Insall-Salvati ratio
The Insall-Salvati ratio or index is the ratio of the patella tendon length to the length of the patella and is used to determine patellar height.
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Usage
The Insall-Salvati ratio is probably the most commonly used measurement to assess patellar height. It is affected by the presence of tibial tuberosity abnormalities (e.g. Osgood-Schlatter disease, osteotomies) and a different technique may be required when these are present 4,5.
Measurement
The Insall-Salvati ratio was initially determined on a 30° flexed lateral knee x-ray and was later applied to sagittal MRI.
Distance lines are used to calculate Insall-Salvati ratio:
- A: patellar tendon length (TL): length of the posterior surface of the tendon from the lower pole of the patella to its insertion on the tibia
- B: patellar length (PL): greatest pole-to-pole length
Insall-Salvati ratio = A/B (or TL/PL)
Interpretation
On plain radiographs:
- patella baja: <0.8
- normal: 0.8-1.2
- patella alta: >1.2
On MRI, different thresholds are used due to intermodality differences in measurement technique (e.g. indirect vs direct visualization of the patellar tendon). The following is usually accepted as reasonable 1:
- patella baja: <0.74
- normal: 0.74-1.5
- patella alta: >1.5
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Knee pathology
The knee is a complex synovial joint that can be affected by a range of pathologies:
- bone and cartilage
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knee fractures
- distal femoral condyle fracture
- tibial plateau fracture (classification)
- patella fracture
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avulsion fractures of the knee
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- biceps femoris avulsion fracture
- iliotibial band avulsion fracture
- patella fracture
- posterior cruciate ligament avulsion fracture
- reverse Segond fracture
- Segond fracture
- semimembranosus tendon avulsion fracture
- Stieda fracturechronic avulsion injuries
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- osteochondral defects
- osteochondritis dissecans of the knee
- patterns of bone bruise in knee injury
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knee fractures
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- anterior cruciate ligament mucoid degeneration
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- lateral collateral ligament tear
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- posterolateral corner injury
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- fat pad
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