The failure rate is very low, 1.2% in my observation study (2001 to 2006)
The whole loss rate after 1 to 15 years is under 2% (estimate).

Late losses after several years are almost exclusively caused by peri-implantitis. 
The etiology of the early losses (missing osseointegration) is unclear because of the low number of cases.
The risk of loss is high in situations without primary stability.  

With all implant losses the sinus augmentation had ossified so well that there could be a successful reimplantation three to five months after the explantation.

Early loss: The ossified augmentation allowed a simple reimplantation without sinus lift.

Case 2
Missing primary stability.
Despite high risk of loss implantation was performed at one sitting as in several cases with missing primary stability the implant osseointegrated without any trouble and in case of a loss a simple reimplantation can be performed.


Case 3
Loss likely because of insufficient hollowing of the full prosthesis.

The implant bed was prepared with a hollow cylinder during the reimpantation. The removed augmentation cylinder showed good ossification.


Case 4 (picture gallery)
Missing primary stability