title: Survival of short implants is improved with greater implant length, placement in the mandible compared with the maxilla, and in nonsmokers.
abstract: METHODS
  The authors conducted a search of MEDLINE and EMBASE databases for the period January 1980 to October 2009. This was supplemented by searching reference lists of literature obtained. There was no language restriction applied. Two authors reviewed the search yield for relevance, disagreement was resolved by consensus discussion, and the selected articles deemed relevant for full-text review were read by one reviewer. The relevant articles selected were judged against inclusion/exclusion criteria. Included studies were restricted to randomized controlled trials (RCTs) or prospective cohort studies. Only studies with partially edentulous applications of at least 5 implants shorter than 10 mm followed for more than 1 year were included. No (alumina)-zirconium implants or mini-implants for orthodontic anchorage or short implants used for cantilevered prostheses were included. A validity assessment using methodological criteria for cohort and RCTs was accomplished by 2 reviewers.
  
  
  METHODS
  The focus of the review was to determine the prognosis of short (<10 mm) implants in the partially edentulous patient. The authors sought to identify important influences on survival rate by conducting subgroup analyses where the subgroups included individual implant lengths shorter than 10 mm, smoking, implants in the mandible versus maxilla, and bone augmentation procedures. The 2-year estimated survival rate was selected based on evidence suggesting that after 1 year the implant survival rate is considered to be constant, an important aspect for survival function estimation.
  
  
  METHODS
  To allow a pooled estimate of prognosis from multiple studies, the estimated failure rate per year and estimated implant survival rate after 2 years was determined for each study. The estimated failure rate per study was determined as a function of number of implant failures and total implant "exposure" time. Exposure in this context means the time an implant is in vivo and, hence, exposed to failure risk. Consequently, exposure time includes implants followed the entire study, those up to the time of failure, and those followed to a premature study end (eg, patient death, moving, refusal to continue). When study data were not provided separately for the short implants in a publication, a proportional exposure (short implants/total implants χ overall exposure time) was determined. The 2-year survival calculation made from the estimated failure rate assumed a constant rate of implant failure (considered to follow a Poisson distribution). Additional analyses sought to identify independent effects on failure based on implant surface topography (rough vs machined), maxilla versus mandible, smoking status, and augmentation.
  
  
  RESULTS
  Twenty-nine studies were identified that met both the inclusion criteria and methodological requirements of the 1353 articles identified in the literature search. Of these 29 studies, 28 were prospective cohort studies and 1 was an RCT. The mean follow-up for the studies was 3.7 years, with a range from 1.6 to 8.1 years. There were 2611 short implants in the identified studies ranging in length from 5.0 mm to 9.5 mm. Table 1 provides the outcome data by implant length, including the number of implants in each group, the estimated annual failure rate (expressed as a percentage), and the 2-year estimated failure rate (expressed as a percentage). Overall, 5-mm implants were calculated to have a 2-year survival rate of 93%, whereas 9-mm implants had a 2-year survival rate of 98%. Additional analyses revealed that for all implant lengths, both rough and machined (smooth) implants had similar failure rates (0.008 and 0.010, respectively). Implants placed in the maxilla had a significantly greater failure rate than those placed in the mandible (0.010 and 0.003, respectively). Estimated failure rates in studies that excluded smokers were twice as low as those seen in studies that included heavy smokers (0.004 and 0.080, respectively). No significant difference in estimated failure rate was seen for implants placed with simultaneous augmentation compared with no augmentation (0.007 and 0.010, respectively).
  
  
  CONCLUSIONS
  The findings from this systematic review are important in that collectively they support consideration of placement of short implants in partially edentulous patients. The estimated data showed a tendency for improved survival rate with increasing implant length, implant placement in the mandible compared with the maxilla, and for implants placed in nonsmokers.
authors.affiliation: Mayo College of Medicine, 200 First Street SW, Rochester, MN 55905, USA. carr.alan@mayo.edu
authors.forename: Alan B
authors.initials: AB
authors.lastname: Carr