A thumb saddle joint endoprosthesis is an artificial replacement of the thumb saddle joint, which is located between the first metacarpal bone and the large polygonal bone (Os trapezium). If wear of the thumb saddle joint, known as rhizarthrosis, occurs, the thumb saddle joint endoprosthesis is an option for surgical treatment.
The thumb saddle joint is located between the first metacarpal bone and the large polygonal bone(os trapezium). The saddle-shaped structure of the joint enables the thumb to have a large range of movement, particularly in relation to the long fingers.
However, gripping movements cause the joint surfaces to edge up, which leads to a punctual pressure load on the joint cartilage. The increased pressure load and slackening of the joint capsule lead to premature wear of the joint cartilage, also known as rhizarthrosis. Hormonal changes are discussed as the cause of ligament laxity. Menopausal women are more frequently affected by the disease.
The wear and tear of the articular cartilage in the thumb saddle joint leads to a narrowing of the joint space and painful rubbing of the bones of the polygonal bone and the first metacarpal bone against each other. As the disease progresses, the metacarpal bone slips away from the polygonal bone due to the slackening of the capsular ligament.
Wear and tear of the thumb saddle joint leads to increasing stiffness and associated pain on movement and reduced strength during gripping movements. The partial dislocation of the joint causes swelling over the saddle joint.
On clinical examination, there is tenderness over the thumb saddle joint and reduced strength when gripping. X-rays of the thumb saddle joint confirm the diagnosis of rhizarthrosis. X-rays can also reveal wear between the large and small polygonal bone.
The changes visible on the X-ray are used to classify the condition into four stages (according to Eaton/Littler).
Sometimes there are radiological changes in the sense of rhizarthrosis, but no symptoms. In this case, surgery should be avoided.
Early stages of rhizarthrosis can be treated with
can be treated.
One possible surgical procedure for the treatment of rhizarthrosis is endoprosthetic replacement of the thumb saddle joint. This procedure has been used for over 15 years with increasing success.
The implantation of a thumb saddle joint endoprosthesis avoids the disadvantages of resection arthroplasty (removal of the large polygonal bone) such as residual loss of strength and shortening of the thumb ray.
The thumb saddle joint prosthesis used is constructed like a small hip joint prosthesis. It consists of a socket and stem as well as a ball head. Thanks to its modular design, it can be individually adapted.
The thumb saddle joint endoprosthesis is inserted using a press-fit technique after milling the socket bearing and rasping the stem. The grooves in the cup enable a secure connection to the bone and avoid the disadvantages (loosening, dislocation) of screw cups.
Allergic reactions to the material used (titanium) are not known. The rough coating with hydroxyapatite enables a connection with the bone and thus avoids the disadvantages of the ceramic prosthesis due to bone resorption at the contact surface.
The operation is performed with the arm drained of blood and under plexus anesthesia (arm anesthesia) or general anesthesia. An inpatient stay of 4 to 5 days is necessary.
Immediately after the operation, there may be swelling of the hand, which is treated by consistent elevation and cooling with ice.
After the operation, a forearm plaster cuff with thumb enclosure is applied and remains in place for 10-14 days. The thumb is then immobilized in a circular cast for a further 10 days.
After removal of the cast immobilization, intensive physiotherapy can begin in the 4th week, whereby a thumb orthosis should be worn at night for 2 - 3 months. As a rule, the hand can be fully used again after 6 weeks. Sports activities are possible without restriction after 3 months.
No operation is without risk! The weak point of every thumb saddle joint endoprosthesis is the socket component. Due to the high pressure loads, it must be securely anchored here. Sometimes it turns out during the operation that a secure cup fit is not possible. The surgical procedure must then be changed to the aforementioned suspension arthroplasty.
Despite the tissue-sparing surgical technique, the cup may become loose. If sufficient bone tissue is available, a larger cup can be implanted or a modified resection arthroplasty can be performed. Pop-out of the head from the cup is very rare and can be remedied by using a longer head/neck component.
Prosthetic replacement of the thumb saddle joint is a useful addition to resection arthroplasty for rhizarthrosis. A shortening of the thumb ray and a reduction in strength can be avoided with the thumb saddle joint endoprosthesis.
Aftercare and convalescence are significantly shorter. Complications such as loosening and dislocation are extremely rare thanks to the further development of thumb saddle joint endoprostheses, particularly with the use of innovative double mobility technology, and have significantly increased safety both during implantation and in the post-operative course. Should they nevertheless occur, the tried-and-tested surgical procedure of suspension arthroplasty can be performed without complications.
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