Iatrogenic proximal femur hoop-stress fracture is a recognised complication of uncemented hip arthroplasty. It has a reported incidence of two to three percent and increases patient morbidity. We describe a novel technology that predicts fracture in real-time by less than one minute. Four proximal femora from red deer (Cervus elaphus), similar size to human proximal femora, were prepared to accept an uncemented hip arthroplasty femoral rasp (Finsbury Orthopaedics) and then mounted in a loading machine. The femora were fresh-frozen, defrosted and kept at room temperature in 0.9% saline swabs. The rasp was forced into each femur in repeated loading cycles every 10 seconds, in steps of 100N increasing from 200N to over 2000N until fracture, in a manner to simulate surgery. One sensor was attached to the surface of the proximal femur and one to the femoral rasp. The sensor outputs were recorded, analysed and displayed on a PC using a software algorithm to show signal energy (joules) and amplitude (decibels). The proximal femur was coated with specular marking paint to permit real-time 3-D digital image correlation (DIC) analysis. DIC is an established tool in engineering fracture analysis and utilises two spatially orientated video cameras to measure surface strain and fracture. The femur was observed by the human eye and loaded in cycles until a fracture was seen. The moment of fracture was marked in the recording timeline. DIC was used to confirm fracture.Introduction
Method
Interbody fusion is increasingly widely used as a treatment for intervertebral disc disorders, but the biomechanics of the procedure are not well understood. The compressive loads through the spine are largely carried by the implant or bone graft, which typically rests on a relatively small area of the vertebral body. As the compressive strength of the bone is very low, subsidence of the implants into the vertebral bodies is a common clinical complication. Previous biomechanical studies of spinal fusion have concentrated on the stiffness of the constructs, which is important in promoting fusion. Preliminary studies have shown that there are large differences in compressive strength between different implant systems, and gave an insight into the biomechanical factors that are important in determining the strength of spinal fusion constructs. This paper reports part of a larger on going study comparing anterior and posterior fusion systems, with various methods of fixation. A major problem in interpreting the results of these tests is to distinguish between initial settling of the implants and the onset of failure to construct. We have developed a novel technique using acoustic emission monitoring to detect microcracking in the bones, which allows the onset of failure to be distinguished from initial bedding in of the implants. Two implant systems were tested, the Syncage and the Contact fusion cage. The cages were implanted into porcine lumbar spines at L4-L5, and the implanted motion segment was then dissected out. Steel plates were mounted on each end using bone cement to ensure an even distribution of load through the vertebral body. The complete constructs were then loaded in compression, using acoustic emission sensors to detect microcracking in the bones. The load was cyclically increased in o.5kN steps until failure occurred. The acoustic emission technique gave a sensitive indication of the onset of damage in the bones and allowed the initial settling of the implant under load to be identified. Using cyclic unloading and reloading, it was possible to accurately identify whether this damage had weakened the construct or increased its strength by redistributing stress concentrations. Initial results indicate that the Contact fusion cage fails at a much lower load than the Syncage in this model; this is ascribed to the very small contact areas between the cage and the vertebral body, which results in high compressive stresses in the bone. Under large compressive loads it appears that the constructs become unstable, and fail by buckling and plastic collapse of the vertebral bodies. Various failure models are therefore possible depending on which part of the vertebral body starts to collapse first.
