Fracture near press-on interlocking enhances callus mineralisation in a sheep midshaft tibia osteotomy model.

Introduction: Factors which impair fracture healing after intramedullary (IM) nailing of long bone fractures range from surgical and biological factors to mechanical parameters. Mechanical parameters known to prolong bony consolidation are share forces at the site of the fracture. Fracture near press-on interlocking reduces share forces directly at the fracture site and is hypothesised to enhance callus mineralisation. A sheep model of midshaft tibia osteotomies evaluates the technique.

Materials And Methods: Fracture near interlocking was achieved by surfacing a custom made nail with special hutches that enable firm screw seating on top of the nail ("golf ball" structure). Virtual (fine element analysis (FEA)) and biomechanical pilot tests were completed before in vivo application in 12 adult female German black sheep. Midshaft tibia osteotomy was performed creating a subcritical 7 mm gap for delay in union. One group (n=6) was treated with reamed IM nailing employing the custom made nail and in addition to proximal and distal standard interlocking a fracture near press on interlocking was employed. A second group of six sheep without additional press on interlocking served as control. 10 weeks after operation the quality of fracture healing was determined by micro-CT.

Results: The FEA showed that axial loading up to 4000N did not lead to implant fatigue. Fracture near press on interlocking led to significantly more callus mineralisation compared to the conventional interlocking procedure (0.567 g/cm(3) ± 0.106 g/cm(3) versus 0.434 g/cm(3) ± 0.0836 g/cm(3), p=0.043).

Conclusions: Fracture near press on interlocking increases callus mineralisation in a subcritical osteotomy model in sheep. The results indicate that the reduction of share forces at the fracture site after nailing procedures may be effective in reducing the time until bony consolidation.