Coronal Implant Positioning and Alignment Predict the Range of Motion After Cervical Disc Arthroplasty.

STUDY DESIGN: Retrospective cohort study. OBJECTIVE: To determine whether coronal implant position and implant alignment on first postoperative radiographs predict flexion-extension range of motion (ROM) following cervical disc replacement (CDR). SUMMARY OF BACKGROUND DATA: CDR is a motion-preserving alternative to anterior cervical discectomy and fusion. While sagittal alignment factors have been studied, the impact of coronal implant positioning on ROM preservation remains poorly understood. METHODS: This retrospective radiographic study evaluated 236 CDR levels at a single academic institution. On first postoperative anteroposterior radiographs, we measured superior and inferior anatomic axis deviation (AAD), superior and inferior mechanical axis deviation (MAD), disc level deviation (DLD), and plumbline deviation (PD). Flexion-extension ROM was assessed at final follow-up at the disc space (DS ROM), functional spinal unit (FSU ROM), and C2-7 levels. Pearson correlations, multivariable linear regression adjusted for age, sex, and body mass index, dose-response quartile analysis, and receiver operating characteristic curve analysis were performed. RESULTS: The mean follow-up was 1.8±1.7 years. Superior MAD demonstrated a significant negative correlation with FSU ROM (r=-0.149, P=0.023), which persisted after multivariable adjustment (β=-0.36, P=0.032). Dose-response analysis revealed that patients in the best alignment quartile had significantly higher FSU ROM than those in the worst quartile (10.2 vs. 8.0 degrees, P=0.046). Every 3 degrees of MAD corresponded to ∼1 degree of FSU ROM loss. Superior MAD >3.8 degrees was associated with a 2.27-fold increased odds of poor FSU ROM (P=0.024). CONCLUSIONS: Coronal MAD significantly impacts FSU ROM following CDR. Surgeons should target MAD <3 degrees to optimize motion preservation. LEVEL OF EVIDENCE: Level III-therapeutic study.