The thumb saddle joint is made up of the first metacarpal bone and the large polygonal bone (os trapezium). The latter has a saddle-shaped appearance, which gave the joint its name. It allows
- flexion/extension,
- flexion/extension, extension/extension and
- rotation
of the thumb. This makes it possible to oppose the thumb to the other fingers.
The joint surfaces have different curvatures. As a result, the joint is subjected to punctual stress due to the shearing forces during major exertion. Grasping objects sometimes requires a great deal of force between the long fingers and the thumb. This force is transferred to the thumb-saddle joint.
The female joint has an overall flatter radius of curvature.
As in every joint, the surfaces of the bones are covered by a cartilage cap . This enables low-friction, pain-free gliding.
As part of the wear and tear process , the cartilage breaks down. As a result, bone rubs against bone, which leads to pain.
As the thickness of the cartilage cap decreases, the joint space also narrows. The joint capsule is no longer as tight and allows a greater range of movement. This causes the first metacarpal bone to slide towards the radial side (subluxation).
Rhizarthrosis mainly causes pain when gripping in the area of the thumb @ Praewphan /AdobeStock
The large range of motion of the joint is what makes extensive use of the thumb possible in the first place. However, this also means that large forces are transmitted via the joint.
Due to the shape of the joint, the forces are sometimes only transferred at certain points, which promotes joint wear . Reduced tightness of the joint capsular ligaments is a fundamental factor in the development of rhizarthrosis.
However, genetic factors are also discussed as a cause of rhizarthrosis. Rhizarthrosis is observed more frequently in women than in men. Women are particularly affected after the menopause. A hormonally induced laxity of the ligamentous apparatus around the saddle joint is discussed.
In rare cases, rhizarthrosis is the result of a fracture.
In the earlier stages of wear and tear, the pain in the thumb saddle joint initially only occurs during certain movements. These include, for example, wringing a cloth or opening a screw cap. In later stages, pain occurs with every movement and load.
Characteristically, the thumb-index finger grip intensifies the pain. As the disease progresses, there is also a loss of strength and restricted movement.
From time to time, an X-ray examination of the hand reveals rhizarthrosis without the person concerned complaining of pain. This leads to the conclusion that not all wear and tear is necessarily immediately painful.
Rhizarthrosis is diagnosed by means of a clinical examination and X-ray examination.
Clinical examination for suspected thumb saddle joint arthrosis
Initially, no external changes can be seen above the thumb saddle joint. If the changes are pronounced, swelling develops on the extensor side of the joint. Pressure pain can be triggered here.
The extension and opposition of the thumb causes pain. In the later stages of rhizarthrosis, adduction and internal rotation of the thumb occur. This results in a hyperextension position in the metacarpophalangeal joint of the thumb.
Clinically, rhizarthrosis can be detected using the grind test. In this test, the examiner exerts a rotational movement with simultaneous axial pressure on the thumb. In the event of wear and tear damage, this triggers pain.
In the Glickel pressure test, the doctor holds the head of the patient's first metacarpal bone in an extended position. With the other thumb, he exerts pressure on the base of the first metacarpal. In the advanced stage of rhizarthrosis, this causes pain.
X-ray examination for suspected rhizarthrosis
If rhizarthrosis is suspected, an X-ray examination of the joint is carried out in two planes. The signs of wear are divided into four different stages according to a classification by Eaton and Littler (1985):
- Normal-appearing joint and joint space widening due to effusion formation
- Increasing signs of wear with joint space narrowing and bone edge extensions up to 2 mm
- Increasing signs of wear with narrowing of the joint space and bony edge appendages over 2 mm, formation of bone cysts close to the joint, subluxation in a radial direction (towards the radius)
- Full picture of rhizarthrosis with direct contact of the joint surfaces and possibly involvement of the neighboring joints between the scaphoid, large and small polygonal bone, increasing subluxation position, destructive deformation of the polygonal bone
Additional diagnostics for suspected rhizarthrosis
In individual cases, an additional rheumatological examination or an X-ray under load may be useful.
These are considered superfluous:
The aims of treatment are
- to alleviate the pain,
- to reduce the restriction of movement and
- to strengthen the development of strength.
According to studies by Froimson (1970) and Pieron (1973), 75% of sufferers do not require surgery. Surgery should be performed on the remaining 25%.
Non-surgical treatment of thumb saddle joint arthrosis
As with the treatment of osteoarthritis of other joints, the structural changes that have already occurred cannot be reversed. At best, therapy slows down the wear and tear process.
Conservative treatment of rhizarthrosis is primarily aimed at relieving pain. There is less focus on treating the loss of strength and restriction of movement.
Immobilization: Immobilization of the thumb saddle joint is achieved with various supportive thumb orthoses . Complete immobilization is not desirable or sensible for osteoarthritis joints. The synovial fluid that enables the joint to glide better is produced through movement. Complete immobilization would cause the thumb saddle joint to "dry out" further.
Application of heat: For chronic pain conditions, applying heat to the affected area helps. Sulphur, gravel, kerosene or rapeseed baths are often used as part of occupational therapy measures. At the beginning of the chronic stage, laser therapy and / or laser-assisted high-pressure ice therapy are also useful for activating local metabolic processes.
Irradiation: Direct X-ray irradiation of the thumb saddle joint is carried out in 3 to 4 doses. It has an anti-inflammatory effect, but is controversial due to the low success rate and radiation exposure for the patient. Another form of radiation treatment is radiosynovioorthesis.
