Biomechanics of Lumbar Intervertebral Disc Herniation
Original Editors -Melanie Hsu as part of The University of Waterloo Clinical Biomechanics Project
Description[edit | edit source]
A human lumbar spine has evolved over many centuries to allow for weight bearing properties, however it has limited mobility compared to other segments of the spine. Lumbar spinal segments are composed of intervertebral discs, which are fibrocartilaginous joints that have biomechanical limitations, but still allow for everyday activities. The intervertebral disc is composed of three components: the annulus fibrosus, the nucleus pulposus, and the vertebral endplates. These three components allow for mobility, shock absorption, and load distribution properties in a healthy lumbar spine.
Definition[edit | edit source]
There are various types of degenerative disc disease, however a common degenerative condition is an intervertebral disc herniation. A herniation occurs when the nucleus pulposus content protrudes through the lamellae of the annulus fibrosus, resulting in a displacement of the disc outside of the intervertebral space. Due to the physiological shape of an intervertebral disc, protrusion commonly occurs at the posterior-lateral site which may protrude into the spinal canal. When the nucleus content begins protruding out this results in stress on the collagen fibers, which when increased, can initiate delamination of the annulus and loss of proper distribution of the vertebral endplates. This results in reduction of stability and increased load bearing for the healthy lumbar arch to maintain.
Biomechanical Features[edit | edit source]
Each individual component of the intervertebral disc has different biomechanical features. It is important to understand how each function and how they are affected by degeneration.
Nucleus Pulposus[edit | edit source]
The highly hydrated component known as the nucleus pulposus contains collagen type 2 fibers and proteoglycans. This component has isotropic properties allowing it to pressurize and withstand loads. In addition to these isotropic properties, biphasic properties are exhibited when shear forces are encountered. Also, to support axial spinal loads, other components of the disc, the end-plate cartilage and annulus fibrosis can increase the intra-discal pressure of the nucleus pulposus. This resulting deformation can increase stress placed on the surrounding collagenous annulus fibers.
Annulus Fibrosus[edit | edit source]
Unlike the nucleus pulposus, the annulus fibrosus contains both anisotropic and non-linear properties and is composed of laminated collagen fibers. These fibres are located around the nucleus pulposus arranged in layers of lamallae. These layers alter depending on placement, on the posterior side the layers are thinner while on the anterior and lateral regions they are thicker. In addition, collagen type varies by region as type two and type one collagen are found in the innermost and outermost layers respectively. The annulus objective is to restrain the pressurized nucleus and prevent protrusion of nucleus content. To prevent this protrusion, the collagen fibres are oriented at either 30 or 60 degrees to the load axis, and between each lamellae they differ in orientation. These differing orientations increase the stiffness of the annulus to resist exposed tensile stress. These stiffer properties also differ by region with the anterior and superficial layers being stiffer than the posterior and inner layers respectively. In addition to being stiffer, the peripheral layers are also able to match the mechanical properties of the adjacent vertebrae.
Inter-lamellar mechanics[edit | edit source]
The inter-lamellar properties allow for the macroscopic annular mechanisms. To examine the properties of the inter-laminar matrix, Gregory and Callaghan performed a peeling test showing the formation of clefts. Clefts are formed when the elastic property of the matrix is broken down due to a presented strain such as a disc prolapse. The elastic property contained in the matrix helps tightly bind the lamellae together, however when the elastic property is loosened this increases the chance of a disc herniation.
Vertebral Endplate[edit | edit source]
The vertebral endplate is the cartilaginous region preventing nucleus displacement to the superior or inferior vertebrae. It provides nutrient transport and mechanical strength to the intervertebral disc. While placed under stress, it contains pressure dispersing properties and is responsible for controlling disc movement. Endplate failures can include rim fractures and plate avulsions that have been linked to disc herniations.
