Analysis of Primary Stability in Two Designs of Ultrashort Implants: In Vitro Study

This in vitro study evaluated the influence of macro and microscopic implant design, drilling protocol, and bone density on the primary stability of 4 mm ultrashort dental implants, aiming to provide evidence-based guidance for their use in severely atrophic posterior jaws.

Two implant systems were compared: test group (Klockner Essential Cone^® conical implants with polished neck, diameters 4.0 mm [B1] and 4.5 mm [B2], shot-blasted and acid-passivated surface) and control group (Straumann Standard Plus^® 4.1 mm parallel-walled implants with SLA^® surface). A total of 722 implants (n = 30 per condition) were inserted into natural bone blocks simulating Lekholm and Zarb type II (cortical-dominant) and type III (medullary-dominant) bone qualities. Fifteen experimental conditions were tested, combining three main drilling protocols: (1) manufacturer's standard preparation, (2) horizontal under drilling (final diameter 3.5 mm), (3) vertical overpreparation (1 mm deeper), and (4) combined vertical + horizontal restriction. Primary stability was assessed by insertion torque (measured with a calibrated Tohnichi^® torque wrench) and Implant Stability Quotient (ISQ) using Penguin^® resonance frequency analysis (RFA) in two perpendicular directions. Subjective insertion ease and complications were also recorded.

The conical macrogeometry with progressive, dense V-shaped threads provides significantly better primary mechanical anchorage than parallel-walled designs in ultrashort (4 mm) implants. Within the limitations of this ex vivo animal bone model study, the results indicate that different drilling protocols significantly influence the primary mechanical stability with insertion torque ≥ 25 Ncm and ISQ ≥ 55 of ultra-short implants, as measured by insertion torque and ISQ values. Certain drilling protocols resulted in higher insertion torque and ISQ compared to others, particularly in Type II and Type III bone.