THE FACET ORIENTATION CIRCLE - A NEW PARAMETER FOR FACET JOINT ANGULATION IN THE LOWER LUMBAR SPINE

Study design. A descriptive quantitative evaluation was done of the tr ansverse orientation of the lower lumbar facet joints as measured by c omputed tomography scanning. Objectives. To evaluate a new parameter f or facet joint angulation in the transverse plane (the ''facet orienta tion circle'') and to obtain reference values for this new parameter. Summary of Background Data. In other studies, both in vitro and in viv o, the angulation of the facet joints has been measured in degrees rel ative to the frontal or sagittal plane. These methods have some limita tions. The parameter used in the present study has not been described in the literature. Methods. Lower lumbar facet joint orientation was m easured in 212 vertebral levels of 123 consecutive patients using the facet orientation circle parameter. Patients with degenerative or deve lopmental abnormalities of the lumbar spine were excluded, as were tho se with technically inadequate computed tomography studies. Results. M ean facet orientation circle diameters (+/- SD) were: L3-L4, 43.7 +/- 10.5 mm; L4-L5, 63.8 +/- 26.4 mm; and L5-S1, 82.3 +/- 23.5 mm. Differe nces between right and left sides were: L3-L4, 6.3 +/- 7.1 mm; L4-L5, 13.9 +/- 25.3 mm; and L5-S1, 17.9 +/- 16.2 mm. Intra-observer variabil ity was 2.3%. Conclusions. Measurement of facet joint angulation using the facet orientation circle is possible and reproducible. The morpho metric data presented may be useful as a reference for biomechanical a nd clinical research of facet joint orientation and asymmetry.