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Insall-Salvati Index: Definition & Normal Values
The Insall-Salvati Index is a radiological measure used to assess patellar height. It describes the ratio between patellar tendon length and patellar length.
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The Insall-Salvati Index is a radiological measure used to assess patellar height. It describes the ratio between patellar tendon length and patellar length.
What is the Insall-Salvati Index?
The Insall-Salvati Index (also referred to as the Insall-Salvati Ratio) is a measurement used in orthopaedics and radiology to assess the position of the kneecap (patella) relative to the knee joint. It was first described in 1971 by orthopaedic surgeons John N. Insall and Edmond Salvati and has since become a standard tool in the clinical evaluation of knee conditions.
The measurement is performed on a lateral X-ray of the knee taken at approximately 30 degrees of flexion. The index is calculated as the ratio of two distances:
- Length of the patellar tendon (from the inferior pole of the patella to the tibial tuberosity)
- Length of the patella (greatest diagonal diameter of the kneecap)
The formula is: Insall-Salvati Index = Patellar Tendon Length / Patellar Length
Normal Values and Interpretation
In a healthy knee, the Insall-Salvati Index is approximately 1.0, with values between 0.8 and 1.2 generally considered normal.
- Index > 1.2 (Patella alta): The kneecap is positioned too high. This can lead to instability, a tendency toward dislocation, and anterior knee pain.
- Index < 0.8 (Patella baja / Patella infera): The kneecap is positioned too low. This is often seen after surgery or injury and is frequently associated with limited range of motion.
Clinical Significance
The Insall-Salvati Index plays a key role in the evaluation and treatment planning of various knee conditions. It is particularly used in the assessment of:
- Patellar dislocation: Patella alta is a significant risk factor for recurrent dislocations of the kneecap.
- Patellofemoral pain syndrome: Anterior knee pain that may be partly caused by abnormal patellar positioning.
- Preoperative planning: The index is routinely measured before knee-preserving procedures or total knee arthroplasty.
- Postoperative follow-up: After surgery on the knee joint or patellar tendon, the index is used to evaluate the outcome of the intervention.
Modifications of the Insall-Salvati Index
Because the classic Insall-Salvati Index can be inaccurate in certain cases (e.g., after surgical alteration of the tibial tuberosity), modifications have been developed:
- Modified Insall-Salvati Index (Grelsamer and Meadows, 1992): This version relates patellar tendon length to the articular surface length of the patella rather than its total length, improving accuracy in cases of anatomical variation.
How the Measurement is Performed
The measurement is standardly performed on a lateral knee X-ray under the following conditions:
- Knee flexion angle of approximately 30 degrees
- Patient positioned standing or supine depending on the protocol
- Clear visualisation of the patella, patellar tendon, and tibial tuberosity
Patellar height can also be assessed using MRI (magnetic resonance imaging) or ultrasound, although the Insall-Salvati Index is primarily a radiographic measurement tool.
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. Clinical Orthopaedics and Related Research. 1992;282:170-176.
- Diederichs G et al. Radiological measurement of patellar height: a comparative study of six methods. The Journal of Bone and Joint Surgery. 2010;92(14):2045-2052.
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