The Incidence and Classification of the Severity of Heterotopic Ossification Around Femoral Endoprostheses

Introduction: Although several studies observe and report the incidence of heterotopic ossification (HO) following total hip arthroplasty, to our knowledge, no study has characterized HO around femoral endoprostheses for reconstruction in cases of tumor resection or massive bone loss. Methods: A grading system for HO around a femoral endoprosthesis was developed based on the Brooker Classification System. We then retrospectively reviewed all patients who underwent primary femoral endoprosthetic reconstruction (proximal, distal, or total) between 2010 and 2017 with minimum 6-month follow-up at a single institution. Radiographs from the most recent follow-up were used to classify the pattern of HO using the defined classification system. Intra- and interrater reliabilities of the classification system were determined. Electronic medical records were reviewed for patient demographics and further information on treatment. Results: Forty-one femoral endoprostheses in 41 patients were included (22 proximal, 15 distal, and four total femur arthroplasties). Mean age at surgery was 53 ± 23 years (range 9 to 91 years), and 22 were female (54%). Inter- and intrarater agreement for HO classification was 87% (κ = 0.82) and 91% (κ = 0.88), respectively. In total, an 83% incidence of HO was found after femoral endoprosthetic arthroplasty. All proximal femur arthroplasties and 66.7% of distal femoral arthroplasties exhibited some degree of HO. Patients with HO were markedly older than those without (57 ± 21 vs. 30 ± 19; P = 0.003). Discussion: The proposed classification system has high reproducibility and agreement. Our data suggest that HO is very common after femoral endoprosthetic reconstruction and is more predominant around the hip compared with the knee. Most HO was noted around the diaphyseal stem of the prosthesis and away from the joint.