Shape memory attachment as a possible modality to prevent peri-implant disease—a narrative review

Introduction

Since the discovery of bone healing around titanium chambers by Dr. Branemark in 1952, implants have advanced in technology, materials, and knowledge. Implants are fundamentally restored with one of two ways: screw or cement. The main advantages of screw-retained implant crowns are retrievability and application in minimal interocclusal spaces. However, the disadvantages of screw-retained restorations are the effect of the access hole on esthetics and occlusion and difficulty in obtaining passive fit in multiple splinted units due to the lack of spatial relief (i.e., cement space). Cement-retained implant restorations are advantageous in that esthetics are enhanced due to the absence of an access hole, there is more freedom in terms of implant position/angulation, and the protocol for restoration is similar to a traditional tooth (1). The greatest disadvantage to cement-retained implant restorations is the inability to remove excess cement at the crown margin, despite proper cementation protocols and presence of shallower subgingival margins (2-4). The effect of excess cement on peri-implant tissues has been heavily reported in the literature. Peri-implant mucositis includes acute complications such as swelling, exudate, bleeding, deeper probing depths, and peri-implantitis typically also includes radiographic loss of peri-implant bone often associated with residual cement around cement-retained implant crowns. Also, more severe complications such as implant loss has also been related to cement-retained implant restorations. These reported complications can arise from weeks to even years after cementation (5,6).

While screw retained restorations seem to be popular to overcome the negative effects of cement retention, the higher mechanical fracture rate and lack of ideal occlusal contact spots are reasons used to not utilize these screw-retained restorations. The aim of this review was to generate discussion around a novel crown retention method using shape memory alloy to eliminate the risk of residual cement induced peri-implant diseases while keeping the restoration retrievable. We present the following article in accordance with the Narrative Review reporting checklist (available at https://fomm.amegroups.com/article/view/10.21037/fomm-21-118/rc).

Methods

A search on the English literature pertaining to peri-implant diseases and its associated risk factors was conducted. Specially the using of shape memory alloys to retain prosthetics and restorations was also sought (see Table 1).

Discussion

Cement-retained restorations have a poorer outcome measure on certain assessment tools. Weber et al. evaluated the health of peri-implant tissues surrounding cement- and screw-retained implant restorations with parameters such as plaque index, bleeding index, and gingival levels. Peri-implant soft tissues had a more favorable response to screw-retained than cement-retained crowns. Throughout the 3-year follow up period, cement-retained implant crowns showed increased bleeding with a consistent level of plaque while screw-retained crowns had bleeding indices that were stable with decreased plaque indices over time (7). A comparison of the marginal gaps between cement- and screw-retained implant crowns revealed significantly larger marginal gaps in cement-retained crowns (54.4±18.1 µm) than screw-retained crowns (8.8±5.7 µm). Additionally, the size of the marginal gap was dependent on the type of cement (glass ionomer 57.4±20.2 µm, zinc phosphate 67.4±15.9 µm) (8). Korsch et al. investigated the type of luting cement (methacrylate vs. zinc oxide eugenol) and association to peri-implantitis. For cases luted with methacrylate cement, 62% of implants presented with excess cement and significantly more bleeding and suppuration than crowns luted with zinc oxide eugenol cement. The amount of excess methacrylate cement was related to the radiolucent property of the cement and inability to adequately detect and remove it from the sulcus (9). These studies indicate that cement-retained implant restorations have a greater risk of peri-implantitis compared to screw-retained implant restorations not only due to increased plaque retention attributed to the roughness of the crown-implant interface, but also due to the influence of cement type on marginal gap size and amount of excess cement.

A prospective study by Wilson reported that 81% of cases presenting with peri-implant disease showed evidence of excess dental cement around the restoration (10). Removal of the excess cement led to no clinical and no endoscopic signs of peri-implant disease in 74% of these cases. In another retrospective study by Korsch et al., 59.5% of cement-retained implants included in the study presented with excess cement, with 80% of implants presenting with bleeding on probing and 21.3% of implants presenting with suppuration (11). With removal of the residual cement and careful re-cementation with Temp-bond, there was a 76.9% resolution of bleeding on probing and no evidence of suppuration (12). The results of these studies reveal a positive relationship between peri-implantitis and residual cement. A significant number of implants with peri-implant disease had a resolution of clinical signs through removal of the excess cement, however, the fact remains that the disease process was established and the risk of peri-implantitis is not eliminated completely, but merely decreased. The peri-implant complications associated with residual cement has been shown to range from bleeding on probing to complete implant loss (5,6). When the complications are reversible, treatment modalities such as removal of cement are appropriate, but with irreversible complications, palliative treatment is not sufficient. Different precise cementation protocols and careful handling have been proposed to minimize excess cement, but studies have shown that extrusion of cement is inevitable in a traditional cement-retained implant restoration (2). Thus, the literature directs the focus to completely avoiding the intraoral use of cement in implant restorations. There are more recent friction-fit implant restorations that have been reported as case reports (13). They appear to eliminate the intra-oral cementation process, however, the long-term implications of removal and reseating of the same restorations remain unknown.

