Figure 1. Lateral radiograph of the right knee.
Figure 2. Sagittal magnetic resonance imaging of the knee.
A 25-year-old construction worker presents to the office with pain in his right knee following a fall sustained at work. He was walking down a hill when his feet slipped and his right knee hyperflexed. The patient felt a “pop” in the knee as he fell and experienced immediate onset of pain and inability to bear weight. On physical examination, the patient is unable to extend his right knee. Lateral radiograph and sagittal magnetic resonance imaging (MRI) of the right knee is obtained (Figures 1 and 2).
Submit your diagnosis to see full explanation.
An extensor mechanism injury should be suspected when a patient is unable to perform knee extension or hold their leg straight in the air. The extensor mechanism of the knee consists of the quadriceps tendon, patella, and the patellar tendon. Injury to any of these structures may impair knee extension; this is commonly referred to as an extension lag.1
Injuries to the quadriceps and patellar tendons typically occur with a forceful quadriceps contraction of a flexed knee. Quadriceps tendon ruptures typically occur in patients >40 years of age while patellar tendon ruptures typically occur in patients aged <40 years.1,2 Patellar fractures are the most common reason for an extensor mechanism injury and generally occur with a direct impact to the knee.1
Lateral radiograph of the patient’s knee identifies patella alta, also known as a high-riding patella. The length of the patellar tendon (measured from the tibial tubercle to the inferior pole of the patella) should be roughly equal to the length of the patella. MRI of the patient’s knee shows a complete rupture of the patellar tendon with detachment off the inferior pole of the patella and attenuation of the tendon. Patellar tendon ruptures require timely surgery with primary repair of the tendon back to the inferior pole of the patella.1,2
Dagan Cloutier, MPAS, PA-C, practices in a multispecialty orthopedic group in the southern New Hampshire region and is editor in chief of the Journal of Orthopedics for Physician Assistants.
1. Watts E. Patella tendon ruptures. OrthoBullets website. https://www.orthobullets.com/knee-and-sports/3024/patella-tendon-rupture. Updated September 28, 2018. Accessed June 8, 2020.
2. Garner MR, Gausden E, Berkes MB, Nguyen JT, Lorich DG. Extensor mechanism injuries of the knee. J Bone Joint Surg Am. 2015;97(19):1592-1596.