What limb length discrepancy actually means after hip arthroplasty

'One leg feels longer than the other' is among the most unsettling things a patient can notice after hip replacement. The clinical term is limb length discrepancy (LLD), and it takes two quite different forms. True, or structural, LLD occurs when component malposition alters the actual bony length of the leg — for example, a femoral stem seated too deeply or a cup placed too high. Functional LLD, by contrast, involves no real change in bone length; instead, pelvic tilt, residual muscle imbalance, or an altered gait pattern creates the appearance of unequal legs. Each type calls for a different management path: structural LLD that persists and causes significant disability may ultimately require revision surgery, whereas functional LLD often improves as muscles recover and gait normalises in the months after the operation.

Published prevalence figures span a wide range — somewhere between 3% and 30% of total hip arthroplasty patients, depending on how LLD is measured and what threshold is used to define it. Real-world outcomes put this in perspective: roughly 60% of patients achieve leg-length equality within 0.5 cm, and only around 9% are left with a discrepancy exceeding 1 cm. Even perceived LLD resolves spontaneously in the majority of cases within a year.

None of this is merely a matter of patient comfort. LLD ranks among the most common reasons for litigation against orthopaedic surgeons after hip replacement, which is why reducing its incidence is both a clinical and a practical imperative.

Consequences patients notice — and why they persist

Walking out of rehabilitation with what feels like one short leg is disconcerting in a way that is difficult to dismiss. Patients typically notice a rhythmic hitch in their stride, a tendency to lean to one side, or a nagging ache across the lower back and pelvis that was not part of the original hip problem. These are the predictable downstream effects of a leg-length mismatch: the pelvis tilts to accommodate the shorter limb, the lumbar spine compensates with a lateral curve, and the gait pattern reorganises around the asymmetry rather than correcting it.

The reassuring finding — from a 2025 prospective cohort — is that 49% of patients reported a perceived length difference after surgery, yet 73% of those cases resolved completely within one year, without any further intervention. The likely explanation is soft-tissue adaptation: as recovering muscles regain tone and proprioceptive signalling from the hip recalibrates, the brain's perception of limb symmetry gradually normalises. Most patients who experience this kind of perceived discrepancy early on should expect it to improve substantially over the first several months.

Even so, precision surgery reduces rather than eliminates the risk. A meaningful minority of patients will continue to perceive a difference despite technically sound implant placement — a reminder that proprioceptive and psychological factors shape how length is experienced, quite separately from what a radiograph measures. For those in whom concern persists, the nature of the underlying cause — whether structural or functional — determines what kind of help is likely to be worthwhile.

How the surgical approach affects intraoperative leg-length accuracy

The choice of surgical approach shapes how accurately a surgeon can judge leg length while the operation is under way — and a 2025 comparative study of 358 patients provides the clearest published evidence of that effect. Patients who received hip replacement via the direct anterior approach (DAA) ended up with a mean LLD of 3.0 mm, compared with 4.2 mm in those treated through a posterolateral approach (p = 0.027). The DAA group also reported less perceived length difference and achieved better Harris Hip Scores at six weeks.

The mechanical explanation centres on two things: patient position and soft-tissue tension. The direct anterior approach is performed with the patient lying flat (supine). In this position, gravity acts symmetrically on both sides of the pelvis, and the soft-tissue envelope around the hip remains largely intact — muscles and capsule on both the front and back of the joint continue to exert balanced tension. That tension gives the operating surgeon a living, dynamic reference point. When the trial components are seated and the hip reduced, the resistance and symmetry of the soft tissues communicate, in real time, whether the leg length and offset match the pre-resection state.

Posterolateral approaches place the patient on their side (lateral decubitus). In that position, the weight of the pelvis shifts relative to the floor and bony landmarks — the greater trochanter, the ischium, the iliac crest — no longer sit in a neutral plane. Small but meaningful errors in apparent limb length become harder to detect by landmark measurement alone. When the short external rotators are also divided to gain access, the last remaining source of dynamic soft-tissue feedback is removed: the surgeon is left with geometric checks — calipers, pin measurements, fluoroscopic comparisons — but no tensioned tissue to confirm that what has been measured is functionally equivalent to what was there before.

Muscle-sparing posterior approaches aim to recover that tensioned feedback even when supine positioning is not used — a distinction explored in the next section.

How SPAIRE preserves the feedback that guides length and offset judgement

SPAIRE — the muscle-sparing posterior approach developed and refined by Professor Paul Lee — addresses that loss of functional reference directly, by not creating it in the first place. Where conventional posterior approaches divide the piriformis and obturator internus to expose the joint, SPAIRE leaves both tendons and the native posterior capsule intact throughout the procedure.

Those two tendons are the short external rotators that run behind the hip joint, stabilising the femoral head in the acetabulum. Keeping them attached means the posterior soft-tissue envelope continues to exert its normal physiological tension from the first incision to the last. When trial components are seated and the hip reduced, the surgeon can feel whether the tissue tension matches the pre-resection state — not just whether a caliper reading corresponds to a preoperative template. That distinction matters: bony measurements confirm geometry; preserved soft-tissue tension confirms functional equivalence.

