Biomechanics of Lumbar Intervertebral Disc Herniation: Difference between revisions

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== 1. Description ==
== 1. Description ==


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.1
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.<ref name=":0">Vital, J., & Cawley, D. (2020). Spinal Anatomy Modern Concepts (1st ed. 2020.). Springer International Publishing. https://doi.org/10.1007/978-3-030-20925-4</ref>


== 2. Definition ==
== 2. Definition ==
   
   
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.2 Due to the physiological shape of an intervertebral disc, protrusion commonly occurs at the posterior-lateral site which may protrude into the spinal canal.1 When repetitive stress, disc height loss, and overall stiffness occurs, 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.3 This results in reduction of stability and increased load bearing for the healthy lumbar arch to maintain.  
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.<ref name=":1">TAMPIER, C., DRAKE, J., CALLAGHAN, J., & MCGILL, S. (2007). Progressive Disc Herniation : An Investigation of the Mechanism Using Radiologic, Histochemical, and Microscopic Dissection Techniques on a Porcine Model. Spine (Philadelphia, Pa. 1976), 32(25), 2869–2874. <nowiki>https://doi.org/10.1097/BRS.0b013e31815b64f5</nowiki></ref> Due to the physiological shape of an intervertebral disc, protrusion commonly occurs at the posterior-lateral site which may protrude into the spinal canal.1 When repetitive stress, disc height loss, and overall stiffness occurs, 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.<ref name=":2">Nerurkar, N., Elliott, D., & Mauck, R. (2009). Mechanical design criteria for intervertebral disc tissue engineering. Journal of Biomechanics, 43(6), 1017–1030. https://doi.org/10.1016/j.jbiomech.2009.12.001</ref> This results in reduction of stability and increased load bearing for the healthy lumbar arch to maintain.  


== 3. Biomechanical Features ==
== 3. Biomechanical Features ==


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.
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.
3.1. Nucleus Pulposus
3.1. Nucleus Pulposus
The highly hydrated component known as the nucleus pulposus contains collagen type 2 fibers and proteoglycans.4 This component has isotropic properties allowing it to pressurize and withstand loads.3,4,5 In addition to these isotropic properties, biphasic properties are exhibited when shear forces are encountered.6 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.3 This resulting deformation can increase stress placed on the surrounding collagenous annulus fibers.3
 
The highly hydrated component known as the nucleus pulposus contains collagen type 2 fibers and proteoglycans.<ref name=":3">Iatridis, J., Nicoll, S., Michalek, A., Walter, B., & Gupta, M. (2013). Role of biomechanics in intervertebral disc degeneration and regenerative therapies: what needs repairing in the disc and what are promising biomaterials for its repair? The Spine Journal, 13(3), 243–262. https://doi.org/10.1016/j.spinee.2012.12.002</ref> This component has isotropic properties allowing it to pressurize and withstand loads.<ref name=":2" />,<ref name=":3" />,<ref>Wang, P., Wang, P., Yang, L., Yang, L., Hsieh, A., & Hsieh, A. (2011). Nucleus Pulposus Cell Response to Confined and Unconfined Compression Implicates Mechanoregulation by Fluid Shear Stress. Annals of Biomedical Engineering, 39(3), 1101–1111. https://doi.org/10.1007/s10439-010-0221-1</ref> In addition to these isotropic properties, biphasic properties are exhibited when shear forces are encountered.<ref name=":4">Johannessen, W., & Elliott, D. (2005). Effects of degeneration on the biphasic material properties of human nucleus pulposus in confined compression. Spine (Philadelphia, Pa. 1976), 30(24), E724–E729. https://doi.org/10.1097/01.brs.0000192236.92867.15</ref> 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.<ref name=":2" /> This resulting deformation can increase stress placed on the surrounding collagenous annulus fibers.<ref name=":2" />
 
