| Summary: | Background: Heterotopic ossification (HO) may occur after multiligament knee injuries (MLKIs), with the potential to cause significant functional deficits. However, risk factors for HO after MLKI are not well understood. Purpose: To identify the incidence and risk factors for HO after MLKI. Study Design: Case-control study; Level of evidence, 3. Methods: Patients were included if they had undergone treatment for an MLKI by a single surgeon at a level 1 trauma center between 2001 and 2023, with radiographs at the time of injury and at a minimum 6-month follow-up. All data were obtained from electronic medical records. Two senior musculoskeletal radiologists reviewed all cases for the presence of HO in the most recent radiographs, using the initial radiographs obtained at the time of injury for comparison. Patients were grouped by whether they had developed HO after MLKI. Univariable analyses were performed to compare patient, injury, and treatment characteristics between groups. Odds ratios (ORs) for HO were assessed by multivariable logistic regression, adjusted for ligament injury classification, mechanism of injury, documented dislocation, central nervous system (CNS) trauma, and knee-spanning external fixation (KSEF). Results: The studied cohort included 128 patients, of whom 28 were excluded due to insufficient follow-up. Of the remainder, 35 (35%) developed HO, with a mean follow-up of 35.17 months. In the univariable analysis, the HO group had significantly higher rates of documented dislocation, CNS trauma, and KSEF. In the multivariable analysis, KSEF (OR, 8.51; 95% CI, 21.81-31.717; P = .001) and CNS trauma (OR, 6.63; 95% CI, 1.507-29.209; P = .012) remained independent predictors of HO after MLKI. Conclusion: The incidence of HO after MLKI was 35%. KSEF and CNS trauma were independently associated with HO. Recognition of these risk factors may help identify at-risk patients for closer monitoring and consideration of HO prophylaxis and should therefore be included in the decision-making strategy for KSEF versus early surgical treatment after MLKI.
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