Considering the necessity of guide planes

To enhance the effectiveness of direct retainers, as many guide planes as possible should be utilized. Although guide planes are more effective in tooth-supported edentulous areas, they also help guide the path of insertion in distal extension dentures. The effectiveness of guide planes is proportional to their number and height (Figure 8-8). They may be naturally present on the axial surfaces of certain abutment teeth or may be shaped by mouth preparation.