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

Abstract

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.

Keywords: arthroscopic transosseous; axillary nerve; rotator cuff.

Figure 1.

A diagrammatic representation of the oblique sagittal view through the axillary nerve over the lateral aspect of the humerus. Line PQ represents the horizontal plane, and its perpendicular line RS represents the oblique coronal plane. Let MN be the path of the axillary nerve over the lateral aspect of the humerus and W be the point where the drill wire passes through this section. Then, line WZ, perpendicular to MN, will be the actual minimum distance between the nerve and the wire, and line WO, drawn perpendicular to the horizontal axis, will be the distance measured on the oblique sagittal view. Assuming that the axillary nerve is nearly horizontal, that is, angle NOQ or OWZ is very small, the difference between the 2 distances will be nonsignificant.

Figure 2.

Selection of the 3 sections for measuring the angle of the bone tunnel and the thickness of the overlying bone bridge on oblique coronal magnetic resonance imaging. A, anterior section; B, bicipital groove; M, middle section; P, posterior section.

Figure 3.

Axes drawn on the middle section on oblique coronal magnetic resonance imaging. Line SI passes through the centers of 2 circles drawn inside and touches the walls of the medullary canals of the humerus. Line OY is parallel to SI. Line OX is perpendicular to OY, thus representing the horizontal axis. Line OA, passing through the axillary nerve, creates an angle of 76.5° with the horizontal axis in this case. A circle with a radius of 5 mm is drawn centered over the nerve, and line OB is tangential to this circle. It creates an angle of 68.4° with the horizontal axis.

Figure 4.

Measurement of the thickness of the bone over the tunnel. Line B₁B₂ is drawn parallel to line OB, such that it is tangential to the margin of the greater tuberosity. The distance between these 2 lines (8.8 mm in this case) gives the greatest thickness of the overlying bone.

Figure 5.

Measurement of the thickness of the bone over the tunnel for different angles of drilling. Here, line OD is drawn at an angle of 60° with the horizontal axis, and line D₁D₂ is parallel to line OD and tangential to the margin of the greater tuberosity. The distance between these 2 lines (6.5 mm) gives the thickness of the overlying bone.

Figure 6.

Measurement of the distance of the exit point of the tunnel over the lateral surface of the humerus (point T) from the tip of the greater tuberosity with a drill angle of 60°. Line O₁X₁, parallel to the horizontal axis, is drawn through the tip of the tuberosity. Another parallel line TX₂ is drawn through point T, and the perpendicular distance between these lines is measured (18.9 mm). The distance of the nerve from the tip of the greater tuberosity is also measured in a similar way (distance between O₁X₁ and O₃X₃, which is 37.0 mm in this case).

Figure 7.

Graph showing the safe limit (mean – 2SD) for the thickness of the bone over the tunnel for the 3 imaging sections at different angles of insertion.

Figure 8.

Graph showing the safe limit (mean – 2SD) for the distance of the lateral end of the tunnel from the tip of the greater tuberosity (GT) for the 3 imaging sections at different angles of insertion.

Figure 9.

The placement of the jig through the lateral portal for drilling of the wire at 60° to the horizontal axis of the humerus. The direction of drilling can also be referenced directly from the longitudinal axis of the humeral shaft. The jig in the figure is fixed at 60°, and the goniometer is at 30°.