3.2. Annulus Fibrosus  
3.2. Annulus Fibrosus  
Unlike the nucleus pulposus, the annulus fibrosus contains both anisotropic and non-linear properties and is composed of laminated collagen fibers.3,8 These fibres are located around the nucleus pulposus arranged in layers of lamallae.1 These layers alter depending on placement, on the posterior side the layers are thinner while on the anterior and lateral regions they are thicker.1,3,4 In addition, collagen type varies by region as type two and type one collagen are found in the innermost and outermost layers respectively.4 The annulus objective is to restrain the pressurized nucleus and prevent protrusion of nucleus content.3 To prevent this protrusion, the collagen fibers are oriented at either 30 or 60 degrees to the load axis, and between each lamellae they differ in orientation.6 These differing orientations increase the stiffness of the annulus to resist exposed tensile stress.9 These stiffer properties also differ by region with the anterior and superficial layers being stiffer than the posterior and inner layers respectively.7 In addition to being stiffer, the peripheral layers are also able to match the mechanical properties of the adjacent vertebrae.3, 7
3.2.1. Inter-lamellar mechanics
The inter-lamellar properties allow for the macroscopic annular mechanisms.,3,4 To examine the properties of the inter-laminar matrix, Gregory and Callaghan performed a peeling test showing the formation of clefts.10 Clefts are formed when the elastic property of the matrix is broken down due to a presented strain such as a disc prolapse.10 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.10
3.3. Vertebral Endplate
The vertebral endplate is the cartilaginous region preventing nucleus displacement to the superior or inferior vertebrae.1 It provides nutrient transport and mechanical strength to the intervertebral disc.3 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.11
3.4. Overall Mechanics
The intervertebral disc can deform to a degree, but after reaching a threshold, going outside its “neutral zone”, it will stiffen. 4 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 nucleus pulposus volume causing tension on the lamellae fibers 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.8, 10


== 4.Treatment ==
Unlike the nucleus pulposus, the annulus fibrosus contains both anisotropic and non-linear properties and is composed of laminated collagen fibers.<ref name=":2" />,<ref name=":5">Gregory, D., Bae, W., Sah, R., & Masuda, K. (2012). Anular delamination strength of human lumbar intervertebral disc. European Spine Journal, 21(9), 1716–1723. https://doi.org/10.1007/s00586-012-2308-x</ref> These fibres are located around the nucleus pulposus arranged in layers of lamallae.<ref name=":0" /> These layers alter depending on placement, on the posterior side the layers are thinner while on the anterior and lateral regions they are thicker.<ref name=":0" />,<ref name=":2" />,<ref name=":3" /> In addition, collagen type varies by region as type two and type one collagen are found in the innermost and outermost layers respectively.<ref name=":3" /> The annulus objective is to restrain the pressurized nucleus and prevent protrusion of nucleus content.<ref name=":2" /> To prevent this protrusion, the collagen fibers are oriented at either 30 or 60 degrees to the load axis, and between each lamellae they differ in orientation.<ref name=":4" /> These differing orientations increase the stiffness of the annulus to resist exposed tensile stress.<ref>Holzapfel, G., Holzapfel, G., Schulze-Bauer, C., Schulze-Bauer, C., Feigl, G., Feigl, G., Regitnig, P., & Regitnig, P. (2005). Single lamellar mechanics of the human lumbar anulus fibrosus. Biomechanics and Modeling in Mechanobiology, 3(3), 125–140. https://doi.org/10.1007/s10237-004-0053-8</ref> These stiffer properties also differ by region with the anterior and superficial layers being stiffer than the posterior and inner layers respectively.<ref name=":6">Galante, J. (1967). Tensile Properties of the Human Lumbar Annulus Fibrosus. Acta Orthopaedica, 38(S100), 1–91. https://doi.org/10.3109/ort.1967.38.suppl-100.01</ref> In addition to being stiffer, the peripheral layers are also able to match the mechanical properties of the adjacent vertebrae.<ref name=":2" />,<ref name=":6" />
4.1. Phases of healing 


Healing an intervertebral disc herniation is much like the healing of tendons.12 There are three stages of healing through rehabilitation therapy.  
3.2.1. Inter-lamellar mechanics


