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Os Trapezium – Anatomy, Function and Conditions

The os trapezium is a small carpal bone located at the base of the thumb. It plays a key role in thumb mobility and stability of the wrist.

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The os trapezium is a small carpal bone located at the base of the thumb. It plays a key role in thumb mobility and stability of the wrist.

What is the Os trapezium?

The os trapezium (also known as the trapezium bone or os multangulum majus) is one of the eight carpal bones of the human hand. It is part of the distal row of the carpus and is situated directly at the base of the thumb. Its name derives from its trapezoid-like shape.

Anatomy and Location

The carpus consists of two rows of four bones each. The os trapezium belongs to the distal row and is strategically positioned among several surrounding structures:

Proximally, it articulates with the scaphoid bone (os scaphoideum).
Medially, it borders the trapezoid bone (os trapezoideum).
Distally, it forms a joint with the first metacarpal bone, creating the so-called carpometacarpal joint of the thumb (articulatio carpometacarpalis pollicis), also known as the saddle joint of the thumb.

The thumb saddle joint is one of the most mobile joints in the human body, enabling the wide range of thumb movements including the uniquely human opposition movement (bringing the thumb across the palm to meet the other fingers).

Function

The os trapezium serves several important functional roles:

Foundation of the thumb saddle joint: It forms the proximal joint surface for the first carpometacarpal joint, enabling the versatile movements of the thumb.
Force transmission: During gripping movements, forces are transferred from the thumb through the os trapezium to the rest of the wrist and forearm.
Wrist stability: Together with the neighboring carpal bones, it contributes to the overall stability and integrity of the wrist.

Clinical Relevance

Thumb Saddle Joint Osteoarthritis (Rhizarthrosis)

One of the most common conditions affecting the os trapezium is rhizarthrosis, also called basal thumb arthritis or carpometacarpal osteoarthritis. This condition involves the progressive degeneration of the cartilage between the os trapezium and the first metacarpal bone. Typical symptoms include pain at the base of the thumb, swelling, reduced grip strength, and limited thumb mobility. Women are more frequently affected than men, particularly after menopause.

Fractures

Although fractures of the os trapezium are relatively uncommon, they can occur due to direct trauma or falls onto an outstretched hand. They are often associated with injuries to the scaphoid or surrounding ligamentous structures. Symptoms include localized tenderness, swelling, and restricted movement.

Trapeziectomy

In cases of advanced rhizarthrosis that no longer respond to conservative treatment, the os trapezium can be surgically removed. This procedure is known as a trapeziectomy and results in significant pain relief and improved quality of life for most patients.

Diagnosis

Conditions involving the os trapezium are typically diagnosed using the following methods:

X-ray (radiograph): The standard imaging method for assessing fractures, signs of osteoarthritis, and joint space changes.
Computed tomography (CT): Provides detailed three-dimensional visualization of fractures and bony changes.
Magnetic resonance imaging (MRI): Particularly useful for evaluating ligament injuries, cartilage damage, and early signs of arthritis.
Clinical examination: Provocation tests such as the grind test (axial compression and rotation of the thumb metacarpal) can indicate the presence of rhizarthrosis.

Treatment

Conservative Approaches

Immobilization with a thumb splint or orthosis
Anti-inflammatory medications (e.g., non-steroidal anti-inflammatory drugs, NSAIDs)
Physiotherapy to strengthen the surrounding musculature
Intra-articular corticosteroid injections for pain relief

Surgical Approaches

Trapeziectomy with or without tendon interposition
Implantation of a thumb saddle joint prosthesis
Arthroscopic procedures for early-stage osteoarthritis

References

Gray's Anatomy: The Anatomical Basis of Clinical Practice. 42nd Edition. Elsevier, 2020.
Spies, C.K. et al. - Basal Thumb Arthritis. Deutsches Aerzteblatt International, 2019; 116(1-2): 1-8. DOI: 10.3238/arztebl.2019.0001
Standring, S. (Ed.) - Gray's Anatomy: The Anatomical Basis of Clinical Practice. 41st Edition. Churchill Livingstone, Elsevier, 2016.

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