Comparative Study Between M6‐C and Mobi‐C Cervical Artificial Disc Replacement: Biomechanical Outcomes and Comparison with Normative Data

Abstract

Objective

Cervical spondylosis affects a huge proportion of the middle‐aged population. Degenerative changes can occur in multiple regions of the cervical spine typically affecting the joints, intervertebral discs and endplates. These changes lead to compression of adjacent nervous structures, which results in radiculopathic and myelopathic pain. Various treatment modalities are currently available with non‐surgical approaches the initial go to if there is no symptomatic cord compression. Anterior cervical discectomy and fusion, or arthroplasty are the two common surgical approaches if non‐surgical treatments fail to relieve symptoms of the patients or there are signs of central cord compression. However, studies have shown that there is an increased risk of adjacent segment disease related to fusion. Cervical disc arthroplasty aims to restore normal range of motion (ROM) in patients with pain and disability due to degenerative disc disease resistant to conservative care. Two common disc prostheses used include M6‐C and Mobi‐C. Both prostheses comprise a mobile polymer segment sandwiched between two metal endplates with mechanisms resembling an actual intervertebral disc. This study aims to compare the kinematics associated with these prostheses, against the normal range of motion in the non‐degenerative population.

Method

Patients who underwent M6‐C or Mobi‐C disc replacements by the senior author from 2012 to 2015 were identified at a single tertiary institution. Routine 3‐month postoperative lateral radiographs were analyzed for flexion and extension ROM angles at the involved vertebral level by two independent authors. Data was compared to previous published studies investigating cervical spine ROM of asymptomatic patients.

Results

There was no statistical significance in the difference of overall flexion range between M6‐C and Mobi‐C prostheses. However, overall range of extension of Mobi‐C was greater compared to M6‐C (P = 0.028). At C_5–6, the range of flexion for both implants were similar but lesser compared to asymptomatic patients (P < 0.001). Range of extension was greater in the Mobi‐C group (14.2° ± 5.1°) compared to the M6‐C (7.3° ± 4.6°) (P = 0.0009). At C_6–7, there were no statistical differences in both range of flexion and extension between the two prostheses and asymptomatic patients (P > 0.05).

Conclusion

The early results regarding restoration of ROM following cervical arthroplasty using either M6‐C or Mobi‐C prosthesis are encouraging. Long‐term follow‐up studies are necessary to observe the change in ROM over time with physiological loading and wear patterns.

Introduction

The cervical and lumbar regions are two main weight‐bearing components of the spine. This results in them being commonly affected by age‐related degenerative changes, also known as spondylosis. Cervical spondylosis affects more than half the middle‐aged population with up to 84% related to cervical disc disease1, 2. With age, changes occur to multiple regions of the cervical spine. These include the loss of water content in the nucleus pulposus, leading to decreases in disc height, annular tears leading to disc herniation, osteophyte formation, stenosis of the central canal, and sclerotic changes of the endplates3, 4, 5. In addition, range of motion of the cervical spine reduces as we age6. Although radiographic findings do not completely correlate to symptomology, the above changes affect a significant amount of the population5. Age‐related spondylosis often results in compression of the adjacent nervous structures, resulting in either myelopathy, radiculopathy, or both7, 8. Patients with nerve root compression (radiculopathy) often experience severe neck and arm pain and other sensations such as burning and tingling. Weakness of upper limbs may also be a presenting symptom1, 8. When the central canal is severely narrowed, compression of the central cord results in myelopathic symptoms, such as limbs numbness, posture instability, poor coordination and incontinence9.

Various treatment modalities are currently in practice to address the above conditions. Non‐surgical approaches are generally the initial approach. Oral anti‐inflammatory agents and analgesia are commonly prescribed. This is followed by epidural injections of corticosteroids. Up to approximately 60% of patients reported long‐term alleviation of symptoms with the latter approach10. Decompressive surgical intervention is an alternative for patients who experience radiculopathy without myelopathy but a definitive approach for patients with cord compression8, 11.

