to 50.1 years, and the mean follow-up time ranged from 32.3 to 74.4 mo. The artificial cervical discs included Bryan, Baguera-C, Discocerv, and Mobi-C. The surgical procedure included single-level CDR, double-level CDR, triple-level CDR, and hybrid surgery. Study information and level of evidence are listed in Table 1. Detailed information on ABL including the prevalence, course, effects, and outcomes are summarized in Tables 2-4 and Figure 2.

Table 1

Studies included in the systematic review

Ref.	Yr	Study type	Level of evidence	Number and type of artificial disc	Sex	Age (yr)	Follow-up
Ren et al[22]	2011	Prospective and nonrandomized study	3	Total (Bryan): 45 patients and 51 implants	19 Female; 26 Male	46 (31-50)	Mean 35 (24-70) mo
Hacker et al[23]	2013	Randomized controlled study	2	Total 51 patientsBryan: 32 patientsPrestige-LP: 19 patients	27 Female; 24 Male	-	1, 6, 12, 24 wk and 12, 24, 48, 60 mo
Kim et al[24]	2015	Prospective registry with retrospective analysis	3	Total (Bryan): 37 patients	13 Female; 24 Male	45.4 (27-55)	1, 3, 6, 12, 24, 36 mo, mean 60.1 (42-113) mo
Heo et al[25]	2017	Retrospective observational study	3	Total (Baguera-C): 48 patients	30 Female; 18 Male	50.1 ± 7.4	1, 6, 12, 24 mo, mean 32.3 ± 3.3 mo
Kieser et al[26]	2018	Retrospective observational study	3	Total 145 patients and 193 implants Bryan: 32 patients, 56 implants, Discocerv: 38 patients, 44 implants; Baguera-C: 44 patients, 56 caudal implants; Mobi-C: 31 patients, 37 implants	78 Female; 67 Male	45 (25-65)	3, 6, 12, 24, 36, 48, 60 mo, mean 6.2 (5-10) yr
Kieser et al[27]	2019	Retrospective observational study	3	Total 114 patients and 156 implants; Bryan: 32 patients, 56 implants; Discocerv: 38 patients, 44 implants; Baguera-C: 44 patients, 56 caudal implants	65 Female; 49 Male	45.3 (28-65)	6 wk, 3, 6, 9, 12, 24 mo, 5 yr, maximum 8 yr

Table 2

Summary of the combined data of anterior bone loss in cervical disc replacement

Implant	Prevalence	Course	Effect	Outcome
Total	41.84% (159/380)	Occur within 3 mo postop; Does not progress after 12 mo postop; Self-healing in several cases	Higher VAS score within 12 mo postop in severe cases; Exposure or subsidence of implants in severe cases	No significant difference compared with patients without bone loss at the last follow-up
Bryan	22.16% (39/176)
Discocerv	47.73% (21/44)
Baguera-C	62.50% (65/104)
Mobi-C	91.89% (34/37)

VAS: Visual analogue score; postop: Postoperatively.

Table 4

Effects, outcomes, and risk factors of anterior bone loss

Ref.	Radiological effects	Clinical effects	Treatment	Potential risk factors
Ren et al[22]	Grade 4 HO in 1 patient with > 2 mm ABL distance, grade 2 HO in 1 patient; only 3 patients had HO in this study	No effect	Monitored	Micromovement of implant
Hacker et al[23]	Kyphosis of shell angle and FSU angle; Implant subsidence	Recurrent neck and arm pain persisting 52 mo postop	Revision fusion surgery at index and lower adjacent level	Low virulence bacterial infection of endplates at arthroplasty level
Kim et al[24]	Ossification of ALL at the inferior adjacent level in 1 patient	No effect	Monitored	Stress shielding; Friction and wear debris between the anterior flange and vertebra (less likely)
Heo et al[25]	No effect	Grade 3 ABL had significantly higher mean VAS score for neck pain at 6 mo (5.3 vs 1.8) and 12 mo (4.7 vs 1.8) postop	Monitored	Stress shielding;good motion function of implants
Kieser et al[26]	A significant relationship with HO	A lower NDI score 5 yr postop (P < 0.1)	Not reported	More operative levels; less of traction of ALL; surgical exposure
Kieser et al[27]	Exposure or subsidence of implant in severe ABL	No effect	Monitored	Grade 1-2: direct anterior vertebral injury including heat necrosis or resected ALL; Grade 3-4: avascular necrosis caused by injury to nutrient vessels

HO: Heterotopic ossification; FSU: Functional spine unit; ALL: Anterior longitudinal ligament; postop: Postoperatively; ABL: Anterior bone loss; VAS: Visual analogue score; NDI: Neck disability index.

