A 45 y/o M with PMhx of HTN and ESRD on dialysis presents to the ED w/ knee pain after slipping at work. On exam, he is unable to extend his leg. An X-ray is done and shown below. What's the diagnosis? (scroll down for answer)

Answer: Quadriceps tendon rupture

• At first glance, lateral XR does not show obvious deformity....however: you may notice calcifications just superior to the patella (representing retracted tendon) 
• Quadriceps tendon ruptures tend to occur in individuals > 40, while patellar tendon ruptures occur more frequently in individuals < 40
• Overall quadricep tendon ruptures are more common (risk factors- rheumatologic disease, renal failure, DM, chronic steroid use) 
• High riding patella ("patella alta") on lateral films is more frequently seen in patellar tendon ruptures
• Low riding patella ("patella baja") may be seen on lateral film with complete quadricep tendon tear (intact patellar tendon displaces patella inferiorly) 
• Xrays may be normal!
• In both injuries, classic exam finding is inability to extend the knee
• You can often palpate a defect just above the patella in quadriceps tendon ruptures
• Orthopedic consultation is always warranted for these cases
• Remember-- always consider knee dislocation in any patient that has a severely injured knee that is unstable!
• Dislocations typically occur traumatically, but can also occur spontaneously, especially in obese individuals 
• They are a surgical emergency due to possible injury of the popliteal artery (ischemia risk) and peroneal nerve
• Timely reduction is essential, with documentation of a neurovascular exam before and after reduction
• Extemity should then be splinted with knee at 20 degrees of flexion to prevent re-dislocation

References:

Bengtzen RR, Glaspy JN, Steele MT. Knee Injuries. In: Tintinalli JE, Stapczynski JS, et al., eds. Tintinalli’s Emergency Medicine: A Comprehensive Study Guide. 8th ed. New York, NY: McGraw-Hill; 2016: (Ch) 274.