The objective of this prospective, randomized, pilot study is to determine whether the Conelog® connection is suitable for splinted/non-splinted cemented crowns in the chewing centre (focus only on implant-abutment connection) and whether implants with the Conelog® connection and a length of 7mm are suitable for use in the molar region of the lower jaw.

In order to avoid sinus augmentation in the upper jaw and a vertical augmentation in the lower jaw, various manufacturers offer short implants of 6-8mm length. The prevention of augmentative measures, especially in the mandible, is a fundamental component of the minimization of invasiveness while at the same time improving the functional capability. The benefit lies in the possibility to place implants without prior bone augmentation even in difficult anatomical conditions. This significantly reduces the invasiveness which on the one hand allows a higher number of patients to be treated with implants (especially morbid patients and anxiety patients) and on the other hand makes the treatment more predictable, with less risk, cost-efficient and considerably faster. Moreover, new scientific insights are gained in the area of short implants.

As with all implants, the risks associated with the study consist of intraoperative complications (e.g. damage of adjacent structures), failure of the implant (e.g. impairment of wound healing, fracture of the implant body), the abutment or abutment screw (e.g. loosening, fracture), or the crown on the abutment (veneer fracture, scaffold fracture).

The objective is to determine whether implants with the Conelog® connection and a length of 7mm are suitable for splinted/non-splinted monolithic lithium disilicate crowns cemented on individualized titanium abutments in the chewing centre of the mandible. The main indication for short implants is severe atrophy of the jaw, which creates a longer distance from the implant shoulder to the occlusal plane. An unfavourable crown-to-implant ratio means higher stress on the screws, the implant-abutment surface and possibly on the osseointegration of the implants. This raises the question whether splinted crowns have a positive effect on implant survival compared to non-splinted crowns.

The two 7mm Conelog ® implants are either used in the area of the second premolar and the first molar or the first and second molars of the mandible. The diameter of the implant depends on the width of the alveolar bone and ranges from 3.8 to 4.3 mm.

In case of a bilateral free-end situation, both quadrants are treated as part of the study, and both treatment variants are applied by using splinted crowns on one side and individual crowns on the other (split mouth design).

20 patients will be included in the study, which will be divided into two equally sized groups of 10 patients. All patients are recruited by the three clinical investigators at the Department of Prosthodontics and Biomaterials, Centre for Implantology, Medical Faculty, Rheinisch-Westfälische Technische Hochschule Aachen (RWTH) Aachen University Hospital. In order to achieve sufficient patient numbers, the population of Aachen will be informed via print media.

Each patient receives a single dose of antibiotics one hour prior to implant surgery. A mucoperiosteal flap was mobilized and the implants were placed as recommended by the manufacturer, using a surgical template as a guide. Healing abutments were screwed, and submerged healing is used for all implants. Second stage surgery is performed after 3 months. The prosthetic restoration is completed within 4 weeks after exposure.

Patients in group A will receive splinted monolithic lithium disilicate crowns cemented on individualized titanium abutments. Patients in group B will receive 2 non-splinted monolithic lithium disilicate-crowns cemented on individualized titanium abutments.

The cementation is done with Multilink Implant ® on the machined abutments, which have been cleaned for 5 minutes in an ultrasonic cleaner with 95% alcohol. The crowns are conditioned with hydrofluoric acid for 20 seconds and silanized with Monobond Plus® for 60 seconds. The day of insertion of the restoration is marked as baseline.

The examination performed at baseline and the follow up after 6 months includes occlusion, probing depth, plaque index and the condition of the ceramic surface of the crowns. Additionally, radiographs with customized positioning jigs are taken at baseline, after 1, 3, and 5 years.

Inclusion Criteria:

• female and male subjects older than 18 years
• absence of all molars and, optionally, the second premolar in at least on quadrant of the lower jaw
• the periodontal situation of the other teeth is healthy or periodontal treatment has already been successfully performed
• the antagonists are natural teeth or treated with fixed dentures
• good health according to the ASA Physical Status Classification System one or two (ASA 2010)
• good oral hygiene and an at most moderate tobacco consumption
• bone height above the N. alveolaris inferior of at least 11mm and the precondition of a one-stage implantation and augmentation
• signed consent

Exclusion Criteria:

• Indication for large augmentations of the jaw bone
• Bone abundance with minimal risk of injury to neighbouring structures when using two 11 mm or longer implants
• Psychological disorder
• substance abuse
• removable dentures (with or without implants)
• pregnancy
• patients under age or unable to reason
• persons who are placed in an institution on a judicial or administrative order