Figure 2

Summarized grading systems of the included studies. ABL: Anterior bone loss, colored in gray; VAS: Visual analogue score; NDI: Neck disability index.

Overview of ABL after cervical disc arthroplasty

ABL was identified on lateral X-rays in all six studies, and had the following two signs: first, the ventral-inferior part of the cranial vertebra or the ventral-superior part of the caudal vertebra at the arthroplasty level disappeared; second, the cortical bone margin of the ABL area still existed and shifted dorsally.

The prevalence of ABL ranged from 3.13% to 91.89%, with a combined overall prevalence of 41.84%, and it differed depending on the implant type. The Mobi-C showed the highest prevalence of ABL (91.89%), followed by the Baguera-C (60.42%-64.29%, 62.50% after combination), Discocerv (47.73%) and Bryan (3.13%-57.14%, 22.16% after combination). In the study by Kieser et al[26], the cranial vertebra showed a higher ABL rate.

ABL occurred within 3-6 mo postoperatively and did not progress after 12 mo postoperatively in most cases. Kieser et al[26,27] reported a “self-healing phenomenon” of ABL in several cases, but they did not present other radiological evidence to prove that the self-healing was not caused by HO. In the study by Hacker, one patient with ABL received revision fusion surgery to relieve symptoms[23].

The grading system of ABL

Three studies reported the grading system of ABL based on the lateral radiographs (Table 2). Heo et al[25] divided the ABL into three grades: Grade 1 refers to a small part of the vertebra with ABL; Grade 2 indicates a larger portion of the vertebra with ABL compared with Grade 1, but the implant was not exposed; Grade 3 refers to significant bone loss with exposure of the implant. This non-quantitative grading method is ambiguous when classifying Grade 1 and 2 ABL. The authors then assigned Grade 1 and 2 ABL into the minor change Group (without exposure of implants), and assigned Grade 3 into the major change group (with implant exposure). The prevalence of ABL was 43.75% in the minor change group and 16.67% in the major change group. This grading system is not suitable for artificial discs with an anterior flange such as Bryan, because a small portion of ABL would cause implant exposure. Besides, an obvious ABL may be assigned to the minor change group due to the posteriorly positioned implant.

Kieser et al[26] introduced a quantitative grading method based on the lateral radiographs. They calculated the ratio of the bone loss distance and the initial length of the endplate and then divided the ABL into three groups: mild (ratio ≤ 5%), moderate (5% < ratio ≤ 10%), and severe (ratio > 10%). The prevalence of ABL was 48.7% in the mild group, 11.92% in the moderate group, and 3.11% in the severe group, respectively. Severe ABL caused implant exposure or subsidence.

In 2019, Kieser et al[27] slightly revised their grading system. They divided severe ABL into two subgroups: severe with endplate collapse (four patients), and severe without endplate collapse (one patient, who showed a self-healing process). However, in their grading system, there was no significant difference among the groups in terms of radiological or clinical outcomes. Therefore, this grading system is not instructive in clinical work.

Outcomes of ABL

All patients with ABL achieved similar clinical outcomes at the last follow-up, compared with those who did not develop ABL. In the study by Ren et al[22], three patients had ABL, and they did not complain of any subjective discomfort. Two patients had ABL < 2 mm and Grade 2 HO was observed in one of them; one patient had ABL > 2 mm, and grade 4 HO was noted in this patient. It is noteworthy that the authors only observed HO in three patients in this study.

Hacker et al[23] reported one patient with ABL who suffered from recurrent neck and arm pain after surgery for 52 mo. The shell angle was kyphosis at the last follow-up, as well as the functional spine unit angle. This patient received revision surgery, in which the Bryan disc was removed and two-level anterior cervical fusion was performed.

Kim et al[24] described the outcomes of ABL in three patients. The average distance of ABL was 2.57 mm (range 2.0-3.0 mm). These three patients achieved satisfying clinical outcomes without any change in symptoms during follow-up, and their pre- and post-operative read only memory were in the average range of this study. Ossification of the anterior longitudinal ligament (ALL) at the inferior adjacent level was noted in one patient at the two-year follow-up.

