Failure of acrylic external skeletal fixators (ESFs) has been shown to occur at the pin-acrylic interface. The mechanical properties of this interface are governed by the thickness of the acrylic surrounding the pin, thus the diameter of the column, and the centring of the pin in the column. This study aimed to evaluate the biomechanical effects of pin centring in acrylic columns of ESFs in a fracture gap model. Twenty-four type I ESFs were created with pins placed centric (n=12) or eccentric (n=12) in the acrylic columns. Both groups were subjected to axial compression and four-point bending tests. Failure point, yield point and stiffness were compared between groups. Significance was set at P < .05. Eccentric pin positioning decreased yield and failure points by 27% and 28% respectively during bending. Pin positioning had otherwise no influence on the other biomechanical properties tested. These results are in agreement with a recent study which found a decrease in column stiffness with eccentric pins, however only the acrylic columns themselves were subjected to axial compression. The in vitro nature of this present study cannot reproduce the complexity of a fracture but allows for standardization. Cyclic loading tests were not performed but would have provided additional relevant information. Versatility of acrylic ESFs is an advantage in challenging clinical situations but pin centring in the column can sometimes be difficult to achieve. This study showed the importance of properly centring the pins in the acrylic columns of ESF constructs.
