Musculoskeletal System Flashcards
What are the five common metabolic bone disorders?
- Primary hyperparathyroidism
- Rickets / osteomalacia
- Osteoporosis
- Paget’s disease
- Renal osteodystrophy
What are the symptoms of metabolic bone disease?
Metabolic - Hypo/hypercalcaemia - Hypo/hyperphosphataemia • Low phosphate→renal damage • High phosphate→precipitation with calcium to form calcium phosphate causing widespread tissue damage
Bone
- Pain
- Deformity
- Fractures
How is calcium stored in bone?
As inorganic hydroxyapatite
What makes a bone strong?
1) Mass
2) Material properties (matrix and mineral)
3) Microarchitecture
- Trabecular thickness
- Trabecular connectivity
- Cortical porosity
4) Macroarchitecture
What are the different ways of assessing bone structure and function?
1) Bone histology
2) Biochemical tests
3) Bone mineral densitometry (e.g. osteoporosis)
4) Radiology
What are the age related changes in bone mass?
Men always have higher bone mass than women.
Attainment of peak bone mass <30
Consolidation ∼30-40
Age related bone loss >40
Women have a faster loss of bone mass during the menopause
What biochemical investigations are performed in bone disease?
Serum:
- Bone profile
(calcium, corrected calcium [albumin], phosphate, alkaline phosphatase)
- Renal function
(creatinine, parathyroid hormone, 25-hydroxy vitamin D)
- Urine
(calcium/phosphate, NTX)
What are the biochemical changes in osteoporosis?
Calcium: normal Phosphate: normal Alkaline Phosphatase: normal Bone Form: ↑⟷ Bone Resorption: ↑↑
What are the biochemical changes in osteomalacia?
Calcium: normal or ↓
Phosphate: ↓
Alkaline Phosphatase: ↑
What are the biochemical changes in Paget’s disease?
Calcium: normal
Phosphate: normal
Alkaline Phosphatase: ↑↑↑
Bone Form: ↑↑
What are the biochemical changes in primary hyperparathyroid?
Calcium: ↑
Phosphate: ↓
Alkaline Phosphatase: normal or ↑
Bone Resorption: ↑↑
What are the biochemical changes in renal osteodystrophy?
Calcium: normal or ↓
Phosphate: ↑
Alkaline Phosphatase: ↑
What are the biochemical changes that occur in metastases (metabolic bone disease)?
Calcium: ↑
Phosphate: ↑
Alkaline Phosphatase: ↑
Bone Resorption: ↑
How is normal calcium homeostasis maintained?
1) Calcium is absorbed (mainly in the jejunum and ileum); either passive (not controlled; inefficient) or active (vitamin D controlled)
2) Exchange of Ca with bone
3) Kidneys filter blood and have compulsary loff of Ca per day
What is a normal total calcium in the serum?
2.15-2.56mmol/L
What is the total calcium in serum comprised of? What percentage are these of total calcium? What causes a shift in the percentages?
46% is protein bound
47% is free (ionized)
7% is complexed
Alkalosis causes a shift from free to protein bound calcium
What causes PTH to be released? What effect does the release of PTH have?
↓ plasma Ca²⁺ = ↑ PTH
Bone:
↑ resorption = release of Ca and phosphate
Kidney:
↑ phosphate excretion
↑ calcium reabsorption
↑ calcitriol formation → ↑ intestinal CaHPO₄ absorption
What is the rapid response system for low serum Ca?
PTH release
What also binds to the PTH receptor? Why is this clinically relevant?
PTHrP
A lot of this is produced during breastfeeding and by some tumours (which induces hypercalcaemia)
How is PTH release controlled?
The parathyroid gland monitors serum Ca through the calcium sensing receptor
Even at high calcium levels there is a base-line PTH secretion
There is a set-point (half of the maximal suppression of PTH)
What effect does PTH have on the kidney?
1) Internalises the Na/phosphate transporter from the membrane of the PCT to prevent reabsorption
2) Binds to receptor, activating Ca transport protein. Ca then binds to an intracellular protein which is transported through the cell and into the interstitial space by either a Ca²⁺ATPase or a Ca exchanger
How does PTH induce bone resorption?
