Biomechanical evaluation of the tension band wiring principle. A comparison between two different techniques for transverse patella fracture fixation

Purpose: The aim of this study was to investigate the validity of the dynamic compression principle of tension band wiring in two techniques for patella fracture treatment. Methods: Twelve human cadaveric knees with simulated transverse patella fractures were assigned to two groups for treatment with tension band wiring using either Kirschner (K-) wires or cannulated screws. Biomechanical testing was performed over three knee movement cycles between 90 degrees flexion and 0 degrees full extension. Pressure distribution in the fracture gap and fracture site displacement were evaluated at the 3rd cycle in 15 degrees steps, namely 90 degrees -75 degrees -60 degrees -45 degrees -30 degrees -15 degrees -0 degrees extension phase and 0 degrees -15 degrees -30 degrees -45 degrees 60 degrees -75 degrees -90 degrees flexion phase. Results: Mean anterior / posterior interfragmentary pressure in the groups with K-wires and cannulated screws ranged within 0.16-0.40 MPa / 0.12-0.35 MPa and 0.37-0.59 MPa / 0.10-0.30 MPa, respectively. These changes remained non-significant for both groups and loading phases (P >= 0.171). Mean anterior / posterior fracture site displacement for K-wires and cannulated screws ranged within -0.01-0.53 mm / 0.11-0.74 mm and 0.11-0.55 mm / -0.10-0.50 mm, respectively. Anterior displacement remained without significant changes for both groups and loading phases (P >= 0.112). However, posterior displacement underwent a significant increase in the course of knee extension for K-wires (P <= 0.047), but not for cannulated screws (P >= 0.202). Significantly smaller displacement at the posterior fracture site was detected in the group with cannulated screws compared to K-wires at 60 degrees and 75 degrees extension phase (P <= 0.017), as well as at 45 degrees, 60 degrees and 75 degrees flexion phase (P <= 0.018). The critical value of 2 mm displacement at the posterior fracture site was not reached for any specimen and fixation technique. Knee extension was accompanied by synchronous increase in quadriceps pulling force. Conclusions: Tension band wiring fulfills from a biomechanical perspective the requirements for sufficient stability of transverse patella fracture fixation. It should, however, rather be considered as a static fixation principle than a dynamic one. Tension band wiring with cannulated screws was found advantageous over Kirschner wires in terms of interfragmentary movements at the posterior fracture site. (C) 2017 Elsevier Ltd. All rights reserved.