4.1.1. Initial inflammatory phase
The inter-lamellar properties allow for the macroscopic annular mechanisms.,<ref name=":2" />,<ref name=":3" /> To examine the properties of the inter-laminar matrix, Gregory and Callaghan performed a peeling test showing the formation of clefts.<ref name=":7">Gregory, D., & Callaghan, J. (2011). An examination of the mechanical properties of the annulus fibrosus: The effect of vibration on the intra-lamellar matrix strength. Medical Engineering & Physics, 34(4), 472–477. https://doi.org/10.1016/j.medengphy.2011.08.007</ref> Clefts are formed when the elastic property of the matrix is broken down due to a presented strain such as a disc prolapse.<ref name=":7" /> 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.<ref name=":7" />
During this phase, the affected individual is required to rest from all activity as initial recovery involves promotion of neovascularization and inflammatory mediators are utilized for tissue remodelling.12Another factor to enhance healing is proper spinal positioning to minimize pain within an individual’s range of motion.13 Individuals neglecting to rest 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.13 One technique includes hip hinging training which emphasizes the importance of ergonomics to protect the lumbar spine.13 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.  
4.1.2. Repair phase
The second phase of healing introduces controlled mobilization movements by placing tension on fibers so the collagen matrix is repaired by the fibroblast content.12 Cyclic micromovements are shown to aid in the healing process of the vertebral endplates.13 Performing passive isometric repetitive flexion of the spine promotes regeneration of the longitudinal ligament and outer annulus while spinal flexion when positioned on one side decrease the risk of protrusion of the nucleus by reducing spinal compressive loading.12,13 In addition to flexion, rotational mobilization promotes sliding of the interlamellar matrix and reduces scar tissue formation.12 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.12, 13
4.1.3. Remodelling phase
The final rehabilitation phase involves slow continuous progression into dynamic rotational movements to promote regeneration of the nucleus pulposus and annulus content.13 In preparation for daily activities, low loads are safely introduced alongside these dynamic movements.13 In addition, eccentric exercises are implemented to regenerate collagen content and redevelop biomechanical function of the intervertebral disc.12 According to a study completed by Gagne and Hasson, lumbar extension exercises are also beneficial when combined with mechanical traction therapy as they improve function and provide pain relief.14 Rehabilitation including stabilization and resistance training exercises increases muscle strength and proprioceptive abilities in the lumbar spine.15 Afterwards, when the individual is able to adequately move effectively and pain-free, the individual can slowly transition into returning to normal routine.
4.2. Surgical treatments
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.2,16,17 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.16 This involves injecting hydrogels and water-in-oil emulsion biomaterials to replace the components of the nucleus.4, 16 This strategy has shown to restore angular stiffness when exposed to cyclic flexion and extension.16 The replacement was also able to reform in shape, decreasing risk of herniation and degeneration.16
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.4 An adhesive and sutures are shown to withstand repetitive fatigue.4 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.4,18 Usage of these methods have been shown to imitate the same strength integrity of a healthy disc.


3.3. Vertebral Endplate


The vertebral endplate is the cartilaginous region preventing nucleus displacement to the superior or inferior vertebrae.<ref name=":0" /> It provides nutrient transport and mechanical strength to the intervertebral disc.<ref name=":2" /> 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.<ref>Rajasekaran, S., Bajaj, N., Tubaki, V., Kanna, R., & Shetty, A. (2013). ISSLS Prize Winner: The Anatomy of Failure in Lumbar Disc Herniation: An In Vivo, Multimodal, Prospective Study of 181 Subjects. Spine (Philadelphia, Pa. 1976), 38(17), 1491–1500. https://doi.org/10.1097/BRS.0b013e31829a6fa6</ref>


== References ==
3.4. Overall Mechanics
<references />
1. Vital, J., & Cawley, D. (2020). Spinal Anatomy Modern Concepts (1st ed. 2020.). Springer International Publishing. https://doi.org/10.1007/978-3-030-20925-4


2. TAMPIER, C., DRAKE, J., CALLAGHAN, J., & MCGILL, S. (2007). Progressive Disc Herniation : An Investigation of the Mechanism Using Radiologic, Histochemical, and Microscopic Dissection Techniques on a Porcine Model. Spine (Philadelphia, Pa. 1976), 32(25), 2869–2874. https://doi.org/10.1097/BRS.0b013e31815b64f5
The intervertebral disc can deform to a degree, but after reaching a threshold, going outside its “neutral zone”, it will stiffen. <ref name=":3" /> 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 nucleus pulposus volume causing tension on the lamellae fibers 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.<ref name=":5" />, <ref name=":7" />


3. Nerurkar, N., Elliott, D., & Mauck, R. (2009). Mechanical design criteria for intervertebral disc tissue engineering. Journal of Biomechanics, 43(6), 1017–1030. https://doi.org/10.1016/j.jbiomech.2009.12.001
== 4.Treatment ==
4.1. Phases of healing 


