Normal Insall-Salvati Ratio Size on Radiography
The Insall-Salvati ratio is a standardized measurement used to assess patellar height relative to the patellar tendon length on lateral knee radiographs. It is calculated by dividing the greatest diagonal length of the patella by the length of the patellar tendon, measured from the inferior patellar pole to its tibial tuberosity insertion. Accurate assessment of patellar height is essential in evaluating anterior knee pain, patellar instability, and surgical planning for patellofemoral disorders.
Normal Reference Values
| Orientation | Measurement |
|---|---|
| Lateral | 0.8-1.2 |
Clinical Significance
A normal Insall-Salvati ratio falls between 0.8 and 1.2. A ratio greater than 1.2 indicates patella alta (high-riding patella), which is associated with increased risk of patellar instability, subluxation, and dislocation due to delayed engagement of the patella within the trochlear groove during knee flexion. Conversely, a ratio below 0.8 indicates patella baja (low-riding patella), which may result in patellofemoral pain, reduced quadriceps efficiency, and is commonly seen following total knee arthroplasty or tibial tubercle osteotomy.
Clinicians should be aware that the Insall-Salvati ratio is best applied in skeletally mature patients with an intact patellar tendon. Patellar tendon rupture, prior surgery, or significant joint effusion can all affect measurement accuracy. Alternate indices such as the modified Insall-Salvati or Caton-Deschamps ratio may be preferred in specific clinical scenarios.
- Patella alta — patellar instability and recurrent dislocation
- Patella baja — post-surgical complication or chronic patellar tendinopathy
- Osgood-Schlatter disease — may affect tendon length measurement in adolescents
- Patellar tendon rupture — falsely elevates the ratio
- Prior tibial tubercle advancement procedures — alters baseline ratio
Reference: Biedert RM. Patellofemoral Disorders, Diagnosis and Treatment. Wiley. (2005).
Imaging Notes
The Insall-Salvati ratio is measured on a true lateral radiograph of the knee obtained at approximately 30° of flexion, which is the position that best defines the patellar tendon under physiologic tension. The patellar tendon length is measured along its posterior surface from the inferior patellar pole to the posterior superior aspect of the tibial tuberosity, while the patellar length is measured as its longest diagonal axis. Strict lateral positioning is critical — even minor rotation can foreshorten or elongate patellar dimensions, introducing significant measurement error.