Musculoskeletal Disorders and the Workplace Low Back and Upper Extremities (2001) / Chapter Skim
Currently Skimming:
Tissue Mechanobiology
Tissue Mechanobiology
Pages 184-218
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 184... ...
The purpose of this chapter is to systematically review basic biology studies in order to determine to what degree they support an association between loading and tissue injury, especially at load levels well below those that cause tissue disruption. Biological plausibility, one of the Bradford Hill criteria for causality (see Chapter 3)
|
From page 185... ...
This is followed by reviews of the biological responses of six tissues vertebral bone, spinal disc, tendon and ligament, muscle, peripheral nerve, and spinal nerve root to loading. These reviews are based on systematic evaluations of the scientific literature.
|
From page 186... ...
Signals from nociceptors are transmitted by the peripheral nerve to cells in the spinal cord. Damage to the peripheral nerve (e.g., carpal tunnel syndrome, spinal root compression)
|
From page 187... ...
The types of load considered, along with the biological and mechanical responses, are appropriate to the tissue. VERTEBRAL BONE AND SPINAL DISC Structural and Functional Properties The intervertebral disc is a complex structure consisting of four distinct tissues: the nucleus pulposus, the annulus fibrosus, the cartilaginous endplates, and the adjacent vertebral bodies (Figure 5.1~.
|
From page 188... ...
The fibers run in the same direction within a given layer, but opposite to those in an adjacent layer. The cartilaginous endplates cover the end surfaces of the opposed vertebral bodies and serve as the upper and lower surfaces of the intervertebral disc; they are composed predominantly of hyaline cartilage.
|
From page 189... ...
Disc bulge can also occur when fibrocartilage proliferates within the substance of the annulus fibrosus (Yasuma, 1990~. Disc herniation occurs when disc material escapes through a fissure in the annulus fibrosus, which, like a bulge, can place pressure on nerve roots or the spinal cord.
|
From page 190... ...
Both of these geometric changes can adversely affect patients by accelerating facet joint arthritis and by causing mechanical impingement on the adjacent spinal cord or nerve roots. In these cases, alterations in nerve function secondary to chronic compression are thought to be the primary mediators of back pain (Devor, 1995~.
|
From page 191... ...
Vertebral body compressive strength is strongly correlated with its cross-sectional size and bone density (Brinckmann, Biggeman, and Hilweg, 1989a, 1989b) , making it feasible to predict noninvasively in humans.
|
From page 192... ...
Tissue stress developed during spinal loading can influence disc biology. Within vertebra, stress can stimulate cells to produce more bone in areas of high stress or remove bone in areas of low stress.
|
From page 193... ...
Within the nucleus and annulus, spinal loading can alter tissue water content (via creep, as discussed above) and tissue shape, leading to altered cell metabolism.
|
From page 194... ...
The fibers and fascicles are enclosed in a thin film of loose connective tissue called endotendon or endoligament. The whole tendon is wrapped in a connective tissue called the epitenon, which in turn is surrounded by the paratenon, a loose, areolar connective tissue (Figure 5.2~.
|
From page 195... ...
For example, a pinch force of 4 kg is likely to require between 70 and 230 N of tensile force in the flexor digitorum superficialis tendon (Dennerlein et al., 1998~. The mechanical fatigue properties of human tendon have been evaluated by subjecting tendon to cyclical loading (Schechtman and Bader, 1997; Hubbard and Chun, 1988~.
|
From page 196... ...
Instead, at surgery there are tears in the tendon with disorganized collagen, vascular hyperplasia, and fibroblast proliferation near cleavage planes in the tendon (Kraushaar and Nirschl, 1999; Coonrad and Hooper, 1973~. It has been proposed that these disorders characterized by degenerative changes be called tendinosis (Kraushaar and Nirschl, 1999~.
|
From page 197... ...
Mechanisms of Injury The mechanism of injury for the various tendon disorders may vary depending on local anatomy and the forces experienced by the tendon and adjacent tissues. A limited number of well-designed animal models have been developed to investigate mechanisms of injury by studying the effects of repetitive motion and loading on the adaptation and pathological changes of soft tissue; Archambault and colleagues (Archambault, Wiley, and Bray, 1995)
|
From page 198... ...
Recent studies have suggested that peritendinitis due to repeated loading may be mediated by an early inflammatory response. Using microdialysis techniques, it has been observed that metabolism is accelerated and is accompanied by an elevation of prostaglandin E2 and thromboxane B2 in the peritendinous region of the human tendon with dynamic loading (Langberg et al., 1999~.
|
From page 199... ...
Skeletal muscle is an elongated, contractile tissue that generates force and shortens when activated to contract by stimuli from alpha (oc) motor neurons that originate in the spinal cord.
|
From page 200... ...
Damage or injury to skeletal muscle invariably occurs as a result of external forces that exceed the tolerance limits of the muscle's passive (e.g., connective tissue) and active contractile structures; the nature of the damage is directly related to skeletal muscle structure and the molecular mechanism of force generation.
|
From page 201... ...
filament lattice from both directions toward the center of each sarcomere. The thick filament myosin heads are the molecular force generators that form cross-bridge attachments to the thin filaments and pull the thin filaments toward the center of the sarcomere, thereby generating force and shortening.
|
From page 202... ...
