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1. Developmental bone diseases

2. Developmental bone diseases (DBD) These diseases include the following: Achondroplasia . Osteogenesis imperfecta . Ospeopetrosis . Paget’s disease. Mucopolysaccharidosis . Osteoporosis (primary and secondary).

3. In this session the following diseases will be discussed: Achondroplasia . Osteogenesis imperfecta . Osteopetrosis . Paget’s disease of bone.

4. Achondroplasia The most common cause of dwarfism . Incidence: 1/25000 birth. It is a hereditary defect: Autosomal dominant. Mutation in fibroblast growth factor receptor 3 (FBGFR3).

5. Pathogenesis: Activation of fibroblast growth factor receptor 3 (FBGFR3) inhibits cartilage synthesis at the epiphyseal growth plate. This leads to decreased enchondral bone formation and premature ossification of the growth plates.

6. Clinical features (generally): Large head with frontal bossing. Hypoplasia of the mid face and low nasal bridge with narrow nasal openings. Normal trunk, with short extremities . The fingers appear short.

7. Achondroplasia, adult Large head with frontal bossing. Short arms. Normal trunk. Short legs. Normal intelligence. Normal reproductive ability. Normal life span

8. Achondroplasia in relation to a normal child

9. Osteogenesis imperfecta (OI), (Brittle bone disease): Group of closely related genetic disorders. Types of osteogenesis imperfecta : 8 types, but 2 only will be discussed: Type I. Type II.

10. Type I: (mild form) Collagen is of normal quality but is produced in insufficient quantities. Clinical features: Bones fracture easily. Slight spinal curvature. Loose joints. Poor muscle tone.

11. Osteogenesis imperfecta X-ray of the lower limbs of an infant. Showing multiple bone fractures and shortening.

12. Discoloration of the sclera, usually giving them a blue-gray color because it becomes thin. Early loss of hearing in some children. Slight protrusion of the eyes.

13. Blue sclera in OI

14. Type II: (severe form) Collagen is abnormal in quality & quantity. Most cases die within the first year of life due to respiratory failure or  intracerebral hemorrhage. Severe respiratory problems due to underdeveloped lungs. Severe bone deformity and small stature.

15. Pathogenesis: Caused by qualitative or quantitative abnormal type-I collagen synthesis. There is a mutation in the genes that code for alpha-1 and alpha-2 chains of collagen molecule, leading to weakness in the bone and this leads to bone fractures .

16. Microscopic appearances: In OI due to quantitative defects: Thick osteoid seams and reduced intercellular matrix. The number of osteoclasts and osteocytes are normal. Bone trabeculae are thin and disorganized. Lamellar bone is seen in the diaphysis and metaphysis.

17. In OI due to qualitative defects: Increased number of osteocytes and increased vascular channels. Reduced cortical bone thickness. Woven bone is seen, with minimal osteoid bone and no lamellar bone.

18. Osteopetrosis (marble bone disease): Hereditary disease. Clinical forms: Autosomal recessive characters: Affects infants and children. There are multiple fractures. Early death due to anemia, infection or hemorrhage. Autosomal dominant characters: Affects adults. There are fractures. There is mild anemia. Cranial nerve impairment.

19. Pathogenesis: Normal bone growth is achieved by a balance between bone formation by osteoblasts and bone resorption by osteoclasts. In osteopetrosis , the number of osteoclasts may be reduced, normal, or increased. Dysfunction of osteoclast mediates the pathogenesis of this disease.

20. Effects: Increased bone density and thickening of bone cortex. The thickened bone is fractured easily. Myelophthisis (narrowing and fibrosis of bone marrow, which may lead to pancytopenia) Extramedullary hematopoiesis. Cranial nerve compression: Due to narrowing of cranial foramina. May leads to blindness, deafness and facial palsy.

21. Hydrocephalus due to obstruction of CSF flow. X-ray findings: Symmetrical generalized osteosclerosis . Long bones may have metaphyseal widening.

22. X-ray of the pelvis in osteopetrosis

23. Paget’s disease of bone (osteitis deformans) Definition: Localized disorder of bone remodeling, resulting in excessive bone resorption followed by disorganized bone replacement producing thickened but weak bone.

24. Begins after age of 40. Poly- ostotic (85%), monostotic (15%). Causes: May be caused by viral infections ( Paramyxovirus , measles & others). Genetic predisposition (defect in specific regions of chromosome 5 & 6). Environmental (air pollution may play a role).

25. Common sites affected: Skull, Pelvis, Femur, Tibia & Vertebra. Pathogenesis: The pathogenesis of Paget's disease is described in 3 stages: Osteoclastic ( osteolytic ) activity. Mixed osteoclastic -  osteoblastic  activity. Sclerotic activity.

26. Microscopic appearances: Osteolytic phase: resorption of bone by numerous large osteoclasts. The mixed phase ( osteolytic and sclerotic): woven bone formation with few lamellar bone, but poorly mineralized, so fracture is easy. The lamellar bone is deposited in mosaic pattern, puzzle-like , and this is diagnostic for Paget disease. The sclerotic phase: thick lamellar bone and fibrotic bone marrow.

27. Osteolytic phase in Paget’s disease

28. Mixed phase “Mosaic bone” in Paget’s disease

29. Paget’s disease, tibia X-ray shows well-defined two areas of osteolysis in diaphysis of tibia.

30. Clinical features and complications of Paget disease: Asymptomatic in most cases. In some cases: bone pain and deformities. Fractures. Warmth over the bone due to high vascularity. Osteoarthritis. Visual problems. Headache. Osteosarcoma in 1% of cases.