Strain of the facet joint capsule during rotation and translation range-of-motion tests: an in vitro porcine model as a human surrogate

Abstract

BACKGROUND CONTEXTPrior data about the modulating effects of lumbar spine posture on facet capsule strains are limited to small joint deviations. Knowledge of facet capsule strain during rotational and translational intervertebral joint motion (ie, large joint deviations) under physiological loading could be useful as it may help explain why visually normal lumbar spinal joints become painful. PURPOSEThis study quantified the strain tensor of the facet capsule during rotation and translation range-of-motion tests. STUDY DESIGN/SETTINGStrain was calculated in isolated porcine functional spinal units. Following a preload, each specimen underwent a flexion/extension rotation (F/E) followed by an anterior/posterior translation (A/P) range-of-motion test while under a 300 N compression load. METHODSTwenty porcine spinal units (10 C3–C4, 10 C5–C6) were tested. Joint flexion/extension was imposed by applying a ±8 Nm moment at a rate of 0.5°/s, and translation was facilitated by loading the caudal vertebra with a ±400 N shear force at a rate of 0.2 mm/s. Points were drawn on the exposed capsule surface and their coordinates were optically tracked throughout each test. Strain was calculated as the displacement of the point configuration with respect to the configuration in a neutral joint position. RESULTSCompared to a neutral posture, superior-inferior strain increased and decreased systematically during flexion and extension, respectively. Posterior displacement of the caudal vertebra by more than 1.3 mm was associated with negative strains, which was significantly lower than the +4.6% strain observed during anterior displacement (p≥.199). The shear strain associated with anterior translation was, on average, −1.1% compared to a neutral joint posture. CONCLUSIONSThese results demonstrate that there is a combination of strain types within the facet capsule when spinal units are rotated and translated. The strains documented in this study did not reach the thresholds associated with nociception. CLINICAL RELEVANCEThe magnitude of flexion-extension rotation and anterior-translation may glean insight into the facet capsule deformation response under low compression (300 N) loading scenarios. Further, intervertebral joint motion alone, even under low compression loading, does not appear to initiate a clinically relevant pain response in the lumbar facet capsule of a nondegenerated spinal joint.