Studies of muscle electrical activity have confirmed the preferential activation of small motor units during repetitive, low-intensity, stereotypical movement (Sj0gaard and McComas, 1995; Jensen, Pilegaard, and Sj0gaard, 2000~. Because the skeletal muscle fibers of these low-threshold small motor units carry a disproportionate burden, they are referred to as "Cinderella" fibers (Hagg, 1991~.
|
From page 203... ...
Fatigue of skeletal muscle fibers is failure of the fiber to contract in response to continuing or motor neuron stimulation (Bigland-Ritchie, Furbush, and Woods, 1986~. While the fatigued fibers are not themselves permanently damaged, they can put other motor units at risk of structural damage due to inappropriate recruitment or excessive strain from external loads.
|
From page 204... ...
However, for repetitive contraction of skeletal muscle fibers under conditions leading to damage, loss of force may be accompanied only by clinical symptoms and pain as precursors to measurable structural damage (Sj0gaard and Jensen, 1997~. It is hypothesized that loss of force generation is the result of molecular damage to the myosin heads, or cross-bridges; the methods for testing this hypothesis in intact muscle have not been developed.
|
From page 205... ...
Since slow-twitch fibers are also characteristic of the small motor units recruited for low-force, repetitive, endurance work, they are the most vulnerable for being contracted to the point of muscle damage. Small muscles used for low-force, repetitive work are at risk for this type of damage (Lindman et al., 1991; Dennett, 1998; Larsson et al., 1988)
|
From page 206... ...
External loads or work that cause sarcomeres to lengthen during active cross-bridge attachment are very likely to result in structural damage. Evidence of muscle damage may include loss of active force generation capacity, inflammation, necrosis, hemorrhage, and connective tissue tearing.
|
From page 207... ...
The relationship of amplitude and duty cycle of passive stretch in injury appears more complex. Cycling of passive stretch has been shown to cause a decrease in maximal force generation without abnormal microscopic or ultrastructural changes (Lieber and Friden, 1988; Lieber, Woodburn, and Friden, 1991~.
|
From page 208... ...
firing rate, decreased motor unit firing rate, and decreased skeletal muscle force generation (Bongiovanni, Hagbarth, and Stjernberg, 1990~. The effects on skeletal muscle per se are not so well documented, and the decrease in skeletal muscle maximal force generation is due at least in part to reduced firing of or motor neurons (Farkkila, 1978; Farkkila et al., 1980~.
|
From page 209... ...
In studies of rats subjected to eccentric contractions, researchers have demonstrated that aged skeletal muscle fibers are more easily injured by single and multiple eccentric contractions, muscle fibers regenerate less, and structural and functional recovery is not complete (Zerba, Komorowski, and Faulkner, 1990; Brooks and Faulkner, 1990, 1996; Carlson and Faulkner, 1989~. However, extensive studies of the effects of eccentric contraction on aged human muscle have not been published.
|
From page 210... ...
The amount of connective tissue in and surrounding the nerve varies by level. For example, nerves located superficially in the limb or parts of the peripheral nerve that cross a joint contain an increased quantity of connective tissue, possibly as a response to repeated loading (Sunderland, 1978~.
|
From page 211... ...
The fascicles are embedded in a loose connective tissue called the epineurium (epi)
|
From page 212... ...
The study indicated that when extraneural pressure fluctuates rapidly, the effect on nerve function is associated with the mean value of the pressure waveform, rather than the minimal or peak value. Long-Term Effects of Nerve Compression The long-term biological effects of brief, graded nerve compression have been studied in several animal models using small inflatable cuffs (Powell et al., 1986; Dyck et al., l990~.
|
From page 213... ...
However, spinal canal pathology that compromises the neural space, such as disc herniation or protrusion, spinal stenosis, and degenerative disorders, can create high risk of injury, even under what might be considered moderate physical exposures. Furthermore, nerve roots do not possess so much protective connective tissue as do the peripheral nerves, which makes them particularly sensitive to mechanical and chemical irritation.
|
From page 214... ...
The vessels from the periphery and from the spinal cord meet in the proximal one-third of the nerve root; it has been suggested that this region is particularly vulnerable to injury from ischemia. The blood-nerve barrier in the nerve root is not so well developed as in peripheral nerves; this creates a higher risk of edema.
|
From page 215... ...
For instance, mechanical stimulation of nerve roots or peripheral nerves results in nerve impulses of short duration; these impulses are prolonged if the nerve tissue had been exposed to mechanical irritation by a chronic gut ligature (Howe, Loeser, and Calvin, 1977; Cavanaugh, Ozaktay, and Vaidyanathan, 1994~. Severe mechanical deformation, such as ligation of the nerve root, is generally not painful (Chatani et al., 1995; Kawakami et al., 1994a, 1994b)
|
From page 216... ...
To some degree, the ultrastructural damage due to cyclical loading may be repaired as long as the time frame for the repair and remodeling is not long relative to the rate of damage and as long as the remodeling mechanism is not overwhelmed. With a repair system in place, one would expect a load-duration or a load-repetition threshold below which there is no damage accumulation and a disorder would never manifest.
|
From page 217... ...
For the spinal nerve roots, adjacent tissue compression may release cytokines that stimulate pain transmission. Age can influence the mechanical and biological properties of bone, disc, muscle, and nerve.
|
From page 218... ...
The role of gender as a covariate in Me response of tissues to cyclical loading has not been 1nvesUgated.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.