Introduction: Posterior free end edentulous areas are more prevalent among population. Absence of posterior abutments to support and retain partial dentures affects the prognosis of prostheses. A problem of support, retention and stability is usually associated with distal extension removable partial dentures.
Objectives: To evaluate and compare the effect of implant locations on the retention of the prostheses of implant tooth assisted partial overdenture for mandibular Kennedy class II with implant located in first premolar area and second molar area. Second objective was to evaluate the wear of female parts of the attachments attached to these implants.
Materials and methods: The study was conducted on two parallel groups according to the different implant locations: Group (A) implant in first premolar area, Group (B) implant in second molar area. Two acrylic models with Kennedy class II were used in this study. Two implants with (2.3mm length and 3.6mm width), two positioner attachments were attached to implants and inserted in the previous locations for both groups. Each group had 9 sets of attachments with 3 different colored plastic matrices (27 matrices) with different retention force.  A metallic overdenture with acrylic resin base and teeth was constructed for each group. By using the universal testing machine, vertical load dislodgment forces were measured at anterior of the saddle of the denture and posterior of the saddle of the denture for both groups at the beginning of the study (initial retention) and after 50,000 cyclic with load of 20 K at first molar area, the retention test was repeated and the mean value was recorded. All retention parts were imaged under stereomicroscope to study wear and attrition of internal components. All data from both groups in this in-vitro study was gathered, tabulated and statistically analyzed.
Results:In vertical dislodging, Group (B) posterior of the saddle, recorded the highest retention force. In Group (B) anterior of the saddle recorded the lowest retention force. After 50,000 cyclic loading, Group (B) posterior of the saddle was higher than Group (A) in retention force. Dimensional changes and wear scratches appeared in the metal housing and plastic matrices in central and outer lines.
Conclusions: The distal position of the implant with positioner attachment has great retention during vertical dislodgment compared with mesial implant location with positioner attachment.