Contribution of loading conditions and material properties to stress shielding near the tibial component of total knee replacements.

This communication reports important preliminary results of a parametric analysis into the stress shielding effects of loading conditions and material properties of a total knee replacement (TKR) prosthesis. A previously developed finite element (FE) model of the proximal tibia that incorporated orthotropic and heterogeneous bone properties was used. Tibiofemoral joint compression and soft tissue (ligament and muscle) forces were also included to better represent the loading condition in the tibia. Stress shielding effects were studied for a prosthesis similar to a commercially available model. Results from the model show that the hypothesis of relatively higher Young's modulus of implant compared to bone as the primary cause of stress shielding is not sufficiently descriptive. Loading conditions as a result of altered bone or implant condylar surface geometry, load placement on the condylar surface, and load pattern created by the TKR are at least as important or, in some cases, more important factors in observed stress shielding immediately post-operation. This finding can be used to focus new implant design on altered loading conditions as well as material selection.

Geometrical properties of the ovine tibia: a suitable animal model to study the pin-bone interface in fracture fixation?

To determine whether dimensional scaling (relative to the human) is necessary for screwed pins used in externally applied fracture fixation studies on sheep, geometrical data were determined for six ovine tibiae. Each tibia was potted relative to a lengthwise reference axis and sectioned at 5 per cent length intervals over its central 80 per cent. Enlarged (280 per cent) images of each cross-section were digitized at 1 mm increments around the periphery of the periosteal and endosteal surfaces, the data were digitally filtered, and geometrical properties were computed to include cross-sectional area A, maximum and minimum second moments of area (Imax and Imin), polar second moment of area J, and effective polar second moment of area J(eff). Proportional scaling of geometrical properties with respect to bone length (L2 for A, and L4 for second moments of area) significantly (p < 0.000001) decreased the coefficient of variation in data by an average 36 per cent. From 30-90 per cent distal, J(eff) for the ovine tibia is smaller but within 7 per cent of J--in stark contrast with the human tibia, where J(eff) has been reported as 70-80 per cent of J over the same tibial length. While previous ovine studies involving external fixator pins have employed the same diameter of pin as has been used in humans (that is 5 or 6 mm), a 'first-order' approximation of the data for A, Imax, Imin and J(eff) suggests these pins should be scaled down to 4 mm and 4.75 mm respectively for use on the ovine tibia over the range 25-80 per cent distal along its length.

Sterilization of Fuji pressure-sensitive film.

Fuji Prescale film is a pressure-sensitive medium which produces a characteristic pink stain on the application of pressure. Up to a saturation level, increases in pressure will produce a denser stain, thereby providing a method of determining pressures within the interface between two articulating surfaces. The relationship between the magnitude of applied pressure and the optical density of the resulting stain is non-linear; this relationship also varies with ambient temperature and humidity, in addition to load rate, and therefore requires a calibration procedure prior to use. The use of Fuji prescale film for recording interface pressures within the joint space in vivo has been widely reported; however, the object of this study was to assess the effects of sterilizing this medium, with a view to future in vivo applications. Samples of Fuji film were sterilized using a standard ethylene oxide (ETO) gas process and their subsequent pressure-recording properties were compared to a control group of samples. The 'optical-density vs pressure' relationship for the sterilized group was significantly different from that of the control group (paired Student's t-test, P < = 0.001); however, both groups provided reliable data across the same pressure-range and both exhibited an excellent degree of repeatability (coefficient of variation < 2.5%). It was concluded that Fuji film will continue to produce pressure-stains following ETO sterilization; however, the calibration of this film will only be valid if it is conducted using film from the sterilized group.

A quantitative assessment of orthoses for stabilization of the anterior cruciate ligament deficient knee.

Knee braces are used to restore functional stability to joints which have become unstable as a result of the traumatic disruption of the anterior cruciate ligament. A number of quite different designs of knee brace have evolved and are used clinically in the management of these injuries although to the authors' knowledge little quantitative data are available on their relative mechanical characteristics. This paper reports the results of static testing carried out to determine the stiffness characteristics of 24 commonly used braces. The data obtained indicated clear differences between different brace designs with the stiffest having values closely approaching that achieved physiologically in uninjured joints, and the least effective having minimal stiffnesses. It was found that the mechanical performance of any individual brace was determined by three factors: the mechanical characteristics of the individual components from which it was constructed, the structural integrity of its design, and the interaction of the brace with the limb during loading.