Microscale bone interlocking enhances osseointegration strength on the rough surface of 3D-printed titanium implants: experimental and finite element analysis

The advent of 3D-printing technology, which is capable of on-demand fabrication, has ushered in a new era for fixed implant prosthodontics. Over the past decade, immediately loaded 3D-printed titanium implants have demonstrated predictable clinical outcomes in human jaws, highlighting their superior osseointegration strength, which is attributed to their increased surface roughness. However, the biomechanical mechanisms underlying this enhanced osseointegration strength remain elusive, thereby impeding the standardization and broader clinical application of 3D-printed titanium implants. Experimental 3D-printed titanium implants were fabricated via selective laser melting (SLM), and conventional sandblasted and acid-etched titanium implants (CNC-SLA) served as the control group. Implant surfaces were characterized with scanning electron microscopy, surface profilometry, energy-dispersive X-ray spectroscopy, and a contact angle meter. Implants (n = 10) were surgically inserted into the femoral condyle of New Zealand rabbits. At weeks 1, 2, and 8, micro-CT and undecalcified histological sections were used to assess histological osseointegration (n = 6), whereas removal torque analysis was performed to evaluate osseointegration strength (n = 4). At week 8, microscale finite element analysis of different bone-implant interfaces was conducted to predict the peri-implant bone strain under multidirectional implant loading. The surface roughness of the SLM implants was significantly greater than that of the CNC-SLA implants. Histological osseointegration assessments revealed equal levels of SLM and CNC-SLA implants at weeks 1, 2, and 8. Notably, after week 2, bone interlocking phenomenon appeared on the SLM implants. The removal torque for the SLM implants at week 2 were significantly greater (P < 0.05) than that for the CNC-SLA implants at the same time point and was comparable to the CNC-SLA implants at week 8 (P = 0.775). The removal torque for the SLM implants at week 8 was further increased. Microscale finite element analysis revealed that the rough surface of the SLM implants dispersed harmful strains at the bone-implant interface into the surrounding bone, thereby mitigating the risk of damage to the bone-implant interface. The rough surface of 3D-printed titanium implants fosters microscale bone interlocking and alleviates peri-implant bone strain concentration, which is a promising biomechanical basis for osseointegration strength.