There are no oral surgeons or periodontists who regularly contribute to
this newsgroup anymore (though Dave King will occasionally show up).
    There is no set "protocol" as such.  There has to be enough bone volume
to support an implant fixture of adequate size.  While the forces placed
on the upper front teeth are certainly not as great as those placed on
molars, the forces are placed generally not along the long axis of the
teeth.  Furthermore, the quality (ie: density) of the anterior maxillary
bone is generally much poorer than the mandible (esp. the posterior
mandible).  So having adequate bone volume is an important
consideration.  Esthetics of the ultimate restoration may also be a
factor, depending upon the length of the upper lip.

Steve