Sensory Mapping of Lumbar Facet Joint Pain

Chronic pain is defined as pain lasting more than three months duration. Between one third and one half of the population of the UK are affected by chronic pain. This corresponds to just less than 28 million adults. Chronic low back pain is an important contributor to the chronic pain burden. Up to a third of UK's population is affected by low back ache each year and 20% visit their general practitioner for advice. The result is that nearly 2.6 million people are seeking medical care for back pain in a year. This translates into a huge economic burden on the already stretched healthcare budgets. The health care cost of low back ache was estimated to be £1632 million in 1998.4 However the indirect cost of low back pain mainly due to lost productivity is much higher and conservative estimates put it at £3440 million in 1998. It would not be hard to imagine what the present economic burden of chronic low back pain would be if inflation is factored in to the above figures. Apart from the fact that chronic low back pain has a huge economic impact, it does cause significant loss of function from an individual's perspective, damaged relationships and difficulties at workplace.

Lumbar facet joint pain

Mechanical low back pain is the most common cause of chronic low back pain. Anatomical structures, such as facet joints, sacro-iliac joints, posterior longitudinal ligaments and muscles have been found to generate and contribute to the mechanical low back pain. The lumbar zygapophyseal (facet) joints are the pain generators in 15% to 45% of patients with chronic low back pain. Variety of interventional treatment modalities have been used to treat facet joint pain. Intraarticular injections of steroids and radiofrequency (RF) ablation of the nerve supply of the facet joints are the most widely practised. A recent NICE guideline (NG 59) has recommended radiofrequency denervation but not steroid injections for proven lumbar facet joint pain.

The lumbar facet joints are innervated by the medial branches of posterior primary ramus of the spinal nerve (L1-L5). Each of these posterior primary rami divide into medial, intermediate and lateral branches (except L5 posterior primary ramus). The medial branches innervate the facet joints and the multifidus muscle. The intermediate branches innervate the muscle longissimus thoracis. The lateral branches innervate the iliocostalis lumborum muscle. The lateral branches of L1, 2 ,3 supply the skin over the lower back.

The nerve supply - medial branch and dorsal ramus have been the target for RF interventions to relieve facet joint pain. Anatomical studies in 1980s found that facet joint has a dual innervation - for instance, the L4-5 facet joint derives its innervation from the medial branches of dorsal rami of L3 and L4. The medial branches of L1-4 and the posterior primary ramus of L5 have a relatively fixed anatomical course, thereby rendering themselves good targets for interventions. This forms the basis of targeting two nerves to denervate one facet joint. The success rate of RF denervation varies from 50 to 90% .9, 10 The varied success rate is often attributed to single diagnostic blocks and procedural variations. But anatomical variation is not often considered. A recent study has showed that there is significant variation in the individual nerve supply to facet joints which may add to the false -negatives of diagnostic blocks and inadequate pain relief from radiofrequency denervation. One way of improving the success rate is to reproduce the pain by electrical stimulation prior to RF treatment.

Lumbar medial branches and fifth lumbar dorsal ramus have been electrically stimulated not only in healthy volunteers but also in pain patients. A recent study has shown that supratheshold stimulation of facet joints in healthy volunteers results in local and referred pain.However none of the studies have correlated the suprathreshold stimulation to map the painful area. Hence we propose to map the painful area by suprathreshold stimulation.

Rationale for Study Based on conventional anatomical studies, two medial branches are denervated to treat pain from one facet joint. But this does not take into account of the anatomical variation of individuals, thereby potentially over or under treating the pain. In this study the investigators will map the painful areas by stimulating the medial branches, thus estimating how necessary it is to lesion the medial branches according to the standard practice.