Acromioclavicular (AC) joint dislocations are commonly caused by high-energy trauma in young male adults with a direct impact on the shoulder [1]. While Rockwood type I and II instabilities are usually treated non-operatively, high-grade instabilities require operative stabilization [1]. In cases of failed operative or non-operative treatment, chronic instability might develop. Chronic AC joint instability can be present as follows: periscapular muscle fatigue, symptomatic dyskinesis, or “SICK” scapula syndrome with scapular malposition, inferior medial border prominence, pain and malposition, functional instability, and dyskinesis of scapular motion [2, 3]. The indication for operative reconstruction in chronic AC joint instability is an individual decision, depending on the activity-related impairments and the extent of patients’ complaints [4], In the case of acute AC joint instability, surgical treatment allows for an intrinsic healing response of the ruptured ligaments, which may result in satisfactory outcomes [5,6,7,8,9,10,11]. In chronic cases, the torn ligaments lose their healing potential over time, due to degenerative changes. Therefore, the ligaments must be biologically augmented by a tendon graft. An interval of more than 21 days after trauma is commonly defined as the border between acute and chronic injuries [12]. Still, among shoulder surgeons, this time interval remains controversial.

Surgical treatment of chronic AC joint instability can be performed using either open techniques, such as with a hook plate, or with arthroscopically assisted techniques [10, 12,13,14]. Due to the potential detection of concomitant intraarticular injuries and the advantages of a minimally invasive approach, the number of arthroscopically assisted techniques is increasing [15].

Nonetheless, data on clinical results and recommended techniques for treating chronic AC joint instability are limited. The aim of this study was to evaluate clinical and radiological results after treatment for chronic AC joint instability. We hypothesized that (1) operative treatment would significantly improve patients’ outcome in short-term follow-up (2) without a significant difference between the two groups.

Materials and methods

Patient selection

The Trauma Committee of the Society for Arthroscopy and Joint Surgery (Gesellschaft für Arthroskopie und Gelenkchirurgie, AGA) initiated this prospective multicenter study. Five level I trauma centers participated and included patients. All patients with chronic high-grade AC joint instability after failed operative or non-operative treatment were included in the study and prospectively monitored. High-grade AC joint instability was defined as Rockwood type III–V. A minimum interval of 3 months between trauma and index surgery was required to define a chronic situation. All patients had to be between 18 and 65 years of age. Patients with pre-existing pathologies beside AC joint injury and related surgery which limited shoulder function, strength, or movement were excluded.

Surgical technique

To restore stability, patients were treated either openly, using hook plate fixation (Fig. 1), or by an arthroscopically assisted technique using a flip-button device (Fig. 2). An additional autologous gracilis tendon graft for coracoclavicular (CC) and AC ligament reconstruction was included in each technique. The arthroscopically assisted technique was performed according to the technique published by Scheibel et al. [16]. The CC flip-button device (TightRope, Arthrex, Naples, FL, USA) was inserted through a 4-mm drill hole. Two clavicular 4‑mm drill holes positioned at the former CC ligaments and one through the coracoid were drilled for augmentation of the CC ligaments. The graft was pulled through and fixed with two tenodesis screws (each 4 × 10 mm) at the clavicle. The end of the graft was used for AC augmentation. An acromial 4‑mm drill hole was created, ascending from lateral and pointing to the proximal acromial edge of the AC joint. The graft was pulled through the acromion, shuttled back subcutaneously, and sutured to the remaining end of the graft. In the hook plate group, the graft was also pulled through two 4‑mm drill holes in the clavicle, but just wrapped around the coracoid without an additional drill hole. The ends of the graft were sutured above the clavicle. The remaining ends of the graft were then sutured to the acromion in terms of an AC augmentation.

Fig. 1
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a Rockwood type V lesion in the anteroposterior view with b horizontal instability in the Alexander view. c Hook plate stabilization with additional tendon graft augmentation. d Result at final follow-up

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Fig. 2
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a Rockwood type III lesion in the anteroposterior view with b horizontal instability in the Alexander view. c Arthroscopically assisted stabilization using a flip-button device with additional tendon graft augmentation. d Follow-up at 9 months

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Rehabilitation protocol

During the first 6 weeks after hook plate implantation, motion was limited to 90° flexion and abduction. From the seventh week on, a free range of motion was allowed. Implant removal was performed 3 months after surgery. In the arthroscopic group, shoulders were protected in a sling for 6 weeks. During the first 3 weeks, patients were allowed only passive motion up to a flexion and abduction of 45°. From the fourth week, patients performed active/assistive motions up to a flexion and abduction of 45°, and up to 60° in weeks 5 and 6. From the seventh week on, free motion was allowed. After 12 weeks, muscle strengthening exercises were initiated in both groups.

