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 supplied as two parts, a liquid monomer and a powder polymer, and cures by strongly exothermic free-radical addition polymerisation.

A grout, not a glue. PMMA does not bond chemically to bone or metal. It flows into the interstices of cancellous bone and hardens in situ, locking the prosthesis to bone by mechanical interlock (interdigitation). As a load-transferring grout it converts point loading at the implant surface into distributed loading across a larger bony bed.

Three themes. Its mechanical behaviour dictates both how the cement is handled and how it eventually fails. Most exam questions are answered by the chemistry (monomer versus polymer, initiator versus accelerator), the mechanics (why mantle defects cause loosening, and how vacuum mixing and pressurisation improve durability) and the safety (BCIS and thermal necrosis).

The liquid. Typically a 20 mL ampoule of monomer with two additives:

• Methylmethacrylate (MMA) - the reactive monomer; volatile, flammable and pungent
• N,N-dimethyl-p-toluidine (DMPT) - a tertiary amine accelerator, which drives the reaction at room temperature
• Hydroquinone - a stabiliser (inhibitor) that prevents premature polymerisation during storage

The powder. Typically a 40 g sachet:

• Pre-polymerised PMMA (and co-polymer) beads - the bulk filler
• Benzoyl peroxide (BPO) - the initiator, which decomposes to free radicals
• A radiopacifier, barium sulphate (around 10%) or zirconium dioxide - renders the mantle visible on radiographs
• Optionally, a heat-stable antibiotic powder such as gentamicin (see antibiotic-loaded cement, below)

Polymerisation. On mixing, BPO reacts with the DMPT accelerator to generate free radicals. These attack the C=C double bond of MMA and propagate a growing PMMA chain (addition polymerisation), while the pre-formed beads swell and become embedded in the newly polymerised matrix. As monomer converts to polymer the volume shrinks by around 2-5%.

The exotherm. Bench measurements on bulk specimens peak at roughly 80-90°C. The temperature at the living bone-cement interface is considerably lower, because blood flow and the heat sink of bone and implant carry heat away, but it can still enter the range capable of thermal osteonecrosis.

The four phases of curing. Mixing, sticky, doughy and hard, in that order ("Make Some Dough Hard"):

1. Mixing - wetting the powder; the cement is sandy and runny
2. Sticky - it adheres to gloves; do not touch it
3. Doughy - the working phase: non-sticky and packable, when the implant is inserted and the cement pressurised
4. Hard - viscosity has risen to a solid and the cement is set, with the exothermic peak; do not move the implant

Working time. Total handling time is roughly 8-12 minutes and is temperature dependent. Warming the components or the theatre shortens working time; cooling the monomer, or canal lavage with cold saline, lengthens it.

Viscosity. High-viscosity cements pass quickly into the doughy phase, with little sticky phase and an early, long working window. Low-viscosity cements stay runny longer and are favoured for vertebroplasty and pressurised injection.

Load it in compression. The cement is strongest in compression and weakest in tension and shear, so the surgical aim is a uniform, void-free, fully interdigitated mantle that is loaded in compression. Voids, mantle defects and thin or eccentric mantles act as stress risers: they crack in tension, and the cracks propagate to aseptic loosening.

Mixing. Vacuum mixing removes entrained air, reducing porosity and substantially improving tensile fatigue life; it is the dominant evidence-based modifier of strength. Centrifugation is an alternative method of reducing porosity.

Delivery. Pressurising doughy cement forces it deeper into trabecular bone and maximises micro-interlock, with a target penetration of roughly 2-5 mm. Preparing the bone bed, with pulsatile lavage to remove fat and marrow and then drying it, improves interface strength.

The generations. Cementing technique is described in generations, and the third is the modern standard:

The grade. The quality of a femoral cement mantle is graded on the immediate post-operative radiograph by the Barrack classification (Barrack, Mulroy and Harris).

Why it matters. The grade 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. Keep it distinct 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.

Definition. An intra-operative syndrome of hypotension, hypoxia and/or loss of consciousness occurring around the time of cementation, prosthesis insertion or joint reduction. It can include arrhythmia and ranges to cardiac arrest, and it is a leading cause of intra-operative death in cemented hip surgery, particularly cemented hemiarthroplasty for fracture.

