PMMA Bone Cement

Polymethylmethacrylate (PMMA) is the self-curing acrylic cement that revolutionised joint replacement after Sir John Charnley introduced it for low-friction arthroplasty in the early 1960s. It is not an adhesive: it is a load-transferring grout that locks a prosthesis to bone by flowing into the interstices of cancellous bone and hardening in situ, converting point loading at the implant surface into distributed loading across a larger bony bed.

Supplied as a two-part system (a liquid monomer and a powder polymer), PMMA cures through free-radical addition polymerisation that is strongly exothermic. Its mechanical behaviour - excellent in compression, poor in tension and shear, and viscoelastic (creeps over years) - dictates both how it is handled and how it eventually fails. Mastery of three themes answers most exam questions: the chemistry (monomer vs polymer, initiator vs accelerator), the mechanics (why mantle defects cause loosening and how vacuum mixing and pressurisation improve durability), and the safety (BCIS and thermal necrosis).

Components

Polymerisation chemistry

When powder and liquid are combined, BPO reacts with the DMPT accelerator to generate free radicals, which attack the C=C double bond of MMA and propagate a growing PMMA chain (addition polymerisation). The pre-formed beads swell and become embedded in newly polymerised matrix. The reaction is exothermic; bench measurements on bulk specimens approach 80-90°C, and the interface temperature in vivo is moderated by blood flow and the heat sink of bone and implant but can still reach a range capable of thermal osteonecrosis.

The 4 phases of curing

The phases are Mixing then Sticky then Doughy then Hard (mnemonic M-S-D-H, "Make Some Dough Hard"). Total handling time is roughly 8-12 minutes and is temperature dependent: warming the components or the theatre shortens working time, while cooling the monomer or canal lavage with cold saline lengthens it. High-viscosity cements pass quickly into the doughy phase (early, long working window with little sticky phase); low-viscosity cements stay runny longer (favoured for vertebroplasty and pressurised injection).

Setting changes to know (mnemonic HOST)

• H - Heat released (exothermic, 80-100°C)
• O - Oxygen-free chains form (free-radical addition polymerisation)
• S - Shrinkage of volume (around 2-5% as monomer converts to polymer)
• T - Toughening/hardening (viscosity rises to a solid)

The cement is strongest in compression and weakest in tension/shear, so the surgical aim is a uniform, void-free, fully interdigitated mantle that is loaded in compression. Mantle defects, voids and thin/eccentric mantles act as stress risers that crack in tension and propagate to aseptic loosening.

Optimising cement quality

• Vacuum mixing removes entrained air, reducing porosity and substantially improving tensile fatigue life - the dominant evidence-based modifier of strength.
• Centrifugation is an alternative porosity-reducing method.
• Pressurisation of doughy cement forces it deeper into trabecular bone, maximising micro-interlock (penetration of roughly 2-5 mm is the target).
• Bone-bed preparation - pulsatile lavage to remove fat/marrow and dry the bed - improves interface strength and reduces embolic load.

The whole technique section turns on producing a "uniform, void-free, fully interdigitated mantle," and mantle defects are named as the stress risers that cause loosening — so the examiner expects the standard radiographic grading of mantle quality, the Barrack classification (Barrack, Mulroy and Harris).

• The grade is read off the immediate post-operative radiograph and predicts durability: Grade C and D mantles are associated with higher rates of aseptic loosening, which is why third-generation technique aims for a Grade A "white-out."
• Distinguish this mantle-quality grade from the later assessment of loosening by progressive radiolucent lines and stem migration mapped to the femoral zones, which is covered in the Gruen zones topic.

When the blood pressure crashes during cemented arthroplasty, BCIS is the lead diagnosis but is diagnosis of exclusion - rule out the mimics.

Antibiotic-loaded cement (ALBC) is referenced throughout — as an additive, in the registry evidence and the controversies — and the MCQ notes that only heat-stable agents can be used. The underlying pharmacology is examinable in its own right.

Why only certain antibiotics

• The agent must survive the exothermic polymerisation and remain active in set cement, so only heat-stable drugs are used; it must also be added as a powder (a liquid antibiotic would further weaken the cement) and be water-soluble to elute. Beta-lactams are heat-labile and are not used.
• The work-horse agents are the aminoglycosides (gentamicin, tobramycin) and vancomycin, used alone or combined; commercial "low-dose" prophylactic cement carries roughly 0.5 to 1 g per 40 g packet, whereas "high-dose" hand-mixed cement (for spacers/established infection) carries several grams.

