Through an extra-oral approach, an oblique incision was made bilaterally along the sagittal axis between the facial tuberosity and the inferior orbital rim. The skin and periosteum were elevated separately, and the bony facial sinus wall was exposed on both sides of the maxilla (Fig. 1a).

A 12-mm large and 8-mm high antrostomy was prepared using a burr (H254E Komet Dental, Trophagener Weg 25, Lemgo, Germany) and the bone plate was removed bilaterally (Fig. 1b). The sinus mucosa was subsequently carefully elevated with sinus floor elevators, exposing the medial bony plate of the maxillary sinus (Fig. 1c). A landmark was positioned, twisting a steel wire through a hole above the access window (Fig. 1d).

After sinus mucosa elevation, the obtained space was filled with a biphasic calcium phosphate (60% HA, 40% β-TCP) (Easy-graft^TM CRYSTAL; Sunstar GUIDOR, Etoy, Switzerland) on both sides (Fig. 1d).

Experimental and control sites were randomly chosen. The control site was covered with a polylactic acid blended with a citric acid ester membrane (GUIDOR® matrix barrier; Sunstar Americas Inc., Chicago, IL, USA) and secured with a minimum amount of cyanoacrylate glue (Indermil® x fine; Henkel-Loctite Corp. Dublin, Republic of Ireland) accurately positioned in four points (one at each corner; Fig. 1e, f). Cyanoacrylate glue was stored at 4 °C and used immediately after leaving the refrigerator, to keep the viscosity low. At the experimental site, similar procedures were applied (Fig. 2a–c), and the bony window was repositioned in place and secured with the four points of cyanoacrylate (one at each corner; Fig. 2d). Suturing in layers was then performed.

Gentamicin (Gentamicin-5® Bela-Pharm GmbH, Vechta, Germany) 8 mL/100 kg was administered every 12 h during the first postoperative day, and every 24 h during the following 3 days. The sheep were kept in an animal house individually, in a roofed shed to reduce distress after surgery, with a concrete floor. The boxes were cleaned daily and the animals had free access to water. The diet was based on a balanced specific food made of cereals, protein, and concentrates of vitamins and minerals, with added green forages. The wounds were cleaned by the veterinarians of the centre every day during the first week, and then inspected three times per week for clinical signs of complications for the duration of the experiment.