A 50-year-old male presented to the emergency room following a motor vehicle accident. He was diagnosed with a C5-C6 fracture- dislocation and immediately underwent a fixation surgery of C5 and C6 vertebral body. After an unremarkable postoperative course, he was transferred to inpatient rehabilitation with an C6/C6 American Spinal Injury Association (ASIA) A tetraplegia. Twenty days after the operation, he began to have intermittent high fever with a temperature of 38.4℃. On postoperative day 35, his right thigh was noted to be edematous. The circumferential difference between the lower extremities measured at 15 cm above the knee was 6.6 cm (right 39.6/left 33.0 cm). His white blood cell count was 11,230 cells/uL, erythrocyte sedimentation rate (ESR) was 31 mm/hr and C-reactive protein (CRP) was 6.24 mg/dl. Our differential diagnosis included deep infection including osteomyelitis, heterotopic ossification, and deep vein thrombosis (DVT) at the time because of high fever and increased inflammatory markers such as CRP, ESR and leukocytosis. Plain film of the pelvis showed no abnormalities (Fig. 1-A). CT angiography of lower extremities demonstrated no evidence suggesting deep vein thrombosis. Three phase bone scintigraphy showed mildly increased blood pooling and bone uptake in both proximal femurs. A thigh contrast MRI showed intramuscular fluid collections that appeared hyperintense on T2-weighted imaging (T2WI) and hypointense with a hyperintense rim on T1-weighed imaging (T1WI) while T1WI after fat saturation and gadolinium enhancement showed peripheral enhancement. Fluid collection was also noted in both hip joints. Diffusely increased signal intensity on T2WI was noted in both buttocks and proximal thighs involving both iliopsoas, vastus, gluteus, adductor and right sartorius muscle groups. Diffusely increased thickness and signal intensity in the subcutaneous tissues of both buttocks and thighs were observed (Fig. 2). This finding suggested pyomyositis. Sono-guided aspiration of the collections was cultured for antibiotic sensitivities. There was no growth of microorganism and empiric antibiotics (ampicillin/sulbactam) were used for two weeks.

Although his fever subsided and inflammatory markers normalized with antibiotics, the swelling persisted, while limitation of motion, especially in hip external rotation and flexion contracture, worsened. In follow-up, simple radiographic imaging after ten days showed overt ossified lesions (Fig. 1-B). Indomethacin was prescribed and gentle stretching and range of motion exercises were performed. In addition, he was given radiotherapy with 2000 cGy in 10 fractions on the area. Range of motion was significantly improved. On follow-up MRI 40 days after the initial MRI, the extent of high signal intensity on T1WIs was decreased. Multiple hypointense amorphous materials consistent with calcification were observed around both lesser trochanteric areas. Loculated fluid collections from the previous study had resolved.

A 55-year-old man complained of left hip pain after slipping. He was diagnosed with an intertrochanteric fracture of left femur and underwent closed reduction and internal fixation. His past medical history is significant for a left basal ganglia lacunar infarct two years prior, right intertrochanteric femoral fracture treated by closed reduction and internal fixation. On postoperative day 29, CRP was elevated and he had a mild fever with a temperature of 37.9℃. The focus of inflammation was uncertain. On postoperative day 34, he presented with swelling around his left hip and upper thigh. As the hemoglobin level persistently decreased, CRP level remained elevated and thigh swelling worsened, we strongly suspected hemorrhage or infection around the operation site rather than deep vein thrombosis or HO. Simple X-ray showed no abnormal findings. A very small amount of fluid was detected on the left thigh by ultrasound but aspiration was not carried out due to insufficient volume. Bone scintigraphy obtained to exclude the possibility of HO showed no definite abnormal findings. On the MR scan, diffuse enhancement and increased signal intensity in the T2 weighted images were noted in the left proximal femur and hip muscles. These findings suggested pyomyositis (Fig. 3) and empiric antibiotic therapy (cefazolin) was administered for one week. A soft tissue mass with cloudy ossification around the left femoral neck was observed in simple X-ray taken one week later. Finally, the diagnosis of HO was made and overly aggressive passive range of motion exercise was avoided to prevent any trauma. Active-assistive and gentle passive range of motion were carefully initiated to maintain available joint motion and to avoid progressive contractures.