Week 9 Lecture - skeletal health Flashcards
Bone structure, composition and remodelling:
- Trabecular/ Spongy bone: network of cross bridges filled with red bone marrow – found at ends of bone (lighter, more rapidly turnover)
- Cortical/ compact bone: dense, consisting of longitudinal cylindrical structures (osteons) – provide strength
- Outside = periosteal surface (periosteum)
- Inside = endosteal surface (endosteum)
Bone tissue composition:
Ground substance:
- 2/3rd of bone matrix
- Very hard but very brittle
- Minerals, predominantly crystals of hydroxyapatite Ca10(PO4 )6 (OH)2 with other calcium salts and ions
- One of its Function – store and release calcium
Protein:
- 1/3rd of bone matrix
- Most abundant protein type 1 collagen fibres – strong fibre that helps to reinforce the structure
Bone cells:
- Only 2% of bone mass
- Mesenchymal stem cells => => osteoblasts => osteocytes (mature bone cells)
Osteoclasts
Dissolve away old bone
Resorb bone
Osteoblasts
Secrete osteoid (protein) & mature into osteocytes.- form new bone.
Some of these osteoblasts will turn into osteocytes to maintain the bone.
Osteocytes
coordinate the remodelling of bone by signalling
Osteoid
Mineralised to form new bone
Measurement of bone mineral density:
- DXA – BMD: g/cm2 (most common) – provides a 2D image of the bone – tells us how much bone there is in a given area
- pQCT & Clinical QCT- BMD: g/cm3, bone shape – allows 3D assessment
Schematic of BMD according to age
BMD tends to increase to around 30 years old and then declines with age
Higher in men than women
Substantial decline in women after menopause as levels of oestrogen declines – increases bone reabsorption – release more calcium - an imbalance in bone remodelling, resulting in more bone being broken down than formed
BMD can decline by around 1/3rd with menopause
Osteoporosis
“Osteoporosis is a systemic skeletal disease characterised by low bone mass and microarchitectural deterioration of bone tissue (structural changes e.g., thinning of cortical bone), with a consequent increase in bone fragility and susceptibility to fracture” (WHO 1993)
- May also lose some trabeculae – make bone weaker and more likely to fracture
- Can affect respiratory function
- Result in loss of height
- Hip fractures are very common- they take a very long time to heal
Risk factors for osteoporosis
- Demographic: age, gender (female), family history
- Skeletal: low BMD, bone geometry (shape of the bones)
- Clinical: oestrogen deficient, cancer treatments, glucocorticoid use (supress immune responses), T2DM
- Extra-skeletal: falls risk, low BMI (less padding, less muscle), smoking
Bone shape and bone property:
- Smaller diameter of bone has a larger BMD
- However, larger diameter of bone has greater bending strength
- Wider bones haves a thinner cortical thickness
Cortical thickness in femoral neck fractures cases compared to controls
Cortical thickness is related to fracture risk
Superior part of femoral neck was significantly thinner in people who had a femoral neck fracture
- Increase risk with thinning of superior femoral neck
Mechanostat theory – proposed by HM Frost
Loading bone causing the bone to get fractionally smaller – applying forces to bones causes strains
- Applying greater stresses causes greater strains
Mechanoststat theory suggests this stress is detected by osteocytes and these cause the bones to adapt – signal for more bone to be added. Bone then becomes thicker as an adaptation to increases loading (stress)
- Continued stress (e.g., jumping) will eventually lead to in less strain as the bone is getting stronger
Strains in bones and the consequential bone change:
A lack of loading on the bone causes bone weakening
Increasing the force moderately acting on a bone – we will see an increase in bone mass – more osteons added to strengthen the bone
Extreme overload (adding much greater forces on the bone) can cause microdamage or even fractures
- We want to moderately overload the bone to increase strength. When adapted we moderately increase force again
- Gradual progressive moderate overload is what is required to increasing bone strength
Cellular responses to changed loading:
Osteocytes detect heighten strains to signal to osteoblasts
Modelling – changing the bone shape by forming new bone for strength
Targeting remodelling – when microdamage is present (e.g., from overloading), death of osteocytes may occur – this will signal for the damage parts of the bone to be removed by osteoclasts.
When inactive/ oestrogen deficiency – there can be a loss of osteocytes – and old bone will not be replaced with new bone – gradual loss of bone overtime – trabeculae may become thinner and weaker
With age bones tend to get wider – this greater width gives greater strength in bending to cope with the loss in bone density that can happen with age
Section through cortical bone of a young man
- Old fully mineralised osteons appear white (mature)
- New osteons with lower, mineralisation appear darker
- New dark osteons are apparent at the periosteal surface
- Targeted remodelling – replace old osteons with new osteons
- On the outside of the bone we form new osteons remodelling
Exercise and bone in children:
- RCT in children (aged 5.9-9.8 yr). Jumping group: performed 100 drop jumps from a 61cm block, 3 times per week for 7months . Stretching group: stretching exercises 3 times per week for 7 months.
