Effects of acromioclavicular and coracoclavicular ligament stability on acromioclavicular joint dislocation: insights from a cadaveric study

Abstract

Background

Acromioclavicular (AC) joint (ACJ) dislocation can lead to superior clavicular instability when the AC and coracoclavicular (CC) ligaments are torn. No previous study has assessed the effects of combined AC–CC ligament resections in fresh-frozen cadavers with preserved soft tissues around the thorax and shoulder girdle. This study aimed to develop such an ACJ dislocation model and evaluate stability following ligament resections.

Methods

Nine fresh-frozen cadaver shoulders (mean age, 86.6 years) without clavicular fractures or ACJ osteoarthritis were used. Each specimen included the thoracic spine, scapula, clavicle, and shoulder. Biomechanical testing was performed with a customized system to assess displacement and evaluate superior and posterior stability. Three conditions were compared: intact ligaments, AC ligament resection, and AC–CC ligament resection.

Results

Superior translations were 0.0 mm (intact), 1.1 mm (AC resection), and 9.6 mm (AC–CC resection). Posterior translations were 0.0 mm, 3.2 mm, and 9.0 mm, respectively. The AC–CC resection group showed significantly increased translations compared to the intact and AC resection groups. No significant difference was observed between the intact and AC resection groups in superior translation. Posterior translation increased progressively from intact to AC and then AC–CC resection.

Conclusions

This is the first study to assess both superior and posterior ACJ stability using cadavers with preserved soft tissues. Our findings demonstrate the importance of the AC and CC ligaments in maintaining ACJ stability. Notably, the AC ligament contributes to posterior stability, indicating the need for reconstruction to achieve overall joint stability.

Level of evidence

Controlled laboratory study.

INTRODUCTION

Acromioclavicular (AC) joint (ACJ) dislocation leads to superior clavicular instability when the AC and coracoclavicular (CC) ligaments are torn. Surgical treatment may be indicated for dislocations of Rockwood classification type 3 or higher [1]. Many CC ligament reconstruction methods have been reported; however, AC ligament reconstruction has received little attention [2,3]. Although previously performed, CC ligament reconstruction poses the risk of persistent posterior instability [3]. Conversely, the AC ligament, which attaches obliquely from the posterior clavicle to the acromion, prevents posterior clavicular dislocation [4].

AC ligament reconstruction is essential for overall stability and has been shown to increase posterior stability [5]. Previous biomechanical studies on ACJ stability have been conducted on dissected bones, muscles, and ligaments using AC or CC ligament resection [2,5,6]. We wanted to examine the stability of the ACJ in combined AC–CC resection to better reflect the pathology of ACJ dislocation. As a preliminary test, we performed traction on the clavicle with dissected bones, muscles, and ligaments using AC or CC ligament resection, but the ACJ immediately ruptured, and we were unable to examine its stability. The presence of soft tissue was essential to provide greater stability. To the best of our knowledge, no studies have yet evaluated AC–CC ligament resection in freshly frozen cadavers with soft-tissue preservation, including the entire thoracic region, ligaments, and joint capsule around the shoulder joint, along with the AC and CC ligaments. Consequently, the stability of the ACJ in freshly frozen cadavers with intact soft tissue remains unclear.

In this study, we aimed to create an ACJ dislocation model using fresh frozen cadavers with soft-tissue preservation around the shoulder girdle to evaluate stability after AC and CC ligament resections. We hypothesized that AC ligament resection would lead to reduced posterior stability, whereas AC–CC ligament resection would result in reduced superior and posterior stabilities. We aimed to provide a comprehensive understanding of ACJ stability by examining the effects of ligament resection in a cadaveric model that closely mimics clinical conditions.

METHODS

The study was conducted in accordance with the principles of the Declaration of Helsinki. It was approved by the Institutional Review Board of the Graduate School of Medicine, Chiba University, Japan (No. M10156). The requirement for informed consent was waived.

Specimen Preparation

Nine fresh-frozen shoulders (six right and three left) from six deceased donors (three matched pairs) with preserved thoraces were included, excluding those with clavicular fractures or ACJ osteoarthritis. The mean age of the donors was 86.6±7.0 years (range, 71–95 years). Three and six shoulders were obtained from male and female donors, respectively. A power analysis was performed based on the primary outcome variable of displacement (mm) when the ACJ was forced with superior and posterior torque of 0.6 N·m to determine the number of specimens required. The effect size for our power analysis was based on the difference in displacement reported in other studies (β=0.2, α=0.05) [2,5,7-9]. The power analysis indicated that seven specimens per group were needed; therefore, we evaluated nine shoulders.

