Long Leg Alignment Films

For Medical Students

What It Shows

A long leg film includes the hip, knee and ankle in one standing image, so the relationship between the three can be seen. 5.3.7 covers the mechanical axis.

A knee radiograph cannot show alignment, because alignment is defined by joints outside the field of view. 5.3.1 covers the anteroposterior view.

It matters because every alignment measurement and every osteotomy plan starts here, and an estimate from a short film is not a measurement. 5.16.5 covers osteotomy planning.

How It Is Acquired

Take it standing, with weight on both legs, because alignment under load is the quantity of interest. 5.18.5 covers weight bearing imaging.

Position both patellae facing forwards, since rotation changes the projected alignment without changing the limb. 5.2.5 covers magnification.

Keep the knees extended, because a flexion deformity projects as apparent varus or valgus. 5.3.10 covers the deformity analysis.

Include a calibration marker at the limb's depth if any length is to be measured. 5.2.5 covers magnification.

Use a long source to detector distance, which reduces the divergence across a long field. 5.2.1 covers the generation of X-rays.

What Normal Looks Like

Hip, knee and ankle centres visible and identifiable, which is what the measurement needs.

Both patellae projected centrally, indicating neutral rotation.

Extended knees without an apparent flexion gap. 5.3.10 covers the deformity analysis.

A mechanical axis passing close to the knee centre. 5.3.7 covers the mechanical axis.

What Abnormal Looks Like

A mechanical axis passing medial or lateral to the knee centre. 5.3.7 covers the mechanical axis.

An obliquity of the joint line relative to the mechanical axis. 5.3.9 covers the joint line obliquity.

Deformity located in the femur, the tibia, the joint itself or more than one of them. 5.3.10 covers the deformity analysis.

Compartment changes consistent with the deformity's direction. 5.3.20 covers the alignment changes.

MRCS Level

How It Is Acquired

In a viva the expected answer gives the positioning requirements as part of the measurement rather than as preamble.

The expected points are the standing position, the patellae forwards, the extension, the calibration, and the source distance.

Say that a short film cannot give alignment. 5.3.1 covers the anteroposterior view.

Say that rotation and flexion both corrupt the measurement. 5.3.10 covers the deformity analysis.

Say that an angle survives the projection where a length does not. 5.2.5 covers magnification.

What Abnormal Looks Like

The reasoning expected is that alignment is the relationship between three joint centres, so any two of them outside the image make the quantity undefined rather than approximate. 5.3.7 covers the mechanical axis.

Rotation moves the projected position of a condyle without moving it, which is why the patellae forward rule is a measurement condition and not a tidiness preference. 5.2.5 covers magnification.

A flexed knee projects the condyles differently, so flexion contributes a spurious coronal angle. 5.3.10 covers the deformity analysis.

Standing matters because the limb's alignment under load includes the laxity of the compartment, which a supine film excludes. 3.13.7 covers the dynamic alignment.

How Good It Is

The long leg standing film is the reference standard for coronal alignment in routine practice and is sensitive to every positioning error.

Its best contribution is the mechanical axis and the location of the deformity, which nothing else in routine use provides. 5.3.10 covers the deformity analysis.

It is limited by rotation, by flexion, by magnification for lengths, and by being a projection of a three dimensional deformity. 5.16.8 covers three dimensional reconstructions.

No figures appear for accuracy on this page, under decision 028, and no angle values or source distances appear either: the alignment angle conventions and their normal ranges are published quantities and this environment cannot check them.

FRCS Level

How Good It Is

At decision level this film is the input to every realignment decision, so its quality is the plan's quality.

An osteotomy's correction is planned from these measurements, and a rotated film plans the wrong correction. 5.16.5 covers osteotomy planning.

An arthroplasty decision uses the deformity's magnitude and whether it corrects, which this film plus a stress view supplies. 5.16.6 covers arthroplasty planning.

Locating the deformity decides which bone is cut, which is the measurement most easily lost by cropping the film. 5.3.10 covers the deformity analysis.

A clinically obvious deformity still needs measuring, because the eye does not distinguish femoral from tibial. 4.5.2 covers varus alignment.

Where It Misleads

The commonest error is planning from a rotated film. 5.2.5 covers magnification.

A flexion deformity is present and the coronal angle is taken at face value. 5.3.10 covers the deformity analysis.

A supine or short film is used because the long standing film was not available. 5.3.1 covers the anteroposterior view.

A length is measured with no calibration marker. 5.2.5 covers magnification.

The deformity is assumed tibial because most varus is, rather than located. 5.3.8 covers the anatomical axis.

Fellowship Level

Where It Misleads

At fellowship level the long leg film is a two dimensional measurement of a three dimensional deformity, and the practice has moved further than the film has.

Rotation is the dominant error and there is no reliable way to detect residual rotation from the image beyond the patellar position, which is a coarse check. 5.16.3 covers rotational profiles.

Coronal, sagittal and rotational deformity coexist and interact, and this film measures one of them, so a plan built on it alone is incomplete by construction. 5.16.8 covers three dimensional reconstructions.

Weight bearing computed tomography measures all three under load, which is the combination realignment surgery actually needs, and its availability is limited. 5.18.5 covers weight bearing imaging.

Alignment targets have moved from a single neutral goal towards reproducing a patient's own pre-arthritic alignment, which makes the measurement's precision matter more than it did. 3.13.5 covers constitutional varus.

The consistent recommendation is to measure from a film acquired to protocol and to say so when it was not, which is the chapter's recurring discipline. 5.20.6 covers reporting osteoarthritis.