A thumb saddle joint endoprosthesis is an artificial replacement for the thumb saddle joint. It is located between the first metacarpal bone and the trapezium bone (os trapezium). If the thumb saddle joint becomes worn—a condition known as rhizarthrosis—a thumb saddle joint prosthesis is one option for surgical treatment.
Background Information on the Thumb Saddle Joint and Rhizarthrosis
The thumb saddle joint is located between the first metacarpal bone and the trapezium bone (os trapezium). The saddle-shaped structure of the joint allows for the thumb’s wide range of motion, particularly its ability to oppose the other fingers.
However, during grasping movements, the joint surfaces come into contact at an angle, which leads to localized pressure on the articular cartilage.
The increased pressure and a loosening of the joint capsule lead to premature wear of the articular cartilage, also known as rhizarthrosis.
Hormonal changes are thought to be a possible cause of this laxity. Women going through menopause are more frequently affected by this condition.
The wear and tear of the articular cartilage at the thumb saddle joint leads to a narrowing of the joint space. It also results in painful friction between the bones of the trapezium and the first metacarpal.
As the condition progresses, the loosening of the joint causes the metacarpal bone to slip down off the trapezium.
Symptoms of Rhizarthrosis of the Thumb Saddle Joint
Degeneration of the thumb saddle joint leads to increasing stiffness and, as a result, pain during movement and reduced strength during grasping motions. Due to partial dislocation of the joint, swelling occurs above the saddle joint.
Diagnosis of rhizarthrosis
During the clinical examination, tenderness is present over the thumb saddle joint, along with reduced grip strength.
X-rays of the thumb saddle joint confirm the diagnosis of rhizarthrosis. The X-rays may also reveal wear and tear between the large and small trapezoid bones.
Based on the changes visible on the X-ray, the condition is classified into four stages (according to Eaton/Littler).
How is rhizarthrosis treated?
In some cases, radiological changes indicative of rhizarthrosis are present, but there are no symptoms. In such cases, doctors should refrain from performing surgery.
Early stages of rhizarthrosis can be treated with the following measures:
- supportive orthoses
- Physical therapy
- Laser and cryotherapy
- radiosynoviorthesis or
- local cortisone injections
Surgical treatment of rhizarthrosis using a thumb saddle joint endoprosthesis
One possible surgical procedure for treating 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 trapezium), such as:
- Persistent loss of strength and
- shortening of the thumb
The thumb saddle joint prosthesis used is structured like a small hip joint prosthesis. It consists of a socket, a stem, and a ball head. Thanks to its modular design, it can be customized to the individual patient.
Procedure for Implanting a Thumb Saddle Joint Prosthesis
After milling the socket and rasping the stem, surgeons insert the thumb saddle joint prosthesis using the press-fit technique.
The grooves in the cup ensure a secure bond with the bone and prevent complications such as:
- loosening and
- dislocation of screw-retained sockets
There are no known allergic reactions to the material used (titanium). The rough hydroxyapatite coating facilitates bonding with the bone and avoids the disadvantages of ceramic prostheses. This occurs through bone resorption at the contact surface.
The surgery is performed with the arm in a bloodless position (arm extended) under brachial plexus anesthesia (arm numbing) or under general anesthesia. A hospital stay of 4 to 5 days is required.
Swelling of the hand may occur immediately after surgery. This can be treated by consistently elevating the hand and applying ice.
Follow-up Care After Thumb Saddle Joint Endoprosthesis Implantation
After surgery, doctors apply a forearm cast that includes the thumb, which remains in place for 10–14 days. This is followed by further immobilization of the thumb ray in a circular cast for an additional 10 days.
Once the cast is removed, the patient can begin intensive physical therapy in the fourth week. At night, the patient must wear a thumb brace for 2–3 months.
As a rule, the patient can fully use the hand again after 6 weeks. Sports activities are possible without restriction after 3 months.
Possible Complications and Risks of a Thumb Saddle Joint Prosthesis
No surgery is without risk! The weak point of any thumb saddle joint endoprosthesis is the acetabular component. Due to the high compressive loads, secure anchoring must be achieved here.
Sometimes it becomes apparent during surgery that a secure fit of the socket is not possible. In that case, the surgeon must switch to a different surgical procedure, known as suspension arthroplasty.
Despite the use of tissue-sparing surgical techniques, the socket may become loose. In such cases, if sufficient bone tissue is available, the surgeon can implant a larger socket. A modified resection arthroplasty is also an option.
Dislocation of the head from the socket is very rare and can be corrected with a longer head-neck component.
Conclusion: Advantages of the thumb saddle joint endoprosthesis
Prosthetic replacement of the thumb saddle joint is a useful adjunct to resection arthroplasty in cases of rhizarthrosis.
Shortening of the thumb ray and a reduction in strength can be avoided with the thumb saddle joint endoprosthesis.
Postoperative care and recovery are significantly shorter. Complications such as loosening and dislocation are extremely rare with thumb saddle joint endoprostheses based on Double Mobility Technology.
Safety is significantly increased both during implantation and in the postoperative course. Should complications nevertheless arise, the tried-and-true surgical procedure of suspension arthroplasty can be performed without difficulty.
