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Effect of the Increase in the Height of Lumbar Disc Space on Facet Joint Articulation Area in Sagittal Plane

Study Design. Computerized tomography (CT) of the lumbar spine cadaveric specimens was used to evaluate the effect of increasing the height of the disc space in the lumbar spine to the facet joint articulation in the sagittal plane. Objective. To show how the facet joint articulation is affected by increasing the height of the disc space in the lumbar spine. Summary of Background Data. The Charité Artificial Disc (DePuy Spine, Inc., Raynham, MA) was successful in relieving low back pain in the majority of patients, yet there was still a significant number of patients who did not obtain pain relief, or their pain even worsened. The etiology of their pain is still not known. To our knowledge, no study has addressed the effect on the facet joints when the disc height is increased. Methods. CT images passing through the center of the L3–S1 facet joints (sagittal plane) were obtained from 15 cadaveric lumbar spine specimens. The articulation overlap of facet joints in sagittal plane from the L3 to S1 was measured. A 1-mm incremental increase to a total 5 mm in disc space height was performed to simulate the changes seen in disc replacement. The change in the facet joint articulation overlap in sagittal plane at normal and each displacement was then measured. There were 5 lumbar spine specimens dissected to validate the technique and standardize the measurements. Mean, percentages, and standard deviation values were calculated for all measured dimensions. Results. No significant difference was found between the measurements on CT and gross specimens (P > 0.05). In 15 specimens, the mean facet joint articulation overlap on the sagittal plane was: 16.29 ± 1.20 mm (left) and 16.22 ± 1.16 (right) at the L3–L4 level; 17.81 ± 1.18 mm (left) and 17.74 ± 1.18 mm (right) at the L4–L5 level; and 18.18 ± 1.18 mm (left) and 18.23 ± 1.15 mm (right) at the L5–S1 level. There is no significant difference between the measured values on left and right sides (P > 0.05). Each 1-mm incremental increase in disc space at the L3–L4 level translated to a decrease in the facet joint articulation overlap in the sagittal plane by 6%, and the mean facet joint space increased 0.4 mm. At the L4–L5 level, the articulation overlap decreased by 6%, and the facet joint space increased 0.5 mm. At the L5–S1 level, the articulation overlap decreased by 4%, and the facet joint space increased 0.7 mm. Conclusions. There is a significant decrease of the facet joint articulation overlap in sagittal plane and an increase in the facet joint space following an increase in the lumbar disc space. The inappropriate increase of the height of disc space will result in facet joint subluxation.