A finite-element based parametric study of an orthopaedic bone-plate in situ

This study is an examination of some of the factors which affect the load carrying capacity of a plated bone and indicates how the results generated might lead to improved implant design and utilization. The study is confined to a consideration of gross mechanical behaviour of the metal and the cortical bone in the mid-shaft region of a human femur.

The primary analytical tool used was the finite element method, implemented through use of the SApv computer code. . The three-dimensional finite element model developed includes fully anisotropic elastic properties for bone and also includes the presence of a bone fracture realistically simulated by no-tension "gap" elements.

The mechanical response of the bone-plate-screw structure is influenced by many variables. The factors considered in this study include patient weight, bone inside and outside diameters, plate length, distances between screws and angular orientation of the fracture plane. A series of computer "experiments" was designed and implemented to permit assessment of the importance of the parameters studied.

The finite element model was verified experimentally using a strain gauge instrumented bone plate attached to a segment of embalmed human femur. The model gave stress results of the same order of magnitude as the experiment.

The parameter sensitivity study done using the model, indicated that stresses in the structure are most sensitive to changes in plate length, bone diameters and angulation of the fracture. Closure of the gap between fracture surfaces is also strongly influenced by plate length and is also affected by compression plating. Overall, one can conclude from the observed levels of stress, that static loading alone will probably not cause fracture failures of metal or bone, but under the influence of cyclic loading and the corrosive action of body fluids, such failures are highly probable.