Myositis Ossificans in Short External Rotator Muscles of the Hip: Temporal Changes on CT.

Introduction Swelling in the short external rotator muscles (SERMs) in patients with cerebrovascular disease may regress to non-traumatic myositis ossificans. This study investigates the temporal changes in CT findings of myositis ossificans, aiming to provide exploratory insights for diagnostic accuracy and clinical management. Methods We retrospectively reviewed 1,469 CT examinations of the pelvis conducted at our institution between 2001 and 2023, totaling 436 cases of cerebrovascular disease. We selected 20 patients showing swelling of SERMs, including six patients with bilateral swelling. Of the 20, only patients with at least one follow-up CT, six cases (eight hip joints), were selected for analysis, including two cases with bilateral involvement. The intervals between swelling, calcification, and ossification were calculated, with calcification defined as a high-attenuation area (≥100 HU) and ossification as peripheral calcification with internal fat density. Results All eight hip joints exhibited calcification, either concurrently or sequentially with swelling. The mean interval from swelling to calcification was 12 ± 5.3 days, and 112 ± 102 days from swelling to ossification in six hips that developed ossification. In two hip joints, calcification disappeared without ossification developing. In all cases, the affected hip corresponded to the side of the paralyzed lower limb. On initial examination, all patients exhibited fever, two with fever of unknown origin. Conclusion This study determined temporal progression in CT findings of myositis ossificans in swelling of SERMs of patients with cerebrovascular disease. Further, detecting swelling, calcification, or ossification in SERMs in follow-up CT is informative in excluding infections or neoplastic conditions.