A 35-year-old man comes to the emergency department due to right knee pain. The patient was participating in a recreational football game when he jumped to catch a ball and landed with his foot planted and knee partially bent. He heard a loud popping sound, felt immediate pain, and has been unable to walk since. The patient smokes half a pack of cigarettes a day and drinks a few beers on the weekend. BMI is 35 kg/m². There is a large effusion over the right knee. The patient can bend the right knee but is unable to straighten his leg due to weakness. He is unable to bear weight on the knee but has no instability. X-ray of the right knee shows a large effusion and a low-lying patella with no fractures noted. Which of the following is the most likely diagnosis in this patient?

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Explanation: 11ec25c8_d974_09a1_bc81_cfc1650e1b8d_MD0003_00 At full activation, the quadriceps muscle-comprised of the rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius-generates extreme force across the knee joint. Sudden, forceful contraction, as can occur in deceleration from a fall or in certain athletic activities, can cause rupture of the quadriceps-patellar tendon complex. Tendon injuries can be categorized in relation to the patella: Quadriceps tendon tears (proximal to the patella in the rectus femoris tendon): the patella rides low (as seen in this patient), indicating an intact connection to the tibia, with a palpable defect above the patella Patellar tendon tears (distal to the patella): the patella rides high, often with a palpable defect below the patella (the nomenclature can be confusing as the patellar tendon is actually a ligament) (Choice C) This patient, who is unable to bear weight after landing on flexed knee, has a large knee effusion and a low-lying patella; this presentation suggests quadriceps tendon rupture (proximal to the patella). Typical symptoms include an audible pop, rapid swelling, and inability to actively extend the knee against gravity. Risk factors include chronic kidney disease, hyperparathyroidism, and anabolic steroid abuse. Exposure to fluoroquinolone antibiotics may also increase the risk. The diagnosis and extent of the injury can be assessed on MRI. Management of complete tears of the quadriceps tendon requires surgical intervention. MRI (Choice A) Tears involving the anterior cruciate ligament are common in middle-aged athletes and can cause rapid-onset hemarthrosis. Extensive swelling may mask ligamentous instability in the acute phase, but knee extension against gravity is intact. (Choice B) Meniscal injuries typically present with slow-onset swelling, with locking or catching during range-of- motion testing. Weight bearing and knee extension are intact. (Choice D) Popliteal cyst rupture typically presents with acute pain, ecchymosis, and swelling in the posterior knee and calf. Although this injury occurs most commonly in patients with underlying knee pathology (eg, osteoarthritis), weight bearing and joint stability are unaffected. Educational objective: Sudden, forceful contraction of the quadriceps muscle can cause rupture of the quadriceps-patellar tendon complex. Symptoms include an audible pop, rapid swelling, and inability to actively extend the knee against gravity. In tears of the quadriceps tendon (proximal to the patella) the patella rides low, with a palpable defect above the patella. References: Quadriceps tendon rupture. (http://www.ncbi.nlm.nih.gov/pubmed/29494011)

A 35-Year-Old Man Comes to the Emergency Department Due to Right