Objective: This study was designed to assess the shear bond strength of 3D printed Polymethylmethacrylate (PMMA) material to two different types of repair composite resin materials following two different surface roughening techniques.

Materials and methods:
Thirty-six discs were 3D printed from PMMA liquid resin, aged and then allocated to 3 groups (n=12 in each) according to the surface roughening method: Group P: As printed and untreated, Group G: discs were grinded using diamond stone and Group A: discs were air abraded by 53 um alumina particles. Universal adhesive was applied, and light cured. Then, half of the discs was bonded to Flowable composite resin, and the other half was bonded to injectable composite resin according to the standards of shear bond strength test (SBS). All discs were stored in distilled water for 7 days and then SBS test was carried out using a universal testing machine. The mode of failure was observed using a digital stereomicroscope. All calculated data was sent for statistical analysis.

Results: Grinded PMMA discs repaired with Injectable composite showed the highest statistically significant difference (21.32 ±1.71) MPa in comparison to untreated (9.52 ±1.86) MPa and air-abraded ones (14.05±4.47) MPa and also higher than Grinded PMMA repaired with flowable composite (11.87 ±4.16) MPa.
Conclusions: It is attainable to establish an acceptable bond with PMMA. Grinded PMMA had a greatly enhanced repair bond strength to injectable composite resin.