Biomechanical analysis of minimally invasive crossing screw fixation for calcaneal fractures: Implications to early weight-bearing rehabilitation

Background: Minimally invasive fixation using crossing screws was believed to produce satisfactory clinical outcome whereas its stability in early weight-bearing remained controversial. This study aimed to analyze the biomechanical stability of minimally invasive fixation during balanced standing and walking stance, and provide evidence for early rehabilitation. Methods: A finite element model of foot-ankle-shank complex was reconstructed based on computed tomography and magnetic resonance images, and validated by plantar pressure of the model participant. A Sanders III calcaneal fracture was created on the model, and then fixed using crossing screws. The predicted stress distribution, fracture displacement, Bohler's angle and Gissane's angle were compared between the intact calcaneus and fracture model with the fixation. Findings: Postoperatively, the concentrated stress appeared at the junction of the calcaneus and its surrounding tissues (especially Achilles tendon, plantar fascia and ligaments) during standing and walking stances, and the stress exceeded the yield strength of trabecular bone. The longitudinal screws sustained the highest stresses and concentrated at the tips and the calcaneal tuberosity junction. The displacement of posterior joint facet, Bohler's angle, and Gissane's angle were within the acceptable range either standing or walking after the fixation. Interpretation: Early weight-bearing standing and walking after minimally invasive fixation may cause high stress concentration thereby induce calcaneus stress fractures and other complications like plantar fasciitis and heel pain, so it should not be supported. The peri-calcaneus tendons, i.e., Achilles tendon and plantar fascia, play key roles in the stabilization of the calcaneal fracture after operation.