Purpose: The aim of the following study was to assess and compare the effect of treatment with conventional complete dentures, two implant retained mandibular overdenture and four implant supported mandibular overdenture on mandibular posterior residual ridge resorption over a 5-year period.
Material and methods 24 patients were selected from previous studies, where (group I) included 8 patients previously treated with conventional complete dentures. The second group (group II) 8 patients received mandibular overdenture using two osseointegrated implants, (group III)8 patients received implant supported mandibular overdenture using two implants in canine region and two implants in the first molar region connected with rigid bar. Posterior residual ridge resorption was evaluated for all groups using proportional area measurements.
Results: After 5years of prosthetic treatment, regarding posterior residual ridge resorption, statistically significant bone resorption had occurred for all the three groups, also statistically significant difference was reported when comparing the two-implant overdenture group to the four-implant supported overdenture group.

Conclusion Four implant supported overdenture with the posterior implants in the first molar region showed the least amount of posterior ridge resorption after 5 years of treatment in comparison to conventional complete dentures or two implant retained overdenture.