Effect of initial biologic width on crestal bone loss- a clinical retrospective study of 1-5 years

Background Limiting the marginal bone loss is a prerequisite for long-term stability of implants. Previous studies have evaluated the factors affecting marginal bone loss, including the mucous thickness, the depth of placement. The biologic width will form when the implants are uncovered, which unvaryingly exposes the bone around the implant neck to the oral cavity and starts bone loss, and the biologic width is formed by dissected mucous and the depth of placement of implant at initial stage. Aim/Hypothesis The purpose of this study was to evaluate the short-term effect of the mucous thickness, the depth of placement and the initial biologic width on the marginal bone loss. Material and Methods The clinical and radiographic data of 181 delayed-loaded Ankylos Implants in 140 patients were collected after 1–5 years follow-up. The radiographic examination was taken after implant placement, before the second stage surgery, and after 1–5 years follow-up. The mucous thickness, the depth of placement and crestal bone loss were calculated medially and distally. The initial biologic width was defined as either the thickness of the mucous when the implant was positioned supracrestally, or the sum of the thickness of the mucous and the depth of the implant when the implant was placed below the bone level. Implants were divided into 3 groups based on the initial biologic width, Group A (width less than 3 mm, include 3 mm), Group B (between 3 mm and 4 mm, include 4 mm), and Group C (width more than 4 mm). Significance was set to 0.05. Results Implants in Group A had 0.4(0.1.0) mm bone loss mesially and 0.3(0.0.8) mm distally. For Group B, the bone loss values were m- 0.3(0.0.8) mm and d- 0.3(0.0.7) mm. Group C implants experienced bone loss of m- 0.7(0.2.1.1) mm and d- 0.6(0.1.1.0) mm. Kruskal-Wallis test was applied to find differences between groups. There was significant difference between groups (P = 0.011). Kruskal-Wallis test for implants placed below, at, or above the bone level yielded a statistically significant difference (P = 0.000). The calculation of Spearman's correlation coefficient between the mucous thickness and the marginal bone loss shows negative correlation (rs=−0.243, P = 0.001), while the depth of placement and the marginal bone loss show the positive correlation (rs=0.477, P = 0.000). Conclusions and Clinical Implications The crestal bone loss was found to be the positively correlated to the depth of placement and had negative correlated to the mucous thickness. The result may suggest that the closer the initial biologic width is to the final biologic width, the less bone loss is, and if the initial biologic width is not adequate, bone loss may occur to ensure the proper development of biologic width.