How common is leg length change after hip replacement

Concern about leg length is one of the questions patients most commonly raise before hip replacement — and it is a legitimate one. Leg length discrepancy (LLD) after total hip arthroplasty (THA) means the operated leg ends up measurably longer or shorter than the other side once the new joint is in place. Published incidence figures range from 1% to 27% of primary THA cases, a spread wide enough to cause understandable anxiety — but that range largely reflects differences in how researchers measure discrepancy rather than a uniformly high rate of clinically significant problems.

Real-world outcomes are more reassuring. In clinical series, around 60% of patients are within 0.5 cm of equal leg length after surgery, and only approximately 9% show a difference greater than 1 cm (Sarangi & Bannister, 1997). Where discrepancies do occur, reported magnitudes typically average 3–17 mm — meaningful in some cases, but far from the extreme end of the published range.

One further nuance is worth knowing early: patients consistently notice lengthening more acutely than shortening. A leg that feels longer after surgery does not always mean it is structurally longer — sensation and anatomy can diverge, a distinction that becomes important when understanding why symptoms sometimes appear without any true change in bone length.

Because LLD carries both functional consequences and medico-legal sensitivity, it is an area that modern surgical planning — including the choice of operative technique — addresses directly and specifically.

What actually causes leg length discrepancy after hip arthroplasty

Two quite different mechanisms sit beneath the single label 'leg length discrepancy', and conflating them leads patients down the wrong management path.

True (structural) LLD means the operated leg is physically longer or shorter than the other side — a measurable difference in bone length caused by component malpositioning, an implant sized slightly large or small, or a deliberate intraoperative decision to accept a few millimetres of extra length. That last scenario is worth dwelling on: surgeons sometimes consciously choose a slightly larger implant or allow modest soft-tissue tension to achieve a stable joint. In a hip where dislocation risk is elevated, gaining stability by accepting a small length difference is a recognised clinical trade-off, not a surgical error — and it is one that should be explained to patients before they go to theatre.

Apparent (functional) LLD is arguably more prevalent, and it is real — not imagined. Think of the difference between a table with one short leg and the same table placed on an uneven floor: the wobble looks identical, but the fix is completely different. In apparent LLD, structural leg lengths are equal, yet pelvic tilt, hip abductor tightness, or lumbar stiffness creates a convincing sensation of unevenness. Pre-existing severe osteoarthritis compounds this picture: years of cartilage and bone loss may have already shortened the arthritic limb before surgery, so once normal anatomy is restored, the operated leg can feel comparatively long — even though it is, structurally, exactly where it should be.

Understanding which category applies to a given patient is clinically essential: true structural LLD and apparent functional LLD follow entirely different management pathways.

How soft-tissue preservation reduces leg length discrepancy risk

The approach a surgeon takes to access the hip joint directly influences how accurately leg length can be controlled during the operation — and this is where technique choice becomes clinically meaningful for LLD risk.

In standard posterior approaches, and in many lateral techniques, the short external rotator tendons — piriformis, obturator internus, and related structures — are divided to reach the joint. Once cut, those tendons no longer exert their natural tension, which removes a key intraoperative calibration tool: the surgeon loses direct tactile feedback about how the reconstructed leg length and femoral offset compare with the pre-operative anatomy. Anterior and direct anterior approaches (DAA), as well as SuperPATH, handle soft tissues differently and each carries its own trade-offs around nerve exposure, patient anatomy, and equipment requirements — but most require varying degrees of soft-tissue release, and none preserves the full posterior tendon complex intact throughout the procedure.

SPAIRE (Saves Piriformis And (Obturator) Internus with Repair of (Obturator) Externus) — the muscle-sparing posterior approach that forms the basis of Professor Paul Lee's routine practice in hip replacement — leaves these tendons completely intact. Because the posterior capsule and short external rotators remain under their natural physiological tension throughout implantation, the surgeon retains a reliable anatomical reference: the resistance felt through intact tissue indicates, in real time, when leg length and femoral offset are appropriate. Cut the tendons and that reference disappears.

The intact obturator internus tendon also creates what is termed a 'strap effect' — a passive mechanical tether directly over the posterior femoral head. This provides joint stability without the surgeon needing to achieve it through additional leg lengthening, which is precisely the situation in which small but unwanted structural LLD tends to arise. Furthermore, the tendons' own nerve endings resume normal feedback immediately after surgery, rather than waiting the months it typically takes for divided and repaired tissue to re-establish its neural connections — a distinction that supports earlier active protection of the new joint.

