CORR Insights®: Does Screw Location Affect the Risk of Subtrochanteric Femur Fracture After Femoral Neck Fixation? A Biomechanical Study

Where Are We Now? Surgeons generally choose cannulated screws to fix stable femoral neck fractures in older patients. While the screw configuration and number of screws can vary, surgeons in North America largely support the use of the apex-distal triangle with three screws [10]. But the most-distal point of fixation has been implicated as a cause of subtrochanteric femoral fractures with an incidence of 0.78 to 5.7% [1, 5-8]. While these case series did not all use the same form of fixation, they have a common denominator—the most caudal point of fixation was placed distal to the inferior edge of the lesser trochanter. Furthermore, a large number of these fractures occurred in the first 3 months after surgery with minimal trauma in elderly patients. In the current study, Crump and colleagues [4] performed a biomechanical study using normal density and osteoporotic Sawbones and confirmed that more-distal placement of cannulated screws was associated with an increased risk of subtrochanteric femur fractures. Specifically, when the screw start sites were distal to the most-inferior edge of the lesser trochanter, the authors observed these fractures, but no fractures were seen in samples that had more-proximal screw placement. The relatively transverse and short oblique fracture pattern noted in the study also is consistent with what many case reports have observed [1, 5-8]. Load to failure was also lower when the most distal screw was started distal to the lesser trochanter compared to more proximal [4]. The laboratory work by Crump and colleagues is in general concordance with previous clinical observations [1, 5-8]. To determine why that is the case, it is beneficial to review some studies with data that ties in well with the focus of the current study. For example, Oakey and colleagues [10] postulate that screw placement at the proximal lateral femoral cortex acts as a stress riser. This is supported by the fact that Bechtol’s work showed that placing a drill hole through tension side of bone decreased loading strength by 30%, while placing a drill hole on the compression side did not change bending loads [2]. A later study supported this finding by showing that a 2.8 mm drill hole in the femur reduced the energy-absorbing capacity of the femur by 55% and increased local stress by 1.6 times [3]. It is important to note that surgeons are routinely using relatively large diameter (6.5 mm to 7.3 mm) cannulated screws in this region. These screws utilize guidewires that range from 2.8 mm to 3.2 mm and errand paths with these guide wires have been implicated as a potential reason for subtrochanteric stress fractures. A recent finite element analysis (FEA) study found increased stress levels when unused pin sites were located either anterior or posterior to the most caudal pin site for fixation of femoral necks, suggesting that multiple guide-wire passes can contribute to subtrochanteric femur fractures [9]. This is supported by the fact that another recent FEA study concluded that the occurrence of iatrogenic subtrochanteric fracture increases when the most caudal screw is located distal to the lesser trochanter [11]. Where Do We Need To Go? Our goal, of course, is to achieve stable, solid fixation of the reduced femoral neck without exposing patients to the risk of postoperative subtrochanteric femur fracture. A simple solution would be to not place screws below the inferior border of the lesser trochanter in elderly patients. Unfortunately, in some patients who have a valgus femoral neck, or when there are lower Pauwel’s angles (which calls for screw placement in relative valgus to provide adequate compression of the fracture), the screws may need to be below the lesser trochanter. A possible solution in these select instances could be to use a sliding hip screw or newer implants that have telescoping cannulated screws that lock into a laterally based side-plate. The use of a side plate may decrease the stress concentration that occurs on the lateral cortex of the femur. While I do not advocate the routine use of these implants because of their increased cost compared to cannulated screws, I think they may have a role when faced with a start site below the lesser trochanter. How Do We Get There? Biomechanical and FEA studies could be done to test the above solutions. To start, a methodology similar to the above discussed studies could be utilized with a side plate study arm. If the results of these studies show decreased stress concentration and lower rates of subtrochanteric femur fractures when implants are placed below the lesser trochanter, then surgeons can consider altering their treatment strategies. It is also critical that surgeons try to avoid additional stress risers in the lateral cortex of the femur. For example, we should avoid multiple large-caliber guide wire passes during placement of cannulated screws. This can be done by perforating the lateral cortex of the femur with 0.06 inch K-wire in the location of the start site for the cannulated screw, instead of larger cannulated screw guide wire. If the trajectory is deemed appropriate on AP and lateral imaging then the K-wire can be advanced 5 mm and a 4.5 mm cannulated drill can be placed over this wire to fine tune the trajectory prior to reaching the fracture. The drill can then be removed and replaced with the appropriate guide wire, which can be advanced into the femoral head. This technique will prevent multiple large-guide wire passes. Finally, surgeons should avoid placing screws too close together and avoid apex-proximal triangular configuration.