Calcar referenced tip apex distance (CalTAD) – A better measuring tool to assess lag screw position in pertrochanteric femur fractures treated with twin screw cephalomedullary nails

Literature comparing TAD and CalTAD values in twin screw cephalomedullary (CM) nails in pertrochanteric femur fractures is sparse. In twin screw CM nails, the lag screw is placed in the inferior half of femoral head to accommodate derotation screw in the upper half. As the tip of the lag screw moves away from apex of femoral head, TAD increases. There is a need for an alternative measurement method for twin screw CM nails. The aim of this study was to evaluate screw position in twin screw CM nails by measuring TAD and CalTAD and analyse how these two measurements vary. Postoperative radiographs of patients who underwent fixation of pertrochanteric femur fractures with twin screw CM nails were retrieved from Medsynapse PACS of our institution. TAD and CalTAD were measured in a total of 76 radiographs and the data was analysed. The mean TAD and CalTAD were 26.7 and 21.8 respectively. TAD was less than 25mm in 32 patients (42 %) and CalTAD was less than 25mm in 54 patients (71 %). Among 44 patients with TAD>25mm, 26 patients had CalTAD<25mm. Although TAD was more than 35mm in 9 patients, none of them had CalTAD more than 35mm. In twin screw CM nails, the lag screw is placed in the inferior half of femoral head. This results in high TAD values even though screw is placed deep in the subchondral bone. For an inferiorly placed lag screw, CalTAD may be a better measuring tool to assess screw placement.