Facet joint orientation in spondylolysis and isthmic spondylolisthesis.

Study Design: The orientation of facet joints (FJs) in a normal population and isthmic spondylolisthesis (IS) population was assessed using magnetic resonance imaging in the lumbar spine.

Objective: To document the difference in FJ orientation (FJO) between a normal population and a population with spondylolysis of L5 and IS. SUMMARY OF BACKGROUND [corrected]

Data: Spondylolysis and IS have both a familial and mechanical etiology, yet the phenotypic expression of the familial etiology is unknown except for the observation of spinal bifida occulta. Other posterior element abnormalities are unrecognized, and any FJO abnormality below the pars defect has been ignored because of presumed previous mechanical defunctioning by the development of that pars defect at an earlier age. The recognition of multilevel sagittal FJO in L4/5 degenerative spondylolisthesis (DS), raises the possibility that more proximal segment examination may reveal FJ variations in IS.

Methods: Magnetic resonance imaging scans were used to measure the orientation of the FJ at L3/4, L4/5, and L5/S1 in 30 individuals with normal scans, and 30 patients with IS. The angular measurement recorded was in relation to the coronal plane. Repeated measurements confirmed the validity of the method.

Results: Mean measurement of axial FJO at L3/4 and L4/5 was 51.1 and 42.5 degrees in the controls, and 45.2 and 35.0 degrees in IS. The more coronal angulation at the levels above a pars defect in IS was highly statistically significant (P = <0.001 at L3/4 and P = <0.0001 at L4/5). At L5/S1, orientations were the same (39 degrees) in each group.

Conclusions: Relative coronal FJO in the lumbar spine may be the phenotypic expression of the familial etiology of IS. This may result in increased stress concentration in the pars between or below coronally oriented FJs. These more coronal FJOs in IS may also explain the common observation of retrolisthesis at L4/5 above IS when the L4/5 disc degenerates, lateral overhang of the L4/5 FJ to the L5 pedicle entry point above an IS, and the rare combination of DS at L4/5 and IS at L5/S1 when both disorders are separately common. This latter observation can be explained by the observation that DS occurs in those individual with sagittal lumbar facets, and that IS occurs in those with more coronal FJs.