Cystic Lesions

Sung Wook Seo

Young Lae Moon

Francis Young-In Lee

This chapter includes cystic lesions which may occur in bone. Epidermal inclusion cysts and ganglions may also occur in soft-tissues, where the pathophysiology is similar. Simple bone cysts and aneurysmal bone cysts represent distinct clinical entities which occur within bone only.

Epidermal Inclusion Cyst

Epidermal inclusion cysts are more commonly seen as soft-tissue lesions, but may involve bone, and present in either case as painful digital lesions which typically require surgical extirpation.

Pathogenesis

Etiology

For both soft-tissue and intraosseous epidermal inclusion cysts, penetrating trauma causes epidermal tissue to be deposited deep within the tissues

Pathophysiology

Epidermal tissue traumatically deposited within the deeper tissue grows and causes cyst formation
Cystic lesion filled with keratinous materials lined with flattened squamous epithelium

Diagnosis

Physical Examination and History

Clinical Features

Painful, often swollen mass most often within distal digit
History of penetrating trauma in many cases

Radiologic Features

Intraosseous geographic radiolucent lesion
Distal phalanx of fingers common

Pathologic findings

Cystic cavity filled with keratinaceous debris

Diagnostic Workup Algorithm

Differential diagnosis includes intraosseous synovial or ganglion cyst
Diagnosis usually apparent radiographically, confirmed histologically

Treatment

Surgical Indications/Contraindications

Painful lesions bone can be curetted and grafted

Degenerative Cyst (GEODES)

Pathogenesis

Etiology

Related to underlying degenerative arthritis

Pathophysiology

Damage to cartilage theorized to allow fluid intravasation within bone, leading to cyst formation
Pathology: Cystic lesion filled with fluid or gelatinous or proteinaceous material, demarcated by fibrocartilaginous tissues (Fig. 5.3-1)

Diagnosis

Physical Examination and History

Clinical Features

Dictated by manifestations of underlying arthritis
Pain, local tenderness, variable stiffness, loss of motion

Radiologic Features

Plain x-ray
- Cyst: geographic, smooth, often sclerotic borders with central radiolucency within epiphysis immediately adjacent to joint, often on both sides of joint
- Variable associated arthritic changes:
  - loss of joint space, osteophytes, subchondral sclerosis, subluxation may be present
  - some joints show few or no degenerative changes
MRI
- Fluid-filled well defined lesions adjacent to joint with communication to joint usually obvious
- Typically homogenously dark on T1W, bright on T2W sequences
- Enhancement only peripherally

Pathologic findings

Cystic cavity filled with serous fluid, proteinaceous, or gelatinous material and lined with flattened fibrocartilaginous tissues

Diagnostic Workup Algorithm

Usually evident radiographically without the need for histological confirmation
- In presence of established arthritis, plain radiographs often suffice
- In absence of other radiographic signs of arthritis, MRI may be useful
Differential diagnosis includes other epiphyseal lesions
- PGCAT: Pigmented villonodular synovitis (PVNS), giant cell tumor of bone, chondroblastoma, clear cell chondrosarcoma aneurysmal bone cyst, tuberculosis (and other cause for Brodie’s abscess)

Treatment

Surgical Indications/Contraindications

If diagnosis is clear, direct treatment towards underlying joint arthritis
If diagnosis not clear, biopsy may be necessary

Figure 5.3-1 Histopathology of degenerative cyst. (A) Schematic shows white cyst below thinned (degenerative) cartilage, shown in black. (B) Intraosseous cysts within subchondral bone with overlying fibrillated cartilage.

Simple Bone Cyst (Unicameral Bone Cyst)

Unicameral bone cyst is a serous or serosanguineous fluid-filled cavity which typically behaves in an inactive fashion, not causing symptoms until pathological fracture occurs through the cyst. The clinical presentation, location, and radiographic features are classic enough that the diagnosis may usually be readily established. Treatment continues to evolve, and is site dependent, but often involves aspiration and injection.

Pathogenesis

Etiology

Unknown but theories suggest epiphyseal plate defect or venous outflow obstruction

Epidemiology

Ages
- Peak incidence 4–10 years of age
- 85% within first two decades
Gender: Males > females 3:1
Distribution
- Proximal humerus most common
- Proximal femur #2
- Calcaneus #3
- Other relatively common sites: proximal tibia, ilium
- Most common sites in adults: calcaneus and ilium

Pathophysiology

Unknown

Classification

Active position: Immediately juxtaposed to growth plate
- Not to be confused with Enneking stage 2 benign (active) classification
Inactive position: Growth plate no longer adjacent to cyst
Not to be confused with Enneking stage 1 benign (inactive) classification