Joints
Patellofemoral joint (art. patellofemoralis)
The back of the patella against the patellar surface of the femur. Not an independent joint but a pulley: the patella lengthens the moment arm of the quadriceps by roughly a third, so the same muscle force produces more extension moment.
- Joint type
- Gliding joint inside the knee capsule, one degree of freedom with guided translation
- Body region
- Knee
- Latin name
- Articulatio patellofemoralis
Movements, axes and normal values
| Movement | Plane | Axis | Normal range | End feel |
|---|---|---|---|---|
| Patellar glide | sagittal | transverse (mediolateral) | 5-7 cm of distal travel from full extension to full flexion | springy |
| Translation | frontal | sagittal (anteroposterior) | mediolateral displacement of about a quarter of the patellar width with the knee extended | firm springy (ligament) |
- Close-packed position
- Around 90 degrees of knee flexion, where the contact area is largest
- Resting (loose-packed) position
- Full extension, with the patella riding above the trochlea
- Capsular pattern
- No pattern of its own: it shares the capsule with the knee joint and follows that pattern.
- Arthrokinematics
- The patella glides distally in flexion and proximally in extension, tilting about a transverse axis as it goes. The contact area migrates: at 20 degrees only the distal pole makes contact, at 90 degrees the proximal part does. Contact pressure therefore rises to several times body weight on stairs.
- Stability
- Passively the depth of the trochlea and the lateral retinaculum, actively the vastus medialis obliquus holding the patella medially. The Q-angle determines how much lateralising force the quadriceps exerts.
- Palpation
- With the quadriceps relaxed the patella can be shifted sideways; the facets are reachable under the medial edge. Crepitus during resisted extension is felt with a flat hand on the patella.
- Clinical
- Patellofemoral pain syndrome is the most common knee complaint in runners: pain around or behind the patella on stairs, squatting and prolonged sitting. Lateral dislocation occurs with a shallow trochlea or a large Q-angle, usually in flexion with valgus and external rotation of the lower leg.
- Source
- Speciële anatomie van het houdings- en bewegingsapparaat (HU, 2013); Reader kinesiologie (HU, 2016); Kapandji, Physiology of the Joints vol. 2