Facet orientation and tropism: associations with facet joint osteoarthritis and degeneratives.

Back pain and its sequelae place an enormous burden on society, health care systems, and the economies of developed countries7,17. Degenerative changes in the spine can be a potential cause of back pain, and therefore an understanding of the relationship between pathoanatomic abnormality and advanced degeneration is of importance from a clinical and public health perspective. The purpose of this study was to investigate associations between facet joint tropism and facet joint orientation and two common lumbar degenerative spinal disorders: facet joint osteoarthritis (OA) and degenerative spondylolisthesis (DS). The facet joints are the only synovial joints in the spine, with hyaline cartilage overlying subchondral bone, a synovial membrane and a joint capsule. Facet tropism and facet orientation may have a relationship to degenerative changes in the spine, either as the cause of degenerative changes or as the result of abnormal forces produced by degeneration. Facet orientation is the angle of the facet joint in the transverse plane relative to the sagittal plane (Figure 1). Facet tropism is defined as asymmetry of the left and right vertebral (zygapophyseal) facet-joint angles, with one joint having a more sagittal orientation than the other4. Despite extensive studies with some conflicting results2,3,6,9,11,12,26, the clinical significance of facet tropism and orientation is not yet well established. Figure 1 Schema of facet orientation measurement. Reference plane was defined by posterior aspect of vertebral body. Sagittal line was drawn though spinous process, 90° to the reference plane. Facet orientation (α) was measured automatically at ... Facet tropism and facet orientation have been proposed as factors associated with lumbar facet joint OA. One cadaveric study examining this relationship found an association between facet tropism and facet sclerosis, though no association between facet tropism and a composite score of cartilage degeneration and sclerosis12. Conclusions from this study are limited due to the very small sample size involved (n=22) and the somewhat arbitrary criteria used to grade degeneration. Another study examining the relationship between facet orientation and facet joint OA in 111 patients9 found a significant association between facet orientation and OA, but little association between facet tropism and OA. Prior studies also have found significant associations between facet orientation, facet tropism, and DS2,3,6,9,11,26 in simple univariate analyses, but have largely failed to account for simple covariates such as age, sex and BMI. The relationship between facet tropism and orientation, and facet joint OA and DS, while adjusting for important covariates, has yet to be clearly defined. Due to precise demonstration of osseous details13,15,24 and relatively low cost, computed tomography (CT) is the preferred method for imaging lumbar facet joint OA. Abnormalities of the facet joints that can be demonstrated and categorized by CT include osteophyte formation, hypertrophy of articular processes, joint space narrowing, vacuum joint phenomenon, synovial and subchondral cysts, and calcification of the joint capsule5,25. CT also allows easy identification of spondylolisthesis, particularly when sagittal reformats are available. The aims of the present study were: 1) to document the distribution of facet orientation and prevalence of facet tropism in the community based sample; 2) to evaluate the association between facet orientation and tropism and age, sex and BMI; and 3) to evaluate the association between facet orientation and tropism, and facet joint OA and DS.