Object This study aimed to investigate the incidence rate of heterotopic ossification (HO) after implantation of Coflex interspinous devices. Possible risk factors associated with HO were evaluated. Methods The authors retrospectively analyzed patients who had undergone single-level (L4–5) implantation of a Coflex device for the treatment of lumbar spinal stenosis. Patient data recorded were age, sex, height, weight, body mass index, smoking habits, and surgical time. Heterotopic ossification was identified through lumbar anteroposterior and lateral view radiographs. The authors developed a simple classification for defining HO and compared HO-positive and HO-negative cases to identify possible risk factors. Results Among 32 patients with follow-up times of 24–57 months, HO was detectable in 26 (81.2%). Among these 26 patients, HO was in the lateral space of the spinous process but not in the interspinous space in 8, HO was in the interspinous space but did not bridge the adjacent spinous process in 16, and interspinous fusion occurred at the level of the device in 2. Occurrence of HO was not associated with patient age, sex, height, weight, body mass index, smoking habits, or surgical time. Conclusions A high incidence of HO has been detected after implantation of Coflex devices. Clinicians should be aware of this possible outcome, and more studies should be conducted to clarify the clinical effects of HO.
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