A BIOMECHANICAL EVALUATION OF THE LONG STEM INTRAMEDULLARY HIP SCREW

Despite the advantages associated with short-stem intramedullary hip s crew devices for the treatment of intertrochanteric fractures, recent reports have shown an increased incidence of femoral shaft fractures a fter their insertion. These findings led to the hypothesis that an int ramedullary hip screw with a longer stem may more effectively redistri bute loads to the distal end of the femoral shaft, where they may be m ore readily absorbed by the increased bony cross-sectional area. To ch aracterize the load patterns of a long-stem device in the femur, 10 fr esh-frozen adult femurs were instrumented with unidirectional strain g auges. A total of eight strain gauges were placed in the direction of principal femoral strains on the medial and lateral surfaces of each f emur. Each femur was held in a steel vice at 15 degrees of adduction i n the coronal plane and vertical in the sagittal plane. The femurs wer e then subjected to successively increasing vertically applied compres sive loads from 0 N to 1,400 N at 200-N increments using a servohydrau lic testing machine. Strain values were recorded at each load after a 5-min equilibration period. Each femur was tested under five condition s: (a) intact, (b) after insertion of the long-stem intramedullary hip screw device, (c) with an experimentally created two-part fracture, ( d) with a stable four-part fracture, and (e) with an unstable four-par t fracture with the posteromedial fragment removed. Half the femurs we re randomly assigned to have two distal interlocking screws placed bef ore fracture. The remaining half were loaded without distal interlocki ng screws. The results indicate that the loads on the femur with inter trochanteric fractures are redistributed such that the proximal femur is subjected to significantly lower strains. Moreover, even though str ain values in the distal metaphysis at the site of load transfer were relatively high, they did not differ significantly from the values rec orded in the intact femur. Thus, the long-stem intramedullary hip scre w device transmits progressively decreasing load to the proximal femur with increasing fracture instability and redistributes this load thro ughout the distal femur without significantly increasing distal femora l strain values.