Critical role of the acromioclavicular ligament in posterior stability: a paradigm shift in acromioclavicular joint reconstruction

For decades, surgical treatment of acromioclavicular (AC) joint dislocations has predominantly focused on coracoclavicular (CC) ligament reconstruction, following principles established by Weaver and Dunn in 1972 [1]. However, mounting clinical evidence of persistent instability following isolated CC reconstruction has challenged this paradigm. Hattori et al. [2] featured in this issue, provide compelling biomechanical evidence that fundamentally changes the perspective on AC joint stability. This noteworthy cadaveric investigation is the first to evaluate both superior and posterior stability using fresh-frozen specimens with preserved soft tissue architecture around the entire shoulder girdle.

Previous biomechanical studies by Dawson et al. [3] and Dyrna et al. [4] utilized dissected specimens with removed muscle and soft tissue components, potentially altering the natural biomechanical environment. The importance of the study of Hattori et al. [2] is in preserving the entire thoracic region, scapula, clavicle, and shoulder components, including deltoid and upper trapezius muscle origins and insertions. This methodological rigor, particularly the preservation of surrounding soft tissue architecture, directly addresses limitations and inconsistencies highlighted in previous systematic reviews of AC joint biomechanics research [5]. The largest contribution of Hattori et al. [2] is demonstrating the essential function of AC ligaments in posterior stability through quantitative measurement, demonstrating progressive increases in translation of 0.0 mm in intact specimens, 3.2 mm after isolated AC ligament resection, and 9.0 mm following combined AC+CC ligament resection. This is evidence that AC ligaments are essential stability components.

For superior stability, the minimal translation with isolated AC ligament resection (1.1 mm) versus dramatic increase following combined resection (9.6 mm) confirms the primary role of CC ligaments while highlighting the synergistic effect of the two ligament systems. These findings provide a scientific foundation for concerning clinical observations. Scheibel et al. [6] reported radiographic posterior instability in 42.9% of patients following CC ligament reconstruction alone, with significantly inferior cl inical outcomes. Hattori et al. [2] explains why CC ligament reconstruction alone may be insufficient.

Further supporting these findings, Nakazawa et al. [7] demonstrate that AC ligaments run obliquely at 30° to the joint surface, optimally positioning them to resist posterior translation, as revealed by Hattori et al. [2]. This anatomical insight aligns with recent biomechanical investigations. Morikawa et al. [8] demonstrated that AC ligament complex transection reduced resistance to posterior translation to less than 25% compared to native ligaments. Saier et al. [9] showed that isolated CC reconstruction did not provide adequate horizontal stability compared to combined AC and CC reconstruction. Beitzel et al. [10] found that AC ligament reconstruction by direct wrapping around the AC joint was most stable, emphasizing the importance of AC ligaments in anatomic reconstruction, as validated by Hattori et al. [2].

Despite compelling biomechanical evidence, clinical translation remains challenging. The systematic review by Jordan et al. [11] revealed that, while biomechanical studies consistently show enhanced posterior stability with combined AC-CC reconstruction, comparative clinical studies found no significant differences in functional outcomes. This paradox highlights the complexity of translating laboratory findings to patient outcomes and may be attributed to the multifactorial nature of shoulder function, limitations of current outcome measures, or methodological variations in study designs. Recent systematic reviews found clinical evidence of improved outcomes with AC joint repair and reconstruction, although comparison between studies was difficult due to methodological variations [12].

Recognition of horizontal instability has led to development of more than 150 AC joint repair or reconstruction techniques [13]. Current concepts emphasize that 80% of horizontal stability is provided by an intact superior-posterior capsuloligamentous complex [14]. Recent finite element modeling shows that different ligament bundles play substantial roles during various shoulder motions [15]. These advances support the comprehensive reconstruction approach validated by Hattori et al. [2]. Emerging techniques such as combined AC-CC reconstruction methods using dermal allografts and modern suture anchor constructs represent the type of comprehensive approaches that are expected to benefit most from this new understanding of AC ligament biomechanics [8,16]. These methods specifically address both vertical and horizontal stability components identified by Hattori et al. [2].

The study of Hattori et al. [2] represents a watershed moment in the understanding of AC joint biomechanics. By demonstrating AC ligaments' essential function in posterior stability through methodologically rigorous testing with preserved soft tissue architecture, the study provides scientific justification for comprehensive reconstruction approaches. While clinical studies have not yet demonstrated clear superiority of combined reconstruction approaches, the biomechanical advantages clearly demonstrated by Hattori et al. [2] are robustly supported.

This paradigm shift toward comprehensive AC joint reconstruction acknowledges that successful treatment must address the multidirectional nature of AC joint instability. The findings suggest that future surgical approaches should systematically evaluate and address both CC ligament-mediated vertical stability and AC ligament-mediated horizontal stability to optimize clinical outcomes. The era of isolated CC ligament reconstruction as standard care may be replaced by comprehensive strategies acknowledging the importance of AC ligaments to joint stability. This study provides crucial biomechanical foundation for that evolution.