Radiographic outcome measures

Preoperatively, all patients were graduated according to the Rockwood classification using bilateral anteroposterior stress views of the AC joint. According to Rockwood, an increase in CC distance of 25 to 100% is defined as a Rockwood type III injury, an increase of more than 100% as type V [17, 18]. Bilateral Alexander views and an axillary view of the affected shoulder were used for detecting static or dynamic horizontal instability [19]. A difference of more than one clavicle shaft was defined as horizontally instable. Further radiographs were performed postoperatively and at a follow-up. The changes in CC distance were compared preoperatively, postoperatively, and at a follow-up. Furthermore, a comparison was made with the contralateral shoulder. Horizontal stability was assessed by a further Alexander view at follow-up.

Furthermore, preoperatively performed surgeries or conservative treatments were recorded. Intraoperatively detected concomitant injuries were documented.

Clinical outcome measures

Functional outcome measures (Subjective Shoulder Value [SSV], Constant Score [CS], Taft Score [TFS], and Acromioclavicular Joint Instability Score [ACJI]) were recorded preoperatively and at follow-up. The TFS and ACJI are AC joint-specific scores evaluating subjective, objective, and radiological parameters, with 0 points being the worst and 12 (TFS) or 100 (ACJI) being the best result.

Statistics

Results were compared between the different timepoints and the two treatment groups (group 1: arthroscopically assisted; group 2: hook plate). Descriptive statistics for quantitative variables were presented as means, standard deviations, and ranges. Results were tested for normal distribution using the Shapiro–Wilk normality test. A t-test for independent means was used for normally distributed results. If not distributed normally, the Mann–Whitney U test was used as a two-way analysis of variance for independent factors. Categorical variables were compared using the Fisher exact test. A p-value <0.05 was considered statistically significant. Statistical analyses were performed using SPSS for Windows (IBM SPSS Statistics 25, Armonk, NY, USA).

The local ethics committee of each clinic approved the study protocol. All work complied with the principles laid down in the Declaration of Helsinki. All included patients provided written informed consent.

Results

A total of 43 patients were included into this study between February 2011 and November 2013. Mean age was 44 ± 12 (range 18–64) years and four patients (9%) were female. In 21 cases the dominant shoulder was affected. The injury was classified as Rockwood type III in 17 patients (40%) and Rockwood type V in 26 patients (60%). Of the Rockwood type III patients, 16 had horizontal instability (Rockwood type IIIB). The indication for surgery was due to failed non-operative treatment in 30 patients (70%), whereas 13 patients (30%) had had previous surgeries, in some cases multiple. Six patients had previously been stabilized with a hook plate, four with a polydiaxanon suture (PDS) augmentation, two with a double tightrope, and one with a single tightrope. Further revision surgeries were necessary because of an infection. Index surgeries were an arthroscopically assisted technique (group 1) in 32 patients (74%) and hook plate stabilization (group 2) in 11 patients (26%). In group 1, four patients (13%) had a concomitant intraarticular lesion (1 × SLAP-II lesion, 1 × partial SSC rupture type Fox & Romeo I, 2 × PASTA lesions). No patient was noted to have an intraarticular lesion in group 2.

Subsequently, 35 patients (81%) were available for a follow-up (Table 1). Nine patients refused further participation or were not available for follow-up examinations. Mean follow-up was 12 ± 3 (range 6–20) months. All hook plates were removed before the final follow-up examination. All functional outcome measures significantly improved through operative treatment (Table 2). No difference in the functional result was seen between the two treatment groups (Table 3). Comparing the clinical results of patients after initially failed operative and non-operative treatment, there was no statistical significance for SSV (p = 0.294), CS (p = 0.682), TFS (p = 0.487), and ACJI (p = 0.645).

