Journal of - Spinal Research Foundation

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SPINAL RESEARCH FOUNDATIONA. Agarwal, V. Kaul, A. Agarwal, V. Goel/The Journal of the Spinal Research Foundation 7(2012) 36–46Figure 2: Bi-level spinal segment showing normal bone (topvertebra), osteoporotic bone (middle vertebra) and a vertebralcompression fracture due to osteoporosis (bottom vertebra). Thetwo adjoining vertebrae along with the ligaments and the intervertebraldisc (consisting of gel-like nucleus pulposus surroundedby concentric layers of annulus fibrous) form a functional spinalunit– FSU. Modified and adapted from Clinica Neuros. (2010).Vertebroplasty. Retrieved July 27, 2012, from: http://neuros.net/en/vertebroplasty.phpIVD is somewhat cylindrical in shape and is madeup of three structures: annulus fibrosus (AF), nucleuspulposus (NP), and cartilaginous end plates (CEPs). NPis the central hydrated region of the IVD and developshydrostatic pressure due to the compressive stresses exertedon it. 7 NP is surrounded by laminated AF circumferentiallyand by CEPs at the cranial and caudal ends.AF and CEPs absorb the hydrostatic pressure developedby the NP and prevent it from protruding out to the adjoiningareas like the spinal canal. IVD allows bendingand rotation of the spine and helps in the transmissionof loads all the while dissipating some of the energy.The ligaments that stabilize the spine are passivestructures: anterior longitudinal ligament (ALL), posteriorlongitudinal ligament (PLL), ligamenta flava(LF), ligamentum nuchae (LN), interspinous ligament(ISL), intertransverse ligament (IL), and capsular ligament(CL). Apart from these, there are additional ligamentspresent in the upper cervical region. 8A functional spinal unit (FSU) consists of twoadjacent vertebrae, the intervertebral disc and all adjoiningligaments between them (Figure 2). It is thesmallest physiological motion segment of the spine toexhibit the key anatomical and biomechanical characteristicssimilar to those of the entire spine. 9Changes in Anatomy and Disease Manifestationwith AgeWith age, most of the spinal structures show degenerativechanges and some of these are described below:Trabecular Architecture and Cortical ThicknessIn a vertebral body, trabecular bone tends to becomemore rod-like and its thickness decreases withage. Also, there is an increase in the anisotropy (theresistance to forces is no longer in all directions) of thetrabecular structure. This increased anisotropy allowsincreased axial load carrying capacity. However, thereis a down side: increased vulnerability to fractures dueto off-axis impacts. 10Cortical thickness also changes with age and osteoporosis.In an osteoporotic spine, upper thoracicvertebral bodies have 15% thinner ventral corticalbone when compared with control specimens. Lowerthoracic and lumbar vertebral bodies also show a 30%decrease in the dorsal cortical thickness compared tocontrol. Even in a normal spine with no signs of osteoporosis,a significant decrease in the cortical thicknessbelow the T8 vertebral body has been found. 11Osteoporosis and Vertebral Compression Fractures (VCF)Osteoporosis is a condition of skeletal fragility inwhich bone strength gets reduced to the extent thatfractures occur with minimal force, often during routineactivities. Primary osteoporosis is classified intotwo major types: one related to menopausal estrogenloss (type-I) and the other to aging (type-II). Type Iosteoporosis signifies a loss of trabecular bone aftermenopause in women, while type II osteoporosis suggestsa loss of cortical and trabecular bone in both menand women as a result of age-related bone loss. Oneof the major predictors of osteoporosis is bone mineraldensity (BMD). On average, BMD reaches its peak at24-25 years of age and decreases thereafter. 12-1437 Journal of The Spinal Research Foundation FALL 2012 VOL. 7 No. 2
FALL 2012Spines in Motion: The Biomechanics of the SpineA VCF is the collapse of a vertebra due to traumaor a weakened vertebra in a subject with osteoporosis(Figure 2). VCFs are associated with pain and progressivevertebral body collapse resulting in spinal kyphosis.The initial acute pain can be incapacitating andmay become chronic in some cases. Chronic pain isa result of incomplete vertebral healing accompaniedby progressive vertebral body collapse, pseudarthrosisat the involved region, or altered spinal kinematics.Prolonged inactivity after a VCF can lead to furtherbone loss, muscle attrition, and an increased risk ofadditional fracture. 12Disc DegenerationIVDs undergo wear and tear with age, which is exacerbatedby lower nutrition supply and waste exchange(Figure 3). 15, 16 Intervertebral disc degeneration (IDD)begins as early as 20 years of age, and autopsies suggestthat 97% of individuals over 50 years of age have discdegeneration. 17-20 Manifestation of degeneration beginswith decrease in the proteoglycan content of the NP andhence its water content, which results in reduced intradiscalpressure and height. Radial tears, fissures, protrusions,osteophyte formations, facet joint arthritis, andSchmorl’s nodes (IVD protrusions into the vertebralbody) may form along with pain, worsening the degreeof degeneration. 15, 21 Disc degeneration is classified interms of grades ranging from grades I to V: Grade I refersto disc with homogeneous and distinct NP and AF,with normal disc height. On the other extreme, grade Vrefers to inhomogeneous and indistinct NP and AF, withcollapsed disc space. 22Annular Tear and Disc HerniationAnnular tear mostly accompanies IDD, but itcould also occur early in the cascade due to strenuousactivities (Figure 3). This condition leads to extrusionof disc nucleus into the spinal canal, leading to herniation.Confined herniation occurs from the protrusionof nucleus pulposus, with outermost layer of AF remainingintact. 23, 24Figure 3: A diagram showing concentric and radial annular tearswith nucleus pulposus extruding into spinal canal causing discherniation. Modified and adapted from Lynn Kerew ChiropracticCorporation. (2012). Annular Fissures: Definition and Treatment.Retrieved July 27, 2012, from: http://lynnkerewchiropractic.com/blog/annular-fissures-definition-and-treatmentSpinal Stenosis, Osteoarthritis, and SpondylolisthesisFacet joints undergo degeneration due to overuseor high loading conditions. Typically, facet degenerationstarts with degeneration of cartilage, leadingto synovial space inflammation and joint space narrowing,ultimately resulting in osteophyte formation.This may lead to stenosis or spondylolisthesis. 25-27 Ithas been suggested that disc degeneration precedesfacet degeneration. 26, 28, 29Hyaline cartilage traverses the synovial joint surfaceof a facet joint and is critical for its proper function.Its loss is a common finding with age. Radiographically,this can be seen as reduction in joint spaceon magnetic resonance imaging (MRI). In most cases,this loss is accompanied by a variable amount of inflammatorydestruction of the cartilage and synoviallined joint capsule, known as inflammatory pannus.Depending on the severity and duration of the disease,the cartilage may be completely lost and the barebones may start to rub against each other, leading toeburnation, i.e. new bone formation. It is achieved byincreasing its surface area through osteophyte formation(Figure 4). This is called osteoarthritis. 30Spondylolisthesis is the slippage of one vertebrarelative to the adjacent vertebra, and it often resultsfrom asymmetric degeneration of either the IVD orthe facet joints. The imbalance in stresses, due toasymmetric degeneration of these joints, can lead toFALL 2012 VOL. 7 No. 2Journal of The Spinal Research Foundation 38
Page 2 and 3: SPINAL RESEARCH FOUNDATIONTHE JOURN
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Page 7 and 8: FALL 2012Issue OverviewMarcus M. Ma
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