What leg length discrepancy feels like after hip replacement

After a hip replacement, some patients notice that one leg feels slightly longer or shorter than the other. It might show up as a need to put a heel raise in one shoe, a sense that the pelvis tilts when standing upright, or an altered walking pattern — a subtle hitch or lean that wasn't there before surgery. Others describe a nagging discomfort in the lower back or the operated hip that doesn't seem to fit the expected recovery picture. These are the hallmarks of leg length discrepancy (LLD): a recognised postoperative complication in which the two limbs are no longer equal, either in actual bone-and-implant geometry or in how the body perceives them.

There are two distinct types. Structural LLD reflects a genuine difference in limb length created by how the implant components sit within the body. Functional LLD arises instead from soft-tissue tension, pelvic tilt, or muscle imbalance — the limbs may measure the same, yet the patient experiences inequality because the body is compensating. Both types can produce similar symptoms; distinguishing between them matters because management differs.

Mild inequality — broadly under 10 mm — is relatively common after hip arthroplasty and frequently goes unnoticed. Many patients find that as muscles re-balance and recover over the months following surgery, any perceived discrepancy settles without intervention. Where the difference exceeds roughly 10–15 mm, however, the effect on gait and daily comfort becomes more meaningful, and patient-reported satisfaction after hip replacement correlates strongly with how accurately limb length has been restored. LLD is among the most commonly cited sources of postoperative complaint.

Persistent or worsening inequality — particularly if accompanied by a changed walking pattern or ongoing pain — warrants a clinical review rather than indefinite waiting. Raising the concern early gives the treating team the clearest picture of whether the issue is likely to self-correct or needs further assessment.

The surgical variables that create leg length inequality

Four interlocking variables determine final limb length during hip arthroplasty, and surgeons must control all of them simultaneously — errors in any one compound the others.

The first is femoral offset: the sideways reach of the femoral head from the shaft, measured as a horizontal distance between the femoral head centre and the long axis of the femur. Offset governs how taut the abductor muscles are held. Under-restore it and the limb shortens while the abductors lose tension, producing a Trendelenburg-type limp; over-restore it and the limb lengthens, stressing the joint and soft tissues in the opposite direction.

The second variable is cup height — how high or low the acetabular component is seated in the pelvis. This sets the fixed reference point from which every length calculation flows. A cup placed even a few millimetres too high repositions the hip's centre of rotation upward and, in effect, lengthens the limb before the femoral side is even addressed.

Stem seating depth and implant sizing form the third layer. A stem driven too proud, or a neck option one size longer than planned, adds length; a deeply seated stem or a shorter neck option subtracts it. Think of these components as stackable blocks — each decision nudges the final height up or down.

Finally, pre-existing pelvic tilt or obliquity can mask or amplify inequality that is already present before surgery. A pelvis that tilts in compensation for a stiff or arthritic hip may make the limbs appear equal on a standing X-ray when they are not — or unequal when they are. Identifying this pattern at the templating stage is essential; attempting to correct it at the operating table misreads the problem.

Because each variable interacts with the others, LLD is rarely the product of a single misjudgement. Accurate restoration depends on controlling all four together — which is why both careful preoperative planning and precise intraoperative technique matter.

How preoperative templating sets the foundation for equal legs

Before any incision is made, the surgical team works through a detailed planning exercise on calibrated radiographs — a process known as digital templating. Using scaled images of the patient's pelvis and femur, the surgeon maps out exactly where each component will sit: the acetabular cup height, femoral stem size, neck length, and the resulting limb length and femoral offset. This pre-surgical mapping is, by a considerable margin, one of the strongest predictors of postoperative accuracy.

One of templating's less obvious but clinically important functions is accounting for pre-existing pelvic obliquity. As s2 noted, a tilted pelvis can make limbs appear equal on a standing X-ray when they are not. Templating performed on correctly calibrated images allows the surgeon to identify this distortion and set a target that corrects for it — rather than blindly reproducing whatever asymmetry the arthritic hip has introduced.

The plan produced at this stage becomes the intraoperative reference. The surgical team checks component placement against templated predictions in real time; without that baseline, visual and tactile assessment alone is vulnerable to systematic drift, particularly in complex anatomy.

Templating does not, however, remove the need for skilled execution. It defines the intended destination; whether the approach and measurement technique can reliably reach it is a separate question — one the following section addresses.