4. Iatridis, J., Nicoll, S., Michalek, A., Walter, B., & Gupta, M. (2013). Role of biomechanics in intervertebral disc degeneration and regenerative therapies: what needs repairing in the disc and what are promising biomaterials for its repair? The Spine Journal, 13(3), 243–262. https://doi.org/10.1016/j.spinee.2012.12.002
Healing an intervertebral disc herniation is much like the healing of tendons.<ref name=":8">Adams, M., Stefanakis, M., & Dolan, P. (2010). Healing of a painful intervertebral disc should not be confused with reversing disc degeneration: Implications for physical therapies for discogenic back pain. Clinical Biomechanics (Bristol), 25(10), 961–971. https://doi.org/10.1016/j.clinbiomech.2010.07.016</ref> There are three stages of healing through rehabilitation therapy.  


5. Wang, P., Wang, P., Yang, L., Yang, L., Hsieh, A., & Hsieh, A. (2011). Nucleus Pulposus Cell Response to Confined and Unconfined Compression Implicates Mechanoregulation by Fluid Shear Stress. Annals of Biomedical Engineering, 39(3), 1101–1111. https://doi.org/10.1007/s10439-010-0221-1
4.1.1. Initial inflammatory phase 


6. Johannessen, W., & Elliott, D. (2005). Effects of degeneration on the biphasic material properties of human nucleus pulposus in confined compression. Spine (Philadelphia, Pa. 1976), 30(24), E724–E729. https://doi.org/10.1097/01.brs.0000192236.92867.15
During this phase, the affected individual is required to rest from all activity as initial recovery involves promotion of neovascularization and inflammatory mediators are utilized for tissue remodelling.<ref name=":8" /> Another factor to enhance healing is proper spinal positioning to minimize pain within an individual’s range of motion.<ref name=":9">Vangelder, L., Hoogenboom, B., & Vaughn, D. (2013). A phased rehabilitation protocol for athletes with lumbar intervertebral disc herniation. International Journal of Sports Physical Therapy, 8(4), 482–516.</ref> Individuals neglecting to rest 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.<ref name=":9" /> One technique includes hip hinging training which emphasizes the importance of ergonomics to protect the lumbar spine.<ref name=":9" /> 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.  


7. Galante, J. (1967). Tensile Properties of the Human Lumbar Annulus Fibrosus. Acta Orthopaedica, 38(S100), 1–91. https://doi.org/10.3109/ort.1967.38.suppl-100.01
4.1.2. Repair phase 


8. Gregory, D., Bae, W., Sah, R., & Masuda, K. (2012). Anular delamination strength of human lumbar intervertebral disc. European Spine Journal, 21(9), 1716–1723. https://doi.org/10.1007/s00586-012-2308-x
The second phase of healing introduces controlled mobilization movements by placing tension on fibers so the collagen matrix is repaired by the fibroblast content.<ref name=":8" /> Cyclic micromovements are shown to aid in the healing process of the vertebral endplates.<ref name=":9" /> Performing passive isometric repetitive flexion of the spine promotes regeneration of the longitudinal ligament and outer annulus while spinal flexion when positioned on one side decrease the risk of protrusion of the nucleus by reducing spinal compressive loading.<ref name=":8" />,<ref name=":9" /> In addition to flexion, rotational mobilization promotes sliding of the interlamellar matrix and reduces scar tissue formation.<ref name=":8" /> 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.<ref name=":8" />, <ref name=":9" /> 


9. Holzapfel, G., Holzapfel, G., Schulze-Bauer, C., Schulze-Bauer, C., Feigl, G., Feigl, G., Regitnig, P., & Regitnig, P. (2005). Single lamellar mechanics of the human lumbar anulus fibrosus. Biomechanics and Modeling in Mechanobiology, 3(3), 125–140. https://doi.org/10.1007/s10237-004-0053-8
4.1.3. Remodelling phase 


