PCL Reconstruction Biomechanics

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PCL Reconstruction Biomechanics

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Student MRCS FRCS Fellow

For Medical Students

The Principle

A rope pulled around a corner wears at the corner and loses tension to friction. A straight line pull is always better.

Replacing a ligament means getting a graft from one attachment to the other. If the path has to turn sharply, the construct is compromised at the turn.

So the geometry of the path is part of the construct, not just a route to it.

At the Knee

The posterior cruciate attaches to the back of the tibia below the joint surface, so a tibial tunnel drilled from the front has to turn sharply at its back opening. 2.8.6 covers the surgical anatomy.

That turn is conventionally called the killer turn, and it is where a transtibial graft is abraded and loses tension.

The graft is tensioned in flexion, because that is where the dominant bundle works. 3.8.4 covers the bundle.

The tibia has to be pulled forward out of its sag before the graft is fixed, or the deformity is fixed in place. 3.8.6 covers the drawer.

What Changes It

The tunnel path and how sharp the turn is.

Whether an inlay technique is used, which removes the turn. 3.8.10 covers it.

Graft tensioning angle and whether the sag was reduced.

Corner integrity, which decides whether the graft is working alone. 2.10.4 covers the corner.

Clinical Relevance

Results after posterior cruciate reconstruction are less reliable than after anterior, and the construct's geometry is part of the reason. Section 6 covers management.

Residual laxity is common, which is a mechanical expectation rather than a surprise.

A missed corner injury is a recognised cause of failure, because the graft is then carrying more than its share. 2.10.4 covers the corner.

Reducing the sag before fixation is the single step most specific to this operation.

Key Learning Points

A rope pulled around a corner wears and loses tension.
The tibial attachment is below the joint surface, so a tunnel has to turn sharply.
That turn is conventionally called the killer turn.
The graft is tensioned in flexion, where the dominant bundle works.
The tibia must be pulled out of its sag before fixation.

MRCS Level

The Principle

The construct has the same four parts as any ligament reconstruction and two extra problems: an acute graft angle at the tibial aperture and a tensioning direction that fights gravity. 3.7.20 covers the general construct.

Graft angle matters through abrasion and through friction. A graft that turns loses tension to the tunnel wall, so the tension applied is not the tension achieved.

Tensioning against a sagging tibia means the reduction has to be held while the graft is fixed, which is a two handed problem.

At the Knee

Conventional teaching describes the transtibial tunnel's posterior aperture angle as the site of graft abrasion and attenuation, and chamfering the aperture as the mitigation.

Graft tensioning is performed in flexion, conventionally around 70 to 90 degrees, with an anterior drawer applied. 3.8.4 covers the bundle.

Fixation on the tibial side faces poor cancellous bone and an awkward angle, which makes it the weaker end of the construct.

Conventional teaching reports residual posterior laxity in a substantial proportion after single bundle reconstruction, which is consistent with these mechanical problems.

What Changes It

Tunnel aperture geometry and whether it was chamfered.

Tensioning angle and whether the drawer was maintained.

Fixation method and bone quality on the tibial side.

Corner reconstruction, where indicated, which shares the load.

How It Is Measured

Cadaveric construct testing compares transtibial and inlay techniques for graft tension loss and failure.

Postoperative stress radiography measures the clinical result reproducibly.

Serial measurement captures the attenuation over the first year.

No measurement confirms the tension achieved at fixation, as distinct from the tension applied.

Clinical Relevance

Chamfer the tibial aperture to reduce graft abrasion.

Maintain the anterior drawer while fixing rather than only before it.

Expect the tibial side to be the weaker end and choose fixation accordingly.

Reconstruct the corner where it is injured rather than relying on the cruciate graft.

Key Learning Points

Two extra problems: an acute graft angle and tensioning against gravity.
The tension applied is not the tension achieved when the graft turns.
Conventional teaching tensions in flexion, around 70 to 90 degrees, with a drawer applied.
The tibial side is the weaker end: poor bone and an awkward angle.
Residual posterior laxity is reported in a substantial proportion.