Donaldson grading. The most quoted classification:

• Grade 1 - moderate hypoxia (SpO2 under 94%) or a systolic BP fall over 20%
• Grade 2 - severe hypoxia (SpO2 under 88%), a systolic BP fall over 40%, or unexpected loss of consciousness
• Grade 3 - cardiovascular collapse requiring CPR

Pathophysiology. The cause is multifactorial. Pressurised insertion embolises marrow fat, cement and air into the pulmonary circulation, raising pulmonary vascular resistance and straining the right ventricle. Circulating MMA monomer causes vasodilation and direct myocardial depression, and histamine release and complement and thrombin activation also contribute.

Who is at risk. The patient factors are advanced age, ASA III or IV, pre-existing cardiopulmonary disease, pulmonary hypertension and an osteoporotic or wide canal. The surgical factors are the femoral component at the hip, long-stem implants, pathological or metastatic bone and intramedullary pressurisation.

Reducing the risk. Four measures reduce the risk or prepare the team for it:

• Identify high-risk patients before surgery, and consider an uncemented or non-pressurised technique and invasive monitoring
• Pulsatile lavage and drying of the canal before cementing, which reduces the embolic load
• Suction or venting and retrograde gun filling, to limit intramedullary pressure spikes
• Surgeon-to-anaesthetist communication ("cement going in"), so that euvolaemia, 100% oxygen and vasopressors are ready

If collapse occurs. Stop, and give 100% oxygen, fluids and vasopressors, with adrenaline and CPR for Grade 3.

A diagnosis of exclusion. When the blood pressure crashes during cemented arthroplasty, BCIS is the lead diagnosis, but the mimics must be ruled out first.

Which antibiotics. The agent must survive the exothermic polymerisation and remain active in set cement, so only heat-stable drugs are used. It must be added as a powder, since a liquid antibiotic would further weaken the cement, and it must be water-soluble to elute. Beta-lactams are heat-labile and are not used.

Agents and doses. The work-horse agents are the aminoglycosides (gentamicin, tobramycin) and vancomycin, used alone or combined. Commercial "low-dose" prophylactic cement carries roughly 0.5-1 g per 40 g packet, whereas "high-dose" hand-mixed cement, for spacers or established infection, carries several grams.

Elution. 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.

Porosity trades strength for elution. Elution rises with porosity and surface area, so hand-mixed cement elutes more but is mechanically weaker, while vacuum-mixed cement is stronger but elutes less. It is a deliberate trade-off, the same one seen between a spacer (maximise elution) and a load-bearing primary mantle (maximise strength). Combining two antibiotics can enhance the elution of each.

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

Related pages: Periprosthetic Joint Infection for the disease that antibiotic-loaded cement is meant to prevent and that high-dose spacer cement is used to treat; Revision THA for what happens when the mantle fails, and for the cement-removal problem that cementing creates for the next surgeon; Cement-in-Cement Revision for the technique that exploits an intact mantle rather than removing it; Calcium Phosphate Cements for the osteoconductive, non-exothermic alternative used to fill metaphyseal defects, which is a different material solving a different problem; and Fat Embolism Syndrome for the embolic mechanism that pressurised cementation shares and that underlies much of BCIS.

Cemented or uncemented hemiarthroplasty for hip fracture. The WHiTE 5 randomised trial (N Engl J Med, 2022) showed that cemented hemiarthroplasty gave a modestly but significantly better quality of life at 4 months and fewer periprosthetic fractures, with no excess mortality, reinforcing the guideline preference for cement. Surgeons must still balance this against the risk of BCIS in the frailest patients.

Routine antibiotic-loaded cement 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 versus targeting high-risk patients remains debated, set against cost, resistance and theoretical strength reduction.

Venting and pressurisation to prevent BCIS. Bone venting and "low-pressure" techniques are intuitively protective, but the evidence is limited; lavage and patient selection have stronger support.

Thermal necrosis in vivo. The clinical relevance of thermal necrosis versus monomer toxicity for loosening is still argued; the bench peak is not the interface temperature (above).

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.