Metabolic Bone Disease 2- Clinical Biochemical Features of Bone Disease
Anil Chopra
1. Explain how basic biochemical tests can be used to assess mineral metabolism and
bone turnover.
2. Explain how Bone Densitometry can be used to diagnose Osteoporosis, and to
monitor response to treatment.
3. Describe the biochemical and clinical features of osteoporosis, osteomalacia,
Paget’s disease and hyperparathyroidism; and the basics of treatment
Metabolic bone diseases are
characterised by a decrease in bone
density and bone strength either by
increasing bone reabsorption or
decreasing its formation (or both).
Bone is metabolically active tissue
consisting of an interaction between
osteocytes and osteoblasts.
Osteoblasts synthesise bone matrix,
regulate the deposition of
hydroxyapatite (calcium-containing
mineral of bone) and secrete molecules
to control the proliferation and action
of osteoclasts. They arise from
pluripotential mesenchymal cells.
Osteoclasts breakdown and reabsorb
bone, dentine and calcified cartilage. They
play an important role in the modelling of
growing bone, and maintenance of shape.
They are also activated by parathyroid
hormone to regulate the levels of calcium
in the blood.
Assessing Bone Function
 Radiology e.g. osteomalacia, Paget’s
disease
 Bone mineral densitometry, e.g.
osteoporosis
 Bone histology
 Biochemical tests
 Serum
 calcium
 corrected calcium
 albumin
 phosphate
 parathyroid hormone
 25-hydroxy vitamin D
 Urine
 NTX (Cross-linked N-telopeptides of
type I collagen)
 Calcium
 Phosphate
The majority of the calcium in the body is
stored in bone however, that which is stored in
the blood is stored:
- 46% bound to proteins
- 46% free ionised
- 7% complexed
The level is controlled, minute by minute by parathyroid
hormone.
Parathyroid hormone is an 84 amino acid peptide but only
N1-34 are active. It is dependent on Mg and has a plasma
half life of around 8 minutes. It activates receptors in bone,
kidney and the gastrointestinal tract – these receptors can
also be activated by PTHrP – parathyroid hormone related
protein.
The varying rate and amount of release of PTH is a steep
inverse sigmoidal function in relation to Cao2+ in vivo.
MINIMUM- at this level, there is still a baseline release of
PTH SET POINT – this is half the level of suppression of PTH,
and at this point, even small peturbances in Ca2+ cause large
changes in PTH.
Hyperparathyroidism: primary hyperparathyroidism is caused mainly by a
pituitary adenoma, or hyperplasia. It is occasionally caused by an inherited syndrome.
It can cause
1. Increase serum calcium, by absorption from bone/gut/kidney
2. Decrease serum PO4, as increased absorption is overcome by marked renal
excretion
3. Increase urine calcium excretion, as increased renal resorption is overcome by the
hugely increased filtered load
4. Increase markers of bone resorption, thus secondarily formation
Clinical Features
- Stones : Renal colic, nephrocalcinosis, CRF
- Abdominal moans: Dyspepsia, pancreatitis
Constipation, nausea, anorexia
- Psychic groans: Depression, impaired
concentration, Drowsy, coma.
- Thirst Polydipsia
- Polyuria due to impaired reabsorption of water and
sodium.
- Tiredness
- Fatigue
Frusemide
- Muscle weakness
- Increased susceptibility to fractures.
-
Management of Hyperparathyroidism
- BMD (bone mineral density) for osteoporosis
- Ultrasound for renal stones
- 24hr CrCl for renal failure.
Can treat either with surgery or with use of bisphosphonates and calcimimetics.
Osteomalacia: inadequate Vitamin D activity leads to defective mineralisation of the
cartilaginous growth plate (before a low calcium).
Symptoms
- Bone pain and tenderness (axial)
- Muscle weakness (proximal)
- Lack of play
Signs
- Age dependent deformity
- Myopathy
- Hypotonia
- Short stature
- Tenderness on percussion
Causes
Dietary
Gastrointestinal
Renal
Resistance
Lack of sunlight
Decreased production with age.
Small bowel malabsorption/ bypass
Pancreatic insufficiency
Liver/biliary disturbance
Drugs- phenytoin, phenobarbitone
Chronic renal failure – renal phosphate loss.
