Surgical Relevance

For Medical Students

Structure and Location

Three features of the patella constrain almost every operation on it.

Its blood supply enters anteriorly and through the lower pole, so the surface the surgeon works on is the surface the bone is fed through.

It is inside the extensor mechanism, so anything that divides it divides the mechanism.

Its bone stock is finite and does not come back, so each procedure that removes some constrains the next.

Relations

Anteriorly: the quadriceps fibres and the vascular ring on the bone surface.

Medially: the retinaculum in layers with the medial patellofemoral ligament deepest, attaching at the superomedial border.

Laterally: the retinaculum with deep transverse and superficial oblique fibres.

Behind: the trochlea and the cartilage surface, the thickest in the body.

Function

Every operation here is a trade between stability, load and blood supply.

Tightening the medial side improves stability and can increase medial load. Releasing the lateral side reduces lateral tether and divides part of the blood supply. Removing bone reduces thickness and the moment arm with it.

Clinical Relevance

Patellar fracture fixation aims to restore the extensor mechanism first and the articular surface second, because the mechanism is what the patient uses.

A complete lateral release was once routine and is now used sparingly, and the reason is the blood supply on this page.

Resurfacing in arthroplasty removes cartilage and replaces it with a component, and restoring the original overall thickness is part of the operation.

MRCS Level

Structure and Location

For instability surgery the structures named are the superomedial border for the medial patellofemoral ligament, the lateral retinaculum for release or lengthening, and the tubercle below for transfer. Only the first two are on the patella.

For arthroplasty the measures are patellar thickness before and after, the position of the component on the cut surface, and the amount of lateral facet left uncovered.

The cut surface in resurfacing is bone whose density is higher laterally, so a component sits on asymmetrical support.

Relations

The vascular ring lies on the anterior surface with vessels entering the middle third and the lower pole. A lateral release divides the superior lateral and inferior lateral contributions; a medial arthrotomy divides the medial ones. Doing both divides most of the ring.

The medial patellofemoral ligament attaches at the superomedial border close to the vastus medialis insertion, and the two are dealt with together.

The fat pad lies behind the tendon and is routinely excised or disturbed in arthroplasty, which changes tendon excursion and is a source of postoperative anterior pain.

The infrapatellar branch of the saphenous nerve crosses the medial side of the approach and is the sensory structure most often sacrificed.

Blood Supply and Innervation

The anastomotic ring is the limiting structure in combined approaches. The practical rule is that a medial arthrotomy and a complete lateral release together leave the patella relying on the remaining anterior soft tissue, and osteonecrosis is a recognised consequence.

Lateral lengthening rather than complete release exists to reduce that risk while achieving the same soft tissue effect.

Denervation of the anterior surface by circumpatellar cautery is used to reduce pain and is a deliberate interruption of the innervation described on 2.4.3. Its effect on blood supply is part of the argument about it.

Function

In instability surgery the aim is to restore restraint in the first part of flexion without tethering the patella through the rest of the range, and the femoral attachment governs that more than the patellar one.

In arthroplasty the aim is a patella that tracks centrally on a component with the original composite thickness restored. Overstuffing limits flexion; understuffing shortens the moment arm.

In both, the lateral retinaculum is a variable to be adjusted rather than a structure to be removed, and the change in thinking is based on the blood supply.

Clinical Relevance

Avoid combining a medial arthrotomy with a complete lateral release where the alternative exists, and document what was divided.

Measure patellar thickness before resection and restore the composite, rather than resecting to a target depth.

Leave the uncovered lateral facet in mind: a component that does not cover the whole cut surface leaves bone articulating with the component groove.

In a young patient with instability, prefer the procedure that preserves bone stock, because the patella cannot afford a second operation that removes more.

FRCS Level

Relations

In revision the relations that matter are the ones previous surgery changed: how much bone remains, how much of the vascular ring survives, and whether the retinacula are scarred into a fixed position.

A patella that has been resurfaced, released and transferred has none of its original relations intact, and planning from normal anatomy will be wrong.

Function

Patellar anatomy sets a ceiling on what can be achieved. A thin patella cannot be resurfaced; a devascularised one cannot be osteotomised; a maltracking one cannot be stabilised by a procedure that does not address the variable at fault.

Recognising the ceiling is the difference between a staged plan and a sequence of disappointments.

Clinical Relevance

In revision arthroplasty with a thin patella, the options are leaving it unresurfaced, a biconvex or augmenting component, patelloplasty, or resection. Each is a decision about remaining bone and blood supply. 7.79 owns the comparison.

In recurrent instability after a failed reconstruction, re-examine all four tracking variables rather than revising the same procedure.

Patellar osteonecrosis is usually iatrogenic and usually cumulative, and the anatomy that explains it is the anterior entry of the supply.

Extensor mechanism disruption with a deficient patella is the end of the salvage ladder, and the anatomical reason is that the mechanism and the bone are the same structure.

Fellowship Level

Clinical Relevance

Four situations where the patella is the limiting structure rather than one of several.

The multiply operated patella with instability. Bone stock, vascularity and scarred retinacula all constrain, and the decision is often which of the four tracking variables can still be corrected rather than which should be. Trochleoplasty enters here, because the trochlea may be the only variable left untouched.

The thin or deficient patella at revision arthroplasty. Resurfacing needs bone and perfusion. Where either is absent, leaving the patella unresurfaced is a decision rather than a failure, and the anatomical point is that the mechanism survives without a component but not without the bone.

Patellar osteonecrosis after combined approaches. The supply enters anteriorly through the middle third and the lower pole, and this is the one complication where the anatomical cause and the surgical cause are the same sentence.

Extensor mechanism reconstruction. Allograft and synthetic options exist and all of them have to be anchored to or around a patella. Where the patella is gone or unusable, the reconstruction becomes tibia to quadriceps and the moment arm is lost permanently.

In all four the anatomy on the first four pages of this chapter is unchanged. What changes is that there is nothing spare.