This review looked at whether there is any difference in failure rates between short implants (minimum length 7mm) and longer implants (≥10 mm).  A recent review by  Telleman et al also reviewed the prognosis of short <10mm implants.

For this particular review the authors searched Medline, Embase, the Cochrane Central Register of Controlled Trials (CENTRAL) and hand searched 29 journals including prospective observational trials. The Newcastle–Ottawa scale (NOS) was used to assess study quality.

Short implants demonstrated a significantly higher overall failure rate compared with longer implants odds ratio (OR) 1.8 [95% CI 1.3–2.5] with significant differences observed in the anterior and posterior maxilla, while no effect could be seen in the mandible (see table). No influence of implant diameter and denture type on the failure rate of short implants was revealed.

[table id=3 /]

The authors  concluded that:

The present meta-analysis confirms…that implant diameter increase cannot compensate for length reduction.

…results are in accordance with systematic reviews that observed increased failure rates of implants shorter than 7mm.

Pommer B, Frantal S, Willer J, Posch M, Watzek G, Tepper G. Impact of dental implant length on early failure rates: a meta-analysis of observational studies. J Clin Periodontol. 2011 Jul 3. doi: 10.1111/j.1600-051X.2011.01750.x. [Epub ahead of print] PubMed PMID: 21722154.