Anterior cervical decompression and fusion (ACDF) is an established procedure to treat cervical myelo‐radiculopathy with the secondary negative consequence of sacrificing the natural function of the disc, while placing increased stress on adjacent spinal levels12, 13, 14, 15, 16. The intervertebral disc of the affected level is removed via an anterior exposure and a fusion cage is used to fill up the space after distraction. Bone grafts (usually autograft or allograft) are used to facilitate fusion17, 18. In contrast, an artificial mobile disc is being introduced into the emptied disc space. The cervical total disc replacement maintains motion and decreases adjacent‐level stress and degeneration19, 20. Studies have shown that the rate of adjacent segment disease increases with fusion, especially when there are other asymptomatic changes21. When compared to total disc replacement, the rate of adjacent segment disease was higher in the fusion cohort22.

Normal cervical flexion–extension movement and center of motion has been extensively studied23. Cervical spine range of motion and sagittal balance are measures of severity of cervical pathology, surgical outcomes, and functional impairment. Cervical disc arthroplasty aims to preserve spine range of motion (ROM)24. There are numerous cervical arthroplasty prostheses on the market, with a wide variety of design principles. Two commonly used prostheses in Australia include the M6‐C (Spinal Kinetics, Sunnyvale, CA, USA) and Mobi‐C (LDR Spine USA, Austin, TX, USA); however, these devices differ significantly in materials and design.

The Mobi‐C prosthesis is composed of two endplates made of cobalt, chromium, and 29‐molybdenum alloy, with non‐constrained polyethylene insert25. The self‐retaining teeth are designed for optimal anchorage and stability25. Mobi‐C mobility is controlled by movement of the superior plate which re‐positions the mobile insert on the inferior plate26. The inferior plate contains two lateral stops which limit the mobility of the insert and reduce the potential for expulsion of the insert27. This achieves five degrees of freedom (Fig. 1).

Figure 1.

Mobi‐C disc prosthesis. Reproduced from Alvin et al.

The M6‐C prosthesis consists of a compressible poly‐carbonate core. The compressible polymer is designed to stimulate the stiffness and function of the nucleus while the surrounding poly‐ethylene fiber is designed to stimulate the annulus28. The polymer sheath prevents tissue ingrowth and extrusion of any material from inside the disc, which may occur with wear‐and‐tear. The core mimics the natural disc, allowing flexion–extension movement and six degrees of freedom. Endplates anchor the core on the superior and inferior surfaces, with a very different design principal to the Mobi‐C. The end‐plates are coated with porous titanium to promote bone contact surface area, and osseo‐integration (Fig. 2)28.

Figure 2.

Cutaway view of single‐piece M6‐C compressible prosthesis. Reproduced from Lauryssen et al.

To date there has been no comparison between two prostheses with a compressible core and woven annulus design versus a mobile insert sliding core such as the M6‐C and Mobi‐C in regards to their kinematics. The purpose of this study is to conduct a brief review in the interest of overall ROM and kinematics of different segments of the cervical spine, particularly C_5–6 and C_6–7. Second, this study aims to evaluate the functional range of motion on flexion and extension movements for patients with these prostheses in the early phase of their recovery (3 months) in comparison with normative data from asymptomatic patients. A discussion is then carried out to comment on the results of this study.

Materials and Methods

Asymptomatic Patients

To document the ROM of the cervical spine in flexion and extension at the specific levels C_5–6 and C_6–7, a literature search from date of database inception up to July 2016 was performed using the Medline database via the PubMed search engine using the following search terms: “asymptomatic,” “normal,” “biomechanic,” “kinematic,” “cervical,” and “spine” to identify studies investigating cervical spine kinematics in asymptomatic patients. Studies where flexion and extension angles at individual vertebral levels were not available and studies utilizing cadaver data were excluded.

Ethics and Registry

At a single tertiary hospital institution, patients who underwent cervical disc replacement (CDR) for degenerative cervical disc disease using the M6‐C or Mobi‐C disc were identified. The study was registered in the Australian New Zealand Clinical Trials Registry, ACTRN12616000832471p. Ethics for this study was approved by the South Eastern Sydney Local Health District. All patients provided consent to access data from their radiological outcomes studies.