Heo et al[25] addressed the clinical effects of ABL. In their research, patients in the major change group (implant exposure) suffered from obvious neck pain postoperatively. The visual analogue scores for neck pain at 6 mo and 12 mo after surgery were 5.3 ± 2.0 and 4.7 ± 1.7, respectively, in the major change group; and 1.8 ± 1.1 and 1.8 ± 0.9, respectively, in the minor change group. Statistical differences were found between the two groups. The motion function was preserved at the last follow-up in the patients with ABL.

Kieser et al[26,27] reported that ABL caused no statistical difference in visual analogue score for arm or neck pain at any time after surgery, although the neck disability index score showed a decreasing trend at the last follow-up in patients with ABL (18.9 in the non-ABL group, 11.2 in the mild group, 10.1 in the moderate group, and 9.0 in the severe group, P = 0.094). Implant exposure or subsidence was observed in patients with severe ABL. Additionally, a significant relationship between Grade 4 HO and ABL was found in their research.

Potential risk factors and mechanism of ABL

Kieser et al[26,27] analyzed the potential risk factors for ABL. In their retrospective studies with a minimum follow-up of five years, they found that the number of levels operated was the only risk factor. Other variables including age, sex, cervical alignment, read only memory, and implant position were not related to ABL. They hypothesized that the extensive exposure and drilling of vertebra might cause a direct osteolytic insult, and the more the dissection and soft-tissue stripping during multilevel surgery, the greater the bone insult. In addition, Kieser found that the cranial endplate was exposed to a greater risk of ABL, which could be due to resection of the ALL. The ALL attaches tightly to the inferior portion of the ventral surface of the cervical vertebrae, and this region would be non-loaded and subsequently resorbed after resection of the ALL.

In 2019, Kieser et al[27] proposed the mechanism of ABL of different degrees. The direct heat necrosis of the vertebra and resection of the ALL may cause Grade 1-2 ABL. Injury to nutrient vessels of the vertebra and subsequent avascular necrosis may lead to grade 3-4 ABL.

Heo et al[25] considered that the stress shielding effects played an important role in ABL. They observed a complete cortical bone margin of the ABL area based on computed tomography scans, and did not find any radiolucent osteolytic lesion adjacent to the implants. This opinion was consistent with that of Kim et al[24].

Kim et al[24] also speculated that the wear debris of artificial discs might induce the inflammatory response, and the inflammatory factors would recruit pre-osteoclasts and initiate the bone remodeling process.

Hacker et al[23] believed that low virulence bacterial infection of the endplate may account for the ABL in their case. They evaluated the tissue samples around the implant and did not find any pathological agent or culture except for macrophages.

In contrast, Ren et al[22] thought that ABL did not look like a low-grade infection. They hypothesized that micromovement would cause mechanical damage to the adjacent vertebra and subsequent bone loss.

DISCUSSION

Peri-implant bone loss has been widely reported in large joint arthroplasty[29-31]. However, studies on bone loss after CDR are lacking. By reviewing the literature, we classified the mechanism of ABL into three major hypotheses: stress shielding, micromovement, and injury to nutrient vessels.

The stress shielding effect has been documented as one of the factors of bone loss in hip and knee replacement[29]. In CDR, this term may be more accurately named as stress concentration. Physiologically, the axial loads transfer evenly and gently from one vertebra to the intervertebral disc, then to another vertebra. However, this mechanical system is altered after disc arthroplasty; the axial loads would transfer through the implant. Therefore, the stress would concentrate on where the implant is located. On the one hand, some artificial discs show obvious stress concentration in the anterior part of the endplates[32,33]. The hyper-pressure in the anterior region can activate the process of bone resorption[34]. On the other hand, the artificial disc does not cover the endplate sufficiently[35,36]. For the non-keel designed artificial disc, for instance, Discocerv and Baguera-C, the anterior part of the endplates cannot be covered, which may lead to hypo-pressure in this region. Based on Wolff’s law, the anterior vertebral hypo-pressure from the artificial disc would initiate bone resorption. Consistent with this hypothesis, previous finite element studies showed that stress was distributed in the covered area of the endplates at the surgical level[37,38]. In addition, Wang et al[21] noted that patients with ABL showed a significantly more lordotic disc angle (which may shift the axial load posteriorly), compared with those who developed anterior HO. Accordingly, stress concentration may be one major reason for the development of ABL.

