Prevalence of heterotopic ossification following total disc replacement: A prospective, randomized study of two hundred and seventy-six patients

Background: Despite reports of good clinical outcomes in patients treated with lumbar and cervical disc replacements, varying degrees of heterotopic bone have been observed around these devices. The purposes of the present study were to determine the prevalence of heterotopic ossification following lumbar disc replacement and to investigate whether heterotopic ossification results in loss of motion or negatively affects clinical outcome. Methods: All preoperative and postoperative radiographs from a completed prospective, randomized, United States Food and Drug Administration-regulated trial comparing replacement with the CHARITÉ Artificial Disc with anterior interbody arthrodesis were analyzed. In each of 276 patients treated with disc replacement, heterotopic ossification was categorized with use of a validated 5-point radiographic classification system both preoperatively and at all protocol-specified follow-up intervals to two years. The range of motion on flexion and extension radiographs made preoperatively was compared with that on radiographs made two years postoperatively, and the motion was correlated with the presence or absence of heterotopic ossification. Similarly, validated clinical outcome measures were correlated with the presence or absence of heterotopic ossification at two years. Results: The prevalence of heterotopic ossification in the 276 consecutive patients treated with lumbar disc replacement with the CHARITÉ Artificial Disc was 4.3%. There were four cases of Class-I heterotopic ossification and eight cases of Class-II heterotopic ossification. In five of the twelve patients, heterotopic bone was visible as early as six weeks postoperatively, and eleven of the twelve patients had evidence of heterotopic ossification by three months postoperatively. The postoperative range of motion exceeded the preoperative range in all of the patients with heterotopic ossification. With the numbers available, no difference in either the range of motion or the clinical outcome at twenty-four months postoperatively was found between the patients who had and those who did not have heterotopic ossification. Conclusions: Heterotopic ossification is infrequent in patients treated with the CHARITÉ Artificial Disc, and it does not impact the range of motion or clinical outcome. Level of Evidence: Prognostic Level II. See Instructions to Authors for a complete description of levels of evidence.