To determine the relationship of multiple variables, including the Injury Severity Score (ISS), closed head injury (CHI), and timing and type of surgery to formation of motion-limiting heterotopic ossification (HO) following knee dislocation.

Longitudinal observational study.

University level 1 trauma center.

Thirty-five consecutive patients with 36 knee dislocations (OTA fracture and dislocation classification 40-D) admitted over a 26-month period.

Admission ISS, Glasgow Coma scale (GCS) scores, CHI, timing (> or < 3 weeks from injury) and type (open or arthroscopic) of surgery, number of cruciate ligaments reconstructed, medial surgical procedure, and eventual presence or absence of motion-limiting HO.

A classification system for HO was developed ranging from none (type 0) to ankylosing (type IV) HO. Twenty-nine patients with type 0-III HO recovered an average range of motion of 126 degrees at an average of 14 months (group A). Six patients formed ankylosing type IV HO (group B). The ISS in group A ranged from 9 to 26. ISS in group B ranged from 26 to 50 (P < 0.001). Regarding the formation of type IV HO, the sensitivity of an ISS >/=26 was 100%, the specificity was 97%, and the positive predictive value was 86%. Patients in group B had a greater incidence of documented CHI (P < 0.025). Timing and type of surgery, number of ligaments reconstructed, and whether or not the patient had a medial surgical procedure had no statistical influence on degree of HO formation.

An ISS of 26 seems to be a discrete boundary above which patients with knee dislocation are at extremely high risk for type IV HO formation if undergoing surgical reconstruction and below which patients are likely spared this complication. The presence of a CHI is a significant factor in type IV HO formation, although harder to quantify. None of the remaining independent variables studied were significantly related to ankylosing type IV HO formation.