Primary stability is attributed to bone quantity and quality, implant macrodesign and surgical technique. [12–18] Surgical procedure for implant placement in the edentulous posterior maxilla must be planned with caution due to limited anatomical structure and critical bone quality. [1, 2] Especially, SFE might be required to place the implants in the atrophic maxilla, and primary stability should be achieved with the preexisting bone during simultaneous implant placement. Previous studies clearly demonstrated that the effect of implant macrodesign on primary stability. [12–18] Above all, thread pattern and implant body shape can contribute to primary stability and can be selected by the surgeons during preoperative planning. This study aimed to compare the primary stability depending on the macrodesigns between the novel type of implant and other types in a SFE simulated model. In addition, the effect of the lengths on primary stability was also evaluated.
At first, an ex vivo model was used to analyze primary stability. Bone blocks such as bovine rib, bovine kneecaps, porcine iliac crest and cow femur were used to evaluate primary stability in the previous studies[14–18, 21]. In this study, all implants were inserted from cross-sectional cancellous bone, not including cortical bone, which simulated poor maxillary bone. The thickness of 5 mm was designed to simulate the preexisting maxillary bone during SFE and all implants should penetrate completely through the blocks and this model was supposed to be valid.
BLX group with tapered design, comparatively wider thread pitches (1.125 mm) and larger thread depth and width presented the highest MIT and ISQ values among the implants with the length of 10 mm. Regarding thread pitches, although the previous review noted that the implants with more threads could achieve a higher bone-implant contact and stronger resistance to vertical load, the effects of all factors of thread designs on primary stability has been still unclear. However, a previous report showed that thread pitches, depth and width were associated with primary stability. Furthermore, a recent previous study showed that narrower threads could create higher primary stability. This study showed the features of BLX which were designed to enhance primary stability could enhance MIT and ISQ values in a SFE simulated model, although the features of BLX group were not in agreement with some previous findings. In addition, BLT group showed significantly higher MIT than BL and SP groups, not TE group. BLT and TE groups had tapered designs and comparatively narrower threads pitches (0.8 mm), and these might play an important role in higher MIT and ISQ values. Although BL and SP groups had parallel designs, the former had more threads due to narrower thread pitches (BL vs SP: 0.8 vs 1.25 mm), resulting in significantly higher MIT and ISQ values. These results suggested that multiple factors were associated with primary stability (MIT and ISQ values), even in a SFE simulated model that was designed to evaluate primary stability with a limited bone volume. A recent previous study that compared the primary stability between BLX implants and TE implants in a similar model (sinus lift-simultaneous implant insertion model) indicated that TE implants with wider diameter showed higher ISQ values. The difference between this previous study and the present study was bone condition for implant placement. The bone specimens in this previous study was consisted of cortical bone and cancellous bone, and it was suggested that TE implants could indicated higher ISQ values because TE implants with tapered neck design could achieve higher primary stability at cortical bone. A SFE simulated model in the present study was more severe situation, which was only cancellous bone, and it was supposed that BLT and BLX groups could enhance primary stability through other mechanisms
The limited bone volume might allow the placement of only short length implant. This study also evaluated the effect of implant length on primary stability. This study clearly demonstrated that BLX groups could achieve significantly higher MIT and ISQ values than SP groups regardless of the length of implants. The effect of implant length on primary stability (ISQ values) was investigated in a previous study and it suggested higher ISQ values in longer implants. However, available bone thickness was limited in the present study. Interestingly, BLX group demonstrated significantly higher MIT values than BLX short and both SP groups, although there were no differences ISQ values between BLX and BLX short groups. The differences between BLX group and BLX short group were not only the length but also the thread pitches (BLX: 1.125 mm, BLX short: 0.9 mm). Unfortunately, the detailed mechanisms of BLX macrodesigns for enhancing primary stability could not be elucidated. However, this simulated model clearly identified the effective features in the achievement of primary stability. One more unfortunate thing was that the effects of the length on primary stability were not investigated in all types of implants. Further studies will be expected in a similar situation.
This study had several limitations. Though primary stabilities of five implant types were investigated, these results cannot directly be applied to further osseointegration (secondary stability) and loading bearing capacity after the healing period. Satisfactory clinical outcomes of implants after SFE with or without graft materials have been reported,[8–11] although primary stability is one of the most important factors for osseointegration. Clinical investigations using several types of implants for SFE and simultaneous placement would be favorable to compare the macrodesigns and primary stabilities among these implants.