Saddle Joints

1. Overview

A saddle joint, also known as a sellar joint, is a type of synovial joint characterized by articulating surfaces that are concave in one direction and convex in the other—resembling a saddle. This unique structure allows movement in two planes, including flexion/extension and abduction/adduction, with a small degree of axial rotation. Saddle joints offer greater mobility than hinge joints but less than ball-and-socket joints.

2. Location

The most notable and functionally significant saddle joint in the human body is the first carpometacarpal (CMC) joint of the thumb:

• Located between: The trapezium (a carpal bone) and the base of the first metacarpal (thumb).

• Additional example: The sternoclavicular joint is sometimes functionally classified as a saddle joint.

3. Structure

Saddle joints are biaxial synovial joints with reciprocally concave and convex articular surfaces:

• Joint surfaces: Each surface is shaped like a rider sitting on a saddle—concave in one direction and convex in the perpendicular direction.

• Synovial membrane: Lines the joint capsule and secretes synovial fluid for lubrication.

• Joint capsule: Encloses the articulating surfaces, providing support and mobility.

• Ligaments: Surround and reinforce the joint—especially the anterior oblique, dorsoradial, and intermetacarpal ligaments in the thumb CMC joint.

4. Function

Saddle joints enable biaxial movement—allowing motions in two primary planes:

• Flexion and extension: Bending and straightening movements.

• Abduction and adduction: Moving away from and toward the central axis of the body or hand.

• Opposition (in thumb): Unique movement combining flexion, abduction, and rotation, allowing the thumb to touch other fingertips.

This functionality makes the thumb highly versatile, playing a key role in gripping and manipulating objects.

5. Physiological role(s)

Saddle joints contribute to essential hand functions and upper limb dexterity:

• Fine motor skills: The CMC joint of the thumb enables precise movements required for writing, pinching, and tool use.

• Opposability: Allows thumb to oppose fingers, a trait unique to humans and some primates.

• Load distribution: Transfers force during grasping and weight-bearing through the upper limb.

• Functional versatility: The combination of mobility and stability allows for complex hand postures and tool manipulation.

6. Clinical Significance

Due to their high mobility and mechanical load, saddle joints are prone to wear and pathology, especially in the thumb:

• Osteoarthritis (thumb CMC joint):

  • Degeneration of the articular cartilage from repetitive use; most common cause of thumb base pain in older adults, especially postmenopausal women.

  • Symptoms include pain, stiffness, swelling, and decreased grip strength.

• Joint instability:

  • Ligament laxity or trauma can lead to subluxation or instability of the joint, affecting thumb positioning and grip.

• De Quervain’s tenosynovitis:

  • Although not directly a saddle joint issue, this inflammatory condition affects tendons crossing near the CMC joint and may cause confusion in diagnosis.

• Surgical interventions:

  • For severe arthritis, options include trapeziectomy, ligament reconstruction, tendon interposition, or joint replacement.

• Rehabilitation:

  • Conservative treatments include splinting, NSAIDs, corticosteroid injections, and physical therapy to strengthen surrounding muscles and reduce joint strain.