16 Although the prevalence of isolated facet joint pain is debatable, the presence of facet joint arthrosis in different age groups is clear. Eubanks et al. examined prevalence rates of facet arthrosis on 647 cadaveric lumbar spines. Fifty-seven percent of samples between 20 and 29 years of age and 93% of the samples between 40 and 49 years of age had evidence of facet arthrosis. By the age of 60, 100% of the samples had prominent facet arthrosis. The highest prevalence and the greatest severity of arthrosis were found at L4–L5 [15]. Facet joint pain can be diagnosed and treated with facet joint injections and eventually, with radiofrequency lesioning of the medial branches. These are some of the most common interventional pain procedures performed with over 175,000 Medicare billings recorded in 2001 [16]. This review article will provide an overview of the anatomy, pathophysiology, diagnosis, and treatment of facet joint-mediated pain. Anatomy Previous studies have demonstrated that both the capsule [17, 18], and synovial folds [19, 20] of facet joints possess nociceptive nerve endings. Pain sensation from the capsule and synovium are transmitted through the medial branches of the dorsal ramus of spinal nerves. In addition, the medial branches also supply the multiﬁdus muscle, ligaments, and the periosteum of the vertebral arches and spines [21, 22]. The facet joint in the lumbar spine is innervated by the medial branch of the dorsal ramus of the nerve exiting at the same level and also the medial branch of the nerve one level above. For example, when considering the L4–L5 facet joint, innervation is supplied by the medial branches originating from the L3 and L4 nerves. In this example, the medial branch from the L3 nerve supplies the inferior articular process of the L4 vertebrae. This is equivalent to the superior articulation of the facet joint. Likewise, the medial branch of the L4 nerve supplies the superior artic- ular process of the L5 vertebrae, which also represents the inferior articulation of the L4–L5 facet joint. As each medial branch passes inferiorly, it lies in a groove along the medial–posterior surface of the transverse process [23–26]. The medial branch courses over the transverse processes one level inferior to where it origi- nates. This is related to the fact that there is a C8 nerve but no C8 vertebrae. For example, the C6–C7 facet joint is innervated by the medial branches of C6 and C7. However, the C7–T1 facet joint is innervated by the medial branches of C7 and C8. The facet joints of T1–T2 are innervated by the medial branches of C8 and T1. This pattern continues in the lumbar spine. Of note, the anatomy of the L5–S1 facet joint differs from its lumbar counterparts. It is Curr Rev Musculoskelet Med (2009) 2:15–24 Fig. 1 Anatomic features in the lumbar spine. Adapted from illustration by Stephen Ponchak MD in Walsh NE. Nociceptive Pain. In Raj PP, editor: Pain Medicine a Comprehensive Review, second ed. Elsevier Science, 2003. With permission innervated by the medial branch of L4 and the dorsal ramus of L5. The L5 dorsal ramus courses along a groove formed between the base of the S1 superior articulating process and the sacral ala [23, 26] (Fig. 1). The articulating surface of the facet joints is covered by a layer of hyaline cartilage. Surrounding each facet joint is a thin ﬁbrous capsule lined with synovial membrane. The joint capsule plays an important role in the degree of motion obtainable secondary to the ability of the capsule to resist ﬂexion moments [27]. In comparison to cervical facet joint capsules, the lumbar capsules are shorter and more taught, resulting in a lesser degree of ﬂexion obtained in the lumbar spine in comparison to the cervical spine. Experiments in which the joint capsules in the lumbar spine were excised resulted in increased lumbar range of motion in the sagittal plane [28]. The orientation of lumbar facet joints has important functional and clinical consequences. For example, facet joints oriented relatively more parallel to the sagittal plane, such as at L2–L3 and L3–L4, allow limited rotational movements and anatomically favor ﬂexion and extension movements. In contrast, the L4–L5 facet joints, with increased coronal angulations, facilitate greater rotational movements [29–31]. Both facet joint orientation and facet joint tropism (an asymmetry in the angles of two facet joints at the same level) have been implicated as important variables leading to facet joint-mediated pain. Masharawi et al. examined facet joint morphology in 240 human cadavers. They noted facet joint tropism was much more common in the thoracic spine as compared to the lumbar spine [32]. This suggested that asymmetry of facet joints in the lumbar spine may be