The Challenge of Plate-Bone Construct Stiffness: A Swinging Pendulum

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The Challenge of Plate-Bone Construct Stiffness: A Swinging Pendulum

第一作者:Erika Jasmin Mitchell 编号 : #127600#

Fracture plating has evolved in parallel with better understanding of the biology and mechanics of bone healing. Although treatment of the simple plane fracture with compression plating and absolute stability has been successful in most instances, clinicians still largely rely on experience and judgment to decide how to best treat comminuted fractures and fractures in osteoporotic bone where compression is not an option. Stephan M. Perren’s strain theoryand studies that followed have taught us that, for bone to heal, there needs to be an ideal amount of micromotion at the fracture site. Too much, and the callus never consolidates; too little, and no bone is formed. Locking fixation seemed to be an excellent solution to the problem of poor fixation in osteoporotic bone; however, with its use in some instances, we created a new problem of constructs with such stiffness that osteocytes at the fracture are not signaled to form callus.

One solution to this new problem has been far cortical locking, where the near cortex is effectively overdrilled to allow for increased micromotion while still using locking screws to maintain the benefits of fixed angle stability. This technique arose from the observation that healing callus in locked constructs is often seen predominantly on the far side of a locking plate construct where more micromotion occurs. By destabilizing the screw’s fixation in the cortex on the side of the plate, this bone-inducing micromotion could be created circumferentially while still maintaining a fixed angle construct. Early clinical studies using this technique have demonstrated good results with this method in distal femoral fractures.

In their study, “Dynamic Stabilization with Active Locking Plates Delivers Faster, Stronger, and More Symmetric Fracture-Healing,” Bottlang et al. examine healing in an animal fracture model with use of a new method to obtain far cortical locking. The active locking plate is titanium and uses an elastic element that allows the locking screw to slide in a controlled manner within the screw hole in the plate, providing up to 1.5 mm of axial motion at the fracture site. As such, more symmetric motion is created between the near cortex and the far cortex. Axial stiffness of this plate-bone construct on a standard 3-mm ovine osteotomy gap model used in this study was 64% to 89% less than that of standard locking plate constructs, with greater stiffness at higher loads as the motion of the screws becomes limited by the compression of the elastic element in the holes of the plate. The authors demonstrated that the active construct led to a significant improvement in fracture-healing with increased circumferential callus and increased torsional strength of the healed bone when compared with standard locked plating in this fracture model.

This study, along with others on far cortical locking fixation, provides further evidence that relative stability with circumferential healing can be successfully achieved while maintaining the fixed angle stability of locked plating. However, it is still important to further investigate this technique in a clinically relevant setting. A simple gap osteotomy treated with fully locked plating fails to truly replicate the complexities of a multifragmentary fracture and bridge plating. Treating a simple plane fracture in young, healthy bone with fully locked screws and leaving a 3-mm gap, as this study models, would almost certainly lead to nonunion. This study’s comparison of active plate fixation with a construct that is expected to create nonunion is less meaningful than a comparison with a more standard method of bridge plating such as the far cortical locking technique with overdrilling, nonlocked or hybrid plating, or locked plating with a longer working length, all of which would increase micromotion and would improve healing potential. With such comparisons, we could determine if there is an advantage of locking fixation at all in this model, as well as if the active construct improves healing. The study also attributes the success of the active implant to motion at the fracture site, but we cannot be certain that improved healing is not at least partially due to a fracture gap that is potentially halved in width in the active group once the sheep is weight-bearing. It remains unclear what amount of dynamization would occur in these constructs with toe-touch weight-bearing, which is routinely used clinically after lower-extremity plating, because the ovine model is fully weight-bearing.

Locking-screw technology arose from a need to improve fixation in poor-quality bone where standard screws fail to provide adequate stability. However, the increased stiffness of these locking constructs, particularly in good-quality bone, can lead to nonunion despite the improved attempt to preserve soft tissue and blood supply. Multiple techniques have thus been suggested to decrease stiffness in locking constructs to bring motion back into the ideal window for bone healing. These include the use of titanium rather than steel implants, unicortical locking screws, hybrid fixation, increased working length with longer plates, and far cortical locking techniques.

Since the advent of plating for fractures, techniques have been developed to increase or decrease the stiffness of the bone plate construct while respecting the soft tissues and periosteal blood supply. It is still necessary to critically evaluate when locking is necessary, particularly with healthy bone quality. The major advantage of locking fixation is improved stability in osteoporotic or metaphyseal bone where the softer bone can promote more motion and a less stiff construct and fixed-angle locking screws maintain alignment of the fracture. Additionally, stable fixation can be obtained with minimal injury to the periosteal blood supply. If using locked bridge plating, the surgeon must have an understanding of bone healing and of these techniques to prevent a construct that is too rigid (or too unstable) for osseous union. As described in this study, far cortical locking with a specialized plate may be one method for optimizing fracture micromotion.