Clinical calculator
Insall-Salvati Calculator
Insall-Salvati Index
The Insall-Salvati index is the ratio of patellar tendon length to patellar length measured on a lateral knee radiograph; a ratio between 0.8 and 1.2 is normal, above 1.2 indicates patella alta, and below 0.8 indicates patella baja.
What the Insall-Salvati score means
Insall-Salvati scores run from 0 to 3, where a higher score is better. The bands below are conventional reporting categories, not diagnostic cut-offs.
- Patella baja (infera)0–0.79
The patella sits lower than expected, which is seen after patellar tendon shortening, in achondroplasia, and following knee arthroplasty or tibial tubercle surgery.
- Normal patellar height0.8–1.2
The patellar tendon length is within the normal range relative to patellar length.
- Patella alta1.21–99
The patella sits higher than expected, a recognised risk factor for patellar instability, and associated with patellofemoral pain and chondromalacia.
Published criteria and reliability
The published cut-offs that define this instrument, and the measured agreement between observers using it. Each is reported with the population it was derived in.
| Type | Value | Subscale | Population | Follow-up | Source |
|---|---|---|---|---|---|
| Criterion | > 1.2 defines patella alta | — | Adults, lateral radiograph | — | Insall J 1971 |
| Criterion | < 0.8 defines patella baja | — | Adults, lateral radiograph | — | Insall J 1971 |
| Criterion | > 1.3 associated with markedly increased patellar instability risk | — | Patellar dislocation cohorts | — | Dejour H 1994 |
Criterion is a published cut-off, definition or performance figure belonging to a classification or decision rule — it is not a change score and should not be read as one.
How the score is calculated
Stated in full so the calculation can be independently checked, and reproduced by hand if needed.
Insall-Salvati index = patellar tendon length ÷ patellar length. Patellar tendon length is measured from the inferior pole of the patella to its insertion on the tibial tubercle. Patellar length is the greatest diagonal length of the patella. Because both values are lengths on the same image, the ratio is independent of radiographic magnification and of patient size.
Instrument profile
- Developer
- John Insall and Enrique Salvati
- Institution
- Hospital for Special Surgery, New York
- First published
- 1971
- Items
- 2
- Response format
- Two linear measurements in millimetres
- Score range
- 0 to 3
- Direction
- Higher score = better
- Completion time
- About 1 minutes
- Regions
- Knee
- Conditions
- Patellar instability, Patellofemoral pain, Patella alta, Patella baja
- Procedures
- Tibial tubercle osteotomy, MPFL reconstruction
- Observer agreement
- Interobserver ICC typically 0.86–0.95
- Validity
- The most widely reported patellar height index, though the Caton-Deschamps and Blackburne-Peel indices reference the tibial plateau rather than the tendon insertion and are preferred after tubercle surgery.
Normative values
Reference values from published cohorts, for comparison against the score you calculated.
| Population | Age | Mean | SD |
|---|---|---|---|
| Asymptomatic adults | — | 1.02 | 0.13 |
| Recurrent patellar dislocation | — | 1.25 | 0.18 |
When to use the Insall-Salvati
Well suited to
- Assessing patellar height in suspected patellar instability or patellofemoral pain.
- Pre-operative planning where patella alta may warrant distalisation.
Limitations and poor fits
- After tibial tubercle osteotomy or patellar tendon repair, where the tendon insertion is no longer anatomical — use Caton-Deschamps instead.
- In skeletally immature patients where the tubercle apophysis is not ossified.
- When the patellar tendon is not clearly visible on the lateral radiograph.
Insall-Salvati compared with other instruments
Instruments measuring a related construct, with the practical difference between them.
| Instrument | Items | Range | What it adds |
|---|---|---|---|
| Caton-Deschamps | 2 | 0–2 | The Caton-Deschamps index is the ratio of the distance from the inferior pole of the patellar articular surface to the anterosuperior corner of the tibial plateau, divided by the length of the patellar articular surface, where 0.6 to 1.2 is normal, above 1.2 is patella alta and below 0.6 is patella baja. |
Copyright and permitted use
Published openly because it is the question every clinician and researcher asks before using an instrument, and almost nobody answers it.
A published radiographic measurement method. No licence is required to calculate or describe it.
References
- Insall J, Salvati E. Patella position in the normal knee joint. Radiology. 1971.doi:10.1148/101.1.101PMID 5111961
- Dejour H, Walch G, Nove-Josserand L, Guier C. Factors of patellar instability: an anatomic radiographic study. Knee Surgery, Sports Traumatology, Arthroscopy. 1994.PMID 7584186
- Seil R, Müller B, Georg T, Kohn D, Rupp S. Reliability and interobserver variability in radiological patellar height ratios. Knee Surgery, Sports Traumatology, Arthroscopy. 2000.PMID 11061294
Editorial status
This page has not yet been signed off by a named clinical reviewer. The scoring logic is stated in full above so it can be independently verified, and every threshold is traceable to its source. See our editorial methodology.