Introduction
Acromioclavicular (AC) dislocation involves complete loss of articular contact; a dislocation that is not untreated, is treated conservatively or is treated unsuccessfully by surgery is defined as chronic or inveterate [
1,
2]. The AC and coracoclavicular ligaments contribute to anterior–posterior and superior–inferior joint stability, respectively [
3]. Complete instability requires rigid fixation of the coracoclavicular ligaments to counteract the AC joint laxity that induces posterior translation of the clavicle. The classification of AC dislocation into 6 degrees of severity, as devised by Rockwood et al. [
4], is still the one most commonly used. While there is consensus on the conservative treatment of types I and II, there is still debate over whether types III to V should be managed surgically [
5,
6]. Among the surgical approaches developed to treat acute and chronic AC dislocation, some authors [
5,
16,
17,
31] have recommended procedures that restore the original joint anatomy and congruity [
7]; a number of these techniques use biological or synthetic means [
8,
9].
This study compares the clinical and radiographic outcomes of surgical AC joint stabilization performed in 40 patients with chronic dislocation using a biological graft or a synthetic ligament.
Discussion
From 1861 [
14] to the present, 60 different surgical procedures have been devised to treat acute and chronic AC joint dislocation, but finding the gold standard has proved an elusive task. In 1972, Weaver–Dunn [
15] proposed the transposition of the coracoacromial ligament to the lateral portion of the clavicle. This approach involves sacrificing the coracoacromial ligament (a humeral stabilizer). The interest in this type of technique, which is based on the assumption that AC joint reduction and anatomical restoration provide more satisfactory outcomes [
16], has recently been revived by the introduction of synthetic ligaments [
17,
18] and biological grafts [
16,
19]. Techniques based on the transposition of the patient’s tendons that show resistance to cyclic loading, similiar to rigid osteosynthesis (screws, plates, pins, metal or synthetic cerclage) [
20,
21] but with lower rates of intra- and postoperative complications, were developed to address these problems [
8,
19,
22‐
24]. Bailey [
25] was the first to report the results of tendon transposition; Dewar and Barrington [
26] used only coracoid transposition and obtained better mid-term outcomes compared with the Weaver–Dunn procedure in young patients [
27]. Although transposition of the coracoid with the conjoint tendon reinforces the reconstructed coracoacromial ligament, it involves a greater risk of coracoid fracture and musculocutaneous nerve injury; furthermore, bone cerclage may result in coracoid or clavicle osteolysis.
Materials that are used for artificial ligaments include polyester, Dacron
®, Dupont
®, Wilmington
®, Notthingam
® [
8,
23], carbon fiber [
28], polytetrafluoroethylene (Gore-Tex
®) [
29], and PET (LARS LAC
®) [
24]. The characteristic interwoven fibers and the porosity of the synthetic ligament promote fibroblast colonization and make the ligament biocompatible and resistant to traction and torsion; nonetheless, intolerance, inflammation, and rejection have been described [
30]. Tendon autografts or allografts were initially used in salvage procedures after failed coracoacromial ligament reconstruction [
19]. The most widely used allografts are semitendinosus [
19], gracilis, hallux extensor [
31], and peroneus brevis tendons [
18]. Biocompatibility, resistance, and rigidity of the system used for joint reduction are crucial for postoperative stability in chronic AC joint dislocation.
Although good outcomes of synthetic [
21] and biological grafts [
16] have been (separately) described in several reports, no single study has, to our knowledge, used both materials and compared them. Although anatomical AC joint reconstruction cannot restore original stability to the joint, tendon grafts provide greater resistance and rigidity than the Weaver–Dunn procedure [
29]. Analysis of the results of our study disclosed significantly greater clinical scores in the “biological” compared with the “synthetic” group at both follow-up time points, with mean intergroup differences in Constant–Murley score of >29 points at 1 year and >8.9 points at 4 years, and mean differences in modified UCLA score of 6 points at 1 year and 2.8 points at 4 years.
Eleven out of 40 patients were previously surgically treated using different surgical techniques, which affected the articular biomechanics of the AC joint in different ways, and consequently influenced the homogeneity of the study population. In these patients, we found a higher incidence of periarticular ossifications, clavicular osteolysis, and fibrous adhesions intraoperatively, which made it more difficult to expose the clavicle and acromion. Furthermore, the passage of the graft under the coracoid required a longer surgical step due to the thickening of the surrounding soft tissues. Despite these difficulties, we did not find any significant effects on the clinical scores and AC joint stability based on the X-rays for this subgroup of patients.
Our clinical findings are consistent with the aforementioned case-series studies describing the use of synthetic or biologic grafts. Specifically, Carofino et al. [
16] reported a significant difference between preoperative and postoperative clinical scores when using a semitendinosus allograft. On the other hand, Morrison et al. [
21] reported satisfactory early and midterm outcomes using a synthetic graft. Postoperative radiographic assessment showed three complete AC joint dislocations that negatively affected the clinical scores, while subluxations were only associated with poor subjective satisfaction in 20 % of group B patients. None of the remaining radiographic measures investigated correlated with clinical outcomes. Coracoclavicular ossification is usually related to surgical exposure of the coracoclavicular space [
32], but it is unclear how its onset, site, and extension affects clinical outcomes. Although the incidence of clavicular osteolysis is greater in patients managed surgically than in those managed conservatively [
33], this has been related to the biomechanical effects of AC injury rather than to the surgical procedure per se [
32]. In this study, patients with clavicular osteolysis were more numerous in the “synthetic” group; in this group, the sizes of the osteolytic areas increased in 90 % of the patients, and the only patient with dislocation complicated by clavicular fracture was treated with the LARS LAC
® ligament. The number of osteolytic areas and their sizes in our 40 patients were not related to loss of postoperative AC alignment, consistent with other reports [
34,
35]. AC joint stability is not related to poor clinical outcome [
36], whereas clavicle malrotation or anteposition may contribute to arthritic changes [
21,
36]. Comparison of our patient groups showed a greater rate of osteoarthritis in the “synthetic” than in the “biological” group (80 vs 40 %), with no significant correlations with clinical scores or X-ray evidence of instability found for either group. A number of considerations can be drawn from these findings:
(i)
Postoperative AC joint stability is the main factor affecting final outcome; the best results were recorded in patients with completely stable joints.
(ii)
Although the synthetic graft is effective from a biomechanical standpoint, graft shredding and wear and bone remodeling around the screws can compromise mechanical strength over time, particularly in elderly patients and in those with poor clavicle bone thickness or osteoporosis.
(iii)
Biological grafts provide joint stability in the axial and the coronal planes through suture of the lateral stump of the graft to the acromion, a finding confirmed by recent [
16] and earlier [
3] studies; axial stability appears more difficult to restore using a synthetic graft.
(iv)
Biological grafts are fixed to the clavicle with resorbable screws and are a valuable option when treating patients with postoperative recurrence of dislocation due to synthetic graft failure.
The major limitations of this study are the small sample size, the lack of inter- and intraobserver data, and the absence of patients treated with tendon autografts.
In conclusion, our findings show that biological grafts provide biocompatible, durable, and effective reduction, as well as better clinical outcomes and radiographic findings than synthetic ligaments, and thus represent the most reasonable alternative to the Weaver–Dunn [
15] procedure in shoulders with chronic AC joint instability. Graft fixation to the clavicle is the major weakness of both procedures and should be improved.