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Open Access Highly Accessed Research article

Biomechanical analysis of the lumbar spine on facet joint force and intradiscal pressure - a finite element study

Ching-Sung Kuo*, Hsuan-Teh Hu, Ruey-Mo Lin, Kuo-Yuan Huang, Po-Chun Lin, Zheng-Cheng Zhong and Mu-Lin Hseih

BMC Musculoskeletal Disorders 2010, 11:151  doi:10.1186/1471-2474-11-151

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Lumbar facet joint asymmetric ��contralaterality��

Brian Sweetman   (2012-12-04 14:21)  NHS email

Dear Editor,

Kuo et al (1) used finite element modeling to study, amongst other things, contralateral facet joint forces induced by lumbar axial rotation. Unexpectedly, twisting to the left generated greater forces than when twisting to the right.

We also used the term ��contralateral��, but in a clinical context, to describe how twisting the body to the left can induce pain in a symptomatic right facet joint, and vice versa with the mirror-image effect occurring. When twisting the body the other way to the right, it can hurt on the left. This probably happens because the facet joints are behind the center of axial rotation in the spine. Thus, the posterior elements of the vertebra swivel in the opposite direction to the way the front of the patient twists. Their biomechanical studies use the term ��contralateral�� to refer to force magnitudes rather than the pain as in our clinical studies.

At the end of their article they also mentioned that rotation entrains an element of coupled lateral flexion that might warrant further studying. Our studies also suggested this, and indeed that lateral flexion may have had an even greater role in defining our clinical type of contralaterality than did rotation.

We would like to present some overlap evidence to suggest that these different discipline contralateralities have much the same implications. Our studies included cluster analyses, which were performed on 301 low back pain cases (2, 3), and discriminant analyses performed on a separate more recently prepared database of 1641 consultations (4).

It was found that our type of contralaterality was the most specific classification test for what we were subsequently to believe was the facet joint syndrome. Previously, researchers had found pain on lumbar extension, rotation or lateral flexion did not distinguish the facet joint anaesthetic injection responsive cases. This is because all the other syndromes also hurt with such back moves. These other syndromes induce pain that is felt on the same side as the direction of twist or lateral flexion (ipselateral) or that is felt in the midline or bilaterally equally on both sides of the back. If they had distinguished the contralaterality pain response with rotation or lateral flexion, they would, presumably, have demonstrated the anaesthetic responsive link with the facet joints.

Kuo et al. showed with their model that greater facet joint forces were induced contralateral to the direction of twist as shown in earlier studies. However, they were to find that the contralateral facet joint forces were even greater with left axial rotation than with right rotation. Thus, the right-sided facet joints ��suffered�� the greater forces. The contralaterality effects seemed unbalanced.

Our large database showed a similar disparity. Chi-squared analysis also indicated that this is particular to the facet joint syndrome. When the lumbar pain at rest hurts on one side (unilateral), it occurs in twice as many patients with pain on the right side of the back (52) as would have been expected from overall figures (25.8) and greater than occurred on the left (31). This preponderance would be elicited with left rotation (or left lateral flexion) bringing out right sided pain, which matches Kuo��s left rotation, causing greater forces particularly in the right sided facets. Similar findings of asymmetry in contralaterality, and that it is in the same direction, might suggest that such greater forces predispose to more facet joint pain problems on the right.

On inspection, our database showed that in those facet joint cases with pain on both sides, which is worse on one side than the other (lopsided), the imbalance is not so distinctive, though still in the same direction. Also, our earlier study with smaller numbers of facet joint cases (30) showed the imbalance in the opposite direction. This difference vanished at follow up three weeks later after the treatments. Were the left sided problems less painful? Most authors of the clinical facet joint syndrome studies, referred to above, have not included these kind of laterality statistics in their articles with the exception of Schwarze et al (5). Again, they only had small numbers of facet joint syndrome cases (26). This may explain why none of their variables showed a statistically significant relationship to ��double diagnostic injection blocks��. The best p value was for contralaterality though, once again, it was with right rotation, which is the opposite to our larger data base and the Kuo study. It is interesting to inspect these borderline statistics, but there is obviously a need for further work in this field.

The contralaterality defined facet joint syndrome showed a weak, though statistically significant, correlation with a number of clinical features. Those features were not nearly as useful diagnostically as rotation and lateral flexion contralaterality. The work of Kuo et al. goes some way towards validating our contention that contralaterality is the long sought after feature that identifies the facet joint syndrome at the initial clinical examination. Further, we hope our paradigm can supplement the clinical implications of the finite element models.

Sweetman BJ, Sweetman SJ

Email: bjsweetman@hotmail.com
Swansea, UK.

References
1. Kuo CS, Hu HT, Lin RM, Huang KY, Lin PC, Zhong ZC, Hseih ML. Biomechanical analysis of the lumbar spine on facet joint force and intradiscal pressure; a finite element study. BMC Musculoskeletal Disorders 2010 Jul 5; 11: 151 - 63.
2. Sweetman BJ. Numerical classification of common low back pain. MD thesis, London University, 1985.
3. Sweetman BJ. Low Back Pain; some real answers. tfm Publishing Ltd, Harley SY5 6LX, UK. 2005.
4. Sweetman SJ, Sweetman BJ. Database in preparation 2012.
5. Schwarzer AC, Aprill CN, Derby R, Fortin J, Kine G, Bogduk N. Clinical features of patients with pain stemming from the lumbar zygapophysial joints. Is the lumbar facet syndrome a clinical entity? Spine 1994; 19 (10):1132 -7.

Competing interests

None

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