Cemented fixation
PMMA cement is a grout that creates mechanical interlock between implant and prepared bone.
Good cement technique aims for:
- clean, dry cancellous bed
- distal restrictor when appropriate
- retrograde cement delivery
- pressurisation
- central stem position
- continuous cement mantle without major voids
Cement is strong in compression but relatively weak under tensile and fatigue loading. Malalignment and mantle defects create stress concentrations.
Good cemented fixation depends on:
- canal preparation
- haemostasis and lavage
- cement restriction
- retrograde cement introduction
- pressurisation
- correct stem insertion
- controlled mantle
A cemented stem may be polished taper-slip or composite-beam in philosophy; these designs behave differently and should not be mixed conceptually.
Cementless fixation
Initial stability is mechanical; long-term fixation depends on bone ongrowth or ingrowth.
Requirements include:
- good bone contact
- appropriate press fit
- limited micromotion
- suitable porous or rough surface
Hydroxyapatite is osteoconductive and can accelerate bone apposition on selected implants, but coating does not compensate for poor primary stability.
Stress shielding
A very stiff stem can carry a disproportionate load, reducing load transfer to proximal femur. Extent depends on material, size, geometry and fixation pattern.
Choosing cemented versus cementless
Consider:
- age and biological bone quality
- femoral morphology
- fracture risk
- surgeon familiarity
- implant track record
- registry and long-term evidence
There is no absolute age cut-off that determines fixation.
Templating
Templating aims to predict:
- centre of rotation
- cup size and position
- femoral stem size
- neck cut
- offset
- leg length
Check radiographic calibration and positioning before trusting measurements.
Templating is a plan, not a substitute for intraoperative assessment.
Why template
Templating helps anticipate:
- acetabular component size and position
- femoral stem size
- neck cut
- centre of rotation
- leg length
- offset
- unusual anatomy
- implant inventory needs
Digital templating depends on reliable radiographic calibration. Apparent dimensions are affected by magnification and patient positioning.
Acetabular goals
Plan to restore the anatomical centre of rotation where practical while obtaining secure bone contact.
Important concerns include:
- medial wall
- superolateral coverage
- protrusio
- dysplasia
- bone defects
- previous hardware
Femoral goals
Evaluate:
- canal width
- metaphyseal shape
- femoral offset
- neck-shaft angle
- version
- deformity
- previous osteotomy
The planned stem should achieve stable fixation without avoidable fracture or excessive stress concentration.
Uncemented fixation
Uncemented stems require adequate initial mechanical stability for osseointegration.
Fixation may be metaphyseal, diaphyseal or combined depending on design.
Excessive undersizing risks micromotion and subsidence; excessive oversizing risks fracture and stress shielding.
Leg length and offset
Leg-length restoration should be balanced against stability and nerve safety.
Offset influences:
- abductor tension
- joint reaction force
- impingement
- stability
Failure to restore offset can produce limp or instability even if length appears correct.
Intraoperative checks
Use:
- anatomical landmarks
- trial reduction
- stability testing
- leg-length assessment
- soft-tissue tension
- imaging when required
FRCS synthesis
Templating predicts a reconstruction; it does not dictate it. The final implant must satisfy stable fixation plus restoration of hip biomechanics.