• Background There is a growing interest in restoring function in the posterior maxilla and mandible with dental implants. Next to this, these implants must be restored with crowns which are subject to minimal complications. Companies have introduced the option of screw-retaining and zirconia as crown material. The combination of posterior implants and screw-retained zirconia crowns are widely used, but prospective research on clinical performance and patient centered outcomes are scarce. Even more, 5-years data are lacking.
• Main research question To evaluate the clinical performance of Astra Tech Implants EV and Atlantis CustomBase Abutments with full-contour zirconia Atlantis Crowns in the posterior maxilla and mandible. The primary objective of the study is marginal bone level changes by radiological assessments at 5-years follow-up. Secondary objectives are implant and restoration survival, condition of peri- implant mucosa and patients' satisfaction.
• Design (including population, confounders/outcomes) The study design is an observational study of a group of patients which were treated 5 years ago with a dental implant and a screw-retained implant-supported zirconia restoration because of a missing tooth in the posterior region. Outcomes: primary outcome is the change in marginal peri-implant bone level 5 years after placing the definitive restoration. Secondary outcome measures will be implant and restoration survival, peri-implant mucosa health and patients' satisfaction using a questionnaire.

• Introduction and rationale Nowadays, the use of dental implants for oral rehabilitation is a generally accepted treatment modality. During the first years of implant dentistry, dental implants were used mainly for the restoration of edentulous mandibles with implant-supported prostheses. Yet, there is a shift towards the application of dental implants for single-tooth replacements, supported by long-term studies reporting excellent survival rates. Not only in the aesthetic region, but also in the posterior maxilla and mandible there is a growing interest in restoring function. Dental implants show a good performance in the posterior region; in recent years attention has been shifted towards a better initial stability to reduce treatment time. Next to this, implants must be restored with crowns which are subject to minimal complications. Specially for the posterior region, companies have introduced the option of screw-retaining and zirconia as crown material, of which less technical complications are mentioned. The combination of posterior implants and screw-retained zirconia crowns are widely used, but prospective research on clinical performance and patient centered outcomes are scarce and 5-years results are lacking.

RESEARCH QUESTION To evaluate the clinical performance of Astra Tech Implants EV and Atlantis CustomBase Abutments with full-contour zirconia Atlantis Crowns in the posterior maxilla and mandible after a 5-years follow-up.

STUDY DESIGN The study design is an observational study of a group of 50 patients which were treated 5 years ago with a dental implant and a screw-retained implant-supported zirconia restoration because of a missing tooth in the posterior region.

Inclusion Criteria:

• Patients referred to the department of Oral and Maxillofacial Surgery 5 years ago and treated with a dental implant and a screw-retained implant-supported zirconia restoration because of having a missing tooth in the posterior region. At the time of treatment:
• The patient was 18 years or older;
• The missing tooth was a premolar or molar in maxilla or mandible;
• Sufficient healthy and vital bone to insert a dental implant with a minimum length of 8 mm and at least 4.2 mm in diameter with initial stability > 45 Ncm
• The implant site was free from infection;
• Adequate oral hygiene (modified plaque index and modified sulcus bleeding index ≤ 1);
• Sufficient mesio-distal, bucco-lingual, and interocclusal space for placement of an anatomic restoration;
• The patient was capable of understanding and giving informed consent.

Exclusion Criteria:

• Medical and general contraindications for the surgical procedures;
• Presence of an active and uncontrolled periodontal disease;
• Bruxism;
• Smoking
• A history of local radiotherapy to the head and neck region.

Study population: Patients referred to the department of Oral and Maxillofacial Surgery 5 years ago and treated with a dental implant and a screw-retained implant-supported zirconia restoration because of having a missing tooth in the posterior region. At the time of treatment:

• The patient was 18 years or older;
• The missing tooth was a premolar or molar in maxilla or mandible;