Septic arthritis is most commonly secondary to a bacterial infection with less common, more indolent infections resulting from fungal or mycobacterial causes. Predisposing factors include elderly patients, diabetes mellitus, immunosuppressed patients, rheumatoid arthritis, skin infection, IV drug abuse, and previous joint manipulation including joint prosthesis, recent joint surgery and intra-articular corticosteroid injections.10
Septic arthritis is caused by hematogenous spread (where the presenting sign can be bacterial endocarditis), direct inoculation of the joint from corticosteroid injection, surgery or trauma, or from spread of adjacent infection into the joint space. One retrospective review of 191 cases of septic arthritis found that 72% of cases were thought to arise from hematogenous spread.n
The majority of cases in adults is caused by Staphylococcus aureus
and occur in the larger peripheral joints including knees, wrists, shoulders, elbows, ankles and hips. Smaller joints are rarely affected including the sternoclavicular joint, sac-roiliac joint, pubic symphysis and the spinal facet joint.
One or more predisposing factors was seen in 38-58% of patients diagnosed with facet joint septic arthritis with the most common being concomitant infection and im-munosuppression (most notably diabetes mellitus, liver disease, transplant patients, long-term corticosteroid use and malignancy) P
Another predisposing factor is underlying joint disease which is reportedly found in almost 50% of cases of septic arthritis.12
Related to underlying facet disease, there was a retrospective study where 209 consecutive lumbar spine MRIs were reviewed regardless of patient history or clinical indication which revealed that 41% of patients were found to have facet synovitis based on signal abnormality within the joint capsule and peri-articular region.13
A high index of suspicion is needed to prevent a delay in diagnosis and therapy. Mean time from symptom onset to diagnosis has been reported to be 36-43 days with a large range from 2 days to 6 months.1,14
This delay in diagnosis can result in increased patient morbidity and highlights the need for consideration of this disease in the differential diagnosis of patients presenting with neck/back pain, fever and with any of the risk factors discussed above. Further evaluation includes lab work up (white blood cell count, ESR, CRP, and blood cultures) followed by appropriate imaging studies.
Plain radiographs are not sensitive in diagnosing early disease as radiographic findings may not be evident for weeks to months following onset of symptoms.1,14-18
However there are a few reports where facet joint space widening suggesting joint pathology was noted at 4 and 21 days after onset of symptoms.14,15
Radionuclide studies including Technitium-99m MDP bone scan, Gallium-67, and In-111 labeled white blood cell scans have shown very high sensitivity for this entity as early as one week after onset of symptoms.2
Tc-99m MDP bone scan can be helpful for assessing osteoblastic activity or bony remodeling secondary to infection. However, the low specificity of this test limits its utility in diagnosing septic arthritis. Ga-67 and more recently In-111 are being used to evaluate for infection/inflammation with very high sensitivity, more specificity than Tc-99m bone scans and improved spatial resolution with the implementation of SPECT and co-registered SPECT-CT images.
Non-contrast CT is able to show joint space widening, pre-existing joint disease, bony erosions and either a fluid collection or soft tissue air that could suggest abscess formation. CT is also very helpful for establishing the diagnosis via obtaining synovial fluid for isolation of the organism and for drainage of the affected joint. MRI is the imaging modality of choice for diagnosing facet joint septic arthritis due to its high sensitivity, specificity and soft tissue contrast.15,17,19-20
MRI is also essential for therapeutic planning.
Soft tissue gadolinium enhancement may be seen on MRI within 2 days from the onset of symptoms.17
Reportedly 81% of cases show epidural and/or paraspi-nal extension of the infection,1
and MRI is superior at demonstrating extension into the epidural or disc spaces, paraspinal soft tissues, vertebra and abscess formation. Many case reports of facet joint septic arthritis are associated with epidural or paraspinal abscesses.1,2,5-8,15,19-23
It may be difficult to distinguish on imaging whether the infection started in the facet joint or if the infection spread to the facet joint. Some authors have postulated that the incidence of this infection may be underestimated if the infection decompresses into the surrounding paraspinal tissues or epidural space prior to diagnosis.17
Facet joint septic arthritis should be considered when a patient presents with back pain, fever and elevated inflammatory markers (ESR and CRP), however the presentation and at risk populations are nearly identical to that of spondylodiscitis. Over 90% of patients present with pain, roughly 75% present with fever and about 33-50% present with neurologic symptoms. Facet joint septic arthritis may be suspected in patients with unilateral symptoms or when there is a more rapid symptom progression (4 weeks) compared to the typical presentation of the more common spondylodiscitis (2-3 months).2
Other differential diagnosis considerations include non-pyogenic infection such as Tuberculosis, degenerative or inflammatory arthritis and malignancy. Lytic or destructive lesions involving the posterior elements are most often neoplastic in etiology. One retrospective study using CT to determine infection vs. tumor in the spine noted that severe neurological impairment was more common with spinal infection (39%) than tumor (14%).24
Complications of septic arthritis of the spinal facet joint include chronic pain, joint/bony destruction, pyomyositis, abscess (epidural, psoas muscle and paraspinal), neurologic sequelae, spondylodiscitis, endocarditis, meningitis, septic emboli and rarely death. In one review of 42 patients excluding cases involving pediatric patients, IV drug users and prior surgical instrumentation/surgery, the most severe complications were paraplegia in one patient and death during surgery in another patient.1
The majority of patients in the case reports recovered fully or had minimal residual pain following treatment.
Patients are typically treated with long-term (at least 6 weeks) parenteral antibiotics followed by oral antibiotics or a combination of percutaneous drainage and long-term antibiotics. Open arthrotomy and surgical drain-age/debridement is typically reserved for the patient with infection refractory to antibiotic trial or with acute neurological compromise. MRI is less helpful in assessing for treatment response as soft tissue enhancement can persist following clearance of infection. Treatment response can be assessed using the patient's subjective improvement in symptoms and improvement in inflammatory serum markers.
In conclusion, septic arthritis of the facet joint is an uncommon infection that requires a high clinical suspicion for accurate and timely diagnosis. This infection shares many features with septic arthritis in the more commonly affected large peripheral joints as well as with spondylodiscitis. The incidence of this entity is increasing and MRI will continue to play an important role in diagnosis and surgical planning. Lastly, while most patients recover with little or no neurologic sequlae, prompt diagnosis and definitive therapy are essential for decreasing patient morbidity and mortality.