A biomechanical study of the function of the femoral fracture brace
The cast-brace treatment of femoral shaft fractures, and functional fracture bracing in general, allows controlled motion of the fractured bone ends and loading of the fracture. Traditionally this treatment would not have been considered as the concept of fracture treatment was to eliminate movement at the site of injury. However, early in the 1950's functional fracture bracing for tibial fractures became an additional acceptable treatment and this was shortly followed by an extension to include femoral fractures. Despite the fact that this form of treatment produces good results there has been little scientific measurements to establish the basic criteria on which this treatment is founded and which may allow improvements to be suggested. Fifty-seven patients were fitted with modified cast-braces. Transducers, attached at the fracture level and the knee, enabled estimates of fracture and knee loads to be made during static weight-bearing as a function of period of healing. It was found that the fracture load appeared to be controlled by a form of feedback mechanism such that the fracture load was optimised throughout treatment. The feedback operated two regulatory mechanisms; limb load moderation as a coarse control and changes in the amount of load transferred from limb to cast above the fracture (off-loaded) as a fine control. The position of the fracture had an effect on the load to which it was subjected, upper third fractures, with less cast into which to off-load, having the highest loads and "being most difficult to treat. This study was extended to measure fracture loads during the stance phase of gait. For this a two dimensional gait analysis system was developed comprising of an Apple II micro-processor, disc drive, 'home-built' force plate and TV movement analysis system. The eleven patients in this part of the study had multicomponent transducers mounted at the fracture level in the brace and reflective markers at the hip, fracture level, knee and ankle. Muscle lines of action were obtained by grouping together muscles having the same primary action and ignoring antagonistic activity and the centroid of each group was obtained for eleven levels in the thigh. Force plate and spatial data were collected and used to obtain the fracture load by substitution into the equilibrium equations obtained from free body analysis of the limb distal to the fracture. This study showed that the fracture load was between 2.5 and five times body weight at the time of brace removal. The difficulty in controlling fractures of the upper third, in patients with fat thighs, and in bilaterally fractured patients was well illustrated. It was also found that the fixed ankle in the brace caused a large skeletal transient to be transmitted to the fracture just after heel strike. This was found to be greatly reduced by having a free ankle with hinges and a heel cup. Intra-cast pressure measurements during static weight bearing showed that the top of the brace was an important area for load transfer and that the brace had an upper limit to the degree of off-loading that it could provide. These results also suggested that the limb in the brace was of more or less constant volume during treatment and this was confirmed by the results of the Nuclear Magnetic Resonance (NMR) studies. These latter results showed that whilst the fractured limb volume remained constant the volume of muscle in that limb rose by about and the amount of fat fell by about 3%. The difference in the two values is accounted for by an increase in callus formation and changes in an unidentifiable fluid constituent of the limb. The brace is thus seen to provide sound fracture union and rehabilitation of the patient providing the thigh portion of the brace is a snug fit and well moulded. There are no risks associated with this form of treatment, and the clinical results are as good as or better than those obtained by other techniques, provided that the brace is fitted properly, many of which carry the normal risks of surgery.