The lumbar mamillo-accessory foramen: a study of 203 lumbosacral spines.

We have examined 203 lumbar and sacral skeletal specimens and have noted the frequent occurrence of a mamillo-accessory foramen, formed by the ossification of the mamillo-accessory ligament. The dorsal ramus of the lumbar nerve passes through this foramen. At the L5 level, this foramen is found frequently: in 26% of the cases on the left side and in 13.5% on the right. In some cases it is seen as a simple, deep notch. It is found much less frequently at L4 and almost never seen above. It is equally rare at the sacral level where it is formed by a narrowing and closure of the groove between the zygapophyseal joint and the sacral lateral mass. The authors propose that these bony foramina are a manifestation of osteoarthritic changes and that they could, in certain cases, irritate or compress the dorsal ramus along its passage.