A 45-year-old man presents to the emergency department after a dashboard injury to his right knee during a motor vehicle accident. On examination, he has significant knee pain and swelling. A lateral knee X-ray is obtained. The structure marked **A** in the diagram is the patella. On careful inspection, you note that the patella appears to be positioned lower than expected relative to the femoral condyles, and the Insall-Salvati ratio is measured at 0.75. Which of the following is the most likely associated injury?

Explanation

## Why "Quadriceps tendon rupture with patella baja" is right The clinical presentation of a low-riding patella (patella baja) with an Insall-Salvati ratio of 0.75 (normal 0.8–1.2) is pathognomonic for quadriceps tendon rupture. When the quadriceps tendon ruptures at its insertion on the superior pole of the patella, the patella loses its proximal support and migrates inferiorly, creating the characteristic patella baja appearance on lateral X-ray. This injury typically occurs in older patients (around 50 years and above) with predisposing factors such as chronic tendinopathy or fluoroquinolone use, though direct trauma can also cause acute rupture. The patient would present with inability to extend the knee against gravity and a palpable suprapatellar defect. (Gray's Anatomy 42e Ch 80; Apley 10e) ## Why each distractor is wrong - **Patellar tendon rupture with patella alta**: Patellar tendon rupture produces the opposite radiographic finding—patella alta (high-riding patella) with an elevated Insall-Salvati ratio (>1.2), not the low-riding patella described in this case. The patella rises when the inferior attachment is disrupted. - **Patellar dislocation with MPFL tear**: Patellar dislocation is primarily a clinical diagnosis with lateral tracking on examination (J sign) and apprehension on palpation. While it may be associated with effusion and hemarthrosis on X-ray, it does not produce the characteristic patella baja finding. The Insall-Salvati ratio remains normal in uncomplicated dislocation. - **Transverse patellar fracture with intact extensor mechanism**: An undisplaced transverse patellar fracture with an intact extensor mechanism would not alter the vertical position of the patella relative to the femoral condyles. The Insall-Salvati ratio would remain within normal limits. Patella baja is not a feature of patellar fractures. **High-Yield:** Patella baja (low-riding patella, Insall-Salvati <0.8) = quadriceps tendon rupture; patella alta (high-riding patella, Insall-Salvati >1.2) = patellar tendon rupture. [cite: Gray's Anatomy 42e Ch 80; Apley 10e]