The biomechanical effects and clinical perspective of aging on the vertebral column
The extent of aging within the musculoskeletal system during the life course affects the quality and length of life (Roberts et al., 2016). The primary features of an aging spine are loss of trabecular structure in the bones, damaged articular cartilage and narrowing of the intervertebral discs and together cause pain and mobility loss (Roberts et al., 2016). Age-related bone loss and osteoporosis in the elderly population increase the risk of fractures.
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The spinal column is made up of 33 vertebrae – 7 in the cervical region, 12 in the thoracic region, 5 in the lumbar region, 5 in the sacral region and 4 in the coccygeal region. The functional spine unit (FSU) consists of two vertebrae, fibrocartilagenous disc, facet joints, and the spinal ligaments. The biomechanical purpose of the spinal system is to allow trunk movement, provide structural support and protect neural elements (Oxland, 2016). Ferguson and Steffen (2003) stated that between adjacent vertebrae there is very little movement possible, but the totality of these movements throughout the spine increases mobility in all major planes.
Normal aging of the spine causes spinal degeneration of the vertebral body, the intervertebral disc, and the surrounding structures altering the motion segment (Ferguson and Steffen, 2003). As biomechanical properties of the spine alter, changes occur in the stress-strain relationship and the translation of the instantaneous axis of rotation (IAR) from the usual position (Iorio, Jakoi and Singla, 2016).
Aging of the vertebral bodies
The vertebral body derives its significant compressive strength from its trabecular bone. According to Keaveny et al (2001), the trabecular bone is the spongy, porous type of bone found in the spine and at the end of other long bones.
Giambini et al., (2014) stated that the main cause of alteration in the trabecular architecture and loss of bone minerals was osteoporosis. Reduction in daily loading leads to rapid bone loss, and disuse alone may result in osteoporosis, furthermore leading to fractures.
As a bone age, the trabeculae can be lost by two mechanisms; biological and mechanical overload.
Biological overload- Due to age-related reduction in bone formation, some trabeculae become thin leading to osteoclasts reabsorbing the vertical trabeculae and breaking the trabecula. The horizontal trabeculae get absorbed but do not get replaced leading to loss of bracing effect on the vertical trabeculae. During menopause, women have less oestrogen and thus leads to early loss of trabeculae (Gong et al., 2010).
Mechanical overload- Due to high local stress on the spine, the trabeculae are fractured and results in disruption of the trabecular network (Badiei, Bottema and Fazzalari, 2007).
A study measuring the kinematics of the lumbar spine in the elderly with decreased bone mass density combined skin-mounted sensor data with radiographic data to investigate the new aspects of lumbar kinematics in subjects with different bone mass density. In this study, subjects with osteopenia or osteoporosis did not have the same predictable motion patterns compared to those with normal bone density. It was therefore concluded that changes in kinematic behaviors were related to morphological changes as well as altered neuromuscular functions (Ma et al., 2009).
Aging of the facet joint
A study by Morimoto et al., (2019) concluded that the angles of facet decreased with age at L4/5 and L5/S1 in women in the axial plane and at L4/5 in men and L3/4 and L4/5 in women in the sagittal plane.
The only synovial joints in the spine are the facet joints, with hyaline cartilage covering the subchondral bone (Benoist, 2003). Biomechanically the facet joints provide posterior load-bearing assistance, stabilizing the motion segment in flexion and extension, blocking axial rotation and forward sliding of the lumbar vertebrae, and also protecting the disc from extreme torsion (Iorio, Jakoi and Singla, 2016).
Abbas et al., (2011) mentioned that it is well recognized that a facet joint arthrosis is an age-dependent event, and osteophytes form as an adaptive alteration to stresses in the facet joint capsule. These marginal osteophytes could impinge on intervertebral foramen and foramen transversarium. Recurrent episodes of facet strain may predispose to degenerative changes (Kalichman and Hunter, 2007).
It is usually believed that degenerative changes of the facets are a result of disk degeneration (Benoist, 2003). Facet deterioration could damage the surrounding spinal structures, such as the intervertebral disc, nerve roots exiting the spinal column and the spinal cord leading to pain (Jaumard, Welch and Winkelstein, 2011).
Aging of the intervertebral disc
The intervertebral discs are composed of a central highly hydrated, gel-like tissue called the nucleus pulposus (NP). Each nucleus pulposus is surrounded by a highly oriented annulus fibrosus (AF). Both tissues are encircled between the vertebral endplates (VEP) (Roberts et al., 2016).
According to Chaffin and Ashton-Miller, (1991) disc degeneration (spondylosis) normally starts in the second decade of life and continues gradually thereafter. Degenerative changes are promoted by the placement of large loads on the spine. Microscopic deterioration begins in the nucleus and spreads to the annulus. By the fifth decade, 97% of all lumbar discs show microscopic degeneration, with changes being evident earlier in males than in females (Chaffin and Ashton-Miller, 1991).
Age-related biochemical changes, causes water and proteoglycan concentrations to reduce in the nucleus pulposus in degenerated intervertebral discs leading to a fall in pressure of the nucleus (An, Masuda, and Inoue, 2006). Additionally, it was found that the sagittal diameter of the nucleus pulposus reduced nearly by 50% with age (Newell et al., 2017).
Research by Ferguson and Steffen, (2003) stated that based on the measurements of the viscoelasticity of human nucleus pulposus, the changes in the mechanical properties proposed a shift from a fluid-like behavior to a more solid like behavior with degeneration.
