composite cements

1. Properties

• Bisphenol-a-glycidyl dimethacrylate (bis-GMA) resins have been used since the late 1970’s in orthopaedic applications (pedicle screws augmentation).  Those cements were developed in order to offset such disadvantages of PMMA  as exothermic reaction, possibility of the release of unreacted monomer in the circulatory system and modification of the initial composition of the PMMA (changes in the monomer-to-polymer-ratio and addition of contrast materials).
• The research was also directed to obtain more biocompatible, easy-to-handle cement with sufficient radiopacity and with good biomechanical properties.
• One of these composite cements is Cortoss® developed by Orthovita (Malvern, USA).

Figure 7: Presentation of Cortoss® synthetic cortical bone void filler. 

• This product was already used in craniofacial surgery and for dentistry.  Now a multicenter study has evaluated Cortoss® as bone void filler for percutaneous vertebrobplasty.
• Major components that make up Cortoss® are:
  • Bisphenol-a-glycidyl dimethacrylate (Bis-GMA)
  • Bisphenol-a-ethoxy dimethacrylate (Bis-EMA)
  • Triethylene glycol dimethacrylate (TEGMA)
  • Glass and ceramic fillers (to stimulate bone apposition)
  • Barium boroaluminosilicate glass (for radiopacity and strength)
  • Silica (for improved viscosity)

• A study compares the biocompatibility and interfacial bond strengths of Cortoss® synthetic cortical bone void filler with a PMMA use in percutaneous vertebroplasty (Simplex® P) implanted into rabbit femurs for up to 52 weeks and in sheep long bones for up to 78 weeks.
• This study showed that new periosteal and endosteal bone were formed within defect sites filled with either both of the cements but that the initial response was greater with Cortoss® than with the PMMA. Concerning the bone formation, new blood vessels invaded the periphery of Cortoss® implants whereas PMMA was unreactive.

• Both cements were surrounded by bone in the long term but half the Simplex® P specimens were separated from bone by a layer of fibrous connective tissue at 24 weeks.
• In terms of displacement forces, this study shows an augmentation with time for both cements but these displacement forces were greater for a rod held in place with Cortoss® than with PMMA.  A relative strength difference of 4,5 Newton was observed between the two cements after 24 weeks. This difference is attributed to a faster initial bone response and a greater degree of mineralization around Cortoss®.
• Another advantage of composite cements is the low temperature at which they become solid. The major temperature does not exceed 58 °C.
• This property avoids adjacent tissues alterations but also avoids the cell coagulation.

• Other properties of Cortoss® are described next:
  The fatigue strength is measured to 10 million cycles in load compression for both Cortoss® and PMMA cements.
  Cortoss® survived 10MM cycles at 80 Mpa in compression.
  Cortoss® survived 10MM cycles at 15 Mpa in tension.
  PMMA failed at 10MM cycles at 45 Mpa in compression.
  PMMA failed at 1MM cycles at 15 Mpa in tension.
  Highest creep of Cortoss® in 24 hours was 22% (at 180 Mpa).
  Highest creep of PMMA in 24 hours was 82% (at 80 Mpa).
  
  

2. Modalities of use

a) Preparation of the cement

• Cortoss®, contrary to PMMA, does not need any manual mix of the 2 components (monomer and polymer).
• Cortoss® is a two-part paste system that is packaged sterile in a delivery cartridge.  Disposable mix tips blend the 2 pastes automatically at the time of injection.  The cement is also never mixed at once (“mix-on-demand system”) that allows physicians making the injection not to hurry.

• While the exotherm of Cortoss® remains low, the temperature of the material as well as the temperature of the body can affect the set time.
• The optimal temperature of Cortoss® to be used is as close as possible to 20°C. Higher temperature will reduce the setting time.

• To obtain a good fluoroscopic visualisation, there is no need to modify Cortoss®. It contains over 65% of radiopaque fillers.

b) Injection of the cement

Cortoss® system use syringes and catheters for the injection in the vertebral bodies.

Figure 14:  Material used for Cortoss® (delivery gun, cartridge of pastes, syringes and catheters).

• 1 cc Luer-Lock syringe is connected to a catheter and both are filled with the cement using the delivery gun and the mix-tips. After that, the catheters are inserted successively in the needle until obtaining the optimal filling of the vertebral body.
• Concerning the viscosity, Cortoss® remains at the same viscosity state during a large percentage of its set time until a “snap-set”.
• This could be an advantage or an inconvenience:
• An advantage, for an easier injection during all the filling time of the vertebrae.
• An inconvenience, because of the continuous risk of leakage during all the injection.  After approximately 8 minutes, fast setting would occur with risk of blockage of the needle inside the vertebrae.

Figure 15: Treatment of an osteoporotic compression fracture with Cortoss® . Lateral fluoroscopic view.  Introduction of the catheter through the needle.

Figure 16: Injection of Cortoss® in the vertebral body under lateral fluoroscopic guidance.  Good radio-opacity.

Figure 17: Lateral fluoroscopic view.  Final result.

Figure 18: Axial CT. Good distribution of Cortoss® in the vertebral body.

• Moreover, in our experience, the catheter-and-syringe system for cement injection is also not so easy and safe.  It compels the physician always to prepare new syringes during the injection of the same vertebral body.  The pressure of the injection created by the fingers of the physicians varies with each person and the progression of the cement through the catheter does not stop immediately when the physician removes the pressure.  The major consequence of these drawbacks is an increase risk of cement leakage.