Nucleotomy Alters Mechanical Function Following Cyclic Loading and Unloaded Recovery of Human Discs

The intervertebral disc plays a critical role in supporting loads, permitting spinal motion, and dissipating energy. Unfortunately, it is also commonly degenerated, resulting in altered spinal mechanics and low back pain. Nucleotomy is a common treatment for herniated discs and is also used experimentally to simulate degeneration.[1] The procedure, which involves a posterior annular incision and removal of a portion of the nucleus pulposus (NP), has also been shown to alter disc mechanics. These changes include acute changes of decreased NP pressure, decreased disc height, and increased neutral zones.[2, 3] Cyclic studies have shown that trans-endplate nucleotomy permanently alters creep mechanical properties of sheep discs.[4] However, the effects of annular nucleotomy on the cyclic properties of human discs have not yet been studied. This work studied the mechanical effect of annular nucleotomy on human discs subjected to physiological axial cyclic loading.

Plane of vertebral movement eliciting muscle lengthening history in the low back influences the decrease in muscle spindle responsiveness of the cat

Proprioceptive feedback is thought to play a significant role in controlling both lumbopelvic and intervertebral orientations. In the lumbar spine, a vertebra's positional history along the dorsal-ventral axis has been shown to alter the position, movement, and velocity sensitivity of muscle spindles in the multifidus and longissimus muscles. These effects appear due to muscle history. Because spinal motion segments have up to 6 degrees of freedom for movement, we were interested in whether the axis along which the history is applied differentially affects paraspinal muscle spindles. We tested the null hypothesis that the loading axis, which creates a vertebra's positional history, has no effect on a lumbar muscle spindle's subsequent response to vertebral position or movement. Identical displacements were applied along three orthogonal axes directly at the L6 spinous process using a feedback motor system under displacement control. Single-unit nerve activity was recorded from 60 muscle spindle afferents in teased filaments from L6 dorsal rootlets innervating intact longissimus or multifidus muscles of deeply anesthetized cats. Muscle lengthening histories along the caudal-cranial and dorsal-ventral axis, compared with the left-right axis, produced significantly greater reductions in spindle responses to vertebral position and movement. The spinal anatomy suggested that the effect of a lengthening history is greatest when that history had occurred along an axis lying within the anatomical plane of the facet joint. Speculation is made that the interaction between normal spinal mechanics and the inherent thixotropic property of muscle spindles poses a challenge for feedback and feedforward motor control of the lumbar spine.

Lumbar Spinal Mechanics in Pure Bending: Influence of Gender, Spinal Level, and Degeneration Grade

Gender differences have been identified in normal and traumatic motions of the spine. In the cervical region, spinal motions in females were significantly greater than in males during identical dynamic acceleration pulses [1]. Static cervical range of motion was also shown to be greater in female volunteers [2]. In the thoracic region, gender differences were identified in compressive and tensile elastic moduli [3]. Although male volunteers had slightly greater lumbar spine mobility, the difference was not statistically significant [4]. Another study reported that female lumbar specimens were somewhat more flexible than male specimens [5]. Lumbar spinal motions are clinically important in the diagnosis of abnormalities and instability. Increased motions occur secondary to instability and may indicate a need for spinal stabilization. However, although previous studies have provided baseline data for lumbar motions [6], possible variations in spinal motions between males and females may lead to inaccurate diagnosis. Therefore, the purpose of this investigation was to define lumbar spinal motions on a level-by-level basis to determine statistically significant differences between males and females and at varying levels of degeneration.

Does Placement of the Axis of Rotation of the Cervical Spine Affect Motion Segment Mechanics During Flexion and Extension?

Current in vitro testing methodologies remain limited in the ability to explore spinal mechanics. The gold standard of flexibility testing has traditionally focused only on evaluating rotational components of motion within a motion segment unit (MSU). While such data may be applied towards evaluation of the Center of Rotation (CoR) of a joint, many systems lack the needed sensitivity. The result is that there is currently no consensus on the location of the CoR of the spine. Further, very limited data or insight can be gathered as to the precise kinematic or dynamic state of the MSU, the influence of surgically implanted motion restoration devices, or the influence of subtle changes to an implanted device.

Quantitative morphology of the human and porcine mid-lumbar interspinous ligament

SummaryAnimal models are essential in spine research for evaluating implants and for studying spinal mechanics. Several studies have compared the geometrical characteristics of animal and human vertebrae, but few studies have compared the structure of the spinal ligaments. The purpose of this study was to systematically quantify the collagen fibre orientation of the porcine and human interspinous ligament and thereby allow clearer interpretation of function. Human and porcine lumbar spine segments were loaded with a 10 Nm pure-flexion moment and chemically fixed. The sagittal plane collagen fibre orientation in the mid-lumbar interspinous ligaments was quantified by examining histological sections using a plane-polarized light macroscope and custom analysis software. The specimens showed collagen fibres in a posterior-cranial orientation originating from the superior aspect of the spinous process of the inferior vertebra and merging into the supraspinous ligament. There were not any statistically significant differences in interspinous ligament collagen fibre orientation between the human and porcine specimens. The middle and ventral spaces between the spinous processes of the human specimens contained loose disorganized collagen, skeletal muscle, and voids. The main load-bearing component of porcine and human interspinous ligament at the midlumbar level appears to be the dorsal portion, which is oriented at approximately 77-79 degrees with respect to the mid-disc plane. This dorsal aspect has a long moment arm and therefore is well suited to prevent excessive flexion. The similarity of the interspinous ligament morphology suggests that the porcine lumbar spine is a good model of the human lumbar spine.

SIGNIFICANCE OF THE ANNULUS PROPERTIES TO FINITE ELEMENT MODELING OF INTERVERTEBRAL DISCS

Current mathematical material laws work quite accurately with conventional engineering materials because they are linear and isotropic. These laws are much less effective, however, at representing living tissues. Biomechanical engineers therefore often face the problems of modelling material non-linearities, particularly with the soft tissues, muscles and tendons of the human body. The non-linear and often anisotropic structure of these materials makes any mathematical representation very difficult. In particular, the lack of a good general mathematical model of the intervertebral disc has hampered the study of spinal mechanics, as the disc annulus is a nonlinear fibrous tissue with highly directional properties. Previous studies have concentrated on the overall behavior of discs and this has been largely explained but knowledge is still very limited on the effect of the individual disc components. This means that current mathematical models are poor when it comes to describing a prolapsed disc, where there has been failure of at least one of the components. This finite element study described here focuses on the disc annulus properties and their effect on disc behavior. The novelty of this study lies in the material formulation of the annulus fibrosus, which changes its Young's modulus according to a non-linear curve.