A Brief Synopsis of AFP's "Evaluation of Patients Presenting with Knee Pain: Part II. Differential Diagnosis"

Evaluation of Patients Presenting with Knee Pain: Part II. Differential Diagnosis
WALTER L. CALMBACH, M.D., University of Texas Health Science Center at San Antonio, San Antonio, Texas MARK HUTCHENS, M.D., University of Texas at Austin, Austin, Texas
http://www.aafp.org/afp/2003/0901/p917.pdf
Am Fam Physician. 2003 Sep 1;68(5):917-922.

     The differential diagnosis of knee pain can be broken down by age and location of pain. In children, the three most common conditions are patellar subluxation, tibial apophysitis, and patellar tendinitis. Patellar subluxation is when the patella moves out of it's track, usually laterally. It may be seen in a teenage female who states that her knee "gave way". The dislocation may lead to hemarthrosis and swelling.  Tibial apophysitis is also known as Osgood-Schlatter disease. The pain is typically found at the tibial tuberosity. The pain in intermittent. It is worse when the patient is squatting, walking on stairs, repetitively jumping or during quadricep contraction. Pain is reproduced with passive knee hyperflexion or knee extension against resistance. Patellar tendonitis presents with vague anterior knee pain ad tenderness. Knee extension against resistance will reproduce the pain. SCFE (slipped capital femoral epiphysis) is also seen primarily in children. It is seen in overweight children. They will present with a flexed and externally rotated hip while sitting. Hip pain will be seen with passive internal rotation or extension of the hip. The disease is best seen with CT of the femoral head. Osteochondritis dissecans is degeneration and re-calcification of the articular cartilage and bone. The pain is poorly localized. There is morning stiffness as well. Joint mice ("loose bodies") may be seen in the joint which will exacerbate locking and catching of the joint. 
     Overuse injury is a common cause of adult knee pain. Anterior knee pain may be due to patello-femoral syndrome (chondromalacia patellae). This presents as patellar tenderness with prolonged sitting. Pain may be reproduced with anterior, lateral, or medial pressure to the knee. Medial knee pain may be due to medial plica syndrome. This presents as medial knee tenderness without effusion, which may become inflamed from overuse. Pes anserine bursitis is medial knee pain over the anteromedial aspect of the knee. This will worsen with repetitive use and may be confused with MCL problems. Pain may be reproduced with valgus stress (as well as the MCL, so it is not very helpful). There is effusion at the insertion of the medial hamstrings, but not at the knee joint. 
     Lateral knee pain in the adult may be due to tendinitis of the iliotibial band on the lateral femoral condyle. It is another overuse injury seen in runners and cyclists. The pain is seen at the lateral knee. Noble's test is done by having the supine patient flex and extend his or her knee during pressure over the lateral condyle. Trauma was explained in the previous article so please refer to that one.
     Infection of the knee is a serious condition. Risk factors include corticosteroid use and a weak immune systems. The joint will be red, warm, swollen and tender. Arthrocentesis will show an elevated WBC count (above 50,000/mm3), at least 75% PMNs, elevated protein, and a low glucose concentration.  Possible pathogens include S. aureus, H. influenzae, N. gonorrhoeae, and Streptococcus. There may be an elevated ESR .
     In older adults, the most typical problems are OA, gout/pseudogout, and bakers cyst.  OA in the knee is worse while weight bearing and better with rest. It is asymmetrical. Morning stiffness persists for less than 30 minutes. X rays will show joint space narrowing, sclerosis, and osteophyte formation. Gout of the knee will present with intense inflammation. Synovial fluid will have sodium urate crystal and negative birefringent rods. If it is pseudogout  the crystals will be calcium pyrophosphate and there will be positive birefringent rhomboids. The synovial fluid will be clear or partly cloudy. The WBC will be 2,000-75,000/mm3, high protein, and lower glucose concentration.