Accessory Obturator Nerve

Fig. 10.1 Cadaveric example of a right-sided accessory obturator nerve. The small threadlike nerve above the scissors is the accessory obturator nerve. Note its course over the pubis. More deeply below the scissors, the obturator nerve is seen coursing out the obturator foramen.

Fig. 10.2 Cadaveric example of a left-sided accessory obturator nerve (lower arrow). The psoas major is retracted laterally to show the obturator nerve (upper arrows). In this specimen, the accessory obturator nerve crosses over the pubis to join the anterior branch of the obturator nerve.

10.2 Origin

When present, the AON arises from L3 or more commonly L3 and L4 between the roots of the femoral and obturator nerves. It can also arise from L2, L3, and L4; or L2 and L3; or from the obturator nerve directly. ⁴^, ⁵ Katritsis et al found it was formed by roots from the anterior primary divisions of L3 and L4 (63.6%) or L2, L3, and L4 (10.6%), or L2 and L3 (7.6%), or L3 (6.1%), or from the trunk of the obturator nerve (12.1%). ⁶ Ellis reported one case in which the AON arose from the trunk of the obturator nerve. ⁷ Quain et al described the obturator nerve as originating in association with the anterior crural nerve in two cases. ⁸

10.3 Anatomy

The prevalence of the AON has consistently been reported as ranging from 10 to 30%. ⁹^, ¹⁰^, ¹¹ Population samples in individual studies have been too small for reliable estimates of the overall prevalence of the AON. Most studies of it have failed to record gender or unilateral bias. Sim and Webb and Akkaya et al reported a greater prevalence in females and of left-sided AON, but these results could have been misleading owing to the low numbers of specimens. ¹²^, ¹³ The largest study by Katritsis et al, which examined 1,000 plexuses, revealed no gender difference in the prevalence of an AON in the lumbar plexus, but there was still a left-sided dominance in unilateral cases. This suggests a lack of association between side dominance and gender.

The AON branching from the trunk of the obturator nerve (12.1%) ⁶ is incompatible with the proposal that it be termed the accessory femoral nerve. Misidentification of the AON can lead to surgical complications such as those in a case reported by Jirsch and Chalk, ¹⁴ which demonstrated the importance of these variations in surgical practice. In this case, the AON was thought to be the obturator nerve, which led to the obturator nerve being injured during elective laparoscopic tubal occlusion. Techniques such as magnetic resonance imaging (MRI) and intraoperative nerve stimulation can be used to locate it. ¹²

10.4 Course

In 100% of the plexuses examined that had an AON, Katritsis et al found that it passed 2 to 3 cm anterolateral to the obturator nerve and medial to the psoas major toward the obturator foramen, but instead of passing through the canal, it passed over the superior pubic ramus, staying medial to the psoas muscle. Woodburne described the AON as passing directly over the pubic ramus under the femoral vein. Once it crosses the pubic ramus, the nerve descends dorsally to the pectineus muscle, where it typically divides into three branches: one entering the anterior hip joint, one entering the dorsomedial aspect of the pectineus muscle, and one passing medially to anastomose with the anterior branch of the obturator nerve. ¹¹^, ¹⁵ In a rare case reported by Rohini et al, the AON emerged on the medial side of the psoas major, entered the femoral triangle, divided into the three typical terminal branches, and passed superficially to the pectineus muscle instead of deep to it. ¹⁶

10.5 Variations

Katritsis et al studied 1,000 plexuses (132 with AON) and found that 36.4% of AONs had variant origins. Although most of these variations were not drastically different, it is important to recognize that the AON can derive from the trunk of the obturator nerve or the anterior crural nerve. ⁸ Multiple variations of the three terminal divisions have been reported. Katritsis et al saw that after supplying the pectineus, the AON branched off behind the pectineus muscle and supplied the anterior branch (14.3%), posterior branch (4.65%), or trunk of the obturator nerve (6.1%), or the femoral nerve (2.3%). Woodburne also reported that a single branch supplying the adductor longus is not uncommon, along with other additional branches. A very common variation of the AON makes it the sole innervation of the pectineus muscle rather than the typical dual innervation with the femoral nerve. ¹¹ Quain et al described a small cutaneous branch that supplies the inner thigh and upper proximal inner leg. Allen and Shakespeare reported a similar finding of the AON anastomosing with the obturator nerve and supplying cutaneous innervation to the skin of the inner thigh. ¹⁷

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