Bone Void Filler vs Bone Cement (PMMA): Key Differences

In the operating room, "bone cement" can mean three different things. Usually it means PMMA. Sometimes it means a calcium phosphate cement. Occasionally it means a magnesium-based filler that happens to set hard. Because the names overlap, a basic difference gets lost. PMMA is a permanent polymer designed to carry load. A resorbable bone void filler is a scaffold the body is meant to replace with bone over time.

They do different jobs, and in most cases the defect itself decides which one you need.

PMMA bone cement and resorbable bone void fillers compared

	PMMA bone cement	Resorbable bone void filler
What it is	Acrylic polymer formed from liquid monomer and powder	Mineral-based material, such as calcium sulfate, calcium phosphate or magnesium phosphate
How long it stays	Permanently	Broken down over time, at a rate that varies by material. Some apatite calcium phosphates can persist for a long time.
Load-bearing role	Yes. It is used to fix implants and stabilize vertebrae.	Generally no. Most are cleared for non-structural voids.
Setting	Exothermic polymerization	Depends on the product. Self-setting cements harden in minutes. Granules and many putties do not set.
Bone interface	Mechanical interlock with no remodeling	Osteoconductive, so bone grows into the material as it resorbs
Visibility on X-ray	Usually from an added radiopacifier	Many mineral fillers are radiopaque on their own

What PMMA bone cement is used for

PMMA has been part of orthopedics since the 1960s, when Charnley used it to fix hip replacements. Its main uses today are:

• Arthroplasty. It fixes joint replacement components, acting as a grout between implant and bone.

• Vertebral augmentation. It is used in vertebroplasty and kyphoplasty for osteoporotic compression fractures.

• Tumor surgery. It fills defects after tumor resection, where immediate strength matters and remodeling isn't expected.

• Infection surgery. Antibiotic-loaded spacers and beads are used in staged procedures.

Trade-offs of PMMA

PMMA's strengths come from being permanent and stiff. The same properties set its limits.

• Heat. PMMA hardens through an exothermic reaction. A review in the Journal of Clinical Orthopaedics and Trauma puts polymerization temperatures in the body at about 82 to 86 °C, and notes that keeping the cement layer thin helps limit it (Vaishya et al., 2013).

• Permanence. It stays unless it is surgically removed. For a younger patient, or anyone likely to need revision, a block of cement in the defect can make later surgery harder.

• No remodeling. Bone does not grow into PMMA, so the defect is never biologically restored.

PMMA also has well-documented procedure-specific risks, such as bone cement implantation syndrome in cemented arthroplasty. These belong in the clinical literature for each procedure and are outside the scope of this comparison.

What resorbable bone void fillers are used for

Resorbable fillers are used where the aim is new bone rather than permanent support. Typical uses include:

• Metaphyseal voids in fractures, with a plate providing stability

• Cavities after curettage of benign lesions

• Contained, non-structural defects in revision surgery

• Fusion procedures, often combined with autograft or allograft

How fast a filler resorbs depends on what it is made of. Calcium sulfate tends to go quickly. Apatite calcium phosphate cements degrade slowly, while brushite cements are more soluble and resorb faster (Gelli and Ridi, 2023). The magnesium vs calcium phosphate cement review covers those differences in more detail.

Why self-setting fillers aren't a substitute for PMMA

Calcium phosphate and magnesium phosphate cements are mixed into a paste and harden in place, so on the back table they can look a lot like PMMA. The differences are fundamental:

• They are mineral, not polymer.

• They are designed to be resorbed, though the rate varies.

• When cleared as bone void fillers, they are not intended to provide structural support during healing.

Some can be drilled once set, so a screw can pass through the filled area. That makes them easier to work with, but it doesn't make them a replacement for PMMA where load-bearing is needed.

Choosing between PMMA and a bone void filler

A useful starting question is: does this defect need permanent, immediate mechanical support from the material itself? It isn't a universal rule. Many cases combine approaches, and patient factors can change the answer. It does narrow the options quickly:

• If the material has to carry load, as in implant fixation, vertebral augmentation or some tumor reconstructions, PMMA or another structural option such as hardware or structural graft is usually considered.

• If the aim is new bone in a contained, non-structural void, a resorbable filler is often used, with hardware providing stability where needed.

Patient age, the chance of future revision, infection risk, how contained the defect is and the product's cleared indications all shape the final choice.

Where Bone Solutions fits

Mg OSTEOCRETE and Mg OSTEOINJECT are magnesium-based bone void fillers, not PMMA alternatives. They are cleared for bony voids that are not intrinsic to the stability of the bony structure, and they can be placed before or after final fixation. In the extremities and pelvis they can be used as an adjunct to conventional rigid hardware fixation, supporting the bone fragments during the surgical procedure only. Once set, they act as a temporary support medium and are not intended to provide structural support during the healing process. Rx only. See each product's IFU for full indications, contraindications and warnings (FDA K251522).

Questions about whether a void in your case fits these indications? Contact our team for product information.

Frequently asked questions

Is PMMA a bone void filler?

PMMA is a bone cement. It can fill a defect, but it is permanent and load-bearing, and it does not remodel into bone. "Bone void filler" usually refers to resorbable materials.

Is bone cement permanent?

PMMA bone cement is. It stays in the body unless it is surgically removed. Mineral "cements" used as bone void fillers, such as calcium phosphate and magnesium phosphate, are designed to resorb.

Why does PMMA get hot?

PMMA hardens through an exothermic polymerization, reaching about 82 to 86 °C in the body according to Vaishya et al. That is why cement layers are kept thin.

Can a bone void filler replace bone cement?

Not where the material itself has to carry load. Cleared bone void fillers are not intended to provide structural support during healing.