Normal Modified Insall-Salvati Ratio Size on Radiography
The modified Insall-Salvati ratio is a radiographic index used to assess patellar height relative to the patella tendon insertion, measured on a true lateral view of the knee. Unlike the original Insall-Salvati method, it uses the length of the articular surface of the patella rather than the total patellar length, improving reproducibility. Accurate measurement is clinically important because abnormal patellar height is associated with patellofemoral pain, instability, and degenerative joint disease.
Normal Reference Values
| Orientation | Measurement |
|---|---|
| Lateral | <2 |
Clinical Significance
A normal modified Insall-Salvati ratio is less than 2. The ratio is calculated by dividing the distance from the inferior pole of the patellar articular surface to the tibial tubercle by the length of the patellar articular surface. Values at or exceeding 2 indicate patella alta (high-riding patella), while an abnormally low ratio suggests patella baja (low-riding patella), each carrying distinct clinical implications.
Patella alta is associated with increased lateral patellar instability, recurrent subluxation, and early chondral wear of the lateral trochlea. Patella baja is more commonly seen as a postoperative complication — particularly after tibial tubercle osteotomy or total knee arthroplasty — and can cause anterior knee pain and reduced quadriceps efficiency. A key pitfall is ensuring the lateral radiograph is obtained with the knee in 30° of flexion and the image is a true lateral projection, as rotational errors significantly affect the measurement.
- Patella alta — recurrent patellar instability or subluxation
- Patella baja — post-surgical complication (e.g., tibial tubercle osteotomy)
- Patellofemoral pain syndrome
- Trochlear dysplasia with concomitant high-riding patella
- Quadriceps or patellar tendon pathology altering patellar position
Reference: Biedert RM. Patellofemoral Disorders, Diagnosis and Treatment. Wiley. (2005).
Imaging Notes
The modified Insall-Salvati ratio is measured exclusively on lateral knee radiography. The knee should ideally be positioned in approximately 30° of flexion to allow the patella to engage the trochlear groove. On the lateral view, two measurements are made: the length of the patellar articular cartilage surface (from inferior to superior articular margin) and the distance from the most distal point of the patellar articular surface to the tibial tubercle insertion of the patellar tendon. The ratio of tendon insertion distance to articular patellar length is then calculated.
Careful attention to patient positioning is essential — even mild rotation can distort the apparent patellar position and yield inaccurate ratios. Digital PACS tools with precise caliper measurement are recommended. The modified method offers improved inter-observer reliability over the classic Insall-Salvati ratio, particularly in patients with congenital or postoperative changes to patellar shape.