polymethylmethacrylates

a) Properties

• It is the first and most widely used cement in vertebroplasty.  Its mechanical properties allow a durable result to be obtained in the treatment of osteoporotic compression fractures and tumor invasion with low complication rates.
• PMMA cements are, due to their low viscosity, easy to inject into vertebral bodies.
• The composition of PMMA consists of:
• Methylmethacrylate/dimethyl-p-toluidin monomer (liquid)
• Methylmethacrylate/methylacrylate polymer (powder).
• PMMA is generally unreactive in bone formation. Pathological examination demonstrates a fibrous tissue layer between implanted PMMA and bone.
• During the polymerization of the PMMA, the temperature of the cement reaches 80° to 120°Celsius.  Moreover, this exothermic reaction could potentially damage adjacent tissues in case of cement leakage.  Although, this thermal effect induces cell coagulation that could be useful in the pain management of malignant tumors (e.g., metastatic lesions).

Figure 1: Exotherm temperature profile of Cortoss® and PMMA as a function of time.

Figure 2:  78-week PMMA histology (sheep).  Soft tissue encapsulation of prepolymerized PMMA.

• The mechanical properties of PMMA are detailed in the next chart:

b) Modalities of use

1) Preparation of the cement

• Before the injection, the two components (20 g of powder and 20 ml of liquid monomer) of PMMA (Osteopal®-Biomet Merck, Palacos® or Simplex® P-Stryker Howmedica Osteonics) need to be well mixed in order to obtain a homogenous mixture.

Figure 3: Mixing of the monomer (liquid) and the polymer (solid).

• The monomer-to-polymer ratio recommended by the manufacturers is 0,5ml/g. However, most of the time, physicians do not respect this ratio in order to obtain a longer setting time and to decrease the viscosity of the cement.  This modification may increase the unreacted monomer (liquid) content available to enter the circulatory system and induce arterial hypotension, cardiac or neurological dysfunction.
• The radiopacity of those cements is also not sufficient for fluoroscopic monitoring in vertebroplasty procedures.   To obtain a safe radiopacity of the cements, a radio-opaque contrast should be used.  Those agents available are tungsten, tantalum, barium sulphate, titanium, zirconium or gadolinium powders.
• Meanwhile, the different changes in and manipulation of the cement composition (addition of contrast, modification of the polymer-to-monomer ratio, withdrawal of some quantity of cement mixture) increase the risk of sterility mistakes and alter the mechanical properties of the cement, especially the strength and the compressive properties.

Figure 4: Cement compressive modulus as a function of the monomer-to-powder ratio for Simplex® P.

• Moreover, those different changes of the initial composition of the cement induce a modification of the approval of the cement by the different authorities and  manufacturers.
• Therefore, there are now new “ready for use” PMMA cements for vertebroplasty (Osteopal® V- Biomet Merck, Vertebroplastic® - Johnson and Johnson®) which allow safe injection without manipulation of the cement.

 Figure 5: Presentation of the Osteopal® V (specific cement for vertebroplasty).

• In this cement, the addition of zirconium to the composition makes it radiopaque enough for fluoroscopic monitoring.  The composition of one of this cement (Osteopal® V) that we use in our departement is:
• Powder (26 g): polymethyl acrylate, methyl methacrylate, zirconium dioxide, benzoyl peroxide
  Liquid (10 ml): methyl methacrylate, N,N-dimethyl-p-toluidine.
  This new presentation allows an easier, faster and safer injection of the cement into vertebral bodies.

b) Injection of cement

• PMMAs have a long setting time (8 minutes,  depending on the ambient temperature) and during that time, the cement keeps almost the same viscosity. Those two conditions allow a safe injection in the bone with an appropriate injection set.
• The optimal temperature is as close as possible to 20°C.  Higher temperatures will reduce the setting time.
• In our department we use an injection set with a special bevelled needle for the introduction of the cement into the vertebral body.

Figure 6: Injection set (Cemento®-Optimed Merck)

• After the positioning of the needle, the pressure syringe allows us to maintain constant pressure during the injection of the cement by slowly turning a screw. 
• When minor leakage occurs, we can immediately stop the injection by removing the pressure inside the syringe to avoid more significant leakage (see Vertebroplasty with PMMA section).