THA Templating and Preoperative Planning

Preoperative planning for total hip arthroplasty is the process of deciding how the arthritic or fractured hip will be reconstructed before surgery. Templating is one part of that process: it estimates component size and position. The wider plan also covers exposure, fixation, leg length, offset, centre of rotation, stability risk, blood loss, implant inventory and bailout options.

Every hip needs a plan. A routine osteoarthritic hip still needs an AP pelvis, a lateral view and a clear plan for cup and stem sizing. A complex hip needs more. Dysplasia, protrusio, post-traumatic deformity, previous osteotomy, retained metalwork, childhood hip disease, fracture of the femoral neck, obesity, osteoporosis and spinal stiffness can all make a standard template misleading.

What templating does well. It clarifies the intended hip centre, the component size range, the neck cut, the offset option, the leg-length goal and the equipment list. It also makes the surgeon think the case through before the incision.

Planning is biomechanical reconstruction. The native hip transmits load through a centre of rotation, a femoral offset, an abductor lever arm and a leg-length relationship, and THA changes these variables. If the reconstruction shortens offset, the abductors work at a disadvantage and the hip may feel weak or unstable. If length is overcorrected, the patient may have nerve stretch, pain or dissatisfaction.

Cup and stem errors. If the cup is placed too high or too lateral, hip mechanics and wear may suffer. If the femoral stem is undersized or malaligned, fixation may fail.

What the plan must set. These are the most important planning variables:

• Hip centre: restore or deliberately reconstruct the centre of rotation
• Cup position: size, medialisation, inclination, version, coverage and screw plan if needed
• Femoral fixation: cemented, cementless, hybrid, modular, long stem or special implant
• Neck cut: level and orientation that allow the planned stem and length restoration
• Offset: femoral and global offset, abductor tension and impingement-free motion
• Leg length: planned correction balanced against nerve and soft-tissue safety
• Version and stability: combined anteversion, impingement, spinopelvic mechanics and head/liner choice

For templating, classification should make the case easier to plan. The most useful one is a practical complexity assessment rather than a single named system: a standard primary, a complex primary, or a hip at high stability risk.

Clinical assessment before templating is an operative planning assessment as well as the diagnosis of arthritis.

History. Ask about pain, walking distance, walking aids, instability symptoms, previous hip surgery, childhood hip disease, fracture history, spine disease, neurological disease, infection risk, anticoagulation, metal allergy history and the patient's priorities. Clarify any perceived leg-length inequality and shoe raises, and whether the patient would prefer equal length, stability or nerve safety if a trade-off is unavoidable.

Examination. Examine gait, the Trendelenburg sign, fixed flexion deformity, abduction contracture, true and apparent leg length, pelvic obliquity, lumbar stiffness, rotational profile, hip range of motion, abductor power, knee and foot deformity, and the neurovascular baseline. The examination helps explain why the radiographic plan may not match the functional limb length.

Pelvic obliquity. When it is fixed, apparent leg length may not equal bony leg length. Plan a standing assessment and counsel carefully.

Neurological symptoms. Baseline documentation is essential, and nerve stretch risk may limit lengthening. Document the preoperative status and avoid excessive lengthening.

Lumbar stiffness, abductor weakness and a previous femoral osteotomy also change the template; the planning response to each is set out in the Classification tables.

Management here is the planning process itself: the decisions made before the incision that determine whether the operation is straightforward. The plan has three parts: the acetabular plan, the femoral plan, and the backup for when the hip does not behave as templated.

Acetabular PlanFemoral PlanLength and OffsetBackup Plan

Plan the cup before the stem. The hip centre affects length, offset, abductor mechanics and cup coverage. Identify the teardrop, native acetabular floor, superolateral acetabular margin and intended hip centre, then choose a cup size that gives stable bone contact without excessive medialisation, over-reaming or loss of bone stock.

Hip centre

Decision

Restore native centre or deliberately plan a safe alternative.

Key risk

High or lateral centre can impair mechanics.

Cup size

Decision

Estimate size and adjacent sizes.

Key risk

Oversizing can fracture or remove bone; undersizing can be unstable.

Medialisation

Decision

Use controlled medialisation to improve coverage when appropriate.

Key risk

Protrusio or thin medial wall needs protection.

Inclination and version

Decision

Plan safe orientation adjusted for anatomy and spinopelvic risk.

Key risk

Malposition increases instability, edge loading and wear.

Defect management

Decision

Plan screws, graft, augments, cage or custom options when needed.

Key risk

Unrecognised deficiency leads to poor fixation.

Acetabular Planning Steps

Step	Decision	Key risk
Hip centre	Restore native centre or deliberately plan a safe alternative.	High or lateral centre can impair mechanics.
Cup size	Estimate size and adjacent sizes.	Oversizing can fracture or remove bone; undersizing can be unstable.
Medialisation	Use controlled medialisation to improve coverage when appropriate.	Protrusio or thin medial wall needs protection.
Inclination and version	Plan safe orientation adjusted for anatomy and spinopelvic risk.	Malposition increases instability, edge loading and wear.
Defect management	Plan screws, graft, augments, cage or custom options when needed.	Unrecognised deficiency leads to poor fixation.

