THA Leg Length Discrepancy

Leg length discrepancy (LLD) after total hip arthroplasty is a leading source of patient dissatisfaction and of negligence complaints, despite being a recognised and often unavoidable consequence of the procedure. Even with modern techniques, some degree of LLD is almost universal.

The litigation claim. "The commonest cause of litigation after THA" is widely repeated but rests on small medicolegal case-series rather than population data. Say a leading source, and let the documentation argument carry the weight.

Why it matters. LLD affects gait mechanics, patient satisfaction and medicolegal risk. Small discrepancies, under 10mm, are usually well tolerated, but the patient's perception often exceeds the measured LLD. The key is documentation: proof of preoperative assessment and intraoperative diligence protects against litigation even if LLD occurs.

How often. With careful technique most primary THAs achieve a residual LLD under 10mm. Larger discrepancies, over 15mm, are less common but cause functional gait changes, and revision and dysplasia carry higher rates of significant LLD. Up to 10mm of natural LLD exists in 60-95% of the population (O'Brien 2010).

Risk factors.

• Developmental dysplasia, with its high hip centre
• Severe deformity (Crowe III-IV DDH)
• Revision surgery: bone loss and soft-tissue laxity
• Inadequate templating, the most preventable cause

The patient. Some patients tolerate LLD better than others. High-demand patients, fixed spinal deformity and bilateral disease require extra caution, so baseline gait and spine pathology are documented before surgery.

Perception. O'Brien imposed discrepancies on volunteers: about 97% perceived a 10mm discrepancy, and no subject perceived a 5mm increase as uncomfortable. Beyond the millimetres, psychological factors influence perception, and body image and expectations play a major role.

The medicolegal picture. Whittingham-Jones (2012) found uncemented stems less forgiving of LLD and emphasised preoperative planning where the anatomy is abnormal. Documentation is the defence, because it proves due diligence.

True and apparent length. True leg length runs from the ASIS to the medial malleolus. Apparent length runs from the umbilicus to the medial malleolus and reflects pelvic obliquity. THA aims to restore true length while accounting for fixed pelvic tilt and spinal deformity.

Femoral offset. The perpendicular distance from the centre of rotation of the femoral head to the long axis of the femur. Restoring it is essential, because inadequate offset forces the surgeon to lengthen the leg to achieve stability and abductor tension. Restored offset:

• Restores the abductor moment arm, reducing limp
• Gives stability without excessive lengthening
• Improves range of motion
• Normalises gait efficiency

Offset errors. Under-restoring offset leads to compensatory lengthening for stability; over-restoring it risks impingement. An offset error of 5mm can translate to 10mm of LLD. Template offset first, then adjust length, and measure both length and offset intraoperatively.

The acetabular side. Teaching on length restoration concentrates on the femur, but the acetabular component is an equal determinant of both length and offset, as the Weber trial's use of global offset makes explicit.

The hip centre. The vertical position of the cup sets the acetabular contribution to leg length. A cup placed superiorly, a high hip centre, shortens the limb relative to anatomic, and restoring the centre of rotation inferiorly to its anatomic level lengthens it. This is why a high, medialised cup tempts the surgeon into compensatory femoral overlengthening to regain stability.

Global offset. Global (total) offset is acetabular offset plus femoral offset. Acetabular offset is the horizontal distance from the cup centre of rotation to the pelvic (teardrop) reference. Medialising the cup reduces acetabular offset and therefore global offset, weakening the abductor lever arm exactly as an under-restored femoral offset does; restoring the anatomic centre of rotation preserves global offset and avoids the trap of lengthening the leg to chase abductor tension.

Soft-tissue tension. The soft-tissue envelope determines the acceptable change in length. Overlengthening stretches neurovascular structures; excessive shortening creates laxity and instability.

LLD is classified by its cause, present before surgery or created by it, and by whether the patient's perception matches the measurement.

History. Ask about:

• Baseline gait: did the patient limp before surgery, or use walking aids
• Shoe modifications: prior use of a shoe lift indicates tolerance
• Spine: fixed scoliosis or kyphosis, spinal fusion, low back pain
• Contralateral hip: arthritis or a prior THA affects the comparison
• Expectations: tolerance for minor LLD; some patients fixate on perfect symmetry
• Functional goals: high-level athletics versus basic mobility

Examination. Measure both sides identically, and look for the cause of any shortening: bone loss, joint destruction or soft-tissue contracture.

