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A Biomechanical Analysis of the Sliding Hip Screw

1990; Lippincott Williams & Wilkins; Volume: 4; Issue: 2 Linguagem: Inglês

10.1097/00005131-199004020-00005

1531-2291

Robert J. Meislin, Joseph D. Zuckerman, Frederick J. Kummer, Victor H. Frankel,

Orthopaedic implants and arthroplasty

There is general agreement that the implant of choice for intertrochanteric fractures is the sliding hip screw (SHS). However, considerable differences of opinion exist as to which plate angle—varying from 130 to 150°—is preferred. Thus far there has been no cadaver-based biomechanical analysis of this problem. To examine these questions, we determined the effect of plate angle on plate strain and proximal medial femoral strain distribution in cadaver femurs fixed with 130, 135, 140, 145, and 150° SHS after experimentally produced stable and unstable intertrochanteric fractures. Twenty-four fresh adult cadaver femurs were assigned randomly to either the 130, 135, 140, 145, or 150° SHS group. Each femur was radiographed and bone mineral density was determined by dual-photon absorptiometry. Multiple-strain gauges were affixed to the femur, with specific focus on the proximal femur and plate. Femurs were loaded at 25° adduction in increments of 70 N from 0 to 1,800 N in a servohydraulic testing machine. Femurs were tested in a progressive manner: (a) intact femur; (b) intact femur with SHS inserted; (c) a stable two-part intertrochanteric fracture reduced with SHS; (d) a four-part fracture with the posteromedial fragment (PMF) reduced anatomically by a lag screw; (e) the same fracture with the PMF rotated 180° and held in place by a lag screw to approximate a “near-anatomic” reduction; and (f) the same fracture with the PMF discarded. Screw sliding measurements were determined at regular intervals throughout each test. While the plate strain gauge demonstrated higher tensile strain in the 130° SHS group than in the other groups for all tests performed, the differences were not significant (p > 0.05). For each test, the tensile strain was lowest for the 140° SHS and highest for the 130° SHS. Less compressive strain was measured in the medial “calcar” region as the fracture became more unstable. However, the compressive strain was almost identical for all angle groups when a two-part or four-part fracture with perfect reduction was performed. Analysis of the sliding characteristics showed more screw sliding with the 150° nail than with the 130° nail (p < 0.01) or 140° nail (p < 0.05). Even though the 150° nail plate was found to achieve the greatest amount of screw sliding, it produced neither optimal compressive medial femoral loading nor a decreased plate tensile loading. The 150° SHS also “cut out” in the majority of these samples when tested to failure. The 130° nail jammed and bent for two specimens once the PMF was discarded. The matching of the plate angle to the anatomic neck-shaft angle was not found to significantly effect strain distributions (p > 0.05). The argument for using a plate angle closest to the biomechanical axis of the hip or one that is closest to the anatomic neck-shaft angle may be somewhat overstated. The goal of producing a fracture-implant combination with optimal biomechanical loading characteristics may be achievable using an SHS with a plate angle of 135 or 140°. This may limit the need for maintaining and producing large inventories of SHSs of varying plate angles.