Roll and Glide Mechanisms

For Medical Students

The Principle

Two curved surfaces in contact can move against each other in three ways. One can roll on the other, it can glide across it, or it can spin on the spot.

Rolling moves the contact point on both surfaces. Gliding moves the contact point on one surface and not the other.

A wheel on a road rolls. A tyre on ice glides. The knee does both at once, and the proportion changes through the range.

At the Knee

The femoral condyle is far larger in circumference than the tibial surface it sits on. Conventional teaching gives the femoral articular arc as roughly twice the length of the tibial one.

Pure rolling through flexion would therefore carry the femur off the back of the tibia before the knee reached full flexion.

Pure gliding would drive the same point of the femur against the same point of the tibia throughout, concentrating load and wear in one spot.

Combining them keeps contact within the plateau and spreads it across the surface, which is the mechanical reason the knee is built this way.

What Changes It

Flexion angle. Conventional teaching gives more rolling in early flexion and more gliding as flexion deepens.

Which side of the knee. The lateral condyle rolls further than the medial, which is the source of the rotation coupled to flexion. 3.2.9 covers coupling.

The menisci, which move with the condyles and keep a congruent contact as the condyle travels.

Whether the limb is bearing weight, because compression changes how readily a surface glides.

Clinical Relevance

The knee has no single hinge point, which is why a brace with one fixed axis cannot match the joint through a full arc.

Load is spread over a large area of each surface rather than concentrated, which is part of why cartilage survives decades of use.

A knee that loses its posterior meniscus loses part of the guidance for this motion, and the contact pattern changes. 3.6 covers meniscal biomechanics.

Restoring a plausible roll and glide relationship is one of the design problems in knee replacement.

MRCS Level

The Principle

Describe the relationship as a ratio rather than as a sequence. At any flexion angle the motion is some rolling and some gliding, and the ratio shifts rather than switching.

The quantity usually reported is contact point translation, because it is what rolling and gliding differ in and what imaging can follow.

Posterior femoral rollback is the summary term for the net posterior migration of the contact point with flexion, and it is the quantity arthroplasty design tries to reproduce.

At the Knee

Conventional teaching describes rolling as dominant through roughly the first 20 to 30 degrees of flexion, with gliding taking an increasing share beyond that.

The two compartments differ. The medial contact point moves relatively little through mid flexion while the lateral translates considerably, which places the axis of rotation medially.

In extension the motion reverses: the condyles glide posteriorly and roll anteriorly as the knee straightens, which is the same relationship read in the other direction.

The cruciates are part of the mechanism rather than bystanders. Their geometry constrains how far the condyle can translate at each angle, which 3.7 and 3.8 cover.

What Changes It

Cruciate integrity, since a deficient anterior cruciate changes the contact path and shifts it posteriorly in the loaded knee.

Posterior tibial slope, which alters the resting position the rollback starts from.

Meniscal integrity, particularly the posterior horns, which guide the condyle at the flexion angles where gliding dominates.

Open against closed chain, which changes which surface is fixed and therefore how the same relationship is described.

How It Is Measured

Fluoroscopic contact point tracking through a loaded task is the usual in vivo method, and it measures the nearest point rather than the true contact area.

Magnetic resonance imaging in incremental flexion gives position without load, which is a different condition rather than a static version of the same one.

Cadaveric rigs with pressure sensitive film measure area and position together, in a specimen with no muscle.

Instrumented implants measure in vivo loading directly in a replaced knee, which is a small and selected population.

Clinical Relevance

Expect a cruciate deficient knee to sit and to travel differently rather than only to translate further on testing.

Preserve the posterior horns where possible, on the mechanical ground that they guide the contact path.

Read a rollback figure with its loading condition attached, since unloaded and loaded measurements differ.

Account for tibial slope when a knee's resting contact position looks abnormal.

FRCS Level

At the Knee

Treat rollback as a design requirement rather than a description. A cruciate retaining replacement relies on the posterior cruciate to produce it, a posterior stabilised design produces it with a cam and post, and each choice trades a different thing.

Paradoxical anterior sliding is the recognised failure of that aim: the contact point moves anteriorly with flexion instead of posteriorly, and the knee loses the lever arm that deep flexion needs.

The same quantity explains why a ligament reconstruction can restore a measured limit and leave the contact path shifted, with the cartilage consequences that follow. 3.5 covers contact mechanics.

What Changes It

Implant geometry, particularly the sagittal radius and the insert conformity, which together decide how much the path is imposed.

Posterior cruciate tension in a retaining design, which is a surgical variable rather than an implant one.

Soft tissue balance and component rotation, both of which change which compartment translates further.

Tibial slope, native or created, which shifts the whole path and is modifiable by osteotomy. 3.13 covers alignment.

How It Is Measured

Fluoroscopic kinematic studies of replaced knees are where paradoxical motion was described, and they measure a selected group performing a prescribed task.

Cadaveric simulators compare designs under a repeatable load profile, without the variability that makes the clinical question interesting.

Instrumented implant telemetry gives load rather than position, so the two literatures answer adjacent questions.

No routine clinical measurement of contact path exists, which is why design arguments rest on laboratory work and outcome data rather than on per patient measurement.

Clinical Relevance

Tension the posterior cruciate deliberately in a retaining design, since rollback depends on it.

Consider paradoxical motion when a well aligned replacement gives poor deep flexion without pain.

Address slope where it is shifting the contact path rather than treating it as an incidental measurement.

Read a design comparison by the task its kinematics were measured in.