Drilling trials were performed using drilling boards incorporating pairs of 22 mm polyethylene tubes mounted horizontally. The tubes were premarked with 20, 0.5 mm deep notches along the centre of their upper surface representing the starting point for each drilling trial. Volunteers were instructed to drill 20 straight holes across the tube until they penetrated both walls. Kirschner wires were inserted through each of the drill holes until they made indentations into the base board. Deviation of each mark from mid-line was measured using digital callipers. The measured values were used to calculate deviation angles to the left (negative values) or right (positive values). Trials were performed with the drill and guide held in the surgeon's line-of-sight (LOS) and with 300 mm offset (OFF). A systematic error (aiming bias) was identified in all individuals. Overall, left-hander's drilling action was skewed to the left and right-hander's drilling action was skewed to the right. Using LOS technique, mean overall bias was 2.3° (range 0°–7.7°) to the left for left-handers and 1.5o (range 0.3°–3.1°) to the right for right-handers. Surprisingly, aiming bias was greater for LOS than for OFF, although the difference for this comparison was not statistically significant.
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