Abstract
Summary
Nucleotomy almost doubles the transmitted forces on the facet joints in human lumbar spine, regardless of the amount of removed nucleus pulposus.
Introduction
Low back pain involves the lumbar facet joints in 15% to 45% of the cases. The surgical intervention, nucleotomy, might also lead to painful facets with a high risk; however, its mechanism is yet to be fully understood. The aim of this study is to reveal how a small amount of nucleus removal can change the force transmission on the facets.
1.1.1.1 Methods
Nine human lumbar motion segments with a median age of 48 years (interquartile range: 8.3) were tested quasi-statically (0.2 Hz) in extension and flexion (+/−5°) under constant compression loading (700 N). Specimens were tested in vitro by a servo-hydraulic test rig (MTS, USA) equipped with an additional superstructure enabling independent actuation of axial compression, extension-flexion and anterior-posterior shear on the top of the specimen. Forces and moments were measured beneath the specimen. The test method is based on recording intact and nucleotomy performed (range of removed nucleus 0.888–1.708 g) kinematics, which were applied in sequence, at different reduction stages of the same specimen. In the last steps, the facets were the only force transmitting structures, therefore, the load cell beneath the specimen revealed the force transmission on the facets.
1.1.2 Results
The average proportion of force transmitted through the facets increased significantly due to nucleotomy for both an intact capsular ligament (caps. lig.) (p=0.008) from 8.6% (5.2) to 15.8% (6.2), and without a caps. lig. from 8.4% (2.3) to 10.3% (4.9, p = 0.008). Amount of removed nucleus did not show statistically significant correlation to the force increase on facets due to nucleotomy (p>0.19). Forces on the facets were the highest in extension: fully intact 10.7% (6.6), nucleotomy with caps. lig. 19.0% (9.6), intact without caps. lig. 9.37% (3.42), and nucleotomy without caps. lig. 12.6% (6.5).
Discussion
Results show, that even a small amount of nucleus removal increase the transmitted forces on facet joints, independent of the amount of removed nucleus. Therefore, a surgeon's decision on the amount of nucleus to remove might perhaps be based on aspects other than facet loading - for example, the avoidance of reherniation. Since facet loading is highest in extension, patients after nucleotomy should perhaps limit this motion as far as possible.
