This study aimed to investigate the effect the coracoid bone tunnel location had on the treatment of ACJ dislocation using a single-tunnel CC ligaments fixation with the Dog Bone™ button. Firstly, a cadaveric study was performed, followed by the implementation of FEA to further prove the validity of the cadaver study results. Overall, when the coracoid bone tunnel is located 5 mm anterior to the center of the coracoid base (along the axis of the coracoid), the clavicle gained greater rotational stability.
Previously, several postoperative complications have been reported following a single-tunnel CC ligament reconstruction of an ACJ dislocation using the Dog Bone™ technique. Shin et al.  reported a 33% rate of loss of reduction of more than 50% in 18 patients managed with a single-tunnel, adjustable-loop suspensory device. Cook et al.  reported that in 8 of 10 repairs (80%) intraoperative reduction was lost at an average of 7.0 weeks (range, 3–12 weeks), and four patients (40%) required revisions. Tunnel widening was universally noted, and the holding suture was prominently described as the failure mode in most patients. Dalos et al.  reported that tunnel widening was observed for Dog Bone™ technique and was located in the inferior parts of the clavicle and superior parts of the coracoid. However, the specific reason of postoperative tunnel widening remain unclear.
Based on the results of the cadaver study for G2, the angles of pronation and supination of the clavicle were 20.50 (19.50, 21.25) °, 20.00 (18.75, 21.25) °, respectively. There was no significant difference between the G2 and Gn groups; however, there was a significant difference between the G2 and G0 groups. For G1, there was no significant difference between the G1 and Gn groups, as well as the G1 and G0 groups. For G3, there was no significant difference between the G3 and G0 groups; however, there was a significant difference between the G3 and Gn groups. Therefore, the clavicle in the G2 group had better rotational stability in response to an external force.
Subsequently, FEA was implemented and the displacement nephogram of FE models shows that the maximum displacement of M1 was located in front of the distal end of the clavicle (1.5610 mm) and the clavicle tended to supinate. The location of M2 at the middle position of the distal end of the clavicle (0.4244 mm) and the displacement direction of the distal clavicle are almost vertical. Finally, M3 was located behind the distal end of the clavicle (2.4420 mm) and the clavicle tended to pronate. Since the coracoid bone tunnel is located in the center of the coracoid process base, it is relatively backward in M1. Thus, when the resultant force of the load was added to the distal clavicle it occurred in front of the distal clavicle, indicating that the displacement is greatest in front of the distal clavicle, which tends to supinate. For M2, the resultant force appears in the middle of the distal clavicle, so the displacement of the middle of the distal clavicle is the largest, making the distal clavicle move vertically. For M3, because the coracoid bone tunnel is located at the distal end of the coracoid process, its position is relatively forward. Therefore, the resultant force occurs behind the distal end of the clavicle, so the displacement behind the distal end of the clavicle is the largest and causes the tendency of the clavicle to pronate.
Generally, when the coracoid bone tunnel is located in the center of the base of the coracoid process or 10 mm anterior to the center of the base (along the axis of the coracoid), the action of external force promotes supination or pronation in the clavicle. This is consistent with the results of our cadaver study where the M2 clavicle has better rotational stability in response to an external force. Moreover, a supination or pronation movement in the clavicle causes the FiberWire to cut the clavicular and coracoid bone tunnel, which explains the postoperative tunnel widening. Hence, when the coracoid bone tunnel is located in the G1 site or 10 mm anterior to the G1 site (along the axis of the coracoid), the risk of a postoperative clavicular and coracoid fracture may be increased. Likewise, the erosion of clavicular and coracoid bone may be promoted by buttons and may also increase the risk of a clavicular or coracoid button failure. These complications may directly or indirectly lead to an ACJ dislocation reduction failure or re-dislocation.
In addition, the peak value of von Mises stress and the strain LE along the FiberWire were both smaller in M2 than in M1 and M3. This indicated that M2 has a stronger total construct.
Previous investigations have analyzed the location selection of the coracoid bone tunnel. Kummer et al.  used a combination of synthetic bone models and cadaveric scapulae to assess the effect of tunnels on the coracoid strength. Kummer and co-authors reported that the cadaveric specimens were more prone to fracture when tunnels were placed in the distal coracoid compared to the base. Campbell et al.  used six matched pairs of cadaveric scapulae to study the effect of the coracoid bone tunnel position on the treatment of an ACJ dislocation using Dog Bone™. They reported that the ultimate load for the centered tunnels in the distal coracoid absorbed a significantly higher ultimate load and energy compared to the eccentric tunnels. Although, they did not find a difference between the distal tunnels and the tunnels at the base. However, their study ignored the rotational stability of the clavicles. After research we found that: whereby during a single-tunnel reconstruction of the CC ligament, the different positions of the coracoid bone tunnel will lead to altering rotational stabilities of the clavicle. These will ultimately result in numerous cutting degrees of the FiberWire to the clavicle and coracoid bone tunnels. Indeed, these cuts may lead to either the clavicle and coracoid bone tunnels widening, which may potentially promote clavicular or coracoid fractures and other postoperative related complications. Therefore, to reduce the occurrence of these possible complications, our suggestion is that during a single-tunnel CC ligaments fixation of an ACJ dislocation using a Dog Bone™ button, the coracoid bone tunnel should be positioned 5 mm anterior to the center of the coracoid process base, along the axis of the coracoid. Although this would create a moment arm on the coracoid process, but a distance of 5 mm would not be too far from the center of the coracoid process base, which still provided considerable strength. Therefore, in general, we still recommended that the coracoid bone tunnel be located in 5 mm anterior to the center of the coracoid process base (along the axis of the coracoid).
As the research into ACJ dislocations increases, our current research aims are to explore a method that can both achieve ideal reduction and fixation alongside maximizing the recovery of ACJ mobility. Our results provide more information for orthopedic surgeons using a single-tunnel reconstruction of CC ligament with Dog Bone™.
Deficiencies of the study
Several limitations within this study should be considered. Firstly, the use of cadaveric specimens during biomechanical testing does not accurately mimic the real situation in vivo with the various forces involved. Therefore, some degree of error is likely to occur . Secondly, using cadaveric specimens for biomechanical testing does not provide information relating to biological healing. Consequently, result validation could not be performed on the long-term effects of the grafts on the clavicle and coracoid processes under different experimental conditions . Thirdly, differences in scapula position can also change the relation between the clavicle and the coracoid. Is it possible that the rotational stability of the clavicle may be secondary to the angle between the line that intersects the insertion point in the clavicle and coracoid and the horizontal axis, for example? And thus, the further research in this field is needed. Finally, the application of the FEA in orthopedics was limited by the simplification of the model, whereby some of the more complex anatomies in the bone modeling phase was omitted. Therefore, the results obtained are often questioned . The addition of the load to the models was only performed in a single direction, which was chosen because it accurately represented the clinical situation. However, in patients the loads applied to the construction would likely be multidirectional. It is possible that loads applied in other directions could lead to earlier or other causes of reduced ACJ dislocation failure and our conclusions cannot be directly applied to these situations .
Our further studies will focus on clinical application of ACJ treatment using Dog Bone™ buttons. Their clinical effects will be investigated.