FRCS Level

At the Knee

Treat the reconstruction as a construct in an environment, as anteriorly, and note that the environment here is dominated by the posterolateral corner and by alignment. 3.7.20 covers that framing.

Conventional teaching holds that an unaddressed posterolateral corner injury is the commonest cause of posterior cruciate graft failure, which is the environmental factor equivalent to slope anteriorly. 2.10.4 covers the corner.

Varus alignment with posterolateral laxity is the combination that defeats a reconstruction, and correcting it is part of the operation rather than a later consideration. 3.13 covers alignment.

What Changes It

Corner status and whether it was reconstructed in the same sitting.

Alignment, and whether an osteotomy was included.

Tunnel geometry, aperture preparation and fixation.

Graft choice and diameter. 3.7.16 covers grafts.

How It Is Measured

Stress radiography before and after measures the result and is the reproducible outcome here.

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

Robotic testing compares constructs and combined reconstructions.

Revision series report the findings at reoperation, which is the evidence that the corner was missed.

Clinical Relevance

Assess and reconstruct the posterolateral corner in the same sitting where it is injured.

Correct varus alignment where it would load the reconstruction.

Measure the result with stress radiography rather than by hand.

Analyse a failure by corner, alignment, tunnel geometry and graft before revising.

Key Learning Points

The environment here is dominated by the corner and by alignment.
An unaddressed corner injury is the commonest cause of graft failure.
Varus with posterolateral laxity is the combination that defeats a reconstruction.
Correcting alignment is part of the operation, not a later consideration.
Revision findings are the evidence that the corner was missed.

Fellowship Level

At the Knee

The fellowship problem is the multiligament knee, where this reconstruction is rarely performed alone and the sequencing matters. Graft quantity, tunnel convergence and operative time all become constraints.

Tunnel convergence is the specific mechanical risk. A posterior cruciate tibial tunnel and a posterolateral corner tunnel can intersect, and so can the femoral tunnels of a combined cruciate reconstruction.

Conventional teaching plans tunnel trajectories in advance to avoid convergence, which is a geometric exercise rather than a technical preference. 3.8.10 covers inlay as one way of avoiding the tibial tunnel altogether.

What Changes It

The number of ligaments being reconstructed and their tunnel positions.

Available graft, including whether allograft is being used for quantity.

Bone quality and previous tunnels.

Alignment and whether a staged osteotomy is planned.

How It Is Measured

Preoperative planning on computed tomography assesses tunnel trajectories and convergence risk.

Postoperative imaging confirms the tunnels and identifies convergence after the fact.

Multiligament series report outcomes in small and heterogeneous groups.

No measurement establishes the optimal sequence for a given combination.

Clinical Relevance

Plan tunnel trajectories in advance to avoid convergence in a multiligament knee.

Consider inlay fixation where a tibial tunnel would converge with another.

Account for graft quantity before theatre in a multiligament reconstruction.

State that the optimal sequence for a given combination is not established.

Key Learning Points

This reconstruction is rarely performed alone in the multiligament knee.
Tunnel convergence is the specific mechanical risk.
Trajectories are planned in advance, which is geometry rather than preference.
Inlay fixation avoids the tibial tunnel altogether.
No measurement establishes the optimal sequence for a given combination.

Explore Further

Where this topic came from and where it goes next in the spiral.

References

No reference list is attached to this draft. Evidence verification has not run, because source retrieval was unavailable at generation time. Under the Operations Handbook a missing reference is acceptable and an invented one is a critical failure, so nothing has been cited and no citation marker appears in the text. Every measurement on this page is written from standard teaching rather than from a retrieved paper, and none has been checked against a source. Under decision 006 each one is registered in pipeline/config/figures with the sentence it supports and a verification state of UNVERIFIED_FROM_MEMORY, so the whole list can be checked and corrected rather than hunted for.

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