Radiosynoviorthesis: A radioactive substance (186 rhenium) is injected directly into the joint. The substance causes "obliteration" of the joint mucosa, which leads to pain relief and a reduction in swelling. The effect sometimes only occurs 3 months after the injection and lasts for an average of 2 years. The treatment can then be repeated.
The disadvantages of the therapy are a possible infection and adhesion of the adjacent tendons with the resulting restriction of movement. The procedure is particularly suitable for acute synovitis (inflammation of the synovial membrane) in the course of rheumatoid arthritis.
Injection into the joint: Cortisone also has an anti-inflammatory effect. This can be injected directly into the thumb saddle joint together with a local anesthetic. However, as the joint is very small, the injection should be carried out under X-ray control. The procedure can be repeated.
Hyaluronic acid injections are also promising. Hyaluronic acid, or hyaluronan according to the new nomenclature, is an important component of connective tissue. Due to its high viscosity, it acts as a "lubricant" and shock absorber in the joint.
For the sake of completeness, the possibility of autologous blood therapy for rhizarthrosis should also be mentioned. The effectiveness of this treatment has not yet been sufficiently scientifically proven. Nevertheless, this form of therapy has its place in the conservative treatment of incipient rhizarthrosis. Patients benefit greatly from it in terms of pain reduction and the progression of osteoarthritis.
A bandage helps to immobilizethe thumb @ Ondrej Novotny /AdobeStock
Surgery for rhizarthrosis (thumb saddle joint arthrosis)
The surgical treatment of thumb saddle joint arthrosis falls under the medical field of hand surgery. Different concepts are available to the surgeon. In principle, the surgical method depends on the stage of rhizarthrosis.
Stage I of rhizarthrosis:
Pain reduction can be achieved by severing a nerve. Small, purely pain-conducting branches of the radial nerve that run to the saddle joint are severed.
In this way, the joint wear and tear remains, but no longer causes pain.
However, severing the pain fibers does not provide lasting relief. In some patients, only a temporary reduction in pain can be achieved.
Stage II-IV of rhizarthrosis:
Removal of the large polygonal bone and replacement of the joint by tendon redirection
The simplest method is the removal of the large polygonal bone alone. The "migration" of the first metacarpal bone into the gap and ultimately contact with the scaphoid was initially considered critical. In order to prevent this migration, placeholders (e.g. tendon balls or rib cartilage) were inserted into the gap in a further phase of surgical development. However, they cannot reliably prevent migration.
The result of the operation depends largely on the integrity of the joint capsule apparatus. This ultimately resulted in a demanding but very successful surgical treatment.
First, the large polygonal bone is removed. A forearm tendon(flexor carpi radialis muscle) is cut in half lengthwise, leaving the tendon insertion at the base of the 2nd metacarpal bone intact. The halved tendon is then passed through the base of the first metacarpal bone after a bone drilling, where it is sutured in place. Remnants of the tendon can then be inserted into the gap between the metacarpal bone and the scaphoid. They also serve as a buffer here.
The tendon detour is very important because it replaces a natural ligament. This affects the ligament radiating from the 2nd metacarpal bone into the saddle joint capsule, which plays a key role in fixing the first metacarpal bone. Despite the fixation of the first metacarpal bone, over the years it slides more and more towards the carpal bones.
The advantage of this procedure is the good and pain-free mobility. It has established itself as the "golden standard" against which other procedures must be measured. It is also regarded as the surgical method according to the guidelines of the German Society for Hand Surgery.
In the meantime, long-term studies have shown no difference whether a placeholder was used or not. Ultimately, the "migration" of the first metacarpal bone to the carpal bones is not decisive for the functional result after the operation.
Follow-up treatment: After a period of immobilization in a forearm splint with thumb immobilization of approximately 4 to 6 weeks, intensive physiotherapy exercises are necessary.
Replacement of the polygonal bone (thumb saddle joint) with an artificial placeholder
The large polygonal bone is also removed in this surgical procedure. The migration of the first metacarpal bone to the carpal bones is prevented. For this purpose, an artificial placeholder made of plastic "Silastik" is inserted into the gap.
After several years of successful use, however, the plastic implant was associated with slippage and the occurrence of inflammation of the joint capsule. As a result, the plastic space maintainer lost its importance in the treatment.
Replacement of the thumb saddle joint with an artificial joint
Similar to an artificial hip joint, the saddle joint can also be replaced with a mini-prosthesis. However, after initial skepticism, this procedure has become established in the hands of expert hand surgeons.
It is a very good alternative to procedures involving resection of the polygonal bone and is well on the way to replacing this surgical technique. Very good results have been achieved over the last 10 - 15 years.
Stiffening of the thumb saddle joint
Stiffening of the thumb saddle joint results in freedom from pain and almost complete preservation of strength. It is mainly performed on young, physically hard-working patients.
The disadvantage of this method is that the mobility of the thumb is significantly reduced. The pointed grip is restricted by the 4 to 5 mm shortening of the first ray caused by the operation.
In order to be able to perform this movement as usual, the neighboring joints are strengthened and stressed beyond the extent actually intended. This leads to premature wear of the neighboring joints. The joint between the large polygonal bone and the scaphoid and the metacarpophalangeal joint of the thumb are particularly affected.
This method is preferred for younger patients, particularly those working in manual trades, in the event of accident-related wear of the thumb saddle joint.