Tunnel Assessment

For Medical Students

What It Shows

The tunnels are the holes drilled in the femur and tibia through which the graft passes, and their position determines how the graft is loaded. 3.7.20 covers the reconstruction biomechanics.

Their position is described against bony landmarks on the lateral femoral condyle wall and the tibial plateau, which makes it a geometric measurement rather than a judgement. 2.7.6 covers the femoral footprint.

It matters because tunnel position is measurable and graft signal is not, so this is where the post-operative study earns its place. 5.7.9 covers cruciate reconstruction.

How It Is Acquired

Find the femoral tunnel on a sagittal image through the lateral femoral condyle wall, where its position along the wall can be judged. 5.5.1 covers the standard protocol.

Find the tibial tunnel on sagittal and coronal images and relate its aperture to the plateau. 2.7.7 covers the tibial footprint.

Assess tunnel width along its length, looking for a tunnel wider than its aperture. 5.17.1 covers post-operative cruciate reconstruction.

Assess the graft's line, since tunnel position and graft line are two views of the same geometry. 2.7.12 covers the surgical anatomy.

Note that tunnel widening and tunnel position are both assessed better on computed tomography, which belongs to its own chapter. 5.16.8 covers three dimensional reconstructions.

What Normal Looks Like

A femoral tunnel aperture on the posterior part of the lateral condyle wall, corresponding to the native attachment. 2.7.6 covers the femoral footprint.

A tibial tunnel aperture in the native footprint, behind the front of the plateau. 2.7.7 covers the tibial footprint.

A graft running parallel to the notch roof between the two. 2.7.12 covers the surgical anatomy.

Tunnels of uniform width with fluid in them early after surgery, which is expected. 5.5.3 covers the post-operative protocol.

What Abnormal Looks Like

A femoral tunnel too far forward on the condyle wall, which tightens the graft in flexion.

A tibial tunnel too far forward, which risks notch impingement in extension. 5.7.13 covers cyclops lesions.

A tunnel wider along its length than at its aperture, which is tunnel widening and affects revision planning. 5.7.14 covers revision cruciate imaging.

A graft that does not run parallel to the notch roof, which is the consequence of either. 2.7.12 covers the surgical anatomy.

MRCS Level

How It Is Acquired

In a viva this is the measurement to volunteer, because it is the one the study makes well.

The expected points are the femoral aperture on the condyle wall, the tibial aperture against the plateau, the tunnel width, the graft line, and the computed tomography boundary.

Say that you would report tunnel position whether or not the graft looks abnormal. 5.20.7 covers reporting the post-operative knee.

Say that a forward femoral tunnel and a forward tibial tunnel fail in different ways. 3.7.20 covers the reconstruction biomechanics.

Say that widening is assessed better on computed tomography. 5.16.8 covers three dimensional reconstructions.

What Normal Looks Like

The reasoning expected is that the native attachments are where the ligament's length stays nearly constant through flexion, so a tunnel at the native site produces a graft that is neither slack nor over-tensioned through range. 3.7.4 covers the bundle function.

A femoral tunnel placed forward lengthens the graft's path in flexion, so the graft is over-tensioned there and either stretches or restricts flexion. 3.7.20 covers the reconstruction biomechanics.

A tibial tunnel placed forward brings the graft against the notch roof in extension, which is the mechanism of impingement and of the nodule that follows it. 5.7.13 covers cyclops lesions.

Tunnel widening reflects the graft tendon interface and the mechanical environment in the tunnel, which is why it matters for whether a revision can be single stage. 5.7.14 covers revision cruciate imaging.

How Good It Is

Tunnel position is the most reliably assessed feature of the reconstructed knee on magnetic resonance imaging.

Its best contribution is separating a technical failure from a traumatic one, which no other observation in the study does. 5.7.11 covers graft failure.

It is limited by fixation artefact at the apertures, by the aperture being oblique to every standard plane, and by width being better measured on computed tomography. 5.2.19 covers artefacts.

No figures appear for accuracy on this page, under decision 028. No tunnel position percentage or millimetre threshold appears either: the quadrant conventions and the widening thresholds are published quantities tied to specific methods and this environment cannot check them.

FRCS Level

How Good It Is

At decision level tunnel assessment is what converts a failed reconstruction into a revision plan.

Correct tunnels with a ruptured graft allow a revision along the same lines. 6.8.1 covers graft failure.

A malpositioned tunnel means the new tunnel has to be placed where the old one is not, which is a design problem rather than a repetition. 6.8.2 covers tunnel malposition.

Widened tunnels that converge on the intended new position are the argument for a staged bone graft, and that decision is made before the patient is listed. 5.7.14 covers revision cruciate imaging.

A forward tibial tunnel with extension loss is an impingement problem with its own answer. 5.7.13 covers cyclops lesions.

Where It Misleads

The commonest error is omitting tunnel position from the report. 5.20.7 covers reporting the post-operative knee.

Fixation artefact obscures the aperture and the position is estimated rather than assessed. 5.2.19 covers artefacts.

Tunnel width is judged on magnetic resonance imaging when computed tomography would answer it. 5.16.8 covers three dimensional reconstructions.

A position measured on one plane is reported as a position, when the aperture is oblique to all of them. 5.5.1 covers the standard protocol.

Early tunnel fluid is reported as loosening. 5.5.3 covers the post-operative protocol.

Fellowship Level

Where It Misleads

At fellowship level tunnel assessment is the clearest case in this chapter of a reliable measurement that the reporting convention does not ask for.

Several position conventions exist and they are not interchangeable, so a position stated without its method is not a measurement. 5.21.2 covers the cruciate imaging literature.

The apertures are oblique to every standard plane, so an accurate position needs reformatting, and reformatting is a computed tomography habit rather than a magnetic resonance one. 5.16.8 covers three dimensional reconstructions.

Widening thresholds that decide single stage against staged revision come from small series with different measurement methods, which is why none is stated here. 5.7.14 covers revision cruciate imaging.

The surgeon needs the position and the report gives the graft signal, and that mismatch persists because nobody has made the position a mandatory field. 5.20.1 covers structured reporting.

Automated measurement of tunnel geometry is a tractable problem and has not arrived, which is a missed opportunity rather than a limitation. 5.18.8 covers artificial intelligence in imaging.