PTH activates osteoblasts/stromal cells to express RANKL on their surface.
RANKL binds to RANK receptors on macrophages which stimulates osteoclasts to initiate bone resorption
What age and gender is primary hyperparathyroidism most common in?
50s
Female 3:1 Male
What are the causes of primary hyperparathyroidism?
Parathyroid adenoma 80% Parathyroid hyperplasia 20% Parathyroid CA <1% Familial syndromes MEN1 2% MEN2A rare HPT-IT rare
How is primary hyperparathyroidism diagnosed?
Elevated total/ionized calcium with PTH levels elevated or in the upper half of the normal range
What are the clinical features of primary HPT?
Thirst, polyuria, tiredness, fatigue and muscle weakness
Stones, moans and psychic groans
Renal colic, dyspepsia, pancreatitis, constipation, nausea, anorexia, depression, drowsy, coma
At what point is a calcium level a medical emergency? Why?
> 3mmol/L
Kidney will shut down
Transporter will shut down so the patient will get rapidly dehydrated
What effect does acute and chronic PTH elevation have on bone resorption?
Acute (pulsatile): anabolic- builds bone
Chronic: catabolic- resorption
What are the actions of vitamin D?
Intestine:
- 1,25(OH)₂ vitamin D activates Ca and phosphate absorption in duodenum
Bone:
- Synergises with PTH, acting on osteoblasts to increase formation of osteoclasts through RANKL
- Increases osteoblast differentiation and bone formation
Kidney:
- Facilitates PTH action to increase Ca reabsorption in distal tubule
What is Rickets? What are the signs and symptoms?
Inadequate vitamin D activity leads to defective mineralisation of the cartilaginous growth plate Symptoms: - Bone pain and tenderness - Muscle weakness - Lack of play Signs - Age dependent deformity - Myopathy - Hypotonia - Short stature - Tenderness on percussion
What are the causes of Rickets/osteomalacia?
Vitamin D related: - Dietary - Gastrointestinal • Small bowel malabsorption / bypass • Pancreatic insufficiency • Liver/biliary disturbance • Drugs (phenytoin, phenobarbitone) - Renal • Chronic renal failure - Rare hereditary • Vitamin D dependent rickets → Type I - deficiency of 1α-hydroxylase → Type II- defective VDR for calcitriol
What hormone also causes phosphate loss in the PCT, as well as PTH? What metabolic bone disorder does this cause?
FGF-23
Osteomalacia/Rickets
What two hormones control phosphate homeostasis? What is the duration of action of these hormones?
PTH: Fast-acting hormone
FGF-23: Slow, long lasting hormone
What is osteoporosis?
Low bone density
What are the causes of osteoporosis?
1) High bone turnover
Increased bone resorption greater than increased bone formation
2) Low bone turnover
Decreased bone formation more pronounced than decreased bone resorption
3) Increased bone resorption and decreased bone formation
Deficiency in what hormone causes menopausal bone loss? What effect does it have?
Oestrogen
- Increases number of remodelling units
- Causes remodelling imbalance with increased bone resorption (90%) compared to bone formation (45%)
- Remodelling errors- deeper and more resorption pits lead to:
• Trabecular perforation
• Cortical excess excavation
- Decreased osteocyte sensing
Why would high urine calcium not be diagnostic of osteoporosis?
Because urine calcium will be high when a woman is post-menopausal
How is osteoporosis diagnosed? Explain this.
DEXA (Dual energy X-ray absorptiometry)
Measures transmission through the body of X-rays of two different photon energies
Enables densities of two different tissues to be inferred i.e. bone mineral, soft tissue
What is a FRAX? What does it measure and why?
Fracture Risk Assessment Tool
Uses hip bone
The commonest fractures in osteoporosis are vertebral fractures followed by hip fractures.
What propeptides can be measured in the blood and used to indicate bone formation?
P1NP = Procollagen type I N-terminal propeptide
What markers indicate bone resorption?
