José Antonio Blas‑Dobón 1 · Luis Aguilella 2 · Daniel Montaner‑Alonso 1 · Maria Morales‑Suárez‑Varela 3, 4
1 Servicio de Cirugía Ortopédica y Traumatología, Upper Extremity Unit, Hospital Universitario Dr. Peset, Av. Gaspar Aguilar, 90, 46017 Valencia, Spain
2 Upper Extremity Unit, Hospital Universitario de La Ribera, Alzira, Spain
3 Unit of Public Health and Environmental Care. Department of Preventive Medicine and Public Health, Food Sciences, Toxicology and Legal Medicine, School of Pharmacy, University of Valencia, Valencia, Spain
4 CIBER in Epidemiology and Public Health (CIBERESP), Madrid, Spain

Abstract
Introduction Arthroscopic transosseous rotator cuff repair can be performed with an external guide, although the proximity to the axillary nerve raises safety concerns. The aim of this study is to determine the safety of different drilling angles regarding the axillary nerve.
Materials and methods We performed a bone tunnel in the greater tuberosity in 17 fresh frozen shoulders, using an external guide at four different angles: 40°, 50°, 60°, and 70°. At each angle, we measured the distance between the drill and the axillary nerve, the distance from the acromion to the skin incision point, and the perimeter of the arm at the axilla.
Results The distance to the axillary nerve was safe with the guide at an angle of 40°, 50° and 60°, but not at 70° (p = 0.001).
We found significant differences between all four angles (p < 0.05). Regression analysis demonstrated the influence of the guide angle in all measurements assessed (p < 0.001). There was no association between the measurements taken and the axillary perimeter (p > 0.5).
Conclusions Arthroscopic transosseous rotator cuff repair with an external guide does not pose a risk for the axillary nerve using angles of 60° or less.
Keywords Rotator cuff · Transosseous repair · Shoulder arthroscopy · Axillary nerve · External guide