Medial Reconstruction Biomechanics

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Medial Reconstruction Biomechanics

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Student MRCS FRCS Fellow

For Medical Students

The Principle

Replacing a structure means reproducing what it did, where it did it, and at the length it did it at.

If two structures were doing different jobs, replacing one leaves the other job undone.

Making a replacement too short constrains the joint. A ligament that is too tight stops the joint moving as well as stopping it opening.

At the Knee

Medial reconstruction replaces the superficial medial collateral ligament for valgus and the posterior oblique ligament or deep layer for rotation. 3.9.3 and 3.9.5 cover them.

The superficial ligament's graft runs from the femoral epicondyle to the distal tibial attachment, reproducing its length. 2.9.7 covers the surgical anatomy.

The corner limb runs more obliquely and is tensioned nearer extension, because that is where the structure it replaces is taut. 3.9.6 covers the corner.

Each limb is tensioned at the flexion angle where its native structure works, which is why the two are tensioned separately.

What Changes It

Attachment positions, which set each limb's length change pattern.

Tensioning angle for each limb.

Graft choice and availability.

Whether a cruciate reconstruction is being done at the same time.

Clinical Relevance

Over-constraint is the characteristic error here, and it presents as loss of flexion rather than as laxity. Section 7 covers technique.

Reconstructing only the superficial ligament leaves rotational laxity, which is the mechanical reason anatomic reconstruction is specified. 3.9.2 covers rotation.

Most medial injuries heal and do not need this operation, which is why the indication is narrow. Section 6 covers management.

7.38 covers the repair and reconstruction technique.

Key Learning Points

Replacing a structure means reproducing its job, its position and its length.
Medial reconstruction replaces two structures: one for valgus and one for rotation.
Each limb is tensioned at the angle its native structure works at.
Over-constraint presents as loss of flexion rather than as laxity.
Reconstructing only the superficial ligament leaves rotational laxity.

MRCS Level

The Principle

The construct's job is to reproduce two reciprocal length change patterns with two limbs, which is a harder problem than reproducing one. 3.9.9 covers the reciprocal division.

Attachment position dominates, as it does in cruciate reconstruction, because position sets the length change pattern. 3.7.20 covers that principle.

Tensioning angle is the second variable and it differs between the limbs, which is the feature specific to this reconstruction.

At the Knee

Conventional teaching describes anatomic medial reconstruction with a femoral attachment at the native superficial ligament origin, a tibial attachment at the native distal insertion, and a separate posteromedial limb.

The superficial limb is conventionally tensioned at around 20 to 30 degrees of flexion and the posteromedial limb nearer full extension, matching the native structures. 3.9.5 covers the posterior oblique ligament.

Over-constraint restricts flexion and raises medial compartment contact pressure, which is a cartilage consequence of a technical error. 3.5.2 covers contact pressure.

Repair rather than reconstruction is possible acutely, particularly for a displaced distal avulsion. 3.9.3 covers that pattern.

What Changes It

Attachment positions for both limbs.

Tensioning angle for each limb separately.

Acute repair against chronic reconstruction, which differ in what tissue is available.

Concurrent cruciate reconstruction and its rehabilitation.

How It Is Measured

Robotic testing compares reconstructions for restored valgus and rotational restraint against the intact knee.

Valgus stress radiography measures the coronal result clinically.

Range of motion measurement detects over-constraint.

No measurement confirms either limb reproduces its native length change pattern.

Clinical Relevance

Place both limbs anatomically, since position sets the pattern.

Tension each limb at its own angle rather than at one angle for both.

Check flexion after fixation, since over-constraint presents that way.

Consider acute repair where a displaced distal avulsion is present.

Key Learning Points

The construct reproduces two reciprocal length change patterns with two limbs.
Attachment position dominates, as in cruciate reconstruction.
Conventional teaching tensions the superficial limb at 20 to 30 degrees, the posteromedial nearer extension.
Over-constraint restricts flexion and raises medial contact pressure.
No measurement confirms either limb reproduces its native pattern.

FRCS Level

At the Knee

Treat the indication as the harder question than the technique. Most medial injuries heal, so the operation is for the knee that did not heal, the one with rotatory laxity, or the displaced avulsion. 3.9.6 covers the corner.

Over-constraint is the error to avoid and it is avoided by reproducing length rather than shortening it. A medial reconstruction that is too tight produces a stiff knee and loads the medial compartment.

In a combined injury the sequence matters. Conventional teaching fixes the cruciate first and then addresses the medial side, so the medial tension is set against a reduced knee. 3.9.10 covers combined injury.

What Changes It

Whether the knee healed and what laxity remains.

Rotatory laxity, which is the operative indication.

Alignment, particularly valgus. 3.13 covers alignment.

Sequence in a combined reconstruction.

How It Is Measured

Valgus stress radiography before and after measures the coronal result.

Examination under anaesthetic assesses rotation, which no instrument quantifies.

Range of motion at follow up detects over-constraint.

Robotic testing compares constructs and is where the tensioning angles come from.

Clinical Relevance

Reserve the operation for failed healing, rotatory laxity or a displaced avulsion.

Reproduce length rather than shortening it, to avoid a stiff knee.

Fix the cruciate first and set the medial tension against a reduced knee.

Correct valgus alignment where it would load the reconstruction.

Key Learning Points

The indication is harder than the technique, because most medial injuries heal.
Over-constraint produces a stiff knee and loads the medial compartment.
Reproduce length rather than shortening it.
Fix the cruciate first, then set medial tension against a reduced knee.
Rotatory laxity is the operative indication.

Fellowship Level

At the Knee

The fellowship problem is the chronic valgus knee with medial laxity, where the ligament and the alignment are both wrong and each makes the other worse. 3.13 covers alignment.

Reconstructing the medial side in an uncorrected valgus limb asks the graft to carry the load that stretched the original, which is the same argument as reconstructing a cruciate in an uncorrected slope. 3.7.20 covers that framing.

Conventional teaching corrects the alignment, by distal femoral osteotomy where the deformity is femoral, before or with the ligament reconstruction.

What Changes It

The magnitude and level of the valgus deformity.

Whether the lateral compartment is intact, since correction loads it.

Graft availability for a two limb reconstruction.

Whether a cruciate reconstruction is also required.

How It Is Measured

Long leg standing radiography measures the alignment and localises the deformity.

Valgus stress radiography measures the ligamentous component.

Stress views separate bony deformity from soft tissue laxity, which changes the correction.

Combined procedure series are small and report outcomes without separating components.

Clinical Relevance

Correct a valgus deformity before or with a medial reconstruction.

Localise the deformity to the femur or the tibia before planning.

Separate bony deformity from soft tissue laxity on stress views.

Account for graft quantity where two limbs and a cruciate are all required.

Key Learning Points

The chronic valgus knee with medial laxity has two wrong things reinforcing each other.
A graft in an uncorrected valgus limb carries the load that stretched the original.
Correct the alignment before or with the ligament reconstruction.
Stress views separate bony deformity from soft tissue laxity.
Combined procedure series do not separate the components.

Explore Further

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

References

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