Week 9 reading - Exercise prescription for osteoporosis

Question 1

Osteoporosis definition:

Answer 1

low BMD: ≤ -2.5 T-score from DXA or presence of fragility fracture(s) or both.

Question 2

Optimum Mechanical Load Parameters to Build Bone:

Answer 2

• Static loading is not a remodelling stimulus, thus the first principle of exercise intervention for bone health is that loading should be dynamic – loading must be applied cyclically.
• The second principle of exercise intervention for bone health; bone loading (at physiological frequencies) must exceed the regular bone strain milieu to induce an adaptive response - loading must be greater than that to which a bone is usually exposed.
• The third important principle of exercise prescription for bone, is, if the load is sufficient to stimulate the adaptive response, few load cycles are required each bout.
• The fourth principle of exercise prescription for bone, is that a fast rate of loading may be more osteogenic than slow.

Question 3

Biology of a bone:

Answer 3

Bone comprises of organic material (collagen), an inorganic matrix (hydroxyapatite (calcium + phosphate), and water.

Question 4

Cortical bone

Answer 4

Dense and ‘ivory-like’ – properties well suited to its function of support and protection. It forms the external part of the long bones and is thickest in the shaft, where it encloses a cavity filled with yellow, fatty marrow.
- The outer membrane covering cortical bone, facing the soft tissue, is the periosteum,

Question 5

Trabecular bone

Answer 5

• Trabecular bone is much less dense than cortical bone is made up of a lattice of thin, calcified struts (trabeculae) that form along the lines of the functional requirements.
• Trabecular bone makes up 40% of vertebrae but only 1% of the mid-radius.
• Facing the medullary cavity in a long bone, and covering the trabeculae of trabecular bone, is the endosteum

Question 6

3 types of cells in bones:

Answer 6

Osteocytes, osteoblasts and osteoclasts
- The activities of osteoblasts and osteoclasts are closely coupled during bone remodelling.

Question 7

Osteocytes

Answer 7

Mature cells embedded within small cavities in bone.
Their main role is to activate bone turnover and regulate extracellular calcium concentration.

Question 8

Osteoblasts:

Answer 8

Produce bone matrix and build new bone. After a delay, osteoblasts fill the cavity made by osteoclasts, with a volume of new bone which then undergoes remineralisation

Question 9

Osteoclasts

Answer 9

Remove old bone. They are abundant at the surfaces of bone undergoing erosion and secrete enzymes that create an acid environment to resorb (digest) old bone, creating a cavity.

Question 10

Bones and ageing

Answer 10

• Bone resorption initiations bone formation and, under most circumstances, restores lost bone. However, as age advances, less new bone is formed than is resorbed in each remodelling cycle, leading to bone loss and structural damage
• In older people, increased turnover enhances age-related bone loss.

Question 11

Oestrogen helps to conserve bone mass by limiting bone resorption and turnover

Answer 11

It also increases intestinal calcium absorption and reduces calcium excretion - both bone-conserving effects. Oestrogen withdrawal therefore results in an increase in the intensity of remodelling, accelerating bone loss.

Question 12

Bone strength

Answer 12

The strength of bone reflects the integration of 2 main features: bone mineral density and quality (architecture, organic content, turnover and damage accumulation).

Question 13

Bone mineral density:

Answer 13

The mass of mineral per unit area or volume.
It is measured by imagining techniques- dual energy X-ray absorptiometry (DXA).

Question 14

Measuring the structural properties of bone:
Dual energy X-ray absorptiometry

Answer 14

• DXA uses X-ray beams of two distinct energy levels to distinguish the relative composition of bone and non-bone compartments of the body.
• The measurements are based on the degree to which the X-ray beam is attenuated by the tissues.
• Two measures are derived: bone mineral content – the total grams of bone mineral within a measured region of bone; and bone mineral density (BMD) – the grams of bone mineral per unit of bone area scanned.
• Bone mineral content is highly dependent on bone size so a larger person will have a greater value than a smaller person.
• Limitation of DXA = although it measures all bone within a given area, it does not assess bone architecture, nor does it differentiate between trabecular and cortical bone.
• Advantages of DXA= the low level of radiation exposure and its accuracy and precision. Scans take as little as five minutes and can measure bone at clinically relevant sites.
• The DXA scan is a two-dimensional scan, so BMD is the ratio of bone mineral content to area rather than volume.

Question 15

Quantitative computed tomography:

Answer 15

• This technique uses X-rays to create an image of specific thin layers through the body that are built up to provide a measure of the density of bone.
• It measures bone mineral content, volumetric BMD and axial cross-sectional area.
• It can measure cortical thickness, trabecular bone density and some estimates of strength that take account of both bone mineral content and distribution.
  Limitation = Axial CT scanners involve higher exposure to radiation than DXA and are more expensive. Peripheral QCT (pQCT) has a lower radiation dose and is cheaper but can only measure sites such as lower leg and forearm.

Question 16

High resolution peripheral quantitative computed tomography:

Answer 16

• A purpose-built scanner is used to measure bone mineral content and BMD of the peripheral skeleton, using an X-ray source.
• It has a resolution that is close to trabecular thickness and generates data that can visualise microarchitecture and quantify microarchitectural parameters.
• Advantage = the high resolution allows assessment of microarchitectural parameters.
• Limitation = it can only measure the distal forearm and distal part of the lower leg.