Micromovement of the implant into the vertebra may induce mechanical damage to the peri-implant bone and subsequent bone loss[22,23]. On the other hand, micromotion may cause artificial discs to produce friction and wear debris, which may induce an inflammatory response and peri-implant bone loss[29,39,40]. Basic studies have proved that the wear debris can initiate bone loss via the RANK/RANKL/OPG pathway, and the inhibition of inflammation or RANK/RANKL pathway has protective effects on peri-implant osteolysis[41-43]. In addition, debris can also induce innervation and pain factor production[42].

Although wear debris-induced osteolysis is widely accepted in large joint arthroplasty, this hypothesis is flawed with regard to two aspects in CDR. First, bone loss caused by wear debris is not confined to the anterior portion of the vertebra, but is around the surface of the implant. For instance, Devin et al[44] reported wear debris-induced bone loss after lumbar disc replacement. Massive osteolysis was noted in the central and posterior region of the vertebral body, and debris was scattered in the adjacent bones. Veruva et al[45] described the presence of osteolytic cysts distributed around artificial lumbar discs caused by the debris. These findings suggest that wear debris-induced bone loss is different to ABL in phenotype. Second, ABL was observed within 3 mo in most cases, and had a self-limited course. In contrast, debris-induced osteolysis may be evident on radiographs after a longer time and may be progressive, which was reported by Tumialán et al[46]. They described bone loss which was noted 9 mo after Prodisc-C implantation. The osteolytic process did not stop until the implant was removed. To sum up, wear debris is not a likely cause of ABL.

The third major hypothesis of ABL is the injury to nutrient vessels of the anterior vertebrae. The anatomic study by Dunbar et al[47] demonstrated the abundant nutrient blood supply in the anterior part of the cervical spine[48]. These vessels pass over the longus colli, and penetrate vertebrae mainly on the anterior surface of the cervical vertebrae. Usually, these nutrient vessels are coagulated to reduce intraoperative bleeding, and this may cause avascular necrosis of the anterior part of the endplate. The anterior nutrient foraminae are mainly located in the superior third of the vertebrae, leaving the inferior portion hypo-vascular. Therefore, theoretically, the inferior portion of vertebrae would be at a higher risk of avascular necrosis, and this is consistent with the clinical findings of Kieser et al[27]. Consequently, ABL may be related to avascular necrosis caused by injury to nutrient vessels.

Kieser et al[26] showed that the more levels operated, the higher the risk of developing ABL. A possible explanation for this might be that multilevel surgery would change the biomechanical properties of the cervical spine leading to hyper- or hypo-pressure in the anterior region of the vertebrae, as described earlier. Additionally, we believe the design of artificial discs is another risk factor in the development of ABL. The prevalence of ABL was different among artificial discs, and this could be attributed to the discrepancy in stress distribution of the endplates of different types of artificial discs.

Some authors consider that heat necrosis due to the burring and milling process could inactivate osteocytes of the anterior vertebra, and result in a direct osteolytic insult. However, the findings of Heo et al[25] and Kieser et al[26,27] do not support this assumption. Even though the anterior cortex of the vertebra was carefully preserved during surgery, ABL still occurred. In addition, Wang et al[21] found that the anterior milling ratio and milling angle were comparable between patients with ABL or anterior bone formation. Therefore, heat necrosis may not be the most important risk factor for ABL.

Usually, patients with ABL do not have clear clinical and radiological effects, while severe ABL may produce segmental kyphosis and local pain[23-25]. Due to the self-limited process, most cases do not require intervention. However, we hold the view that early interventions including rehabilitation exercise and pain relief are necessary for these patients. In our center, we have seen patients with ABL complain of persistent neck pain for one year after surgery. Early interventions can improve the satisfaction of surgery and quality of life. In addition, the impacts of ABL on the adjacent segments remain unknown. Further studies with long-term follow-up are needed to clarify the relationship between ABL and adjacent segment degeneration.

Currently, ABL is observed on lateral radiographs. Notwithstanding this method is simple and easy, but it may increase the false positive rate of ABL. Occasionally, the inappropriate irradiation direction of X-rays may give us a false impression that ABL has occurred. For this reason, computed tomography scans are better in evaluating the prevalence of ABL[25]. Moreover, to prevent and identify ABL early, further studies on the mechanism and risk factors of ABL after CDR are needed.

