Richard Bartley MSc MCSP, Betsi Cadwaladr University Health Board, Wales
The facet joints have an important role in maintaining segmental lumbar stability and quality of spinal movement. They are innervated by nociceptive fibres and may be important sources of pain in the low back. However precise diagnosis and effective management of low back pain by direct treatment of the facet joints remains controversial.
Anatomy
The facet, or zygo-apophyseal joint, is a complex synovial joint lying either side of the posterior compartment of the lumbar intervertebral unit. It has a complete capsule and is lined with synovial membrane made up of villi. The ligamentum flavum blends with the medial superior aspects of the joint capsule, acting as a barrier with the exit foramen. The facet joint is supplied by two medial branches of the dorsal rami and each facet joint receives innervation from at least two spinal levels.
Function
The facet joints assist the lumbar discs in resisting compressive forces and protect the posterior annulus from excessive torsion and flexion stress. Maximum pressure through the facet joints occurs in extension, which if extreme can lead to trauma of the capsule. In prolonged standing 16% of the axial load is taken through the facet joints which rises with disc narrowing.
Pathology
Systemic disease (e.g. RA, villo-nodular synovitis) can involve the facet joints, but the cause is idiopathic in most patients with facet joint pain. Capsular tears, haemorrhage, meniscoid/synovial entrapment, even stress fractures are well documented theories to explain facet pain. Osteoarthritis is also a popular explanation but radiographs of degenerated facet joints are equally common in patients with and without low back pain, although one or two studies suggest that very severely degenerate facet joints are more likely to be symptomatic.
Diagnosis
Although it was Ghormely who first coined the term 'facet joint syndrome' in 1933, it was Goldthwait back in 1911 who recognised that these joints can be a source of low back pain. Hirsch in 1963 reproduced low back and leg pain by injecting hypertonic saline into the lower lumbar facet joints. This experiment was reproduced by Mooney and Robertson in 1976 and McCall in 1979. Mooney and Robertson also documented relief of low back pain with injection of local anaesthetic into lower lumbar facet joints. Thus an anaesthetic block or an irritant can confirm the presence of a symptomatic facet joint.
However no radiographic, historical or physical examination findings can definitively diagnosis facet pain as the primary cause of low back pain. Even if provocative or anaesthetic blocks identify a symptomatic facet joint they cannot take into account the complexities of spinal mechanics when the interaction of other pain-sensitive structures within the spinal unit may be contributing to the patient's symptoms.
Injections
If facet joint pain is suspected as a contributing factor in a patient with non-specific mechanical back pain, two choices face the clinician; a diagnostic anaesthetic facet block and/or a therapeutic facet block, possibly including local steroid infiltration. Many radiologists would recommend that the former should always carried out first so that the correct level and specific joint can be identified.
Contraindication to facet injections include bleeding diathesis, local infection, impaired local sensation or suspected 'red flag' signs. Minimum sedation is usually required so that post-block assessments are reliable. Injection accuracy is dependent on fluoroscopy (injection using an x-ray image). A partial arthrogram technique, using contrast medium, helps to further improve needle position and avoid intravascular injection. Patients should keep a pain diary after the procedure so that accurate interpretation of the effects of the injection can be made.
Efficacy of Facet Injections
Schwarzer et al studied 176 consecutive patients with low back pain who had received facet injections under fluoroscopy control. They did not find any clinical features that predicted responses to whether the injection relieved pain or not, except that all responders had pain on extension prior to the injection. There were no predictive correlates with referred pain to the back, buttock, groin, knee or blow the knees. Interestingly no patients with central low back pain responded to the injections.
Revel et al studied 40 patients to identify predictors for facet injections. This study revealed that the responders were more likely to present historically with an absence of exacerbation of symptoms with coughing or rising from sitting, or pain relief when lying down. They also tended to be older patients.
Injections using local steroid (therapeutic injections) have shown mixed results. Lilius et al documented seventy patients out of a total of 109 who had undergone steroid injection achieved pain relief for more than three months (36%). Carette et al found that 46% of their patients maintained pain relief six months post-injection. However both studies were found to have methodological flaws in the research design.
Current practice encourages the use of diagnostic and therapeutic facet blocks be reserved for patients who require additional pain relief to enable them to cope with conventional rehabilitation, or for whom conventional treatment is inappropriate, i.e. in older patients. These injections should always be carried out under fluoroscopy control. They should not be requested on the basis of clinical or radiological findings alone.
References
Dreyfus PH, Dreuer SJ & Herring SA: Contemporary concepts in spine care; lumbar zygoapophyseal (facet) joint injections Spine 1995: No. 18; 20240-2047
Carrette S, Marcoux S, Truchon R et al: A controlled trial of corticosteriod injections into the facet joints for chronic low back pain N Engl J Med 1991: 325; 1002-1007
Goldthwait JE: The lumbosacral articulation; an explanation of many causes of lumbago, sciatica and paraplegia Boston medical and Surgical Journal 1911: 164; 365-372
Ghormely RK: Low back pain with special reference to the articular facets, with presentation of an operative procedure JAMA 1933: 101; 1773-1777
Lilius G, Laasonen EM, Myllynen P, Harilainen A & Gronlund G: Lumbar facet joint syndrome; a randomised clinical trial J Bone Joint Surg (Br) 1989: 71; 681-684
Mooney V & Robertson J: The facet syndrome Clin Orthop 1976; 15: 149-155
Revel ME, Listrat VM, Chevalier XJ et al: Facet joint block for low back pain; identifying predictors of a good response Arch Phys Med Rehabil 1992: 73; 824-82