Deformity Analysis

For Medical Students

What It Shows

Deformity analysis takes an abnormal alignment and works out where along the limb it arises. 5.3.7 covers the mechanical axis.

The same overall deviation can come from the femur, the tibia, the joint itself, or a combination. 5.3.9 covers the joint line obliquity.

It matters because the correction has to be made where the deformity is, and the overall measurement does not say where that is. 5.16.5 covers osteotomy planning.

How It Is Acquired

Start from a standing long leg film with the positioning conditions met. 5.3.6 covers long leg alignment films.

Measure the femoral and tibial contributions separately against their own joint orientation references. 5.3.8 covers the anatomical axis.

Assess the joint line separately from the bones above and below it. 5.3.9 covers the joint line obliquity.

Assess the sagittal plane on the lateral film, because a flexion deformity is part of the analysis. 5.3.2 covers the lateral view.

Compare with the other limb, since a bilateral pattern suggests constitutional rather than acquired deformity. 3.13.5 covers constitutional varus.

What Normal Looks Like

Femoral and tibial contributions each within their usual relationship to their joint lines.

A joint line perpendicular to the mechanical axis. 5.3.9 covers the joint line obliquity.

No sagittal deformity on the lateral film. 5.3.2 covers the lateral view.

A symmetrical pattern between limbs. 3.13.5 covers constitutional varus.

What Abnormal Looks Like

A femoral contribution outside its usual relationship, which locates the deformity in the femur. 5.3.8 covers the anatomical axis.

A tibial contribution outside its usual relationship, which locates it in the tibia. 3.13.3 covers varus alignment.

An oblique joint line with normal bone contributions, which locates it in the joint. 5.3.17 covers joint space narrowing.

A sagittal component, which changes the osteotomy plane as well as its site. 5.16.5 covers osteotomy planning.

MRCS Level

How It Is Acquired

In a viva this is the structured answer to a deformity question, and the structure is the answer.

The expected points are the long film, the separate femoral and tibial contributions, the joint line, the sagittal plane, and the contralateral comparison.

Say that the overall deviation does not locate the deformity. 5.3.7 covers the mechanical axis.

Say that an intra-articular deformity is not corrected by an osteotomy. 5.3.9 covers the joint line obliquity.

Say that you would assess the sagittal plane as well as the coronal. 5.3.2 covers the lateral view.

What Abnormal Looks Like

The reasoning expected is that a limb is a chain of segments, so a deviation at the end is the sum of the contributions along it, and only the sum is directly visible. 5.3.7 covers the mechanical axis.

Separating the contributions means comparing each segment to its own reference rather than to the whole limb, which is why joint orientation angles exist. 5.3.8 covers the anatomical axis.

A correction at the wrong level neutralises the sum and leaves both the original deformity and a compensating one, which is worse mechanically than either. 3.13.9 covers the osteotomy biomechanics.

An intra-articular deformity cannot be corrected outside the joint at all, which is the distinction that changes the operation rather than the plan. 5.3.17 covers joint space narrowing.

How Good It Is

Deformity analysis converts a measurement into a plan, and it is the part of radiographic alignment assessment most often skipped.

Its best contribution is identifying the responsible segment, which determines the site of correction. 5.16.5 covers osteotomy planning.

It is limited by being planar, since a deformity with coronal, sagittal and rotational components is not fully described by two films. 5.16.3 covers rotational profiles.

No figures appear for accuracy on this page, under decision 028, and no joint orientation angle values appear either: those are published quantities with normal ranges this environment cannot check.

FRCS Level

How Good It Is

At decision level this analysis is the operation plan, and its weaknesses are the plan's weaknesses.

A single level bony deformity with a healthy joint is the clearest osteotomy indication, and the analysis establishes both halves. 5.16.5 covers osteotomy planning.

A multi level deformity needs the contributions quantified, because correcting one level alone leaves a residual. 3.13.10 covers the alignment correction principles.

An intra-articular deformity with compartment loss moves the decision to arthroplasty. 5.16.6 covers arthroplasty planning.

A sagittal or rotational component changes the osteotomy, and the plain films give the first and not the second. 5.16.3 covers rotational profiles.

Where It Misleads

The commonest error is correcting the overall deviation without locating its source. 5.3.7 covers the mechanical axis.

An intra-articular deformity is planned as a bony one. 5.3.9 covers the joint line obliquity.

The sagittal plane is not assessed. 5.3.2 covers the lateral view.

Rotation is not assessed at all, because plain films cannot. 5.16.3 covers rotational profiles.

Constitutional deformity is corrected towards a population mean. 3.13.5 covers constitutional varus.

Fellowship Level

Where It Misleads

At fellowship level deformity analysis is a rigorous planar method applied to a problem that is not planar, and the gap is where complications come from.

Rotational deformity interacts with coronal correction, so a plan that ignores rotation can produce a correct coronal result and an abnormal rotational one. 5.16.3 covers rotational profiles.

Joint orientation angle conventions have normal ranges derived from populations, and applying a population range to an individual is the same error the constitutional varus argument identified. 3.13.5 covers constitutional varus.

The analysis assumes the deformity is in the bone or in the joint, and standing films include soft tissue laxity in both, which a stress view separates and clinics rarely obtain. 4.5.2 covers varus alignment.

Three dimensional planning from a computed tomography volume measures all three planes and the segment contributions directly, which makes it the natural successor and brings dose and segmentation with it. 5.16.8 covers three dimensional reconstructions.

The reasonable current practice is a full planar analysis with its limits stated, and a three dimensional study where the deformity is multiplanar or revision. 5.1.3 covers choosing the correct investigation.