Influence of Bicortical or Monocortical Anchorage on Maxillary Implant Stability: A 15-Year Retrospective Study of Brånemark System Implants

Abstract

Influence of Bicortical or Monocortical Anchorage on Maxillary Implant Stability: A 15-Year Retrospective Study of Brånemark System Implants C.J. Ivanoff, K. Gröndahl, C. Berström, et al., Int J Oral Maxillofac Implants. 2000;15:103–110 In the literature, there is considerable debate over the benefits of bicortical stabilization of implants. The purpose of this retrospective study was to evaluate, over a 15-year follow-up period, implant survival and marginal bone loss in the maxilla as a function of either monocortical (MC) or bicortical (BC) implant stabilization. The following selection criteria were used to develop an investigative sample drawn from a patient population treated from 1977 to 1983: edentulous patients were provided with 10-mm long Brånemark implants independent of bone quality. Any patients who were treated with grafts or implants of different length or design were excluded. Based on these selection criteria, a patient population consisting of 20 women and 17 men, having a mean age of 50 years (range 31 to 73 years), was evaluated. The study encompassed 37 maxillary arches giving a total 207 implants for analysis. Radiographic verification via a panoramic image of either monocortical or bicortical stabilization was accomplished by ascertaining if the apical portion of the implant either engaged the anterior or inferior border of the maxillary sinus or floor of the nasal cavity. Following this procedure, three examiners classified 110 implants as MC and 97 as BC. An equal distribution amongst the patient group was found with respect to the ratio of MC or BC implants per patient. Implant survival was assessed by patient records. Marginal bone levels were assessed using intraoral radiographs in relation to a fixed point on the implant. Measurements were made under magnification to the nearest 0.1 mm. Of the original 37 patients, 30 were available after 5 years, 23 after 10 years, and 16 after 15 years. In terms of the drop-out rate’s effect on results, 35% of the MC implants and 45% of the BC implants were lost to follow-up by year 15. Of the 207 implants, a total of 18 (8.7%) failed during the follow-up period. In regards to status of MC or BC stabilization, the survival rates were 96.2% and 84.8%, respectively. The MC group showed four (3.6%) failures out of 110 implants, all four of which fractured. The BC group showed 14 (14.4%) failures out of 97 implants. Of the 14 failures, 11 of the failures were due to fracture (three times higher than MC group), whereas the remaining three failed (all in one patient) due to a loss of integration. Sixteen of the total 18 failures occurred in three patients, who had a high ratio of BC implants. Other variables that may influence implant survival such as smoking, opposing occlusion, cantilever length, and parafunction were not evaluated. As for marginal bone loss, less than 1.0 mm of change was observed over the 15-year period regardless of type of implant anchorage. This study found a four times higher failure rate amongst BC implants versus those with MC stabilization, whereas no difference in marginal bone loss could be observed between the two groups. Although the results of this study should be interpreted with caution, the fact that this paper assesses the concept of either monocortical or bicortical stabilization for implants of equal length is unique.