Musculus extensor carpi obliquus
Also known as the abductor pollicis longus, this muscle originates from the lateral border of the radius. Its tendon crosses obliquely over the dorsomedial carpus and inserts on the proximal medial splint bone (MC2). It abducts the carpus medially and assists carpal extension. Innervation is via the deep branch of the radial nerve (roughly C7–T1). Its tendon sheath is a common site of hygroma (fluid-filled swelling) formation, often from trauma or repetitive irritation.
Origin: Lateral border of the radius
Insertion: Proximal MC2 (medial splint bone)
Action: Abducts the carpus medially; assists carpal extension
Innervation: Deep branch of the radial nerve (C7-T1)
Hygroma of the tendon sheath is relatively common and usually cosmetic, though it can become painful if large or infected. Diagnosis is by palpation and ultrasound. Treatment is usually conservative.
## Overview A carpal hygroma is a fluid-filled subcutaneous swelling on the dorsal aspect of the carpus, usually acquired after repeated trauma. It is distinct from effusion of the carpal joints or t
A soft, non-painful swelling on the inside of the knee is often a hygroma of this tendon sheath. Monitor for heat or lameness; most are managed with rest and protective bandaging. Sudden enlargement or lameness needs veterinary attention.
## Overview A carpal hygroma is a fluid-filled subcutaneous swelling on the dorsal aspect of the carpus, usually acquired after repeated trauma. It is distinct from effusion of the carpal joints or the carpal sheath. True carpal sheath swelling (tenosynovitis) produces a more proximal and palmar swelling and may be associated with lameness when internal structures are injured. ## Causes Repetitive contusion from lying on hard surfaces or hitting the knee produces hygromas. Carpal sheath effusion results from synovitis, tendon injury, or trauma within the sheath. ## Diagnosis Location and ballotability distinguish a hygroma from joint or sheath effusion. Ultrasound confirms the subcutaneous nature of a hygroma and evaluates the carpal sheath contents when swelling is more proximal. ## Treatment Hygromas are managed by protecting the area from further trauma; drainage and injection are sometimes performed but recurrence is common. Carpal sheath tenosynovitis is treated according to the underlying cause (rest, injections, or surgery). ## Recovery and Prognosis Uncomplicated hygromas are cosmetic. Sheath swellings associated with tendon injury have a more variable prognosis. ## Prevention Adequate bedding and knee boots reduce hygroma formation. Signs include: soft fluctuant swelling on the front of the carpus (hygroma) or proximal palmar swelling (sheath); variable lameness
Hygroma of the tendon sheath is relatively common and usually cosmetic, though it can become painful if large or infected. Diagnosis is by palpation and ultrasound. Treatment is usually conservative.
A soft, non-painful swelling on the inside of the knee is often a hygroma of this tendon sheath. Monitor for heat or lameness; most are managed with rest and protective bandaging. Sudden enlargement or lameness needs veterinary attention.
See the Extensor Carpi Obliquus (Abductor Pollicis Longus) on our interactive 3D horse model. Open 3D Model