Patellar Height: Insall-Salvati Ratio Calculator
Patellar Height: Insall-Salvati Ratio Calculator
Divide the patellar tendon length by the patellar length for the Insall-Salvati ratio, with the modified ratio alongside it — and two published sets of cut-offs that disagree about where patella alta begins.
Insall-Salvati and modified Insall-Salvati ratios
Tendon / patella, and tendon / articular surfacePatellar tendon length 46 mm, greatest patellar length 44 mm, patellar articular surface 35 mm, on a lateral radiograph with the knee flexed 30 degrees
Two ratios from three distances
modified Insall-Salvati = patellar tendon length / patellar ARTICULAR surface length
- tendon on top, in both
- the numerator is the same measurement for both ratios: the posterior surface of the patellar tendon from the lower pole of the patella to its tibial insertion. Only the denominator changes
- the denominators are different lengths
- Insall-Salvati divides by the WHOLE patella pole to pole; the modified ratio divides by the ARTICULAR SURFACE only. The articular surface is shorter, so the modified ratio is always larger, and that is why its normal range is 1.2 to 2.0 against 0.8 to 1.2
- transposing the two terms gives the reciprocal
- patellar length divided by tendon length is 1 / the Insall-Salvati ratio, and the two orderings multiply to exactly 1. That identity is how this page’s proof detects a swap — and it only works where the two lengths differ, because at tendon length equal to patellar length both orderings give 1 and every wrong version agrees. The check is made away from that point deliberately
- why the modified ratio exists
- a patella with a long non-articular inferior pole — the Cyrano patella — has a large pole-to-pole length, so Insall-Salvati reads low. The review read for this page records that patellar shape “might result in a higher incidence of patella infera when utilizing the IS index”. The modified ratio removes the shape dependency by measuring only the cartilage
- the two indices not implemented here
- the Caton-Deschamps index (infera below 0.6, norma 0.6 to 1.2, alta above 1.2) and the Blackburne-Peel index (below 0.6, 0.6 to 1.0, above 1.0) both measure from the tibial plateau rather than from the patellar tendon. Neither is built here because no source read for this page states their numerator and denominator distances in text, and a measurement index inferred from a diagram is exactly the kind of plausible error this project refuses to ship
- derivation
- Insall J and Salvati E, “Patella position in the normal knee joint”, Radiology 1971;101:101–104; the modified ratio from Grelsamer RP and Meadows S, Clin Orthop Relat Res 1992;282:170–176. Both citations were read in a reference list; neither paper was reachable, so the measured distances here are quoted from Radiopaedia and the cut-offs from a 2022 SICOT-J instructional review
Worked example
Patellar tendon length 46 mm, greatest patellar length 44 mm, patellar articular surface 35 mm, on a lateral radiograph with the knee flexed 30 degrees
Insall-Salvati = 46 / 44 = 1.05, inside the 0.8 to 1.2 normal range in the SICOT-J review's table
Modified Insall-Salvati = 46 / 35 = 1.31, inside its own normal range of 1.2 to 2.0. The two ratios agree that this patella sits normally, and they have to be read against different numbers to say so
Now transpose the first two measurements: 44 / 46 = 0.96, which is still inside the normal range. A swap of two similar lengths is invisible in the verdict and visible only in the number — which is why the proof for this page asserts that the two orderings multiply to exactly 1 at unequal lengths, rather than checking the case where tendon and patella are the same length and every wrong version agrees
Take a genuinely high patella: tendon 58 mm, patella 42 mm gives 58 / 42 = 1.38. That is patella alta on the SICOT-J table (alta above 1.2) and normal on Radiopaedia's MRI thresholds (normal 0.74 to 1.5). Same knee, two published answers
And a low one: tendon 32 mm, patella 48 mm gives 32 / 48 = 0.67, patella infera on both sets — but check the shape of the patella before accepting it, because a long non-articular inferior pole produces this reading in a normally positioned patella
Four published indices and their cut-offs
| Index | What is divided by what | Infera | Normal | Alta |
|---|---|---|---|---|
| Insall-Salvati | patellar tendon length / greatest pole-to-pole patellar length | below 0.8 | 0.8 to 1.2 | above 1.2 |
| Modified Insall-Salvati | patellar tendon length / patellar articular surface length | below 1.2 | 1.2 to 2.0 | above 2.0 |
| Caton-Deschamps | measured from the tibial plateau — the distances are not stated in text in any source read for this page, so this index is named and not implemented | below 0.6 | 0.6 to 1.2 | above 1.2 |
| Blackburne-Peel | also measured from the tibial articular surface; the SICOT-J review notes it is “necessary to determine the tibial articular surface to draw a joint line”, which is hard on a non-lateral radiograph. Named and not implemented | below 0.6 | 0.6 to 1.0 | above 1.0 |
| Insall-Salvati on MRI, per Radiopaedia | the same two distances, measured on sagittal MRI | below 0.74 | 0.74 to 1.5 | above 1.5 |
Which index is most reliable — the sources disagree
| Finding | Source | What it says |
|---|---|---|
| Insall-Salvati best | van Duivenbode et al, Int Orthop 2016, four independent examiners on weight-bearing lateral radiographs in 30° flexion | “The inter-observer reliability was excellent for the IS and moderate for the other ratios”; categorised, strong for IS, moderate for MIS and Blackburne-Peel, poor for Caton-Deschamps. The numeric intraclass correlations are paywalled and were not read, so none is quoted |
| Insall-Salvati best | A study cited in the SICOT-J review | Its title is “Patellar height measurements: Insall-Salvati ratio is most reliable method” |
| Blackburne-Peel best | Seil et al and Berg et al, as cited in the same SICOT-J review | Both “reported that the BP ratio was most reliable” |
| Use the modified ratio anyway | van Duivenbode et al’s own recommendation | The authors advise the MIS, which “showed the second best reliability” and, per the literature, has better validity — so the most reliable index and the recommended index are not the same one |
Two lengths, one division, and three sets of thresholds
The Insall-Salvati ratio is the patellar tendon length divided by the patellar length: the posterior surface of the tendon from the lower pole of the patella to its tibial insertion, over the greatest pole-to-pole length of the patella itself. It is measured on a lateral radiograph with the knee flexed about 30 degrees, and it answers one question — does the patella sit high, low or normally on the femur?
