Association of facet tropism with lumbar disc herniation

Abstract

Purpose: Facet tropism is defined as asymmetry between left and right facet joints and is postulated as a possible cause of disc herniation. In the present study, the authors used a 3-T MRI to investigate the association between facet tropism and lumbar disc herniation at a particular motion segment. They also examined whether the disc herniated towards the side of the more coronally oriented facet joint.

Methods: Sixty patients (18-40 years) with single level disc herniation (L3-L4, L4-L5, or L5-S1) were included in the study. Facet angles were measured using MRI of 3-T using the method described by Karacan et al. Facet tropism was defined as difference of 10° in facet joint angles between right and left sides. Normal disc adjacent to the herniated level was used as control. We also examined if disc herniated towards the side of more coronally oriented facet.

Results: Twenty-five herniations were at L4-L5 level and 35 at L5-S1. Statistical analysis was performed using the Fischer Exact Test. At L4-L5 level, 6/25 cases had tropism compared to 3/35 controls (p = 0.145). At L5-S1 level, 13/35 cases had tropism as compared to 1/21 controls (p = 0.0094). Of 19 cases having tropism, the disc had herniated towards the coronally oriented facet in six (p = 0.11).

Conclusion: The findings of the study suggest that facet tropism is associated with lumbar disc herniation at the L5-S1 motion segment but not at the L4-L5 level.

Fig. 1

Diagram of the measurement of facet tropism. A line is drawn between the two margins of each of the superior articular facets. The midsagittal line will pass through the centre of the disc (O) and the centre of the base of the spinous process. The angle between the facet line and midsagittal line was measured for each side of the spine. The difference of the right (a) and left (b) facet angles for each patient were calculated (AO = OB)

Fig. 2

a 39-year-old patient had a left sided disc herniation at L5–S1. The L4–L5 disc served as control. b The axial image shows the presence of facet tropism at the level of disc herniation, with the disc herniating towards the side of the sagittally-oriented facet. c The axial cut at the level of the L4–L5 disc (control) shows that the facets are symmetrical

Fig. 3

a 35-year-old male patient had a right sided herniated disc at L4–L5, the L5–S1 level served as control. Axial cuts b, c show symmetry at both the control and HNP levels

Fig. 4

During sagittal spinal movement (flexion–extension) the more coronally orientated (C) facet joint will restrict the movement of the joint in the anteroposterior plane (a) leading to automatic rotation (b) Thus tropism could lead to a combined movement in the spine whenever the articular facet glides on each other, ending with automatic rotation during spinal movement

Fig. 5

The weight of the body (P1 or P2) can be broken down into component of compression perpendicular to the intervertebral disc and a component of shearing (C1 or C2). The shearing force is small at the L4–L5 level and much greater at L5–S1