Fracture Healing and Avascular Necrosis

Question 1

What are the phases of indirect (secondary) bone healing?

Answer 1

• Fracture haematoma and inflammation (blood from broken vessels forms a clot, this releases growth factors (TGF-beta) which stimulate inflammatory cells to come to the area of the fracture.
• Fibrocartilage (SOFT) callus – lasts about 3 weeks, new capillaries organise fracture haematoma into granulation tissue, ‘procallus’, fibroblasts and osteogenic cells invade procallus, make collagen fibres which connect ends together and chondrocytes begin to produce fibrocartilage.
• Bony (HARD) callus – after 3 weeks and lasts about 3-4 months where osteoblasts make woven bone.
• Bone remodelling – osteoclasts remodel woven bone into compact bone and trabecular bone.

Question 2

What is direct (primary) bone healing?

Answer 2

• Artifical surgical situation
• Involves stabilising fracture so no movement under physiological load

Question 3

Which fractures lead to a compromised blood supply to bone?

Answer 3

• Proximal pole of scaphoid fractures
• Talar neck fractures
• Intrascapular hip fractures
• Surgical neck of humerus fractures

Question 4

What is avascular necrosis/osteonecrosis?

Answer 4

• Bone infarction near a joint

Question 5

What is the pathophysiology of avascular necrosis/osteonecrosis?

Answer 5

• Oedema, haemorrhage, fibrilloreticulosis and hypocellularity may be present in bone marrow lesions
• In AVN, necrosis in medullary bone first
• Overlying cartilage receives nutrition from synovial fluid and remains viable
• Dead bone has empty lucanae surrounded by necrotic adipocytes that often rupture and release fatty acids
• Increased interosseous pressure causes fat to come out leading to focal areas with lack of blood supply
• Fatty acids bind calcium and form insoluble calcium soaps
• During healding process, osteoclasts resorb the necrotic trabeculae

Question 6

How is avascular necrosis/osteonecrosis diagnosed and treated?

Answer 6

• X-ray may be normal for months but pathogenic crescent sign (subchondral radiolucency) precedes subchondral collapse
• In late stages loss of sphericity and collapse of the femoral head and joint space narrowing can be seen
• MRI shows increased signal on T2 where oedema is present in bone
• Treat using decompression and joint replacement