Serum CTX
Urine NTX
What is the process of collagen synthesis involved in bone formation?
1) 2 ‘Alpha 1’ and 1 ‘Alpha 2’ chain of type 1 collagen is produced by the osteoblast join
2) Extension peptides are cut off
3) 3 hydroxylysine molecules on adjacent tropocollagen fibrils condense to form a pyridinium ring linkage
Following treatment with anti-resorptive drugs, how long would it take to see results in an osteoporosis patient?
Bone resorption markers fall in 4-6 weeks
Expect a 50% drop of urine NTX by 3 months
Bone mineral density change in 18 months
What is alkaline phosphatase used to diagnose and monitor?
Pagets
Osteomalacia
Boney metastases
What is Chronic Kidney Disease Mineral Bone Disorder?
Skeletal remodelling disorders caused by CKD which contributes directly to vascular calcification
Contributes to excess mortality in CKD
CKD impairs skeletal anabolism, decreasing osteoblast function and bone formation rates
What in renal osteodystrophy? What is the progression of this disease?
Increased serum phosphate and reduction in calcitriol
SO
Secondary Hyperparathyroidism develops to compensate
BUT
Unsuccessful and hypocalcaemia develops
LATER
Parathyroids autonomous (tertiary)
What happens to the parathyroid gland in secondary hyperparathyroidism?
Parathyroid hyperplasia causing huge increase in parathyroid hormone
What are the functions of bone?
1) Mechanical
- support and site for muscle attachment
2) Protective
- vital organs and bone marrow
3) Metabolic
- reserve of calcium
What is the composition of bone, and their proportions?
Inorganic - 65%
Organic - 35%
What is contained within the inorganic portion of bone?
- Calcium hydroxyapatite
- 99% of Ca in the body
- 85% of phosphorous
- 65% of Na and Mg
What is contained within the organic portion of bone?
- Bone cells
- Protein matrix
What are cortical bones?
- Long bones
- 80% of the skeleton
- Appendicular
- 80-90% calcified
Mainly mechanical and protective
What are cancellous bones?
- Vertebrae and pelvis
- 20% of skeleton
- Axial
- 15-25% calcified
- Mainly metabolic- Large surface
What are the two types of bone biopsy?
Closed
- needle - core biopsy usually taken from ASIC (Jamshidi needle)
Open
- For sclerotic or inaccessible lesions perfored under general anaesthetic (e.g. inner aspect of the pelvis)
What are the different types of bone cells?
1) Osteoblasts
2) Osteoclasts
3) Osteocytes
What are osteoblasts?
Build bone by laying down osteoid.
Mononucleated
What are osteoclasts?
Multinucleate cells of macrophage family.
Reabsorb bone or chew bone
What are osteocytes?
Osteoblast-like cells, which sit in lacunae in bone
What is RANK?
Receptor Activator for Nuclear Factor κβ
What inhibits osteoclastogenesis? How?
OPG = Osteoprotegerin
Inhibits RANK/RANKL binding by competing with RANK for RANKL
What turns on the RANK/RANKL system causing osteoclast differentiation?
- PTH
- Cytokines
- Mechanical influences
Anatomically, what are the different types of bones? Give examples of each.
Flat (Intramembranous ossification) - Shoulder blades Long (Endochondral ossification) - Femur Cuboid - Vertebrae
What are the different classifications of bones? Explain the function of each.
1) Trabecular bone (cancellous)
The ends of long bones; porous, contains bone marrow
2) Compact bone (cortical)
Thick, dense, white bone; protective. Forms the hard exterior of bone
3) Woven bone (immature)
Abnormal, except in the base of teeth
4) Lamellar bone (mature)
Forms in response to gravity; thickens/makes bone strong
What is osteopaenia?
Reduced bone mass
Often results in fractures with little of no trauma
What are the three main categories of metabolic bone disease?
1) Related to endocrine abnormality (↓ Vit D; ↑ PTH)
2) Non-endocrine (e.g. age-related osteoporosis)
3) Disuse osteopaenia (lack of gravity)
What do you reach your peak bone mass?
