Two different things with one name.
This is the distinction that explains almost every conversation about leg length.
- True length difference. The bones are actually different lengths, or the hip joint has collapsed, worn or been reconstructed in a way that changes where the leg sits. It is measurable on a radiograph and with a tape.
- Apparent length difference. The bones are the same length, but the pelvis is tilted or the hip is held in a fixed position, so the leg behaves as though it were longer or shorter. Common with a stiff arthritic hip, with a fixed contracture, and with spinal curvature.
Both feel the same to the person experiencing them. Only one is corrected by lengthening or shortening anything.
Before a hip replacement.
An arthritic hip usually shortens. Cartilage is lost, bone wears away at the top of the socket or the head of the femur, and the leg sits a little higher. Many people arrive at surgery with a leg that has been getting shorter for years without them noticing, because the change was gradual and the pelvis compensated.
That is why some people feel longer after a hip replacement even when both legs measure equal. The operation restored length that had been lost, the body had adapted to the shortening, and it now has to adapt back.
How length is planned and controlled.
Leg length is not left to chance. It is:
- Templated before surgery on a calibrated radiograph, planning the level of the femoral neck cut and the position of the socket.
- Measured during the operation, against a reference point, and checked at trial reduction before the final components go in.
- Confirmed on a radiograph taken on the table where the anterior approach is used, because the patient is supine and both hips are directly comparable. This is one of the practical advantages of that approach.
Length is also traded against stability. A hip that is left slightly short may be less stable, so the plan balances the two rather than optimising length alone. Where the two conflict, stability generally wins, and you will be told if that applies to you.
After a hip replacement.
Differences of a few millimetres are common and usually not noticed. A difference large enough to need a shoe raise is uncommon.
What is common is the perception of a difference in the first weeks and months. Causes include pelvic tilt that has not yet corrected, tightness in the muscles on the outside of the hip, and restored length the body has not readjusted to. Most of this settles within three to six months as the soft tissues accommodate.
For that reason a shoe raise is not usually fitted early. Fitting one to a perceived difference that is going to resolve creates a problem rather than solving one. If a genuine difference persists past about three months, it is measured properly and a raise fitted if warranted.
Common questions.
Why does my leg feel longer after a hip replacement?
Usually because it is longer than it was. An arthritic hip shortens over years, and the replacement restores the length that was lost. Your pelvis and spine had adapted to the shorter leg and now have to adapt back. It commonly settles over three to six months.
Will my legs be exactly the same length?
Length is templated before surgery, measured during it, and with an anterior approach confirmed on a radiograph taken on the table. Differences of a few millimetres are common and usually unnoticed. A difference large enough to need a shoe raise is uncommon.
Should I get a shoe raise straight away?
Generally not. Most perceived differences in the early weeks are pelvic tilt and muscle tightness rather than true length, and they settle. Fitting a raise to a difference that resolves creates a new problem. If it persists past about three months, it is measured and addressed.
Can my leg be made shorter?
Length is balanced against stability. Deliberately shortening a hip to match the other side can make it less stable, so the plan weighs both. Where they conflict, stability generally takes priority and you will be told.
Is a small difference a problem?
Usually not. Small differences are extremely common in the general population, including in people who have never had surgery, and most are asymptomatic.
References.
- Desai AS, Dramis A, Board TN. Leg length discrepancy after total hip arthroplasty: a review of literature. Curr Rev Musculoskelet Med. 2013;6(4):336-341.
- Australian Orthopaedic Association National Joint Replacement Registry. Annual Report. Adelaide: AOA.
This page provides general information about a medical condition and a surgical procedure. It is not individual medical advice and does not replace consultation with a qualified practitioner. Outcomes vary between individuals and all surgery carries risk. A referral is needed for a consultation with Professor Daevyd Rodda; without one, you can start with the practice nurse practitioner, who can provide it.