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Pseudoarthrosis "false joint" is the lack of osseous continuity after a fracture or surgical arthrodesis, often with fibrous tissue interposition and persistent motion, typically after ≥6–9 months post-injury or surgery.

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It is a pathological condition, not a congenital joint, and may be symptomatic or discovered incidentally.

Classification

A. Based on Radiographic Appearance

Type	Description
Hypertrophic	Vascular and biologically active → shows callus formation ("elephant foot")
Oligotrophic	Minimal callus; still viable but healing impaired
Atrophic	Avascular and biologically inactive → tapered bone ends, no callus

B. Based on Cause

Type	Notes
Congenital	Seen in neurofibromatosis, especially ‣
Acquired	Most common; follows fracture or failed fusion
Post-surgical	E.g., failed spinal fusion, limb reconstruction

Clinical features

Feature	Description
Persistent pain	At site of prior fracture/fusion
Instability/movement	May feel abnormal motion across site
Palpable gap	In superficial bones like clavicle
Deformity	In long-standing or untreated cases
Hardware failure	Screws or rods may loosen due to unstable nonunion

Common locations:

Site	Clinical Context
Tibia	After trauma or congenital (e.g., NF1-related)
Clavicle	Post-traumatic, especially if displaced
Spine	Post-laminectomy fusion or in ankylosing spondylitis (Andersson lesion)
Forearm (radius/ulna)	Malunion/nonunion in diaphyseal fractures

Special Cases:

Cases	Key features
Spinal Pseudoarthrosis	• Seen after failed spinal fusion (especially lumbar)
• Presents with back pain, instability, or hardware failure
• Confirmed with CT; may need revision surgery
‣	• Pseudoarthrosis in ankylosing spondylitis
• Seen as vertebral body cleft or erosion on imaging
‣	• Strongly associated with neurofibromatosis type 1
• Presents early in childhood with pathologic fracture and nonunion

Radiology

Plain radigraphy:

Feature	Hypertrophic	Atrophic
Callus formation	Abundant	Absent
Bone ends	Sclerotic, rounded	Narrow, tapered, "penciled" ends
Gap	Present	Present
Motion across site	May be inferred (unstable hardware)	More likely with poor healing

Other modalities: