Patella height ratios diagnose the same healthy knees differently

Area of Science:

• Orthopedics
• Radiology
• Biostatistics

Background:

• Patella height is crucial for knee biomechanics and diagnosing conditions like patella alta and baja.
• Established ratios, including Insall-Salvati and Caton-Deschamps, are widely used but their applicability to diverse populations is questioned.
• Normal distribution of these ratios in a healthy, large-scale population remains underexplored.

Purpose of the Study:

• To evaluate if Insall-Salvati and Caton-Deschamps ratios conform to normal distribution in a healthy population.
• To assess the diagnostic accuracy of these ratios for patella alta and baja against population centiles.
• To determine the inter-rater reliability of both patella height measurement methods.

Main Methods:

• Analysis of 434 healthy, skeletally mature knee CT scans using the SOMA system.
• Measurement of patella height ratios using Insall-Salvati and Caton-Deschamps indices.
• Statistical analysis including quantile plotting, centile identification, and kappa statistics for inter-rater reliability.

Main Results:

• The Insall-Salvati ratio's 98th centile aligned with its definition of patella alta (≥1.2), but its 5th centile (0.59) indicated overdiagnosis of patella baja (≤0.74).
• The Caton-Deschamps index's 95th centile (1.3) exceeded its patella alta threshold (≥1.2), and its 5th centile (0.75) suggested underdiagnosis of patella baja (≤0.6).
• Both ratios exhibited poor inter-rater reliability (kappa coefficient = 0.01).

Conclusions:

• Current patella height ratios (Insall-Salvati and Caton-Deschamps) show significant discrepancies when applied to a large, healthy population, leading to diagnostic inaccuracies.
• The diagnostic thresholds for patella alta and baja based on these ratios may not be universally applicable due to their origin from small, select samples.
• Future research should establish normative values for patella height ratios using large population datasets to refine diagnostic criteria.