Inclusion, Exclusion, and Implant Choice

We included patients with symptomatic radiculopathy unresponsive to conservative medical management. All patients had disc osteophyte complex causing neural compression and were treated with discectomy and artificial cervical disc replacement at either single level (n = 59), or double levels (n = 10). We excluded patients with infection, cancer, kyphosis, and osteoporosis. Patients with facet joint arthropathy on bone scan were also excluded.

Patient characteristics including age and gender were collected from a prospective database. All patients underwent radiographic evaluation 3 months postoperatively. The neutral position of the neck was defined as the natural position of the head looking forward when the patient was placed in a standing position. Lateral cervical spine radiographs were taken at active full flexion and extension movements 3 months postoperatively. Flexion and extension angles were measured at the disc level with SpineView software (Surgiview, France). The angle was formed by lines drawn at the superior margin of the lower level vertebral body of the treated segment and the inferior margin of the upper level vertebral body of the adjacent segment. The measurements were conducted by two independent authors. If the recorded measurement of an angle differed between the authors, then the mean angle was selected.

Choice of implant used was a decision between surgeon and patient. In the earlier time period of the study, only the M6‐C prosthesis was approved and available for use, and, therefore, greater numbers of this implant were included in the data analysis. In the later time period, the treating surgeon provided information to patients on both implants and patient choice determined the implanted prosthesis.

Data Analysis

Data was analyzed using a commercially available statistical software package, STATA 13 software for windows (StataCorpLP, Texas, USA). One‐way ANOVA was used to compare outcomes between the groups. The data was expressed as mean ± standard deviation. The result was statistically significant with P‐value <0.05.

Results

Study Population Demographics

During the study period, 2012 to 2015, there were 53 patients with M6‐C and 16 patients with Mobi‐C cervical disc prostheses. Of the 53 patients with M6‐C, there were 16 women and 37 males aged 21–61 years (mean, 45 years). Of the 16 patients with Mobi‐C, there were 9 women and seven men aged 30–60 years (mean, 47 years). In the M6‐C group, the level of implantation was at C_4–5 in 9 patients, C_5–6 in 33 patients, and C_6–7 in 13 patients. In the Mobi‐C group, the level of implantation was at C_4–5 in 1 patient, C_5–6 in 11 patients, and C_6–7 in 6 patients. In the M6‐C cases, 45 cases involved a single level, with 8 in two consecutive levels. In the Mobi‐C cases, 14 cases involved a single level, with 2 in two consecutive levels. No patient in either group has had a prosthesis failure or revision surgery at the operative level. One patient presented after 26 months with adjacent segment degeneration above an M6‐C prosthesis requiring additional disc arthroplasty surgery, with a good result.

Asymptomatic Patients: Literature Review

Two studies were identified in the literature to have flexion and extension angles at individual vertebral levels of the cervical spine of asymptomatic patients29, 30.

One study consisted of 20 asymptomatic patients, of which 13 were female and 7 male, aged 39 to 51 years (mean, 46 years)29. In this study, flexion was 7.4° ± 2.9° at C_5–6 and 7.1° ± 3.9° at C_6–7. Extension was 7.3° ± 3.8° at C_5–6 and 5.4° ± 2.3° at C_6–7.

Another study consisted of 20 patients30. There were 11 women and 9 men. The age range was 20 to 49 years (mean, 31 years). Flexion was 9.3° ± 2.7° at C_5–6 and 8.3° ± 4.0° at C_6–7. Extension was 9.3° ± 3.8° at C_5–6 and 8.3° ± 5.8° at C_6–7.

Overall Range of Movement

For the M6‐C, overall flexion was 4.0° ± 3.0°. For the Mobi‐C, overall flexion was 4.8° ± 3.1°. The difference between the prostheses was not statistically significant (P = 0.4), −0.7° (95% CI: −2.5° to 1.0°).

For the M6‐C, overall extension was 7.6° ± 4.7°. For the Mobi‐C, overall extension was 10.9° ± 6.3°. The difference between the prostheses was statistically significant (P = 0.028), −3.3° (95% CI: −6.2° to 0.4°).

C_5–6 ROM/Kinematics

At C_5–6, flexion was 3.4° ± 2.4° in the M6‐C group and 3.9° ± 2.4° in the Mobi‐C group. Reported values for asymptomatic patients at this level by Anderst et al. and Ordway et al. were 7.4° ± 2.9° and 9.3° ± 2.7°29, 30. The M6‐C and Mobi‐C group have statistically significantly less flexion at this level compared to asymptomatic patients (P < 0.001).

