Biomechanical analysis of compression screw fixation versus standard in situ pinning in slipped capital femoral epiphysis

A slipped capital femoral epiphysis was created in 12 matched pairs of imma ture bovine femora using an anterior-to-posterior-directed shear force. All soft tissues, with the exception of the perichondrial ring, were removed b efore testing. One specimen from each pair was fixed with a single cannulat ed screw in standard fashion, whereas the contralateral specimen was Bred w ith a single screw that compressed the physis. The amount of compression ac hieved was quantified using Fuji film. Standard fixation yielded 1.4 MPa of pressure across the physis; compression fixation yielded 3.2 MPa, a 2.3-fo ld difference (p = 0.0001). The compression fixation was 47% more stiff tha n standard technique (p = 0.030), yet the differences in ultimate strength (p = 0.180) and energy absorbed at failure (p = 0.910) were not statistical ly significant. The stiffness of the compressed specimens remained less tha n that of the intact femora. Single-screw compression fixation of in vitro bovine femora was significantly more stiff than the current, widely used no ncompression fixation technique, yet does not compromise the ultimate stren gth, energy absorbed, or the technical ease of single-implant fixation.