When SPAIRE is combined with Mako robotic-arm assistance, which delivers sub-millimetre precision for bone resection and implant seating, the two tools are complementary rather than interchangeable: soft-tissue preservation addresses the surgeon's intraoperative calibration reference and mechanical stability; robotic guidance addresses the accuracy of bone cuts. No head-to-head randomised trial currently compares SPAIRE directly with other approaches on LLD rates specifically, but the mechanical rationale for preserving posterior soft-tissue tension is well-established in the literature.

Symptoms of leg length discrepancy and when they are clinically significant

A noticeable limp or the sensation that one leg sits lower than the other is the most common way patients first register a discrepancy after hip replacement. Other symptoms include low back or hip pain on the shorter-feeling side, altered balance on uneven surfaces, and — where discrepancy is large enough to place the sciatic nerve under traction — numbness, tingling, or a dull ache running down the back of the thigh and into the calf.

Some degree of asymmetry in the first four to six weeks after surgery is expected and does not confirm a structural measurement problem. Swelling, protective muscle guarding, and altered posture all generate an uneven sensation that tends to settle as the hip begins to load normally. Lengthening is consistently more noticeable to patients than shortening, so a feeling that the operated leg is 'too long' in early recovery is common and usually reflects functional rather than structural change.

In clinical terms, a structural discrepancy of around 1 cm or more is generally the threshold at which formal assessment and active management become appropriate; most patients with smaller differences adapt without intervention. Those with pre-existing lumbar stiffness or pelvic obliquity may perceive even smaller differences as significant — their pre-operative baseline matters considerably when interpreting any post-operative comparison, because what feels like a new problem may partly reflect a pre-existing imbalance now experienced differently.

Nerve symptoms — tingling, numbness, or weakness in the leg — that have not improved by three months warrant clinical review. Prolonged sciatic traction from uncorrected structural LLD is the most serious potential consequence of untreated discrepancy and should not be attributed to normal post-operative adaptation without assessment. The management pathways for each scenario are outlined in the following section.

Recovery from leg length discrepancy: what most patients can expect

For the great majority of patients, the outlook is reassuring. Functional discrepancy — which accounts for most post-operative unevenness — resolves naturally within 6–12 months as the pelvis re-levels and the hip abductors adapt to their new mechanical environment. No intervention beyond time and guided rehabilitation is needed in these cases.

The management pathway follows a clear sequence. Physiotherapy is first-line: a programme targeting hip abductor strengthening, hip flexor flexibility, and pelvic alignment exercises addresses the muscle imbalances that sustain functional discrepancy. Gait retraining helps patients unlearn the protective patterns adopted during the arthritic phase before surgery, which can themselves perpetuate an uneven stride long after the hip has been corrected.

Shoe inserts or heel raises are a practical adjunct for patients with a persisting structural difference once the pelvis has had time to settle — typically from around six to eight weeks post-operatively. They are simple, immediately reversible, and effective for discrepancies that are real but modest in magnitude.

Revision surgery is genuinely uncommon in this context. It is reserved for severe structural cases — usually discrepancies large enough to cause major functional impairment, persistent nerve symptoms, or intractable pain — that have not responded to a full course of conservative management. The vast majority of patients with minor to moderate LLD are managed successfully without returning to theatre.

Understanding this trajectory before surgery matters. Patients who know that early unevenness is expected and usually transient are better placed to engage with physiotherapy rather than interpreting normal adaptation as a surgical failure.

What to discuss at your hip replacement assessment

Before any hip replacement, a structured pre-operative conversation about LLD risk is more useful than a blanket reassurance. Patients with a meaningful pre-existing leg length difference, significant cartilage loss on one side, or established pelvic obliquity carry a higher baseline risk — and benefit from specific planning, not generic reassurance.

Concrete questions are worth preparing. How will the surgeon judge leg length intraoperatively? What anatomical reference does the technique preserve throughout? Is robotic assistance part of the plan, and how does it complement — rather than substitute for — the surgical approach? For patients who already notice some discrepancy before surgery, asking whether any planned correction is structural or functional shapes expectations on both sides of the operating table.

Surgical approach, including how the short external rotators are handled, is a legitimate topic for this discussion. The SPAIRE technique — which informs the clinical perspective in this article, reflecting Professor Paul Lee's specialist practice — preserves posterior tendon tension as a real-time intraoperative reference for leg length and offset. Hip Replacement Lincolnshire, part of the MSK Doctors group, accepts patients without GP referral and offers consultant-led assessment covering approach choice, implant planning, and individual LLD risk factors; visit hipreplacementlincolnshire.co.uk.

A patient who arrives knowing the difference between true and functional LLD, and understanding what drives each, is better placed to ask the right questions — and to interpret the answers honestly, wherever they choose to seek care.