Table 1 Comparison of demographic data between the groups at follow-up
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Table 2 Comparison of functional outcome measures before surgery and at follow-up
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Table 3 Comparison of functional outcome measures between arthroscopically assisted treatment and hook plate stabilization at follow-up
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Vertical stability was significantly improved by the treatment, reducing the CC distance from 19 ± 5 mm (range 10–28) to 8 ± 2 mm (range 4–12; p < 0.001). There was no difference in CC distance compared to the contralateral shoulder (10 ± 2 mm, range 6–13; p = 0.11). The CC distance significantly increased from postoperative to the latest follow-up (13 ± 4 mm, range 6–22; p = 0.001), which was also a significant difference to the contralateral shoulder (p = 0.005). Comparing the groups (Table 4), both improved significantly by treatment (group 1: p < 0.001; group 2: p = 0.005). However, both groups lost reduction in the follow-up period. For group I the CC distance increased from 8 mm ± 2 (range 4–12) to 13 mm ± 4 (range 6–21; p < 0.001), which also differed to the contralateral shoulder (p < 0.001). Group II showed an increase in the CC distance from 10 mm ± 4 (range 5–13) to 12 mm ± 6 (range 6–22; p = 0.65), with a contralateral CC distance of 10 mm ± 2 (range 6–13; p = 0.97). While all patients were classified as horizontally stable after hook plate treatment with regard to the Alexander view at follow-up examination, there were two patients (8%) with horizontal instability after the arthroscopically assisted treatment, one of whom was symptomatic and underwent revision in the course of time. Overall, two patients from group 1 required revision surgery (one arthroscopic AC joint resection due to persistent pain in a stable situation and one re-stabilization with a modified Weaver–Dunn procedure with hook plate retention). The overall revision rate was 6%. No infections or fractures, neither of the clavicle nor the coracoid, were observed. Heterotopic ossifications were observed in two patients with arthroscopically assisted stabilization (8%) and two after a hook plate procedure (22%).

Table 4 Comparison of the coracoclavicular distance for both treatment groups at different timepoints
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Discussion

The aim of this prospective multicenter study was to evaluate clinical and radiological results after treatment of chronic AC joint instability with a hamstring autograft for AC and CC ligament reconstruction. Since data on treatment for stabilization are limited, arthroscopically assisted and hook plate stabilization were compared. Both treatment groups were expected to show comparable clinical and radiological results. Our results showed that operative treatment significantly improved clinical results at short-term follow-up. Comparing the two treatment groups, there was no difference for any of the functional outcome measures. Radiological analysis showed a significantly improved reduction of the AC joint separation after treatment. Nonetheless, reduction was lost over time in both groups, which did not influence the clinical outcome.

The indication for operative intervention in the case of symptomatic chronic AC joint instability is an individual decision based on symptoms, impairments, and the patient’s expectations [4]. To date, there is no consensus regarding operative or non-operative treatment for chronic AC joint instability [20, 21]. Regarding operative treatment, multiple techniques have been described for stabilization, all with the idea of AC joint reduction and additional substitution of the ruptured, degenerative CC ligaments. Over the past couple of years, anatomic reconstructions using free tendon grafts have become more and more common for stabilization (about 60% in the past 3 years) [22]. The torn and degenerative ligaments are biologically augmented by a free tendon graft from the hamstring or tibialis anterior tendon. Free tendon grafts have the advantage of being longer and therefore allow anatomic positioning of the graft. Biomechanically, reconstruction of the CC ligament complex using a semitendinosus tendon showed a slight decrease in structural properties and stiffness, but was almost comparable to the native situation [23]. Mazzocca et al. also biomechanically tested the highest stability for an anatomic reconstruction using a free tendon graft for the trapezoid and conoid ligaments, which almost fully restored the intact state [24].

Barth et al. evaluated prognostic factors for the treatment of chronic AC joint dislocations. Beside a time to surgery of more than 3 months, persisting horizontal instability due to missing AC stabilization was evaluated as a negative prognostic factor [25]. Scheibel et al. previously analyzed persisting horizontal instability as a negative predictor after AC joint dislocation [5]. Biomechanically, cerclage of the AC joint augmenting the AC capsule significantly improved stability [26, 27]. Therefore, additional AC cerclage should be recommended for treatment of chronic high-grade AC joint instability, as performed in all of our cases. Due to the length of free tendon grafts, additional AC cerclage after CC stabilization is possible using the same graft.

