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Case: A 62-year-old woman receiving romosozumab for 3 months underwent extreme lateral interbody fusion (XLIF) for lumbar degenerative spondylolisthesis. From 1 week after surgery, she experienced gradually increasing pain from the right groin to the front of the thigh. Examination revealed ossifying myositis in bilateral psoas major muscles. Etidronate treatment was initiated, improving pain after 4 days. Computed tomography showed lesion disappearance by 3 months after surgery., Conclusion: We report a rare case of myositis ossificans in bilateral psoas major muscles following XLIF surgery, possibly influenced by intraoperative manipulation and romosozumab treatment. Etidronate administration may be effective, as with heterotopic ossification., Competing Interests: Disclosure: The Disclosure of Potential Conflicts of Interest forms are provided with the online version of the article (http://links.lww.com/JBJSCC/C372)., (Copyright © 2024 by The Journal of Bone and Joint Surgery, Incorporated.)

Retinoids are a plausible family of therapeutic agents for fibrodysplasia ossificans progressiva due to their ability to inhibit differentiation of mesenchymal tissue into cartilage and bone. A prospective study was conducted to assess the efficacy of isotretinoin (13-cis-retinoic acid) in the prevention of heterotopic ossification in patients who had fibrodysplasia ossificans progressiva. Eleven anatomic regions were assessed in each of 21 patients by clinical examination, radiographs, and bone scans. An anatomic region was considered to be involved if there was clinical, radiographic, or radionuclide evidence of orthotopic or heterotopic ossification anywhere in the region. There were 143 involved anatomic regions and 88 uninvolved anatomic regions at the beginning of the study. Only one of the 88 anatomic regions that was completely uninvolved at the beginning of the study became involved during isotretinoin therapy. However, 16 of the 21 patients (76%) had major flare ups develop in 38 of 143 (27%) previously involved anatomic regions while administered isotretinoin therapy. Isotretinoin at steady state doses of 1 to 2 mg/kg per day decreased the incidence of heterotopic ossification at uninvolved anatomic regions compared with an external control group, as long as the medication was started before the appearance of any orthotopic or heterotopic ossification in that anatomic region. The data did not allow the determination of whether isotretinoin was effective or detrimental in preventing disease flareups in regions that had even minimal orthotopic or heterotopic ossification at the time the therapy began. Extreme caution should be exercised when using this medication in patients who have fibrodysplasia ossificans progressiva.

There is no known effective therapy for fibrodysplasia ossificans progressiva. Ethane-1-hydroxy-1-diphosphonate has been studied because of its inhibitory effect on bone mineralization and its potential to impair the rapid ossification process observed after acute episodes of fibrodysplasia ossificans progressiva. In an open study, the effects of intravenously administered ethane-1-hydroxy-1-diphosphonate and oral corticosteroids in the treatment of fibrodysplasia ossificans progressiva was evaluated. Thirty-one fibrodysplasia ossificans progressiva attacks were observed in seven patients during the mean followup of 6 years. In 29 attacks, the authors observed a rapid diminution of local inflammation, swelling, and pain during the first 7 days of treatment. However, despite the ethane-1-hydroxy-1-diphosphonate treatment, 10 new ossifications were observed, causing severe deterioration of joint mobility in all affected patients. In 21 attacks, no new ectopic ossification appeared. The radiologic pattern of preexisting ossifications did not change during the treatment. There are no data in the literature describing the percentage of acute episodes leading to an ossification. The results suggest the possibility that intravenous administration of ethane-1-hydroxy-1-diphosphonate and oral steroids may be helpful for the treatment of fibrodysplasia ossificans progressiva, but more control data on the spontaneous resolution of early flareups are needed.