Arthrokinesiologic Changes with Aging

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Contents I. Structural Changes:1.Periarticular Connective Tissue.2-Articular Cartilage.3-Age-Related Changes in Bone. II. Functional Changes:1- Change in Joint Angular Velocity. A. Reduced Physical Activity: B. Sensorimotor Changes . C. Stiffness in Periarticular Connective Tissue .2-Aged Related Reduction in Joint Rang of Motion.3-Age-Related Influences in Joint Mechanics.

Growing old is usually associated with a reduced level of

physical activity. In fact, the body's physiological responses to

both are quite similar. Furthermore, as one reaches an

advanced age, the chance of being affected by a disease

increases.

The effects of disease, reduced physical activity, and

advanced age often occur simultaneously and may have 'a

combined effect on joint function. Countless other factors,

such as genetics, previous postural habits, and earlier injury,

also interact and influence an aged person's arthrokinesiologic

function.

I. Structural Changes

1.Periarticular Connective Tissue (PCT)

The mechanical properties of PCT change with

advanced age. The structural and functional changes

in the collagen protein account for most change.

Much of the research literature regarding the

effects of advanced age on human PCT is based on

animal research.

Human research in this area obviously cannot be

conducted with rigid experimental control for

variables such as previous physical activity, earlier

tissue trauma, nutrition, or breeding.

Age-related animal research suggests that ligaments and tendons

increase in stiffness and demonstrate a decrease in the maximal length

at which rupture occurs.

A biochemical analysis of aged tissue usually shows an increase in

the relative amount and diameter of collagen and a relative decrease in

water, elastin, and proteoglycan content.

A mechanism to account for the increase in stiffness in age-related

PCT may be the fact that aged collagen shows increased numbers of

cross-links between adjacent tropocollagen molecules.

Increased rates of cross-linking would increase the mechanical

stability of collagen and may explain the increased stiffness in the

tissue.

2-Age-Related Changes in Articular Cartilage Histological observation of

healthy articular cartilage in the aged adult shows that the density of chondrocytes and

the amount of collagen within the extracellular matrix remain

essentially unchanged.

The water content in the tissue, however, does reduce

with advanced age. Dehydrated articular

cartilage may have a reduced ability to dissipate forces

across the joint.

Aged articular cartilage may become more susceptible to mechanical failure. The loss of physical strength of aged cartilage may be due to fragmentation of the collagen network and/or ruptures of the interfiber bonding. Cartilage lesions, often referred to as fibrillated cartilage. Fibrillated cartilage does not tolerate compression and tensile force.

3-Age-Related Changes in Bone The precise shape and density of bone are maintained through life by a balance of mechanical and physiological mechanisms. Mechanical stress stimulates the formation of new bone, whereas the endocrine system functions to ultimately reabsorb old bone. Bone is composed of a network of collagen fibres impregnated with mineral salts (largely calcium phosphate and calcium carbonate), a combination that gives it great density and strength As an individual advances in age and becomes less active, a loss of bone mass per unit volume usually occurs. If the bone becomes excessively brittle and prone to fracture, the condition may be classified as osteoporosis.

This process is characterized by a progressive loss of both fibrous matrix and mineral content; new bone is not made at a rate to replace the natural rate of bone absorption. Decreased bone mass results in a decreased ability of bone to support loads and resist external forces. As an example, consider the relatively common incidence of avulsion fracture of the tibial tuberosity in the aged population. This fracture occurs when the ligamentum patella and tibial tuberosity are pulled free from the shaft of the tibia due to excessive force produced in the quadriceps muscle. The large tensile force developed through the ligamentum patella exceeds the capability of the bone to maintain an intact tibial tuberosity.

The medical impact of osteoporosis in the elderly is significant, particularly evident by the high incidence of fracture of the hip. Hip fracture can lead to significant loss of functional status in the elderly. One study showed that at 1 year after hip fracture, only 33% of persons regained their prefracture status in performing basic activities of daily living. Hip fracture in the elderly often results from high torsional forces created about the shaft of the femur during a twisting motion.

The decline in physical activity and subsequent

diminished stress placed on bone are often associated with

growing old. Therefore, the loss of bone mass and increased

susceptibility to fracture should be considered a normal age-

related process.

The postmenopausal loss of bone in women reflects the

normal physiological role of estrogen in the maintenance of

cortical bone mass.

Bone loss in postmenopausal women can be minimized

somewhat through active dynamic exercise.

Furthermore, regular moderate physical activity in

persons with osteoporosis can reduce the risk of falls and

bone fracture.

