Metabolic and endocrine skeletal disease

OSTEOPOROSIS

RADIOLOGICAL FEATURES

XR

A reduced bone density (50% of bone density has to be lost to be visible) thin or absent trabeculae (with thickened remaining trabeculae due to increased stresses) thinned, irregular or a scalloped cortex (due to endosteal resorption) intracortical tunnelling and porosity (representing enlarged Haversian systems and Volkmann’s canals)

• Spine: a vertical ‘striated’ appearance to the vertebral bodies due to preferential loss of the horizontal trabeculae (this is seen within most vertebrae, in comparison with a haemangioma that affects a single vertebra)

Wedge, biconcave (‘cod-fish’) or crush deformities: the vertebral anterior and central mid-portions withstand any compressive forces relatively poorly

• Sacrum: insufficiency fracture lines are parallel to the SI joint on CT

‘Honda’ sign: a characteristic ‘H’ pattern of radionuclide uptake

MRI/scintigraphy

This is sensitive at detecting femoral neck insufficiency fractures before they are evident radiographically

PEARLS

Insufficiency fractures

These are low trauma fractures due to increased bone fragility

• Common sites: pubic rami sacrum vertebrae calcaneus distal forearm proximal femur vertebral bodies

• The axial skeleton is affected more frequently than the appendicular skeleton (and most commonly within the thoracic and thoracolumbar regions) fractures are uncommon above the level of T7 (consider metastases here)

Postmenopausal osteoporosis (type I)

This is due to an oestrogen lack it occurs 15–20 years after the menopause there is a disproportionate loss of trabecular bone

Senile osteoporosis (type II)

This affects men and women (≥ 75 years) it is due to age-related impaired bone formation associated with secondary hyperparathyroidism (there is a reduced vitamin D production in the elderly) there is a proportionate loss of trabecular and cortical bone

Disuse and reflex sympathetic dystrophy (RSD/Sudeck’s atrophy)

Overactivity of the sympathetic nervous system causing pain, soft tissue swelling and hyperaemia with excessive bone resorption (particularly peri-articularly) it is precipitated by a variety of causes (e.g. following a fracture or related to tumour)

Transient osteoporosis of the hip

A self-limiting condition affecting young and middle-aged patients (M>F) there is a sudden onset of pain with no associated trauma

• XR: reduced proximal femoral bone density

• MRI: this is sensitive for detecting early marrow changes (T2WI: increased SI)

Idiopathic juvenile osteoporosis

This is rare and affects prepubertal children with an acute variable course over a period of 2–4 years (it is reversible and spontaneously remits) it affects the vertebral bodies and the long bones (particularly the distal tibial metaphysis) it may be life threatening if thoracic involvement leads to a kyphoscoliosis (± respiratory failure)

• It needs to be differentiated from osteogenesis imperfecta (there are no blue sclerae)

Osteoporosis of young adults

A mild condition leading to multiple fractures of the vertebrae, metatarsals, ribs and hips (M = F) it is possibly due to inadequate bone mass formation during development

Secondary osteoporosis

See table of main causes

Treatment

For painful osteoporotic vertebral fractures:

• Vertebroplasty: cement (methylmethacrylate) is injected under image guidance into a fractured vertebral body a minimum of 4 weeks has to have elapsed after the initial injury

• Kyphoplasty: a balloon is first used to decompress a fracture prior to cement injection

Main causes of osteoporosis*

Primary
	Juvenile Idiopathic of young adults Postmenopausal Senile
Secondary
Endocrine	Glucocorticoid excess Oestrogen/testosterone deficiency Hyperthyroidism Hyperparathyroidism Growth hormone deficiency (childhood onset)
Nutritional	Intestinal malabsorption Chronic alcoholism Chronic liver disease Partial gastrectomy Vitamin C deficiency (scurvy)
Hereditary	Osteogenesis imperfecta Homocystinuria Marfan’s syndrome Ehlers–Danlos syndrome
Haematological	Thalassaemia Sickle-cell disease Gaucher’s disease
Tertiary
Other	Rheumatoid arthritis Haemochromatosis

VITAMIN D DEFICIENCY

OSTEOMALACIA

DEFINITION

• Vitamin D deficiency resulting in defective mineralization of osteoid in the mature skeleton