Each specimen comprised the entire thoracic region along with the scapula, clavicle, and shoulder components. The origins and insertions of the muscles, including the deltoid and upper trapezius fibers, were preserved. The integrity of the ligaments and joint capsule around the shoulder joint, including the AC and CC ligaments, was retained. The specimens were stored at −20 °C and thawed at 24 °C–26 °C for 24 hours before the experiment and then dissected to expose the clavicular and acromial surfaces and the fibers of the deltoid and upper trapezius muscles (Fig. 1). The anatomical origins and terminations of the deltoid and upper trapezius muscles were preserved, and no external tension was applied. When removing the ACJ joint capsule, disc, and AC ligament, parts of the attached trapezius and deltoid muscles were also removed.

Fig. 1.

Cadaveric specimens were dissected to expose the clavicle (C), acromion (A), deltoid (D), and upper trapezius fibers (T). The anatomical origins and terminations of these muscles also were preserved.

Preparation for the Experiment

Biomechanical testing was performed using a customized shoulder-testing system. Both the inferior angles of the scapula were securely clamped in a vise and affixed to the pedestal (Fig. 2). The distal end of the clavicle was aligned by visually defining the area horizontal to the floor. A pulley mechanism was employed to establish a connection between a universal testing machine (Autograph, Shimadzu Corporation) and the shoulder to create a traction system (Fig. 3) that exerted superior and posterior forces on the distal end of the clavicle. A superior traction force was applied perpendicular to the distal end of the clavicle, while a posterior traction force was applied parallel to the ACJ (Fig. 4). The traction system applied force to a TWINFIX metal anchor (Smith & Nephew) positioned 2 cm from the distal end of the clavicle using a FiberWire high-strength thread (size 2; Arthrex). During the biomechanical testing, the specimens were moistened with 0.9% NaCl to prevent drying.

Fig. 2.

Both inferior angles of the scapula were securely clamped in a vise and affixed to the pedestal.

Fig. 3.

A pulley mechanism was employed to establish a connection between the specimen and the traction system, which exerted a superior and posterior force on the distal end of the clavicle.

Fig. 4.

(A) A superior traction force was applied perpendicularly to the distal clavicle. (B) A posterior traction force was applied parallel to the acromioclavicular joint. Rt: right, ACJ: acromioclavicular joint.

Experimental Protocol

The test began with intact specimens. Loading was conducted three times at magnitudes of 2–5 N, and the displacement was subsequently measured during towing at a force of 30 N (with a torque of 0.6 N·m) and a speed of 1 m/sec to evaluate superior and posterior stability. There were no specimens in which the clavicle anchors loosened or pulled out during testing. The comparative analysis focused on the effects of resection of the intact ligament, AC, and AC–CC on superior and posterior translations. The extent of translation was assessed using video footage, with markers placed on the upper surface of the anterior edges of the clavicle and acromion. After testing, the intact specimen was subjected to AC ligament resection, followed by CC ligament resection. Total superior and posterior translations were used to evaluate ACJ stability.

Statistical Analysis

All statistical analyses were performed using EZR (Saitama Medical Center, Jichi Medical University), a modified version of R Commander designed to add statistical functions frequently used in biostatistics [10]. One-factor analysis of variance and Bonferroni post hoc analysis were used to compare the three groups. Statistical significance was set at P<0.05. Additionally, intraclass correlation coefficients were used to assess the reliability of the measurements, and intra- and inter-examiner reliability were examined. One evaluator measured all cases twice by reviewing the video, 6 months apart, to assess intra-examiner reliability, and then two evaluators independently measured all cases by video to assess inter-examiner reliability.

RESULTS

The extent of superior translation was 0.0 mm (standard deviation [SD], ±0.1), 1.1 mm (SD, ±1.5), and 9.6 mm (SD, ±3.5) in the intact ligament, AC resection, and AC–CC resection groups, respectively (Fig. 5). The intact and AC resection groups exhibited significantly lower extents of translation compared to the AC–CC resection group (P<0.001 and P=0.04, respectively). However, no significant difference was observed between the intact and AC resection groups (P=0.46).

Fig. 5.

Superior translation. The intact and acromioclavicular (AC) resection groups both exhibited lower translation than the AC–coracoclavicular (CC) resection group. However, no significant differences were observed between the intact and resected AC groups. *P<0.01, **P<0.001.

The extent of posterior translation was 0.0 mm (SD, ±0.1), 3.2 mm (SD, ±0.6), and 9.0 mm (SD, ±2.5) in the intact, AC resection, and AC–CC resection groups, respectively (Fig. 6), significantly increasing in the following order: intact, AC resection, and AC–CC resection (P<0.001). The reliability of the measurements was acceptable, with intra-examiner reliabilities of 0.94 and 0.96 for the upper and posterior positions, respectively, and corresponding inter-examiner reliabilities of 0.88 and 0.86. 

Fig. 6.

