Split Femoral Nerve Due to Psoas Tertius Muscle: A Review with Other Cases of Variant Muscles Traversing the Femoral Nerve

Leg pain from lumbar disc herniation is a common presentation. However, certain muscular and peripheral nerve variants may present similarly and represent an unrecognized etiology of femoral nerve dysfunction. Such cases might affect the outcome of specific treatment regimes. Therefore, recognition of these variations in anatomy may be useful to the clinician when treating the patient with medically refractory lower limb pain. Some reports have reported variant slips of the psoas and iliacus muscles, which may split the femoral nerve causing a potential risk for nerve entrapment. Herein, we report a very unusual variant of the psoas muscles, the psoas tertius, which pierced the femoral nerve into two parts. Additionally, the literature of other similar muscle variants is reviewed. Clinicians should be aware of anatomical muscular variants of the posterior abdominal wall and the propensity of such anomalies to result in distortion of regional neural structures. In this regard, the anatomy of the psoas tertius should be known.