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Carpal Boss

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Lateral radiograph showing prominent carpal boss at the level of the carpometacarpal joints.
Lateral radiograph showing prominent carpal boss at the level of the carpometacarpal joints.

Quiz

What is the finding on this radiograph?

  1. Fracture of the lunate bone
  2. Carpal boss of the metacarpal bones
  3. Dislocation of the carpal bones
  4. Normal study

Pathophysiology

The carpal boss is an osseous protuberance along the dorsal base of the second or third metacarpal, between the trapezoid and capitate. The exact pathogenesis is not known. The proposed causes include degenerative osteophyte formation, sequela of prior trauma, carpometacarpal joint partial or complete coalition, exostosis, an accessory capitate bone, or an accessory ossicle -Os Styloideum located at dorsally between capitate, trapezoid and second and third metacarpal

Key Imaging Features 

  • Radiograph– AP and lateral wrist radiograph – the presence of a bony protuberance or an abnormal ossicle along the dorsal aspect of the wrist at the base of the second or third metacarpal.
  • Carpe Bossu view- a lateral radiograph with the hand flexed and supinated 30-40 degrees for better visualisation of the accessory bone
  • Ultrasound- USG is done torule out alternative etiologies of a painful dorsal wrist swelling -ganglion cyst or tenosynovitis of an extensor tendon
  • CT- the presence of a carpal boss or other causes of wrist pain including fracture non-union, osteophyte formation or os styloideum
  • MRI– MRI can identify the carpal boss with additional osseous and soft tissue details including the presence of bone marrow edema, subchondral/subcortical cystic change, and resultant tendinopathy/tenosynovitis and ganglion/bursal cyst formation.
  • Nuclear Medicine: The Tc-99m methylene diphosphonate (MDP) bone scan -abnormally increased radiotracer uptake at the level of a carpal boss which is thought to be secondary to secondary osteoarthritis however it is a non-specific finding.

Imaging Recommendation:

Radiographs are the initial modality of choice, depending on the patient’s symptoms further evaluation with CT or USG is performed.

Top 3 Differential Diagnosis

Top 3 differentials for swelling over the wrist are :

  1. Ganglion cyst– fluid signal intensity cystic lesion adjacent to joint or tendon without causing significant bony abnormality
  2. Tenosynovitis of the extensor tendons– increased fluid within the tendon sheath, thickening of the synovial sheath with or without increased vascularity and subcutaneous oedema. No bony abnormalities.
  3. Fracture non-union of the carpal bones or metacarpal- causing bony swelling over the dorsum of the wrist.

Clinical Features:

  • Symptoms- asymptomatic, dorsal wrist swelling and pain, restricted wrist movements predominantly in the dominant hand due to repetitive use
  • Age/Sex predilection- third or fourth decade of life, no sex predilection
  • Risk factors- prior trauma, repetitive use, ganglion cyst

Treatment:

  1. Symptomatic treatment- behaviour modification, hand therapy, NSAIDs, corticosteroid injection, wrist splint for immobilisation.
  2. Surgical management in selected patients by removing painful surgical boss.

References:

Single best review article:

Porrino J, Maloney E, Chew FS. Current Concepts of the Carpal Boss: Pathophysiology, Symptoms, Clinical or Imaging Diagnosis, and Management. Curr Probl Diagn Radiol. 2015;44(5):462-468. doi:10.1067/j.cpradiol.2015.02.008

Other references:

Conway WF, Destouet JM, Gilula LA, Bellinghausen HW, Weeks PM. The carpal boss: an overview of radiographic evaluation. Radiology. 1985;156(1):29-31. doi:10.1148/radiology.156.1.3923555

 Case co-authored by TeamGyan Member  Dr.Mansi Sarmalkar

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