Bone Cement: How PMMA Works in Orthopedic Surgery

Bone cement is a fast-setting acrylic material, most commonly polymethylmethacrylate (PMMA), that surgeons use to anchor artificial joints to living bone and to stabilize fractured vertebrae. It does not glue bone to metal the way wood glue bonds two boards. Instead, it fills the irregular gaps between an implant and the surrounding bone, creating a tight mechanical interlock that lets the implant bear weight almost immediately. Since its introduction in orthopedic surgery in the 1960s, PMMA bone cement has become one of the most widely used biomaterials in medicine, though its dominance has shifted somewhat with the rise of press-fit, cementless implants that encourage bone to grow directly onto the device.

How Bone Cement Sets Inside the Body

PMMA bone cement arrives in the operating room as two components: a powder containing pre-formed polymer beads and an initiator, and a liquid monomer. When the surgeon mixes them together, a chemical reaction called polymerization begins. The liquid monomer links into long polymer chains around the existing beads, and the mixture transitions from a runny paste to a dough-like consistency, then hardens into a solid within roughly ten to fifteen minutes. Surgeons have a limited working window to press the cement into the bone canal or around the implant before it becomes too stiff to shape.

One well-known downside of this reaction is heat. The polymerization is exothermic, meaning it releases energy as it proceeds. Peak temperatures at the cement-bone boundary can climb high enough that researchers have long worried about thermal damage to surrounding tissue. Composite cements designed to address this problem have shown significantly lower peak temperatures and smaller zones of tissue damage compared to standard commercial PMMA cements, though these modified formulations tend to trade some compressive strength for their cooler setting profiles.¹

Where Bone Cement Gets Used

The two biggest arenas for bone cement are joint replacement and spinal stabilization. In a cemented hip or knee replacement, the surgeon prepares the bone canal, pressurizes cement into it, and seats the implant while the cement is still workable. Once hardened, the cement mantle transfers the patient’s body weight from the metal implant into the bone. In spinal procedures like vertebroplasty and kyphoplasty, cement is injected directly into a collapsed or fractured vertebra to restore its height and rigidity, giving patients rapid pain relief.

Beyond these headline applications, bone cement shows up in tumor surgery (filling voids after a tumor is removed from bone), skull reconstruction, and even dental prosthetics. In dentistry, PMMA resins are the backbone of denture fabrication, though the biocompatibility concerns there mirror those in orthopedics.²

Cemented Versus Cementless Implants

A question that comes up in every joint replacement conversation is whether the implant should be cemented or press-fit. Press-fit (cementless) designs have a roughened or porous surface that encourages living bone to grow directly into the implant over weeks to months. These have gained ground, particularly for hip replacements in younger, more active patients. But cement remains the standard in many knee replacements and in older patients whose bone quality may not support reliable ingrowth.

A large matched comparison of cemented and cementless total knee replacements found ten-year survival rates that were extremely close: about 96% for cemented and roughly 95.5% for cementless, with revision for any reason as the endpoint. Cemented knees had a slightly lower rate of revision for pain, while cementless knees had a slightly lower rate of revision for infection.³ A separate prospective randomized trial at midterm follow-up reported 100% survivorship in both cemented and cementless groups, with no radiographic loosening in either.⁴ The practical takeaway is that both approaches work well; the choice hinges on the patient’s bone quality, age, activity level, and surgeon preference rather than a clear superiority of one method.

Bone Cement Implantation Syndrome

One of the more alarming complications associated with bone cement is bone cement implantation syndrome (BCIS). During cemented hip surgery, pressurizing the cement into the femoral canal forces fat, marrow contents, and cement particles into the bloodstream. In rare cases this triggers a cascade of events: a sudden drop in blood pressure, decreased oxygen levels, and occasionally cardiac arrest. The proposed mechanism involves both the direct effects of absorbed methyl methacrylate monomer and the release of the body’s own inflammatory mediators, which together can push toward multiorgan dysfunction.⁵

Anesthesiologists now anticipate BCIS during cemented hip surgery, monitoring closely at the moment of cement pressurization and stem insertion. Strategies like thorough canal lavage (washing the bone canal before cementing) and maintaining adequate fluid volume help reduce risk. The syndrome is uncommon but has not been fully eliminated, and it remains one of the reasons some surgeons prefer cementless techniques when the patient’s bone can support them.

Monomer Toxicity and Biocompatibility

Even outside the operating room crisis of BCIS, the monomer component of bone cement raises biocompatibility questions. Unpolymerized methyl methacrylate monomer is a volatile, pungent chemical. Studies have documented that residual monomer can irritate skin and mucous membranes, trigger allergic reactions, and in laboratory settings cause effects ranging from liver toxicity to fertility disturbances in animal models.⁶ In practice, proper mixing technique and adequate curing time reduce residual monomer to very low levels in the hardened cement. Operating room staff, who breathe monomer vapor during mixing, face more routine exposure than patients do once the cement has set.

