Native kinematics · 0–120°
How the Native Knee Flexes
Medial side stays. Lateral side rolls back.
From 0° to 120°, this medial-pivot teaching model keeps the medial femoral condyle nearly stationary on the tibia. The lateral condyle translates posteriorly about 20 mm, coupled with about 20° of tibial internal rotation around a medial axis.
Iwaki, Pinskerova and Freeman mapped this pattern in unloaded cadaver MRI. Living-knee MRI studies by Hill and Johal found broadly similar asymmetric motion under unloaded and loaded conditions; loading can bring rotation forward in the flexion arc. Fluoroscopy studies, including Komistek, also show that individual knees and activities vary.
ANATOMIC RESURFACING HOW THE NATIVE KNEE FLEXES
Native knee: medial stability, lateral rollback.
Preparing the 3D knee…
Medial plateau · concaveLateral condyle
Lateral plateau · flat / slightly convex
Fibula marks the lateral side
Play the lecture video
Explore the motion
Three ways to see the same knee.
A contact region is not a rotation center.
The circle marks the posterior flexion-facet center. The pink marker follows the nearest opposing surfaces in this model as it flexes. Watch the surface marker separately from the center.
One knee. More than one motion.
Loading, rotational position and the depth of the bend change the movement. These study diagrams separate those effects from the illustrative 3D knee.
Unloaded bending
Weight-bearing squat
Start together, then watch the lateral centres separate.
Pinskerova et al. (2004), Table I · The same five volunteers in both conditions. Dots show each flexion-facet centre’s travel relative to extension, with starting positions aligned for comparison. The outlines and connecting lines are schematic, not measured femoral orientation. Movement between measured positions is interpolated.
Read the evidence and its limits
Hill et al. (2000) examined 13 unloaded living knees and seven loaded knees. The loaded group showed about 4 mm of medial forward movement. In four knees tested with altered rotational positioning, tibial external rotation suppressed the usual accompanying internal rotation during squatting to 90°. This is a separate cohort from the five-volunteer comparison above; the datasets are not interchangeable.
Iwaki et al. (2000) studied six unloaded cadaver knees. Their pattern included early axial rotation, comparatively little additional rotation to approximately 45°, and greater rotation later. The 3D patient animation uses an illustrative phased trajectory based on this description, not digitised individual-knee data.
Contact location is not condylar translation. Pinskerova et al. measured contact as the closest approach of the subchondral plates on MRI. This is not a pressure map or the full cartilage–meniscus load-bearing area. These related studies and the later review do not establish one universal trajectory, ligament-force behaviour, or clinical outcomes after resurfacing.
From the published figures to motion
Pinskerova & Vavrik’s 2020 chapter revisits the MRI and anatomical work cited above. Compare its original diagrams with the animation.
The center of the posterior circular facet sits inside the femoral condyle. The articular surface stays above the tibial surface.
The medial ends cluster together while the lateral ends sweep posteriorly. Our animated replay is limited to 0–120°.
Sources, measurements, and what this animation represents
The live values are prescribed animation targets, not measurements from this specimen. Medial AP is the flexion-facet-center translation, not the migrating contact point. The animation uses 1 mm medial and 20 mm lateral posterior translation with 19.9° coupled rotation at 120°. The sagittal overlays identify the flexion-facet centers; they do not represent contact patches. The bone positions include a geometric surface-clearance correction, not a force-based cartilage simulation.
The medial-pivot pattern is a teaching reference, not an invariant rule. Hill reported approximately 4 mm medial forward movement in loaded knees. Komistek studied five knees and reported individual variation, including a lateral-pivot subject. These studies do not establish that every native knee has no medial rollback or that loading affects timing alone.
- Iwaki, Pinskerova & Freeman (2000) — unloaded cadaver MRI; medial facet transition around 20°, approximately 20° rotation by 110°.
- Hill et al. (2000) — loaded and unloaded living-knee MRI; similar overall pattern with load-related differences.
- Johal et al. (2005) — weight-bearing and non-weight-bearing interventional MRI.
- Freeman & Pinskerova (2005) — 0–120° medial stability, early contact transfer and lateral rollback; deeper flexion is outside this film.
- Komistek, Dennis & Mahfouz (2003) — in-vivo weight-bearing fluoroscopy; subject and activity dependence.
Bone geometry: Open Knee(s), specimen oks009, mirrored to a right-knee teaching reference. Articular surfaces and ghosted menisci are illustrative. Geometry, animation license and adaptation details. This is not a patient-specific or validated joint-contact simulation.