Biomechanics

For Medical Students

Structure and Location

The patella's mechanical role follows from one piece of geometry. It is a block of bone and cartilage inside the extensor tendon, and it holds that tendon away from the axis about which the knee bends.

The thicker the block, the further the tendon sits from the axis. The further it sits, the greater the turning effect a given muscle force produces.

The patella also sits at the angle between two tendons that do not run in the same direction. The quadriceps pulls up and slightly laterally; the patellar tendon runs down and slightly medially.

Relations

The surfaces in contact are the patellar articular surface and the trochlea. Which part of each is in contact depends on the flexion angle, and 2.4.4 covers the sequence.

The restraints either side are the retinacula, with the medial patellofemoral ligament the main passive restraint to lateral translation and the trochlea's lateral facet the main bony one.

Below, the patellar tendon connects to the tibial tubercle, whose position determines the direction of the force the patella receives from below.

Function

Three consequences follow from the geometry.

The quadriceps gains mechanical advantage. Without the patella the same muscle force produces less extension torque, which is why patellectomy leaves permanent extension weakness.

The joint is loaded in compression. The patella is squeezed against the trochlea by the two tendons pulling at an angle, and that compressive force rises steeply as the knee bends. Conventional teaching puts it at around half body weight in level walking, about 3 times body weight on stairs, and considerably more in a deep squat.

The patella is pushed laterally. The angle between the two tendons has a lateral component, and it is resisted by the trochlea and the medial soft tissues.

Clinical Relevance

Anterior knee pain is commoner going up and down stairs and on squatting than on level walking, and the load figures above are the reason.

Reducing the compressive load is the mechanical aim of several treatments: strengthening the quadriceps so less force is needed, avoiding deep flexion under load, and in surgery moving the tubercle forwards.

The quantitative account of contact pressure, moment arms and the stress distribution belongs to 3.12. This page is orientation only, because that is what the architecture assigns it.

MRCS Level

Structure and Location

The variables are patellar thickness, patellar height, tubercle position and trochlear shape. Three of the four are patellar or tibial and can be altered surgically; trochlear shape largely cannot without reshaping the femur.

Patellar height determines when the patella engages the trochlea. A high riding patella engages later in flexion and spends longer unsupported by bone.

Patellar thickness determines the moment arm directly, which is why composite thickness is restored in arthroplasty rather than resection depth being matched.

Relations

The quadriceps line of pull and the patellar tendon line of pull meet at the patella, and the angle between them in the coronal plane is the basis of the Q angle. Conventional teaching gives a normal value of about 14 degrees in men and 17 degrees in women.

The lateral restraint comes from the trochlear lateral facet and operates only once the patella is engaged. Before engagement the restraint is soft tissue.

The medial patellofemoral ligament is the principal medial passive restraint, conventionally credited with around 60 per cent of the medial soft tissue contribution.

The fat pad behind the tendon is not a restraint but it is a source of pain and it scars, which changes the tendon's excursion.

Blood Supply and Innervation

Load and vascular supply interact in the patella, because the supply enters anteriorly through the surface the quadriceps passes over. A patella loaded in compression is supplied through its tension surface.

Subchondral bone density on the lateral facet is higher than on the medial, reflecting where load habitually passes, and that difference is a response to loading rather than a fixed property.

The anterior retinacular tissue and the fat pad are richly innervated by articular branches of the femoral nerve. Cartilage is not innervated, which is why the source of anterior knee pain is often not the surface that looks abnormal.

Function

The joint reaction force is the resultant of the quadriceps tension and the patellar tendon tension, and it increases with flexion for two reasons: the muscle force needed rises, and the angle between the two tendons closes.

Contact area also increases with flexion, which partly offsets the rising force. That is why pressure does not rise as steeply as load does, and why a patella with a small contact area is at a disadvantage at every angle.

The lateral force component is greatest in early flexion, when the patella is least constrained by bone. That coincidence is the mechanical basis of patellar instability being an early flexion problem.

Clinical Relevance

Anteriorising the tubercle reduces the joint reaction force by increasing the angle between the tendons. Medialising it reduces the lateral force component. They are different operations because they change different variables.

Restoring patellar height corrects when engagement happens, which is why height is assessed before instability surgery rather than after.

In arthroplasty, an overstuffed patellofemoral joint raises the joint reaction force at every angle and limits flexion. An understuffed one reduces the moment arm.

The Q angle and the medial patellofemoral ligament's share of restraint are both written here from standard teaching and registered as unchecked. Both are measured differently by different authors.

FRCS Level

Relations

Every patellofemoral procedure changes one of four relations: the patella to the trochlea, the patella to the tendon above, the tendon below to the tibia, or the retinacula to the patella. Knowing which one a procedure changes is how its failure mode is predicted.

The relations are not independent. Moving the tubercle changes patellar height as well as direction of pull unless the transfer is purely medial.

Function

The patellofemoral joint is the one place in the knee where the surgeon can alter the load by changing geometry rather than by changing the surface. That is what makes tubercle procedures attractive and what makes them easy to overdo.

Load reduction and stability are separate aims that can conflict. Anteriorisation reduces load and does little for stability; medialisation improves stability and can increase medial facet load. A combined transfer trades between them, which is why its direction is planned rather than chosen.

Patellectomy is the extreme case: it eliminates the patellofemoral joint reaction force entirely and loses the moment arm permanently. The trade is never favourable except as a salvage.

Clinical Relevance

Predicting which compartment a transfer will overload is an anatomical exercise done before the operation, from the direction of transfer and the existing cartilage state.

In a patella with isolated medial facet change, medialisation is the wrong direction, and the reason is the geometry on this page rather than any trial.

Patellofemoral arthroplasty replaces the surface without changing the geometry, and tubercle surgery changes the geometry without replacing the surface. Choosing between them is choosing which problem the patient actually has.

All quantitative treatment of contact pressure, moment arms and load distribution sits in 3.12. The figures on this page are orientation values, registered and unchecked, and are not a basis for calculation.