252. Kinematic response of cervical disc arthroplasty in different hybrid constructs

Abstract

BACKGROUND CONTEXT The use of cervical disc arthroplasty (CDA) adjacent to an anterior cervical decompression and fusion (ACDF), a hybrid construct, is an attractive alternative to a multilevel cervical fusion operation. Consider a clinical scenario where a patient with a prior fusion at C6-C7 develops new radiculopathy and/or myelopathy at C5-C6. A surgeon considers a CDA at C5-C6, proximal to an existing C6-C7 ACDF, in hopes of reducing the risk of adjacent segment disease at C4-C5. Another clinical scenario is in the presence of two symptomatic adjacent cervical segments, a surgeon may consider fusing the segment with more severe spondylotic changes such as severe disc collapse and facet arthropathy while performing CDA at the adjacent, more mobile segment. While numerous studies have demonstrated excellent clinical outcomes of primary cervical disc replacement surgery, many questions regarding the biomechanics of CDA in a hybrid construct remain unanswered. PURPOSE (1) How is disc prosthesis motion response influenced by the presence of a fusion proximal or distal to the prosthesis? and (2) How does the number of fused levels in the hybrid construct affect the prosthesis motion? STUDY DESIGN/SETTING Biomechanical study using human cadaveric cervical spines. PATIENT SAMPLE Human cervical spine specimens (C2-T1, N=10, 46±16 years). OUTCOME MEASURES Segmental flexion-extension range of motion (ROM). METHODS Specimens were divided into two equal groups based on age, gender distribution, and preoperative mobility. Segmental motions were assessed under flexion and extension moments of 1.5 Newton-meters. In Group I, the arthroplasty was proximal to one- and two-level ACDF, while in Group 2, the arthroplasty was distal to one- and two-level ACDF. ACDF was performed using a PEEK cage and anterior plate and CDA was performed using the M6-C cervical disc prosthesis (Orthofix Medical, Inc). Within group comparisons were made using a repeated-measures ANOVA and paired-t test, whereas across group comparisons used 2-sample t-test. RESULTS Segment level of implantation: 20 segments underwent CDA. Ten discs were implanted at C4-C5, 5 at C5-C6, and 5 at C6-C7. The mean segmental ROM after CDA was 9.2±4.2 degrees. Segmental level of implantation did not influence the ROM (C4-C5: 8.6±3.7; C5-C6: 9.4±4.1; C6-C7: 10.0±6.1; p>0.7). Number of fused levels in the hybrid construct: The number of fused segments in the hybrid construct did not influence the ROM after CDA (1-level: 9.7±4.6 vs 2-level: 9.9±4.4; p=0.9). CDA proximal vs distal to fusion: CDA above a 1-level ACDF had ROM similar to CDA below a 1-level ACDF (9.4±4.1 degrees vs 10.0±6.1; p=0.76). This result also held for a prosthesis implanted above versus below a 2-level ACDF (9.9±4.6 vs 9.8±5.2; p=1.0). CDA in hybrid construct compared to intact: CDA in a hybrid construct had ROM similar to the ROM of the corresponding segment in the intact spine. This result held true for all hybrid constructs studied regardless of location of prosthesis above or below a fusion, and the number of fused levels. CONCLUSIONS This study simulated a scenario where the patient experiences the same moments before and after surgery. Some authors suggest patients maintain similar ROM before and after surgery. Likely, actual in vivo loading reflects a combination of the two approaches. Results of this study suggest that disc arthroplasty performed adjacent to a 1- or 2-level cervical fusion is likely to maintain the same degree of motion as a primary arthroplasty. These findings provide a cadaveric basis for future in vivo studies to validate these results. FDA DEVICE/DRUG STATUS M6C artificial cervical disc, Orthofix, Inc (Not approved for this indication)