Orthopedics - Oleksa A.P. 2006

Joint Pathophysiology

The mechanical function of a joint is governed by two laws: physical and biological. This specific feature gives biological joints an advantage over various artificial mechanical structures, such as endoprostheses, because, in accordance with biological laws, a normal joint is capable of adapting to the conditions of its functioning. For instance, weightlifters develop bone outgrowths As a result of training, while gymnasts develop excessive joint mobility. This adaptation applies not only to joints, but to bones and Muscles as well.

However, when the Skeleton is overloaded, the body is not always immediately able to adapt, which leads to pathological changes. A case in point is Deutschlander's disease: due to significant overload, the bone and periosteum initially thicken as a manifestation of compensation (Fig. 46), but once the limits of compensation are exhausted, structural remodeling and bone resorption occur at the overload site in the form of a transverse fissure ("stress fracture").

There are two main Types of bone connections: synarthroses and diarthroses.

Synarthroses and their subtypes lack all the components of a true joint—they have no hyaline Cartilage, synovial capsule, or synovial fluid. These bone connections are designed only for minor movements under tensile and compressive forces.

Diarthroses feature a hyaline articular cartilage on the contacting bones, a fibrous capsule, a synovial membrane, a joint cavity containing synovial fluid, and a robust ligamentous apparatus.

The structural design of a joint determines the direction and range of its movements. The most mobile are the shoulder and hip joints, which are classified as ball-and-socket joints. The rounded HEAD of the humerus or Femur allows for multidirectional movement. The head fits snugly into the socket—the articular surface of the adjacent segment. The larger the head and the smaller the socket, the greater the range of motion (for example, in the shoulder joint). In the hip joint, a larger acetabulum somewhat limits the range of motion, but in turn increases the load-bearing area per unit of the femoral head's surface. Movements in these two joints occur through the sliding of the articular heads. The articular cartilage on the femoral head varies in thickness—it is thicker in the central part than at the edges, whereas in the acetabulum it is the reverse: thinner in the center and thicker at the edges. This uneven cartilage thickness is not a sign of Dysplasia, but rather a result of functional expediency, representing the most mechanically efficient shape.

The knee joint is uniaxial, and its movements occur in the sagittal plane. When the knee is flexed and the ligaments are relaxed, a very slight rotational mobility of the lower leg can be detected. The Water/140.html">Anatomical Structure of this joint is unique: the articular surface of the Tibia is flat, while that of the femur is nearly spherical (Fig. 47), which, together with the Ligaments and Muscles, ensures Stability of the knee in both static and dynamic conditions.

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Fig. 46. Deutschlander's disease.

The menisci and fat pads partially compensate for the discrepancy in shapes. This mismatch in the anatomical shapes of the articulating bone ends determines the specific nature of movements in the sagittal plane. During lower leg flexion, movement initially occurs as in a ball-and-socket joint, followed by rolling over the posterior part of the condyles, and then returning to the initial flexion pattern.

The function of any joint is influenced by external and internal loads, as well as their frequency and magnitude. The endurance of joints significantly exceeds the external loads applied to them, which is determined by Muscle force during contraction (e.g., lifting a weight). Therefore, the magnitude of external loads is limited. The internal load of a joint depends on the pressure caused by the mass of the musculoskeletal segments; hence, the internal load of the upper and lower limbs differs, yet remains constant. Thus, joints are subjected to both internal and external loads.

Fig. 47. Diagram of knee joint stability under static and dynamic conditions, and points of maximum articular cartilage loading during movement.

Subjecting a joint with normal internal loading to excessive or frequent external loading can cause pathological Changes in the osteoarticular apparatus. Rapid and prolonged movements, especially under external load, lead to circulatory disturbances in a specific area of the joint. Under normal limb loading, such as walking and running, the frequency and quantity of these loads per unit of time play a critical role. Overload not only causes muscle fatigue, but also has a detrimental effect on ligaments, cartilage, and all constituent elements of the joints. Furthermore, frequent excessive ranges of motion in a joint can lead to joint instability and pathological changes in the cartilage and synovial fluid. Therefore, the optimal conditions for joint functioning involve avoiding overload, with a range of movements that does not reach the Limits of the physiological norm for each individual joint, as movements to extreme positions strain the joint ligaments and affect Muscle Function. However, all of this applies to joints with a normal limb axis.

In cases of valgus or varus deviation of a segment relative to the normal physiological axis of the limb, the directions of internal load forces change. For instance, in Varus deformity of the knee (genu vara) under vertical limb loading, the intra-articular pressure force shifts to the articular surface of the medial condyles of the femur and tibia (Fig. 48). Up to a certain point, this static disturbance is compensated for by the strong lateral collateral ligament of the knee and the muscles, but eventually, the ligament becomes overstretched, the lateral joint space widens, and joint instability and dysfunction ensue. Under METABOLISM/18.html">The Influence of constant overload, the articular cartilage of the medial condyles degenerates and gradually wears away mechanically, while Osteoporosis develops in the lateral condyles due to underloading.

The Human Body attempts to compensate by increasing the load-bearing area of the medial half of the joint through compensatory marginal outgrowths. All of this contributes to The Development of classic osteoarthritis of the knee joint.

Impairment of the static-dynamic function of a single joint leads to functional disturbances in adjacent joints and even the spine, resulting from occupational or Other types of lower limb overload.

Similarly, a state of adaptation followed by maladjustment occurs due to the overload of one leg following an amputation at the thigh or lower leg level of the contralateral limb. Analogous processes also take place in the presence of a shortened limb.

Flexion contracture of the FOOT (pes equinus) leads to hyperextension of the knee with corresponding pathological manifestations and consequences (overstretching of the ligaments causes joint instability, subsequently leading to the development of osteoarthritis).

Fig. 48. In varus deformity of the lower leg, the medial condyles of the tibia and femur are overloaded, and the lateral joint space widens.