Impact of three-dimensional stabilization thread design on biomechanical fixation and osseointegration of endosteal implants

Background: Implant's primary stability is determined by the intimate and immediate contact between the implant and osteotomy wall, whereas secondary stability is primarily influenced by healing chambers that facilitate the bone formation and remodeling processes following placement. Therefore, modifications to macro-geometric parameters are essential to elicit the desired in vivo response and to ensure successful osseointegration. Three-dimensional (3D) stabilization thread forms comprise both curved and linear geometric surfaces across the thread's crest maximizing retention forces while constraining lateral movement under load relative to conventional buttress-threaded implants. Methods: This study utilized Ti-6Al-4V ELI implants with (i) a buttress thread design [Tapered Pro, BioHorizons®, Birmingham, AL, USA] (TP - control) compared to (ii) a novel, patented, 3D stabilization trimmed-thread design (TP 3DS - experimental). Implants were placed in the mandible of sheep (N = 14 sheep, 6 implants per group per sheep) and allowed to heal for 3- and 12-weeks (N = 7 sheep per time point). During implant placement (T = 0 weeks), the maximum insertion torque value (ITV) and implant stability quotient (ISQ) were measured by torque-in testing and resonance frequency analysis, respectively. After the healing periods, subjects were euthanized, and samples harvested en bloc for biomechanical evaluation via lateral loading tests in addition to histomorphometric and nanoindentation analysis. Results: ITV values were significantly lower in the TP 3DS group compared to TP (p < 0.001). Both groups presented ISQ values ≥ 70, indicating high primary stability. Relative to the TP group, TP 3DS exhibited a significant (∼1.85-fold) increase in lateral load at 3 weeks (p = 0.029) and comparable load values at 12 weeks (p > 0.05). No quantitative differences in percentage of bone-to-implant contact (BIC) and bone-area-fraction-occupancy (BAFO) were observed at either time points between the two thread designs (p > 0.05). Similarly, no differences in bone's mechanical properties (Young's modulus (E) and Hardness (H)) between TP and TP 3DS were observed at 3- and 12- weeks (p > 0.05). Qualitatively, scattered microcracks were apparent at the outer threads of the implant, particularly within the TP group, whereas small bone chips were interspersed between threads of the 3DS implant serving as additional nucleation sites for bone formation. Conclusion: The TP 3DS design reduced insertion torque, improved lateral loading competence, and resulted in a healing pattern, that are beneficial during early stages of osseointegration compared to TP implants.