Osteogenesis Imperfecta — Orthopaedic Manifestations

The Core Defect (Conceptual Foundation)

1. Decreased bone mass — less material
2. Increased brittleness — the material that exists is poorly organized

1. Bone Fragility & Fractures

Key concept: OI fractures are not a healing failure — they are a remodeling failure. The fracture "heals" mechanically but the bone never regains normal architecture.

2. Progressive Bowing

• Each fracture heals with slight angular malunion (because the remodeling capacity to straighten it is absent)
• Weight-bearing forces then act on the angulated bone, causing further deformation
• This is most dramatic in the femora and tibiae — the lower limbs carry load and the bone literally bends under physiologic stress over time

3. Ligamentous Laxity

4. Short Stature

• Recurrent fractures → long bones never achieve normal length
• Compression fractures of vertebrae → loss of axial height
• Progressive bowing → functional limb shortening
• Underlying growth plate dysfunction from collagen deficiency

5. Scoliosis

• Vertebral body weakness → wedging → asymmetric collapse drives curvature
• Ligamentous laxity → poor spinal stability → the spine drifts
• Muscle weakness (secondary to disuse and recurrent fractures) → reduced dynamic support
• Scoliosis is progressive and can compromise pulmonary function (a major cause of morbidity in OI)

6. Codfish Vertebrae (Compression Fractures)

• The intervertebral disc is made of nucleus pulposus, which is under hydrostatic pressure and is much stronger than the adjacent weakened OI vertebral bodies
• The disc therefore bulges symmetrically into both end-plates of the weak vertebral body above and below, creating the classic biconcave "codfish" shape
• This is a compression fracture equivalent in OI, not a disc herniation

7. Basilar Invagination

• The skull base is also made of collagen-dependent bone
• In OI, the occiput softens and the foramen magnum region gradually invaginates upward under the weight of the skull
• The odontoid process of C2 can then protrude into the foramen magnum → brainstem compression
• This is a life-threatening complication unique to OI among the skeletal dysplasias

8. Olecranon Apophyseal Avulsion Fracture — Most Common First Sign

• At the olecranon, the triceps tendon attaches to the apophysis (a traction epiphysis)
• Normally, a child pulling to stand or throwing a ball generates tensile force at this site — the bone should easily resist this
• In OI, the weakened bone avulses at physiologic loads — a force that would not fracture normal bone
• This is often the first fracture that brings the child to medical attention, because it occurs with trivial activity and is anatomically unusual (avulsion fractures are rare in normal children)
• It serves as a red flag for an underlying bone fragility disorder

9. Coxa Vara (~10%)

• The femoral neck is under continuous bending stress from body weight
• In normal bone, this is resisted easily; in OI, the weakened femoral neck gradually bends (plastic deformation + repeated microfractures) into a varus position
• This worsens gait, increases the lever arm on the abductors, and can progress to a shepherd's crook deformity in severe cases

10. Congenital Anterolateral Radial Head Dislocation

• Ligamentous laxity (from defective collagen in the annular ligament) is the primary driver
• The radial head is not held tightly within its socket → it subluxes/dislocates
• Anterolateral direction is the typical direction of dislocation in OI
• This is often missed clinically because elbow range of motion may be preserved, but it can eventually cause pain, limited forearm rotation, and a prominent lateral elbow bump

Summary Concept Map

Defective Type I Collagen
        |
   ┌────┴─────┐
BONE (scaffold)    SOFT TISSUE (ligaments)
        |                   |
Poor mineralization    Ligamentous laxity
Decreased mass         → Joint instability
Increased brittleness  → Radial head dislocation
        |
   ┌────┴──────────────────────┐
FRACTURES                 DEFORMITY
 • Olecranon avulsion (1st)    • Bowing (femur, tibia)
 • Recurrent long bone fx      • Coxa vara
 • Compression fx              • Scoliosis
 • Codfish vertebrae           • Short stature
        |
   Abnormal remodeling
        |
   Progressive angulation → Bowing worsens
        |
   Axial skeleton involvement
        |
   Basilar invagination (skull base softening)