Furcal Nerve

The lumbosacral plexus has had less attention than the brachial plexus probably due to there being less injuries to it. ¹ The lumbar plexus is often formed by a contribution from the 12th thoracic nerve, the first three lumbar ventral rami, and the greater part of the fourth lumbar ventral ramus. The lumbosacral plexus is formed by the remaining small part of the fourth lumbar nerve and the fifth lumbar nerve (lumbosacral trunk) ² and the upper sacral nerves. ³ Various types of lumbosacral nerve root anomalies have been described in the literature. The existing classifications of nerve root abnormalities consider convergence, intradural or extradural anastomosis, and division. ⁴^, ⁵

The furcal nerve has been variably defined, but many consider it the nerve that enters into the formation of both the lumbar and sacral plexuses, usually as a result of a nerve dividing, or forking, between the two plexuses ⁶^, ⁷^, ⁸^, ⁹ serving as a link between the two plexuses (▶ Fig. 9.1) ¹—for example, a part of the L4 ventral ramus that joins with L5 ventral ramus to form the lumbosacral trunk. Some, however, have described the furcal nerve as a separate nerve traveling alongside the L4 nerve root in the intervertebral foramen, having separate anterior and posterior root fibers and its own dorsal root ganglion, and thus indicating that it is its own independent nerve root. It has also been found to have fibers contributing to neighboring nerves—26% to the femoral nerve, 18% to the obturator nerve, and 16% to the lumbosacral trunk. ¹⁰ Schaeffer ⁸ has specifically described the furcal nerve as “when the fourth nerve enters into the formation of both lumbar and sacral plexuses, it may be called the furcal nerve, but this name is also applied to any of the nerves that enter into the formation of both plexuses, so there may be one or more furcal nerves.” Others have defined the furcal nerve simply as the entire ventral ramus of the L4 spinal nerve.

Fig. 9.1 Right-sided L3, L4, and L5 ventral rami. Note the L4 contribution (left arrow) to the L5 ventral ramus forming the lumbosacral trunk (right arrow) that continues distally to interconnect with upper sacral ventral rami and thus forming the lumbosacral trunk. The obturator nerve is being retracted laterally. Lateral to the obturator nerve, the large femoral nerve is seen. Depending on the author, the L4 ventral ramus or the connection (left arrow) to the L5 ventral ramus could be termed the furcal nerve in this specimen.

9.2 Variations of the Furcal Nerve

The different deviations are thought to be a result of a deviation from the normal fetal development during the first four weeks of life. ¹ Kikuchi et al performed cadaveric dissections including coronal and sagittal sectioning of the lumbosacral region in order to better delineate the anatomy of the furcal nerve. The furcal nerve was found in up to 93% of their dissections and mainly as L4. The furcal nerve has been referred to as the boundary root. ¹⁰ However, although most furcal nerves arise at the L4 level, they can occur at any level from L1 to S1. ⁵^, ⁷^, ⁸^, ⁹^, ¹¹