Definition
Thumb saddle joint arthrosis (rhizarthrosis) is an arthrosis of the joint between the first metacarpal bone (Os metacarpale I) and the large polygon bone (Os trapezium), which belongs to the carpal bones. The affected palms are saddle-shaped and allow the joint to move in two axes. The combination of both axes results in a mobility that is almost similar to that of a ball joint.
Thumb saddle joint arthrosis, which is caused by wear and tear of the joint surfaces, is the most common arthrosis of the hand in Germany. This disease is present in about 10% of the population, with women being affected 10 times more frequently than men. In most cases the arthrosis occurs on both sides.
Diagnosis
A first indication of damage to the thumb saddle joint is provided to the doctor by the clinical picture of pain and weakness of grip of the thumb, which causes problems in the early stages. Initially, the thumb is increasingly weak, making it difficult to lift objects with thumb, index and middle finger or to open bottles. Soon pain sets in, which occurs in early stages, especially under stress.
As the disease progresses, these pains continue even after the end of the load and can occur at night. The advanced findings of arthrosis of the thumb saddle joint can already be detected during inspection. There is a visible deformation and swelling of the joint.
During palpation (palpation) the patient reports pain. When axial pressure is applied to the thumb with simultaneous internal and external rotations (grind test), rubbing and grinding can be felt in the case of arthrosis of the thumb saddle joint. Here too, the patient reports pain.
The radiological examination provides the typical signs of arthrosis in the X-ray image: Narrowing of the joint space, compression of the bone below the cartilage layer (subchondral sclerotherapy), bone spurs at the edge of the joint surfaces (osteophytes) and cysts. In the early stages, the X-ray image may be unremarkable. Here the cartilage of the joint surfaces is still intact.
Since a computed tomography (CT) causes a higher radiation exposure than a conventional X-ray image and a magnetic resonance imaging (MRI) is more complex and expensive, these two methods of sectional imaging are usually not indicated and are only used when there is uncertainty in the diagnosis. In order to distinguish isolated arthrosis of the thumb saddle joint from rheumatoid arthritis (chronic polyarthritis – inflammation of several joints), a blood sample may be necessary. If rheumatoid arthritis is present, a rheumatoid factor is detectable in the blood in 80% of cases. If the affected person also complains of nocturnal sensory disturbances and pain, a neurological examination is also necessary, as arthrosis of the thumb saddle joint may be associated with carpal tunnel syndrome.