Cup Orientation Targets: Lewinnek and Combined Anteversion

Two numbers anchor cup positioning. The Lewinnek "safe zone" (radiographic) is cup inclination 40 plus or minus 10 degrees and anteversion 15 plus or minus 10 degrees; dislocations cluster outside it, though it is necessary rather than sufficient (many dislocations occur with cups inside it, the basis of the functional-target debate). The second concept is combined anteversion = cup anteversion + femoral stem anteversion, targeted at roughly 25 to 50 degrees (around 37 degrees); Widmer's rule formalises this as cup anteversion + 0.7 times stem anteversion of about 37 degrees, the principle being that cup and stem version must be planned together for impingement-free, stable motion. In the stiff or fused spine the functional cup target shifts (less pelvic-tilt change standing-to-sitting), so a standing and sitting assessment, not the static Lewinnek box alone, should set the goal.

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The femoral plan. Plan the femur from the canal, metaphysis, version, bone quality, neck-shaft anatomy and intended leg length. Canal shape, deformity and bone quality determine fixation. The template should estimate the neck cut and stem size, but the real decision is stable fixation in the patient's bone, which may call for a cemented, modular, short or long stem, or an osteotomy.

Fixation

Decision

Cemented, cementless, hybrid or modular according to bone and patient factors.

Key risk

Poor fixation leads to subsidence, fracture or loosening.

Stem size

Decision

Estimate size and adjacent sizes.

Key risk

Undersizing risks instability; oversizing risks fracture.

Neck cut

Decision

Plan level relative to lesser trochanter and intended centre.

Key risk

Wrong cut can create length or offset error.

Version

Decision

Assess native femoral version and implant control.

Key risk

Excessive anteversion or retroversion contributes to instability.

Special anatomy

Decision

Narrow canal, stovepipe femur, Dorr C bone, bowing or prior osteotomy.

Key risk

Routine implants may not fit safely.

Femoral Planning Steps

Step	Decision	Key risk
Fixation	Cemented, cementless, hybrid or modular according to bone and patient factors.	Poor fixation leads to subsidence, fracture or loosening.
Stem size	Estimate size and adjacent sizes.	Undersizing risks instability; oversizing risks fracture.
Neck cut	Plan level relative to lesser trochanter and intended centre.	Wrong cut can create length or offset error.
Version	Assess native femoral version and implant control.	Excessive anteversion or retroversion contributes to instability.
Special anatomy	Narrow canal, stovepipe femur, Dorr C bone, bowing or prior osteotomy.	Routine implants may not fit safely.

Plan leg length from several landmarks, not one measurement. Compare the teardrops, lesser trochanters, femoral heads, pelvic obliquity and clinical findings. Plan offset as both femoral and global offset, because both affect abductor mechanics.

Restore leg length

How to plan

Use pelvic landmarks and clinical leg-length assessment.

Common mistake

Chasing radiographic equality in a patient with pelvic obliquity.

Restore femoral offset

How to plan

Choose neck option, stem design or high-offset version; have standard, high-offset, modular or lateralised options.

Common mistake

Using a low-offset construct that weakens abductors.

Restore global offset

How to plan

Consider both cup medialisation and femoral offset.

Common mistake

Medialising cup without compensating femur when needed.

Maintain stability

How to plan

Trial reduction, impingement testing and soft-tissue tension.

Common mistake

Accepting instability because the X-ray template looked correct.

Length and Offset Decisions

Goal	How to plan	Common mistake
Restore leg length	Use pelvic landmarks and clinical leg-length assessment.	Chasing radiographic equality in a patient with pelvic obliquity.
Restore femoral offset	Choose neck option, stem design or high-offset version; have standard, high-offset, modular or lateralised options.	Using a low-offset construct that weakens abductors.
Restore global offset	Consider both cup medialisation and femoral offset.	Medialising cup without compensating femur when needed.
Maintain stability	Trial reduction, impingement testing and soft-tissue tension.	Accepting instability because the X-ray template looked correct.

How to Actually Measure Leg Length and Offset on the AP Pelvis

The measurements examiners want are concrete. Leg-length discrepancy: draw a horizontal inter-teardrop line (or a line across the inferior margins of the ischial tuberosities), then measure the perpendicular distance from this line to a fixed, identical femoral landmark on each side — usually the apex of the lesser trochanter; the side-to-side difference is the bony LLD. Femoral offset is the perpendicular distance from the long axis of the femur to the centre of the femoral head (centre of rotation). Acetabular offset is the perpendicular distance from the centre of rotation to a vertical reference (the ilioischial/Köhler line); global offset = femoral + acetabular offset. On the template, plan the neck cut and stem/neck option so the prosthetic centre of rotation reproduces the contralateral offset and the lesser-trochanter-to-centre distance restores length — which is why a reliable, non-deformed contralateral hip is so valuable as the template reference.

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Write the backup into the plan. The planning note should include the expected implant and the backup, because the plan must survive unexpected fracture, poor bone or an unavailable size. This is especially important in dysplasia, fracture, osteoporosis, previous osteotomy, retained metalwork, protrusio and revision-like primary THA.