• Causes: nutritional deficiency malabsorption states and biliary disease (vitamin D is fat soluble and absorbed in the small bowel) chronic liver disease (affecting the initial prohormone hydroxylation step) chronic renal disease (the active metabolite is not produced) drug therapy (e.g. long-term anticonvulsants)

RADIOLOGICAL FEATURES

Looser’s zone (pseudofracture/Milkman’s fracture)

Translucent areas within bone (unmineralized osteoid) are a pathognomonic feature typically bilateral and symmetrical

• Location: typically found at sites of stress: medial aspect of the femoral neck pubic rami lateral border of the scapula and ribs

XR

A radiolucent line perpendicular to the cortex with a slightly sclerotic margin (it does not extend across the entire bone shaft)

• There is no callus formation unless it has been treated with vitamin D (osteoporotic insufficiency fractures often show florid callus formation) fractures occur medially on the concave side of a bone (incremental fractures in Paget’s disease tend to occur on the convexity of a bone)

Scintigraphy

Radionuclide bone scans are more sensitive than XR

• Osteomalacic bone is soft: it can result in bowing of the long bones, protrusio acetabuli and a triradiate deformity of the pelvis

PEARL

• Features of secondary hyperparathyroidism (the hypocalcaemia acts as a stimulus) can be seen

RICKETS

DEFINITION

• Vitamin D deficiency resulting in defective mineralization of osteoid in the immature skeleton abnormalities predominate at the growing ends of bones where endochondral ossification is occurring

• Causes: as for osteomalacia but also including inborn errors of vitamin D metabolism

Location

The most obvious changes are at the metaphyses (the area of most rapid growth) commonly seen around the knee and wrist, the anterior ends of the middle ribs, the proximal femur and the distal tibia

RADIOLOGICAL FEATURES

XR

Initially loss of the normal ‘zone of provisional calcification’ adjacent to the metaphysis

• Later features: a widened growth plate indistinct metaphyseal margins (‘frayed’ metaphyses) metaphyseal splaying and cupping (following weight bearing on uncalcified bone) indistinct and relatively osteopenic epiphyses (± Looser’s zones)

• There may be features of secondary hyperparathyroidism (in response to the hypocalcaemia)

• Harrison’s sulcus: rib in-drawing near the diaphragm

• Craniotabes: softening of the cranial vault

• Rachitic rosary: expanded long bone metaphyses can cause anterior rib enlargement

• Bone deformities: skull bossing delayed fontanelle closure bowing of the long bones (particularly the lower limbs) thoracic kyphosis with a ‘pigeon chest’ genu valgus and varum coxa vara and valga protrusio acetabuli a triradiate pelvis

• Post treatment: XR features of healing lag behind biochemical and clinical improvements (2 weeks)

Harris growth arrest line: with treatment mineralization of the zone of provisional calcification gives a dense white line adjacent to the metaphysis (as a marker of the age at which the rickets occurred)

PEARLS

Vitamin D resistant rickets

This follows defective renal tubal reabsorption of phosphate (with increased renal excretion of calcium and phosphate) normal or elevated vitamin D levels radiographically similar to rickets but refractory to vitamin D therapy it may be seen with X-linked hypophosphataemia or Fanconi’s syndrome

Vitamin D dependent rickets

A group of autosomal recessive conditions

• Type I: a defect in the renal production of the active metabolite

• Type II: end-organ resistance to the active metabolite

Acquired hypophosphataemic rickets (tumour-induced oncogenic rickets)

Seen in association with bone or soft tissue tumours that produce a phosphaturic substance (e.g. a haemangiopericytoma, non-ossifying fibroma, giant cell tumour, fibrous dysplasia, or an osteoblastoma) there is a normal serum calcium

Metaphyseal chondrodysplasias

Mild (Schmit type) or severe (Jansen type) this can mimic rickets but is differentiated by a normal serum biochemistry

	Serum			Urine
	Calcium	Phosphorus	Alkaline phosphatase	Calcium
*Osteoporosis*	N	N	N	N
*Hyperparathyroidism* Primary Secondary Tertiary	↑ N or ↑ ↑	↑ N or ↓	N or ↑ ↑ N or ↑	N or ↑ ↓ N or ↑
*Hypoparathyroidism*	↓	↑	N	↓
*Pseudohypoparathyroidism*	↓	↑	N	↓
*Rickets/osteomalacia* Vit D deficient Vit D refractory Hypophosphatasia	↓ N N or ↑	↓ ↓ N	↑ ↑ ↓	↓ N or ↑