Besides augmenting the CC and AC ligaments, vertical stability has to be restored. For this, either open or arthroscopically assisted techniques are available. Hook plate stabilization is a widely established treatment with rigid and highly stable fixation. Up until now, there have been no clinical results for chronic AC joint instability treated with hook plates. Only McKee has presented his technique for hook plate stabilization in chronic AC joint instability [13]. Regarding treatment of acute AC joint dislocation, Kienast et al. presented the data of 313 patients treated with hook plates. After a mean follow-up of 36 months, TFS averaged at 10.6 points and CS at 92 points [28]. Metzlaff et al. presented the results of 44 patients with a CS of 93 points, TFS of 10.5 points, ACJI of 81 points, and a high return-to-sport rate after 6 months [11]. A slight loss of reduction (less than 50% of clavicle width) was observed in 2 of 20 (10%) patients. Further studies confirm a partial loss of reduction in 3–67% of patients [29,30,31]. Disadvantages of this treatment are the necessity of implant removal 3–12 months after implantation and the subacromial positioning of the hook, which can result in subacromial injuries and erosions or fractures of the acromion [10, 11, 28].

Tauber et al. published their technique and clinical results of 12 patients with a mean follow-up of 49.5 months [2]. The AC joint was stabilized using a semitendinosus tendon. The graft was pulled through holes in the clavicle and around the coracoid process in a figure-of-eight fashion. The ends were wrapped again around the clavicle and the coracoid process and sutured together in the subclavicular space. The graft was protected by a CC cerclage wire or Bosworth screw. The CS improved from 61 to 76 points and pain decreased from 6.2 to 2.3 points. However, implant removal was also necessary with this technique.

Alternatively, the AC joint can be stabilized in arthroscopically assisted techniques using suture-button devices [14, 32]. Implant removal is not required and arthroscopy enables detection and treatment of concomitant intraarticular lesions. Therefore, comparable results are achieved after failed non-operative and operative treatment [21]. However, depending on the technique, a higher risk for horizontal instability can persist.

In 2016, Tauber et al. compared an arthroscopic single- vs. triple-bundle reconstruction [4]. The CS improved significantly for both groups. However, ACJI (85 vs. 58 points) and TFS (10.9 vs. 9.0) were significantly better for the triple-bundle reconstruction. Since clinical outcome is dependent on horizontal stability, this difference regarding AC joint-specific scores resulted from a higher horizontal stability in the triple-bundle reconstruction group.

Jensen et al. used a GraftRopeTM system (Arthrex, Naples, FL, USA) with additional horizontal augmentation of the AC joint [12]. The CS averaged at 84 points and the TFS at 9 points. At follow-up, a significantly increased CC distance of plus 3 mm compared to the contralateral shoulder was observed.

At short-term follow-up, our data confirm the results in the literature, with a significant improvement through operative treatment and good clinical outcomes. Nonetheless, a loss of reduction was observed for both treatment groups, which is comparable to data in the literature [5, 11, 12, 29,30,31]. Although this result was only significant for the arthroscopically assisted technique, significant differences are also to be expected with an increased number of patients treated with a hook plate. Conversely, the clinical outcome was not influenced. A considerable limitation of hook plate stabilization is the requirement of revision surgery for implant removal after 3 to 12 months. Moreover, an arthroscopically assisted technique enables detection of intraarticular lesions, which can easily go undetected in an open procedure [15]. Independent of the stabilization technique, to improve the clinical outcome, horizontal instability has to be reduced by adding an additional horizontal AC joint augmentation.

Milewski et al. describe high complication rates for CC ligament reconstruction techniques with free tendon grafts for coracoid (in up to 20%) and clavicular fractures (11%) [33]. In a current meta-analysis Gowd et al. report infection rates in free tendon transfers for AC joint reconstruction of 7.6%. Neither of these complications were observed in our cohort [8].

Although this is a prospective study comparing two treatment groups, there are some limitations. Follow-up times are quite inhomogeneous, without a minimum follow-up of 2 years. Therefore, the data can only be assessed as short-term results. Furthermore, the comparison between the two treatment groups in terms of functional and radiological outcomes is limited due to the small number of patients treated with hook plates (n = 8). Regarding radiological evaluation, measurement accuracy is limited in plain radiographs. Furthermore, cut-off values to define loss of reduction are missing. Therefore, the presented data are just absolute values of the differences in the CC distance. It remains unclear to which extent an increase in CC distance will have a negative effect on clinical outcome.

Conclusion

Surgical treatment in symptomatic chronic AC joint instability significantly improves clinical outcome in short-term follow-up. Functional results are comparable between an arthroscopically assisted technique and hook plate stabilization. Nonetheless, some loss of reduction is observed.