Bone quality and exposure. Have cemented, cementless, cable or fracture options ready, along with cables, cement, a revision stem, augments and alternative liners. Plan exposure escalation too: extensile exposure, trochanteric osteotomy or hardware removal when needed.

Poor femoral bone

Backup to prepare

Cemented stem, cement restrictor, cement equipment and cables.

Reason

Cementless press-fit may be unsafe.

Calcar crack risk

Backup to prepare

Cables, cerclage passer and longer stem option.

Reason

Immediate fixation prevents propagation.

Acetabular deficiency

Backup to prepare

Screws, augments, graft, larger cup range or revision shell.

Reason

Coverage and fixation may differ from template.

High instability risk

Backup to prepare

Larger heads, elevated liner, dual mobility or constrained options.

Reason

Stability may drive bearing choice.

Retained metalwork

Backup to prepare

Extraction set, broken screw set and alternative approach plan.

Reason

Hardware removal can dominate the operation.

Backup Equipment

Problem	Backup to prepare	Reason
Poor femoral bone	Cemented stem, cement restrictor, cement equipment and cables.	Cementless press-fit may be unsafe.
Calcar crack risk	Cables, cerclage passer and longer stem option.	Immediate fixation prevents propagation.
Acetabular deficiency	Screws, augments, graft, larger cup range or revision shell.	Coverage and fixation may differ from template.
High instability risk	Larger heads, elevated liner, dual mobility or constrained options.	Stability may drive bearing choice.
Retained metalwork	Extraction set, broken screw set and alternative approach plan.	Hardware removal can dominate the operation.

Complex primary THA should be recognised before the day of surgery; the patterns, and the imaging and implants each adds, are tabulated under Classification. Hip centre, acetabular deficiency, femoral canal shape, version, shortening, offset and backup implants all need to be considered before theatre. The cases below show how CT, three-dimensional models and digital component simulation change the plan in dysplasia and severe femoral deformity.

When plain films are not enough. In a severely deformed femur, plain AP templating alone cannot safely select fixation or stem geometry.

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Three-dimensional planning in dysplasia. When deformity makes two-dimensional outlines inadequate, multiplanar CT and a three-dimensional model quantify true-acetabular bone stock, transfer the contralateral hip centre, size and position the cup, and simulate femoral reconstruction.

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Sizing each component, then the construct. In Crowe II dysplasia, the CT reconstruction permits separate cup and stem sizing before their combined reconstruction is checked on the postoperative radiograph; coverage, hip centre, canal fit and offset are planned together. The plan can also show that the stem will not fit. In a Crowe IV post-septic hip the planned stem exceeds the available femoral bony envelope, and recognising that mismatch before theatre triggers alternative stem geometry, osteotomy and fracture-control options.

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Rehearsing the operation. Patient-specific models define the true and pseudo-acetabula and the planned reconstruction before exposure. A useful plan predicts what the surgeon will encounter.

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Templating errors are not theoretical: they appear as operative and postoperative complications.

What poor planning costs. Poor planning causes predictable problems: the wrong components available, a poor exposure strategy, inadequate restoration of leg length or offset, a malpositioned hip centre, an unstable trial reduction, intraoperative fracture, unplanned conversion to cement, metalwork that cannot be removed, or a rushed decision to use an implant that is not ideal.

What good planning buys. It improves theatre efficiency and reduces avoidable surprises. It also improves consent, because the surgeon can explain why the hip is routine or complex, what reconstruction is intended and what alternatives may be needed if the anatomy is different from the radiograph.

THA is one of the most studied operations in orthopaedics, with more than one million primaries performed worldwide each year and registries tracking implant survival across millions of cases. Planning principles are globally consistent, but imaging access and implant availability vary widely.

Global epidemiology

• Hip osteoarthritis is the dominant indication in high-income settings; in many lower- and middle-income regions, sequelae of childhood hip disease, dysplasia, post-traumatic deformity and avascular necrosis make a larger share of the templating workload, raising the proportion of complex primaries.
• Demand is rising with ageing populations; THA volumes are projected to increase substantially over the coming decades across most registry nations.

Society guidance, side by side

Registry signals that inform planning

• Large arthroplasty registries (NJR for England, Wales, Northern Ireland and the Isle of Man; AOANJRR Australia; AJRR US; the Swedish and Norwegian registries; NZJR) track revision and implant survival by fixation, bearing and head size.
• Registry-level findings that shape the plan: larger femoral heads and dual-mobility constructs reduce dislocation-related revision; cemented fixation lowers periprosthetic fracture risk in older patients with poor bone; and implant choice should favour constructs with documented long-term survivorship rather than novel designs without registry data.

High- versus limited-resource practice

Templating is mature but several questions remain genuinely debated, and examiners reward a candidate who can state both sides.

THA templating is a structured reconstruction plan. Start with adequate calibrated imaging, define the clinical goal, then plan the acetabulum, femur, leg length, offset, stability and backup implants. The plan should be specific enough to prepare theatre and consent the patient, but flexible enough to change when the bone, trials or stability assessment prove that the template is wrong.