The thumb saddle joint connects the metacarpal bone of the thumb to the trapezoidal large polygonal bone. As a saddle joint, it allows the biaxial movements of flexion/extension and abduction/angulation. When the two directions of rotation are combined, the thumb saddle joint acts almost like a ball-and-socket joint.
What is the thumb saddle joint?
The thumb saddle joint (Articulatio carpometacarpalis pollicis) provides the connection between the metacarpal bone of the thumb (Os metacarpale primum) and the large polygonal bone (Os trapezium). As a saddle joint, it allows the two perpendicular directions of rotation of flexion/extension and abduction/angulation. The joint is stabilized by several ligaments, and one or two muscles are available for each direction of rotation. The long and short extensors (extensor pollicis longus and brevis muscles) act as extensors. Muscles that are also responsible for abduction, such as the abductor pollicis brevis muscle, also act as flexors in some cases. Motorically, the muscles of the hand are supplied by branches of the ulnar nerve and the median nerve. During grasping actions of the hand, the thumb saddle joint is subjected to particularly high stress, so that degenerative changes can occur in the joint in many people, especially in women after menopause. This is usually osteoarthritis, which in the case of the thumb saddle joint is called rhizarthrosis.
Anatomy and structure
Unlike the other metacarpophalangeal joints of the fingers, the thumb saddle joint is not a ball-and-socket joint, but rather corresponds to that of a typical saddle joint, which is optimized for the two directions of rotation, flexion/extension and abduction/bending, and is designed in principle for the high loads associated with firm grasping movements. Compared to the other metacarpophalangeal joints, the joint capsule of the thumb saddle joint is relatively elastic, although ligaments ensure secure movement in the desired direction. The thumb thus offers the prerequisites for both fine motor and firm gripping movements. A superficial and a deep arterial arch in the area of the metacarpophalangeal joints supply the hand with oxygen and nutrients. The two arterial arches – also called palmar arches – originate as side branches of the ulnar and radial arteries. The thumb saddle joint and its ligaments are supplied mainly by a branch of the deep arterial arch.
Function and Tasks
The main function and role of the thumb saddle joint is to provide equally fine motor and forceful grasping movements. The opposition of the thumb is of particular importance in this regard. Thanks to the two planes of rotation provided by the saddle joint, the thumb is able to exert what is known as a pinch grip with each of the other four fingers. The tip of the thumb is pressed to a greater or lesser extent onto the tip of any finger on the same hand. The tweezer grip can be performed not only with extremely fine motor skills and sensitivity, but also with a strong grip if required. The thumb also offers the possibility of supporting the fingers with the fist grip during necessary holding grips. In this case, the fingers usually hold the weight of the body, which pulls down vertically (acceleration due to gravity), and the thumb prevents the fingers from slipping in the case of forces acting transversely to the force of gravity by means of fist closure or implied fist closure. The thumb saddle joint is also responsible for allowing the thumb to rotate somewhat passively to accommodate certain conditions. Active rotation is not possible because there are no muscles available for rotation. However, it cannot be ruled out that active rotation of the thumb is also possible through training even at a child’s age, because a certain combination of the available muscles could be used for the rotational movement.
Diseases
The thumb saddle joint is subjected to high stress during almost all movements of the thumb, especially during the tweezer grip with the ring finger or little finger, because the joint surfaces shift against each other and thus much smaller surfaces are subjected to higher stresses. This can lead to premature wear of the articular cartilage and arthritic changes in the joint. Osteoarthritis in the thumb saddle joint is called rhizarthrosis.Rhizarthrosis initially manifests itself as movement pain, which in the further course of the disease can also occur at rest and cause considerable sleep disturbances. In advanced rhizarthrosis, movement is restricted and bony projections often form that can be felt and are already recognizable from the outside as deformities. In a special X-ray procedure, the bony adhesions can be made directly visible. Since the advent of the new text messaging culture and smartphone technology, people who excessively use their fingers and thumb for texting or e-mailing, playing games, or other activities on the smartphone have been suffering from pain in their thumb. The phenomenon, which first appeared mainly in adolescents, occurs due to a strain on the thumb caused by unaccustomed movements that occur when texting. The phenomenon, called SMS thumb, is usually due to tenosynovitis at the thumb saddle joint or to a corresponding bursitis. In the so-called Finkelstein test, a crunching of the tendon can often be felt in an advanced stage of SMS thumb. The thumb is closed in a fist and the hand is then jerkily angled towards the little finger. The extremely painful symptoms can quickly become chronic if left untreated, and their healing can take a period of a month or more. In very persistent cases, a small incision into the connective tissue of the bursa, performed on an outpatient basis, can provide relief. The thumb can be moved immediately after the procedure.
