Does using concave abutments improve 24-month outcomes of single posterior implants?

Seijas Naya, F. Bernabeu Mira, J., C. Pérez Jardón, et al. "Influence of Abutment Shape on Implant Marginal Bone Remodeling: A Double-Blind, Randomized 24-Month Clinical Study." Clinical Oral Implants Research 1-11; https://doi.org/10.1111/clr.70085 DESIGN: A 24-month follow-up of a split-mouth randomised clinical trial with two parallel experimental groups: straight aesthetic antirotational abutments (Nueva Galimplant, Sarria, Spain) and concave antirotational abutments (Nueva Galimplant, Sarria, Spain). This continuation study, with 24-month post-loading data, re-analysed the 8 week post-placement and 6 month post-loading data, from the original study¹. The primary outcome defined in the protocol was stability, measured as Implant Stability Quotient (ISQ), and secondary outcomes were Marginal Bone Loss (MBL), and Probing Depth (PD).

30 partially dentate patients (n = 80 implants) who required at least two posterior implants to be restored with single-unit crowns, were recruited at a secondary care academic department. Adults over 20 years old, who smoked less than 5 cigarettes per day, were at least 6 months post-extraction with healed bone able to accommodate 4 mm diameter and 10 mm length implants, without hard or soft tissue augmentation, were considered. Implants were placed transmucosally 4 mm from the future gingival margin, 1 mm below the alveolar crest, utilising 2 mm or 3 mm height abutments. Healing caps were removed at 8 weeks and outcomes measured, the implants were then loaded with crowns and outcomes measured again at 6- and 24-months post-loading.

A per-protocol analysis was done using independent-sample t-tests for dichotomous variables and paired-sample t-tests for intra-group bone level changes. Mixed linear regression models were used to assess the impact of abutment type and height on MBL, with individual variations weighted based on implant count per patient. Generalised Additive Models were also constructed.

There was no statistical difference in stability, measured through ISQ, between groups at 24 months. There was no statistical difference in PD, bleeding and plaque scores either. At 8 weeks post placement, prior to loading, there was a statistically significant (p = 0.002) difference in MBL: straight abutments had an average of 0.54 mm (95% confidence interval 0.35 to 0.73) and concave abutments of 0.18 mm (95% confidence interval 0.05 to 0.31). A linear mixed-effects regression model was used to evaluate changes in marginal bone levels. Overall, implants restored with concave abutments showed significantly less MBL than those with straight abutments. The model also indicated that this difference appeared early after loading and remained consistent over the follow-up period. In addition, greater abutment height was associated with reduced MBL, suggesting a protective effect of increased abutment height independent of abutment type. Two implants 'failed' and one abutment fractured. Of the 12 implants excluded from the analysis, two had abutment fractures and four crown fractures, the rest were lost to follow-up.

Concave abutments in this cohort had the same MBL at 24 months compared to straight abutments and had no effect on implant stability (ISQ) or periodontal indices such as, bleeding on probing, and PD. At 8 weeks there was a small disparity, less than 0.5 mm difference in MBL, but this appears to be clinically insignificant regarding the final 24-month MBL value.