Surgical Anatomy

For Medical Students

Structure and Location

Two footprints and one hazard describe the surgical anatomy. The femoral footprint is on the medial wall of the notch, the tibial footprint is in a depression on the back of the tibia, and the popliteal artery lies behind the second of them.

The femoral footprint is reached from inside the joint. Its anterior border is close to the articular cartilage of the medial condyle, which is the landmark used to find it.

The tibial footprint sits in the posterior intercondylar fossa and runs down the posterior cortex for roughly 1 cm below the joint line. Reaching it means working at the back of the joint.

At arthroscopy the lower edge of the fossa is seen as a step down on the posterior tibia, and that step is the landmark surgeons use to confirm they are low enough.

Relations

Behind the tibial footprint, in order from front to back: the posterior capsule, a fat plane, then the popliteal artery, the vein and the tibial nerve branches.

Conventional teaching places the artery about 1 cm behind the posterior tibial cortex with the knee extended. Flexing the knee increases that distance.

Above the femoral footprint lies the roof of the notch and the anterior cruciate crossing in front.

The meniscofemoral ligaments occupy the space immediately in front of and behind the ligament and are encountered on the way in.

Function

A graft placed to the centre of each footprint reproduces the ligament's line of pull. That is the anatomical aim, and 7.32 covers how it is achieved.

Because the footprints are broad, a single tunnel reproduces part of a footprint rather than all of it.

The ligament's own curve is not reproduced by a straight graft, and the difference is part of what makes this reconstruction harder than an anterior cruciate one.

Clinical Relevance

The artery behind the tibial footprint is the reason posterior cruciate surgery is treated with more caution than anterior cruciate surgery.

Knee position changes the anatomy. The safe distance is not a property of the knee but of the knee in a particular position.

Operative technique belongs to Section 7, chapters 7.32 to 7.37. This page covers what is where.

MRCS Level

Structure and Location

Describe the femoral footprint as a crescent whose anterior border lies within about 3 mm of the cartilage edge. The cartilage is the fixed landmark and the footprint is measured from it.

The tibial footprint is bounded laterally and medially by the two plateaus and inferiorly by the step down on the posterior cortex. Conventional teaching gives the fossa a width of about 13 mm.

The posterior septum divides the posterior compartment into medial and lateral halves. It is the structure a posterior approach to the notch has to deal with, and the landmark that tells a surgeon which half they are in.

A 70 degree arthroscope is used to see the posterior tibia from within the joint, because the angle of the notch hides the lower part of the footprint from a 30 degree view.

Relations

The artery, vein and nerve branches lie together in the fat behind the capsule. Treat them as one structure for the purposes of orientation.

The middle genicular artery crosses the capsule close to the level of the footprint. It is short and tethered, so it does not move away when the capsule is retracted.

The posteromedial corner lies medial to the capsule and the posterolateral corner lateral to it. A combined injury changes what each plane contains, and 2.9 and 2.10 cover them.

The posterior horn of the medial meniscus sits just below and medial to the tibial footprint.

Blood Supply and Innervation

The middle genicular artery supplies the ligament and lies in the field. Both facts matter, and they pull in opposite directions.

Periligamentous vessels in the synovial sheath are lost if the sheath is stripped, so the sheath is tissue worth keeping where a plan allows it.

The tibial nerve branches that supply the ligament accompany the vessels, so a vascular hazard and a neural one occupy the same plane.

Function

The anatomical aim of a reconstruction is to reproduce the line between the two footprint centres. Everything else follows from that line.

A tunnel reproducing the anterolateral part of each footprint restores restraint in flexion. One reproducing the posteromedial part restores it near extension. 2.8.2 and 2.8.3 carry the bundle anatomy.

Because the tibial footprint runs down the cortex, the angle at which a tunnel meets it determines how much of the footprint it reaches.

Clinical Relevance

Identify the cartilage margin first when working on the medial wall, then read the footprint from it.

Confirm the step down on the posterior tibia before accepting a tibial landmark. The step is the reliable one and the fibres are not.

Flex the knee when working near the posterior tibia, because the anatomy moves the artery further away. 7.33 covers how that is used.

Treat a knee with posterior capsular injury as having lost the plane that protects the vessels, and plan from the injured anatomy rather than the textbook anatomy.

FRCS Level

Relations

The two constraints are different at the two ends. At the femur the limit is cartilage; at the tibia it is the neurovascular bundle behind a thin posterior wall.

The fat plane behind the capsule is the protective structure, not the capsule itself. Where that plane has been injured the distance quoted for the artery no longer applies.

The posterior septum and the posteromedial and posterolateral windows define the routes to the back of the joint. Which route is available depends on which corner is intact.

The tibial footprint's extension down the cortex means a transtibial route and a posterior route reach different parts of the same attachment.

Function

Reproducing the footprint centres reproduces the line of pull and not the ligament. The ligament's forward curve and its reciprocal bundles are not reproduced by a single straight graft.

Restraint restored depends on which part of each footprint a tunnel reaches, and the footprints are broad enough that the choice is real. 3.8.3 carries the mechanics.

The angle at which a tunnel meets the posterior cortex determines both how much footprint it reaches and how close its exit lies to the vessels. One variable governs two outcomes, which is why the geometry is argued over.

Clinical Relevance

Assess the posterior capsule and the fat plane on imaging before planning a route. 2.8.5 covers what is visible.

Plan the tibial exit from the step down on the posterior cortex rather than from the joint line, because the footprint is defined by the step.

Consider which part of the range the patient needs restrained before choosing which part of the footprint to reproduce. The anatomy makes that a decision rather than a default.

Protect the synovial sheath where the plan allows. It carries the supply the remaining ligament depends on.

Where the corners are injured, plan the approach from the corner anatomy as well as from the footprints. 2.9 and 2.10 cover the planes.

Fellowship Level

Clinical Relevance

Approach anatomy, not technique, is what separates the available routes. A transtibial route crosses the posterior cortex and therefore passes the vessels at a distance set by tunnel angle and knee flexion. An inlay route reaches the footprint from behind and trades that passage for a posterior exposure.

The anatomical argument between the two is about where the graft turns. A transtibial graft turns at the tunnel mouth on the posterior cortex; an inlay graft does not turn at all. That single difference is the anatomy behind the whole debate, and 7.35 carries the operative content.

Revision anatomy is not primary anatomy. A previous tunnel removes the landmark it was drilled from, and the step down on the posterior cortex may have been breached. Plan revision from what is present rather than from the standard description, and 7.37 covers the rest.

In a multiligament knee the planes behind the capsule are not where a textbook puts them. Assess each corner before assuming a route is available, because the fat plane that carries the vessels may have been displaced or lost.

Where the posterior capsule is injured, treat every quoted distance to the artery as void. The figures come from intact knees and the knee in front of you is not one.

Consider staging where the posterior anatomy is uncertain. The anatomical reason is that a plane which cannot be identified cannot be protected, and 7.36 carries the discussion of combined reconstruction.