What is a subchondral cyst?

What is a Subchondral Cyst?

A subchondral cyst is a fluid-filled or mucoid-filled cavity located in the bone immediately beneath the articular cartilage (subchondral bone) of a joint, representing a pathological feature commonly associated with osteoarthritis, osteochondral injuries, and cartilage repair procedures.

Definition and Anatomical Location

Subchondral cysts are abnormal cavities within the subchondral bone that develop in close proximity to articular surfaces 1. They are characterized by:

• Location: Situated in the subchondral bone layer, directly beneath the joint cartilage
• Appearance on imaging: Present as well-defined lucent (dark) areas on radiographs and fluid-intensity lesions on MRI
• Communication: May or may not connect to the joint cavity through fissures or defects in the overlying cartilage 2, 3

Pathophysiology and Formation Mechanisms

The development of subchondral cysts involves two synergistic mechanisms 2:

Internal Mechanism

• Upregulation of local proinflammatory factors
• Secretion of matrix metalloproteinases (MMPs)
• Enhanced bone turnover and remodeling
• Angiogenesis (new blood vessel formation)
• Biological response to abnormal mechanical loads causing repeated microtrauma 4

External Hydraulic Mechanism

• Synovial fluid forced into subchondral bone through cartilage defects
• Increased intra-articular pressure driving fluid accumulation
• Progressive expansion through continued fluid influx

Clinical Context and Associated Conditions

Subchondral cysts occur in several clinical scenarios:

Osteoarthritis

• Recognized radiographic marker of advanced degenerative joint disease 5, 4
• Commonly found in weight-bearing joints (hip, knee, ankle)
• Associated with other OA features: joint space narrowing, osteophytes, subchondral sclerosis 5
• Present in 91% of femoral heads examined in advanced hip OA 6

Osteochondral Injuries

• Can develop following focal cartilage and bone damage 7
• May appear as "osseous cysts" on imaging when associated with osteochondral lesions
• Often missed on initial radiographs (41% of osteochondral abnormalities) 7

Post-Cartilage Repair

• Early postoperative sign following articular cartilage repair procedures 2
• Can develop after autologous chondrocyte implantation, particularly at anchor sites 3
• May progressively enlarge in the postoperative period

Imaging Characteristics

Radiography

Plain radiographs show subchondral cysts as well-defined lucent areas in the subchondral bone 5. However, radiography has limitations in detecting early or small cysts.

CT Imaging

CT without contrast is particularly helpful for visualizing subchondral cysts 7. CT demonstrates:

• High specificity (99%) for detecting osseous abnormalities 7
• Superior detail of bone architecture
• Variable presence of sclerotic (dense bone) rim around the cyst

MRI Features

MRI shows subchondral cysts as 8:

• Fluid-intensity signal (bright on T2-weighted and STIR sequences)
• May demonstrate surrounding bone marrow edema
• Can reveal communication with joint space
• Sensitivity of 96% for detecting osteochondral abnormalities 7

Composition and Histological Features

Subchondral cysts are heterogeneous in composition 6:

• Fibrous tissue (most common)
• Mucoid substance 3
• Fatty tissue (least common)
• May contain: vasculature, nerve fibers, cartilage islands, bony spicules
• Aggregation of foamy macrophages 3
• Evidence of ongoing bone resorption with osteoclast activity 3

Clinical Significance

Impact on Joint Function

• Associated with focal abnormalities in bone density and cartilage thickness 6
• Cartilage overlying cysts is typically thinner than non-overlying regions
• Bone adjacent to cysts shows increased density compared to normal subchondral bone
• Can cause pain, particularly when associated with bone marrow edema 8

Prognostic Implications

• Marker of disease progression in osteoarthritis 4
• May affect outcomes of cartilage repair procedures 2
• Can influence surgical planning and treatment decisions 7

Common Pitfalls in Recognition

Key caveat: Subchondral cysts must be distinguished from more aggressive processes such as tumors 1. Critical differentiating features include:

• Clinical context (age, joint involvement pattern)
• Associated degenerative changes
• Well-defined margins with or without sclerotic rim
• Location in typical weight-bearing areas
• Absence of aggressive periosteal reaction or soft tissue mass

The presence of subchondral cysts in younger patients or atypical locations should prompt consideration of alternative diagnoses including inflammatory arthropathies, infection, or neoplastic processes.