How elution works

• Release is a surface phenomenon and is biphasic: a high initial burst over the first hours to days, then a prolonged low-level release. Only a small fraction of the loaded antibiotic ever elutes — most stays trapped in the bulk.
• Elution rises with porosity and surface area: hand-mixing (more porous) elutes more but is mechanically weaker, while vacuum-mixing is stronger but elutes less — the same trade-off seen with spacers (maximise elution) versus a load-bearing primary mantle (maximise strength). Combining two antibiotics can enhance the elution of each.

Trade-offs and harms

• High doses reduce mechanical strength, and systemic absorption of aminoglycoside from high-dose cement can cause acute kidney injury; other concerns are allergy, cost, and selection of resistant organisms.
• General prevention and treatment of periprosthetic joint infection are covered in the dedicated PJI topic; here the point is the cement-specific pharmacology.

Global epidemiology and burden

• Hip and knee arthroplasty are among the most common elective operations worldwide, with millions performed annually; cement use varies markedly by region and indication.
• Cemented fixation predominates for hemiarthroplasty in fragility hip fractures and for elderly osteoporotic bone; uncemented fixation predominates for younger primary THA in many high-income registries.
• Periprosthetic joint infection complicates roughly 1-2% of primary arthroplasties and is a major driver of antibiotic-cement use.

Side-by-side guidance

Registry evidence

• Norwegian/Nordic registries: antibiotic-loaded cement reduces infection-related revision in cemented THA (basis of the Engesaeter data above).
• NJR (UK), AOANJRR (Australia), AJRR (US), SHAR (Sweden): consistently report lower early periprosthetic fracture and revision for cemented hemiarthroplasty in the elderly fracture population.
• Registries track cement type, viscosity and antibiotic loading as procedural variables.

High- vs limited-resource practice

• In high-resource settings, vacuum mixing systems, pulsatile lavage and commercial ALBC are routine; anaesthetic teams pre-empt BCIS with invasive monitoring in high-risk cases.
• In limited-resource settings, hand mixing and manually compounded antibiotic cement are common; reliable theatre communication, canal lavage and patient selection remain the highest-value, low-cost measures to reduce BCIS and infection.

• Cemented vs uncemented hemiarthroplasty for hip fracture. The WHiTE 5 randomised trial (N Engl J Med, 2022) showed cemented hemiarthroplasty gave a modestly but significantly better quality of life at 4 months and fewer periprosthetic fractures, with no excess mortality - reinforcing guideline preference for cement. Surgeons must still balance this against BCIS risk in the frailest patients.
• Routine antibiotic-loaded cement (ALBC) in primary arthroplasty. Registry data support a protective effect against infection in hips, but the benefit in knees is weak or absent (less cement, lower local elution). Routine use vs targeting high-risk patients remains debated, set against cost, resistance and theoretical strength reduction.
• High-dose ALBC and elution. Hand mixing increases porosity and antibiotic release (useful for spacers) but weakens cement; vacuum mixing improves strength but reduces elution - a deliberate trade-off between mechanical durability and drug delivery. High-dose aminoglycoside cement carries a real risk of acute kidney injury.
• Venting/pressurisation to prevent BCIS. Bone venting and "low-pressure" techniques are intuitively protective but evidence is limited; lavage and patient selection have stronger support.
• Thermal necrosis in vivo. Bench exotherm peaks (80-90°C) overstate true interface temperature; clinical relevance of thermal necrosis vs monomer toxicity for loosening is still argued.

1. Fernandez MA, Costa ML, et al. Cemented or Uncemented Hemiarthroplasty for Intracapsular Hip Fracture (WHiTE 5). N Engl J Med. 2022;386(6):521-530.
2. Donaldson AJ, Thomson HE, Harper NJ, Kenny NW. Bone cement implantation syndrome. Br J Anaesth. 2009;102(1):12-22.
3. Engesaeter LB, et al. Does cement increase the risk of infection in primary total hip arthroplasty? Norwegian Arthroplasty Register. Acta Orthop. 2006;77(3):351-358.
4. Hinarejos P, et al. Use of antibiotic-loaded cement in total knee arthroplasty. World J Orthop. 2015;6(11):877-885.
5. Chen AF, Parvizi J. Antibiotic-loaded bone cement and periprosthetic joint infection. J Long Term Eff Med Implants. 2014;24(2-3):89-97.