Acromion morphology and bone mineral density distribution suggest favorable fixation points for anatomic acromioclavicular reconstruction

Recent techniques for acromioclavicular (AC) joint reconstruction focus on additional AC cerclage to coracoclavicular (CC)-reconstructions. Due to the specific slim bone morphology at the acromion, there are concerns regarding these additional bone tunnels, as they may predispose to fracture and break out. The purpose of this study was to investigate anatomic properties of the acromion which may help improve surgical techniques directed at injuries to the AC joint. It was hypothesized that via measurements of thickness and density points of increased strength and support could be identified on the acromion.

Eighty-five fresh frozen cadaveric shoulders were used for this study. A standardized 3D-net was developed and thicknesses of the acromion were taken from defined points using a certified caliper. To define the acromial arch, the angle and radius of curvature between the antero-lateral, the highest point of the acromial arch and the postero-lateral aspect of the acromion were measured. Additional bone mineral density (BMD) evaluation was performed on 43 specimens in an anterio-posterior and latero-medial direction using 5-mm slices with a maximum of 10 and 6 slices, respectively.

Median specimen age was 63.0 (range 36) years (55 female, and 30 male). There was no statistical significance between male (62.0, range: 35 years) and female (64.5, range 32 years) regarding age (n.s.). Thickness of acromion points of interest were ranging from 3.5 to 24.3 mm. Median radius of curvature of acromial arch for female was 48.2 (range 92.7) mm and 66.2 (range 85.6) for male (p = 0.019). The median angle for female specimens was 21.4° (range: 44.6°) and 23.3° (range 51.7°) for male (p = 0.047). The latero-medial measurements showed significant difference between the region of interest (ROI): 1 and 4, 5, 6 (p = 0.001, p = 0.001, p = 0.001), 2 and 4, 5, 6 (p = 0.007, p = 0.001, p = 0.001), 3 and 5, 6 (p = 0.001, p = 0.001), 4 and 5, 6 (p = 0.010, p = 0.001). Antero-posterior measurements showed significant difference between the ROI: 1 and 8 (p = 0.031).

The posterior–medial acromion close to the AC joint revealed the highest BMD with an increasing density from lateral to medial. In combination with thickness measurements this region would support additional anatomical fixation of the AC joint using bone tunnels if necessary.

To anatomically reproduce the insertions of the AC ligaments at the acromion, either bone tunnels or anchors are needed. Therefore, several techniques have been developed. This study provides the anatomical data for these techniques and confirms the reconstructive approach of techniques using anatomical points of fixation and orientation.