Based on an earlier animal histologic study, as well as a clinical up-to-40-month study, which referred to comparable apically expandable implants, authors did not report any periapical inflammatory complications. To eliminate the potential risk of deep intrabony microleakage, it is questionable whether equal biomechanical stability values can be achieved only by the macrothread design avoiding any deep microgaps.

In the present study, the crestal bone changes under loading in the first year exceeded that of the second year. Moreover, the differences between the maxilla and mandible in each year were not statistically significant, which only partially agrees with previous findings in the literature. Besides microbiological conditions, there are several biomechanical aspects which influence maintenance of periimplant crestal bone. Conical and parallel surfaces of the implant-abutment connection (internal hexagon) provide rotational stability and reduce microgaps and micromovement. Another important factor is the thickness of the implant shoulder, which might be a weak point in the design of a short implant due to elastic deformity under extra-axial loading. This fact might be the reason for non-inflammatory periimplant crestal bone loss. We addressed this aspect by splitting adjacent implants wherever possible. According to Brenner and co-authors as well as Pommer and co-authors, the following prosthodontic factors are to be considered to avoid screw loosening, component fracture, loss of marginal bone or even loss of osseointegration: crown-to-implant ratio (extra-axial loading), cantilever length, status of opposing dentition, splinting of adjacent implants, occlusal surface relief and dimensions.

Comparable studies displayed at 24 months a crestal bone loss of 0.5–0.6 mm. Other authors reported at 2, 3 and 5 years a mean loss of 0.57, 0.55 and 0.53 mm, respectively, in the mandible (without significant change after 1 year). On the other hand, randomised controlled trials demonstrated 1 year after loading periimplant marginal bone loss of 0.7 mm and 1.1 mm in the mandible which is the same value measured in the present study.

Within the limitations of a pilot study design, low number of implants, single-arm study and short-term follow-up, the results show a basic improvement of functional rehabilitation especially for elderly patients with compromised general and local conditions for implantation. Controversial questions remain on whether (a) short implants are suitable for irradiated patients and (b) there is a need for expandable short implants in the D1 bone. Furthermore, potential bias should be eliminated in future studies by a randomised controlled trial.

Initial results of the ongoing study confirm the feasibility and safety of the employed system. The implant type seems to be useful for all bone qualities and shows high initial and secondary biomechanical stability in the maxilla and mandible. Long-term follow-up will be needed in validating these initial results in a larger 3-year clinical trial. Crestal bone changes should be evaluated in a larger study cohort. The novel system might extend the spectrum in functional rehabilitation.

D1, D2, D3, D4:

Bone quality (density)

FDI:

World Dental Federation

ISQ:

Implant stability quotient

RFA:

Resonance frequency analysis