10. Gregory, D., & Callaghan, J. (2011). An examination of the mechanical properties of the annulus fibrosus: The effect of vibration on the intra-lamellar matrix strength. Medical Engineering & Physics, 34(4), 472–477. https://doi.org/10.1016/j.medengphy.2011.08.007
The final rehabilitation phase involves slow continuous progression into dynamic rotational movements to promote regeneration of the nucleus pulposus and annulus content.<ref name=":9" /> In preparation for daily activities, low loads are safely introduced alongside these dynamic movements.<ref name=":9" /> In addition, eccentric exercises are implemented to regenerate collagen content and redevelop biomechanical function of the intervertebral disc.<ref name=":8" /> According to a study completed by Gagne and Hasson, lumbar extension exercises are also beneficial when combined with mechanical traction therapy as they improve function and provide pain relief. <ref>Gagne, A., & Hasson, S. (2010). Lumbar extension exercises in conjunction with mechanical traction for the management of a patient with a lumbar herniated disc. Physiotherapy Theory and Practice, 26(4), 256–266. https://doi.org/10.3109/09593980903051495</ref>Rehabilitation including stabilization and resistance training exercises increases muscle strength and proprioceptive abilities in the lumbar spine. <ref>Rhyu, H., Park, H., Park, J., & Park, H. (2015). The effects of isometric exercise types on pain and muscle activity in patients with low back pain. Journal of Exercise Rehabilitation, 11(4), 211–214. https://doi.org/10.12965/jer.150224</ref>Afterwards, when the individual is able to adequately move effectively and pain-free, the individual can slowly transition into returning to normal routine.


11. Rajasekaran, S., Bajaj, N., Tubaki, V., Kanna, R., & Shetty, A. (2013). ISSLS Prize Winner: The Anatomy of Failure in Lumbar Disc Herniation: An In Vivo, Multimodal, Prospective Study of 181 Subjects. Spine (Philadelphia, Pa. 1976), 38(17), 1491–1500. https://doi.org/10.1097/BRS.0b013e31829a6fa6
4.2. Surgical treatments 


12. Adams, M., Stefanakis, M., & Dolan, P. (2010). Healing of a painful intervertebral disc should not be confused with reversing disc degeneration: Implications for physical therapies for discogenic back pain. Clinical Biomechanics (Bristol), 25(10), 961–971. https://doi.org/10.1016/j.clinbiomech.2010.07.016
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.<ref name=":1" />,<ref name=":10">Balkovec, C., Vernengo, J., & McGill, S. (2013). The Use of a Novel Injectable Hydrogel Nucleus Pulposus Replacement in Restoring the Mechanical Properties of Cyclically Fatigued Porcine Intervertebral Discs. Journal of Biomechanical Engineering, 135(6), 61004–61005. https://doi.org/10.1115/1.4024285</ref>,<ref>Postacchini, F., & Postacchini, R. (2010). Operative Management of Lumbar Disc Herniation: The Evolution of Knowledge and Surgical Techniques in the Last Century. Advances in Minimally Invasive Surgery and Therapy for Spine and Nerves, 17–21. https://doi.org/10.1007/978-3-211-99370-5_4</ref> Surgical interventions of the intervertebral disc include replicating the mechanical functions of each physiological component of the disc.  


13. Vangelder, L., Hoogenboom, B., & Vaughn, D. (2013). A phased rehabilitation protocol for athletes with lumbar intervertebral disc herniation. International Journal of Sports Physical Therapy, 8(4), 482–516.
For the nucleus pulposus, physicians look to mimic the geometry and biomechanical function thereby restoring overall disc height.<ref name=":10" /> This involves injecting hydrogels and water-in-oil emulsion biomaterials to replace the components of the nucleus.<ref name=":3" />, <ref name=":10" /> This strategy has shown to restore angular stiffness when exposed to cyclic flexion and extension.<ref name=":10" /> The replacement was also able to reform in shape, decreasing risk of herniation and degeneration.<ref name=":10" /> 


14. Gagne, A., & Hasson, S. (2010). Lumbar extension exercises in conjunction with mechanical traction for the management of a patient with a lumbar herniated disc. Physiotherapy Theory and Practice, 26(4), 256–266. https://doi.org/10.3109/09593980903051495
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.<ref name=":3" /> An adhesive and sutures are shown to withstand repetitive fatigue.<ref name=":3" /> 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.<ref name=":3" />,<ref>Sharifi, S., Bulstra, S., Grijpma, D., & Kuijer, R. (2015). Treatment of the degenerated intervertebral disc; closure, repair and regeneration of the annulus fibrosus. Journal of Tissue Engineering and Regenerative Medicine, 9(10), 1120–1132. https://doi.org/10.1002/term.1866</ref> Usage of these methods have been shown to imitate the same strength integrity of a healthy disc. 


