Effects of Malalignment

For Medical Students

The Principle

If a pulley and its rope are not in line, the pulley is pushed sideways every time the rope is pulled.

The misalignment can be in the plane you can see from the front, or it can be a twist you cannot see at all.

A twist is the harder problem, because nothing about the leg looks wrong from the front.

At the Knee

A knock kneed leg moves the hip and the thigh inwards relative to the knee, which increases the outward push on the patella. 3.13.4 covers valgus.

A bow legged leg does the opposite and reduces that push. 3.13.3 covers varus.

A thigh bone twisted so the knee points inwards has the same effect as a knock knee, without looking like one.

A shin bone twisted so the foot points outwards moves the tibial tubercle outwards, which also increases the push. 3.12.12 covers the tubercle.

What Changes It

The shape the bones grew into, which is fixed in adults. 2.4.2 covers ossification.

Previous fracture that healed in a rotated position.

Previous surgery to correct alignment. 7.68 covers the principles.

Position during movement, which can add to the fixed problem. 3.12.7 covers dynamic valgus.

Clinical Relevance

Rotation is missed because it is invisible from the front and needs a scan to measure. 5.12.4 covers the imaging.

Two patients with the same knee x ray can have very different mechanics at the patella.

It explains why some instability does not improve after a soft tissue operation. 3.12.5 covers the ligament.

Chapter 3.5 covers what malalignment does to the main weight bearing part of the knee.

MRCS Level

The Principle

Malalignment acts on this joint through the direction of the extensor vector, which has components in the coronal and the transverse plane.

Coronal malalignment is visible and measurable on a long leg film. Transverse malalignment is neither. 3.13.1 covers the mechanical axis.

The clinical consequence is that the measurable component is routinely assessed and the under recognised component is not.

At the Knee

Genu valgum increases the lateral vector on the patella by moving the proximal line of pull medially relative to the tubercle. 3.13.4 covers it.

Femoral anteversion rotates the trochlear groove internally beneath the extensor mechanism, which is mechanically equivalent to lateralising the tubercle.

External tibial torsion lateralises the tubercle directly, which is why the two rotational abnormalities summate. 3.12.12 covers the tubercle.

Both rotational abnormalities raise the tubercle to groove distance without any coronal abnormality, which is the common trap.

Severe rotational malalignment can produce instability with a normal trochlea and an intact ligament, which is the case that confuses the four factor model. 3.12.8 covers it.

What Changes It

Coronal alignment, which is measured on a long leg standing radiograph. 5.3.2 covers the imaging.

Femoral version, which requires axial imaging through the hip and knee.

Tibial torsion, which requires axial imaging through the knee and ankle.

Previous malunion or osteotomy, which may have corrected one plane and worsened another. 7.68 covers the principles.

Dynamic contribution, which adds to the fixed geometry during loading. 3.12.7 covers it.

How It Is Measured

Long leg standing radiographs for coronal alignment, which is the reliable part. 3.13.1 covers the axis.

Computed tomography or magnetic resonance imaging with hip and ankle included for rotational profile, which is not a routine study.

Tubercle to groove distance, which captures the net transverse effect without saying where it came from. 3.12.12 covers the index.

Clinical rotational profile testing, which estimates version and torsion crudely. 4.5.6 covers it.

The net vector is not measured by any single study, which is the recurring problem in this chapter.

Clinical Relevance

Measure the rotational profile where instability persists despite normal coronal alignment and normal morphology. 3.12.8 covers the assessment.

Read a raised tubercle to groove distance as a net result, then look for its cause rather than correcting it blind. 3.12.12 covers the index.

Expect a soft tissue procedure to underperform where the vector abnormality is rotational. 7.51 covers the operation.

Distinguish this page's question from the tibiofemoral one, because the same malalignment has different consequences in the two compartments. 3.5.6 covers that.

FRCS Level

At the Knee

The transverse plane is where this chapter's under recognised pathology sits, and the reason is historical rather than mechanical: the coronal plane was measurable first.

Femoral anteversion above the normal range is reported in a substantial minority of patients with recurrent instability, and it is rarely measured before a first operation.

It matters because no soft tissue or tubercle procedure addresses it. A derotation osteotomy does, and it is a larger operation than either. 7.68 covers the principles.

Coronal malalignment, by contrast, has a modest effect on this joint and a large effect on the tibiofemoral compartments, which is the opposite emphasis to the rotational case. 3.5.6 covers the tibiofemoral side.

Conventional teaching has moved towards measuring the rotational profile in revision and in severe primary cases, and the threshold for intervening on it is not agreed. 6.153 covers the controversies.

What Changes It

Femoral version, which is the dominant under measured factor.

Tibial torsion, which contributes and is smaller in effect.

Coronal alignment, which matters more for the tibiofemoral joint than for this one. 3.13.8 covers that.

Previous surgery, which may have corrected the measurable plane only.

Dynamic valgus, which adds a modifiable rotational component on top of the fixed one. 3.12.7 covers it.

How It Is Measured

Axial imaging from hip to ankle, which is the only way to measure version and torsion properly and is not obtained routinely.

Three dimensional reconstruction, which measures the net vector and is largely a research method. 3.16.5 covers it.

Tubercle to groove distance, which is the clinical proxy and conflates the sources.

Patellar tendon to trochlear groove distance, which removes tubercle rotation from the measurement and is reported less often.

Normal ranges for femoral version are wide and population dependent, so the threshold question is partly a question about the reference range.

Clinical Relevance

Obtain a rotational profile before revising a failed stabilisation, because an unmeasured rotational cause is a common reason for failure. 3.12.8 covers the factors.

Avoid correcting a raised tubercle to groove distance without knowing whether its cause is tubercle position or femoral version. 3.12.12 covers the index.

State the reference range alongside a version measurement, since thresholds depend on it.

Expect coronal correction to do less for this joint than for the tibiofemoral compartments. 3.5.6 covers the comparison.

Weigh the patient's age, physiology, comorbidities, knee demand, psychological state and their own ideas, concerns and expectations before offering a derotation osteotomy, which is a large operation for a geometric finding.