Saddle joints are an articular form of true joints. They consist of two concave articular surfaces that allow biaxial motion. Osteoarthritis of the thumb saddle joint, in particular, is a common condition that affects this ability to move.
What are saddle joints?
Articulated joints are possessed by the human body in somewhat 140 different places. In these joints, two or more bones meet and connect with each other for an exact fit. This principle of precise fit is also known as the hand-in-glove or lock-and-key principle. The principle refers to the fact that the bones involved in a joint fit into each other as precisely as a hand fits into a glove or a key fits into its lock. A distinction is made between non-genuine joints and genuine joints, which are equipped with a joint space. These so-called diarthroses occur within the body in different variations of form, with each variation of form serving a different purpose. The saddle joint is a form of the true joint characterized by two concave articular surfaces. The surfaces sit on top of each other like a rider in a saddle. Saddle joints are biaxial and thus usually allow four different movements. One of the best-known saddle joints is the joint between the carpal bone and the metacarpal bone, which lies below the thumb. In this context, the talk is also of the thumb saddle joint.
Anatomy and structure
Real joints are enclosed by a so-called joint capsule, which encloses the joint cavity without gaps and thus all functional components of the joint. The joint capsule of real joints consists of an inner and an outer membrane, also known as membrana synovialis and membrana fibrosa. While the inner membrane consists of epithelial dressings of connective tissue, the outer membrane is composed of tight connective tissue. The joint capsule lies flaccidly against the articular surfaces of true joints. Its outer membrane is strengthened by capsular and articular ligaments. Within the joint capsule of real joints lies a viscous fluid: the so-called synovia, which is also called synovial fluid. These anatomical characteristics of real joints also apply to the saddle joint. The bony components of the saddle joint essentially consist of two articular surfaces, one corresponding to the condyle and the other to the socket. The two articular partners of the thumb saddle joint have concave shapes and, unlike the articular surfaces of other joints, are more or less superimposed. The upper joint part sits in the lower joint part like a rider in a saddle. Accordingly, the upper part corresponds to the condyle and the lower part to the socket to accommodate the head with a snug fit.
Function and tasks
Each true joint performs several functions at the same time. Joints connect bones that meet and thus fulfill a stabilizing function. On the other hand, articulating joints also give meeting bones a certain degree of mobility and allow movement on at least one axis. At least two different movements can take place on each axis. Saddle joints are among the multi-axial joints. Characteristically, they are biaxial and in this context resemble joint forms such as the egg yolk joint. On their two axes of motion, they allow at least four movements. For example, they allow lateral movements to the right and left. These splaying movements are called abduction. With the opposing movement of adduction, the return to the starting position takes place. In addition, the movement forms of flexion and extension take place in saddle joints. These are the medical terms for stretching and bending movements. In the case of the saddle joint, there is sometimes also talk of forward and backward movements. With these types of movement, saddle joints such as the thumb saddle joint are involved in numerous movements in everyday human life. Furthermore, the thumb saddle joint is the only joint that allows the thumb to move in opposition to the rest of the fingers. This means that the thumb can be positioned opposite the other fingers of the hand as the only finger. This form of movement is required for grasping, for example.
Diseases
Like all other joints, saddle joints can be affected by functional impairment, inflammation, degenerative phenomena or malalignment, and injury. Degenerative manifestations naturally show up in saddle joints with increasing age. However, as soon as the degree of degenerative change exceeds the age-physiological level, we speak of arthrosis. Risk factors for arthrosis can be, for example, regular overloading of the joint. Malpositions and resulting overloads are also considered a risk for arthrosis. In the case of arthrosis of the thumb saddle joint, there is also talk of rhizarthrosis. The disease mainly affects women over the age of fifty and often occurs in this context on both hands. In addition to mechanical overloading or post-traumatic changes, hormonal influences are discussed as causative co-factors. In addition to initial load-dependent pain, the disease manifests itself in a general instability of the thumb saddle joint, which causes the Os metacarpale I to slide in radial and proximal directions. With slippage, the thumb becomes difficult to abduct. In most cases, the proximal thumb phalanx is simultaneously affected by hypermobility, i.e., overmovement. In the early stages of osteoarthritis, the pain occurs during stress caused by movements in the joint and sometimes radiates into the hand. In later stages of arthrosis, the entire cartilage wears away and the joint surfaces rub against each other without protection. From this stage on, the joint hurts even at rest, with pain increasing further with weight bearing. Joint stiffness can be the result of wear and tear. Of course, saddle joints, like all other joints, can be affected by inflammation. In addition, the bones involved in the joint may experience fractures.