15. Rhyu, H., Park, H., Park, J., & Park, H. (2015). The effects of isometric exercise types on pain and muscle activity in patients with low back pain. Journal of Exercise Rehabilitation, 11(4), 211–214. https://doi.org/10.12965/jer.150224


16. Balkovec, C., Vernengo, J., & McGill, S. (2013). The Use of a Novel Injectable Hydrogel Nucleus Pulposus Replacement in Restoring the Mechanical Properties of Cyclically Fatigued Porcine Intervertebral Discs. Journal of Biomechanical Engineering, 135(6), 61004–61005. https://doi.org/10.1115/1.4024285


17. Postacchini, F., & Postacchini, R. (2010). Operative Management of Lumbar Disc Herniation: The Evolution of Knowledge and Surgical Techniques in the Last Century. Advances in Minimally Invasive Surgery and Therapy for Spine and Nerves, 17–21. https://doi.org/10.1007/978-3-211-99370-5_4
== References ==
 
<references />
18. Sharifi, S., Bulstra, S., Grijpma, D., & Kuijer, R. (2015). Treatment of the degenerated intervertebral disc; closure, repair and regeneration of the annulus fibrosus. Journal of Tissue Engineering and Regenerative Medicine, 9(10), 1120–1132. https://doi.org/10.1002/term.1866

Revision as of 18:03, 21 April 2021

1. 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.[1]

2. 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.[2] Due to the physiological shape of an intervertebral disc, protrusion commonly occurs at the posterior-lateral site which may protrude into the spinal canal.1 When repetitive stress, disc height loss, and overall stiffness occurs, 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.[3] This results in reduction of stability and increased load bearing for the healthy lumbar arch to maintain.

3. 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.

3.1. Nucleus Pulposus

The highly hydrated component known as the nucleus pulposus contains collagen type 2 fibers and proteoglycans.[4] This component has isotropic properties allowing it to pressurize and withstand loads.[3],[4],[5] In addition to these isotropic properties, biphasic properties are exhibited when shear forces are encountered.[6] 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.[3] This resulting deformation can increase stress placed on the surrounding collagenous annulus fibers.[3]

3.2. Annulus Fibrosus

Unlike the nucleus pulposus, the annulus fibrosus contains both anisotropic and non-linear properties and is composed of laminated collagen fibers.[3],[7] These fibres are located around the nucleus pulposus arranged in layers of lamallae.[1] These layers alter depending on placement, on the posterior side the layers are thinner while on the anterior and lateral regions they are thicker.[1],[3],[4] In addition, collagen type varies by region as type two and type one collagen are found in the innermost and outermost layers respectively.[4] The annulus objective is to restrain the pressurized nucleus and prevent protrusion of nucleus content.[3] To prevent this protrusion, the collagen fibers are oriented at either 30 or 60 degrees to the load axis, and between each lamellae they differ in orientation.[6] These differing orientations increase the stiffness of the annulus to resist exposed tensile stress.[8] These stiffer properties also differ by region with the anterior and superficial layers being stiffer than the posterior and inner layers respectively.[9] In addition to being stiffer, the peripheral layers are also able to match the mechanical properties of the adjacent vertebrae.[3],[9]

3.2.1. Inter-lamellar mechanics

The inter-lamellar properties allow for the macroscopic annular mechanisms.,[3],[4] To examine the properties of the inter-laminar matrix, Gregory and Callaghan performed a peeling test showing the formation of clefts.[10] Clefts are formed when the elastic property of the matrix is broken down due to a presented strain such as a disc prolapse.[10] 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.[10]

3.3. Vertebral Endplate

The vertebral endplate is the cartilaginous region preventing nucleus displacement to the superior or inferior vertebrae.[1] It provides nutrient transport and mechanical strength to the intervertebral disc.[3] 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.[11]

3.4. Overall Mechanics

The intervertebral disc can deform to a degree, but after reaching a threshold, going outside its “neutral zone”, it will stiffen. [4] 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 nucleus pulposus volume causing tension on the lamellae fibers 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.[7], [10]

4.Treatment[edit | edit source]

4.1. Phases of healing

Healing an intervertebral disc herniation is much like the healing of tendons.[12] There are three stages of healing through rehabilitation therapy.