Cement Leakage in Spinal Procedures

When cement is injected into a fractured vertebra during vertebroplasty or kyphoplasty, some of it can escape through cracks in the bone wall. This cement leakage is the most common complication of spinal cement procedures. A meta-analysis identified several risk factors: the presence of a cleft (vacuum space) inside the vertebra, disruption of the vertebral cortex, lower cement viscosity, and greater injected cement volume all raised the odds of leakage and subsequent new fractures.⁷

Where the cement leaks matters too. A study examining post-vertebroplasty refractures found that cement leaking into the adjacent disc space carried the highest risk of refracture at a neighboring vertebra, while leakage into the spinal canal or around the vertebral body also increased risk significantly.⁸ Surgeons manage this risk by using higher-viscosity cement, injecting smaller volumes, and monitoring under real-time fluoroscopy to stop injection if cement begins migrating outside the bone.

The integrity of the vertebral walls is another independent predictor of leakage. When a fracture has disrupted the outer shell of the vertebra, there is simply less containment to hold the cement inside.⁹

The Stiffness Mismatch Problem

Hardened PMMA is far stiffer than the spongy cancellous bone it sits inside. This mismatch creates a biomechanical dilemma, particularly in the spine. After a vertebra is filled with rigid cement, the adjacent vertebrae, which are still osteoporotic and compliant, may bear abnormal loads. Some data suggest that this imbalance in the load distribution between the stiff cement-bone composite and the softer neighboring bone contributes to adjacent-level fractures.¹⁰ This has driven research into cements whose stiffness more closely matches cancellous bone, using additives like linoleic acid to soften the cured material without sacrificing too much strength.

What Makes Cement Visible on X-Rays

Pure PMMA is radiolucent, meaning it does not show up on X-rays. Surgeons need to see the cement mantle on follow-up imaging, so manufacturers add radiopaque particles. The two most common agents are barium sulfate and zirconium dioxide. The choice is not trivial, because these particles affect the cement’s mechanical behavior and its biological effects on surrounding bone.

Zirconium dioxide tends to improve tensile strength, fracture toughness, and resistance to fatigue crack growth compared to cement with no radiopacifier at all. Barium sulfate, by contrast, slightly decreases tensile strength, though it still improves crack propagation resistance.¹¹ On the biological side, wear particles from barium-sulfate-containing cement produced significantly more bone resorption at the implant interface than particles from zirconium-dioxide cement or radiolucent cement, suggesting barium sulfate may contribute more to loosening over time.¹²

Antibiotic-Loaded Cement for Fighting Infection

One of bone cement’s most valuable tricks is its ability to carry antibiotics directly to the surgical site. Surgeons mix powdered antibiotics into the cement before it sets, creating a depot that slowly releases the drug over days to weeks. This delivers concentrations at the infection site that would be impossible to achieve through an IV without toxic side effects. Antibiotic-loaded cement spacers are a cornerstone of two-stage revision surgery for infected joint replacements: the infected implant is removed, an antibiotic-loaded spacer takes its place for weeks to months to sterilize the area, and then a new implant goes in.¹³

Not all antibiotics work equally well in cement. Vancomycin and gentamicin maintain sustained, high elution rates over a 30-day period, while some beta-lactam antibiotics drop to nearly undetectable levels within just a few days. All tested antibiotics retained their antimicrobial activity through the heat of polymerization, which was once a major concern. Rifampin maintained good elution but weakened the mechanical properties of the cement.¹⁴

Combining antibiotics can boost performance. A combination of tobramycin and vancomycin yielded roughly 170% higher elution mass and rate across all sample sizes compared to single-antibiotic or alternative combinations.¹⁵ To further enhance release, one research group recommends manually incorporating a high dose of vancomycin into gentamicin-premixed cement and keeping the interim period between stages as short as possible.¹⁶

A persistent problem with antibiotic-loaded spacers alone is that their kill zone extends only a few centimeters from the cement surface. Bacteria at the periphery of a wound can survive and develop tolerance. One laboratory study found that supplementing the antibiotic spacer with antibiotic-loaded calcium sulfate beads placed farther from the spacer suppressed bacterial spreading and prevented tolerant colonies from establishing.¹⁷

Viscosity, Porosity, and Surgical Technique

How the surgeon handles cement during the operation affects how well it performs for years afterward. Cement viscosity at the moment of application plays an outsized role. Higher-viscosity cement creates better contact with bone surfaces and produces a stronger mechanical bond. In one study, standard-viscosity cement produced about 15% greater apposition between cement and bone and roughly 21% stronger constructs compared to low-viscosity cement, likely because thicker cement is better at displacing marrow and resisting blood backflow into the interface.¹⁸

The tradeoffs get complicated. An animal study found that low-viscosity cement, despite sustained pressurization, achieved lower penetration into bone because the thin cement followed the path of least resistance into the venous system rather than pushing deeper into the bone structure. Higher-viscosity cement improved interdigitation with bone but increased the risk of fat embolism.¹⁹ Surgeons have to balance penetration depth and bond strength against the hemodynamic risks of pushing more material into the vascular bed.