There are several limitations in our study. The major limitation is the small sample size of each included study. Studies with large sample size are needed to determine the precise prevalence of ABL. As diverse evaluation systems for ABL were used in the included studies, it is difficult to make accurate assessments on the impacts of ABL. In addition, most studies only used non-keeled artificial discs; thus, ABL in keeled artificial discs, such as Prodisc-C and Prestige-LP, is unknown. Finally, we focused on prospective and retrospective studies, and may have missed a wider discussion on the underlying mechanism of ABL in some case reports or case series. Further studies are required to elucidate these limitations.

CONCLUSION

ABL is common in CDR. ABL occurs within 3-6 mo, and stops 12 mo after surgery. Several cases were noted to have a self-healing process. ABL does not have obvious clinical or radiological effects in most patients, while severe ABL may result in segmental kyphosis and persistent pain. ABL may be related to HO; however further studies are required to confirm this. Multilevel surgery is recognized as a potential risk factor for ABL, and the underlying mechanisms include stress concentration and injury to nutrient vessels. ABL should be considered a complication after CDR as it was associated with neck pain, implant subsidence, and HO.

ARTICLE HIGHLIGHTS

Research background

Anterior bone loss (ABL) after cervical disc replacement (CDR) has attracted considerable concern in recent years. Whether ABL is a radiological anomaly or a complication remains unknown.

Research motivation

Several studies have reported the prevalence, impacts, and outcomes of ABL. However, an overall understanding of ABL is lacking.

Research objectives

This study aimed to comprehensively evaluate ABL after CDR.

Research methods

A systematic review was performed according to the preferred reporting items for systematic reviews guideline.

Research results

The prevalence of ABL ranges from 3.13% to 91.89%, and multilevel surgery may be one of the risk factors for ABL. ABL occurred within 6 mo postoperatively and stopped after 1 year. Severe cases may result in kyphosis, implant subsidence, and neck pain.

Research conclusions

ABL is a common condition after CDR. ABL should be considered a complication after CDR due to its clinical impacts.

Research perspectives

Further studies should clarify the relationship between ABL and adjacent segment degeneration. Further studies on the mechanism and risk factors for ABL are needed. The method used to measure ABL should be improved.

ACKNOWLEDGEMENTS

We wish to thank Dr. Ying-Jun Guo and Dr. Jun-Bo He for advice on the discussion.

Table 3

Summarized data of incidence and identification of anterior bone loss

Ref.	Prevalence	Notification time	Course of ABL	Grading system
Ren et al[22]	Bryan: 5.88% more cases were noted from the figures	Within 6 mo postop	Stopped after 6 mo postop	Not reported; ABL distance: < 2 mm in 2 patients, > 2 mm in 1 patient
Hacker et al[23]	Total: 1.96%, Bryan: 3.13%; Prestige-LP: 0	6 wk postop	Not reported	Not reported
Kim et al[24]	Bryan: 8.11%	3-6 mo postop	Progressed within 6 mo postop; Self-limited	Not reported; Mean ABL distance: 2.57 mm (range 2.0-3.0 mm)
Heo et al[25]	Baguera-C: 60.42%	6 mo postop	Progressed within 12 mo postop; Self-limited	Grade 1, Minor, disappearance of the anterior osteophyte or small minor bone loss: 31.25%; Grade 2, Minor, bone loss of the anterior portion of the vertebral bodies at the operated segment without exposure of the implant: 12.5%; Grade 3, Major, significant bone loss with exposure of the anterior portion of the implant: 16.67%
Kieser et al[26]	Total: 63.73%; Cranial: 62.11%; Caudal: 48.7%; Bryan: 57.14%; Discocerv: 47.73%; Baguera-C: 64.29%; Mobi-C: 91.89%	Not reported	Self-limited within 12 mo postop	Mild, ABL ≤ 5%: 48.7%; Moderate, 5% < ABL ≤ 10%: 11.92%; Severe, ABL > 10%: 3.11%
Kieser et al[27]	Total: 57.05%; Cranial: 54.9%; Caudal: 42.31%; Bryan: 57.14%; Discocerv: 47.73%; Baguera-C: 64.29%	Within 3 mo postop	Benign course; progressed within 12 mo postop then stopped; self-healing in several cases	Grade 1, Mild, ABL ≤ 5%: 45.51%; Grade 2, Moderate, 5% < ABL ≤ 10%: 8.33%; Grade 3, Severe, ABL > 10% without endplate collapse: 0.64%; Grade 4, Severe, ABL > 10% with endplate collapse: 2.56%

ABL: Anterior bone loss; postop: Postoperatively.