Contribution of the perichondrial ring to capital femoral epiphysis in torsion.

The purpose of this study was to determine the torsional strength of the capital femoral epiphysis and the role of the perichondrial ring in resisting torsion and to establish the porcine model as an analogue to human slipped capital femoral epiphysis (SCFE) in order to aid in the development of SCFE treatment. Eight pairs of skeletally immature (12-month-old) porcine femora were obtained from a slaughterhouse. The soft tissue was removed, excepting the perichondrial ring. All left femora had the perichondrial ring removed sharply. Matched pairs were tested by fixing the distal end in a custom jig and by subjecting the proximal end to a torque in the plane of the physis at 0.1 degrees/sec, simulating an external rotation. Torque and angular rotation data were acquired at 100 Hz. Test were stopped at failure, defined as a drop in torque of 30% of the peak value. Femora with the perichondrium removed failed at a mean peak torque of 18.7 NM (SD 3.0) compared with 23.4 Nm (SD 3.6) for the contralateral side, with the perichondrium intact. This represents a mean difference of 4.8 Nm (paired Student t test, P = 0.002). Femora without the perichondrial ring were 79.7% as strong as the right femora without the perichondrial ring were 79.7% as strong as the right. Comparison of the paired femurs showed that the perichondrium contributed an average of 20.3% of torsional strength. Current fixation techniques have not been biomechanically tested in this mode. Some torsional loading through the physis is likely in SCFE. SCFE is the most common hip disorder of adolescent children. The porcine femur serves as an excellent model for this common clinical problem. This model will serve in the development of novel bioabsorbable fixation.

Christopher R. Chuinard, MD, MPH, John L. Williams, PhD, and Thomas Schmidt, MD. Department of Orthopaedic Surgery, University of Missouri-Kansas City, Kansas City, MO.