Long-term outcomes of anterior cervical dynamic implants: motion-sparing or a delayed fusion?

Background context: Use of an anterior cervical dynamic implant (ACDI) is generally considered a nonfusion technique for treating cervical degenerative disorders. However, there is limited research focused on evaluating the long-term clinical and radiographic outcomes of ACDI.

Purpose: To analyze the long-term clinical and radiographic outcomes of ACDI in the treatment of degenerative cervical disorders.

Study design: A retrospective cohort study.

Patients sample: Patients with degenerative cervical disorders who underwent anterior cervical discectomy and dynamic cervical implant (DCI) implantation between May 2012 and August 2020 at our institution were included in this study.

Outcome measures: Clinical outcomes were assessed using the modified Japanese Orthopedic Association (mJOA), visual analog scale (VAS) scores and patient reported satisfaction rate. Imaging assessment parameters included intervertebral height (IH), intervertebral disc height (IDH), C2-7 range of motion (ROM), segmental ROM, the degree of DCI subsidence and anterior migration, heterotopic ossification (HO) as well as adjacent segment degeneration (ASD).

Methods: JOA and VAS scores were obtained through questionnaire. The patient reported satisfaction was rated as very satisfied, satisfied, less satisfied and dissatisfied at the final follow-up. The position of the implants, IDH and IH were evaluated on lateral radiographs. ROM at C2-7, ROM at operated level were measured on dynamic radiographs. Cervical 3 -dimensional computer tomography (CT) and magnetic resonance image (MRI) images were used to assess the presence of HO and ASD. The clinical and radiologic variables between the preoperative period and different follow-up time point were statistically analyzed by unpaired t-tests or chi-square tests. Statistical significance was defined as p<.05.

Results: A total of 92 patients (51 males and 41 females) were included in this study. Among them, there were 36 cases of cervical spondylotic myelopathy, 26 cases of cervical radiculopathy, and 30 cases of myeloradiculopathy. The mean age was 55.1±12.6 years. The number of operated levels was single level in 57 patients, 2 levels in 31 patients, and 3 levels in 4 patients. The average follow-up period was 81.3 months (range: 35-135 months). The mean JOA scores showed a gradual increase at 1 month, 1 year, and the final follow-up (12.0±0.7,13.5±0.8, and14.4±1.1 respectively) compared to the preoperative score (9.1±0.9, p<.01). VAS scores significantly decreased at 1 month, 1 year, and the final follow-up (4.1±0.7, 2.3±0.9, and 2.0±0.8 respectively) compared to the preoperative score (7.2±l .2, p<.01). At the final follow-up, the patient reported satisfaction was rated as very satisfied, satisfied, less satisfied and dissatisfied (79%, 10%, 10%, 1% respectively). Revision surgery was not required for any of the patients during the follow-up period, either due to instrumentation failure or adjacent segmental diseases. In the radiographic assessment, there was a notable increase in IH and IDH after surgery compared to preoperative values (33.0±4.0 mm vs. 30.7±3.0 mm, p<.01 and 6.7±2.4 mm vs. 4.6±0.9 mm, p<.01 respectively), which gradually decreased at 1 year and the final follow-up (IH: 32.1±2.5 vs. 30.9±3.5 p=.024; IDH: 5.3±1.5 mm vs. 4.3±0.6 mm, p=.043 respectively). At the 1-month postoperative follow-up, the segmental ROM exhibited a decrease compared with preoperative values (6.2±1.8° vs. 7.5±2.0° p=.044), followed by an increase at the 1-year follow-up (6.2±1.8° vs. 6.4±1.5° p=.078), but ultimately decreased at the final follow-up (6.4±1.5° vs. 2.9±0.6°, p<.01). HO was observed in approximately 81.5% of cases (75/92), while a great proportion (41.3%) of patients experienced varying degrees of prosthesis subsidence and anterior migration during the follow-up.

Conclusion: At the long-term follow-up, a high incidence of HO, along with varying degrees of subsidence and migration of the prosthesis, were observed in most patients. As the motion preservation capability of the ACDI gradually diminishes, delayed intervertebral autofusion becomes a more likely outcome compared to motion sparing.