An Analysis of Paravertebral Ossification in Cervical Artificial Disc Replacement: A Novel Classification Based on Computed Tomography

Objective Cervical artificial disc replacement ( CADR ) is a new technology in cervical spine surgery. However, CADR may result in paravertebral ossification ( PO ) after surgery and affect the mobility of the related spinal segments. The present widely used assessment method based on X ‐ray tomography cannot provide the position information of PO, and also PO detection rates by X ‐ray are low. The incidence of PO varies dramatically between studies. This study built a novel classification system based on cervical computer tomography ( CT ) scan to re‐evaluate the incidence of PO and its influence on CADR , and also analyzed the predisposing factors of PO . Methods In this retrospective study, 71 patients (from J anuary 2004 to D ecember 2009) who received cervical artificial disc replacement in our hospital were enrolled, and 82 cervical segments were replaced by B ryan discs. The range of motion ( ROM ) of the related cervical segments and scores of neurological symptoms (neck disability index [ NDI ] and J apanese O rthopaedic A ssociation [ JOA ] scores) for both pre‐surgery and last follow‐up were acquired, respectively. After the establishment of a novel grading system for PO based on CT scan, we analyzed the CT images acquired before surgery and at the last time follow‐up. Occurrence and distribution of PO at both time points were calculated. ROM between pre‐surgery and post‐surgery was compared by paired t ‐test stratified by PO stages. One‐way ANOVA was used to compare NDI and JOA scores between high‐grade and low‐grade PO groups after surgery. The χ 2 ‐test was used to evaluate the risk (odds ratio) of predisposing factors in developing high‐grade PO after surgery. Results The CT ‐based classification system has good inter‐observer and intra‐observer reliability. The detection rate of PO by CT scan is higher than for traditional X ‐ray examination. The incidence of low‐grade PO in all 82 segments at last follow‐up is 32.9%. The occurrences of high‐grade PO at preoperational and last follow‐up time are 15.9% and 67.1%, respectively. The high‐grade PO is mainly distributed around the uncovertebral joint. The pre‐surgery and post‐surgery ROM are similar in patients with low‐grade PO at last follow‐up time (9.80° ± 3.65° vs 10.03° ± 3.73°, P = 0.801); however, in patients with high‐grade PO the post‐surgery ROM decreases significantly compared to the pre‐surgery ROM (9.73° ± 4.03° vs 6.63° ± 4.21°, P < 0.001). There is no statistical difference for JOA and NDI scores after surgery between high‐grade and low‐grade PO patients at final follow‐up ( P = 0.264, P = 0.703). The χ 2 ‐test indicates that patients with preoperational existence of PO have a high risk of high‐grade PO after surgery ( OR = 4, P = 0.012). Conclusions The novel CT image‐based PO classification system has good intra‐observer reliability. The incidence of PO after B ryan cervical disk replacement is relative high, and the high‐grade PO is mainly distributed at the uncovertebral joint. The high‐grade PO will affect the ROM after surgery; however, it does not affect the neurologic symptoms.