Risk of fracture of the acromion depends on size and orientation of acromial bone tunnels when performing acromioclavicular reconstruction

Purpose: Current techniques for anatomic repair of the dislocated acromioclavicular (AC) joint aim on reconstruction of the AC ligaments and utilize tunnels drilled through the acromion . This improves the stability of the reconstruction but might also increase the risk of fractures at the acromion. The purpose of this study was to evaluate the fracture risk for the acromion after transacromial tunnel placement for anatomic AC joint stabilization procedure. It was hypothesized that the risk of fracture of the acromion is correlated to size and orientation of bone tunnels commonly used for anatomic AC joint reconstruction.

Methods: A finite element analysis was used to simulate multiple bone tunnels and incoming force vectors (lateral vs. superior). Different tunnels were analysed, horizontal meaning an anterior-posterior orientation versus a vertical inferior-superior orientation through the acromion. Two tunnel diameters were simulated (2.4 vs. 4.5 mm). Furthermore, the tunnel length and distance between tunnels were altered. Forty-five cadaveric specimens (median age: 64 years, range 33-71 years) were utilized for data acquisition. Out of these, 30 specimens were used to evaluate basic tunnel orientations and drill diameters using a MTS 858 servohydraulic test system.

Results: With regard to the tunnel orientation and drill hole size, the loads to failure were limited. The acromion is at higher fracture risk, with a superior to inferior directed incoming force. Position, size and direction of bone tunnels influenced the loads to failure. Horizontal tunnels with a higher diameter (4.5 mm) had the most impact on load to failure reduction. A long horizontal tunnel with a diameter of 4.5 mm reduced the load to failure with medial direction of force to 25% of the native acromion. The identical tunnel with a diameter of 2.4 mm reduced the load to failure to 61%. Both 2.4-mm horizontal tunnels with a medium and short length did not reduce the load to failure.

Conclusion: Tunnels placed at the acromion did not result in an increased risk of fracture. However, descriptive data showed a tendency for an increased fracture risk if tunnels are placed at the acromion, especially in horizontal direction with diameters of 4.5 mm. In addition, the pattern of fracture was dependent on the orientation of the bone tunnels and the size. However, the results indicate a "safe zone" for the placement of bone tunnels within the anterior half of the acromion, which does not affect the loads to failure at the acromion. Therefore, current techniques for anatomic AC joint reconstruction which utilize fixation of grafts or sutures at the acromion are safe within current ranges of tunnel placement and sizes.