Newell et al., (2017) declared that with increasing age, the vertebral endplates begin to calcify and may cause disc degeneration as permeability and metabolite transport reduce. As the spinal segments fuse, the stress-strain curve changes: the elastic zone becomes steeper than usual, demonstrating the incompressibility of the fused segment, requiring a larger load to produce slight deformation (Iorio, Jakoi and Singla, 2016).
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The vertebral endplate gets deformed by micro-fracture and bows into the vertebral body, causing the superior and inferior surfaces to concave. The vertebral endplate is more susceptible to compression fractures (Trout et al., 2006). As one ages the lumbar disc undergoes different pathological conditions, they are classified as disk bulge, disc herniation, disc thinning, and disc degeneration with osteophyte formation (Gallucci et al., 2004).
Aging of ligaments and muscles
The firmness of segments is further strengthened by facet joints, anterior longitudinal ligament, posterior longitudinal ligament, supraspinous ligament and other ligamentous structures (Roh et al., 2005).
As one ages, the ligaments undergo macroscopic and chemical changes. The concentration of elastin increases, causing the tensile properties of the ligaments to decrease resulting in ligamentous weakening, affecting the stabilizing function of the longitudinal ligaments (Benoist, 2003). Increased stiffness of the ligaments could also influence the mechanical strain and disc stability (Vo et al., 2016).
Additionally, Benoist, (2003) explained that degeneration and aging of the ligamentum flavum leads to enhanced thickening and bulging, and is often unveiled during surgery of spinal stenosis.
Age-driven changes in the spinal muscles such as infiltration and fatty deposits affect the overall stability of the spine (Vo et al., 2016). Lumbar extensors tend to decline in their muscle strength, leading to a reduction of 50% between the third and sixth decades of life (Kasukawa et al., 2017).
According to Roh et al., (2005) nerve endings have been described surrounding intervertebral discs within the outer one-third of the annulus fibrosus. Anterior longitudinal ligaments, posterior longitudinal ligaments, and facet capsules have also been shown to have nerve endings, therefore these innervations may provide some evidence for an anatomic etiology of discogenic back pain (Roh et al., 2005).
Pain is present when a spinal component encounters an injury, in addition, the motion may also be changed due to degenerative changes. In such a case the neutral zone (NZ) increases, and in the ‘ball-in-a-bowl’ analogy by Panjabi (2003), the ball moves freely over a larger distance, beyond the pain-free zone. The spinal stability system then reacts actively to reduce the neutral zone by activating muscles or over time by adaptive bracing of the spinal column such as the formation of osteophytes (Panjabi, 2003).
Neurogenic pain is caused by compression of either the foraminal nerve root or the dorsal root ganglion or central spinal stenosis, leading to neurogenic claudication. It is, however, important to differentiate between neurogenic claudication symptoms and vascular claudication symptoms (Ploumis, Transfledt, and Denis, 2007). The pain caused by neurogenic claudication reduces when the patient flexes forward or when sitting. The pain in vascular claudication is released by standing and is often accompanied by skin changes and peripheral vascular loss (Ploumis, Transfledt, and Denis, 2007).
Three common classifications of back pain are:
- Mechanical pain or axial pain is described as sharp or dull, constant or comes and goes. Muscle strain is a common cause of mechanical pain.
- Referred pain which is often characterized as dull and achy, and tends to move around and vary in intensity. Degenerative disc disease is an example of referred pain as pain moves toward the hips and posterior thigh.
- Radicular pain is often described as searing and deep pain, and follows the path of the nerve into the arms or legs and may be accompanied by numbness and weakness. This type of pain is usually caused by compression, injury to spinal nerve root or inflammation. Conditions causing such pain are herniated disc, spinal stenosis or spondylolisthesis (O’Sullivan, 2005; Treede et al., 2015).
A biomechanical experiment carried out by Panjabi, (2003) where fixators were used on the lumbar spine, with the aim to stabilize the spinal fracture in a patient already using an external fixator. This fixation device was used to create instantaneous fusion, to be able to diagnose spinal instability in patients with low back pain. The theory was that the decrease in motion, caused by the use of external fixator would lead to a decrease in pain, therefore aid identifying the spinal level triggering the pain (Panjabi, 2003).
Sirvanci et al., (2008) carried out an investigation on patients with degenerative lumbar spine stenosis, to see if there was a correlation between Oswestry Disability Index (ODI) and MR imaging. The ODI was a questionnaire to be completed by the patients, examining perceived levels of disability in activities of daily living to allocate a subjective score on the level of function.
Some of the activities of daily living included grooming, dressing, bathing, walking across a small room, eating and climbing stairs. As a result, they found that out of 63 patients, 10 patients demonstrated mild disability; 13 patients moderate disability, 25 patients severe disability; 12 patients were crippled and 3 patients were bedridden.
Biomechanics describes joint angles, forces, the transmission of energy across the body, and how joint movements and stability are coordinated. Pain and disability are the clinical expressions of the aging spine.
Benefits of an enhanced understanding of the biomechanics of normal and degenerative spinal conditions identified on imaging grants the ability to counsel patients, treat pathological processes, and determine the effects of both surgical and medical treatment on spinal mechanics. Furthermore, it can be applied to studying the cause, treatment, and prevention of the disorder.
Comprehension of biomechanical consequences of degeneration is essential for the efficiency of treatments of spinal disorders in various patient groups, regardless of etiology. Additionally, knowledge of biomechanics aids the practitioner in identifying posture and movement related problems in people with injuries or diseases (Lu and Chang, 2012). Treatment could vary from soft tissue massage, joint mobilization techniques, osteopathic manipulative therapy, and relevant exercises.
A useful understanding of biomechanics will allow the practitioner to target any observed abnormalities in movement or posture to relieve painful symptoms and reduce the risk of further injuries.
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