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Fixation - cemented versus biological

Principles of bone-ingrowth fixation compared with cemented stem fixation

17 questions 3 source pages

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17 questions
Q1What are the aims and prerequisites of cementless biological fixation?▸
  • Aim: rigid internal fixation + cortical bone contact
  • Prerequisite: viable bone
  • Macroloc = initial fixation
  • Microloc = long term fixation
Q2How is initial rigid fixation achieved?▸
  • Press fit: reaming 1-2mm less than implant (hoop stress)
  • Line-to-line fit: supplemented with screw (cup) or porous coating (stem)
  • Cup: 70% cortical contact, 2/3 rim fit, dome intact
  • Stem: at least 4cm scratch fit
Q3What are the requirements for bone ingrowth?▸
  • Porous proximally or extensively coated
  • Pore size 50-150um; volume 50% (too much may shear)
  • Micromotion < 50-150um (too much = fibrous); gap < 50um
  • May be sintered beads, fiber mesh or tantalum
Q4How does ongrowth differ from ingrowth?▸
  • Ongrowth: grit blasted / plasma spray / HA; always titanium; always extensive (weaker method of fixation)
  • Roughness = average distance from peak to valley, proportional to interface shear strength
  • Theoretically ingrowth is better, but no clinical difference
  • Manufacturing process for porous ingrowth stems can result in diminished fatigue strength properties
Q5What is the role of hydroxyapatite (HA)?▸
  • Ca10(PO4)6(OH)2, an osteoconductive adjunct
  • Benefits shown only in animal models, not in humans
  • Deposited by plasma spray (better than electrochemical); optimal 50um
  • May cause 3rd body wear
Q6Describe the Khanuja classification of cementless stems.▸
  • I single wedge (Taperloc); II double wedge (Summit)
  • IIIA tapered round (Mallory); IIIB tapered spline/conical (Wagner); IIIC tapered rectangular (Zweymuller)
  • IV cylindrical fully coated (Restoration); V modular; VI anatomical
  • Proximally coated rely on metaphyseal press fit; extensively coated on diaphyseal scratch fit; type III at metadiaphyseal junction
Q7How are plasma spray and grit blasting performed?▸
  • Plasma spray: hot air + titanium alloy projected onto the surface
  • Grit blasted: stream of abrasive material -> abrasive wear
  • Ongrowth implants are always titanium and always extensive (weaker fixation method)
Q8How does cement achieve fixation and when is it preferred?▸
  • PMMA acts as a grout, transmitting load from implant to bone via a static stable interface with mechanical interlock into cancellous bone
  • Good in osteoporotic bone (better penetration) and irradiated bone (not reliant on bone ingrowth)
  • e.g. Dorr C, poor bone quality
Q9What are the disadvantages of cemented fixation and where is it used?▸
  • Bone heat necrosis
  • Cardiopulmonary suppression
  • PMMA is strong in compression but weak in shear and tension - not preferred in the cup
  • Used in the stem
Q10What are the fully and partially controllable factors for optimal cement fixation?▸
  • Fully controllable: antibiotic and barium inclusion, vacuum mixing to reduce porosity
  • Fully controllable: insertion pressurisation, avoid vigorous mixing, type of stem used
  • Partially controllable: 2-5mm thick cement mantle, blood and fat inclusion
  • Partially controllable: canal preparation - pulsatile lavage, stress riser
Q11What are the uncontrollable factors affecting cement fixation?▸
  • Cement aging
  • Strain rate
  • Temperature and humidity in the body
Q12Is there an ideal THR implant?▸
  • No ideal implant suits every patient population
  • Largely depends on age and quality of bone - should be individualised
  • Use registry data to help make decisions
  • Lecturer's choice: hybrid, ceramic-on-PE THR
Q13What are the 6 headings for choosing a THR implant?▸
  • Material (stiff material, high fatigue strength, corrosion resistant)
  • Modularity (pros and cons)
  • Femoral stem geometry (cementless or cemented)
  • Fixation to bone
  • Head size
  • Bearing surface
Q14How does material and fixation differ in young versus old patients?▸
  • Young: cementless titanium biological fixation; avoid cement (cyclic loading fails in shear); titanium reduces stress shielding; rule of 50; Swedish registry better 50-70M
  • Old: cemented stainless steel stiff stem to avoid bending in the cement mantle, otherwise will fail like the 3M capital THR
  • Old: immediate stability with proven longevity, 3rd generation cementation, even 2mm mantle (Barrack A); registry better > 70
Q15What head size is preferred and why?▸
  • 32mm
  • Large head increases head-neck ratio -> decreased impingement, better range, decreased dislocation rate
  • Too large -> thin PE thickness (traditionally > 8mm), increased volumetric wear
Q16Which bearing surface would you choose and why?▸
  • Metal on PE has proven longevity
  • Avoids ceramic complications (fracture with malposition, squeaking) and MoM complications (metallosis, ALVAL, pseudotumour)
  • Use highly cross-linked PE of at least 5mm thickness
Q17How is stability achieved with cementless fixation?▸
  • Initial stability: macroloc; press fit vs line to line (extensive coated stem for better frictional fit)
  • Long term: microloc - porous coated (ingrowth) vs grit blasted (ongrowth) +/- HA coating
  • Consider pore size, porosity, gap, micromotion
  • Obtain cortical contact for biological interdigitation and long term fixation
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