30
What are the different types of osteoporosis?
Primary: age, post-menopause
Secondary: drugs, systemic disease (long-term steroids, some chemotherapy, anti-epileptics, thyroid treatment)
What is osteomalacia? What are the different types?
Defective bone mineralisation
1) Deficiency of vitamin D
2) Deficiency of PO₄ - usually chronic renal disease. Can’t make calcium hydroxyapatite
What are the different causes of low vitamin D?
1) Skin (e.g. culture- veils etc, winter)
2) G.I. disease (small bowel malabsorption)
3) Liver disease
4) Renal disease
What are the sequelae from osteomalacia?
- Bone pain/tenderness
- Fracture
- Proximal weakness
- Bone deformity
What type of labelling is performed to investigate osteomalacia?
Tetracycline labelling
Tetracycline is taken up into bones so bone sample can be taken from ASIC to determine the amount of bone growth in ∼21 days
What skeletal changes are seen in hyperparathyroidism?
Osteitis fibrosa cystica
What organs are affected in hyperparathyroidism?
- Parathyroid gland
- Bones
- Kidneys
- Proximal small intestine
What are the different types of hyperparathyroidism?
Primary:
- Parathyroid adenoma (85-90%)
- Chief cell hyperplasia within the parathyroid (rare)
Secondary:
- Chronic renal deficiency
- Vitamin D deficiency (due to feedback mechanism)
What would be shown on a hand X-ray of a patient with hyperparathyroidism? Why?
Lesions within the phalanges (Brown cell tumours)
Bone is broken down and replaced by fibrous tissue
What are the skeletal changes associated with chronic renal disease?
Can have one or a mixture:
1) Increased bone resorption (osteitis fibrosa cystica)
2) Osteomalacia
3) Osteosclerosis (thick bone)
4) Growth retardation
5) Osteoporosis
What are the features of renal osteodystrophy?
1) PO₄ retention (hyperphosphataemia)
2) Hypocalcaemia as a result of ↓ vitamin D
3) Secondary hyperparathyroidism
4) Metabolic acidosis
5) Aluminium deposition
What is Paget’s disease? What are the stages of this disease?
A disorder of bone turnover
Divided into 3 stages:
1) Osteolytic (bone is broken down rapidly)
2) Osteolytic-osteosclerotic (bone built again-mixed)
3) Quiescent osteosclerotic (cycle of break down→reformed)
Who is typically affected by Paget’s disease?
Onset >40y (3% Caucasians >55y)
♀=♂
Rare in Asians and Africans
Mono-ostotic 15% - remainder polyostotic
What are the main sites affected by Paget’s disease?
Spine 76%
Skull 65%
Pelvis 43%
What are the clinical symptoms of Paget’s disease?
- Pain
- Microfractures
- Nerve compression (including spinal nerve and cord)
- Skull changes may put medulla at risk
- Deafness
- ± haemodynamic changes, cardiac failure (↑ cardiac output to bone)
- Hypercalcaemia
- Development of sarcoma in area of involvement (1%- usually young people <30)
What is the haversian canal?
Space in mature bone where vessels run
What is Howship’s lacunae?
The bite mark left by osteocytes
What is Rheumatoid Arthritis?
Chronic joint inflammation that can result in joint damage
- Site of inflammation is the synovium
- Associated with atuoantibodies
- Rheumatoid factor
- Anti-cyclic citrullinated peptide (CCP) antibodies
What is Ankylosing Spondylitis?
Chronic spinal inflammation that can result in spinal fusion and deformity
- Site of inflammation is the enthesis
- No autoantibodies (‘seronegative’)
What are the different types of seronegative Spondyloarthropathies?
- Ankylosing spondylitis
- Reiters syndrome and reactive arthritis
- Arthritis associated with psoriasis (psoriatic arthritis)
- Arthritis associated with gastrointestinal inflammation (enteropathic synovitis)
What is Systemic Lupus Erythematosus?