4.1.1. Initial inflammatory phase

During this phase, the affected individual is required to rest from all activity as initial recovery involves promotion of neovascularization and inflammatory mediators are utilized for tissue remodelling.[12] Another factor to enhance healing is proper spinal positioning to minimize pain within an individual’s range of motion.[13] Individuals neglecting to rest 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.[13] One technique includes hip hinging training which emphasizes the importance of ergonomics to protect the lumbar spine.[13] 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.

4.1.2. Repair phase

The second phase of healing introduces controlled mobilization movements by placing tension on fibers so the collagen matrix is repaired by the fibroblast content.[12] Cyclic micromovements are shown to aid in the healing process of the vertebral endplates.[13] Performing passive isometric repetitive flexion of the spine promotes regeneration of the longitudinal ligament and outer annulus while spinal flexion when positioned on one side decrease the risk of protrusion of the nucleus by reducing spinal compressive loading.[12],[13] In addition to flexion, rotational mobilization promotes sliding of the interlamellar matrix and reduces scar tissue formation.[12] 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.[12], [13]

4.1.3. Remodelling phase

The final rehabilitation phase involves slow continuous progression into dynamic rotational movements to promote regeneration of the nucleus pulposus and annulus content.[13] In preparation for daily activities, low loads are safely introduced alongside these dynamic movements.[13] In addition, eccentric exercises are implemented to regenerate collagen content and redevelop biomechanical function of the intervertebral disc.[12] According to a study completed by Gagne and Hasson, lumbar extension exercises are also beneficial when combined with mechanical traction therapy as they improve function and provide pain relief. [14]Rehabilitation including stabilization and resistance training exercises increases muscle strength and proprioceptive abilities in the lumbar spine. [15]Afterwards, when the individual is able to adequately move effectively and pain-free, the individual can slowly transition into returning to normal routine.

4.2. Surgical treatments

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.[2],[16],[17] 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.[16] This involves injecting hydrogels and water-in-oil emulsion biomaterials to replace the components of the nucleus.[4], [16] This strategy has shown to restore angular stiffness when exposed to cyclic flexion and extension.[16] The replacement was also able to reform in shape, decreasing risk of herniation and degeneration.[16]

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.[4] An adhesive and sutures are shown to withstand repetitive fatigue.[4] 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.[4],[18] Usage of these methods have been shown to imitate the same strength integrity of a healthy disc.


References[edit | edit source]

