Rhizarthrosis is an arthritis of the thumb saddle joint. The thumb saddle joint, or simply saddle joint, is located at the base of the thumb; it is formed by the saddle-shaped polygonal bone belonging to the carpus and the base of the first metacarpal bone, which sits on the saddle like a rider. The wide, flaccid capsule of the joint gives the saddle joint the freedom of movement of a ball and socket joint, allowing the thumb to face the other fingers and splay outward. The heavy stress on the joint explains why the joint wears out and osteoarthritis develops (rhizarthrosis).
Rhizarthrosis: a common condition
Rhizarthrosis is the most common finger and wrist osteoarthritis and affects men and preferably women aged 50 and older. Hormonal influences play a role, but genetic factors also play a role, as a familial clustering is clearly established. Mechanical overloading of the thumb saddle joint, more rarely a rheumatic disease (chronic polyarthritis) or poorly healed fracture with joint involvement can then lead to increased wear of the articular cartilage and be the cause of arthrosis. If additional finger joint arthroses occur, the condition is referred to as polyarthrosis.
Development of rhizarthrosis
As in other arthroses, a change in the cartilage substance with crack formation and increasing cartilage degradation occurs gradually with high stress on the joint or previous damage. As an accompanying reaction of the joint capsule, an inflammatory joint irritation with joint effusion develops, which in turn promotes cartilage degradation. When the cartilage is finally almost completely degraded, bone rubs against bone, resulting in painful restriction of movement. Remodeling processes of the bone result in bony attachments (osteophytes) and thickening of the bone near the joint with narrowing of the joint space.
Symptoms: pain and decreased strength
The primary complaint is pain localized to the lower part of the ball of the thumb at the junction with the carpus. In the early stages, the pain is mainly noticeable with exertion, for example when opening screw caps or wringing out cleaning rags. During these everyday activities, the reduced strength when gripping is also noticeable. In addition, swelling in the area of the saddle joint is also noticeable and it is becoming more and more apparent that the thumb can be splayed less. The thumb lies drawn up (in adduction) and is slightly flexed.
The radiograph clarifies the diagnosis
Medical examination reveals a bulging of the saddle joint and malalignment to the back; spreading of the thumb is restricted. The diagnosis of osteoarthritis of the thumb saddle joint is then unequivocally clarified by the X-ray image, which shows a narrowed joint space. The radiologist differentiates between four stages of rhizarthrosis depending on the extent of the malposition of the joint and the joint wear, which is shown by the bony attachments and the width of the joint space. Because saddle joint osteoarthritis often occurs together with carpal tunnel syndrome, a neurologist’s examination should also be performed if pain and numbness occur at night.
Immobilization and anti-inflammation
It is not possible to cure rhizarthrosis, but it is possible to alleviate the symptoms and improve mobility. The main focus of conservative treatment is to stop the inflammatory process in the joint. This includes sparing and immobilizing the joint with taping and special splints (orthoses) worn during weight-bearing and at night, administration of anti-inflammatory and analgesic anti-rheumatic drugs (NSAIDs), and physical therapy. This is done by:
- Cooling
- Ultrasound
- Electrotherapy
For more severe symptoms, injections of local anesthetics or cortisone can provide prolonged relief; however, such injections into the joint should be used sparingly because of the risk of infection.
Surgery for persistent symptoms
If the symptoms do not respond to these measures or if hyperextension is already evident in the adjacent metacarpophalangeal joint of the thumb, surgery should be considered. There are various procedures to choose from here, which are presented in detail below:
- Resection suspension arthroplasty
- Artificial saddle joint (prosthesis)
- Arthroscopy
Resection suspension arthroplasty
In this most common and proven surgical procedure, the polygon (trapezium) is removed and then a portion of the adjacent wrist flexor tendon is connected to the metacarpal bone of the thumb to fill the cavity and improve stability. The operation, which takes about an hour, has good to excellent results in long-term studies. It is performed either under general anesthesia or with anesthesia of the arm nerve plexus (plexus anesthesia). Aftercare consists of immobilization with a plaster splint for two weeks and subsequent fitting with a thumb bandage for four weeks. This is followed by six weeks of mobilization with physiotherapy and occupational therapy. After approximately three to six months, the patient usually regains his or her former strength and mobility, usually without pain.
Artificial saddle joint (prosthesis).
For implantation, there is a choice between partial and complete prostheses made of plastic, silicone or metal. In this case, immobilization lasts only four weeks before physiotherapeutic follow-up treatment can begin. However, the disadvantage of this procedure is the risk of loosening or fracture of the prosthesis, also the time period for a final evaluation is still too short.
Arthroscopy for rhizarthrosis.
In the early stages of rhizarthrosis, the inflammatory process in the joint can be stopped by removing inflammatory tissue and smoothing the joint surface during arthroscopy. At the same time, the draining pain fibers can be cut so that the perception of pain is reduced. The more extensive resection suspension arthroplasty can thus be postponed for several more years, because rhizarthrosis cannot usually be completely eliminated with this measure.
