Injection Technique for Carpal Boss

Indications and Clinical Considerations

Carpal boss is a relatively uncommon pain syndrome that affects the dorsum of the hand. It is characterized by localized tenderness and sharp pain over the junction of the second and/or third metacarpal joints ( Fig. 95.1 ). The pain of carpal boss is caused by an exostosis of the second and/or third metacarpal joints or, more uncommonly, a loose body involving the intra-articular space ( Fig. 95.2 ). The patient often feels that the pain is worse in the area of the carpal boss after rigorous physical activity involving the hand rather than during the activity itself. The pain of carpal boss may also radiate locally, and untreated, continued damage to the extensor tendons may result in rupture of the tendon, further confusing the clinical presentation ( Fig. 95.3 ).

FIG. 95.1, Proper needle placement for injection of a carpal boss.

FIG. 95.2, Carpal boss (os styloideum). A, Increased accumulation of a bone-seeking radionuclide (arrow) is evident at the side of the carpal boss at the base of the second and third metacarpal bones. B, In a different patient, a typical os styloideum (arrow) is seen.

FIG. 95.3, Intraoperative photograph reveals a large carpal boss and distal stump of the ruptured index finger extensor tendons. The wrist is to the right of the image.

On physical examination, pain can be reproduced by pressure on the soft tissue overlying the carpal boss. Patients with carpal boss will demonstrate a positive hunchback sign, with the examiner appreciating a bony prominence under the palpating finger when he or she palpates the carpal boss ( Figs. 95.4 and 95.5 ). The carpal boss may become more evident when the affected wrist is flexed ( Fig. 95.6 ). With acute trauma to the dorsum of the hand, ecchymosis over the carpal boss of the affected joint or joints may be present.

FIG. 95.4, Hunchback carpal sign for carpal boss.

FIG. 95.5, Carpal boss is frequently confused initially with a dorsal ganglion on viewing the dorsal wrist. It generally feels harder with palpation, is positioned more distally than wrist ganglia, and overlies the index and middle finger carpometacarpal joints (arrow).

FIG. 95.6, With wrist flexion, the prominence of the carpal boss becomes strikingly evident (arrow).

Plain radiographs are indicated for all patients with carpal boss to rule out fractures and identify exostoses responsible for the patient’s symptoms. A characteristic volcano-type appearance of the bone is pathognomonic for carpal boss ( Fig. 95.7 ). Based on the patient’s clinical presentation, additional testing may be indicated, including complete blood cell count, sedimentation rate, uric acid, and antinuclear antibody testing to rule out inflammatory arthritis. Magnetic resonance imaging and/or ultrasound imaging of the fingers and bones of the wrist is indicated if joint instability, occult mass, occult fracture, infection, or tumor is suspected and to confirm the diagnosis ( Figs. 95.8 and 95.9 ). Radionuclide bone scanning and computed tomography imaging may be useful in identifying stress fractures of this region as well as the carpal boss that may be missed on plain radiographs of the hand ( Fig. 95.10 ).