• Gait: Trendelenburg, limp, compensatory trunk lean
• True and apparent leg length, bilaterally
• Pelvic obliquity: fixed or compensatory
• Block test: blocks under the short leg until the pelvis is level
• Spine: fixed scoliosis or a compensatory curve

Measuring true length. The clinical protocol:

The patient who feels unequal. Not every patient who reports a "long" or "short" leg after THA has a true skeletal LLD. The task is to separate true (skeletal) LLD from functional (apparent) LLD and from the other causes of a gait that feels asymmetric.

True versus functional length. The surgeon equalises the true (skeletal) length at THA; the patient feels the functional length, which a fixed pelvic obliquity or a hip contracture can distort even when the skeletal lengths are equal. The apparent-length measurement and the check for fixed versus compensatory obliquity exist precisely to separate the two.

Where the obliquity comes from. The source decides whether THA, or a lift, can help:

• Suprapelvic: driven from above by a fixed structural scoliosis or sagittal imbalance. It persists after a perfectly balanced THA, so the patient may still perceive inequality; the spinopelvic and component-positioning consequences belong to the hip-spine relationship and spinopelvic-parameter topics.
• Infrapelvic: driven from below by a hip adduction or flexion contracture, or a knee or ankle deformity. A fixed adduction contracture makes the leg sit functionally short; an abduction contracture makes it sit functionally long. Releasing the contracture at THA corrects this functional component without changing skeletal length.
• Intrapelvic: asymmetry within the pelvic ring itself (post-traumatic, post-radiation, congenital).

Fixed or flexible. A flexible, compensatory obliquity corrects once the hip is balanced, so aim for true skeletal equality. A fixed obliquity can leave the patient feeling unequal even after full skeletal equalisation: document it, set expectations before surgery, and base any shoe-lift trial on the functional (block-test) length rather than the radiographic skeletal difference.

The imaging sequence.

• Standing AP pelvis: the gold standard for measuring LLD and the essential view for bilateral comparison. Weight-bearing on both legs, with the pelvis level (symmetrical obturator foramina); measure from the acetabular teardrops to the lesser trochanters on each side, and assess hip-centre height and pelvic obliquity.
• Lateral hip: for templating. Femoral offset (the perpendicular distance from the femoral axis to the centre of the head) and neck length, stem size and canal width for stem selection, the landmarks for the neck-cut level, and the anterior femoral bow, which affects how deep the stem seats.
• Full-length hip-to-ankle, if indicated: preoperative LLD over 2cm, DDH or bilateral disease. Mechanical axis and true leg length on calibrated images, compensatory knee or ankle changes and fixed deformities; plan gradual correction if the LLD is large.
• CT, optional: severe DDH, post-traumatic deformity or revision with bone loss. It shows bone stock and 3D anatomy for planning custom implants or osteotomy, and is required for robotic-assisted planning.

The inter-teardrop method. The most common method. Draw a horizontal line joining the medial acetabular teardrops, measure the vertical distance from it to the tip of each lesser trochanter, and take the difference. It is standardised and reproducible, independent of the acetabular component, not affected by pelvic tilt, used in most published studies and easy to measure on PACS. Its limits:

• Assumes the teardrops are symmetrical, which they may not be in DDH
• Teardrops are obscured in severe arthritis
• Small measurement errors are magnified
• Needs a good-quality AP pelvis

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The ischial method. Draw the line through the tips of the ischial tuberosities instead and measure to the lesser trochanters in the same way. It is less affected by acetabular asymmetry, so use it when the teardrops are obscured by severe arthritis, prior surgery or DDH, particularly in Crowe III-IV dysplasia where the acetabular anatomy is abnormal.

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Digital templating. PACS or templating software allows precise calibration and measurement, and is most accurate with a calibration marker (a 25mm ball or other known object). Templating improves the accuracy of restoring length and offset and is now standard of care; 3D CT-based planning predicts combined cup and stem size in 96% of cases against 16% for conventional 2D templating (Sariali 2012). The workflow:

1. Calibrate to a known object (femoral head diameter or marker ball), set the PACS scale, and verify it by measuring a known anatomic structure
2. Draw the reference line (teardrop or ischial) and measure to the lesser trochanters; the software calculates the difference in millimetres, accounting for magnification
3. Template the acetabulum first: position, size and inclination
4. Template the femur: stem size, neck length and offset
5. Measure the predicted postoperative length and offset, and document the targets: hip-centre height relative to the teardrops, femoral offset, neck length (stem shoulder to head centre) and the planned change in length

Prevention first. The best approach is prevention: careful planning and intraoperative measurement substantially reduce the incidence of clinically significant LLD. The planning, measurement and technique are set out in the sections that follow; the table below gives the acceptable LLD by scenario.