The arithmetic is trivial and the thresholds are not. A 2022 SICOT-J instructional review tabulates the radiographic cut-offs as patella infera below 0.8, normal 0.8 to 1.2 and patella alta above 1.2. Radiopaedia gives MRI thresholds of below 0.74, 0.74 to 1.5 and above 1.5, attributing the difference to intermodality differences in measurement technique, and its separate article on patella alta gives the threshold as a range, greater than 1.3 to 1.5. So a knee measuring 1.3 is patella alta on one published set of numbers and entirely normal on another. This page prints both and labels its bands with the radiographic set, because a ratio quoted without the modality it was measured on cannot be interpreted.
The second number under the headline is the modified Insall-Salvati ratio, which divides the same tendon length by the patellar articular surface instead of the whole bone. It exists because the Insall-Salvati denominator is shape dependent: a patella with a long non-articular inferior pole — the Cyrano patella — has a large pole-to-pole length and so a falsely low ratio, and the SICOT-J review records that patellar shape may produce a higher apparent incidence of patella infera with the unmodified index. The articular surface is shorter than the whole bone, so the modified ratio is always the larger of the two, and its normal range is 1.2 to 2.0. Reading one ratio against the other’s thresholds is a real and easy mistake.
Two indices are named on this page and deliberately not built. The Caton-Deschamps and Blackburne-Peel indices both measure from the tibial articular surface, and no source read for this page states their numerator and denominator distances in text — only in diagrams. An index inferred from a picture is exactly the plausible, unverifiable error this project refuses to ship, so their cut-offs are tabulated and the measurement is left to a source that states it. Which index is most reliable is itself disputed: one study and one review title favour Insall-Salvati, two other cited studies favour Blackburne-Peel, and the authors of the first recommend the modified Insall-Salvati even though it came second in their own reliability ranking.
This page renders no clinical or surgical decision; it computes the published quantity and states what the literature reports for it, with every threshold and performance figure given with the cohort it came from. A measurement is not a diagnosis, and a figure from a published series is a property of that series rather than a fact about one patient. Agreement between observers is part of what a classification means. A category two experienced readers assign differently on the same film is not a measurement of the fracture, and the published kappa values for these systems run from almost nothing to almost perfect. Each page here prints them with the number of observers and the number of radiographs they came from.
Frequently asked questions
How is the Insall-Salvati ratio measured?
On a lateral radiograph with the knee flexed about 30 degrees, divide the patellar tendon length — the posterior surface of the tendon from the lower pole of the patella to its insertion on the tibia — by the greatest pole-to-pole length of the patella. Both measurements are in the same units, so the ratio is dimensionless.
What is a normal Insall-Salvati ratio?
It depends which published set you use, and they disagree. A 2022 SICOT-J review’s radiographic table gives normal as 0.8 to 1.2, with patella infera below 0.8 and patella alta above 1.2. Radiopaedia’s MRI thresholds are 0.74 to 1.5, with alta above 1.5, and its patella alta article gives the threshold as greater than 1.3 to 1.5. A ratio of 1.3 is abnormal on the first and normal on the second.
Why is there a modified Insall-Salvati ratio?
Because the ordinary ratio divides by the whole patella, which makes it sensitive to the shape of the bone. A patella with a long non-articular inferior pole has a large pole-to-pole length and therefore a falsely low ratio; the SICOT-J review notes that patellar shape may produce a higher apparent incidence of patella infera with the unmodified index. The modified ratio divides by the articular surface only, and its normal range is 1.2 to 2.0.
Can I compare an Insall-Salvati ratio with a modified one?
