Bow Legs and Knock Knees: Varus and Valgus Deformity of the Knee

Most people notice the shape of their legs long before anyone gives it a name. Bow-legged is a varus deformity. Knock-kneed is a valgus deformity. Both describe where the knee sits relative to a straight line from your hip to your ankle.

This page is about adults. Bowing in toddlers is common, usually normal, and generally resolves on its own — a different subject entirely.

Varus and valgus, plainly

The naming describes where the lower leg and foot end up relative to the midline of the body:

That load shift is the whole reason alignment matters. Where the weight goes is where the cartilage wears.

Bow legs (varus)

This is by far the more common of the two. Most people carry a small amount of varus naturally — it is not in itself abnormal.

It can arise two ways, and they are not mutually exclusive:

When wear is genuinely confined to that one compartment, a partial knee replacement may be an option — candidate selection is what makes that operation succeed.

The varus thrust

This is worth knowing about because it is something you or your family may actually be able to see.

A varus thrust is a visible shift of the knee outward at the moment your weight goes through it while walking — the knee appears to kick out sideways with each step, then recover.

It suggests that the inner side of the joint has worn down and the structures on the outer side have stretched, so the knee is no longer held steady when loaded. Mention it if you have noticed it. It is a meaningful finding, not a cosmetic one, and it changes how a knee is assessed.

Knock knees (valgus)

Less common, and it tends to keep different company. Valgus is more often associated with:

If looseness is a feature across several of your joints rather than just this one, that is worth raising — it is covered in knee giving way.

Why valgus behaves differently

Three things make a knock-kneed knee a different proposition from a bow-legged one, and they matter both for how it progresses and for how it is corrected.

It tends to advance faster

Valgus deformity is generally the more aggressive of the two — it tends to progress more rapidly once established.

It changes the ligaments asymmetrically

As the knee angles inward, the MCL on the inner side is stretched out while the LCL on the outer side tightens. That imbalance has to be accounted for when the knee is corrected — simply straightening the bone is not the whole job.

Correction carries a nerve risk

The peroneal nerve runs around the outer side of the knee, and it is the nerve responsible for lifting the foot. Correcting a valgus deformity puts tension on that side of the knee, so there is a higher risk of peroneal nerve injury than with a varus correction.

How alignment is actually assessed

Alignment cannot be judged properly from a knee X-ray taken lying down. Assessing deformity means seeing the leg standing and loaded, and where the deformity is significant that means imaging the whole limb from hip to ankle, not just the knee.

The examination matters as much as the film — how the knee behaves when you walk, whether there is a thrust, and whether the deformity corrects when it is not bearing weight all tell us something the picture alone does not.

Correcting it: replacement versus osteotomy

There are two ways to straighten a deformed knee, and I have a clear preference.

Osteotomy

An osteotomy cuts and realigns the bone to shift weight off the worn side. It is a genuine joint-preservation procedure and it has a real place — for carefully selected younger patients with significant malalignment and cartilage still worth preserving.

But it comes with things that are worth knowing:

Correcting it during the replacement

Where arthritis is already established, the majority of deformity can be corrected through the joint at the time of a knee replacement.

That is a simpler operation. No waiting for bone to heal, no nonunion risk, and — most importantly — one procedure that corrects everything rather than a realignment now and a replacement in several years.

Does it always need correcting?

No — and I want to be clear about that, because appearance alone is not a reason to operate. Plenty of people live with a degree of bow-leggedness perfectly well.

Deformity becomes relevant when it is:

Even at the time of a replacement, the goal is not always a perfectly neutral leg. I use a hybrid approach — mechanical alignment as the framework, with adjustment toward what your knee actually was — and where a knee sits in significant varus I may accept some of that rather than force it fully neutral, depending on how the gaps measure and how the soft tissues tension. That reasoning is set out in how your knee replacement implant is chosen.

Correcting alignment is one of the goals of a knee replacement, and it is one of the things patients notice most — often in how they stand and walk as much as in the relief of pain. If the shape of your legs has been changing, or someone has told you your knee kicks out when you walk, that is worth having looked at. When it becomes part of a surgical conversation is covered in signs you may need a knee replacement.