The established discrepancy. Once LLD is present, management is set by its magnitude and the patient's symptoms, after a sciatic palsy from overlengthening has been excluded.

Algorithm

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By magnitude. Most LLD under 20mm is managed non-operatively, with shoe modifications and physiotherapy.

The shoe lift. Most patients need one once the LLD is over 20mm; between 10 and 20mm it is the patient's preference. Under 10mm a lift often makes symptoms worse through altered biomechanics; the table offers one below that level only when the patient asks. Trial before a permanent prescription, and prescribe with care:

• Start low, at 50% of the measured LLD
• An external lift is easier to adjust, and to remove if uncomfortable
• An internal lift is more cosmetic but limited to 10mm
• A full-length insole is better than a heel lift alone

Physiotherapy. Gait training teaches energy-efficient patterns, core strengthening addresses a compensatory trunk lean, hip abductor exercises restore normal biomechanics, and flexibility work addresses soft-tissue contractures.

Revision. The last resort, with high risks and uncertain outcomes, and rarely indicated for isolated LLD.

• Absolute, urgent: sciatic nerve palsy from overlengthening. Decompress and revise to shorten immediately; the risk of permanent foot drop increases with delay.
• Relative, after 6-12 months: LLD over 20mm with failed conservative management, documented functional impairment (gait analysis, energy expenditure), component malposition contributing to the LLD, patient demand, and a patient fully informed of the risks and benefits.
• Do not revise: LLD under 15mm, conservative management not attempted, perception exceeding the measured LLD (a psychological issue), comorbidities that prohibit surgery, or litigation pending (wait for its resolution).

Planning a revision. Identify the cause first: cup position, femoral offset, neck-cut level or stem depth. Template the revision strategy, ensure there is adequate bone stock for the reconstruction, and consent for the risk of persistent LLD.

• Too long: lower the cup (limited by bone), shorten the femoral neck cut (modular neck), downsize the head diameter, or revise to a shorter stem
• Too short: distal femoral strut allograft (rarely), a constrained liner, or accept the discrepancy

What revision achieves. Correction of the LLD succeeds in 70-80%, at the risk of new complications (infection, dislocation, loosening), and it may not fully resolve the patient's dissatisfaction.

No single measurement technique is 100% reliable. Best practice is to use at least two different methods and cross-check the results.

Early, within 6 weeks. Sciatic or femoral traction palsy; wound breakdown from the tension of overlengthening; dislocation from inadequate soft-tissue tension after shortening; and dissatisfaction, because the patient perceives the LLD at once.

Late, after 6 months. Chronic back pain from compensatory spinal changes, hip abductor weakness from the altered biomechanics of an offset error, and contralateral knee pain from overload. Litigation belongs here too: a lawsuit may be filed up to 3 years later.

Prognosis. It depends on the magnitude of the LLD, the patient and the management. Most patients with minor LLD, under 10mm, adapt well and are satisfied. Larger discrepancies often require intervention, but in the majority can be managed successfully without an operation.

Predictors of satisfaction. Good prognostic factors:

• Preoperative counselling that set expectations realistically
• A small LLD, under 10mm
• Gradual onset, the body adapting over months
• Restored offset
• Good communication, with the surgeon responsive to concerns
• No spine pathology, so compensation is flexible

Poor prognostic factors:

• No preoperative discussion, so the patient feels blindsided
• A large LLD, over 15mm
• High expectations, a perfectionist personality
• Inadequate offset
• A fixed spinal deformity that cannot compensate
• Bilateral hip disease, with the other THA for comparison

Function by management. Outcomes differ by strategy:

Follow-up. Adaptation takes months, and surveillance continues after it:

Litigation. LLD is a leading allegation in THA negligence claims, but most cases are defensible with proper documentation. A successful defence requires proof of:

1. Preoperative measurement and documentation of the LLD
2. An informed consent discussion specific to LLD risk
3. Templating, with the target values documented
4. Intraoperative measurement, with multiple methods
5. Postoperative follow-up and management, with conservative care attempted before any revision

The vulnerable case. Missing documentation, a generic consent without an LLD discussion, no templating or intraoperative measurement, and delayed or inadequate follow-up each leave the surgeon exposed. So does the result: an outcome within the acceptable range (under 15mm) is defensible, whereas component malposition contributing to the LLD, or an LLD over 20mm without explanation, makes the case vulnerable. Claims are often filed well after surgery, so keep excellent records and communication throughout follow-up.