No. They have different denominators and different normal ranges — 0.8 to 1.2 against 1.2 to 2.0 — and the modified ratio is always the larger of the two because the articular surface is shorter than the whole bone. A modified ratio of 1.3 is normal; an Insall-Salvati ratio of 1.3 is patella alta on the radiographic thresholds.
Why are the Caton-Deschamps and Blackburne-Peel indices not calculated here?
Because no source read for this page states their measured distances in text. Both measure from the tibial articular surface and both are shown in diagrams rather than described, and an index reconstructed from a picture would be a plausible guess rather than a reproduction. Their published cut-offs are tabulated on this page — Caton-Deschamps 0.6 to 1.2, Blackburne-Peel 0.6 to 1.0 — so a reader who has the measurement can use them.
Does a high ratio mean the knee needs surgery?
No, and this page makes no such statement. The ratio describes where the patella sits relative to the femur, which is one radiographic finding among many. The source material also warns that an off-angle beam or non-standard positioning can produce a spuriously abnormal ratio, and that reliability depends on identifying the tibial tuberosity — which is harder after arthroplasty or a tubercle procedure.
Related calculators
References
- Insall J, Salvati E. Patella position in the normal knee joint. Radiology. 1971;101:101–104. Citation read in the reference list of the SICOT-J instructional review below; the paper itself was not reachable. The ratio’s definition is therefore quoted from Radiopaedia, and the cut-offs from the SICOT-J table.
- Blackburne JS, Peel TE. A new method of measuring patellar height. J Bone Joint Surg Br. 1977;59:241–242. Caton J, Deschamps G, Chambat P, Lerat JL, Dejour H. Patella infera. Apropos of 128 cases. Rev Chir Orthop Reparatrice Appar Mot. 1982;68:317–325. Grelsamer RP, Meadows S. The modified Insall-Salvati ratio for assessment of patellar height. Clin Orthop Relat Res. 1992;282:170–176. All three citations read in the same SICOT-J reference list; none of the three papers was reachable, so this page implements only the two indices whose measured distances it could source, and names the other two.
- Evaluation of patella height in native knees and arthroplasty: an instructional review. SICOT-J. 2022;8:36. Table 2, “Common radiographic indices used for classification of patella height”, gives patella infera / norma / alta cut-offs as: Insall-Salvati “<0.8” / “0.8–1.2” / “>1.2”; modified Insall-Salvati “<1.2” / “1.2–2.0” / “>2.0”; Caton-Deschamps “<0.6” / “0.6–1.2” / “>1.2”; Blackburne-Peel “<0.6” / “0.6–1.0” / “>1.0”. The review records that “The IS and MIS indices depend either on the length of the patella tendon” and on identifying the tibial tubercle insertion, that “the patella shape might result in a higher incidence of patella infera when utilizing the IS index”, “particularly in Cyrano patella”, and that the IS intraclass correlation was lower after operation “possibly due to difficulty identifying the tibial tuberosity”. It also records the reliability literature disagreeing with itself: one cited study is titled “Patellar height measurements: Insall-Salvati ratio is most reliable method” while “Seil et al. and Berg et al. reported that the BP ratio was most reliable”.
- van Duivenbode DC, Stavenuiter MHJ, Burger BJ, van Dijke CF, Spermon J, Hoozemans MJM. The reliability of four widely used patellar height ratios. Int Orthop. 2016;40(3):493–497. Four independent examiners, weight-bearing lateral knee radiographs in 30° of flexion. “The inter-observer reliability was excellent for the IS and moderate for the other ratios”; for categorised values it was strong for IS, moderate for MIS and BP, and poor for CD. “The intra-observer reliability was excellent for the IS, MIS and CD, and strong for the BP.” The authors nevertheless advise the MIS, which “showed the second best reliability” and has better reported validity. The numeric intraclass correlations are behind a paywall and were not read, so none is quoted here and the abstract’s own words are used instead.
- Radiopaedia. Insall-Salvati ratio. The two distances verbatim: the numerator is “patellar tendon length (TL)”, “the length of the posterior surface of the tendon from the lower pole of the patella to its insertion on the tibia”; the denominator is “patellar length (PL)”, the “greatest pole-to-pole length”. “Insall-Salvati ratio = A/B (or TL/PL).” The ratio “was initially determined on a 30° flexed lateral knee x-ray and was later applied to sagittal MRI”, it “is affected by the presence of tibial tuberosity abnormalities”, and on MRI the page gives patella baja “<0.74”, normal “0.74-1.5” and patella alta “>1.5” — a different set of thresholds from the SICOT-J table above, which is why both are printed here.
- Radiopaedia. Patella alta. Gives the alta threshold as a range rather than a number: “the Insall-Salvati ratio is greater than 1.3-1.5”. Also that “An off-angle x-ray beam or nonstandard positioning may result in a spuriously abnormal ratio” and that the standard position is the knee “flexed at 30 degrees”.
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