Findings:
Stretching group increased their bone mineral content – as they are children – increase bone overtime
The jumping group increased their bone mineral content much more – higher stressors on the bone to increase strength
After 7 years, there was still a significant difference between the jumping group and the controls – bone mineral density can be maintained
Typical pQCT scans of the tibia and fibula in athletes with different adolescent sport history
- Runners (lost of anteroposterior loading) – thickening of tibia at the front but thin on medial aspect of the tibia
- Controls – thinner all the way round
- Hockey players (multidirectional loading) – cortical of the tibia was thicker all the way round
- Exercise during adolescent has long lasting effects on bone shape as well as density
Meta-analysis of exercise effects on BMD in premenopausal women
- High load resistance training (few reps) – produced the greatest benefits at the spine
- High impact exercise (e.g., jumping) had the greatest benefits at the hips
- A combination of both had significant effects of the spine and hip
Athletes who produce large strain rates in their sports had substantially greater section modules (strength in bending) – up to 40% greater
Exercise and fracture risk
- According to observational studies:
- Women who exercise have 38% fewer fractures
- Men who exercise have 45% fewer fractures compared to less active colleagues
- People who exercise may be more healthy in general?
- According to controlled trials:
- Exercise reduced fracture by 25% compared to control groups
- Exercise reduces risk of osteoporotic fracture
Bone density and bone structure can increase bone strength and reduce fracture risk
RCTs of exercise effects on fractures and BMD in postmenopausal women:
Cochrane meta-analysis found:
- Exercise modestly benefitted BMD: spine 0.9%, total hip 0.4%, trochanter 1.0%
- Most effective forms of exercise: high load progressive resistance training (typically progressing to >80% 1RM), combination of resistance training with impact (plyometric) exercise (e.g., aerobics, jumping)
High magnitude resistance and impact training in women with low BMD: the LIFTMOR trial:
- 8-month RCT in 101 women with low BMD
- Found significant benefits on Bone density loss even in women with low BMD
- Femoral neck BMD net benefit: +2.2%
- Lumbar spine BMD net benefit: +4.1%
- Spines also became straighter – better posture etc
Hiphop study: bone mass and geometry by DXA:
- 50 multidirectional hops per day for 12 months: Compare changes between 2 legs (hopping leg vs control leg).
- As older men they were losing bone in control leg and had benefits in bone density in hopping leg
- Larger increases in section modulus (strength in bending)
- CT scans looking at cortical thickness showed that cortical thickness increased the most at the top part of the femur (reduced risk of hip fracture)
- Can target the bone to suspectable regions where weakness can contribute to fracture
Fall prevention exercise - Cochrane review of 108 RCTs found
- Exercise reduced fall rate by 23%
- Reduced fall related fractures by 27%
- Reduced falls requiring medical attention by 39%
Exercise types:
- Balance exercise reduced fall rate by 24%
- Tai Chi reduced rate of falls by 19%
- Challenge balance (reduced base of support, move centre of gravity e.g., stand on one leg and alter centre of gravity)
- Lower limb strength training e.g., tiptoe walking
- > 3hr/week of individualised exercise according to the capabilities of the individual
Uk consensus statement on PA and exercise for osteoporosis: Royal osteoporosis society
Bone strengthening:
* Progressive resistance exercise: Exercise loading spine and hip - 2-3days/week. Build up to 3 sets of 8-12 RM. Bone starts losing mechanosensitivity over time
* Impact exercise: 50 moderate impacts per day with rest breaks. For those with vertebral fracture history (>12 weeks) low impacts, 20 mins/day
Reducing risk of falls:
* Prioritise for those with poor balance
* Strength and balance training: 2-3 days/ week
Spine awareness:
* Prioritise for those with back pain/ vertebral fracture history. Spine extensor exercise: <10 x 3-5sec holds, 2-3 days/week, daily for painful fracture – maintain endurance of the spine extensor muscles by doing repeated holds etc to improve posture. Safe moving and lifting: bend at knees and hips, avoid spine flexion, twisting.
Lecture summary:
- The skeleton is continuously modelled and remodelled according to applied forces (mass and shape)
- Activities exerting high loads (e.g., high load resistance training, impact/plyometric) exercise can increase density and change shape of loaded bones
- Exercise in childhood and adolescence can increase bone mass and produce robust bone shape, providing resilience to later osteoporosis. See larger changes in childhood that can be maintained into adulthood.
- Resistance and impact exercise can have small effects on bone density and shape even in older adults, reducing risk of osteoporotic fracture
- Strength and balance training can reduce fall risk.
- Exercise is fundamental to recommendations for osteoporosis prevention and management but there is a shortage of adequately training practitioners to deliver this. We lack physiotherapists, most gym instructors lack knowledge on osteoporosis
