Impact of Cantilever Design on Clinical, Radiographic, and Volumetric Parameters: a Retrospective Study

Implant rehabilitation of posterior edentulous areas of the mandible is very complex because the presence of noble structures, such as the inferior alveolar nerve (NAI), often requires major reconstructive procedures to achieve adequate bone volume before implant placement.

To overcome these potential problems and provide patients with sufficient function and esthetics, the use of single-unit crowns (SCs) supported by implants with cantilever extensions was introduced. The length and design of cantilever extensions are associated with possible detrimental effects on implant and prosthetic survival. In light of the above, the objective of the present study is to evaluate the impact of cantilever design in the posterior areas of the mandible on clinical, radiographic, and volumetric parameters with an 8-year follow-up.

Inclusion criteria

• Patients aged ≥18 years;
• Patients with partial edentulism;
• Patients with systemic health or controlled medical conditions;
• Patients with healthy or treated periodontal conditions;
• Patients enrolled in regular supportive periodontal therapy (SPT);
• Patients without clinical signs of oral parafunctions (bruxism or clenching);
• Presence of 1 osteointegrated dental implant in posterior region (premolar or molar) of mandible, after transmucosal placement and healing at 3-6 months;
• Tissue level solid-screw implants with a sand-blasted and acid etched (SLA) surface with an endosseous diameter of 4.1 mm, a length of 8, 10 or 12 mm, a shoulder diameter of 4.8 mm and a supracrestal machined neck with a height of 1.8 mm (Straumann Dental Implant System, Institute Straumann AG); Use of prefabricated titanium abutments;
• Monolithic zirconia SCs manufactured by digital flow; Screwed SCs with a mesial or distal cantilever extension;
• Alternative crown design (ACD) with eccentric implant insertion, with crown-like-premolar and with distal or mesial cantilever with ovoid conformation. In the latter case, the lower abutment cross-section in the region between the crown-bridge connector mimics the shape of a root, creating an "interradicular" access route to facilitate oral hygiene maneuvers;
• Absence of occlusal contacts or guidance on the cantilever extension at baseline;
• Opposing dentition consisting of natural teeth or fixed or removable restorations;
• Availability of periapical radiography at baseline (i.e., loading of SCs) and at the analyzed timings (after 1, 3, 5, and 8 years);
• Availability of clinical measurements, such as pocket probing depth (PPD), plaque index (PI), bleeding index at probing (BOP), suppuration at probing (SUP), keratinized mucosa (KM), and mucosal recession (MR) taken at baseline (i.e., at loading of the SCs) and at 8 years after SCs loading, at four sites/implant with a Michigan graded periodontal probe (Deppeler SA); Availability of intraoral scans at baseline (i.e., at loading of the SCs) and at the analyzed time points (after 1, 3, 5, and 8 years after SCs loading) and executed using 3 Shape TRIOS 3, Germany GmbH.

Exclusion criteria

• Untreated or active periodontal diseases;
• Type I implant placement;
• SCs with cantilever extension replacing mandibular and maxillary incisors and canines; - - SCs with cantilever extension replacing posterior maxillary;
• SCs with cantilever extension supported by implants with a diameter ≤3.5 mm;
• SCs supported by hollow-screw and hollow-cylinder implants;
• Technical complications, such as fractures of implants, abutments, and the prosthetic artifact;
• Smoking habit (more than 15 cigarettes/day);
• Drug or alcohol abuse;
• Uncontrolled systemic diseases that could affect bone remodeling during implant osseointegration; or soft tissue healing (e.g., uncontrolled diabetes mellitus with HbA1c>7, osteoporosis);
• Use of medications that could affect bone remodeling during implant osseointegration or soft tissue healing (e.g., steroids, antiresorptive medication, radiotherapy);
• Autoimmune and/or inflammatory diseases affecting the oral cavity;
• Neurological or psychiatric handicap that could interfere with oral hygiene;
• Pregnancy or lactation periods.