Posterolateral Corner

For Medical Students

Structure and Location

The posterolateral corner is the region behind the lateral collateral ligament and lateral to the posterior cruciate ligament.

Three structures are primary: the lateral collateral ligament, the popliteus tendon and the popliteofibular ligament. 2.10.1, 2.10.2 and 2.10.3 cover each.

Several more are secondary: the posterolateral capsule, the arcuate ligament, the fabellofibular ligament when present, the lateral head of gastrocnemius, biceps femoris and the iliotibial band.

The lateral side has three layers. Layer one is the iliotibial band and the superficial part of biceps, layer two the patellar retinaculum and patellofemoral ligaments, layer three the capsule in two laminae.

The collateral ligament lies in the superficial lamina of layer three and the popliteus tendon in the deep lamina, which is the split that makes the lateral side separable at operation.

Relations

In front: the iliotibial band and Gerdy's tubercle, which 2.11.1 covers.

Behind: the posterior capsule and the lateral head of gastrocnemius.

Below and lateral: the fibular head and styloid, where two of the three primary structures attach.

Posterolateral to all of it: the common peroneal nerve, outside the capsule and behind the biceps tendon. 2.10.6 covers it.

Function

The corner is the primary restraint to varus and to external rotation of the tibia.

It works with the posterior cruciate ligament. Each protects the other, which is why losing one loads the other.

It resists hyperextension, mainly through the capsule and the arcuate complex.

Clinical Relevance

An unrecognised corner injury is the commonest reason a cruciate reconstruction stretches out. The graft takes a load the corner should have taken.

The dial test at 30 and 90 degrees separates a corner injury from a combined corner and posterior cruciate injury. 3.10.7 covers how.

A varus thrust on walking points to the corner rather than to a cruciate.

Check the peroneal nerve in every corner injury. The two are close enough that one injury often involves both.

MRCS Level

Structure and Location

Give the three layer scheme in a viva and say whose it is. Layer one, iliotibial band and superficial biceps. Layer two, patellar retinaculum and the patellofemoral ligaments. Layer three, capsule, split by the inferior lateral genicular artery into a superficial lamina carrying the collateral and fabellofibular ligaments and a deep lamina carrying the coronary ligament, the arcuate ligament and the popliteus tendon.

The arcuate ligament is a Y shaped capsular thickening over the popliteus tendon. Its lateral limb runs to the fibular styloid and its medial limb across the back of the knee.

The fabellofibular ligament runs from the fabella, or from the posterolateral capsule where no fabella exists, to the fibular styloid. It is present in a substantial minority of knees.

The fibular styloid carries the popliteofibular ligament, the arcuate ligament's lateral limb and the fabellofibular ligament when present. Three attachments on one small process is why a styloid avulsion is a corner injury rather than a fracture.

Relations

The inferior lateral genicular artery is the landmark for the lamina split in layer three. It runs between the two.

The popliteal hiatus opens into the corner from inside the joint, so the compartment can be entered from the joint as well as from outside it.

The common peroneal nerve lies outside all three layers, behind biceps femoris. It is not a component of the corner, and it is the structure the corner is approached around.

The lateral head of gastrocnemius covers the posterior part of the corner and is retracted rather than divided in most approaches.

Blood Supply and Innervation

Supply is from the lateral superior and lateral inferior genicular arteries, with contributions from the popliteal artery through the posterior capsule.

The lateral side is less well vascularised than the medial, which is the anatomical reason acute lateral repair is less reliable than acute medial repair.

Nerve supply is from the lateral and posterior articular branches, including the recurrent articular branch of the common peroneal nerve.

Function

Varus restraint belongs mainly to the collateral ligament and external rotation restraint mainly to the popliteofibular ligament and the popliteus tendon.

The corner and the posterior cruciate ligament share posterior restraint. Sectioning either increases the load on the other, which 3.10.9 covers.

External rotation laxity increased at 30 degrees but not at 90 suggests the corner alone; increased at both suggests the corner and the posterior cruciate. 3.10.7 carries the reasoning.

Biceps femoris is the dynamic component, as semimembranosus is medially. 2.10.5 covers it.

Clinical Relevance

Perform the dial test at 30 and at 90 degrees, and read the pair rather than either alone.

Examine gait. A varus thrust is a corner finding that no table test reproduces as clearly.

Assess the posterior cruciate in every corner injury and the corner in every posterior cruciate injury. The anatomy links them.

Document peroneal nerve function at presentation. Conventional teaching puts nerve injury in a substantial minority of complete corner injuries.

FRCS Level

Relations

The lamina split is the one true interval on the lateral side, as it is the basis for separating the collateral ligament from the popliteus tendon. Everything that makes an anatomical lateral reconstruction possible follows from it.

Three attachments on the fibular styloid and two femoral attachments 18 mm apart put every fixation point in a crowded field, with a nerve two centimetres behind it.

The corner has no anterior border. It becomes the iliotibial band and the anterolateral structures, which 2.11 covers.

Function

Treat the corner as the structure that sets rotational position. A cruciate graft fixed in a knee whose corner is lax is fixed in the wrong rotation, whatever its tension.

Varus alignment and corner function compound. A varus knee loads the corner in stance, so an injured corner in a varus limb is a different problem from the same injury in a neutral one. Section 7 carries osteotomy.

The dynamic component is biceps femoris and no graft reproduces it, the same limitation as on the medial side.

Clinical Relevance

Assess alignment before planning a corner reconstruction. A graft in a varus limb is loaded every step.

Stage the corner before any cruciate reconstruction in a knee with lateral findings, because an unrecognised corner injury is the commonest cause of graft failure.

Identify the peroneal nerve before working in the corner. It is not part of the corner and it is in every approach to it.

Consider reconstruction over repair in a chronic injury, since the lateral vascularity does not support the medial side's healing.

Fellowship Level

Clinical Relevance

Alignment first. A corner reconstruction in a varus limb is loaded at every step by the limb rather than by the injury, so the osteotomy question is asked before the ligament question and not after a graft has failed.

Staging is limited by the examination. Three primary structures produce two usable findings, varus opening and rotation at two angles, so the stage is an inference. Plan the exposure for more than the inference says.

The fibular styloid is the crowded point. Three structures attach to it and a tunnel through it has to miss all three attachments and the nerve behind. Where the styloid is avulsed, the attachments have come as a block and the anatomy is better than it looks.

Femoral tunnel convergence on the lateral condyle is the other constraint. Two attachments 18 mm apart on the surface, two tunnels angled to avoid the notch and each other, and a cruciate tunnel in the same condyle if the knee needs one. The convergence is in three dimensions and the surface measurement does not describe it.

The nerve decides the approach, not the ligaments. It is identified first, protected throughout, and the knee is worked in flexion where that is possible. 2.10.6 carries the anatomy and 7.49 the management of an injured nerve.

Chronic corners are length problems. Lateral tissue that healed long is present, identifiable and useless, and reattaching it without shortening reproduces the laxity it already had.