SLR - February 2011 - Linda S. Oh
Reference: Hak, D., Toker, S., Yi, C., Toreson, J. (2010). The Influence of Fracture Fixation Biomechanics on Fracture Healing. Orthopedics, 33(10), 752-755.
Scientific Literature Review
Reviewed by: Linda S. Oh, DPM
Residency Program: Cambridge Health Alliance, Cambridge, MA
Podiatric Relevance:
Although secondary fracture healing is commonly encountered in podiatric surgery, the goal of achieving optimal callus formation is not always easily attained. This article evaluates the influence of fixation biomechanics on the quality of fracture healing.
Methods:
Minimally invasive locking plates often bridge fracture sites without compression and are designed to promote healing with callus formation. Recently, the question has been raised as to whether locked plate fixation constructs are too stiff and subsequently result in non-unions. Three concepts of fracture healing and fixation were revisited in this article including: strain theory and fracture healing, locked plates, plate length, and screw position. Two additional experimental methods to reduce locked plate stiffness were also investigated including: far cortical locking screws and dynamic locking screws.
Results:
Rigid internal compression fixation will lead to primary fracture healing due to minimized strain. Fracture gap straining is defined as the relative change in the fracture gap divided by original fracture gap. Cortical bone tolerates 2% strain whereas lamellar bone tolerates up to 10% strain, beyond which point fracture healing will not occur. Comminuted fractures tolerate more motion than simple fractures as the strain is dispersed among many fracture gaps. Locked plates often function in a bridging mode, and because they provide a rigid plate-screw interface, they limit motion directly under the plate; however, motion at the far cortex encourages callus formation. Another factor which was considered was increasing plate length in fracture fixation. This decreased the pull out force acting on screws due to the increased lever arm. The empiric recommendation is to place screws in half or fewer of the available holes, considering that screws placed farther from the fracture will promote distribution of stress and lower the chance of plate fatigue failure. Due to concerns over locked plates not providing sufficient fracture site micromotion, methods were explored to increase fracture gap micromotion with axial loading. Bottlang et al developed the concept of far cortical locking. The far cortex locking screw has a smooth shaft with threads at the tip that achieve purchase in the far cortex only, which decreases stiffness of the plating construct. A study performed on sheep tibial osteotomies provided that the callus volume was 36% higher and bone mineral content 44% higher when using far cortical locking implants. Furthermore specimens treated with far locking implants were 54% stronger in torsion and sustained 156% more energy to failure torsion. Doberle et al investigated the use of dynamic locking screws to reduce stiffness. It was found that dynamic locking screws reduced the axial stiffness by 16% with greater interfragmentary motion at the near cortical side.
Conclusions:
The fixation stability at a fracture site is a principle factor in callus formation. When applied without direct interfragmentary compression, locked plates encourage secondary or indirect fracture healing. However, if the interfragmentary gap is too great, adequate callus formation will not occur. A longer plate with fewer screws may help reduce locked plate stiffness and in turn increase interfragmentary motion to the far cortex. It was also found that far cortical locking screws and dynamic locking screws reduced bending stiffness of locked plates encouraging optimal callus formation.