Shear strength of the physis varies with anatomic location and is a function of modulus, inclination, and thickness

Slipped capital femoral epiphysis involves the gradual displacement of the femoral head relative to the neck. Many theories have arisen to explain thi s slip. Frequently cited etiological factors include increases in physeal t hickness and inclination. Slipped epiphysis has been postulated to result f rom shear overload that causes separation through the hypertrophic cellular zone. We sought to answer the following questions: (a) Do significant regi onal differences in strength and stiffness exist within a given physis? (b) Are regional differences in resistance to shear related to thickness and i nclination of the physis? (c) Does physeal compression cause mammillary int erdigitation to begin sooner and increase the resistance to shear before, d uring, and after failure? (d) Does shear failure occur at displacements det ectable by radiography? and (e) Does cleavage occur throughout the entire c olumnar zone, and do the chondrocyte columns remain intact on both sides of the cleavage plane? We prepared beam-shaped microstructural samples from d ifferent sites of the bovine proximal tibial physis. We determined thicknes s, inclination, ultimate stress and strain, modulus, and strain energy dens ity at ultimate stress as a function of location. Using scanning electron m icroscopy, we also examined the entire failed surface of several samples. F orty-eight samples were tested by displacing the epiphysis end anteriorly, without axial (across the thickness) constraint; 41 were sheared while an a verage axial compressive stress of 0.3 MPa was applied to the physis. The p osterior region had the greatest strength and stiffness, lowest physeal thi ckness, and steepest inclination. Compressing the plate did not increase th e shear strength or tangent modulus. Ultimate strength varied inversely wit h thickness and increased when shearing up steeper inclinations; however, i t was more strongly associated with the modulus, implying that additional f actors control both strength and modulus. Scanning electron microscopy reve aled that the plane of fracture differed widely between and within samples, involving all zones of the growth plate. On either side of the fracture, i ndividual chondrocyte columns remained intact, although separated from neig hboring columns.