Os Odontoideum (Degenerative Joint Disease)

Definition

• chronic non-inflammatory disorder of joints
• characterised by:
• articular cartilage softening & destruction
• new bone formation at the joint surfaces & margins (osteophytes)
• subchondral sclerosis & cysts
• capsular fibrosis
• usually dx.of advancing age
• if in young - underlying pathology
• more common in some joints (hip, knee & spine) than in others (elbow & ankle)
• terminal inter-phalangeal OA chiefly affects postmenopausal women (Heberden nodes)
• different in ethnic groups
• OA of the hip rare in Africans
• common in southern European women - high incidence of acetabular dysplasia

Aetiology

• primary or secondary

Etiological factors

• obesity
• genetic & hereditary forms (dysplastic hip, Mseleni hip dx)
• occupation (IPJ’s in cotton mill workers)
• endocrine disorders eg. diabetes, acromegaly
• metabolic disorders eg. Paget’s, gout, CPPD

Increased stress (load per unit area)

• local mechanical factors
• 76 % underlying cause for hip OA
  • dysplasia
  • Perthes
  • SCFE
• >> load
  • deformities affecting the lever system
• << contact area
  • joint incongruity or instability

Weak cartilage

• stiff
  • ochronosis
• soft
  • inflammation (RA, TB etc.)
  • crystal arthropathies

Abnormal support (subchondral bone)

• osteonecrosis - AVN
• soft bones
  • dysplasias
  • OI
  • Paget’s

Pathogenesis

• cartilage ageing occur
  • splitting & flaking of the surface
  • diminished cellularity
  • reduction of the proteoglycan ground substance & loss of elasticity with decrease in breaking strength
  • proteoglycan matrix depletion - release of proteolytic enzymes - collagen failure - chondrocyte damage - cartilage deformation
• articular cartilage distributing forces associated with joint loading
• when loses its integrity ->> forces concentrated in the subchondral bone
  • cyst formation & reactive sclerosis in the zone of maximal loading
• as the articular surfaces become increasingly mal-apposed & the joint unstable
  • osteophyte formation

Pathology

Cardinal features

1. progressive cartilage destruction
2. subarticular cyst formation
3. sclerosis of the surrounding bone
4. osteophyte formation
5. capsular fibrosis

• initially confined to one part of the joint - the most heavily loaded part
• softening, fraying, or fibrillation of cartilage (chondromalacia)
  • joint space narrowing
• progressive disintegration of cartilage -> underlying bone becomes exposeda)
  • subchondral sclerosis & cysts
• joint capsule shows thickening & fibrosis
• osteophytes arise from the edge of the articular surface as bony outgrowths
• marked vascularity & venous congestion -> pain