OBJECTIVE To test whether the use of (i) particulated bone substitute + collagen membrane used for guided bone regeneration (GBR) of peri-implant bone defects renders different results from (ii) particulated bone substitute + collagen membrane + fixation pins and from (iii) block bone substitute + collagen membrane with respect to the volume stability of the augmented region during suturing of mucosal flaps. MATERIAL AND METHODS Twenty peri-implant box-shaped bone defects were created in 10 pig mandibles. Every bone defect was augmented once with each of the following GBR procedures: Granulate (particulated xenograft + collagen membrane), Granulate + Pins (particulated xenograft + collagen membrane + fixation pins), and Block (block xenograft + collagen membrane). Cone-beam computed tomography scans were obtained prior and after blinded wound closure. The horizontal thickness (HT) of the augmented region (bone substitute + membrane) was assessed at the implant shoulder (HT0 mm ) and at 1-5 mm apical to the implant shoulder (HT1 mm -HT5 mm ). The changes of HT during flap suturing were calculated as absolute (mm) and relative values (%). Repeated-measures ANOVA was used for statistical analysis. RESULTS Wound closure induced a statistically significant change of HT0 mm and of HT1 mm in all the treatment groups (P ≤ 0.05). The change in HT0 mm measured -42.8 ± 17.9% (SD) for Granulate, -22.9 ± 21.2% (SD) for Granulate + Pins, and -20.2 ± 18.9% (SD) for Block. The reduction in HT0 mm , HT1 mm , HT2 mm , and HT3 mm for the Granulate procedure was significantly higher as compared to the Granulate + Pins and the Block procedures (P ≤ 0.05). There were no statistically significant differences in the change of HT between the Granulate + Pins and the Block procedures (P > 0.05). CONCLUSION Wound closure induced displacement of the bone substitute resulting in a partial collapse of the collagen membrane in the coronal portion of the augmented site. The stability of the bone substitute and collagen membrane was enhanced by the application of fixation pins and by the use of block bone substitute instead of particulated bone substitute.