Two stainless steel ‘TriMed’ distal radial fracture reduction techniques were tested to compare the relative stability of the two in vitro for a pre-determined fracture pattern. The movement of the bony segments were plotted over time using an ARAMIS 3 dimensional non-contacting displacement mapping system (GOM mbH, Braunschweig, Germany) to give quantitative data. The data was used to calculate the relative motion of the bony segments with the aim of investigating regions of compression across the fracture line, which is thought to accelerate fracture healing, and shear between bony segments, which is detrimental to fracture healing.

Ten third generation adult radius biomechanical model Sawbones (Sawbones, Malmö, Sweden) were cut to simulate AO type C2 fractures with dorsal comminution. Five bones were plated using the TriMed fixed angle volar bearing plate and five were plated using the TriMed radius and ulnar plating technique. Samples were potted and loaded cyclically at 1 Hz via a floating scaphoid-lunate bearing onto the end of the radius at incrementally increasing loads of 100 N – 500 N with 1000 load cycles applied for each load level.

The results showed the radius and ulnar pin-plate configuration allowed greater movement of the articular surface, with relative shear motion and separation between the two segments, although the relative shear movement between the two distal segments was below 2mm, which is considered the definition of failed fixa-tion. With the volar bearing plate the two distal segments moved as single unit and compression with minimal shear was applied across the fracture line to the proximal radius. Thus the radius and ulnar plates allowed shear across all three fracture lines, while the volar plate held the two distal segments fixed relative to each other and allowed compression across the interface with the proximal radius. The ARAMIS system allowed the three dimensional motion of the bony segments to be followed, in particular the relative motion between the segments, indicating the type of healing to be expected clinically. The study demonstrated the value of ARAMIS in investigating the stability of wrist fractures fixations and can easily be adapted to investigate other orthopaedic fixation systems.

Torsional instability of femoral components has not received much attention, and is difficult to detect in conventional radiographs. To test this we designed a system to apply a load in an anteroposterior direction to the head of a femoral component, implanted into a cadaveric femur. Rotation within the bone was measured, using a purpose built transducer, with and without preservation of the neck, with and without cement, and with longitudinal ridges but no cement. The results show that torsional instability may be a problem in uncemented replacement. Preservation of the femoral neck and the use of a ridged prosthesis increases resistance to rotation. Rotational movements occurring in vivo during such activities as climbing stairs and rising from the seated position may contribute to mechanical loosening.