A 58-year-old man presents with acute inability to extend his right knee after a fall on stairs. On examination, there is a palpable gap in the anterior thigh just above the patella, and he cannot perform a straight leg raise against gravity. A lateral knee X-ray is obtained. The measurement marked **D** (Insall-Salvati Ratio) is calculated as 0.65. Which of the following is the most likely diagnosis?

Explanation

## Why Quadriceps tendon rupture with patella baja is right The Insall-Salvati Ratio (marked **D** in the diagram) is calculated as patellar tendon length divided by patellar height. A ratio < 0.8 defines patella baja (low-riding patella). In this clinical scenario—an older patient (58 years) with acute loss of knee extension, palpable defect above the patella, and inability to perform straight leg raise—these findings are classic for quadriceps tendon rupture. The decreased Insall-Salvati ratio (0.65) confirms patella baja, which occurs because the patella is pulled downward when the quadriceps tendon is ruptured and not repaired, or as a sequela of quadriceps tendon injury. According to Apley's Orthopedic Surgery (10e), assessment of the Insall-Salvati ratio is essential in evaluating extensor mechanism injuries; a decreased ratio strongly suggests quadriceps tendon rupture or post-surgical changes. ## Why each distractor is wrong - **Patellar tendon rupture with patella alta**: Patellar tendon rupture causes patella ALTA (high-riding patella), not baja. An Insall-Salvati ratio > 1.2 would be expected, not 0.65. The clinical presentation (palpable defect ABOVE the patella) also localizes the injury to the quadriceps, not the patellar tendon. - **Congenital patella alta predisposing to recurrent dislocation**: Congenital patella alta presents with an increased Insall-Salvati ratio (> 1.2), not a decreased one (0.65). While it does predispose to recurrent patellar dislocation, it does not cause acute loss of knee extension or a palpable defect above the patella. - **Chronic patellar chondromalacia with normal Insall-Salvati ratio**: Chondromalacia patellae is a degenerative condition of articular cartilage and does not cause acute loss of knee extension, palpable defects, or abnormal Insall-Salvati ratios. The clinical presentation is inconsistent with chondromalacia. **High-Yield:** Insall-Salvati Ratio < 0.8 = patella baja = quadriceps tendon rupture or post-surgical sequela; > 1.2 = patella alta = patellar tendon rupture or congenital predisposition to dislocation. [cite: Apley 10e — Extensor Mechanism Injuries and Radiographic Assessment]