This is the first study looking at the reconstruction of the acromioclavicular capsuloligamentous complex using the palmaris longus tendon graft. Biomechanical testing showed that in superior loading, it is as strong in tensile strength and as stiff as the clavicle hook plate in providing superior stability. In posterior-anterior loading, it is as strong and stiff as the ACJ capsuloligamentous repair.
Our study looks at the combined effect of native acromioclavicular and coracoclavicular ligaments, in contrast to other studies [9, 17, 24, 26], which more closely resemble clinical situations where impact forces do not selectively damage either of these ligaments. A combined injury of both these ligaments is required to give a Rockwood type III or more severe ACJ dislocation. Double-bundle reconstitution of the conoid and trapezoid ligaments in Mazzocca's study is innovative, however, AC capsuloligametous repair was not mentioned and testing in the posterior-anterior direction was not performed. In this study, we have shown the pivotal role of the AC capsuloligamentous complex in providing posterior-anterior stability; however, superior stability is provided by either plate or screw augmentation or tendon graft reconstruction. Debski et al also showed that the ACJ capsule confers posterior-anterior stability and the intact coracoclavicular ligament cannot compensate for loss of capsular function during posterior-anterior loading. Failure to augment a coracoclavicular reconstruction will subject the latter to higher risk of failure. Any residual posterior-anterior instability can cause postoperative pain .
Weaver-Dunn reconstruction alone with coracoacromial ligament is insufficient. Incomplete reduction or recurrence of dislocation was reported to be as high as 24% . We found its strength to be one-eighth that of the native combined AC+CC ligaments (801 +/- 75) N. Harris et al reported its strength to be one-quarter that of CC ligaments (500+/-134) N alone. Various augmentation methods have been described.  Although none of the augmentative methods tested restored acromioclavicular stability to normal, all proved superior to the Weaver-Dunn reconstruction alone . In addition, Deshmukh et al showed that, the contribution of Weaver-Dunn transfer to the stability, when combined with augmentative fixation, is negligible at time zero. This further justifies for the need for augmentation.
We found that both BS and clavicle hook plate devices provide adequate augmentation. BS provided 70% and 170% of the tensile strength and stiffness of the native ligaments, respectively. On the other hand, clavicle hook plate provided 50% and 30% of the tensile strength and stiffness of the native ligaments, respectively. The results of BS augmentation are consistent with that reported by Motamedi et al. . Urist found that failure strength, however, was reduced by half if only unicortical purchase was obtained, indicating the importance of accurate screw placement . Disadvantages of this screw fixation technique include complications during screw insertion, screw irritation, infection, pullout and breakage . Early deformity recurrence may occur with early implant removal.
An ideal augmentation device should, biomechanically, have a similar compliance as that of native ligaments. Too stiff a device can predispose to bone breakage and cause joint stiffness ex vivo, whilst too compliant a device can cause premature failure of the Weaver-Dunn construct during rehabilitation. Distal clavicle resection as part of Weaver-Dunn reconstruction, described by Mumford, was thought to prevent postoperative pain and osteolysis . This was shown not to be the case by Browne JE . We found that distal clavisectomy precludes ACJ repair and prevents proper seating of the clavicle hook plate.
Several considerations need to be made when using the clavicle hook plate. Further prebending of the plate may be required to allow optimal sitting on the clavicle. The narrow rectangular-shaped clavicle hook under the acromion surface causes tremendous contact stress and predisposes to acromial fracture during loading. A more rounded disk-shaped anchorage will be ideal. Extreme care must be exercised during the drilling of the anterolateral distal holes since stress fractures have occurred at these sites. Insertion of medial screws may be sufficient. Distal resection of the distal clavicle or the use of autogenous grafts such as semitendinosus or palmaris longus graft for the reconstruction of the acromioclavicular capsuloligamentous complex will preclude the use of clavicle hook plates because of inadequate sitting of the implant on the clavicle. A further consideration ex vivo is that of subacromial impingement which will need to be explored in post-operative patients. The need for implant removal following graft incorporation, as with the coracoclavicular screw fixation, is a disadvantage compared to autologous grafts or biodegradable substances such as Mersilene tapes. The current clavicle hook plate does not address posterior-anterior instability and translation of the acromoclavicular joint. A routine repair or reconstruction of the acromioclavicular joint capsuloligamentous complex can address this problem.
Coracoclavicular ligament reconstruction using tendon grafts have been widely described. There are the advantages of biological integration, no fracture or loosening, no need for implant removal and low morbidity with graft harvesting.
