Despite the continuing prevalence of dental caries in primary maxillary anterior teeth in children, the esthetic management of these teeth remains problematic. Esthetic restoration of primary anterior teeth can be especially challenging due to: small teeth size, close proximity of dental caries to the pulp, relatively thin enamel, lack of surface area for bonding, and issues related to child behavior (15).
In evaluation of gingival health, this study shows better gingival response in zirconia crowns which can be explained by the fact that zirconia is biocompatible and possesses a polished and smooth surface leading to less plaque accumulation and hence less gingival irritation (16–18). This is in accord with results reported in 2014 by Walia et al. (19) who evaluated anterior primary crowns for 129 patients (aged 3 to 5 years old). Their study revealed that zirconia crowns showed improved gingival health, while the other crowns (Composite strip crowns and pre-veneered SSCs) displayed more gingival inflammation. Another recent retrospective study by Holsinger et al. (20) assessing 57 primary anterior teeth treated with zirconia showed significant healthy gingiva in relation to these crowns.
A retrospective study done in 2003 by Kupietzky et al. (9) included 112 composite resin strip crowns found that 43% of the restored teeth showed gingival irritations around the crowns. These findings could be explained as the gingival health of teeth restored with composite strip crowns can be affected by tooth preparation and finishing (21, 22). Unfortunately, upon reviewing the literature there were no sufficient data with regards to gingival response related to primary teeth restored by composite resin strip crowns. Padbury in 2003 (23), suggested placement of the strip crown margin supra gingivally to reduce gingival inflammation. Despite this recommendation being clinically logical, it is considered not applicable in most cases as it will result in poor aesthetics and appearance.
In agreement with our study, Walia et al. (19) who assessed anterior primary crowns for 129 patients (aged between 3 to 5 years old) also reported that zirconia crowns showed improved gingival health due to less plaque accumulation when compared to composite strip crowns and pre-veneered SSCs.
Our data showed that none of the teeth covered with zirconia crowns showed recurrent caries during the entire follow up. In contrast, teeth restored with composite resin strip crowns showed that 6.7% developed recurrent caries in the 12-months follow up. The lack of adequate preventive measures could have contributed in caries recurrence in our community. Important factors that influence development of caries are poor oral hygiene and high cariogenic diet consumed by the patients included in our study.
A recent study by Holsinger et al. (20) reported results similar to our study’s in their evaluation of 57 crowns treated with zirconia for primary anterior teeth in 18 patients. Their study showed no recurrent caries after a follow-up period of 24 months. Talebi et al. (24) evaluated the drawbacks of anterior primary crown restorations in 38 primary anterior teeth of 12 patients aged 3–5 years diagnosed with early childhood caries (ECC). Their results showed recurrent carious lesions at three and 12 months. One case displayed recurrent carious lesions around the margins of the restoration at the 3-months follow-up. While in eight teeth, the secondary caries occurred at the 12-month follow up over the boundaries of the restoration. Johnsen et al. (25) stated that patients diagnosed with ECC had higher tendency to develop recurrent caries after treatment. Another study done in 2000 by Almeida et al., found that young patients having ECC who were managed under general anesthesia to receive resin composite strip crown restorations exhibited significantly higher caries rates versus the control group who were caries free originally.
The greater restoration failure of the composite strip crowns in this study may be explained by the fact that treatment was done under nitrous oxide sedation and physical restrains to manage the children behavior. Eidelman et al. (26) reported that improved results for strip crowns were found in cases done under general anesthesia than those done under sedation. General anesthesia allows treatment to be rendered under theoretically optimal conditions; implying outcomes would be more successful. Success rate between 80% − 88% were found in the studies done by Kupietzky et al. (9); Waggoner et al. (22); Ram and Fuks (27). High failure rate of 51% over a period of two years was seen in a study by Tate et al. (28) where strip crowns were placed under general anesthesia and endodontically treated teeth were included as well. Endodontic treatment can also affect the overall retention as these teeth are usually more damaged as mentioned by Kupietzky et al (29).
Regarding the zirconia crowns, the success rate in this study was 98.3% by the end of the 12 months follow up. Only two crowns failed due to trauma. Current research on the clinical success of prefabricated primary zirconia crowns for primary incisors is still limited. Walia et al. (19) reported the retention rate of zirconia crowns as 100% after 6 months. These crowns have no facial upper structure, as they are made up of solid zirconia leading to no chance of facial veneer fracture as stated by Manicone et al (30). The flexural strength of zirconia oxide materials has been reported to be in the range of 900 to 1,100 MPa. This is approximately twice as strong as alumina oxide ceramics currently in the market and five times greater than standard glass ceramics (30). Another important property is their fracture toughness making them perdurable and a highly strong restoration (31).
Tooth wear in this study was evaluated according to the Smith and Knight Tooth Wear Index (13, 14). Seven teeth accounting for 11.7% of teeth opposing to zirconia crowns showed loss of enamel surface, minimal loss of contour compared to 100% no loss of enamel surface characteristics in strip crown group. This results is in agreement with Walia et al. (19) who found four opposing primary teeth out of 38 zirconia crowns having loss of enamel surface characteristics and minimal loss of contour.
This study adds significant value to the literature with regards to the clinical performance of zirconia crowns in anterior primary molars. Although the zirconia crowns are considered expensive in comparison to strip crowns, we should take into consideration the high failure rate of strip crowns and the need to repeat dental visits and re-treatment of failed strip crowns. This fact may make the zirconia crowns cost effective after all as it has high success rate and minimal need for re-treatment.