Optimal Angle of the Bone Tunnel for Avoiding Axillary Nerve Injuries During Arthroscopic Transosseous Rotator Cuff Repair: A Magnetic Resonance Imaging-Based Simulation Study.

BACKGROUND

Axillary nerve injury and suture cutout through the roof of the tunnel are potential complications of arthroscopic transosseous rotator cuff repair (ATORCR).

PURPOSE

To determine a safe angle of drilling for the bone tunnel during ATORCR such that the axillary nerve is not at risk. The thickness of the bone bridge over the tunnel for different angles of drilling was also determined.

STUDY DESIGN

Descriptive laboratory study.

METHODS

The drilling of a straight tunnel was simulated on 30 magnetic resonance imaging (MRI) scans in the oblique coronal plane by drawing a straight line that passed at a "safe distance" of 5 mm from the axillary nerve and emerging at the medial border of the insertion of the rotator cuff on the greater tuberosity. The angle made by this line with the horizontal axis of the humerus was measured on 3 MRI sections: anterior (passing just posterior to the lateral lip of the bicipital groove), middle (at the most lateral point of the proximal humerus), and posterior (an equal number of cuts away from the middle section as between anterior and middle). The thickness of the overlying bone roof was measured for this line as well as for simulation lines drawn at 50°, 55°, 60°, and 65° with the horizontal axis. A "safe limit," defined as the mean - 2SD, was also calculated.

RESULTS

The axillary nerve was found to be safe, with a safety margin of 5 mm, at drill angles of less than 61.1° and 60.3° in the posterior and middle sections, respectively. The safe limit value for thickness of the overlying bone roof for the tunnel drilled at 60° was 5.0 mm in the posterior section (mean, 8.2 ± 0.3 mm) and 5.5 mm in the middle section (mean, 8.1 ± 0.2 mm). In the anterior section, the minimum safe angle was 57.7°, and the mean thickness of the bone roof for the 55° angle was 6.3 ± 0.2 mm (safe limit, 3.7 mm).

CONCLUSION

Straight bone tunnels in ATORCR surgery should be drilled at an angle of 60° to the horizontal axis of the humerus or 30° to the humeral shaft to ensure the safety of the axillary nerve while at the same time ensuring adequate thickness of the overlying bone roof. The anterior tunnel close to the bicipital groove should be drilled cautiously at 55° to the horizontal axis or 35° to the humeral shaft.

CLINICAL RELEVANCE

The findings of the present study will help the surgeon choose the best angle for drilling tunnels during ATORCR surgery to avoid axillary nerve injuries as well as suture cut-through without the need for any proprietary device.