Elbow to digit measurements as a preoperative adjunct tool to aid intramedullary femoral nail selection – the rule of thumb.

• Elbow to thumb + 3.5 cm can be used to predict nail length for greater trochanteric entry. • Elbow to thumb + 1.4 can be used to predict nail length for piriform fossa entry. • Surgeons can be confident that the appropriate nail will be selected from a range of three, including that predicted by the "rule of thumb" and one increment to either side. Intramedullary femoral nails are measured intraoperatively using fluoroscopy. This study aims to investigate whether distance from the olecranon to fingertips can be used to estimate nail length preoperatively by evaluating agreement of the two measurements. Such preoperative measurements may provide a likely range of nails needed for theatre and serve as an adjunct to intraoperative measurements. In 70 patients, long leg radiographs were used to make linear measurements from the piriformis fossa and greater trochanter to the distal physeal scar of the femur. These were compared to measurements from the olecranon to the tip of each digit and Bland-Altman agreement analysis was performed. Train and Trial groups were formed to develop models for estimation and to then test their accuracy. Bland-Altman analysis revealed a fixed bias, indicative of average difference between measurements, of 1.4 cm when comparing the elbow-thumb length to maximum femoral nail length for piriformis fossa entry and 3.5 cm using greater trochanter entry, informing the "rule of thumb". Elbow-thumb length plus 1.4 cm or 3.5 cm predicts nail length to a range of ± one nail increment. The rule can be used to preoperatively predict a range of three nails from which there is a 94–97% likelihood the appropriate intramedullary nail would be selected. The "rule of thumb" may serve as a simple to use tool for preoperative planning and a technical check to assess the "reasonableness" of intraoperative measurements before implantation of the device. It has the potential to reduce operating time and nail wastage. [ABSTRACT FROM AUTHOR]