Introduction: Over the last 10 years minimally invasive plate osteosynthesis (MIPO) has gained more and more popularity over the conventional open surgical approach (ORIF). Numerous clinical case collection studies reported the MIPO technique as a good, alternative method. While MIPO offers some advantages over ORIF, it also has significant drawbacks, such as more demanding surgical technique and increased radiation exposure. In an attempt to compare these approaches, a previous animal study 1 did not show any significant differences in fracture healing outcome. Using a new developed, standardised severe trauma model on the sheep femur 2, this study examines the effects of the surgical approaches on fracture healing.

Methods: 24 sheep (Merino wethers, mean age 5.6years, mean weight 39.1kg) underwent the trauma model 2 with a severe soft tissue damage and a multifragmentary, distal femur fracture as well as initial stabilisation with an external fixator. After five days of soft tissue recovery, the animals were definitively operated with an internal fixator (LCP) randomised either by a minimally invasive or open approach. The sheep were sacrificed after 4 and 8 weeks (two groups), mechanical testing performed and statistically analysed with ANOVA test.

Results: After 4 weeks, torsional rigidity is significantly higher in the MIPO group (30.1r10.6(SD)%) of fractured to intact bones, p< 0.05) compared to ORIF group (9.8r12.4(SD)%), while ultimate torque also shows increased values for MIPO technique (p=0.11). After 8 weeks, the differences in mechanical properties levelled out, but still higher values for the MIPO group (p=0.36/p=0.26).

Conclusion: In the early stage of fracture healing, minimally invasive plate osteosynthesis shows advanced healing pattern compared to open fixation technique. This advantage seems to level out over time.