Analysis of the bony geometry of the acromio-clavicular joint

The acromioclavicular (AC) joint is very commonly affected by injuries in terms of direct or indirect trauma as well as by degenerative changes. To be able to treat AC joint pathologies of any kind, physicians need a good understanding of the anatomy and mechanics of the AC joint. In the recent past, significant effort has been made analyzing the ligamentous stabilizers of the AC joint [1‐3]. Especially the coraco-clavicular (CC) ligament, composed of the conoid and trapezoid ligament, presents a highly valuable passive stabilizer notably for vertical translation [4].

Regarding the bony anatomy, the clavicle is known for its double curved configuration. Its lateral end is flattened and presents with prominent landmarks on its inferior surface. From the anatomical point of view the acromion articulates with the lateral clavicle via the medial facet with its orientation posteriorly and laterally whereas the acromial articular surface is oriented medially and anteriorly [5]. The average size of the AC joint surface was described as 9 mm in vertical by 19 mm in antero-posterior orientation [6, 7]. The current literature provides sufficient information regarding the rather complex anatomy of the double curved clavicle as it is described that on average of 40 mm medial of the AC joint the clavicle presents an anterior sweep with an increased thickness of the superior cortical bone ranging between 1 and 4.5 mm [2, 8, 9].

Knowing about the numerous studies of the anatomy of the AC region [1, 2, 5, 8‐10], the aim of our study was to evaluate the configuration of the AC joint on the basis of CT Data sets of a young population without pre-existing joint pathologies.

CT-scans of the upper thoracic aperture of 80 patients (40 male, 40 female) with a mean age of 45 ± 8 years were enrolled. 60 CT-scans were multiple trauma CT scans and 20 were post mortem CT-scans.

Both AC joints of each CT scan of the upper thoracic aperture were evaluated, so that in total 160 AC joints were analysed for the anatomic configuration and possible differences (Fig. 1).

Three significantly (p < 0.001) different groups could be identified (see Table 1). With 46.2% (n = 74) of all examinations the AC joint configuration with an overhanging acromion was determined to be the most common (see Fig. 2a). Besides the neutral shaped type (see Fig. 2b) (n = 61) with 38.4%, the overhanging clavicle type (n = 45) (see Fig. 2c) with 15.4% of all examinations was determined to be the rarest.

The summarized angle acromion + right clavicle was determined with 182.8 ± 10.6°, the summarized angle acromion + left clavicle was determined with 183.2 ± 8.8°.

Injuries to the AC joint most commonly result from a direct impact to the shoulder. Although, other possible underlying trauma mechanisms are frequently discussed in current literature [2, 14, 15], only few studies exist, dealing with the influence of the bony joint configuration on the injury pattern [10, 16, 17].

The anatomical and biomechanical analysis of the AC joint resulted in a description of dynamic as well as static stabilizers, with the deltoid muscle and trapezius muscle as primary dynamic and the acromioclavicular (AC) and coracoclavicular (CC) ligaments as primary static stabilizers [6, 18]. Relating to the double curved clavicle, the trapezoid ridge extends anteriorly and laterally across the inferior surface of the lateral third of the clavicle, representing the landmarks of the insertions of the corresponding ligaments [6, 18]. The CC ligament, consisting of the conoid and trapezoid ligament, is known to provide vertical stability [6], whereas horizontal stability is mainly provided by the AC ligament, respectively [14, 19].

On the basis of 41 cases, Urist et al. is known to be one of the first to describe the anatomical variants of the joint structure [10]. He supposed that the different joint configurations might have an influence on the success or failure of AC joint injuries treated in a conservative manner. Although his assumptions might be true, there is still no general concern of the right treatment of AC joint injuries nowadays [20‐22]. Consistent with Urist et al. [10], we were able to identify three major types of AC joint configurations on the basis of CT-data of a healthy population. Our findings support the anatomical variants described by Colegate-Stone et al. [17], although they were based on a much older patient group (mean age 81.2 years) [17]. The overlapping acromion considered as type I presented the most common AC joint configuration (n = 74, 46.2%). The correspondingly measured mean acromion angle accounted for 112.7°. This AC joint type forms a congruency of the joint resulting in a certain projection towards a superior dislocation of the clavicle. Patients with this AC joint type might rather suffer from lateral clavicle fractures than from AC joint dislocation following a direct impact onto the clavicle resulting in superior dislocation of the clavicle. In contrast, a fall on the outstretched arm, considered as an indirect trauma, drives the humeral head in a superior direction into the acromion with a consecutive superior dislocation of the acromion, while the clavicle remains in the same position [4, 14]. In these cases, patients with an overlapping acromion type would rather suffer from AC joint dislocations than lateral clavicle fractures.

However, not only the osseous configuration is responsible for the origin of lateral shoulder injuries resulting from direct or indirect trauma. Especially concerning the fact, that osseous differences might be equalized by the cartilage surface or the discus. Rather multiple factors like the already mentioned dynamic and static joint stabilizers, the force of the impact or the bone quality play an important role as well and need to be carefully considered in this context [18, 23].

Our results show, that the variety of AC joint configurations can be summarized into three different major types: the overhanging acromion, the neutral shaped and the overhanging clavicle type. We assume, that the shown differences in the AC joint congruency play an important role in the development of various shoulder injuries resulting from the same trauma mechanism. To proof our assumptions, further studies are needed.