  1. ↑ 1.0 1.1 1.2 1.3 Vital, J., & Cawley, D. (2020). Spinal Anatomy Modern Concepts (1st ed. 2020.). Springer International Publishing. https://doi.org/10.1007/978-3-030-20925-4
  2. ↑ 2.0 2.1 TAMPIER, C., DRAKE, J., CALLAGHAN, J., & MCGILL, S. (2007). Progressive Disc Herniation : An Investigation of the Mechanism Using Radiologic, Histochemical, and Microscopic Dissection Techniques on a Porcine Model. Spine (Philadelphia, Pa. 1976), 32(25), 2869–2874. https://doi.org/10.1097/BRS.0b013e31815b64f5
  3. ↑ 3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 Nerurkar, N., Elliott, D., & Mauck, R. (2009). Mechanical design criteria for intervertebral disc tissue engineering. Journal of Biomechanics, 43(6), 1017–1030. https://doi.org/10.1016/j.jbiomech.2009.12.001
  4. ↑ 4.0 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 Iatridis, J., Nicoll, S., Michalek, A., Walter, B., & Gupta, M. (2013). Role of biomechanics in intervertebral disc degeneration and regenerative therapies: what needs repairing in the disc and what are promising biomaterials for its repair? The Spine Journal, 13(3), 243–262. https://doi.org/10.1016/j.spinee.2012.12.002
  5. ↑ Wang, P., Wang, P., Yang, L., Yang, L., Hsieh, A., & Hsieh, A. (2011). Nucleus Pulposus Cell Response to Confined and Unconfined Compression Implicates Mechanoregulation by Fluid Shear Stress. Annals of Biomedical Engineering, 39(3), 1101–1111. https://doi.org/10.1007/s10439-010-0221-1
  6. ↑ 6.0 6.1 Johannessen, W., & Elliott, D. (2005). Effects of degeneration on the biphasic material properties of human nucleus pulposus in confined compression. Spine (Philadelphia, Pa. 1976), 30(24), E724–E729. https://doi.org/10.1097/01.brs.0000192236.92867.15
  7. ↑ 7.0 7.1 Gregory, D., Bae, W., Sah, R., & Masuda, K. (2012). Anular delamination strength of human lumbar intervertebral disc. European Spine Journal, 21(9), 1716–1723. https://doi.org/10.1007/s00586-012-2308-x
  8. ↑ Holzapfel, G., Holzapfel, G., Schulze-Bauer, C., Schulze-Bauer, C., Feigl, G., Feigl, G., Regitnig, P., & Regitnig, P. (2005). Single lamellar mechanics of the human lumbar anulus fibrosus. Biomechanics and Modeling in Mechanobiology, 3(3), 125–140. https://doi.org/10.1007/s10237-004-0053-8
  9. ↑ 9.0 9.1 Galante, J. (1967). Tensile Properties of the Human Lumbar Annulus Fibrosus. Acta Orthopaedica, 38(S100), 1–91. https://doi.org/10.3109/ort.1967.38.suppl-100.01
  10. ↑ 10.0 10.1 10.2 10.3 Gregory, D., & Callaghan, J. (2011). An examination of the mechanical properties of the annulus fibrosus: The effect of vibration on the intra-lamellar matrix strength. Medical Engineering & Physics, 34(4), 472–477. https://doi.org/10.1016/j.medengphy.2011.08.007
  11. ↑ Rajasekaran, S., Bajaj, N., Tubaki, V., Kanna, R., & Shetty, A. (2013). ISSLS Prize Winner: The Anatomy of Failure in Lumbar Disc Herniation: An In Vivo, Multimodal, Prospective Study of 181 Subjects. Spine (Philadelphia, Pa. 1976), 38(17), 1491–1500. https://doi.org/10.1097/BRS.0b013e31829a6fa6
  12. ↑ 12.0 12.1 12.2 12.3 12.4 12.5 12.6 Adams, M., Stefanakis, M., & Dolan, P. (2010). Healing of a painful intervertebral disc should not be confused with reversing disc degeneration: Implications for physical therapies for discogenic back pain. Clinical Biomechanics (Bristol), 25(10), 961–971. https://doi.org/10.1016/j.clinbiomech.2010.07.016
  13. ↑ 13.0 13.1 13.2 13.3 13.4 13.5 13.6 13.7 Vangelder, L., Hoogenboom, B., & Vaughn, D. (2013). A phased rehabilitation protocol for athletes with lumbar intervertebral disc herniation. International Journal of Sports Physical Therapy, 8(4), 482–516.
  14. ↑ Gagne, A., & Hasson, S. (2010). Lumbar extension exercises in conjunction with mechanical traction for the management of a patient with a lumbar herniated disc. Physiotherapy Theory and Practice, 26(4), 256–266. https://doi.org/10.3109/09593980903051495
  15. ↑ Rhyu, H., Park, H., Park, J., & Park, H. (2015). The effects of isometric exercise types on pain and muscle activity in patients with low back pain. Journal of Exercise Rehabilitation, 11(4), 211–214. https://doi.org/10.12965/jer.150224
  16. ↑ 16.0 16.1 16.2 16.3 16.4 Balkovec, C., Vernengo, J., & McGill, S. (2013). The Use of a Novel Injectable Hydrogel Nucleus Pulposus Replacement in Restoring the Mechanical Properties of Cyclically Fatigued Porcine Intervertebral Discs. Journal of Biomechanical Engineering, 135(6), 61004–61005. https://doi.org/10.1115/1.4024285
  17. ↑ Postacchini, F., & Postacchini, R. (2010). Operative Management of Lumbar Disc Herniation: The Evolution of Knowledge and Surgical Techniques in the Last Century. Advances in Minimally Invasive Surgery and Therapy for Spine and Nerves, 17–21. https://doi.org/10.1007/978-3-211-99370-5_4
  18. ↑ Sharifi, S., Bulstra, S., Grijpma, D., & Kuijer, R. (2015). Treatment of the degenerated intervertebral disc; closure, repair and regeneration of the annulus fibrosus. Journal of Tissue Engineering and Regenerative Medicine, 9(10), 1120–1132. https://doi.org/10.1002/term.1866
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