The effect of loading in mechanical response predictions of bone lengthening

Intramedullary (IM) distractor nails have become a viable alternative in bone-distraction operations. Upon stabilization of the fractured/dissected limb via the nail, the resulting construct accommodates the load bearing function of the otherwise healthy limb. In establishing design performance targets for these devices, in vitro test conditions are widely accepted leaving the in vivo conditions aside. However, in vivo device failures due to distraction forces necessitate novel modeling considerations. It is especially important to simulate the loads in limb distraction, as this brings the bone-implant construct to a totally different regime than the hip-joint contact force (Point-Force Model, PFM). In this work we used a simplified approach to incorporate ligament stretching due to limb distraction via self-equilibrating spring elements in a finite-element setting (spring-force model, SFM). We compared the effect of loading type on load transmission paths through the locking pins, for these two distinct loading modes, namely, SFM and PFM. The two modes illustrate entirely different load transfer regimes around the bone/nail interface region. In order to avoid high contact stresses between the nail and the bone segments, it is advisable to keep the osteotomy away from the mid-range between the pin connections. It was also seen for both loading modes that including an additional pin at a load transfer location does not significantly alter the load carried by a single pin (the additional pin rather acts as a geometric stabilizer).