A contribution to the calculation of a safe deltoid split

PURPOSE: Traditional teaching suggests that a safe deltoid split should extend no more than 5 cm from the lateral edge of the acromion. However, there are reports of nerves lying within this distance. Our aim was to redefine the safe maximum split and also to study the influence of arm length and position. MATERIALS AND METHODS: Thirty cadaveric shoulders were dissected using the deltoid-splitting approach and the acromion-axillary nerve distance was measured in the neutral position, in abduction, and in adduction. This was correlated to upper arm length. Deltoid splits were measured at the end of 13 deltoid-splitting shoulder operations. RESULTS: The mean acromion-axillary nerve distance was 6.0 cm (SD 0.6; range 4.5–6.5). Abduction brought the nerve closer by 1.5 cm. There was a strong correlation with upper arm length (r = 0.82) but the presence of high individual variability did not allow calculation of a safe deltoid split. The mean deltoid split in 13 open shoulder operations was 3.4 cm. CONCLUSIONS: Taking the mean acromion-axillary nerve distance minus three standard deviations as the safe deltoid split would protect 99.7% of nerves. Therefore we recommend that the maximum deltoid split should be 4.2 cm; this distance would be sufficient to preserve all nerves in our study as well as all those reported by other authors. Splitting the deltoid in abduction should be avoided. CLINICAL RELEVANCE: The traditional 5-cm deltoid split is probably too generous. We believe 4.2 cm is a safer limit.