Distal locking of femoral nails. Mathematical analysis of the appropriate targeting range

Background: During the last decades, intramedullary nailing has become the standard treatment for diaphyseal fractures of long bones. Numerous innovative techniques and devices have been proposed to simplify distal locking. Each has its own limitations and, as a result, the fluoroscopy-dependent "free-hand technique" remains the most popular method. However, radiation exposure to the patient and operating room staff remains a concern.

Methods: Before the development of a new radiation-independent, nail-mounted targeting system, we mathematically analyzed the aiming accuracy that such a system has to achieve. The correctness of this mathematical model was evaluated using a mechanical testing apparatus.

Findings: We found a quite large targeting range for the unimpeded passage of the drill bit through the locking hole of a given nail. Important degrees of nail bending can thereby be compensated. As predicted by the mathematical formula, a 4-mm drill bit passed the distal locking hole of a 320/11mm femoral nail up to a deflection of ±13mm in the coronal plane.

Interpretation: This mathematical model can be considered to be an additional tool for the development of new targeting devices. Combining our mathematical model with data previously published, not only torsional deformation along the longitudinal axis of the nail but also bending in the coronal plane can approximately be neglected. Hence, the three-dimensional aiming process can be simplified to the determination of the interlocking hole of the nail in the sagittal plane provided that the insertion-induced nail deformation in vivo stays in the range of that observed in vitro.

Level of evidence: Level III. Basic sciences control study.