Porosity is another enemy of long-term performance. Air bubbles trapped during hand-mixing act as stress concentrators where fatigue cracks initiate. Reducing porosity through vacuum mixing or centrifugation increases fatigue life, which matters because cemented implants experience millions of loading cycles over a patient’s lifetime.²⁰

Bioresorbable Alternatives

PMMA stays in the body permanently. It does not remodel, does not participate in healing, and if it needs to come out during revision surgery, removing well-fixed cement from a femoral canal risks bone loss and even fracture of the surrounding bone.²¹ This has pushed development of resorbable bone cements that the body gradually replaces with real bone.

Calcium phosphate cements are the most developed of these. An injectable calcium phosphate cement sets at body temperature without a heat spike and hardens into a mineral form chemically similar to natural bone. In animal studies, nearly the entire cement surface was covered with new bone within two weeks, and over time the material remodeled in a pattern resembling normal bone turnover, with specialized bone-resorbing cells breaking down the cement while new bone formed alongside.²² The tradeoff is mechanical strength: calcium phosphate cement reaches compressive strengths around 25 megapascals, far below the roughly 70-100 megapascals typical of PMMA. That limits its use to non-weight-bearing or partially loaded applications like filling bone defects.

Calcium sulfate is another resorbable option, but its rapid degradation has been a persistent headache. Pure calcium sulfate can dissolve in as little as four to six weeks, often faster than new bone can fill the void. In animal models, fusion failed completely when only calcium sulfate was used because the material disappeared before the bone could catch up.²³ Biphasic formulations that blend calcium sulfate with calcium phosphate slow the resorption rate and achieve complete conversion to host bone over four to six months.²⁴ A clinical study of this composite used as a bone void filler after tumor removal found it provided sufficient early bone regeneration for patients to bear full weight within weeks, with no graft-specific complications.²⁵

Making PMMA Behave More Like Living Bone

Rather than replacing PMMA outright, another research strategy is modifying it to encourage bone integration. Standard PMMA triggers the formation of a fibrous tissue layer between cement and bone, a barrier that can contribute to loosening over time. Adding bioactive fillers can change that response. Composite cements incorporating strontium-containing borate bioactive glass stimulated new bone formation directly at the cement-bone interface at eight and twelve weeks, while plain PMMA only produced the usual fibrous tissue layer.²⁶

Bioactive glass additives based on phosphosilicate combined with calcium sulfate have shown the ability to form a hydroxyapatite layer on the cement surface when exposed to body-like fluids, a hallmark of materials that can bond to bone.²⁷ These are still largely laboratory and early animal-study findings, but they represent a clear direction: keep the mechanical reliability of PMMA while coaxing the body to accept it as something closer to a biological partner.

Nanocomposite Cements

At the cutting edge, researchers are embedding nanoparticles into cement to improve multiple properties at once. A recent study tested cement reinforced with calcium phosphate nanoparticles deposited on chitosan-coated carbon nanotubes. The nanocomposite cement reached a flexural strength of about 56 megapascals, compared to roughly 42.5 for unmodified cement, representing more than a 30% improvement. Each component contributed something: the carbon nanotubes provided structural reinforcement, while the calcium phosphate particles improved biological compatibility and load transfer.²⁸ Whether these laboratory gains translate to longer-lasting implants in patients remains to be proven, but the approach illustrates how material science keeps pushing the boundaries of what a simple acrylic cement can do.

Spacer Design in Infected Joint Revisions

When a hip replacement becomes infected and a two-stage revision is planned, the temporary antibiotic spacer can take several forms: a pre-molded off-the-shelf design, a custom-shaped hemiarthroplasty spacer molded in the operating room, or a custom total-hip-style spacer. A multicenter review of 144 patients compared these three approaches and found that for the vast majority of measured outcomes, the differences were not significant. Dislocation rates trended higher with hemiarthroplasty-style spacers, and re-infection rates varied somewhat, but no single spacer type clearly outperformed the others across the board.²⁹ The finding suggests that surgeon familiarity and patient-specific anatomy may matter more than the spacer blueprint itself.