Question 17

Quantitative ultrasound:

Answer 17

• Measurements reflect the nature and extent of the distortion of a short burst of variable-frequency ultrasound pulses as they pass through bone.
• Limitations = include that measurements are not comparable between different models and that there is not the quantity of reference data or consensus on diagnostic thresholds that are available for DXA.
• Advantages = the ultrasound measurements reflect bone microarchitecture as well as bone density. The technique is cheaper than DXA, can be used outside the laboratory and does not involve exposure to ionising radiation.

Question 18

Adaption to load bearing

Answer 18

Exercise strengthens bone and attenuates bone loss; with greater loading, the load-bearing capacity of bone increases – and vice versa.
* The strains (deformations) produced during loading stimulate an adaptive response in bone that derives from osteocytes in the region where the strains are experienced and is thus described as a ‘local response to local loading’.
* Osteocytes coordinate the activity of osteoclasts and osteoblasts through locally acting signalling pathways, to achieve the modelling or remodelling necessary to adjust bone strength to the loads it must bear.
* Osteoclastic activity increases within a day or two of immobilisation, disturbing the balance between bone resorption and formation.

When loading-induced strain exceeds ‘normal’ strains at a particular skeletal site, new bone is added, increasing bone strength.
Any unusual distribution of strain enhances the osteogenic response at a given strain magnitude

Question 19

Osteoporosis:

Answer 19

• Osteoporosis (literally meaning ‘porous bones’) is a skeletal disorder characterised by low bone mass and microarchitectural deterioration of bone tissue, with a consequent increase in fragility and susceptibility to fracture.
• Diagnosis relies on measurements of BMD at the hip as a proxy measure of bone strength.

Question 20

Osteoporotic fractures

Answer 20

• Almost all types of fractures are increased in patients with low BMD, but the most vulnerable skeletal sites are those where trabecular bone predominates.
• The most common sites of osteoporotic fractures are the hip, vertebral bodies and the wrist.

Question 21

Premenopausal women:

Answer 21

High-impact and odd-impact (soccer and racquet games) exercise loadings were both associated with a thicker cortex at the femoral neck, suggesting that these exercise regimens are the most effective against hip fragility.
* Research suggests that participation in high-impact activities during the premenopausal years may have lasting advantages for skeletal health

Question 22

Postmenopausal women

Answer 22

In a cohort of more than 60,000 postmenopausal women in the US Nurses’ Health Study, women who reported at least 24 MET-h week –1 of physical activity had a 55% lower risk than inactive women who reported less than 3 MET-h week –1 of activity (Feskanich et al. 2002). There was evidence for a dose– response relationship.
* It can be concluded that exercise has modest but clinically relevant, site-specific effects in postmenopausal women (Kelley et al 2012).
- Whilst these preserve bone or attenuate the rate of loss, they do not affect bone density as much as pharmaceutical treatments.
- Strength- or odd-impact training or a combination of these appears to be most effective (Howe et al 2011)

Question 23

Falls:

Answer 23

• Intrinsic risk factors for falls include poor levels of muscle strength, range of motion, balance, gait and reaction time – all of which might be improved through regular exercise.
• Systematic reviews of randomised clinical trials that aimed to prevent falls in older adults found that exercise interventions (including balance, leg strength, flexibility and/or endurance training) reduced the rate of falls in community dwelling older people by 21%.

Question 24

Therapy for established osteoporosis

Answer 24

• Pharmacological treatment focuses on agents that reduce bone loss.
• Except for parathyroid hormone which preferentially stimulates osteoblastic activity over osteoclastic activity, each of these agents acts to prevent bone resorption.
• A newer therapeutic agent, denosumab, is a monoclonal antibody that inhibits osteoclast formation, function and survival, thus decreasing bone resorption.
• Calcium supplementation has been reported to decrease the risk of hip fracture (24% reduction with 1,000 mg day-1). Vitamin D supplementation may also be helpful.

Question 25

Physical activity and osteoarthritis:

Answer 25

• Osteoarthritis (the most common form of arthritis) is a disease entity involving all joint components including cartilage, synovial membrane and bone.
• The improved muscle strength and proprioception and reduced body mass index associated with regular physical activity are protective of joints.
• Systematic review and meta-analysis of 44 randomised controlled trials of knee osteoarthritis and ten trials of hip osteoarthritis demonstrated that exercise reduced joint pain and improved physical function in patients with osteoarthritis, with benefits lasting for several months after the treatment stopped

Question 26

The ‘Timed Up and Go’ test:

Answer 26

A simple, quick and widely used clinical performance-based measure of lower extremity function, mobility and fall risk. It measures the time taken by an individual to stand up from a standard armchair, walk a distance of 3 meters, turn, walk back to the chair and sit down.

Question 27

Summary

Answer 27

• Bone is a dynamic tissue that responds to changes in the internal or external environment. Mechanical loading is an important influence on bone remodelling.
• Strain rate and an unusual strain distribution are important determinants of the site-specific osteogenic effects of loading.
• Bone mass creases during growth and reaches a peak towards the ned of the second decade. It usually remains fairly stable until about 50 years of age when progressive loss begins. Particularly in women, age related loss of bone can leas to osteoporosis, compromising bone strength and increasing risk if fracture.
• The potential of exercise to augment bone mineral in the mature skeleton is clear but small. In premenopausal women its effect is mainly conservation of bone, although specific bone loading exercise can lead o modest bone accrual.
• Exercise can decrease the risk of falls through improving balance, strength and maybe neuromuscular coordination.
• Participation in sports involving high-intensity impacts or torsional types of stress increases the risk of osteoarthritis, potentially mediated by joint injury. On the other hand, moderate amounts and intensities of exercise decrease pain and improve function in patients with osteoarthritis of the knee and hip