This can be measured on a lateral knee x-ray or sagittal MRI. Ideally the knee is 30 degrees flexed.
Distance lines used to calculate Insall-Salvati ratio (see Figure 1):
- 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)
The Insall-Salvati ratio (TL/PL) is considered normal between 0.8 and 1.2:
More recent literature 2 suggests that the true normal range is more forgiving: from as low as to 0.74 to as high as 1.50.
Modified Insall-Salvati ratio
Also applied on a lateral 30 degree flexed knee radiograph, the modified Insall-Salvati ratio is measured slightly differently:
- A: distance from the inferior margin of the patellar articular surface (as opposed to the lower pole of the patella itself) to the patellar tendon insertion
- B: length of the patellar articular surface (see diagram).
Modified Insall-Salvati ratio = A/B
The modified Insall-Salvati ratio is considered normal around 1.25, abnormal when >2 which is considered diagnostic of patella alta.
- 1. Grelsamer RP, Meadows S. The modified Insall-Salvati ratio for assessment of patellar height. Clin. Orthop. Relat. Res. 1992; (282): 170-6. Pubmed citation
- 2. Shabshin N, Schweitzer ME, Morrison WB et-al. MRI criteria for patella alta and baja. Skeletal Radiol. 2004;33 (8): 445-50. doi:10.1007/s00256-004-0794-6 - Pubmed citation
- 3. Lin CF, Wu JJ, Chen TS et-al. Comparison of the Insall-Salvati ratio of the patella in patients with and without an ACL tear. Knee Surg Sports Traumatol Arthrosc. 2005;13 (1): 8-11. doi:10.1007/s00167-004-0515-7 - Pubmed citation
- 4. Kadakia NR, Ilahi OA. Interobserver variability of the Insall-Salvati ratio. Orthopedics. 2003;26 (3): 321-3. Pubmed citation