Monomer Conversion and Mechanical Properties of Composites for Maxillofacial Surgery

Objectives: The aim was to develop chemically activated two-paste bone composite cements containing apatite-promoting monocalcium phosphate monohydrate (MCPM) and antibacterial polylysine (PLS). Effects of varying levels of these hydrophilic additives on setting kinetics, and set material water contact angle, strength, modulus, and surface apatite promoting ability were assessed.
Methods: Hydrophobic dimethacrylate monomers (25 wt%) containing initiator or activator were mixed with powder (75 wt%) to give 2 pastes. Powder consisted of dental glass filler with MCPM (8 or 6 wt%) and PLS (4 or 2 wt%). Setting times and final monomer conversions following two-paste mixing were assessed using FTIR (n=3). Water contact angle on cured composites was measured using a goniometer (n=3). Biaxial flexural strength (BFS) and modulus were tested using ball-on-ring testing jig (n=5) after 1-day simulated body fluid immersion. Surface apatite precipitation was assessed using Raman microscopy (n=1) after 7 days. Factorial analysis was used to quantify effects of low versus high additive contents and a formulation with no MCPM or PLS employed as control.
Results: All composites exhibited a delay time of 2-3 minutes followed by rapid polymerisation and greater than 80% final monomer conversion. Hydrophilic additives, reduced mean surface contact angles from 76 to 62 degrees, BFS from 136 to 108 MPa and modulus from 3.6 to 2.7 GPa. High versus low PLS decreased contact angle and BFS by 19 ± 13 % and 12 ± 6% whilst the effect of MCPM level was negligible. A surface Raman phosphate peak (960 cm^-1) attributed to apatite precipitation appeared over time with MCPM present.
Conclusions: All formulations had setting and mechanical properties suitable for bone repair in maxillofacial surgery. High monomer conversions should reduce potential toxic monomers release, whilst increased hydrophilicity and apatite precipitation could enhance cement adaptation to water-containing bone, and promote bone bonding respectively.