Posterior translation. The extent of posterior translation increased in the following order: intact, acromioclavicular (AC) resection, and AC–coracoclavicular (CC) resection. **P<0.001.

DISCUSSION

Previous biomechanical studies on ACJ stability were conducted on dissected bones, muscles, and ligaments and hindered evaluation of stability using actual AC or CC ligament resections [2,5,6]. To our knowledge, this is the first study in which AC and CC ligament resections were performed while preserving the entire thoracic region and scapular, clavicular, and shoulder components. In this study, muscle origins and insertions, including the deltoid and upper trapezius fibers, were preserved. Furthermore, ACJ stability was ensured by retaining the integrity of the ligaments and joint capsule around the shoulder joint, including the AC and CC ligaments. This approach established an ACJ dislocation model that closely resembles biological conditions, enabling the evaluation of both posterior and superior instabilities after AC and CC ligament resection. The findings clearly demonstrated a significantly reduced extent of superior stability in the AC–CC resection group relative to that in the other groups. Moreover, the intact group displayed the greatest posterior stability, followed by the AC and AC–CC resection groups. These results emphasize the importance of the AC and CC ligaments in maintaining ACJ stability and the potential risk of instability after AC–CC resection. Furthermore, our findings demonstrate reduced posterior stability following AC ligament resection, highlighting the importance of the AC ligament in posterior stability. Finally, the decreased posterior stability after AC–CC ligament resection underscores the importance of the CC ligament in posterior stability.

In a cadaveric study conducted by Dawson et al. [2], the superior ACJ translation was quantified using AC joint compression. Their study involved resection of the AC or CC ligaments and repeated measurements, and their findings indicate a reduction in superior stability in the CC resection group. In contrast, the present study demonstrated greater stability in the intact and AC resection groups than the AC–CC resection group, suggesting that the CC ligament was highly significant owing to its superior stability. The present study did not find any significant differences between the intact and AC resection groups. The ability to maintain stability even after AC ligament resection can be attributed to preservation of the anatomical origins and termination of the deltoid and upper trapezius muscle fibers. In previous studies, muscle fibers were often removed. A clinical study demonstrated that repairing these muscles and their associated fasciae during ACJ dislocation surgery enhances ACJ stability [1]. In the present study, the actions taken to preserve these muscles may have contributed to the maintenance of superior stability.

Dawson et al. [2] and Dyrna et al. [11] each provided valuable insights into ACJ posterior stability. Their findings indicated a consistent pattern: AC ligament resection led to a notable reduction in ACJ stability. Dawson et al. [2] reported that posterior stability decreases in the following descending order: intact, CC resection, and AC resection. Dyrna et al. [11] evaluated the AC joint stability in relation to rotation and posterior translation. Their experiments involved immediate testing in several phases, including the intact state, AC resection, and subsequent AC ligament reconstruction, in various configurations. Further tests were conducted after CC ligament resection and subsequent reconstruction using the suture button system. In terms of posterior translational testing, AC ligament resection led to a notable reduction in resistance. However, when subjected to posterior translation, all configurations of AC ligament reconstruction demonstrated a significant increase in the mean resistance force compared to that of the intact group. Interestingly, resection of the CC ligament did not result in significant changes. In rotational testing, AC ligament resection resulted in a significant reduction in resistance torque. All configurations of AC ligament reconstruction notably increased the resistance torque compared to that of the AC resection group. Collectively, these findings emphasize the significance of AC ligament reconstruction in enhancing posterior translational and rotational stability of the ACJ.

Saier et al. [6] assessed the horizontal stabilities of various ACJ configurations. They evaluated translation using bidirectional anteroposterior dynamic loading within the horizontal plane of the ACJ. This assessment involved the use of a servo-hydraulic testing machine that tracked the horizontal ACJ translation over 5,000 cycles of dynamic anterior-directed loading. Their study demonstrated significantly lower horizontal stability in the AC resection group than in the intact group. However, the AC–CC reconstruction group exhibited a level of stability equivalent to that of the intact group. This substantiates the notion that addressing the AC joint alone does not adequately re-establish physiological horizontal ACJ stability [6,8]. Our study demonstrated a decrease in posterior stability after AC ligament resection, highlighting the importance of ligaments in posterior stability. Further reductions in posterior stability with AC–CC ligament resection underscore the importance of the CC ligament in posterior stability.

Previously, CC ligament reconstruction was commonly performed as a stand-alone procedure and was deemed more important than AC ligament reconstruction [12]. However, Scheibel et al. [3] have reported the possibility of residual posterior instability of the distal clavicle after CC ligament reconstruction alone. Radiographic signs of posterior instability were observed in 42.9% of patients, and those with posterior instability exhibited significantly inferior clinical outcomes than those exhibited by patients without posterior instability. Furthermore, Nakazawa et al. [4] conducted an anatomical investigation revealing that the AC ligament does not run straight across the joint surface but obliquely from the anterior part of the acromion toward the posterior part of the distal clavicle at an average angle of 30° to the joint surface. This orientation plays an important role in preventing the distal clavicle from moving posteriorly relative to the acromion.

