Pathogenesis of osteoarthritis

Morphological pathogenesis: how the joint changes

Disease progression is accompanied by a predictable sequence of structural changes that can be clearly visualized using instrumental diagnostics:

• Cartilage damage: First, the turgor of the cartilage tissue decreases, and fissures appear. Gradually, the destruction deepens down to the subchondral bone. As a result, the joint space steadily narrows, and in end-stage disease, it is barely traceable.
• Subchondral sclerosis: Due to the loss of cartilaginous shock absorption, the bone undergoes compensatory densification in the most heavily loaded areas. This osteosclerosis progresses even in late stages, despite concurrent disuse osteoporosis.
• Bone cysts: Microfractures occur in the epiphyseal zone of the subchondral bone. Cysts form in their place (seen on radiographs as oval radiolucencies with clear margins), and some of them may rupture into the joint cavity.
• Osteophytes: Bone spurs appear at the boundary with the hyaline cartilage, at the attachment sites of ligaments and the capsule. Gradually, they increase in size and dramatically alter the configuration of the joint.
• Bone impression: In areas of maximum load, the articular surface "collapses." This leads to dyscongruence (mismatch of the shapes of articular surfaces) and the formation of subluxations.
• Soft tissue changes: Portions of the capsule may ossify. When detached fragments undergo sclerosis within the cavity, so-called "joint mice" (loose bodies) are formed.

Stages of cartilage destruction (ICRS classification)

The International Cartilage Repair Society (ICRS) identifies 4 stages of damage. This classification is critically important because the state of the cartilage determines the treatment strategy and prognosis:

1. Stage I: Softening and decreased turgor (determined by palpation with an arthroscopic hook), shallow superficial fissures.
2. Stage II: Cartilage destruction and delamination of superficial fragments not reaching the subchondral bone.
3. Stage III: Damage to a significant depth, up to a linear transchondral fracture (destruction reaches the bone).
4. Stage IV: Total destruction of both the articular cartilage and the underlying subchondral bone.

Clinical and radiological presentation (N.S. Kosinskaya stages)

N.S. Kosinskaya's classification takes into account both radiological changes and clinical symptoms (pain, contractures, crepitus).

• Stage I: Minor joint space narrowing, small osteophytes, local ossification of the cartilage. Clinically: Moderate limitation of movement. There is no pain at rest; it appears only after prolonged exertion or as "start-up pain" at the beginning of movement.
• Stage II: Joint space narrowing by 2–3 times, subchondral sclerosis, appearance of bone cysts and gross osteophytes. Clinically: Severe pain syndrome (subsides only after prolonged unloading), coarse crepitus. Contractures develop, leading to functional limb shortening, limp, pelvic tilt, and secondary scoliosis (due to altered statics and excessive load on the intervertebral discs).
• Stage III: Joint space is almost absent, pronounced bone impression and surface deformation, severe sclerosis, possible presence of "joint mice". Clinically: Almost complete loss of mobility, with only rocking movements remaining.
• In coxarthrosis (damage to the hip joint), "stiff leg" syndrome develops: the femur moves only together with the pelvis, and the patient walks with crutches.
• In gonarthrosis (damage to the knee joint), a flexion contracture occurs with marked lateral deformity of the axis (most commonly varus).

Kellgren–Lawrence radiological grades

This grading system is based exclusively on radiography data:

Laboratory and instrumental diagnostics

Osteoarthritis is characterized by the absence of systemic inflammation. Complete blood counts and urinalyses show no abnormalities. The ESR remains within 20–25 mm/h, even if secondary synovitis develops. The synovial fluid retains normal viscosity, the cell count does not exceed $5 \cdot 10^3$ per 1 mm³, and the proportion of neutrophils is less than 50%.

Instrumental methods:

• Ultrasound and MRI: Allow visualization of the capsule, ligaments, menisci, precise sizing of cysts, and detection of non-radiopaque cartilage inclusions.
• Radiography: The baseline method for detecting osteophytes, space narrowing, and sclerosis.
• CT: Considered uninformative for diagnosing osteoarthritis.
• Arthroscopy: An endoscopic method for direct visual assessment of cartilage.

For a comprehensive assessment of the severity of large joint damage, the Lequesne Index (Lequesne Index) is used, which sums up points for morning stiffness, pain, pain-free walking distance, and activities of daily living.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Criteria for secondary osteoarthritis against the background of autoimmune pathologies.
• Principles of calculating the Lequesne index in clinical practice.
• Differential diagnosis with gout and Bechterew's disease.
• Conservative treatment: approaches to managing reactive synovitis.
• Mechanism of formation of arthrogenic and myogenic contractures.

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