Normal Caton-Deschamps Index Size on Radiography
The Caton-Deschamps Index (CDI) is a radiographic ratio used to assess patellar height on the lateral knee radiograph, calculated by dividing the distance from the inferior articular surface of the patella to the anterosuperior tibial plateau by the length of the patellar articular surface. Accurate measurement of this index is clinically important because abnormal patellar height is strongly associated with patellofemoral instability, anterior knee pain, and chondral pathology. It remains one of the most widely used and reproducible methods for evaluating patellar position in both pre-operative planning and post-operative assessment.
Normal Reference Values
| Orientation | Measurement |
|---|---|
| Lateral | 0.6-1.2 |
Clinical Significance
A normal Caton-Deschamps Index falls between 0.6 and 1.2. Values exceeding 1.2 indicate patella alta, a high-riding patella associated with increased risk of patellar dislocation, trochlear dysplasia, and anterior knee instability. Patella alta reduces patellofemoral contact area and delays engagement of the patella within the trochlear groove during early knee flexion, predisposing patients to instability and cartilage overload.
Values below 0.6 define patella baja (low-riding patella), which may follow tibial tubercle distalization surgery, patellar tendon contracture, or quadriceps mechanism fibrosis. Patella baja can cause impingement, reduced knee flexion, and anterior pain. A key pitfall is measurement error introduced by tibial plateau osteophytes or malpositioned lateral radiographs; a true lateral view with 30° of knee flexion is essential for reproducibility.
- Patella alta — recurrent patellar dislocation, trochlear dysplasia
- Patella baja — post-surgical (tibial tubercle distalization, total knee arthroplasty)
- Patellar tendon rupture or contracture
- Osgood-Schlatter disease (altered patellar tendon length)
- Skeletally immature patients — growth-related variation in patellar position
Reference: Biedert RM. Patellofemoral Disorders, Diagnosis and Treatment. Wiley. (2005).
Imaging Notes
The Caton-Deschamps Index is measured on a true lateral knee radiograph, ideally obtained with the knee in approximately 30° of flexion and the quadriceps relaxed. Two measurements are required: (1) the distance from the inferior pole of the patellar articular cartilage to the anterosuperior edge of the tibial plateau, and (2) the length of the patellar articular surface. The index is the ratio of these two values. Ensure the posterior femoral condyles are perfectly superimposed to confirm a true lateral projection; rotation introduces significant measurement error. Unlike the Insall-Salvati ratio, the CDI is less affected by variations in patellar tendon length and patellar morphology, making it particularly reliable in patients with bipartite patella or prior patellar surgery.