Normal Patellofemoral Index Size on Radiography
The patellofemoral index is a quantitative radiographic ratio that compares the medial and lateral patellofemoral joint space widths on the axial (sunrise) view of the knee. It provides an objective measure of patellar tilt and lateral compartment compression. Accurate measurement is essential in evaluating patients with anterior knee pain, patellar instability, and suspected lateral patellar compression syndrome.
Normal Reference Values
| Orientation | Measurement |
|---|---|
| Sunrise | <1.6 |
Clinical Significance
A patellofemoral index of less than 1.6 is considered normal on the sunrise radiograph. An elevated index — where the lateral patellofemoral joint space is disproportionately narrowed relative to the medial side — indicates abnormal lateral patellar tilt or compression. Values at or exceeding 1.6 suggest lateral patellar compression syndrome (LPCS), which may manifest clinically as chronic anterior knee pain, peripatellar tenderness, and crepitus.
It is important to recognize that the measurement is technique-dependent; knee flexion angle during the sunrise view (typically 30–45°) and beam angulation must be standardized. Quadriceps contraction state at time of imaging can also influence patellar positioning and alter the index.
- Lateral patellar compression syndrome (LPCS) — most common cause of elevated index
- Patellofemoral osteoarthritis — asymmetric joint space narrowing may affect ratio
- Patellar tilt without subluxation — isolated tilt detectable by index before frank subluxation
- Post-lateral retinacular release — index used to assess surgical correction
- Trochlear dysplasia — abnormal trochlear morphology alters patellofemoral congruence
Reference: Keats TE, Sistrom C. Atlas of Radiologic Measurement. Mosby Inc. (2001).
Imaging Notes
The patellofemoral index is measured on the axial (sunrise) radiograph. The ratio is calculated by dividing the shortest distance across the medial patellofemoral joint space by the shortest distance across the lateral patellofemoral joint space. Consistent knee positioning is critical; the sunrise view is typically obtained at approximately 30–45° of knee flexion with the beam angled cephalad. Excessive flexion reduces sensitivity for detecting lateral tilt, as deep flexion passively stabilizes the patella within the trochlear groove.
When interpreting the index, ensure the image demonstrates symmetric visualization of both femoral condyles, confirming true axial orientation. Suboptimal rotation will artifactually alter the apparent joint space widths and produce an unreliable index value.