Kaplan Fibre Function

For Medical Students

The Principle

A structure running from one bone to another across a joint restrains that joint. A structure running past a joint without attaching restrains the limb.

Anchoring a passing structure to one of the two bones converts it into a joint restraint for the segment beyond the anchor.

So where a structure attaches decides what it restrains, more than what the structure is made of.

At the Knee

The Kaplan fibres are deep attachments running from the iliotibial band to the distal lateral femur, above the lateral epicondyle. 2.11.2 covers the anatomy.

They anchor the band to the femur, so the band's distal part becomes a femur to tibia structure and therefore a knee restraint. 3.11.1 covers the band.

Conventional teaching describes proximal and distal fibre groups with slightly different attachments.

They restrain internal rotation of the tibia, which is the motion the pivot shift involves. 3.11.5 covers internal rotation control.

What Changes It

Whether they are intact, which decides whether the band acts on the knee.

Flexion angle, which changes their tension.

Band tension from the proximal muscles.

Injury to the anterolateral complex as a whole. 2.11.5 covers the complex.

Clinical Relevance

These fibres are one candidate for the structure a lateral tenodesis reproduces, which is part of why the tenodesis is fixed to the femur. 3.11.7 covers the tenodesis.

They are injured in anterolateral complex injury alongside the cruciate, and their loss contributes to a high grade pivot shift. 3.11.4 covers pivot shift control.

They are difficult to see on imaging, which is part of why the anatomy was argued about. 5.10 covers imaging.

Their rediscovery is part of the reason the anterolateral debate reopened. 3.11.9 covers the controversies.

MRCS Level

The Principle

The mechanical point is that a femoral anchor is necessary for rotational restraint, so any candidate structure for anterolateral rotational control must attach to the femur. 3.11.3 covers the other candidate.

That requirement is what makes these fibres mechanically interesting out of proportion to their size.

It also constrains the reconstruction: a construct without femoral fixation cannot do this job. 3.11.7 covers the tenodesis.

At the Knee

Conventional teaching describes proximal and distal Kaplan fibre groups attaching to the distal lateral femoral metaphysis, proximal to the lateral epicondyle and the gastrocnemius origin. 2.11.2 covers the anatomy.

Sectioning them increases internal rotation and the simulated pivot shift in cadaveric testing, which is the evidence for their restraint role. 3.11.4 covers pivot shift control.

They are consistently present in dissection, which distinguishes them from the anterolateral ligament's contested consistency. 3.11.3 covers that structure.

Their attachment is where a lateral tenodesis is conventionally fixed, which is an anatomical coincidence with mechanical content.

What Changes It

Whether they are intact, which is the primary variable.

Flexion angle and band tension.

Concurrent anterolateral complex injury.

Previous surgery in the region.

How It Is Measured

Selective sectioning with rotational loads and a simulated pivot shift measures their contribution.

Dissection studies establish their consistency and their attachment positions.

Magnetic resonance imaging demonstrates them inconsistently, which limits clinical identification. 5.10 covers imaging.

No clinical test isolates them.

Clinical Relevance

Expect a femoral anchor to be necessary for any anterolateral rotational restraint.

Read their consistency in dissection as the point distinguishing them from the anterolateral ligament.

Expect inconsistent demonstration on imaging.

Note that tenodesis fixation coincides with their attachment.

FRCS Level

At the Knee

Treat the Kaplan fibres as the structure that makes the tenodesis a reconstruction rather than an improvisation. If they are the native femoral anchor of the anterolateral restraint, a tenodesis fixed at their footprint reproduces a structure. 3.11.7 covers it.

That reading competes with the anterolateral ligament account, and the two are not mutually exclusive: both are femorally anchored and both restrain internal rotation. 3.11.3 covers the ligament.

Conventional teaching has moved towards describing an anterolateral complex rather than choosing between the candidates, which is the more defensible position. 2.11.5 covers the complex.

What Changes It

Whether the fibres are reconstructed or merely spanned.

Femoral fixation position relative to their footprint.

Flexion angle at fixation and graft tension.

The rest of the complex's state.

How It Is Measured

Robotic testing compares constructs fixed at different femoral positions for restored rotational restraint.

Dissection and imaging studies establish the footprint a construct aims at.

Instrumented pivot shift measures the clinical result without a threshold.

No measurement confirms a construct is reproducing these fibres specifically.

Clinical Relevance

Fix a tenodesis at the Kaplan footprint where the aim is anatomic reproduction.

Treat the complex rather than a single structure as the target.

Expect no measurement to confirm which structure a construct reproduces.

Hold the two candidate accounts as compatible rather than competing.

Fellowship Level

At the Knee

The fellowship question is whether the distinction changes anything operative, and the honest answer is that it changes the femoral fixation point by a small distance and the rationale by a lot.

Where it does matter is revision and hardware. A femoral tunnel for a cruciate graft, a tenodesis fixation and a corner reconstruction all compete for the distal lateral femur, and knowing the footprints is how convergence is avoided. 3.10.8 covers corner tunnels.

Conventional teaching routes the tenodesis deep to the lateral collateral and fixes proximal and posterior to its origin, which places it in the Kaplan region and clear of the collateral's own fixation.

What Changes It

Other tunnels and fixation in the distal lateral femur.

Whether a corner reconstruction is also being performed.

Bone quality and previous hardware.

Graft and strip availability.

How It Is Measured

Computed tomography planning assesses femoral fixation positions and convergence.

Cadaveric work establishes the footprints the planning aims at.

Postoperative imaging confirms what was achieved.

No measurement establishes the optimal fixation point within the region.

Clinical Relevance

Plan distal lateral femoral fixation positions together when several are needed.

Route deep to the lateral collateral and fix proximal and posterior to its origin.

Account for previous hardware in the region.

Accept that the optimal point within the region is not established.