Bone healing: the role of weight bearing and dynamization
In this article, we will discuss the two key aspects of the mechanical environment: weight bearing and dynamization, and how their optimization can lead to improved outcomes, especially regarding lower extremities.
Early weight bearing
Early in the process of fracture healing, the rigidity provided by external fixation compensates the existent high strain forces at the fracture site. If such forces remain too high during this period (inadequate stability), the result is a fibrous non-union. Therefore, external frames should be very stable in the early period to promote bone healing, but also to allow weight bearing as soon as possible.
Weight bearing, which provides axial loading to the fracture and stimulates osteogenesis, will also counteract the paradoxical impairment of bone healing caused by an excessively rigid fixation. As more research is done, the benefits of early weight bearing become more evident, as they induce the necessary strain at the fracture site, without compromising stability or increasing the rate of post-operative complications.
As the healing process advances, the bone can bear a greater load and less stability is required. But for a more flexible fixation to result in a successful response, there must be a remaining degree of intrafragmentary strain to stimulate the healing process. Functional weight bearing, combined with an appropriate amount of motion at the fracture site, will accelerate the healing rate and promote increased stiffness of the fracture callus.
The second key component for successful bone healing is dynamization, or the introduction of controlled motion, which provides an increasing load across the site of osteogenesis as the callus gains stability.
As Ilizarov demonstrated, only a combination of sufficient blood supply, bony stability, and axial loading will provide the perfect environment for osteogenesis. For example, it has been shown that both clinical and mechanical healing in tibial fractures is aided by adding axial micromotion at the fracture site when treated with a dynamic unilateral external fixator.
Some external fixation devices can readily be changed from a stable to a dynamic configuration, thereby allowing for more axial movement between the fracture fragments. The stability of bony fragments accomplished by the rigidity and the design of the external fixator will determine how much weight bearing is allowed.
Factors influencing the early bone healing process
Targeting the mechanical environment in fracture repair can intensify the healing response and accelerate callus formation, but bone healing is a well-orchestrated interaction between mechanical and biological components, requiring careful timing and attention.
There are several factors influencing the time required for bone healing, including the type of fracture and frame used, bone-fixator distance, extent of fracture repair, other traumas in the same or the other limb, level of pain, and the patient’s physical and psychological condition, as well as their social conditions.
Although not many fracture fixation devices have been specifically developed to actively incorporate dynamization in the clinical treatment of fractures, Orthofix ProCallus, the XCaliber and the TL family circular fixators do allow for fracture dynamization. Passive cyclic micromotion can be applied before any weight bearing has started.
The exact timing for progressive loading and early weight bearing depends on whether the fracture is stable or unstable. In the presence of a stable fracture, progressive loading should start 2 to 4 weeks after surgery – initially with 30% weight bearing, then 75%, and up to 100%. With unstable fractures, progressive loading should commence 5 to 8 weeks after surgery, with an initial 15% weight bearing, then 75% within two months after the operation, up to 100% over the following month, but only with radiological evidence of good callus formation. In any case, weight bearing, as tolerated by the patient, is at the surgeon's discretion.
By modulating the biomechanical environment surrounding a fracture, an attentive surgeon may take advantage of weight bearing and dynamization to stimulate the bone repair process and therefore improve patient outcomes.