Classifying Ischial Tuberosity Avulsion Fractures by Ossification Stage and Tendon Attachment

We propose a new classification system for ischial tuberosity fractures in adolescents that is based on the ossification pattern of the apophysis. We performed a retrospective review of patients who were diagnosed with ischial tuberosity avulsion fractures at a single institution from 2008 to 2018. Skeletal maturity and fracture location, size, and displacement were recorded based on initial injury radiographs. The fractures were classified by location as being lateral (type 1) or complete (type 2). Pelvic computed tomography (CT) review demonstrated 5 stages of ossification. We then reviewed pelvic CT and magnetic resonance imaging scans to assess the tendinous insertions at the ischial tuberosity apophysis. Reliability analysis was performed. We identified 45 ischial tuberosity fractures. The mean patient age was 14.4 years (range, 10.3 to 18.0 years). Boys accounted for 82% of the cohort. Forty-seven percent of the fractures were classified as type 1, and 53% were classified as type 2. Type-1 fractures were associated with younger age (p = 0.001), lower Risser score (p = 0.002), lower modified Oxford score (p = 0.002), less displacement (p = 0.001), and smaller size (p < 0.001) when compared with type-2 fractures. Of the 45 patients, 18 had follow-up of >6 months, with 56% going on to nonunion. Nonunion was associated with greater displacement (p = 0.016) and size (p = 0.027). When comparing union rates by fracture type, 33% of type-1 fractures progressed to nonunion, while 78% percent of type-2 fractures progressed to nonunion; however, this difference was not significant (p = 0.153). A review of the advanced imaging indicated that type-1 fractures involved the semimembranosus and conjoined tendons, whereas type-2 fractures also involved the adductor magnus tendon. We propose a new classification system based on the ossification pattern of the ischial tuberosity apophysis that reflects the skeletal maturity of the patient, the size and location of the fracture, and the amount of displacement, and likely predicts the probability of subsequent nonunion. The ischial tuberosity ossifies in a pattern similar to the iliac crest as described by Risser, and this pattern of ossification dictates the size of the ischial tuberosity avulsion fracture fragments and the involved tendons.