Insall–Salvati Ratio (Patellar Height)
Quantifies patellar height on a lateral knee radiograph by comparing patellar tendon length with patellar bone length, detecting patella alta or baja.
The Insall–Salvati ratio is the most widely used index of patellar height. It relates the length of the patellar tendon to the greatest diagonal length of the patella, both measured on a lateral radiograph with the knee flexed about 30°. Because it uses only bone/tendon landmarks it is independent of knee flexion angle and radiographic magnification.
📐 Formula
Insall–Salvati ratio = patellar tendon length / patellar (bone) length
🎯 When to Use
- Lateral knee radiograph in patients with anterior knee pain, patellar instability or recurrent dislocation.
- Suspected patella alta (predisposing to instability) or patella baja (post-surgical, post-traumatic, arthrofibrosis).
- Pre-operative assessment before patellar realignment procedures.
⚙️ How It Works
- Measure the patellar tendon length (TL) from the inferior pole of the patella to its tibial tuberosity insertion.
- Measure the greatest diagonal length of the patella (PL).
- Compute the ratio TL / PL.
- Perform on a true lateral with approximately 30° of flexion for reproducibility.
📊 Interpreting the Result
- Normal ratio is approximately 0.8–1.2.
- Ratio > 1.2 indicates patella alta (high-riding patella), associated with instability and chondromalacia.
- Ratio < 0.8 indicates patella baja/infera (low-riding patella).
⚠️ Limitations & Pitfalls
- Difficult to apply when the tendon insertion or inferior patellar pole is ill-defined (e.g. after tibial tubercle surgery).
- Does not directly assess the patellofemoral articular relationship — the Caton–Deschamps or patellotrochlear indices may be complementary.
- An irregular patellar shape reduces reproducibility.
📚 References
- Insall J, Salvati E. Patella position in the normal knee joint. Radiology. 1971;101(1):101-104.
- Grelsamer RP, Meadows S. The modified Insall-Salvati ratio for assessment of patellar height. Clin Orthop Relat Res. 1992;(282):170-176.
⚕️ For clinical decision support only. Always verify calculations and apply clinical judgment.