Abstract
Using a simple mathematical formulation, the relationship between the position of the lag screw tip (relevant to both intramedullary and extramedullary devices) and the concept of tip-apex distance (TAD) was derived. TAD is widely used in operating theaters as a surgical guideline in relation to the fixation of trochanteric fractures, and in clinical studies as a predictor of lag screw cut-out. In order to visualize better this concept, the locus of points having the same TAD was plotted and the dependence of TAD on the location of the lag screw tip was also reported. It was shown that TAD should be adjusted for the size of the femoral head (a variable which varies a lot according to the sex of the patient) while no correlation was found between TAD and bone morphometry indices obtained from micro-CT data (BV/TV and Tb.Th). Therefore, these results seem to suggest that TAD lacks mechanical justification and that predictors which are based on mechanical properties, such as bone density, should be investigated further.
Highlights
Cut-out of the sliding hip screw from the femoral head accounts for up to 84% of the failures of fixation of extracapsular proximal femoral fractures [1]
tip-apex distance (TAD) is defined as the sum of the distances between the tip of the lag screw and the apex of the femoral head, as measured on the anterior-posterior (AP) view (XAP) and on the lateral view (Xlat), both distances being corrected for radiograph magnification by using the true diameter of the lag screwDtrue as a reference, i.e
Many different lag screw positions can lead to the same TAD, as illustrated in Figure 3, which shows the locus of points in the femoral head having the same TAD, with the apex represented as a blue dot
Summary
Cut-out of the sliding hip screw from the femoral head accounts for up to 84% of the failures of fixation of extracapsular proximal femoral fractures [1]. Intramedullary nailing suffers from the same complication since Lobo-Escolar et al reported a cut-out rate of 3.6% in a study including 916 hip fracture patients [2]. The tip-apex distance (TAD), suggested by Baumgaertner et al [3], is currently considered to be the gold standard amongst predictors of lag screw cut-out. This predictor is relevant to the positioning of both intramedullary nails and sliding hip screws. TAD is defined as the sum of the distances between the tip of the lag screw and the apex of the femoral head, as measured on the anterior-posterior (AP) view (XAP) and on the lateral view (Xlat), both distances being corrected for radiograph magnification by using the true diameter of the lag screwDtrue as a reference, i.e
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