The surgeon, the interval of time since the vasectomy and …a sperm granuloma. Of the three important factors of a reversal, the quality of the fluid at the time of the reversal involves a bit of luck.
When the tube closest to the testicle is cut, fluid emanates from the cut edge and then is examined. The quality of the fluid is an important factor in the success of the reversal. So…clear or slightly milky fluid with whole sperm has a higher success rate than say creamy fluid and sperm parts. Having a sperm granuloma or testicles that just happened to protect the quality of sperm after a vasectomy, gives the patient a higher success rate.
Having good fluid to work with at the time of a vasectomy reversal is simply…good luck.
I’m reading a book on the the North African campaign during WW2. There were many qualities in a general that were important in their advancement. One of the biggest qualities that aided advancement in rank was the general’s history of good luck.
I’d rather be lucky than good.A common surgeon’s aphorism.