Kaplan Fibres

For Medical Students

Structure and Location

The Kaplan fibres are short, strong fibres running from the deep surface of the iliotibial band to the lateral femoral condyle.

They are usually described as two bundles, proximal and distal, attaching to the femur above and behind the lateral epicondyle.

Conventional teaching places the proximal attachment about 50 mm above the lateral epicondyle and the distal one about 30 mm above it, both close to the supracondylar ridge.

They are deep. Lifting the iliotibial band off the femur divides them, which is why they were missed in descriptions based on surface dissection.

They were described by Kaplan in the 1950s, lost from the literature, and rediscovered when rotational instability after cruciate reconstruction sent people back to the lateral side.

Relations

Above them: the lateral intermuscular septum, which the fibres blend with.

Superficial to them: the iliotibial band, of which they are the deep layer.

Deep to them: the lateral femoral condyle and the distal femur.

Below them: the lateral collateral ligament's femoral attachment and, further forward and down, the popliteus tendon's.

Function

They anchor the iliotibial band to the femur. Without them the band would be a strap running past the knee rather than a structure attached to it.

That anchorage is what converts the band's tension into a restraint on internal rotation of the tibia.

They are tightest in flexion, when the band lies behind the axis of rotation. 3.11.2 covers their contribution.

Clinical Relevance

They are divided by any approach that lifts the iliotibial band off the femur, which includes several standard exposures.

Loss of them changes rotational control without changing anything visible on a standard examination of the ligaments.

They are the anatomical reason a lateral extra-articular procedure can affect rotation at all, and 7.30 covers those procedures.

MRCS Level

Structure and Location

Describe them as the femoral attachment of the deep iliotibial band rather than as a separate ligament. They are the anchorage, and naming them as a ligament invites the wrong operation.

The proximal and distal bundles differ. Conventional teaching gives the proximal bundle the larger footprint and the distal bundle the more consistent position.

They blend with the lateral intermuscular septum at their femoral end, so the attachment is to septum and bone together rather than to bone alone.

They are present in essentially every knee when looked for from the deep surface, which is worth stating because their rediscovery was reported as though they were variable.

Relations

The supracondylar ridge is the bony landmark. Both bundles attach along or just behind it.

The lateral superior genicular artery runs in the field and is encountered when the fibres are exposed.

The capsulo-osseous layer of the iliotibial band lies distal and posterior to them and is a separate structure, which 2.11.1 covers.

The lateral collateral ligament's femoral attachment is well distal, so the two fields do not overlap.

Blood Supply and Innervation

Supply is from the lateral superior genicular artery and from periosteal vessels at the supracondylar ridge.

They are more vascular than the mid substance of the iliotibial band, consistent with a short fibre bundle at a bony attachment.

Nerve supply is from the lateral articular branches, and they carry mechanoreceptors near the femoral attachment.

Function

They are the structure through which the iliotibial band restrains internal rotation. Sectioning them increases internal rotation and increases the pivot shift, which 3.11.2 and 3.11.4 cover.

Their tension rises with flexion and with internal rotation together, so they are loaded in exactly the position a pivot shift reproduces.

They act alongside the anterolateral ligament and the capsulo-osseous layer as part of the anterolateral complex, which 2.11.5 treats as a unit.

Clinical Relevance

Expect them to be divided in any approach that elevates the iliotibial band from the distal femur, and record that they were.

Consider them when a knee has rotational laxity after a technically sound cruciate reconstruction, since nothing in the standard examination tests them directly.

Identify the supracondylar ridge as the landmark for their femoral attachment, not the epicondyle.

Preserve them where an approach allows, because their contribution is not reproduced by anything else.

FRCS Level

Relations

Their attachment to septum as well as bone means a repair or a reconstruction that fixes to bone alone reproduces part of the anatomy.

The field they occupy is the same field a lateral extra-articular tenodesis works in, and that is not a coincidence: the procedure reproduces their function.

They lie well proximal to the collateral ligament, so a construct aimed at rotation sits in different bone from a construct aimed at varus.

Function

Treat rotational restraint on the lateral side as the sum of these fibres, the capsulo-osseous layer and the anterolateral ligament, rather than attributing it to one structure. The literature's disagreement is mostly about which name to give a shared function. 2.11.5 holds that argument.

Their loading pattern, flexion with internal rotation, is why a pivot shift persists after an isolated cruciate reconstruction in some knees.

They are short, so their tension is sensitive to attachment position in the way 2.10.3 describes for the popliteofibular ligament. Length error is a large proportion of a short structure.

Clinical Relevance

Consider a lateral extra-articular procedure in a knee with a high grade pivot shift, and understand it as reproducing this attachment rather than adding a new structure. 7.30 carries the technique and 3.11.7 the evidence.

Plan the femoral fixation from the supracondylar ridge and proximal to the collateral ligament, so the two constructs do not compete for bone.

Record division of these fibres in any approach that elevates the band, because a later rotational problem will otherwise be unexplained.

Avoid over-tensioning a construct in this position. Short fibres tightened too far block rotation rather than restrain it.

Fellowship Level

Clinical Relevance

The question this anatomy was rediscovered to answer is why a technically sound cruciate reconstruction can leave a pivot shift. These fibres, the capsulo-osseous layer and the anterolateral ligament are the candidate answer, and they share one function between three names.

A lateral extra-articular procedure is best understood as reproducing this attachment rather than as adding a structure. That framing decides where it is fixed, how it is tensioned, and at what flexion angle, and it is the reason a graft placed distal to the supracondylar ridge behaves differently from one placed on it.

Over-constraint is the risk that this anatomy makes real. Short fibres have little compliance, so a construct in this field tensioned in full extension can leave a knee that will not rotate rather than one that rotates normally.

Division is routine and unrecorded. Several standard lateral exposures elevate the iliotibial band from the distal femur, and the operation note almost never says so, which makes later rotational laxity hard to attribute.

Where a revision cruciate reconstruction is planned in a knee with a persistent pivot shift, this field is examined rather than assumed. The anatomy that would restrain rotation may have been divided at the index operation.