CLASSIFYING ISCHIAL TUBEROSITY AVULSION FRACTURES BY OSSIFICATION STAGE AND TENDON ATTACHMENT

BACKGROUND: Currently, there is no classification system for ischial tuberosity avulsion fractures. HYPOTHESIS/PURPOSE: To provide a new classification system for ischial tuberosity fractures based on the ossification pattern of the apophysis. METHODS: We performed a retrospective records review of patients diagnosed with ischial tuberosity avulsion fractures at our institution from 2008 to 2018. Skeletal maturity (Modified Oxford score [MOS], Risser score), fracture type, size, and displacement were recorded based on initial injury radiographs. We reviewed a large series of pelvic CT and MRI scans from patients aged 10-19 years old to assess the ossification pattern and tendinous attachments of the ischial tuberosity. Pelvic CT review demonstrated a reproducible 5-stage pattern of ossification spanning the age of 13-19 years for males and 12-17 years for females (Figure 1). Review of available CTs and MRIs indicated that the semimembranosus attaches at the most lateral ossification center, followed by the conjoint tendon and adductor magnus as one moves medially (Figures 1). We created a classification system based on location of the ischial tuberosity avulsion fracture: Type 1 (lateral – semimembranosus and conjoint tendons) or Type 2 (complete – semimembranosus, conjoint, and adductor magnus tendons). An A or B descriptor was then added to distinguish minimally displaced (<1 cm) and displaced (≥1 cm) fractures, respectively (Figure 2). RESULTS: We identified 45 ischial tuberosity fractures. Mean age was 14.4 years (range, 10.3–18). Males accounted for 82% of the cohort. Type 1 fractures accounted for 47% of cases and 53% were classified as Type 2. Type 1 fractures were associated with younger age chronological age (p=0.001), lower MOS (p=0.002), lower Risser score (p=0.002), less displacement (p=0.001), and smaller size (p<0.001), when compared with Type 2 fractures (Table 1). Of the 45 patients, 18 had >6 month follow-up with 56% going on to non-union. Non-union was associated with greater displacement (p=0.016) and size (p=0.027). When comparing union rates by fracture location, 33% of Type 1 fractures progressed to non-union, while 78% percent of Type 2 suffered a non-union; however, this difference did not reach statistical significance (p=0.153) (Table 2). CONCLUSION: In younger patients (ages 13-15 years), the lateral ossification centers of the ischial tuberosity, at which the hamstrings attach, are at risk for isolated avulsion injury. However, in older patients (16-18 years), coalescence of the hamstring and adductor magnus ossification centers predispose patients to a combined avulsion injury consisting of a larger fragment and with greater displacement.