Rare Posterior Dislocation of Mobi-C Cervical Disc: A Case Report and Surgical Resolution

Introduction

Cervical degenerative disc disease is becoming an increasingly common condition, resulting in severe pain, neck stiffness, and upper-limb radiculopathy [1]. Many of such cases require surgical intervention, and anterior cervical discectomy and fusion (ACDF) is considered the best treatment option [2]. However, recent studies highlight the role of cervical disc prostheses (CDP) and their advantages compared to ACDF [3]. Studies suggest that CDP can preserve motion at the affected level while potentially reducing the risk of adjacent segments impairment, their earlier degeneration, and loss of mobility and stability [4,5]. CDP is believed to provide better bonestock protection and may help avoid adverse effects such as pseudoarthrosis, donor-site pain, or plating difficulties [6]. Although CDPs are generally associated with low failure rates, they are not entirely failure-free, but only a few studies have investigated long-term outcomes. Two randomized clinical trials suggest lower failure rates at both the treated and adjacent levels for CDP compared to ACDF [2,7]. The reported failure rate for CDP is 2.4% [8]. Cases of device failure, while rare, include complications such as disc loosening, migration of the core or implant, cracking of the outer shell, and asymptomatic postoperative kyphosis. Furthermore, available studies have reported anterior bone loss, heterotopic ossifications, infection, and anterior prosthesis migration as potential complications [9]. Reports from the MAUDE database also document incidents of developing new radicular pain [10] and neck pain [11,12]. Therefore, we report a case of posterior dislocation of the Mobi-C prosthesis used for the replacement of cervical spine segment C6–C7.

Mobi-C was the first cervical disc prosthesis (CDP) to receive FDA approval for 2-level cervical disc arthroplasty. It consists of 2 metal alloy plates and an ultra-high-molecular-weight polyethylene insert in between. Its construction allows for unrestricted cyclic movement, improving the rate of PROM, and lateral self-retaining teeth provide optimal stability and anchorage [4,13]. Other devices, such as the Prestige LP (Medtronic) and Simplify Disc (NuVasive), have since received FDA approval for 2-level cases.

Case Report

A 50-year-old non-smoking woman was admitted to the hospital for surgical treatment of cervical disc herniation. She reported neck pain radiating to the upper limbs, with a predominance to the right one, numbness of the fingers, paresthesia, and vertigo. There were no factors aggravating such symptoms. She denied having loss of grip strength, muscle weakness, or sphincters disorder. Except for hypertension, she reported no chronic illnesses or allergies. Physical examination revealed limited movement of cervical spine to the right side and hyperesthesia at the C6–C7 area. Spurling’s test was bilaterally positive, with a predominance on the right side. The onset of these symptoms was 2 years prior to the surgery. Since then, she had been receiving conservative treatment based on rehabilitation and pain killers. A magnetic resonance imaging (MRI) scan of the cervical spine (Figure 1) showed a right-central C6–C7 disc protrusion with a local spinal cord compression. She underwent a C6–C7 antero-lateral discectomy and posterior osteophytes removal with a CDP implantation (Mobi-C, Zimmer) (Figure 2). There were no perioperative complications, and she was discharged from the ward after 48 hours. Follow-up exams showed proper wound healing, neurological deficits remission, and lessened radicular pain, without complications.

Two and a half years later, she patient presented to the doctor’s office after routine physical activity, reporting neck pain, loss of grip strength, and numbness of the upper right extremity. No impairment of muscle strength was noticed, and cervical spine range of motion was not affected. The Spurling test was bilaterally positive, triggering right C7 radicular pain. The surgical scar was healed, without any signs of inflammation or infection. X-ray and MRI examinations did not provide data for definite diagnosis (Figures 3, 4). However, due to suspicion of intraprosthetic dislocation, a CT scan was performed, which confirmed the initial diagnosis and revealed increased polyethylene wear and its intrathecal displacement, with segmental spinal cord compression.

The patient underwent elective surgery 3 months later. Using an antero-lateral approach with an X-ray assist, the CDP was found and separated from the adjacent tissues. Dislocation of the polyethylene insert towards the spinal canal and loosening of titanium components were observed. A slight local pigmentary reaction of surrounding tissues was noticed. Using an osteotome, the prosthesis elements were separated from C6 and C7 vertebral body endplates and removed (Figure 5). A sample for microbiological analysis was taken from the surgical site. Decompression of the dural sac was achieved. The bonestock was refurbished and the ACDF implantation was implemented. It was the Anchor C (Stryker) 6 mm with 4 degrees of lordosis. It was stabilized with two 10-mm screws and augmented with antibiotic-infused bone allografts (Figure 6).

Postoperatively, satisfactory pain management was obtained. No complications were noticed. Neurological symptoms subsided and grip strength and sensation improved. The patient was discharged from the ward after 2 days. During follow-up appointments, proper wound healing was observed. Results of the microbiological analysis revealed no signs of infection. A 3-month postoperative X-ray examination of screw positioning revealed proper positioning of the intravertebral body cage, no secondary implant displacement, no signs of secondary C6–C7 kyphotization, and there was properly progressive bone fusion (Figure 7). The patient reported satisfactory outcomes with an asymptomatic, full return to her daily living activities and household chores. To date, the patient has not provided us with the results of the recommended CT scan.