The shape-memory implant abutment system was developed as a novel alternative of retaining implant restorations that combines the advantages of and eliminates the limitations of cement- and screw-retained implant restorations. This system involves a nickel-titanium (nitinol) sleeve that has the ability to transform and switch configurations, which allows the prosthesis to lock and unlock onto the abutment (14). In a 6-month pilot clinical study, eight participants were recruited for restoration with the nitinol shape-memory-retained abutment system for a single osseointegrated implant in a posterior quadrant (15). The parameters measured were oral hygiene, probing depths, plaque index, gingival index, proximal contacts, and occlusal contacts. Comparing the baseline values to the follow-up appointment (minimum of 6 months), minimal differences were noted. Six out of 8 participants had probing depths of less than 3 mm, plaque and gingival indices scores of “0”, no visible plaque retention, and absence of peri-implant inflammation throughout the follow-up period. One participant had improved plaque and gingival indices and another participant had indices of “1” that remained unchanged throughout the follow-up period. Participants in the study had no issues or discomfort with the final prosthesis. The results of this study suggest that the shape-memory implant abutment system does not negatively affect peri-implant health, as seen with traditional cement-retention. An evaluation of the wear and retention performance of these shape-memory abutment systems after 6 months of clinical use was completed through scanning electron microscopy and tensile testing, respectively. Results showed no evidence of damage to the shape-memory alloy sleeves and no significant differences in retention values before and after clinical use. The mean retention force of control nitinol sleeves was higher (480±37 N) than that of a commercial resin cement (336.3±188 N). The mean retention force of nitinol sleeves remained unchanged after 5,000 compression load cycles compared to traditional cement which decreased in retention force (15). Furthermore, the application of this shape-memory system is not limited to conventional single unit restorations and can be efficiently used for treatment with full-arch prostheses (16).

The advantages and disadvantages of the various retention types are summarized in Table 2.

Conclusions

It is estimated that in the United States, 178 million people are missing at least one tooth and about 40 million people are missing all their teeth (17). With the advancements in dental implants and associated technologies, implants have become a fundamental and predictable treatment option for many people. Prosthetic rehabilitation with implants is generally screw- or cement-retained, each with its advantages and disadvantages. One of the concerning complications associated with implants is peri-implantitis. Studies in the literature have shown the positive correlation of residual cement and peri-implant disease. The risk of peri-implantitis associated with excess cement poses a significant problem considering that patients are readily rehabilitated with cement-retained restorations which are arguably preferred by many specialists and general dentists over screw-retained restorations (17). The strong association between cement and peri-implantitis behooves us to reappraise cement-retention for implant restorations as a standard of care. The shape-memory implant abutment system is a third method of retention for implant restorations that can be easily incorporated into clinical practice, has excellent clinical performance, does not sacrifice esthetics for retrievability, and eliminates the need for cement. Without the presence of cement, the risk of peri-implantitis can potentially be significantly reduced, and the nitinol shape-memory sleeve seems to be a promising alternative solution to accomplish just that.

Acknowledgments

Funding: None.

Footnote

Provenance and Peer Review: This article was commissioned by the Guest Editor (Ole T. Jensen) for the series “Current Advances in Treatment of Peri-Implantitis” published in Frontiers of Oral and Maxillofacial Medicine. The article has undergone external peer review.

Reporting Checklist: The authors have completed the Narrative Review reporting checklist. Available at https://fomm.amegroups.com/article/view/10.21037/fomm-21-118/rc

Conflicts of Interest: Both authors have completed the ICMJE uniform disclosure form (available at https://fomm.amegroups.com/article/view/10.21037/fomm-21-118/coif). The series “Current Advances in Treatment of Peri-Implantitis” was commissioned by the editorial office without any funding or sponsorship. The authors have no other conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.”

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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