[Multiple positional relationships of nerves arising from the sacral plexus to the piriformis muscle in humans].

The positional relationships between the piriformis muscle and the nerves which arise from the sacral nerve plexus were studied in 514 sides of 257 Japanese adults. These were classified into Types I-XIII and numerous subtypes based on: 1) the number of nerves perforating the piriformis muscle, 2) whether all or part of the nerve perforated the muscle, 3) the order of perforation and position in the muscle, and 4) communications between the nerves. In this paper, the multiple positional relations between the nerves and the piriformis muscle, the frequencies of the various types, and the order of priority concerning the perforation of nerves through the muscle are discussed. 1) The typical case, Type I, in which the piriformis muscle is not perforated by nerves except for a part of the superior gluteal nerve, was found in 309 (60%) of 514 sides. Types III-X, in which the muscle is perforated by additional nerves, were found in 195 sides (38%), and in 175 of these, all or part of the common peroneal nerve passed through the muscle. Types XI-XIII, in which the inferior gluteal nerve and other nerves pass above the piriformis muscle, were found in 10 sides (2%). Among all types, the following were generally seen: Type V, the piriformis muscle is perforated by both the inferior gluteal and common peroneal nerves; Type VII, the muscle is perforated by the two above-mentioned nerves and part of the posterior femoral cutaneous nerve (Fig. 16, Table 1). The common peroneal nerve followed two courses (a combination of over, through, and under the piriformis muscle) in 49 extremities. In the 4 cases of Type X (1%), the tibial nerve was divided into two components due to the intervention of the most caudal bundle of the piriformis or an unknown muscle. The dorsal component passed through the muscle, while the ventral component followed the typical course under the muscle (Figs. 9-11). Therefore, in the above-mentioned 53 cases as well as in other cases, the sacral nerve plexus cannot be divided into ventral and dorsal layers up to the sacral nerve roots, as in typical cases. Type II, in which the piriformis muscle is perforated by only a caudal branch of the superior gluteal nerve, was found in about 16% of 249 cases; this type was accompanied by some of the other types (Table 2). In the extraordinary case of Type XII with Type II, a branch of the superior gluteal nerve passed under the piriformis muscle (Fig. 13).(ABSTRACT TRUNCATED AT 400 WORDS)