PLC Reconstruction Biomechanics

For Medical Students

The Principle

Replacing a group of structures means replacing each one's job, in its own direction, at its own position.

A group containing a muscle cannot be fully replaced, because a graft is passive. Part of the function is lost whatever is done.

If the limb's alignment is loading the group, replacing the group without correcting the alignment repeats the original failure.

At the Knee

Anatomic posterolateral corner reconstruction replaces the lateral collateral ligament, the popliteus tendon and the popliteofibular ligament. 3.10.6 covers the corner.

Each limb has its own attachment positions and its own orientation, because each restrains a different combination. 3.10.5 covers the popliteofibular ligament.

The fibular head takes two of the fixations and is a small, crowded piece of bone. 2.10.7 covers the surgical anatomy.

The popliteus muscle's active roles are not restored, so unlocking and lateral meniscal retraction are permanently altered. 3.10.4 covers the complex.

What Changes It

Which limbs are reconstructed and where they attach.

Fibular bone quality, which decides the fixation.

Alignment, since varus loads the reconstruction. 3.13 covers alignment.

Peroneal nerve position, which constrains the dissection. 2.10.6 covers the nerve.

Clinical Relevance

Corner injuries are reconstructed rather than braced because they heal badly, which is the opposite of the medial side. Section 6 covers management.

Correcting a varus limb is part of the operation in a chronic injury rather than a later consideration.

The peroneal nerve is at risk, and its function is documented before and after. 2.10.6 covers the nerve.

7.45 covers the reconstruction technique.

MRCS Level

The Principle

The construct's job is to restore varus restraint near extension and external rotation restraint through mid flexion, which are two different directions at two different angles. 3.10.1 and 3.10.2 cover them.

Anatomic and non anatomic approaches differ in whether they reproduce the native attachments. A tenodesis restrains a motion; an anatomic reconstruction reproduces a structure.

Limb orientation is as important as attachment position, because a limb in the wrong orientation restrains the wrong motion. 3.10.5 covers that point.

At the Knee

Conventional teaching describes an anatomic reconstruction with femoral tunnels at the native lateral collateral and popliteus origins, a fibular tunnel for the collateral and popliteofibular limbs, and a tibial tunnel for the popliteus limb.

The two femoral origins are close together, so tunnel convergence and bone bridge are technical constraints. 2.10.7 covers the surgical anatomy.

Graft tensioning is conventionally performed with the knee in slight flexion and the tibia in neutral or slight internal rotation, to avoid fixing an externally rotated position.

Conventional teaching reports anatomic reconstruction restoring varus and rotational restraint closer to the intact state than a fibular based tenodesis in cadaveric testing.

What Changes It

Attachment positions and limb orientations.

Tensioning position and rotation of the tibia at fixation.

Fibular and tibial bone quality.

Alignment and concurrent cruciate reconstruction.

How It Is Measured

Robotic testing compares constructs against the intact knee in varus and rotation.

Varus stress radiography measures the coronal result clinically.

The dial test measures the rotational result without an instrument. 3.10.7 covers it.

Postoperative imaging confirms tunnel positions and convergence.

Clinical Relevance

Reproduce attachment positions and limb orientations, since both decide what is restrained.

Fix with the tibia in neutral or slight internal rotation rather than externally rotated.

Plan the femoral tunnels to avoid convergence and preserve the bone bridge.

Measure the coronal result radiographically and the rotational one by dial test.

FRCS Level

At the Knee

Treat the reconstruction as a construct in an environment, where the environment is varus alignment and the cruciates. A corner reconstruction in an uncorrected varus limb carries the load that stretched the original. 3.7.20 covers that framing.

Conventional teaching corrects varus alignment, by proximal tibial or distal femoral osteotomy as the deformity dictates, before or with the reconstruction in a chronic case. 3.13 covers alignment.

The peroneal nerve is the complication that defines the operation's risk profile, and documenting its function before surgery is part of the consent rather than a formality. 2.10.6 covers the nerve.

What Changes It

Alignment and whether an osteotomy is included.

Which limbs are reconstructable and the graft available.

Cruciate status and the sequence of reconstruction.

Previous surgery and the state of the fibular head.

How It Is Measured

Long leg standing radiography measures the alignment that loads the construct.

Varus stress radiography before and after measures the coronal result.

Dial test under anaesthetic measures the rotational result.

Revision series report what was found at reoperation, which is the evidence for the environmental factors.

Clinical Relevance

Correct varus alignment before or with a chronic corner reconstruction.

Document peroneal nerve function before surgery and after.

Reconstruct all three limbs where the graft and bone allow.

Check the dial test under anaesthetic before leaving theatre.

Fellowship Level

At the Knee

The fellowship problem is the multiligament knee with a varus deformity and a stretched corner, where three or four reconstructions and an osteotomy are all indicated and cannot all be done at once.

Staging is the main tool, and the sequence follows the mechanics: correct the environment first where it is driving the failure, reconstruct the structures that others depend on, and accept that a single sitting may not be possible. 3.8.8 covers the comparable posterior problem.

Tunnel convergence is the specific geometric constraint, because a corner reconstruction's fibular and tibial tunnels compete with a posterior cruciate tibial tunnel. 3.8.10 covers the inlay option that avoids one.

What Changes It

Number of reconstructions indicated and graft available.

Tunnel trajectories and convergence risk.

Degree and level of deformity.

Peroneal nerve status, which can make an extensive dissection unwise.

How It Is Measured

Computed tomography planning assesses tunnel trajectories and convergence.

Long leg radiography plans the correction.

Multiligament series report outcomes in small heterogeneous groups.

No measurement establishes the optimal sequence for a given combination.

Clinical Relevance

Plan tunnel trajectories before theatre in a multiligament corner reconstruction.

Correct the deformity that is driving the failure first.

Consider staging where graft, time or soft tissues will not allow one sitting.

Consider an inlay for the posterior cruciate to free tibial tunnel space.