What is a sessile‑appearing exostosis of the proximal humeral metadiaphysis?

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Sessile-Appearing Exostosis of Proximal Humeral Metadiaphysis

A sessile-appearing exostosis of the proximal humeral metadiaphysis is an osteochondroma—a benign bone tumor that grows outward from the metaphyseal region of the proximal humerus with a broad-based, flat or dome-shaped attachment to the bone surface, lacking a stalk or pedicle. 1, 2, 3

Morphological Characteristics

Sessile vs. Pedunculated Configuration:

  • Sessile exostoses have a flat or dome-shaped appearance with a broad-based attachment directly to the bone surface, without a visible stalk 1, 2
  • This contrasts with pedunculated exostoses that have a thin stalk connecting the lesion to the parent bone 1
  • The sessile morphology is clinically significant because sessile osteochondromas carry a higher risk of malignant transformation compared to pedunculated lesions 1

Anatomical Location and Context

Proximal Humeral Metadiaphysis:

  • The metadiaphysis represents the transitional zone between the metaphysis (near the growth plate) and the diaphysis (shaft) of the humerus 4
  • Osteochondromas preferentially develop in the metaphyseal regions of long bones, including the proximal humerus, which is a common site 5, 3, 6
  • These lesions demonstrate corticomedullary continuity with the parent bone 2

Clinical Presentation

Common Symptoms:

  • Palpable mass in the proximal arm region 2, 6
  • Pain, particularly with activity or throwing motions 6, 7
  • Limited range of motion of the shoulder (26% of cases) 6
  • Potential neurovascular compression symptoms, including isolated musculocutaneous neuropathy 8
  • Frictional bursitis from mechanical irritation of surrounding soft tissues 3, 7

Imaging Characteristics

Radiographic Features:

  • Bony exostosis with corticomedullary continuity with the parent bone 2
  • Cartilage cap typically <5 mm thick in benign lesions 2, 9
  • Sessile lesions appear as broad-based masses extending from the bone surface 1, 2

Advanced Imaging:

  • MRI is the preferred modality for assessing cartilage cap thickness and excluding malignant transformation 9
  • MRI can evaluate associated complications including bursitis, neurovascular compression, and soft tissue involvement 3, 7
  • Cartilage cap thickness >2 cm on MRI raises concern for malignant transformation 9

Associated Conditions

Hereditary Multiple Exostoses (HME):

  • Sessile osteochondromas may occur as isolated lesions or as part of HME syndrome 3, 9, 10
  • HME is an autosomal dominant disorder with mutations in EXT1 (65-70%) or EXT2 (30-35%) genes 5, 9
  • In HME, exostoses are preferentially located in the lower extremity, humerus, and forearm 3
  • Exostoses in HME are more commonly sessile than pedunculated 3

Malignant Transformation Risk

Critical Warning Signs:

  • Patients with HME have a 10% lifetime risk of malignant transformation to secondary chondrosarcoma, compared to only 1% risk with solitary osteochondromas 9
  • Sessile morphology carries higher malignancy risk than pedunculated lesions 1
  • Suspect malignant transformation with: new-onset pain, growth of cartilage cap, or cartilage cap thickness >2 cm 9
  • Most malignant transformations occur between ages 20-40 years, though 7% occur before age 20 5

Management Approach

Indications for Surgical Excision:

  • Pain (81% of surgical cases) 6
  • Limited range of motion (26% of cases) 6
  • Neurovascular compression symptoms 8
  • Concern for malignant transformation 7, 9

Surgical Considerations:

  • The deltopectoral approach is most commonly utilized (61% of cases) 6
  • For posteromedial lesions, a direct medial approach achieves more complete excision (79% vs. 51%) 6
  • Incomplete excision correlates with persistent postoperative pain 6
  • Recurrence rate is approximately 6% 6
  • No neurovascular complications were reported in a series of 31 pediatric patients 6

Surveillance Recommendations:

  • For HME patients: baseline whole-body MRI at diagnosis, with periodic imaging after age 20 years 5
  • Consider regular regional MRI of lesions in the pelvis/scapula every 2-3 years 5
  • Physical examination every 6-12 months for signs of growth or malignant transformation 5
  • Rapid growth or increasing pain warrants immediate MRI/CT evaluation 5

References

7
Research

OSTEOCHONDROMA OF THE PROXIMAL HUMERUS WITH FRICTIONAL BURSITIS AND SECONDARY SYNOVIAL OSTEOCHONDROMATOSIS.

JBR-BTR : organe de la Societe royale belge de radiologie (SRBR) = orgaan van de Koninklijke Belgische Vereniging voor Radiologie (KBVR), 2015

10
Research

Multiple hereditary exostoses and enchondromatosis.

Best practice & research. Clinical rheumatology, 2020

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