Technically, "canthoplasty" means "changing the canthus", usually the lateral canthus [the outside corner of the eyelids, where the upper and lower eyelid meet]; this can mean several different things: sometimes it is just tightening the eyelids to prevent postsurgical scarring from changing the position of the eyelid corner, and sometimes it is actually changing the position of the corner of the eyelid, if there is a problem to begin with.
Often, plastic surgeons will use the term "canthoplasty" to describe a tightening of the eyelids at the outside corner of they eyelids.
A lateral tarsal strip is one of the techniques used to achieve these goals. The tarsal strip basically shortens the width of the eyelid slightly to give the eyelid better tone. Where you attach the eyelid back to the bone achieves the "canthoplasty"
Hope this helps.