Standard dental implants require sufficient bone and spacing, which may not be available in all patients. To address these limitations, narrow diameter implants (≤3.5 mm) have been introduced as a less invasive alternative, especially useful for medically compromised patients or those avoiding bone grafts. However, narrow implants made of pure titanium have shown long-term biological and technical concerns.

The introduction of titanium-zirconium (TiZr) alloy has improved mechanical strength, and several studies have demonstrated favorable outcomes with narrow TiZr implants. Yet, evidence remains limited regarding their exclusive use in high-load posterior (molar) regions. Prior studies report conflicting failure rates, potentially linked to prosthetic design and occlusal forces, but lack quantitative analysis of bite forces or prosthetic angles.

Emergence angle, prosthetic platform width, and implant-abutment connection may all influence implant stability and peri-implant tissue health. A newly introduced TorcFit™ connection may offer better stress distribution and torque resistance, but its clinical relevance-especially in narrow implants used for molars-remains untested.

Introduction (background and research question)

Standard diameter dental implants require sufficient bone volume and adequate interdental distance to prevent adverse bone level changes, or unfavorable long-term esthetic outcomes. To overcome site specific limitations, simplify treatment protocols, and expand clinical applications, short (< 10 mm) and narrow diameter (≤ 3.5 mm) implants were introduced. It has been suggested that narrow diameter dental implants would allow a less invasive surgical procedure by reducing the need for bone augmentation, a viable option for medically compromised patients and those averse to the use of bone grafting materials. However, the use of narrow diameter implants of commercially pure titanium (cpTi) has been associated with several biological and technical concerns when used over a long period of function.

The use of titanium and zirconium (TiZr) alloy improved the mechanical properties and several clinical studies showed that narrow diameter TiZr implants had high survival rates and comparable performance to standard diameter cpTi implants across various clinical indications. The clinical outcomes of narrow diameter TiZr were also comparable to standard diameter implants of the same TiZr material when replacing single missing teeth in the anterior or posterior regions of the maxilla or mandible. On the other hand, the use of narrow diameter TiZr implants to support single crown restorations exclusively in the posterior region remains limited. One randomized controlled trial has reported on the clinical outcomes of single narrow (3.3 mm) diameter TiZr implants in molar sites. These studies reported failure rates of 1% and 18% at one- and three-year follow-up, respectively. The failed narrow diameter implants were regular neck (RN) with prosthetic platform of 4.8 mm and were mostly lost because of implant fracture below the internal screw channel. Traumatic occlusal forces were thought to be the reason behind these failures. However, the occlusal wear or the bite forces on these narrow implant-supported crowns were not quantitatively assessed.

While it could be speculated that the wider prosthetic platform of 4.8 mm diameter might have caused unfavorable leverage on the narrow diameter implant body placed in sites of high occlusal load (i.e., single molar sites), wider prosthetic platform in molar sites may provide favorable prosthetic design and emergence profile that could be associated with greater stability of peri-implant hard and soft tissues. Different prosthetic-related factors, particularly the restorative angles and emergence profile, may have significant impact on marginal mucosal and bone levels and increase the risk of developing peri-implantitis. Narrow diameter implants might be associated with implant restorations with an emergence angle of > 30° in molar sites. The long-term outcomes of implant restorations with different emergence angles were recently evaluated in a systematic review. While significant differences were not observed in terms of periodontal and esthetic outcomes, the authors concluded that platform-matched implant restorations with an emergence angle of ≤ 30° might be associated with less changes in peri-implant marginal bone levels when compared to implant restorations with an emergence angle of > 30°. However, the evidence support was of low to moderate certainty and further studies are still required.

Another reason that has been implicated in the biological and technical complications of dental implants is the poor fit and instability of the implant-abutment connection. Narrow diameter Straumann tissue level dental implants have 15° internal cone with either four internal CrossFit® grooves or 8° morse taper synOcta® connection. These configurations allow flexible positioning of the prosthetic components but evidence on using narrow diameter implants with either CrossFit® or synOcta® connection in molar sites remains insufficient. Recently, a new implant-abutment connection has been introduced with the premise of ensuring transmission of high torques. The stability of the new TorcFit™️ implant-abutment connection was shown to be similar to that of the CrossFit® connection. However, the TorcFit™️ connection, with its 7° conical connection and six-sided star-shaped anti-rotational element, may allow transmission of higher torques, maintain a flexible alignment of the prosthetic components, and ensure uniform stress distribution. It is yet to seen to what extent this new implant-abutment connection influences the clinical performance of a narrow (3.3 mm) or standard (4.5 mm) diameter implant in the replacement of single missing molars.

Aim(s) of the study:

The objectives of this single-center three-year randomized controlled trial will be:

To evaluate the clinical, and radiographic outcomes of narrow and standard diameter TiZr implants used to support screw-retained monolithic zirconia crowns in molar sites.

To evaluate the long-term effects of the restorative emergence angle on the peri-implant mucosal level, marginal bone level, periodontal parameters and prevalence of biological complications.

Inclusion Criteria:

• Aged ≥ 18 years.
• Require replacement of single missing molar tooth with dental implant in fully healed sites.
• Controlled oral hygiene (full-mouth plaque and bleeding scores ≤ 25% at baseline).
• Intact adjacent teeth and opposing dentition.
• Fully healed sites (≥ 6 months) with limited bone availability in the horizontal bucco-lingual dimension (residual bone width of ≤ 6 mm) and minimal height of 8 mm.
• Good compliance and commitment to attend follow-up review appointments.
• Willing to provide informed consent.

Exclusion Criteria:

• Localized/generalized periodontitis.
• Any medical condition that may contraindicate implant treatment (Bornstein et al., 2009).
• Bone metabolic disease and/or taking medications that affect bone metabolism.
• Long term use of non-steroidal anti-inflammatory medications.
• History of malignancy, radiotherapy or chemotherapy.
• Pregnant or lactating women.
• Smokers.
• Severe bruxism or parafunctional habits.
• Large occlusal discrepancies.