The Incidence of Heterotopic Ossification in Total Disc Replacements: A Prospective Randomized FDA Study of 276 Consecutive Patients**

• a - DePuy Spine

Study Design: This is a prospective analysis of the incidence of heterotopic bone formation following single level lumbar disk replacement.

Objectives: 1) Determine the incidence of heterotopic bone formation following total disk replacement in consecutive series of patients. 2) Investigate whether the presence of heterotopic ossification correlates with factors such as excessive disk space distraction, loss of motion, or clinical outcome.

Summary of Background Data: Despite good early clinical outcomes for patients undergoing lumbar or cervical disk replacements, varying degrees of heterotopic bone has been observed in the tissues surrounding these implants. The cause, significance, or incidence of heterotopic ossification (HO) after artificial disk replacement remains unknown.

Methods: In an effort to better investigate HO, we analyzed all pre-operative and post-operative radiographs including flexion and extension views from the recently completed, prospective, randomized, FDA, trial comparing the Charite artificial disk replacement with anterior interbody BAK instrumentation and fusion. Over 6900 radiographs were digitized in a central core laboratory and corrected for magnification factors. Using a validated, 5-point radiographic classification system, the degree of HO was categorized for every patient undergoing lumbar disk replacement surgery both preoperatively and at 6-week, 3-month, 6-month, 12- month, 1-year, and 2-year follow-up. The change is disk height (in millimeters) from pre-op to 2 years postop was calculated from lateral radiographs and correlated with the presence or absence of HO. Similarly, the Oswestry disability index (ODI) and visual analog scores (VAS) measured at 2 years were correlated with presence or absence of HO.

Results: The overall incidence of HO in the 276 consecutive patients undergoing Charite lumbar disk replacement was 4.3% (12/276). There were 4 cases of Class I (islands of bone within the soft tissue but not influencing the range of motion. Bone is not between the planes formed by the two vertebral endplates). There were 8 cases of Class II (HO present between the two planes formed by the vertebral endplates but not blocking or articulating between adjacent vertebral endplates). In 5 of the 12 cases (42%), heterotopic bone was visible as early as 6 weeks post-op, and by 3-months post-op, 11 of the 12 cases (92%) had evidence of HO. There were no cases in which the post-operative range of motion did not exceed the pre-operative range of motion. No statistically significant correlation existed between the amount of disc distraction and the presence or absence of HO (p>0.05). No statistically significant correlation existed between clinical outcome and the presence or absence of HO (p>0.05).

Conclusions.: The occurrence of heterotopic ossification around lumbar artificial disk replacements is infrequent and does not impact motion or clinical outcome. These data do not appear to support the hypothesis that tension, due to excessive disk-space distraction, is a cause for heterotopic bone formation in the muscles adjacent to the lumbar disc space.

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