Our findings underscore the importance of the AC and CC ligaments in maintaining ACJ stability and instability associated with AC–CC ligament resection. This indicates a risk of posterior instability in CC ligament reconstruction. Also, AC ligament reconstruction is necessary to ensure overall ACJ and posterior stability.

Jordan et al. [9] conducted a comprehensive systematic review of biomechanical and clinical research studies. Their analysis demonstrated notably enhanced posterior stability in cases where CC ligament reconstruction was combined with AC ligament reconstruction compared to that in cases with only CC ligament reconstruction. However, comparative clinical studies found no significant differences in functional outcomes, complications, or revision rates between CC ligament reconstructions with and without AC ligament augmentation. They concluded that AC–CC ligament reconstruction and CC ligament reconstruction alone could be considered equivalent in clinical practice. However, they did not find substantial evidence to support routine AC ligament reconstruction in clinical settings.

Conversely, Freedman et al. [5] reported that AC ligament reconstruction positively affected both superior and posterior stabilities. The specimens were subjected to anteroposterior and superoinferior testing with the AC ligament intact and compression loads applied to the ACJ. One specimen from each pair was subjected to AC ligament reconstruction using a free intramedullary semitendinosus graft followed by translational testing. After completion of these tests, both intact and reconstructed specimens were subjected to load-to-failure testing via superior clavicle distraction. Remarkably, the reconstructed specimens succeeded in replicating the stability observed in the intact specimens throughout all the translational and joint compression load trials. However, during load-to-failure testing, the reconstructed specimens exhibited significantly reduced linear stiffness, yield load, ultimate load, and absorbed energy ranging from 40% to 48% of those observed for intact specimens.

Walz et al. [8] demonstrated that CC ligament reconstruction enhanced both superior and anterior stability. Their study involved cyclic loading and a load-to-failure protocol conducted in the superior and anterior directions on ACJ specimens tested in the intact state and after CC ligament reconstruction using two TightRope devices (Arthrex). The reconstruction yielded favorable in vitro results, with forces equal to or greater than those observed in intact ligaments.

Collectively, these reports suggest that both AC and CC ligament reconstructions play vital roles in overall ACJ stability, including posterior stability. The results of the present study suggest that the AC is important for posterior stability, and that it is necessary to reconstruct the AC when posterior instability is present in ACJ dislocation. Currently, no universally accepted gold-standard technique is available for both together AC-CC ligament reconstruction. Furthermore, the experiments described in previously cited reports were conducted using specimens from which the soft tissue was excised. Future studies should investigate both together AC-CC ligament reconstruction using cadaveric specimens to preserve soft tissues, as performed in the present study.

A key limitation of this study was the age difference between the specimens used and the typical age range reported in systematic reviews and previous cadaveric studies [7,9,11]. In previous systematic reviews, the mean age of patients who underwent surgery for ACJ dislocation was 25–46.9 years, whereas that of donors in previous cadaveric studies was 49–69.5 years [2,5,6,8,11]. However, in the present study, the mean age of the donors was significantly higher (86.6 years; range, 71–95 years). This age discrepancy could have introduced variations in the conditions and factors related to ACJ stability compared with those in younger patients. Moreover, soft tissues are often damaged in joint or bone injuries, and we are unaware of which soft tissue structures are naturally repaired or the strength and stability of such repairs. In the present study, we used cadavers with soft tissue preservation around the shoulder girdle but without the AC and CC ligaments, along with parts of the trapezius and deltoid muscles attached to the ACJ. In addition, no muscle tension was applied, which may mean that the pathology of dislocation was not accurately reflected. Thus, these findings may differ from those observed under actual clinical conditions. Moreover, only individual loads were evaluated, which may have resulted in biased outcomes. Traction is an important consideration as it increases the soft tissue damage in each application. In the present study, the same specimen was tested in the intact state, after AC ligament resection, and then again after CC ligament resection. Although actively loaded muscles may have influenced our findings, almost all shoulder girdle muscles were preserved, providing closer resemblance to the in vivo situation.

CONCLUSIONS

Using an experimental model of ACJ dislocation established in fresh-frozen cadavers, the present study highlights the importance of the AC and CC ligaments in ACJ stability and the risks associated with AC–CC ligament resection. Notably, these findings indicate the risk of posterior instability with reconstruction of the CC ligament alone. Consequently, AC ligament reconstruction is deemed essential for overall ACJ and posterior stability.