Biomechanical Relevance

For Medical Students

Structure and Location

Three features of distal femoral shape account for most of what the knee does.

The condyles are not circular in profile. Their radius of curvature is larger anteriorly and smaller posteriorly, so the surface in contact near extension is flatter than the surface in contact in deep flexion.

The two condyles are different. The medial is longer front to back and more curved; the lateral is flatter and projects further forward.

The articular surface sits in valgus relative to the shaft, because the shaft runs obliquely from the hip while the joint surface stays close to horizontal.

Relations

The femoral surface works against the tibial plateau, which is concave medially and convex laterally, and against the menisci, which deepen both.

It works against the patella through the trochlea, with the patella held away from the joint by its own thickness.

The ligaments arising from the bone constrain the surfaces to stay in contact, and their attachment positions relative to the flexion axis determine when each is tight.

Function

A larger anterior radius means the contact area is broader near extension, where the knee spends most of its loaded time in walking.

A smaller posterior radius means the contact area narrows in deep flexion, where load is higher but the duration is shorter.

Condylar asymmetry means the two sides do not finish their arc together, so extension ends with rotation rather than stopping squarely. The lateral side travels further and the medial side pivots.

The trochlea's lateral facet standing further forward resists lateral patellar translation in early flexion.

Clinical Relevance

Any implant that replaces this surface has to choose which of its features to reproduce. A single radius design accepts a different relationship between flexion angle and ligament tension than the native knee has.

The valgus of the articular surface relative to the shaft is why a distal femoral cut is made at an angle rather than perpendicular to the bone, and why that angle is not the same in every patient.

MRCS Level

Structure and Location

The features to name are the sagittal profile, the coronal profile, the condylar asymmetry, the trochlear obliquity and the relationship of the articular surface to the shaft.

In the sagittal plane each condyle is better described as a series of arcs than as a circle. In the coronal plane each is convex, against a tibial surface that is concave medially and convex laterally.

The flexion axis of the knee is not a fixed line through the bone. Because the profile is not circular, the instantaneous centre of rotation moves through the range, and the transepicondylar axis is an approximation to it rather than the thing itself.

Relations

The menisci convert a convex femoral surface meeting a flat or convex tibial one into a congruent pair, and they do more of that work laterally where the tibia is convex.

The cruciates attach inside the notch, away from the flexion axis, so their length changes as the surfaces move. The collaterals attach near the epicondyles, close to that axis, and change little.

The extensor mechanism crosses the trochlea, and the patella's thickness sets the distance of the tendon from the joint and therefore the muscle's mechanical advantage.

Blood Supply and Innervation

Load and perfusion are related at the bone surface. Subchondral bone density under the lateral trochlear facet and under the load bearing part of the medial condyle is higher, and that density reflects habitual loading rather than being a fixed property of the bone.

The epiphyseal supply arrives through the capsular and retinacular attachments, so the regions that carry most load are supplied through soft tissue that surgery often disturbs.

Mechanoreceptors in the ligament attachments feed position sense, and their supply follows the articular nerves. 2.7.10 owns proprioception.

Function

Through flexion the femur rolls back and glides, in a proportion that changes with angle. The lateral compartment translates more than the medial, which is the orientation level account of rotation accompanying flexion.

Near extension the larger anterior radius gives a broad contact area and the collaterals are tight, so the joint is stable with little muscular effort. In deep flexion the smaller posterior radius gives a narrow contact area and stability depends more on the cruciates and the muscles.

The patella engages the deeper part of the trochlea in early flexion and gains bony restraint it does not have in extension.

Quantitative treatment of all of this, including contact pressures, moment arms and the kinematic sequence, belongs to 3.13 and to Section 3 generally.

Clinical Relevance

Preserving the bony landmarks preserves the references that alignment decisions use, which is the practical reason landmarks are protected during exposure.

The relationship between femoral shape and ligament tension is why a knee can be stable in extension and unstable in mid flexion after surgery. The two are governed by different parts of the same surface.

Deep flexion loads a small area at high pressure, which is why activities requiring deep flexion are discussed separately in rehabilitation and in implant selection.

Patellar thickness is part of the extensor mechanism's mechanical advantage, so altering it alters the mechanism rather than only the surface.

FRCS Level

Relations

The relation that drives most reconstructive argument is between the femoral surface and the ligament attachments, because surgery alters one and relies on the other.

Resecting the distal femur moves the joint line. Moving the joint line changes the distance from the surface to the collateral attachments, and therefore changes ligament tension at every angle. The attachments did not move; the surface did.

Function

Femoral geometry is the reason flexion and extension gaps can be balanced independently and the reason they interact. A change to the distal cut affects the extension gap; a change to the posterior cut affects the flexion gap; a change to component size or position affects both.

It is also the reason mid flexion behaviour is harder to control than either end of the range. The ends are governed by surfaces the surgeon cut; the middle is governed by the transition between them.

Single radius and multi radius designs are competing answers to the same anatomical fact, which is that the native radius changes. Neither reproduces it exactly and the argument between them is an evidence question owned by Section 7.

Clinical Relevance

Joint line restoration matters because the collateral attachments are fixed. That is the whole of the argument, and it is anatomical rather than empirical.

Mid flexion instability is an anatomical consequence of a surface change rather than a technical error, which is why it is designed around rather than simply avoided.

Osteotomy around the distal femur changes the orientation of the articular surface relative to the mechanical axis without changing the surface itself, which is the opposite trade to arthroplasty. Section 7 owns both.

No number appears on this page. Every statement here has a quantitative version in Section 3, and decision 005 records why the quantities are absent from Section 2.