Medial migration of intramedullary intertrochanteric fracture fixation devices

George Mikhail

doi:10.17918/etd-3015

Intertrochanteric fractures have been fixed for decades with side plates and sliding hip screws, and more recently, with intramedullary devices and sliding lag screws. The most common failure mode of all such implants is superior cutout of the implant due to varus collapse of the femoral head and neck over the implant. The major cause of this type of failure mode is believed to be osteopenic bone. Another rarely reported failure mode, is the medial migration of the femoral neck implant along its axis through the femoral head into the acetabulum. In follow-up x-rays, the femoral neck implant is seen to migrate through the intramedullary nail, and out of the femoral head. In this failure mode, the femoral head and neck do not collapse or may have already collapsed as far as possible, but instead the implant migrates medially on its own. The failure is very rare with few reported incidents over many years of use and hundreds of thousands of fractures. Due to its rarity, this mode has not been sufficiently documented or studied, nor is the cause well understood. It is postulated that the failure could be due to a specific combination of the physiologic properties of the bone, fracture pattern, and implant design. The purpose of this research is to create a biomechanical test model that would reliably reproduce the medial migration phenomenon in a controlled lab setting. Once a robust model is achieved, various implant designs can be tested to determine if they effectively prevent this phenomenon from occurring. It is not our purpose to analyze orpredict the parameters that might induce medial migration. The event is rarely observed in the clinical setting. The parameters that may be contributing to this phenomenon are numerous, difficult to clinically assess, and there are not enough clinical cases to study. However if an implant is to be designed that would eliminate the capability for medial migration it must be tested. This project sets out to create a robust test model that can reliably be used to test designs claiming to fix the problem. During the course of the study we came across situations that may provide insight into other failure modes. Our test construct design iterations sometimes displayed other failures that were not studied before, including the femoral head collapsing along the axis of the blade, but the blade itself not sliding as it is designed. We also discovered that two-screw devices are more prone to the "Z-effect," which is another form of medial migration. This correlated to the fact that the "Z-effect" was commonly reported in the literature with two-screw devices while medial migration was almost never reported due to its very low occurrence rate. The research did successfully create a biomechanical model that recreated the phenomenon of medial migration and can be used to test potential fixes to prevent medial migration. Without a biomechanical test model, all fixes are only theoretical and cannot be proven until thousands of cases have been performed because of the rarity of the failure. We hope that this work can be used to further improve implant designs reducing the number of revision surgeries needed after initial hip fixation.

Medial migration of intramedullary intertrochanteric fracture fixation devices

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Medial migration of intramedullary intertrochanteric fracture fixation devices

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