Finding the physeal midpoint for distal femoral hemiepiphysiodesis utilizing novel anatomic measurements

Distal femoral hemiepiphysiodesis is a minimally invasive procedure used to correct pediatric knee angular deformities by guiding growth across an open physis. Intraoperative fluoroscopy is used to estimate the physeal midpoint for hardware placement; however, no standardized radiographic method has been established to identify this landmark. The purpose of this study was to determine the accuracy and reliability of three novel radiographic landmarks for approximating the distal femoral physeal midpoint. Perfect lateral digitally reconstructed radiographs of the knee were generated from computed tomography scans of pediatric patients using open-source image computing software. The physeal midpoint was identified and compared with three radiographic landmarks: the diaphyseal midpoint, the apex of the physeal undulation, and the inferior point of Blumensaat's line. The anteroposterior distance between each landmark and the physeal midpoint was measured, and intraclass correlation coefficients (ICCs) were calculated to assess reliability. Twenty-seven (71% male and 29% female) patients with a mean age of 13.6 ± 1.7 years were included. Relative to the physeal midpoint, the diaphyseal midpoint was 4.0 ± 2.4 mm anterior, the apex undulation was 1.6 ± 1.2 mm anterior, and the inferior Blumensaat's point was 1.2 ± 1.3 mm posterior. Reliability was good for the diaphyseal midpoint (ICC = 0.799) and apex undulation methods (ICC = 0.889), and excellent for the Blumensaat landmark (ICC = 0.987). These findings suggest that multiple radiographic landmarks can reliably approximate the distal femoral physeal midpoint on perfect lateral radiographs. The inferior point of Blumensaat's line demonstrated the greatest accuracy and reliability and may serve as a useful intraoperative guide for hardware placement during distal femoral guided growth procedures. LEVEL OF EVIDENCE: Level III.