At C_5–6, extension was 7.3° ± 4.6° in the M6‐C group and 14.2° ± 5.1° in the Mobi‐C group. Reported values for asymptomatic patients at this level by Anderst et al. and Ordway et al. were 7.3° ± 3.8° and 9.3° ± 3.8°29, 30. Patients in the Mobi‐C group have statistically significantly more extension at C_5–6 compared to M6‐C patients and to asymptomatic patients (no statistically significant difference between M6‐C patients and asymptomatic patients).

C_6–7 ROM/Kinematics

At C_6–7, flexion was 4.6° ± 2.4° in the M6‐C group and 6.8° ± 4.0° in the Mobi‐C group. Anderst et al. and Ordway et al. report values for asymptomatic patients at this level as 7.1° ± 3.9° and 8.3° ± 4.0°, respectively29, 30.

At C_6–7, extension was 5.0° ± 4.4° in the M6‐C group and 4.2° ± 2.3° in the Mobi‐C group. Anderst et al. and Ordway et al. report values for asymptomatic patients at this level as 5.4° ± 2.3° and 8.3° ± 5.8°, respectively29, 30.

There are no statistically significantly differences between patients in the M6‐C group, the Mobi‐C group, or the two groups of asymptomatic patients for both flexion and extension at C_6–7 (P‐values were greater than 0.05).

Discussion

Artificial cervical disc arthroplasty aims to replicate the anatomical sagittal balance and preserve normal kinematics of the motion segment and, therefore, maintain physiological movement and alignment of the cervical spine31. Abnormal flexion or extension ROM may lead to cervical spine instability and potential surgical intervention, in addition to adjacent segment degeneration at the levels above and below32.

Two commonly used cervical disc prosthesis currently in use throughout the world are the Mobi‐C and M6‐C. There have been no studies to date investigating the ROM of these prostheses and comparing with normal cervical kinematics. Because flexion–extension movement is uniplanar, and has sagittal kinematics, it can be assessed using standard lateral radiographs, as was performed in this study and the studies with asymptomatic patients as a comparator.

The main finding of this study was that Mobi‐C and M6‐C had different flexion and extension angles at a 3‐month time point following surgery when compared to asymptomatic patients. Asymptomatic patients were comparable to this study's population with similar mean age and gender distribution. At C_5–6, both prostheses had reduced flexion compared to normal, while Mobi‐C also had increased extension. However, at C_6–7, this difference was not found. This may be due to reduced sample sizes for the C_6–7 level (n = 16) compared to the C_5–6 level (n = 38). For overall range of movement, the M6‐C and Mobi‐C have similar flexion but different extension.

This study population is representative of the typical population undergoing CDR27. However, the results reflect the short‐term effects of arthroplasty and may not be representative of longer‐term effects. It is not clear if the observed differences lead to stress either at the facet joint at the index level, or at adjacent segments, altering disc homeostasis and subsequently changing flexion and extension angles.

There were a few limitations associated with this study. First, cervical spine kinematics is not exclusively flexion–extension of the neck. Other neck movements such as rotation and lateral bending may not follow the observed patterns described here. Standard radiographs are limited with assessment of lateral bending and neck rotation, due to difficultly in accurate measurement on simple radiographs. Pain may have limited movement for the arthrodesis group. Future studies with larger study populations are needed to explore normal flexion and extension values in asymptomatic subjects and patients with Mobi‐C and M6‐C CDR.

Long‐term follow‐up was specifically not evaluated in this study as data suggests that a small percentage of disc replacement prostheses undergo ossification around the implant with subsequent fusion33, and our focus was to determine the ROM of these implants, without implant wear and other long‐term complications. Future studies are required to assess the long‐term ROM of these implants.

Conclusion

Cervical arthroplasty using both Mobi‐C and M6‐C disc prostheses due to symptomatic degenerative disc/osteophyte complex results in significant restoration of movement at the 3‐month postoperative mark. However, they do not completely reproduce the normal movement and, thus, further research into the biomechanics of disc replacement technology is required.