Vitamin D dependent rickets type I (autosomal recessive, no
1a-hydroxylation)
Vitamin D dependent rickets type II (autosomal recessive,
VDR defect)
Biochemistry
 Serum
 Calcium
N/low
 Phosphate
N/low
 Alk phos
High
 25(OH)Vit D Low/normal
 PTH
High
 Urine
 Phosphate
High
Management
Patients should be X-rayed and ultrasound scanned.
NB: if calcium and vit D levels are normal then the cause could be renal phosphate
loss, either from X-linked hypophosphataemic Rickets (a disease caused by
mutation in PHEX) or oncogenic osteomalacia (caused by a mesenchymal tumour).
Age Related Changes in Bone Mass
The graph shows an increased loss of bone mass and density in post-menopausal
women. This is associated with oestrogen levels:
• Increases the number of remodelling units
• Causes remodelling imbalance with increased bone resorption (90%)
compared to bone formation (45%)
• Enhanced osteoclast survival and activity
• Remodelling errors. Deeper and more resorption pits lead to trabecular
perforation and cortical excess Haversian excavation
• Decreased osteocyte sensing.
Tests in Osteoporosis
Serum Biochemistry:
 Check Vitamin D levels
 Check for secondary endocrine causes of diease
o Hyperparathyroidism – high PTH
o Hyperthyroidism – high T3, low TSH
o Hypogonadis – low testosterone
 Exclude multiple myeloma
 Check urine calcium (may be high in
osteoporosis)
Bone densitometry
 “Gold standard” for BMD is D.X.A (Dual energy
X-ray absorptiometry): Measures transmission
through the body of X-rays of two different photon energies. It enables densities
of two different tissues to be inferred, i.e. bone mineral, soft tissue
Osteoporosis is defined by patients T-score
T-score =
-2.5
-1to -2.5
<-1
OSTEOPOROSIS
OSTEOPAENIA
NORMAL
T-score =
measured BMD – young adult mean BMD
Young adult standard deviation
Complications of Osteoporosis
The most common problem is increase in fracture risk. The most common fracture is
that of the spine, and then the hip.
Treatment for Osteoporosis
Treatment for osteoporosis is decided by Z-score:
Z-score =
measured BMD – aged-matched mean BMD
aged-matched standard deviation
The result is compared to a healthy normal subject matched for age, gender and ethnic
origin. However, certain Situations can interfere with the interpretation of the scores:
• Degenerative change,
• Osteomalacia
osteoarthritis
• Vascular calcification
• Vertebral fractures
• Scoliosis
• Metal artefacts
• Paget’s disease
The criteria for measurement is dependent on age and complications.
Bone Markers
We can use bone markers to give us an idea of bone activity rather than using bone
mineralisation density. It is a dynamic test.
In bone formation the 2 ‘Alpha 1’ and 1 ‘Alpha 2’ chains of type I collagen that are
produced by the osteoblast are joined together.
3 hydroxylysine molecules on adjacent tropocollagen fibrils then condense to form a
PYRIDINIUM ring linkage ; DPD, PYD
The products produced by the collagen breakdown are useful markers for bone
reabsorption as they directly correlate with the amount of bone reabsorbed. These
include:
 Pyridinium (DPD; PYD not bone specific)
 N-terminal telopeptide (NTX)
 C-terminal telopeptide (CTX)
They can be used in the diagnosis of the disease, to predict fracture risk and also to
monitor treatment. Bone markers are higher in osteoporotic patients. There are
however certain problems with the use of bone markers including their
reproducibility, the association with age, the need to correct the levels for Cr, and the
diurnal variation in level.
Measurement of osteoblast function is also useful:
- serum alkaline phosphatase
- osteocalcin
- propeptides of type 1 collagen
o carboxyterminal PICP
o aminoterminal PINP
- BSAP – bone specific alkaline phosphatase: NB increased in Paget’s disease,
osteomalacia, bone metastases, hyperparathyroidism and hyperthyroidism
Treatment of Osteoporosis
1. Optimise calcium and vitamin D
800 units vitamin D3
1g calcium
2. Prevent bone resorption
bisphosphonates
3. Increase bone formation
strontium
PTH
Paget’s Disease: Chronic disorder of excess
focal exaggerated bone remodelling.
There are 2 main causes of Paget’s disease, viral and genetic.
Investigations
Bone markers
Total Alkaline phosphatase only one needed
Bone-specific if normal total alk-p or liver disease
Plain X-rays
to confirm diagnosis
to diagnose complications
Bone scan
to assess extent of disease at diagnosis
rarely repeated
Should be treated with bisphosphonates.