[The superior gluteal nerve. Anatomical study of its extrapelvic portion and surgical resolution by trans-gluteal approach].

INTRODUCTION: The Transgluteal approach (TGa) to the hip, proposed by BAUER, and often used for total arthroplasties, could be responsible, according to some authors for bad clinical results, due to injury of the nervus gluteus superius (NGS). The aim of this study was to verify the nerve's anatomical condition and evaluate the risks of injury during TGa.
MATERIAL AND METHODS: Thirty three dissections of fresh corpses in the lateral position (preceded by a TGa) permitted the estimation of the risk and the measurement of the distances between the nerve and the trochanter major (TM). The NGS was sought in the area of the foramen ischiaticum at the proximal side of the piriformis muscle after cutting the vessels and the fat. It was possible to follow it's branches to the end, but we had to lift the m. gluteus medius (mGM), or to cut it transversally.
RESULTS: Many anatomical variations were found concerning the point of the nerve's division into 2 branches, nearer or farther from the foramen ischiaticum. The upper branch followed the proximal side of the gluteus minimus (GMin) then innervated the GM and the m. tensor fasciae latae (TFL). the lower branch showed a variable distribution of the strings to the three muscles (GM, GMin and TFL). The TFL's branch ended at upper side of the TM. A safe area, over the TM, without any nerve branches could be determined 7 cm above and behind the TM, 5 cm above it's posterior angle and 3 cm above it's anterior angle. During anterior TGa we noted that the coxofemoral dislocation could tear the GM's proximal fibers and threatened the "inter" or intramuscular nerve filaments. The acetabular exposition by retractors, could compress the more frontal branches. On the contrary using the posterior TGa, neither the dislocation nor the exposition seemed to threaten the nerve, which was farther away and more posterior. DISCUSSION: In this study we confirmed the existence of numerous anatomical variations of the NGS and classified them into 4 categories which include those already described in previous publications, of which the first category seems to be the most common. Our distances measured from the nerve to the TM are similar to those previously published, but our safe area is more restrictive than that proposed by Jacobs and Buxton. Respecting the limits of this area reduces the risk of nerve injuries. During the anterior TGa, the nerve is nearer to the TM and more exposed. The muscular mass innervated is large and the functional consequences of frontal injury must not be neglected. During acetabular exposition, the retractors should exert moderate muscular pressure, to avoid crushing them. The respect of the GMin, ensures adequate protection for the nerves situated between the muscles. To avoid muscular tearing during the anterior dislocation it is better to cut the collum femoris in place. The posterior approach seemed to be less dangerous for the nerves and muscles which are farther away.
CONCLUSION: Strictly remaining within the limits of the safe area and carefully separating the muscles, should allow to decrease the risk of NGS injuries during TGa which seems to be more important in the anterior than in the posterior approaches.