Biomechanical comparison of minimally invasive fixation for sanders II calcaneal fractures: a multi-position finite element study from rigid internal fixation to elastic kirschner wire configuration.

Introduction: Minimally invasive surgical treatment is increasingly favored for intra-articular displaced calcaneal fractures with a high risk of soft tissue complications. The calcaneus is the largest tarsal bone in the foot and plays a critical role in weight-bearing and gait mechanics; displaced fractures often result in significant functional impairment. Sanders Type II fractures, characterized by a single major fracture line that divides the posterior articular surface into two major bone fragments, are one of the most common subtypes and present a therapeutic challenge in balancing mechanical stability with soft tissue preservation. Methods: A finite element model of Sanders Type II calcaneal fractures was established to compare five fixation methods under physiological tendon loading: (1) a periosteal locking plate, (2) four hollow screws, (3) four K-wires placed along the screw trajectory, (4) two intra-articular K-wires combined with two standard K-wires, and (5) six K-wires placed at dispersed locations. Outcome measures included maximum fragment displacement, implant-induced bone stress, and implant stress. Results: Rigid internal fixation (hollow screws) provided the best initial stability and the least displacement of the bone fragments. A key finding was that under the most severe static loading condition (dorsiflexion with maximum Achilles tendon force), the maximum fragment displacement for all K-wire configurations (≤0.103 mm) was below both the clinically reported acceptable threshold (≤0.5 mm) and the safe healing range (<1 mm), indicating sufficient initial healing stability. Furthermore, K-wires generated significantly lower stress on the bone than the plate, while cannulated screw fixation carried the lowest risk of implant failure. Based on extended observations from clinical literature, K-wire fixation offers potential advantages including a minimally invasive approach, flexible handling, low implant burden, and the possibility of outpatient removal to avoid secondary surgery. Conclusion: Although rigid implants are biomechanically superior, K-wire fixation provides clinically acceptable initial stability and, combined with its minimally invasive characteristics, represents an attractive alternative. The choice of fixation method requires a comprehensive consideration of individual patient factors (such as bone quality and functional demands) and the overall treatment strategy.