Title:
Biomechanical evaluation of effect of coracoid tunnel placement on load to failure of fixation during repair of acromioclavicular joint dislocations.

Abstract:
To evaluate the effect of entry and exit points of the coracoid tunnel on load to failure and mode of failure, to reduce the incidence of coracoid fractures and acromioclavicular joint repair failures.
This study investigates 5 tunnel placements based on different entry and exit points in the coracoid process: center-center orientation represents perfect placement of the bone tunnel and served as perfect tunnel placement in our study. Four common errors in drilling were then tested and acted as the experimental groups in our study (medial-center, center-medial, lateral-center, and center-lateral). Using 35 cadaveric shoulders (mean age, 68.0 ± 13.0 years), we tested these 5 tunnel orientations using a single repair technique (cortical button) loaded to failure on an MTS 858 Servohydraulic test system (MTS Systems, Eden Prairie, MN). A control group of 7 cadaveric shoulders without the presence of a coracoid tunnel was also tested to determine the type of fracture pattern that occurred.
The coracoids without tunnel drilling fractured in patterns similar to traumatic coracoid injuries. With regard to the 5 tunnel groups, it was found that the loads to failure with center-center and medial-center tunnel placement were significantly higher than those with center-medial, center-lateral, and lateral-center tunnel placement. The failure modes of the former were primarily within the repair constructs, whereas those of the latter were primarily due to bony failure.
Our biomechanical results showed a higher peak load to failure with a center-center or medial-center tunnel orientation, which may lessen the risk of coracoid fracture during drilling with a 6-mm cannulated drill bit.
Proper trajectory of the drill during formation of a coracoid bone tunnel can help reduce the risk of coracoid process fracture and repair failure.