The Influence Of Mastication Forces On Composite Resin Inlays In Vivo *H. Lang, R. Müllejans, R. Nolden, W.H.-M. Raab (Dept. Of Restorative And Preventive Dentistry, University Of Duesseldorf, Germany; 2: Dept. Of Operative Dentistry And Periodontology, University Of Bonn, Germany) The marginal integrity of composite resin inlays is compromised by several factors in vivo (e.g. thermal loading or mastication). Repeated mechanical loading is known to induce marginal disintegration, but the clinical consequences are still unclear. In this 4-year clinical study, the influence of direct loading by an antagonistic tooth on marginal integrity of composite resin inlays was investigated.   In 84 patients molars and premolars were restored with small to middle-sized composite resin inlays. Mesial-occlusal (n=48) or distal-occlusal (n=36) cavities with an approximal margin in dentin were prepared. Each cavity received a zinc phosphate cement base and the dentin was treated using a dentin bonding agent (Syntac®). The teeth were restored with laboratory fabricated composite inlays (Tetric®, luting composite resin: Variolink®). Clinical status was recorded after 6, 12, 24, 36 and 48 months (Ryge criteria) and microscopic assessment of the marginal integrity (epoxy resin replicas) was performed.   One composite resin inlay fractured after 10 month. The evaluations showed little change in clinical status (88% Alpha). The microscopic assessment revealed only minimal leakage at enamel margins (all inlays: 2,2%). Margins below the cemento-enamel junction showed a significant loss of marginal integrity in inlays with occlusal contact to the antagonistic tooth (16,8%, p<0.05). Composite resin inlays not directly loaded by the antagonist also showed a loss of marginal integrity at margins to dentin (4,9%) but there was no statistical significant difference between these margins and margins to enamel (1,9%). In molars with directly loaded restorations marginal disintegration (6,7%) was more pronounced when compared to premolars (3,1%).   Conclusions: Direct loading by the antagonistic tooth is a risk factor for the integrity of dentin margins of bonded composite resin inlays. This is particularly true in molars. In contrast, inlays with no antagonistic contact or margins completely in enamel seem to be at low risk for marginal breakdown within the investigated period.