The basal joint of the thumb is a specialized one. It is a so-called saddle joint because it is formed by the first metacarpal bone on one side and one of the carpal bones in the wrist (trapezium) on the other side. Due to its saddle shape, it has enormous mobility, allowing the thumb to spread away and move towards the palm, as well as oppose the other fingers. In order to grip objects firmly with the hand, great force must exerted by this joint, which over the years can lead to wear and tear (rhizarthrosis) of the joint itself, which is also known as basal thumb arthritis.
Rhizarthrosis manifests itself in the form of pain, which is aggravated through movement and stress. There is also joint stiffness in the mornings and the pain is especially triggered when gripping firmly, for example, when screwing and unscrewing jars. In advanced stages of the disease, the joint space narrows and there may be a displacement of the metacarpal bones towards the thumb (subluxation). Through the rubbing of the bones against each other, there is not only pain, but a building of bone spurs, which worsen the symptoms. These bone changes can be identified by x-rays. Sooner or later, there will usually be a visible thickening of the joint structure and a reduction in strength.
Conservative methods of treatment can reduce the symptoms in the early stages. The immobilisation of the joint using a thumb splint and treatment using pain-killing and anti-inflammatory medicines and creams can temporarily alleviate symptoms. If, however, routine daily activities, such as turning pages in a book are already causing problems and pain is constant, then surgery should be carried out.
Arthroscopy of the saddle joint
In early stages, the arthroscopy of the joint with synovectomy and possible shrinking of the lax capsule ligament come into question. In advanced rhizarthrosis it is also possible to remove part of the trapezium.
In early stages of joint subluxation, a correction to the axis and possible rotation come into question in order to change the stress on the joint and to achieve more centralisation on the axis.
The most common surgical method is resection arthroplasty. This involves removal (resectioning) of a part of the joint surface of the trapezium through an incision in the wrist. In order to maintain function and stability of the wrist, the first metacarpal bone is stabilized through a tendon reconstruction. In this way, slipping of the thumb and shortening thereof is prevented.
After surgery, one must expect a long rehabilitation period. Initially, the thumb is set in plaster for 6 weeks or immobilised in a shorter plastic splint after the swelling has gone down. Afterwards, the thumb is exercised but the splint still worn when put under stress. One must reckon it will take 3-4 months before the maximum achievable freedom from symptoms.
On the whole, the total strength of the hand achieved after surgery is not equal to that of a healthy hand, however, it will be significantly better than before surgery. The operation is also advantageous in that it makes pain free movement possible again.
Fusion of the saddle joint is necessary under exceptional conditions, for example, when there are various signs of paralysis in the hand (e.g., in a high spinal injury).
Saddle joint prosthesis
There has been as yet no real long-term experience in the insertion of artificial thumb joints. However, there are certain prostheses coming onto the market that offer promising results.
Spacers made from pyrocarbon are implanted into the basal thumb joint areas as well as other areas of the carpal bones (e.g., STT-joint, joint between scaphoid and greater and lesser multangular bones) or also to replace a part of the scaphoid bone.
It is also possible today to partially fuse the carpal bones. As a result, good mobility usually remains and there is significantly less pain and aggravation in the joint (see SLAC- and SNAC-Wrist in wrist arthrosis).
Whichever of the listed treatment methods are used depends on the condition of the remaining cartilage area and which functions are possible from it.