Evaluation of polymethylmethacrylate adhesion: a comparison of direct onlay versus screw anchoring techniques

Polymethylmethacrylate (PMMA) is still the most frequently used alloplastic material for calvarial reconstruction, especially when dealing with large bony defects. It is strong, provides good protection to the underlying cerebral structures, and is stable and minimally reactive. One of the disadvantages of this material is its tendency to become loose over time because of its poor adherence to bone. Onlay miniscrews in improving PMMA's adhesion to bone have been previously proposed. A series of experiments were conducted to evaluate whether placement of screw anchors will significantly improve the adhesion force between the polymer and bony surface.

Four fresh-frozen cadaver heads were used for this experiment. The PMMA preparation and setup time strictly followed manufacturer guidelines. Two experimental groups were created: (1) PMMA was placed on the subperiosteal bony surface with increasing surface areas (areas: 1-20 cm2), and (2) a standard area of 16 cm2 PMMA was placed on the bony surface with an increasing number of titanium miniscrews (number of screws: 0-5). The force required to separate the material from the underlying bone was assessed using a digital pull force gauge (Imada DPS-44) through vertical traction. The experiments were undertaken in triplicate; the results were statistically analyzed using Student t test.

Experiment 1: increasing forces were required as the surface area of PMMA application increased (1.2-42.3 N). The most consistent measurements with a low SD were obtained on the 16-cm2 implant, which was chosen for experiment 2. A 16-cm2 area would allow for the placement of up to 5 screws without technical difficulty. Experiment 2: higher forces were needed to detach the material, with increasing screw placement (1, 79; 2, 132.5; 3, 194.2; and 4 and 5, >196.1 N). In 73 of 75 experiments, the screws remained attached to the PMMA after separation. When the PMMA alone on a 16-cm2 surface area was compared with the placement of one or more screws, the force of adhesion significantly increased for all groups (P < 0.01). There was a 2.6x increase in this force with 1 screw, 4.4x with 2, 6.4x with 3, and 6.5x with 4 or more screws. Three or more miniscrews provided sufficient stabilization to anchor an implant firmly in place while resisting large traction forces.

Although greater surface areas of PMMA will increase the adhesion force between the polymer and bone, a clinically and statistically significant increase in this force may only be achieved with the use of miniscrews.