The obturator internus contributes a second, more specific advantage. Its tendon passes directly over the back of the femoral head, creating what is sometimes described as a 'strap effect' — a dynamic biological tether that tightens predictably when the hip is loaded in a reduced position. If leg length or femoral offset is slightly out, that strap does not sit quite right, and the surgeon registers the discrepancy before the wound is closed.

At the cellular level, Golgi tendon organs and muscle spindles within the intact short external rotators provide continuous afferent signalling to the central nervous system. Conventional approaches sever these mechanoreceptors at the moment of incision, leaving the hip neurologically blind to positional feedback until soft-tissue healing occurs, weeks later. SPAIRE preserves that proprioceptive continuity from implantation onwards.

The honest caveat is that this mechanistic explanation, while anatomically well-grounded, currently rests on case-series data and anatomical reasoning rather than a large prospective randomised trial with LLD as its primary endpoint. Published head-to-head comparison of SPAIRE against conventional posterior approach on this specific outcome has yet to appear in the literature.

Robotic assistance alongside muscle-sparing technique

Precision technology and soft-tissue preservation answer different questions — and a striking finding from a 2025 multicentre study illustrates why that distinction matters.

Robotic-arm assistance such as the Mako system improves the measurable accuracy of implant placement: bone resection can be executed to sub-millimetre tolerance, and cup anteversion and inclination are better controlled than in conventional manual technique. That geometric precision translates into smaller measured LLD. Yet when the same 2025 study followed patients for three years and asked them whether they perceived a leg-length difference, the robotic and conventional groups did not differ significantly. The stronger predictors of perceived discrepancy were preoperative LLD and functional hip scores — factors rooted in proprioception and neuromuscular adaptation, not implant geometry.

The implication is straightforward: technology addresses the skeleton; soft-tissue preservation addresses the nervous system. Measured LLD and felt LLD are related but not identical, and closing the gap between them requires both. Navigation data from revision surgery reinforces the adjunctive role of technology — navigated revision cases averaged 3.7 mm postoperative LLD versus 4.9 mm in non-navigated cases — but navigation still cannot restore the mechanoreceptor feedback that a severed tendon no longer provides.

SPAIRE, as practised at this clinic, integrates Mako robotic assistance as standard. The result is precision implant placement executed within a neurologically intact soft-tissue environment: the robot confirms the geometry; the preserved piriformis and obturator internus confirm the function. Each component addresses a source of LLD that the other cannot reach.

Patient suitability and what to expect from assessment

SPAIRE is a specialist technique — not the automatic choice for every patient who needs hip replacement. Body habitus, hip anatomy, and prior surgical history all influence whether the approach is appropriate. Patients with significant femoral deformity, severe obesity, or a previous posterior hip operation are typically assessed for alternative techniques, and an experienced surgeon will adjust the plan accordingly rather than apply a single exposure to every case.

For any patient approaching hip replacement with concern about leg-length accuracy, three questions are worth putting directly to the surgical team: which approach is planned, and how does it support intraoperative length control? How will leg length and femoral offset be monitored during the procedure — by caliper, navigation, or robotic guidance? And what is the realistic range of residual discrepancy for this anatomy and any preoperative difference?

Those questions are not confrontational — they are precisely what a pre-operative assessment should address, alongside formal leg-length and offset templating and objective gait analysis to quantify any existing functional discrepancy before the operation begins.

Professor Paul Lee, whose clinical approach informs the SPAIRE-specific content on this site, is available for consultant-led assessment through Hip Replacement Lincolnshire — part of the MSK Doctors group — at Sleaford (NG34) and Grantham (NG31). No GP referral is required, and appointments can be arranged at hipreplacementlincolnshire.co.uk. The right surgical approach for any individual patient is something that emerges from that clinical conversation, not from a preoperative assumption — which is precisely why the assessment matters.

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2. [2] Can robot-assisted total hip arthroplasty improve the incidence of self-reported leg length discrepancy? A Multicenter, Propensity Score Matching Comparative Study.. (2025). https://doi.org/10.1016/j.arth.2025.02.056 https://doi.org/10.1016/j.arth.2025.02.056
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4. [4] Leg-length discrepancy in revision total hip arthroplasty. (2025). https://doi.org/10.1302/2633-1462.68.BJO-2024-0149.R3 https://doi.org/10.1302/2633-1462.68.BJO-2024-0149.R3
5. [5] Comparison of leg length discrepancy after total hip arthroplasty: Direct anterior and posterior lateral approach. (2025). https://doi.org/10.1371/journal.pone.0318953 https://doi.org/10.1371/journal.pone.0318953
6. [6] Piriformis-Sparing vs. Conventional Posterior Approach in Total Hip Arthroplasty: A Retrospective Analysis of the Functional Outcomes. (2025). https://doi.org/10.3390/medicina61040609 https://doi.org/10.3390/medicina61040609