Introduction The mechanisms underlying chronic back pain are not well understood, however, disc degeneration and facet joint arthrosis have been suggested to be two major pain sources. Nitric oxide (NO) is an oxygen free radical which is involved in variety physiological and pathological events. Increased concentrations of NO have been demonstrated with direct or indirect methods in temporomandibular (Takahashi T et al. Oral Surg Oral Med Oral Pathol Oral Radiol Endod.1999) and knee joints (Karan A et al. Clin Rheumatol. 2003) with osteoarthrosis. The aims of the study were 1. To investigate if real-time NO concentrations can be measured in the perifacetal region and 2. To estimate perifacetal NO levels in patients with facet joint osteoarthrosis associated chronic low back pain and compare it with that of healthy controls, and 3. To investigate if perifacetal NO levels is discriminative for subsets of patients with facet joint osteoarthrosis.

Methods Patients with at least six months duration of chronic low back pain with signs of facet joint osteoarthrosis on CT and/or MRI, were included in the study (n=26). Healthy volunteers were enrolled as controls (n=7). A detailed questionnaire including visual analogue scale (VAS) was completed by the patients before and six weeks after the NO measurements. Nitric oxide was measured with a custom designed electrochemical real-time nitric oxide sensor (World Precision Instruments, Sarasota, Fl). The NO sensor has a detection limit of less than 0.3 nM, a linear response to NO concentrations up to 100 μM and can discriminate between NO and closely related substances such as nitrite (Zhang X et al; Electroanalysis 2002). The NO sensor was inserted into the facet joint through a 20 gauge needle under fluoroscopic guidance in patients and controls. All patients received corticosteroids (0.4 ml Celestone®) and local anaesthetic (0.5–1.0 ml Marcain®) in the perifacetal region following the NO measurements. Descriptive parameters are expressed as mean (± SEM) and Mann-Whitney’s test was used for statistical comparison between groups.

Results It was possible to obtain NO measurements from all participants. No adverse effects were noted. The patients with chronic low back pain demonstrated 3-fold higher concentrations of nitric oxide in the perifacetal region compared to the healthy controls (1.66±1.39 vs. 0.46±0.37 nM, p=0.007). No association between nitric oxide concentration and pain-duration or pain-level (VAS) was detected. However, patients with a positive response to local anaesthetics and corticosteroid injection (detected as a reduction of VAS at a minimum of 20 mm) at the six week follow-up visit had 25% higher concentrations of nitric oxide when compared to patients who had a less than 20 mm decrease in VAS. p=0.02

Discussion The study demonstrates that measurement of NO with a real time-sensor around the facet joints is feasible and safe. The findings of higher concentrations of NO in the perifacetal region in chronic low back patients compared to healthy controls indicate that the degenerative process of the joints may cause increased NO production. Patients that responded to corticosteroid/local anaesthetic infiltration had higher NO concentrations in the perifacetal region compared to patients without response. This observation indirectly suggests a more pronounced inflammatory process in the responding patients.