Overall Mechanics[edit | edit source]
The intervertebral disc can deform to a degree, but after reaching a threshold, going outside its “neutral zone”, it will stiffen.  This translates to how a human lumbar spine contains limited mobility to prevent spinal instability. A healthy functioning disc, when experiencing an applied load, increases intradiscal pressured measured inside the nucleus pulposus causing tension on the lamellae fibres supporting the compressive loads. The pressurization force caused by the nucleus pulposus is then transmitted to the vertebral endplates. Therefore, experiencing an intervertebral disc herniation decreases how much the disc can normally withstand., 
Treatment[edit | edit source]
Phases of healing[edit | edit source]
Healing an intervertebral disc herniation is much like the healing of tendons. There are three stages of healing through rehabilitation therapy.
Initial inflammatory phase[edit | edit source]
During this phase, joint protection is recommended as initial recovery involves promotion of neovascularization and inflammatory mediators are utilized for tissue remodelling. Another factor to enhance healing is proper spinal positioning to minimize pain within an individual’s range of motion. Individuals neglecting to rest the injured region can thus prolong the inflammatory phase and delay the initial healing process. In terms of therapeutic care, stabilization and balance focused treatments are prioritized early as stabilization exercises can provide symptom relief and management for afflicted individuals. One technique includes hip hinging training which emphasizes the importance of ergonomics to protect the lumbar spine. Promoting these fundamental movement patterns will aid in maintaining the natural curvature of the lumbar region throughout the complete motion without placing additional strain on other extremities.
Repair phase[edit | edit source]
The second phase of healing introduces controlled mobilization movements by placing tension on fibres so the collagen matrix is repaired by the fibroblast content. Cyclic micromovements are shown to aid in the healing process of the vertebral endplates. Performing isometric exercises of the spine flexors promotes regeneration of the longitudinal ligament and outer annulus. Passive rotational mobilization performed by trained professional may promote sliding of the interlamellar matrix and reduces scar tissue formation. Alongside these treatments, therapists can perform manual therapy to re-educate muscle movements and promote pain reduction as these can lower stress placed on the intervertebral disc. 
Remodelling phase[edit | edit source]
The final rehabilitation phase involves slow continuous progression into dynamic rotational movements to promote regeneration of the nucleus pulposus and annulus content. In preparation for daily activities, low loads are safely introduced alongside these dynamic movements. In addition, eccentric exercises are implemented to regenerate collagen content and redevelop biomechanical function of the intervertebral disc. According to a study by Gagne and Hasson, lumbar extension exercises are also beneficial when combined with mechanical traction therapy as they improve function and provide pain relief. Rehabilitation including stabilization and resistance training exercises increases muscle strength and proprioceptive abilities in the lumbar spine. Afterwards, when the individual is able to adequately move effectively and pain-free, they can slowly transition into returning to a normal routine.
Surgical treatments[edit | edit source]
For more severe cases, surgical treatments may be implemented to regain normal function of the disc. These cases are considered if all non-operative measures have not improved the overall treatment. Surgical treatments are most commonly microdiscectomies as they have become the gold standard, due to their low complication and high satisfaction rates. Surgical interventions of the intervertebral disc include replicating the mechanical functions of each physiological component of the disc.
For the nucleus pulposus, physicians look to mimic the geometry and biomechanical function thereby restoring overall disc height. This involves injecting hydrogels and water-in-oil emulsion biomaterials to replace the components of the nucleus.  This strategy has been shown to restore angular stiffness when exposed to cyclic flexion and extension. The replacement was also able to reform in shape, decreasing risk of herniation and degeneration.
Closure of the annulus fibrosus is an essential component for proper function of the intervertebral disc. This closure utilizes a bioactive adhesive to reconcile micro gaps and an anchor for load support. An adhesive and sutures are shown to withstand repetitive fatigue. Physicians have also performed annulus fibrosus replacements using scaffolds containing porous matrices drawn from other biological materials such as glycosaminoglycans to promote proliferation and cell adhesion. Usage of these methods have been shown to imitate the same strength integrity of a healthy disc.
References[edit | edit source]
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