Chronic tissue inflammation in the presence of antibodies directed against self antigens
- Multi-site inflammation but particularly the joints, skin and kidney
- Associated with autoantibodies:
- Antinuclear antibodies
- Anti-double stranded DNA antibodies
What are the different types of connective tissue diseases?
- Systemic lupus erythamatosus
- Inflammatory muscle disease: polymyositis, dermatomyositis
- Systemic sclerosis
- Siogren’s syndrome
- A mixture of the above: ‘Overlap syndromes’
What HLA is associated with Rheumatoid arthritis?
HLA-DR4
What HLA is associated with Systemic Lupus Erythematosus?
HLA-DR3
What HLA is associated with Ankylosing Spondylitis?
HLA-B27
Where are MHC class I and class II cells found?
Class I: All cells
Class II: APCs
What antigen is presented by MHC class I? Give examples
Endogenous (intracellular)
- Viral peptides
- Tumour antigens
- Self-peptides
What T cell recognises antigen presented by MHC class I? What is the response?
CD8 +ve T cell
(cytotoxic T cell)
Response: Cell killing
What antigen is presented by MHC class II? Give examples
Exogenous (extracellular)
- Bacterial peptides
- Self-peptides
What T cell recognises antigen presented by MHC class II? What is the response?
CD4 +ve T cell
(helper cell)
Response: Antibody response
What is the MHC peptide binding site made up of?
Walls: α-helical structures
Floor: β-pleated sheet
What is the pathological mechanism of Ankylosing Spondylitis?
Currently thought that the disease is due to abnormalities in both HLA-B27 and the interleukin-23 pathway.
HLA-B27 has a tendency to misfold and this causes cellular stress that trigger interleukin-23 release and triggers interleukin-17 production by
- Adaptive immune cells i.e. CD4 +ve Th17 cells
- Innate immune cells e.g. CD4 -ve CD8 -ve (‘double negative’) T cells
In what rheumatological conditions do you not see auto-antibodies?
- Osteoarthritis
- Reactive arthritis
- Ankylosing spondylitis
- Gout
What auto-antibodies are present in systemic vasculitis?
Antinuclear cytoplasmic antibodies
What auto-antibodies are present in rheumatoid arthritis?
- Rheumatoid factor
- Anti-cyclic citrullinated peptide antibody
What auto-antibodies are present in systemic lupus erythematosus? What is the significance of each?
- Antinuclear antibodies (ANA)
All cases, non-specific - Anti-double stranded DNA antibodies (anti-dsDNA) (HIGH)
Specific, serum level correlates with disease activity - Anti-cardiolipin antibodies (anti-phospholipid antibodies)
Associated with risk of a/v thrombosis in SLE - Anti-Sm antibodies (ribonucleoprotein antigen)
Specific for SLE
Patient will also have low complement levels
What auto-antibodies are present in diffuse systemic sclerosis?
- Anti-Scl-70 antibody
also termed antibodies to topoisomerase-1
What auto-antibodies are present in limited systemic sclerosis?
- Anti-centromere antibodies
What auto-antibodies are present in Dermato-/Polymyositis?
- Anti-tRNA transferase antibodies
e. g. histidyl transferase (also termed anti-Jo-1 antibodies)
What auto-antibodies are present in Sjögren’s syndrome?
No unique antibodies but typically see:
- Antinuclear antibodies -Anti-Ro and anti-La antibodies
- Rheumatoid factor
What auto-antibodies are present in Mixed connective tissue disease?
- Anti-U1-RNP antibodies
When would you see anti-Ro antibodies and anti-La antibodies in a patient?
Secondary Sjögren’s syndrome
Neonatal lupus syndrome (transient rash in neonate, permanent heart block)
When would you see anti-ribosomal P antibodies in a patient?
Cerebral lupus
If a patient has antinuclear antibodies when would further tests screen for?
- Anti-Ro
- Anti-La
- Anti-centromere
- Anti-Sm
- Anti-RNP
- Anti-ds-DNA antibodies
- Anti-Scl-70
Cytoplasmic antibodies include:
- Anti-tRNA synthetase antibodies
- Anti-ribosomal P antibodies