Anatomy

For Medical Students

Structure and Location

The patella is the largest sesamoid bone in the body. It lies within the quadriceps tendon, in front of the trochlea of the femur.

It is roughly triangular with the apex pointing down. The anterior surface is rough and convex, crossed by vertical ridges where the quadriceps fibres pass over it. The posterior surface is articular over its upper three quarters, with the lower quarter non articular and giving attachment to the patellar tendon at the apex.

A vertical ridge divides the articular surface into a medial and a lateral facet. The lateral facet is the larger of the two. At the extreme medial edge is a third narrow strip, the odd facet, separated from the medial facet by a faint second ridge.

Relations

Above: the quadriceps tendon, into which the patella is incorporated rather than merely attached.

Below: the patellar tendon from the apex to the tibial tubercle. Because the patella lies within the extensor mechanism, the structure above it and the structure below it are continuous through it.

Behind: the trochlea, with articular cartilage on both surfaces and synovial fluid between.

On each side: the medial and lateral retinacula, with the medial patellofemoral ligament the deepest and strongest medial structure.

Below and behind: the infrapatellar fat pad, filling the space between the patellar tendon and the front of the joint.

Function

The patella does two things. It holds the extensor tendon away from the femur, which increases the mechanical advantage of the quadriceps. And it spreads the load of that tendon over a cartilage surface rather than letting it bear directly on bone.

It also protects the front of the joint, which is the least important of the three but the one most people name first.

Its cartilage is the thickest in the body, of the order of 4 to 5 mm at the centre, which reflects the load that passes through the joint.

Clinical Relevance

Because the patella is inside the extensor mechanism, a transverse fracture through it divides the mechanism, and the patient cannot extend the knee against gravity.

The odd facet only articulates in deep flexion, which is why it is a site of change that may be asymptomatic in someone who does not squat.

The patella is palpable and mobile in extension and becomes fixed as the knee flexes and it engages the trochlea. That is the basis of examining it.

MRCS Level

Structure and Location

Describe the patella by surface. Anterior: rough, convex, vertically striated, with the quadriceps fibres passing over it and a vascular foramina pattern visible on a dry specimen. Posterior: articular above, non articular below, divided by a vertical ridge into a larger lateral and a smaller medial facet, with the odd facet at the medial margin.

The articular surface is further divided by two faint transverse ridges into superior, middle and inferior zones, so the surface is conventionally described as having six areas of contact. Which of them is in contact depends on the flexion angle, and 2.4.4 covers that.

The superior border is thick and gives the quadriceps insertion; the apex is distal and gives the patellar tendon origin; the medial and lateral borders receive the retinacula.

Relations

The quadriceps inserts in layers: rectus femoris most superficially onto the superior border, the vasti medially and laterally, and vastus intermedius deeply onto the superior border behind rectus. That layering is why a quadriceps tendon repair is described by layer.

The medial retinaculum has layers, with the medial patellofemoral ligament the deepest and the main passive restraint to lateral translation. The vastus medialis obliquus inserts into the superomedial border and acts through it.

The lateral retinaculum has deep transverse fibres attaching to the patella and superficial oblique fibres continuous with the iliotibial band.

The infrapatellar fat pad lies behind the tendon and in front of the joint, and its upper part reaches the lower pole of the patella.

Blood Supply and Innervation

The patella is supplied by an anastomotic ring formed on its anterior surface from the genicular arteries: the superior medial and lateral, the inferior medial and lateral, and the descending genicular. Vessels enter mainly through the middle third of the anterior surface and through the lower pole.

Because the supply enters anteriorly and distally, a transverse fracture tends to devascularise the proximal fragment, and a wide lateral release divides part of the ring.

Innervation of the anterior surface and the retinacula is from articular branches of the femoral nerve through the nerves to the vasti, with the recurrent branch of the common peroneal contributing laterally and the infrapatellar branch of the saphenous nerve supplying the overlying skin.

Function

The patella increases the moment arm of the quadriceps by holding the tendon away from the joint. Remove it and extension still occurs but takes more force for the same torque.

It also changes the direction of the quadriceps pull. The muscle pulls up and slightly laterally; the patellar tendon runs down and slightly medially; the patella sits at the angle between them and transmits the difference into the trochlea.

Cartilage thickness is greatest where contact pressure is highest, which is the centre of the surface. 3.12 owns the quantitative account.

Clinical Relevance

The blood supply entering anteriorly and distally is the reason patellar fracture fixation is described in terms of preserving soft tissue rather than achieving perfect reduction alone.

The six zone description of the articular surface is the framework used to record where cartilage change sits, and it matters because the zone determines the flexion angle at which the lesion is loaded.

Patellectomy loses the moment arm, and the resulting extension weakness is permanent. It is a last resort for that anatomical reason.

In arthroplasty, resurfacing removes part of the thickest cartilage in the body and replaces it with a component of fixed thickness, so restoring overall patellar thickness is part of restoring the mechanism.

FRCS Level

Relations

The relation that constrains most surgery here is the vascular ring on the anterior surface. Every anterior exposure crosses it and every lateral release divides part of it, so the question is how much and in what order.

The medial patellofemoral ligament's patellar attachment at the superomedial border is the other fixed relation, and it is close enough to the vastus medialis insertion that the two are addressed together.

Function

Patellar thickness is a component of extensor mechanism geometry, in the same way tubercle position is. Change it and the moment arm changes.

This is why restoring composite thickness in arthroplasty matters more than matching the resection depth: the mechanism responds to the distance of the tendon from the joint, not to how much bone was taken.

A patella left too thick overstuffs the joint and limits flexion; too thin and the mechanism is slack and the bone is at risk. Neither is corrected by soft tissue.

Clinical Relevance

In patellar fracture, preserving the inferior pole and the anterior soft tissue preserves the blood supply, and the fixation strategy follows from that rather than from the fracture pattern alone.

In arthroplasty, resurfacing decisions turn on cartilage state, patellar thickness and tracking. 7.51 and 7.79 own the decision; the anatomy that constrains it is here.

In instability surgery the patellar side of the medial patellofemoral ligament is reconstructed at the superomedial border, and fixation there has to respect a bone whose blood supply enters from the front.

Patellar bone stock is finite and non renewable. Every procedure that removes some of it constrains the next one, which is the main anatomical argument for conservatism in a young patient.