The accessory obturator nerve and the innervation of the pectineus muscle.

There are a number of puzzling circumstances related to the occurrence of the accessory obturator nerve and the innervation of the pectineus muscle. What is the frequency of occurrence of the nerve? What is its pattern of innervation? Does the pectineus muscle belong to the ventral or the dorsal division of the thigh musculature? Is its position in the ventral or the dorsal division reflected in its innervation? Does the muscle have, perhaps, a double derivation? Eisler (1891) in his discussion of the lumbosacral plexus of man, set the pattern for the current description of the origin, course, and occurrence of the accessory obturator nerve. He reported finding it in 8 of 32 cases, a frequency of 25% . Oddly enough, subsequent references to Eisler’s study have consistently attributed to him a 29% occurrence for the nerve. There are relatively few reports in the literature of studies on the accessory obturator nerve, and those that exist are based on small numbers of specimens. De Sousa (’42) reported a 19% occurrence; Kaiser (’49) found it in two of 24 sides, an 8.3% frequency. It is equally obvious that Eisler’s sample was too small to give reliable results. Failure to find the nerve nearly so often as reported was the stimulus for a frequency study in this laboratory. Beginning in the summer of 1953, most of the cadavers used in our teaching laboratories were observed for the occurrence and distribution of the accessory obturator nerve. At the termination of the study of 550 sides in 1959, the nerve had been found in 48 specimens, an occurrence of 8.7%. The unreliability of small groups of specimens is apparent in the various segments of this study. One group of 102 sides yielded only three nerves, whereas another group of 99 sides showed 17 nerves. No group, large or small, approached any closer than this last one to Eisler’s sample. A computation half way through the study likewise showed the frequency of occurrence of the nerve to be just under 9%. A statistically reliable study which has not been cited because of its incorporation in an extensive embryological investigation was reported by Bardeen in 1906. He found the accessory obturator nerve to be present in 21 of 250 specimens, a frequency of 8.4%. If our samples were combined, the Occurrence of the accessory obturator nerve could be based on 800 sides and would show a frequency of 8.6%. The small accessory obturator nerve arises from the lumbar plexus by roots from the third and fourth lumbar nerves which emerge between the roots of origin of the obturator and femoral nerves (fig. 1). It parallels the obturator nerve along the medial side of the psoas muscle but runs somewhat more ventrally. Characteristically, the accessory obturator nerve passes deep to the expansion of the psoas minor tendon to cross the superior ramus of the pubis directly on the bone and behind the femoral vein. The nerve descends on the deep or dorsal aspect of the pectineus muscle and divides in the interval between this muscle and the capsule of the hip joint. Typically the nerve breaks up into three branches: one enters the front of the capsule of the hip joint; one passes medially to anastomose with the anterior branch of the obturator nerve; and one penetrates the dorsomedial aspect of the pectineus muscle to supply a portion of that muscle. One or other of these parts may be lacking, and, on Occasion, there are additional branches of the nerve. A