Stress analysis of a partially slotted intramedullary nail.

A finite element analysis of the Arbeitsgemeinschaft für Osteosynthesefragen/Association for the Study of Internal Fixation (AO/ASIF) intramedullary femoral nail was performed to study the failure of the nail from circumferential cracking near the slot tip. These failures are evidently the result of a stress-concentrating effect owing to the partially slotted nail design. Several finite element models were created of the proximal one-fourth of the nail. One model of the nail incorporated the cloverleaf profile as it is presently manufactured, and one had a circular cross section. An additional three models were created with alternative slot-tip geometries: a narrowed slot, a tapered slot, and a widened slot. Antero-posterior (AP) and medio-lateral (ML) bending loads and torsion loads were applied in two fundamentally different loading modes: (1) loads that were applied on both sides of the slot tip (spanned); and (2) loads that were self-equilibrating distal to the slot tip (non-spanned). For the load cases studied, for all models, the stresses predicted from the finite element models were locally highest at the junction between the open and closed cross sections. Alternative slot-tip shapes had a marked effect on the predicted stresses, in one case reducing maximum stress by 40%. However, no alternative slot tip shape was uniformly superior for all load cases. Therefore, until the in vivo loading modes are known more precisely, an alternative slot-tip shape cannot be proposed.