The largest joint in the body, the knee joint, is a trochoginglymus-type joint, which provides flexion and extension as well as a limited range of medial and lateral rotation. The joint has two articulations: (1) between the femur and tibia and (2) between the patella and femur (Fig. 136.1). The joint’s articular cartilage is susceptible to damage, which, if left untreated, will result in arthritis with its associated pain and functional disability. Osteoarthritis of the joint is the most common form of arthritis that results in knee joint pain and functional disability, with rheumatoid arthritis and posttraumatic arthritis also causing arthritis of the knee joint. Less common causes of arthritis-induced knee joint pain include the collagen vascular diseases, infection, villonodular synovitis, and Lyme disease. Acute infectious arthritis of the knee joint is best treated with early diagnosis, with culture and sensitivity of the synovial fluid, and with prompt initiation of antibiotic therapy. The collagen vascular diseases generally manifest as a polyarthropathy rather than a monoarthropathy limited to the knee joint, although knee pain secondary to the collagen vascular diseases responds exceedingly well to ultrasound-guided intra-articular injection of the knee joint.

Patients with knee joint pain secondary to arthritis, tears of the menisci, and collagen vascular disease-related joint pain complain of pain that is localized to the knee and surrounding area. Activity makes the pain worse, with rest and heat providing some relief. The pain is constant and characterized as aching. Sleep disturbance is common with awakening when the patient rolls over onto the affected knee. Some patients complain of a grating, catching, or popping sensation with range of motion of the joint, and crepitus may be appreciated on physical examination.

Functional disability often accompanies the pain associated with many pathologic conditions of the knee joint. Patients will often notice increasing difficulty in performing their activities of daily living, and tasks that require walking, climbing stairs, and walking on uneven surfaces are particularly problematic. If the pathologic process responsible for the patient’s pain symptomatology is not adequately treated, the patient’s functional disability may worsen, and muscle wasting, especially of the quadriceps and ultimately a frozen knee, may occur.

Plain radiographs are indicated in all patients who present with knee pain as not only intrinsic knee disease but also other regional pathology may be perceived as knee pain by the patient (Fig. 136.2). Based on the patient’s clinical
presentation, additional testing may be indicated, including complete blood cell count, sedimentation rate, and antinuclear antibody testing. Magnetic resonance imaging or ultrasound of the knee is indicated if avascular necrosis or meniscal tear is suspected (Figs. 136.3, 136.4 and 136.7).

The knee joint is comprised of the articulation between the rounded condyles of the distal femur and the condyles of the proximal tibia below and the patella anteriorly (see Fig. 136.1). The articular surface of the knee joint is covered with hyaline cartilage, which is susceptible to arthritis from a variety of causes (Fig. 136.8). Because the vascular supply to the articular cartilage is tenuous, the knee joint is susceptible to avascular necrosis. Posterior and lateral joint support is provided by a dense joint capsule with the suprapatellar and prepatellar bursae taking its place anteriorly. Additional posterior stability is provided by the oblique popliteal ligament as well as the posterior cruciate ligament with the anterior cruciate ligament providing anterior stability. Medial and lateral support is provided by the medial and lateral collateral ligaments (see Fig. 136.8). The knee joint is provided with two unique cartilaginous articular discs, the medial meniscus and lateral meniscus, which partly divide the joint space and serve as both shock absorbers to prevent the condyles of the femur and tibia from abrading each other (see Fig. 136.8).