Patients aged 18+ who required an implant in the posterior mandible were eligible for this study.

Exclusion criteria were the following:

general contraindications to implant surgery

insufficient oral hygiene and periodontitis

patients with a history of severe periodontitis

bone augmentation required

smokers

substance abuse

uncontrolled diabetes

severe cardiovascular problems

treated or under treatment with intravenous amino-bisphosphonates

pregnant or lactating.

The study was conducted in accordance with the standards of the Declaration of Helsinki of 1983 and was approved by the ethics committee of the FEKI (Freiburger Ethik-Kommission International, Feki Code: 014/1210). All 40 recruited patients were informed about the design and aim of this study, and written consent was obtained. The randomization to control group (n = 20) and test group (n = 20) was achieved using a sealed envelope system at time of surgery.

All patients had to undergo a professional dental hygiene treatment in advance. One hour prior to surgery, patients were given an antibiotic single shot prophylaxis (600 mg clindamycin). PRF was obtained from each patient of test and control groups and treated according to the PRF protocol with an IntraSpin™ table centrifuge and collection kits provided by Botiss (Zossen, Germany).

After anesthesia with Ultracain® DS-forte (articaine + adrenaline 1:100,000), crestal, lingual, and buccal tissue thickness was tested using an endodontic micro-spreader (Spreader ISO 30, Dentsply Maillefer®) with a silicon stop (illustration 1). The instrument part penetrating the soft tissue was measured with an endodontic longimeter. Measured data was rounded off to the nearest half millimeter (mm).

The initial preparation of the split-flap was carried out the same way in test and control groups using microsurgical instruments. After a crestal incision with a microsurgical blade (SM69, Swann Morton LTD®, Sheffield, England), the split-thickness flap was sharply prepared by elevating the area of the single tooth gap to the middle of the adjacent teeth. The periosteum was split to receive a tension-free adaption of the flaps (illustration 2). Fully threaded titanium implants (Nobel Speedy Replace®, Nobel Biocare, Zurich, Switzerland) were inserted at bone level with primary stability. The implants varied in diameter (narrow platform 3.5 mm, regular platform 4.0 mm, wide platform 5.0 mm) and in length (10 mm, 11.5 mm, 13 mm) (illustrations 3, 4, and 5). For the further procedure, patients were now randomized by a dental assistant using a sealed envelope system. In the test group, the tissue was augmented with a PRF membrane using a double-layered technique. In the control group, the implant treatment was realized without mucosa thickening (illustrations 6 and 7). Flaps were sutured with a non-absorbable polyvinylidene fluoride suture (Seralene®, Serag Wiessner, Naila, Germany) (illustration 8).