Summary

A robotic cadaveric biomechanical comparison of three different acromioclavicular joint reconstruction implants

Abstract

Background

Persistent acromioclavicular joint (ACJ) disruption following high grade injuries can cause symptomatic multi-directional instability. The importance of horizontal plane stability is increasingly recognized, but most studies have only imaged the ACJ in the coronal plane in order to measure superior elevation of the clavicle, while imaging of the transverse plane is difficult. There is little evidence of the ability of current implant methods to restore native ACJ laxity in the vertical and horizontal planes.

Purpose

To measure the ability of three implant reconstructions to restore native ACJ stability.
Study design: Controlled laboratory study.

Methods

Three groups of nine fresh-frozen shoulders were mounted into a robotic testing system. The scapula was stationary and the robot displaced the clavicle to measure native anterior, posterior, superior and inferior (A,P,S,I) laxities at 50N force. The ACJ capsule, conoid and trapezoid ligaments were cut and the ACJ was reconstructed using one of three commercially available systems. Two systems (Lockdown and Infinity-Lock) wrapped a tape around the clavicle and coracoid, the third system (Dog Bone) passed directly through holes in the clavicle and acromion. Two modified reconstructions were also tested. The laxities were remeasured. The data for A-P and S-I laxities were analyzed by ANOVA and repeated-measures Student t-tests with Bonferroni correction to contrast each reconstruction laxity versus the matching native ACJ data for that set of 9 specimens, and to examine contrasts among the reconstructions.

Results

All three reconstructions restored the range of A-P laxity to that of the native ACJ. However, the coracoid loop devices shifted the clavicle anteriorly, so the anterior laxity was larger than the native laxity (p<0.0167) and the Lockdown overconstrained posterior laxity (p<0.001). For range of S-I laxity, only the Dog Bone reconstruction did not differ significantly from native ligament restraint. The mean S-I position was elevated 2mm for Infinity-Lock and Dog Bone procedures and 4mm for Lockdown, which had residual superior laxity (p<0.001). The addition of an Internal Brace to the Dog Bone reconstruction, across the anterior aspect of the ACJ, increased anterior laxity.

Conclusion

Only the reconstruction that passed directly though tunnels in the clavicle and coracoid restored all laxity measures (A,P,S.I) to the native values. The tape implants wrapped around the bones allowed relatively greater superior laxity to persist and led to the unloaded position of the clavicle being anteriorized, with resulting anterior laxity underconstrained and posterior overconstraint. The devices that were wrapped around the coracoid and clavicle were less effective at restoring superior stability than a reconstruction method that passed directly in a superior-inferior axis through both clavicle and coracoid. These differences may be due to the orientations of the devices between the bones and the difficulty of avoiding soft tissue interposition.

Clinical relevance: This study provides novel ‘time zero’ data that compared the ability of three commercially available ACJ implant systems to restore both anterior-posterior and superior-inferior stability of the ACJ. These data will assist surgeon choice of reconstruction method.