Surgical procedures Randomization was performed prior to surgical therapy by a distribution system by sealed cards. One experienced periodontist (AS) performed all surgical procedures instrumented by 2nd and 3rd year periodontal residents (RG, ML). Two different approaches for the treatment of infrabony defects were compared in a randomized controlled double blind clinical trial. Neither the examiners nor the patients were aware of the type of procedure performed. Both surgical procedures were performed under local anaesthesia and the same surgical access to the bony defect was performed. An envelope full thickness mucoperiosteal flap was raised buccal and lingually following simplified papilla preservation incisions. All granulation tissue was removed from the defects and the roots were thoroughly scaled and planed using hand and ultrasonic instruments. The test group was treated by filling the infrabony defect with EMD/BC and the control group with EMD alone. Previously, root surfaces adjacent to all defects were treated with PrefGel® (24% EDTA gel) for 2 minutes according to manufacture's instructions. The root was subsequently rinsed with saline and the bleeding was controlled with the use of gauzes. Topical application of EMD on the root surface in an apical-coronal direction was performed in both study groups. In the test group, the defect was filled in addition with a mixture of the remaining EMD in the syringe and BC. For both treatments, the mucoperiosteal flap was re-positioned and sutured with a non-absorbable suture (Monofilament 6/0) with a horizontal mattress suture and simple suture technique.