Surgical Anatomy

For Medical Students

Structure and Location

The distal femur is approached from the front, the medial side or the lateral side. There is no posterior approach to the bone itself, because the popliteal vessels lie on the capsule behind it.

The anterior approach splits or passes beside the extensor mechanism. The medial and lateral approaches use the border of a muscle rather than its substance: vastus medialis medially, vastus lateralis and the lateral intermuscular septum laterally.

Relations

In front, from skin inwards: skin, subcutaneous fat, the deep fascia, the quadriceps tendon and retinacula, then the capsule and the suprapatellar pouch, then bone.

Medially: the saphenous nerve and its infrapatellar branch cross the approach, the long saphenous vein runs behind the medial border of the tibia further distally, and the descending genicular artery lies deep near the adductor tubercle.

Laterally: the iliotibial band is the first layer of consequence, the superior lateral genicular artery runs deep to it above the condyle, and the lateral intermuscular septum carries perforating vessels that bleed when it is crossed.

Behind everything: the popliteal artery, vein and tibial nerve, with the artery closest to the bone.

Function

Each approach trades exposure against something. A wider anterior exposure costs extensor mechanism integrity. A more posterior medial or lateral exposure costs proximity to the neurovascular structures.

Skin flaps around the knee depend on the genicular anastomosis, and that supply runs mainly from medial to lateral. A new incision placed lateral to an old one can leave the strip between them poorly supplied.

Clinical Relevance

The structure most often injured in an anterior approach is the infrapatellar branch of the saphenous nerve, which produces a numb patch lateral to the scar.

The structure whose injury matters most is the popliteal artery, and it is behind the bone rather than in any approach. It is injured by displacement and by instruments passed from front to back, not by the exposure itself.

MRCS Level

Structure and Location

Name the planes. Medial parapatellar: through the retinaculum medial to the patella, with the quadriceps tendon split proximally. Subvastus: beneath vastus medialis, leaving the extensor mechanism intact. Midvastus: through the substance of vastus medialis obliquus. Lateral parapatellar: lateral to the patella, through the lateral retinaculum.

Lateral approach to the shaft and metaphysis: between vastus lateralis and the lateral intermuscular septum, elevating the muscle forward off the septum.

Posteromedial exposure of the condyle: anterior to the medial head of gastrocnemius, which holds the popliteal contents back.

Relations

Anteromedial: the infrapatellar branch of the saphenous nerve crosses the distal part of the incision obliquely. The saphenous nerve itself emerges between sartorius and gracilis tendons further proximally and medially.

Anterolateral: the lateral retinaculum, then the superior lateral genicular artery running transversely deep to the iliotibial band above the condyle.

Lateral shaft: perforating branches of the profunda femoris pierce the intermuscular septum and are the reliable source of bleeding in that plane.

Posterior: popliteal artery on the capsule, vein superficial to it, tibial nerve most superficial, common peroneal nerve leaving laterally toward the fibular neck. The artery is tethered at the adductor hiatus above and the soleal arch below.

Deep medial: the descending genicular artery and the adductor hiatus lie just proximal to the adductor tubercle, so dissection above that landmark changes in character.

Blood Supply and Innervation

The skin of the anterior knee is supplied by the genicular anastomosis through perforators that arrive predominantly from the medial side. This is the anatomical basis for using the most lateral of several previous incisions when re-operating, so the medially based supply to the flap is preserved.

The extensor mechanism's own supply runs in the retinacula and in the fat pad, and a wide lateral release divides part of it.

Innervation of the anterior capsule is from the femoral nerve through the nerves to the vasti; the nerve to vastus medialis runs on the deep surface of the muscle and is the structure at risk in a midvastus split.

Function

A subvastus approach preserves the extensor mechanism and the nerve to vastus medialis, at the cost of exposure and of being harder in a stiff or heavy limb.

A medial parapatellar approach gives the widest exposure and divides part of the extensor mechanism, which then has to hold a repair.

A midvastus approach sits between the two and crosses the territory of the nerve to vastus medialis.

Clinical Relevance

In a knee with previous incisions, choose the most lateral usable one, because the skin supply arrives medially.

Protect the infrapatellar branch of the saphenous nerve by keeping the distal dissection where the branch is not, and warn the patient that numbness lateral to the scar is common whatever is done.

Treat loss of foot pulses after a distal femoral injury or a realignment as arterial until proven otherwise. The artery's tethering means it does not tolerate the displacement that the bone does.

Expect bleeding when the lateral intermuscular septum is crossed, and control it before proceeding rather than after.

FRCS Level

Relations

The relations that change a plan are the ones altered by previous surgery. Scar obliterates the subvastus plane first, because it depends on a mobile muscle belly. The parapatellar plane survives, which is why it is the fallback.

Posteriorly, capsular scar brings the popliteal structures closer to the bone rather than further from it, and flexion no longer reliably moves them back.

Function

Exposure is a decision about which structure to accept damage to. The extensor mechanism, the skin flap and the medial nerve supply are the three currencies, and no approach pays in none of them.

A stiff knee, a previous lateral incision and a thin flap each shift the choice, and they do so independently of the procedure planned.

Clinical Relevance

In revision arthroplasty, extensile options extend the parapatellar approach proximally or detach the extensor insertion distally. Both are anatomical decisions about which end of the mechanism to compromise, and Section 7 owns the technique.

In a knee with multiple previous incisions, a plastic surgical opinion before rather than after the incision is an anatomical judgement about the flap, not a courtesy.

Where the popliteal artery is at risk, the anatomical fact that matters is its tethering rather than its position. Proximal and distal control is possible; control at the level of the condyles is not.

Fellowship Level

Clinical Relevance

Four situations sit outside the standard approaches and each is defined by anatomy rather than by technique.

The ankylosed or severely stiff knee. The extensor mechanism is short and adherent, and the decision is where to lengthen it. Proximal options work within the quadriceps and distal options work at the tubercle. The anatomical difference is that a proximal lengthening heals in muscle and a distal one has to heal in bone.

The knee with a compromised flap. Perforator anatomy decides this, and the relevant fact is the medial origin of the supply. Any plan that leaves a narrow strip of skin between two incisions is relying on a watershed.

Tumour and massive bone loss around the distal femur. The approach is dictated by the lesion rather than the joint, and the structure that constrains everything is the neurovascular bundle behind the bone. Resection proximal to the adductor hiatus and resection distal to it are different operations because the artery is tethered at that point.

The infected knee with sinuses. Sinus tracts mark the planes that are already compromised, and incorporating them is an anatomical choice about which flap survives.

In all four the structures named on this page do not change. What changes is how much of each one can be spent.