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Metabolic and Endocrine Skeletal Disease


Osteoporosis

Osteoporosis

Definition

  • A systemic skeletal disease characterized by a low bone mass and micro-architectural deterioration of the bone tissue with a consequent increase in bone fragility and an associated susceptibility to fracture

  • It is the most common metabolic disorder affecting 50% of woman and 20% of men (> 50 years old) ▸ due to:

    • ▪

      Reduced bone accumulation during development

    • ▪

      Bone resorption outstrips new bone formation during later life (the ratio of osteoid matrix to hydroxyapatite mineral is normal)

  • WHO definition: this is based on the bone mineral density (BMD) ▸ a T-score is the standard deviation of the BMD compared to a young healthy reference ▸ it is not appropriate for use in children (where you use > 2SD below the mean BMD matched for age, gender and ethnicity)

    • ▪

      Normal: BMD > -1SD below the young adult mean (peak bone mass)

    • ▪

      Osteopenia: BMD between -1 and -2.5SD below the young adult mean

    • ▪

      Osteoporosis: BMD > -2.5SD below the young adult mean

Clinical presentation

  • It can be asymptomatic ▸ there can be insufficiency fractures presenting with pain (e.g. vertebral crush fractures) ▸ the pain resolves spontaneously after 6–8 weeks unlike a more sinister pathology ▸ vertebral fractures may result in an increasing thoracic kyphosis

  • Bone loss typically begins during the 4 th decade (females) or 5 th and 6 th decades (males)

Location

It is more prominent in areas rich in trabecular bone (e.g. the vertebrae, pelvis, ribs and sternum)

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 ▸ with a posterior fracture, consider metastases/myeloma

  • 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/pelvis 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)

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

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

Osteoporosis. Reduced bone density, thinned cortex and reduced number of trabeculae, those which remain appearing more prominent. *

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

Osteoporosis – grading vertebral fractures. Vertebral fractures are strong predictors of future fractures (×5 for vertebral fracture and ×2 for hip fractures) so it is important that they are accurately and clearly reported by radiologists. The higher the grade of vertebral fracture, the higher the risk of future fracture. *

Transient osteoporosis of the hip. STIR image of the left hip in a 42-year-old man with transient osteoporosis. Note homogeneous bone marrow oedema pattern without focal linear signal abnormalities, which would suggest a fracture, or deformities as might occur in avascular necrosis of the femoral head with subchondral fracture. **

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)

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

Scintigraphy

Radionuclide bone scans are more sensitive than XR

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) ▸ thin ‘ghost-like’ rim of mineralisation at the periphery of the metaphysis

  • 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) ▸ initially separated by translucent osteoid, which may be mistaken for a metaphyseal fracture (NAI)

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 FGF23, a phosphate and vitamin D regulating hormone (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

Osteomalacia. The pathognomonic XR feature is the Looser's zone – radiolucent, unmineralized linear areas perpendicular to the cortex and which may have a sclerotic margin (demonstrated in the medial aspect of the left femoral neck). *

Rickets. There is ‘fraying’ of the visible metaphyseal margins. †

Rickets. There is splaying of all the visible metaphyses, with widened epiphyseal plates. In addition there is bowing of the femora and bones of the lower leg. †

Patent sacroiliac joints, dense bones, and bowed femora in X-linked hypophosphatemic osteomalacia. ©35

Rickety rosary. Widening of the anterior ribs is demonstrated (arrows). Metaphyseal changes are also seen in the proximal humeri. †

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

Looser's zone. There is lucency with surrounding sclerosis in the lateral border of the scapula. †

Endocrine Bone Disorders

Hyperparathyroidism

Definition

Primary hyperparathyroidism

This is due to increased parathyroid hormone production (most parathyroid tumours are functionally active)

  • Causes: a single parathyroid adenoma (80%)* ▸ hyperplasia (15–20%) ▸ carcinoma (1%)

    • ▪

      * multiple adenomas are seen in 4% of cases

Secondary hyperparathyroidism

This is induced by a fall in the serum calcium

  • Causes: vitamin D deficiency ▸ intestinal malabsorption of calcium ▸ chronic renal failure (causing a lack of the active vitamin D metabolite)

Tertiary hyperparathyroidism

This occurs in long-standing secondary hyperparathyroidism when an autonomous adenoma develops within the hyperplastic parathyroid glands

Clinical Presentation

  • It is often asymptomatic, but can present with: renal stones and nephrocalcinosis ▸ hypertension ▸ pseudogout (chondrocalcinosis) ▸ osteoporosis ▸ peptic ulcers ▸ acute pancreatitis ▸ depression ▸ proximal muscle weakness ▸ lethargy

Radiological Features

  • 95% have no radiological abnormalities (as a result of effective early therapy)

  • Subperiosteal erosions of cortical bone: pathognomonic ▸ initially affects the radial aspects of the middle phalanges of the index and middle fingers – if it is not seen here then it is unlikely to be identified elsewhere ▸ tufts of the distal phalanges

  • Other sites (indicating more severe and long-standing disease): distal phalanges (acro-osteolysis) ▸ proximal medial tibial cortex ▸ outer ends of the clavicles ▸ symphysis pubis ▸ ribs ▸ vertebral bodies (Schmorl's nodes) ▸ sacroiliac joints ▸ proximal humeral shaft

    • ▪

      There can be loss of the lamina dura of the teeth

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