Discussion

There have been a few reported cases of Mobi-C dislocation; most report anterior intraprosthetic migration and involve other types of implants or refer to different cervical levels. Another case of posterior migration was described by Prod’homme et al, reporting C6–C7 plate osteolysis and polyethylene oxidation in a 40-year-old man, in which this phenomenon may have been triggered by long-term smoking [14]. Our patient was a non-smoker and her neurological symptoms occurred several months later. Tsermoulas et al found an anterior expulsion of the inferior prosthesis plate, whereas the superior component was intact. They assumed the reason for implant failure was its lack of restraint, which led to migration, probably after the patient’s excessive retching and vomiting [15]. Pelletier and Gillie reported an atraumatic anterior dislocation of C4–C5 CDP in a 57-year-old man (smoker) just 1 month after implantation. They suspected that the small anteroposterior diameter of the superior C5 plate caused prosthesis loosening and migration with excessive motion [16]. Altorfer et al identified migration as one of the 3 most common complications associated with the Mobi-C prosthesis [17]. Kandemir et al reported an explant analysis of a Mobi-C cervical disc, in which the insert underwent in vivo migration. An increased roughness of the articulating surfaces of the superior and inferior endplates was noticed, while the roughness of both surfaces of the polyethylene insert decreased. Notably, the articulating surface of the inferior plate lost its polished appearance [18]. Such findings suggest the insert was not operating between the endplates. Migration itself might have led to imbalanced loads at the contact areas that could result in wear-related complications such as osteolysis. DiCesare et al reported a case series of 4 men who had postoperative complications with the Mobi-C cervical disc implant, all presenting with focal neck pain without radiculopathy, unlike our patient. In all cases, the implant was locked in a fixed kyphotic position without migration. Three patients underwent conversion to cervical fusion, and all experienced complete symptom relief after the second surgery, as did our patient. Two patients required reoperation within a week, while 2 others needed revision surgery 2.5 years later, which indicates different causes for early versus delayed failures [19]. In a patient with prior multiple fusions, failure was likely due to increased biomechanical load on the Mobi-C. Another patient received a second Mobi-C implant, which has not failed, suggesting the initial failure was not patient-related and demonstrating the need for precise surgical technique. Pitsika and Nissen described a case of progressive symptomatic spinal cord compression due to nucleus migration from a Mobi-C prosthesis, similar to that observed in our patient [20]. They assumed the complication was related to slowly progressive implant failure. Despite lacking the above-mentioned risk factors, our patient still experienced failure of the prosthesis. She had pain and numbness in her right upper limb right after an average daily workout routine after 31 months. It was found that the prosthesis had a kyphotic alignment, and no implant osseointegration was found intraoperatively. In our case the symptoms were not as acute and severe as in some other reports in the literature, allowing the revision surgery to be postponed and planned thoroughly. The reasons for failure have not yet been determined. The surgery was performed by a highly experienced team, the patient had no comorbidities, and no intraoperative difficulties were encountered. The quality of the bone was solid. Intraoperatively, the area from under the plate was swabbed and cultured, and no bacterial contamination was found. The postoperative outcome was also satisfactory.

To reduce the incidence of displacement, we try to use high-speed rotary drills as little as possible to save the bone stock and its surface while preparing the prosthesis bed. Depending on the surgeon’s needs and intraoperative conditions, bone curettes and Kerisson rongeurs are utilized. It allows preserving the vascular layer of the baseplate and increasing the odds of obtaining solid and stable prosthesis adhesion to the bone surface of adjacent vertebrae.

For some time, authors have tried to avoid implanting simple modular prostheses without constrained elements, such as Mobi-C. In such prosthesis, the polyethylene insert is prone to excessive wear and damage, which can be a risk factor for prosthesis loosening and failure. We prefer constrained models such as ROTAIO (Signus Medizintechnik) or modular types in which the mobile insert is enclosed within a protective capsule to prevent displacement and particle release, similar to the M6 (Orthofix).

Nevertheless, posterior intraprosthetic dislocation is a very rare complication of CDP implantation, and the Mobi-C still seems to be a reliable implant. Further studies are needed to reveal possible CDP failures and help surgeons understand the reasons for certain complications.

Conclusions

Implantation of CDP seems to be a reliable procedure that can provide satisfactory clinical efficacy, with a high success rate, and complications involving the migration of prosthesis elements are rare. Posterior intraprosthetic dislocation of the Mobi-C is an extremely rare complication that requires specific surgical intervention. The present case demonstrates long-term follow-up and proves ACDF is an effective salvage method. The exact reasons for failure have not been determined yet. Further investigation is necessary to understand failure mechanisms, and improvement of CDP design is needed to avoid such complications.