Introduction: Fragility of the bone is widely regarded as a cause of Colles’ Fracture particularly in middle aged or elderly women[1]. However not every fall results in fracture of the wrist. The normal volar angle of the distal radius is said to be about 10 degrees although in one study the mean volar angulation was found to be 12 degrees with a range from 4 to 23 degrees[2]. We hypothesised that the volar angle of the distal radius or the position of the wrist at impact could affect where the peak stresses occurred during a fall onto the outstretched arm. We investigated the effect of these two variables on the location and magnitude of the peak stresses using finite element analysis.

Materials and Method: A finite element model of the distal radius was constructed in MARC (MSC software, USA). The model was developed from CT data of the right wrist of a 46 year old male. The data was examined by edge detection software (Materialise, Belgium). The inner and outer boundaries of the cortex were imported as curves into MARC. A surface mesh of the distal radius was constructed, from which a 3D solid mesh of the distal radius was generated automatically. The volar angle was modified to represent between 5 to 25 degrees in 5 degree increments. The wrist position was also changed for each volar angle. This varied in 5 degree increments from 0 to 35 degrees, and then at 45, 75 and 90 degrees. Material properties assigned to cortical and cancellous bone were 20GPa and 6GPa respectively with a Poisson’s Ratio of 0.3. The model consisted of 17660 8 noded hexahedral elements and was fully fixed at the cut end of the proximal radius. For each volar angle a load of 500N and 400N was applied perpendicularly to the articular surface across the scaphoid and lunate fossa respectively. The magnitude and location of peak stresses in the proximal and distal radius were recorded.

Results: Results show that the location and magnitude of peak stresses vary as a result of wrist position. Distally the stress rises with increasing dorsiflexion and at 35 degrees exceeds the load to failure. The volar angle does not influence the stresses unless it is 20 degrees or more. Proximally the volar angle had no effect, but if the wrist is in more than 75 degrees of dorsiflexion then the peak stresses exceeded the load to failure.

Conclusion: Results show that a fall onto the outstretched arm will produce differential stresses in the radius depending on the position of the wrist at impact. The volar angle affected the stresses in the distal radius at greater than 20 degrees but proximally it did not. Proximally stresses above 130MPa (when the wrist is in more than 75 degrees of dorsiflexion) will subject the wrist to fracture[3]. Distally (when the wrist is in more than 35 degrees of dorsiflexion) with high volar angles (greater than 20 degrees) is likely to produce the conditions for a fracture (cancellous bone has been reported to fail as a result of fracture at 50 MPa [4] and for osteoporotic bone at 0.44MPa [4].