Abstract
INTRODUCTION:
The purpose of this study was to determine if a short femoral stem (Lima Corporate, Udine, Italy) would result in a strain distribution which mimicked the intact bone better than a traditional length stem, thereby eliminating the potential for stress-shielding.
METHODS:
A 2 mm thick moldable plastic (PL-1, Vishay Micromeasurements, Raleigh, NC) was contoured to six fourth-generation composite femoral bones (Pacific Research Laboratories, Vashon, WA). The intact femurs were then loaded (82 kg) in a rig which simulated mid-stance single limb support phase of gait (Figure 1). During testing, the femurs were viewed and video recorded through a model 031 reflection polariscope. Observing the photoelastic coating through the polariscope, a series of fringes could be seen, which represented the difference in principal strain along the femur. The fringes were quantified using Fringe Order, N, as per the manufacturers technical notes. In order to analyze the strain distribution, the femur was separated into 6 zones, 3 lateral and 3 medial, and the maximum fringe order determined. Upon completion of testing of the intact femur, the short length femoral stem was inserted and tested, and finally the traditional length femoral stem was inserted and tested. Anterior and lateral radiographs were obtained of the femur with each femoral stem in order to confirm proper alignment.
RESULTS:
Fringes formed in a similar pattern for all femurs, intact and with stems. The fringes first occurred medially and laterally in a proximal-distal direction and radiated outward, decreasing in fringe order, toward the neutral axis of bending (anterior and posterior). The magnitude of the fringe order, N, remained the same or increased in the proximal to distal direction. This became more prominent, particularly on the lateral side, with the traditional length femoral stem, when a distal migration of the fringes was seen compared to the intact femur (Figures 2 and 3). Medially, with the traditional length femoral stem, the fringes remained but were of a lower magnitude than the intact femur. The femoral strain pattern, resulting from implantation of the short length femoral stem, was found to closely match the intact femur. X-rays confirmed proper alignment of all implants.
CONCLUSIONS:
The distal migration of strain seen with the traditional length femoral stem was indicative of potential stress shielding. As an alternative, this study suggests that the short length femoral stem most closely replicates the strain distribution of the intact femur and may limit this type of failure.