In this study, we used the palmaris longus tendon graft, in addition to the Weaver-Dunn procedure, to reconstruct the acromioclavicular capsuloligamentous complex and augmented it with a 5 mm Mersilene tape which looped around the coracoid process and clavicle. The tendon graft may benefit from augmentation with the tape to protect the repair, limit the amount of possible stretching and counteract the weakening effects of revascularization. This reconstruction had tensile strength not significantly different from the clavicle hook plate with superior loading and similar to ACJ capsuloligamentous repair with posterior-anterior loading. We also noticed that its flat cross-sectional area, superior-inferiorly, also helped in its sitting across the acromioclavicular joint. It therefore served a dual function of stabilizing the acromioclavicular joint in both the posterior-anterior and superior directions while protecting the concurrent WD reconstruction. The palmaris longus tendon grafts used here for ACJ ligament reconstruction were about 10 cm long, as opposed to the 16 cm palmaris longus graft used by Lee et al for coracoclavicular ligament reconstruction .
Grutter and Petersen showed that, when tested in coronal plane only, the Weaver-Dunn reconstruction, palmaris-longus tendon graft and flexor carpi radialis graft achieve tensile strength 59%, 40% and 95% that of the native ACJ capsule . These were in contrast to our findings, with the WD and palmaris longus reconstruction achieving, in the coronal plane, 14.7% and 34% that of the combined native ligaments and in the sagittal plane, 8% and 19.6% that of the combined native ligaments. The discrepancy in results arose because Grutter et al compared the tensile strength of the reconstruction with that of the native ACJ capsule only (simulating grade II and below injury) whereas we compared the tensile strength of our reconstruction with that of the combined native AC + CC ligaments (simulating grade III and above injury).
Suture failures were noted in WD reconstruction, ACJ repair and PL-mt reconstruction. This was not surprising given the fact that the sutures were weaker in tensile strength compared to that of the CAL, ACL or palmaris longus ligament, as shown by Harris et al.  and Lee et al. . Native ligaments failed at mid-substance at the low strain rate used in our study. However, most specimens will have bony avulsion if high strain rates are used. Therefore, the crosshead speed or strain rate must be specified to suit the purpose of one's study. For clavicle hook plates testing, the probability of a clavicle fracture is dependent on the bone size and amount of bone bridge in between drill holes. On the other hand, a strong fixation on the clavicle will result in plate failure by acromial deformation or fracture.
The strengths of the current study were observed. Firstly, baseline tensile strength results of the native ligaments were made available for comparison with other reconstructive and augmentative groups, in both coronal and sagittal planes. Secondly, the testing setup has 3 degrees of freedom and allows firm hold on the scapular blade and proximal clavicle. It allows plastic bending of the acromion, coracoid process and distal clavicle, which further simulates a real-event injury. In-situ precise testing of native ligaments and sequential repair or augmentation were performed without removing the specimens from the testing apparatus [18, 19]. Thirdly, the methods used to measure failure load and failure displacement for each group were precise, objective and reproducible. Loads to failure were consistently applied in both coronal and sagittal directions for various conditions tested in each specimen because the specimens were not removed from the test rig when the reconstruction or augmentation procedures were performed. Fourthly, reconstructive and augmentative procedures were performed in conjunction with the modified Weaver-Dunn procedure and finally, the failure loads of the native ligaments were measured in comparison with the capsuloligamentous reconstruction and the various augmentative repairs. The palmaris-longus tendon graft-mersilene tape graft was tested uniquely in reconstructing the ACJ, in contrast to other studies where it was used to reconstruct the CC ligament.
Concurrently, a few limitations of this study were also seen. Firstly, tensile loading was performed only in the superior axis at a much lower strain rate than that which would have occurred during injury. Secondly, repetitive testing on a bony specimen may cause plastic deformation of the clavicle and acromion and predispose to bony failure in some specimens; conversely, the ligament repair and reconstruction were performed on uninjured joints and did not account for any damage to the coracoid or clavicle which may accompany the injury. Thirdly, the cyclic and static viscoelastic properties of the native ligaments and fatigue properties of the clavicle hook plate and coracoclavicular screw, have not been determined. Finally, it has been shown that all of the soft tissues at the acromioclavicular joint function synergistically, in a complex manner, to provide joint stability. Thus, traumatic disruption of the acromioclavicular joint capsule is thought to result in abnormal joint kinematics and load transmission, factors that increase the possibility of postinjury pain, instability, and degenerative joint disease. These are factors which could not be tested in this study [8, 27].