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ESSENTIALS OF DIAGNOSIS
Effusion can occur with intra-articular pathology (eg, OA, meniscus and cruciate ligament tears).
Acute knee swelling (due to hemarthrosis) within 2 hours may indicate ligament injuries or patellar dislocation or fracture.
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General Considerations
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The knee is the largest joint in the body and is susceptible to injury from trauma, inflammation, infection, and degenerative changes. Table 41–4 shows the differential diagnosis of knee pain. The knee is a hinge joint. The joint line exists between the femoral condyles and tibial plateaus. Separating and cushioning these bony surfaces is the lateral and medial meniscal cartilage, which functions as a shock absorber during weight bearing, protecting the articular cartilage. The patella is a large sesamoid bone anterior to the joint. It is embedded in the quadriceps tendon, and it articulates with the trochlear groove of the femur. Poor patellar tracking in the trochlear groove is a common source of knee pain, especially when the cause is atraumatic in nature. The knee is stabilized by the collateral ligaments against varus (lateral collateral ligament) and valgus (medial collateral ligament) stresses. The tibia is limited in its anterior movement by the anterior cruciate ligament (ACL) and in its posterior movement by the posterior cruciate ligament (PCL). The bursae of the knee are located between the skin and bony prominences. They are sac-like structures with a synovial lining. They act to decrease friction of tendons and muscles as they move over adjacent bony structures. Excessive external pressure or friction can lead to swelling and pain of the knee bursae. The prepatellar bursae (located between the skin and patella) and the pes anserine bursa (which is medial and inferior to the patella, just below the tibial plateau) are most commonly affected. Joint fluid, when excessive due to synovitis or trauma, can track posteriorly through a potential space, resulting in a popliteal cyst (also called a Baker cyst). Other structures that are susceptible to overuse injury and may cause knee pain following repetitive activity include the patellofemoral joint and the iliotibial band. OA of the knees is common after 50 years of age and can develop due to previous trauma, aging, activities, alignment issues, and genetic predisposition.
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Acute hemarthrosis represents bloody swelling that usually occurs within the first 1–2 ...