A 46-year-old female with a history of diabetes and morbid obesity presents to the emergency department (ED) with difficulty walking after she tripped on a curb and fell onto her right knee. You obtain X-rays (Figure 1). What is your suspected diagnosis? What is your initial workup in the ED? What is your management and disposition?
Figure 1. AP/lateral x-ray of the right knee. Author’s own images.
Pearl: Patellar tendon ruptures are most frequently seen in men under 40 years of age. This is opposed to quadricep tendon ruptures, which are typically seen in older patients . Common comorbidities include diabetes mellitus, chronic renal failure, and gout. Rupture usually occurs at the inferior pole of the patella.
1. X-rays: Can show a superiorly displaced patella, known as patella alta (Figure 1), as well as avulsion fractures from the inferior pole of patella and, less frequently, the tibial tuberosity .
Pearl: Insall-Salvati ratio (ratio of patellar tendon length to diagonal length of patella) >1.2 suggests patellar tendon tear (Figure 2) [2,5]. See Figure 3 for the typical location of the patella on the lateral knee x-ray. For more on patella alta x-rays, please see the associated SplintER Series article.
Figure 2: Insall-Salvati index. Case courtesy of Dr Wael Nemattalla, Radiopaedia.org, rID: 10329.
Figure 3: Normal lateral knee radiograph demonstrating the typical location of the patella; case courtesy of Dr. Matt Skalski, Radiopaedia.org, rID: 30420.
2. Ultrasound: Useful, but operator dependent and has been associated with a false positive rate of up to 33.3% in one study [2,4].
3. MRI: The gold standard, but may not be feasible or necessary in the emergency department . See Figure 4 below for an MRI demonstrating a rupture of the patellar tendon.
Figure 4. MRI showing a sagittal view of complete rupture of the patellar tendon. Author’s own images.