II. Functional Changes

1-Aged related change in Joint Angular Velocity

A. Reduced Physical Activity: Decreased velocity of joint movement in the aged seems to parallel a natural decline in overall physical activity. The elderly often assume a more sedentary life-style. This life-style may be chosen due to a combination of personal, family, cultural, or socioeconomic reasons. The decline in physical activity may also be related to actual age-related physiological changes in the sensorimotor systems, such as decreased muscle strength or decreased vision. Excessive medication; debilitating medical problems or poor nutrition; and a general overcautiousness, coupled with a fear of falling, are additional factors that may contribute to the decreased physical activity.

B. Sensorimotor Changes. The general responsiveness of the nervous system tends to slow with

advanced age .

This slowing may partially account for a decline in physical activity

and subsequent slowed joint movement.

Age-related changes in the nervous system include increased

reaction times; increased rate of loss of brain cells; altered level of

neurotransmitter production; and a decreased acuity of the auditory,

vestibular, and visual systems.

Possibly, the slowed movement displayed by many elderly is simply

a natural mechanism that provides additional time to adequately

interpret and process incoming environmental stimuli.

C. Stiffness in Periarticular Connective Tissue Increased PCT stiffness may be another contributor to slowed movement in the aged. Increased levels of resistance to joint motion has been measured directly in the elderly. To discuss the implications of this concept, consider an example where an elderly person demonstrates slowed neck rotation to the left, let us say, in response to the call of his/her name. The motion of left cervical rotation requires a concentric contraction of the left rotators. These muscles must provide sufficient force to rotate the neck as well as elongate the antagonistic right rotator muscles.

Increased intramuscular connective tissue stiffness in the right rotator group, for example, could act as a resistance to the left rotation motion. Attempts at increasing the velocity and subsequent amount of left cervical rotation may increase the resistance offered by the right muscle group's intramuscular connective tissue. Significant resistance offered by these tissues would reduce the productive power output of the intended motion of left rotation.

2-Aged Related Reduction in Joint Rang of Motion

The loss of passive range of motion in the elderly is often progressive and subtle. This reduced magnitude of joint movement may exist even in the absence of pathology. Healthy adult men and women tend to have greatest joint mobility in their 20s,with a gradual decrease thereafter. The loss of range of motion is highly variable across joint and subject; however, joint flexibility is clearly inversely related to age. Females tend to lose range of motion at a slower rate than males and that joints of the upper extremity remain more flexible than the joints of the lower extremities.

What factors could account for this rather strong association between advanced age and a progressive decrease in joint range of motion? To consider this question, a few prerequisites for full active range of joint motion should be recognized:First, full range of motion requires that the articular surfaces allow a tracking for movement without undue physical interference. Second, a sufficient motor drive with adequate sensory feedback is needed from the neuromuscular system. Third, the PCT must possess a stiffness level that does not inhibit a joint's full range of motion.

Several factors may impede full active or passive range of motion in the elderly: 1-Age-related changes may occur in the joint from previous injury, occupation, or poor posturing. 2-Subsequent excessive joint wear may predispose osteophyte formation and incongruities at the articular surfaces. These factors, in conjunction with increased viscosity of the synovium, calcification of articular cartilages, and increased fatigability of muscle, could

all interfere with full joint motion.

3-Age-Related Influences in Joint Mechanics

Increased stiffness in PCT in the aged may have significant influence on joint arthrokinematics. Consider the motion of active glenohumeral abduction to full range. To achieve this motion, all PCT and muscle that have the potential to produce a glenohumeral adduction torque must be elongated. Furthermore, the head of the humerus must be able to stretch a pouch formed by the inferior aspect of the glenohumeral capsule. The inferior slide of the humeral head is part of the natural arthrokinematic pattern of full abduction .

Significant capsular and ligamentous stiffness may interfere with the natural translations that constitute the arthrokinematics of abduction. Increased tissue resistance to any expansion of the capsule, for example, would inhibit the descent of the humeral head. This may cause the head of the humerus to roll superior on the glenoid without the necessary compensatory inferior glide.

The head of the humerus may impinge on the supraspinatus tendon or make contact with the coracoacromial arch, thus limiting further abduction. Increased transarticular forces may result since greater muscle forces may be needed to rotate and/or translate the bones against the resistance imparted by the stiffer capsule. Abnormal muscle synergies may also result over time, since, as in the previous abduction example, the serratus anterior may have to develop greater and longer duration forces to upward rotate the scapula on the thorax in efforts to assist the shoulder abduction.