At what angle is the articular surface of the patella (kneecap) in contact with the femur (thigh bone)?

Patellofemoral Contact Angle and Anatomy

The articular surface of the patella contacts the femur at varying angles throughout knee flexion, with contact beginning at approximately 10-20° of flexion and progressing proximally on the patella as flexion increases, reaching the odd facet only at deep flexion angles beyond 90-135°.

Contact Pattern Throughout Knee Flexion Range

Early to Mid-Flexion (0-90°)

• From extension to 90° of flexion, a band of contact sweeps across the patella from the inferior to superior pole, systematically engaging different portions of the patellar articular surface 1
• At approximately 90° of flexion, the contact area is located in the proximal half of the articular surface and is continuous over the medial and lateral facets 2
• The patella holds the quadriceps tendon away from the femur during this range, serving as a mechanical lever 1

Deep Flexion (90-135°+)

• Between 90° and 135° of flexion, the patella undergoes significant rotation, and the ridge between the medial and odd facets engages the femoral condyle 1
• At approximately 135° of flexion, separate medial and lateral contact areas form, with the medial contact area limited to the odd facet 1
• In maximum active flexion (mean 140°), the odd facet engages in approximately 75% of knees 2
• At maximum passive flexion (mean 156°), the contact area on the lateral facet moves distally and decreases significantly, while the odd facet engages in 90% of knees 2

Dynamic Contact Area Changes

Quantitative Contact Measurements

• Peak mean patellofemoral contact area reaches 228.7 ± 173.6 mm² at 40° of knee flexion during active extension 3
• The total contact area remains relatively constant from 90° to maximum active flexion (3.43 ± 0.70 cm² vs 3.62 ± 0.72 cm²), but decreases significantly at maximum passive flexion (2.96 ± 0.78 cm²) 2
• Contact area is significantly different at 25°, 30°, 35°, and 40° of knee extension, demonstrating the dynamic nature of patellofemoral articulation 3

Contact Centroid Migration

• During extension, the contact centroid and peak strain locations track medially on both femoral and patellar cartilage 3
• The contact area divides into two parts (odd and lateral facets) and moves distally in deep knee flexion 2

Clinical Significance of Contact Patterns

The Odd Facet as a Special Zone

• The odd facet represents a habitual non-contact area during normal functional range of motion (0-90°), only engaging during deep flexion beyond 90-135° 1
• The ridge between the medial and odd facets is subject to high loads during deep flexion, which correlates with specific cartilage lesion patterns 1

Implications for Pathology

• Lateral patella tilt is associated with reduced contact area (21.6% to 29.1% reduction in ACL-reconstructed patients) and increased risk for early-onset osteoarthritis 4
• Lateral patellar tilt is negatively correlated with contact area across all knee angles (r-values ranging from -0.32 to -0.66), suggesting that malalignment reduces the load-bearing surface 4

Surgical Considerations

Component Positioning in Total Knee Arthroplasty

• Central positioning of the patellar component results in optimal patellofemoral mechanics with the most evenly balanced patellar facet contact pressures 5
• Medial positioning of the patellar component increases patellar tilt and elevates contact pressures at the medial facet at higher flexion angles 5
• Lateral positioning increases contact pressures at the lateral facet during flexion 5

Assessment of Patellar Tracking

• Axial (tangential) radiographs at weight-bearing demonstrate the degree of patellar tilt or subluxation and are the primary radiographic method for assessing patellar tracking 6
• Weight-bearing axial radiographs provide superior assessment of patellofemoral kinematics compared to non-weight-bearing views 6

Common Pitfalls

• Assuming the patella maintains constant contact throughout the entire articular surface during all flexion angles is incorrect—contact is highly dynamic and angle-dependent 1, 2
• Evaluating patellar alignment without weight-bearing conditions underestimates the true functional tracking abnormalities 6
• Failing to recognize that the odd facet only engages in deep flexion may lead to misinterpretation of cartilage lesions in this region 1