Intraoperative measurement and what robotic assistance adds

Even the most thorough preoperative plan cannot fully anticipate what happens once the patient is on the table. Anaesthesia relaxes muscle tone, the pelvis shifts as it settles against the surgical supports, and the displacement of soft tissue during exposure changes the mechanical relationships that templating was based on. These intraoperative forces mean that a perfectly calculated plan can drift by several millimetres before the first component is even trialled.

Surgeons counter this with a layered set of intraoperative checks: established bony landmarks (the lesser trochanter, greater trochanter, and acetabular landmarks) give fixed reference points; trial reductions with provisional components allow direct assessment of limb length before any final implant is committed; and careful tensioning of the surrounding soft tissue provides a dynamic sense of whether the reconstructed hip sits in balance.

Robotic-arm assisted technology — the Mako system being a widely used example — adds an objective, real-time dimension to this process. By tracking bone resection and implant positioning to sub-millimetre accuracy against the preoperative plan, it functions as a continuous cross-check on surgical judgement rather than a replacement for it. Importantly, robotic assistance is not tied to any single approach: it complements both standard posterior techniques and muscle-sparing methods such as SPAIRE with equal effect.

Taken together, preoperative templating and intraoperative robotics form the measurement framework within which the operation is executed. What the chosen surgical approach then contributes — specifically, what tissue it preserves — adds a further, distinct layer of accuracy that templating and robotics alone cannot replicate.

How preserved soft tissue in SPAIRE guides femoral offset

Retaining the piriformis and obturator internus tendons intact throughout the procedure gives the operating surgeon something that templating and robotics cannot fully replicate: a living anatomical reference that registers, in real time, whether femoral offset has been accurately restored.

In a standard posterior approach, those tendons are released to gain access to the joint. The physiological tension they carry — the intrinsic pull reflecting the native relationship between the femoral head and the acetabulum — disappears the moment they are cut. The surgeon then reconstructs offset by reference to templated measurements and bony landmarks alone; the tissue-level signal that would have confirmed or contradicted those measurements is no longer present.

SPAIRE keeps those attachments continuous throughout implantation. As trial components are placed and reduced, the preserved soft tissue transmits resistance proportional to how far the reconstructed position deviates from the patient's anatomical baseline. The intact tendons act as a plumb line fixed in place: the surgeon can feel the deviation before committing to final fixation, rather than inferring it from external measurements after the fact. That tactile layer operates independently of templating and robotics — it does not replace them but cross-checks them from a different direction.

The obturator internus, which in SPAIRE passes directly over the posterior femoral head at or slightly below the true centre of rotation, simultaneously generates what surgeons describe as a 'strap effect': a biological tether that resists posterior subluxation and contributes to the technique's posterior stability. Offset guidance and dislocation resistance arise from the same anatomical decision.

Lateral and anterior approaches preserve different tissue planes and do not share this specific posterior-tendon feedback mechanism, so the comparison is most meaningful against standard posterior technique. Published data on SPAIRE-specific LLD rates relative to that standard currently rests on mechanistic evidence and surgical case series rather than head-to-head randomised trial data — a reflection of where this relatively recent technique sits in its research cycle. The clinical rationale informing the approach as practised here, including Professor Paul Lee's application of it, draws on that evidence base alongside outcome data from centres including the Exeter Hip Unit.

Getting a pre-surgical assessment for leg length concerns

Patients who already carry a measurable limb length difference before surgery — through a previous hip procedure, longstanding pelvic obliquity, or progressive joint disease — benefit most from early specialist assessment. An accurate baseline established before templating begins gives the surgical plan a fixed reference point; templating against the wrong starting position compounds, rather than corrects, any pre-existing inequality.

Suitability for SPAIRE is assessed individually. Anatomy, body habitus, prior surgical history, and the degree of soft-tissue change all influence whether a tendon-sparing posterior approach is appropriate. For some patients, an anterior, lateral, or standard posterior approach may be a better anatomical fit. That judgement belongs to a surgeon experienced across the full range of options rather than to any single-technique preference, and SPAIRE carries the same family of procedural risks as other approaches — including residual limb length inequality — so realistic expectations are part of the conversation from the outset.

A consultant-led pre-surgical assessment typically includes clinical examination, calibrated weight-bearing radiographs, and an open discussion of approach options — SPAIRE, standard posterior, lateral, and anterior — with their respective tradeoffs for stability, nerve risk, and recovery. Specialist hip replacement assessment is available without GP referral at Hip Replacement Lincolnshire, part of the MSK Doctors group.

Three questions are worth raising directly at any such consultation: What is my current leg length difference, and how was it measured? What offset and limb-length target are you planning for the reconstruction? And which surgical approach suits my anatomy, and what are the tradeoffs?