This question is one of the most common reasons for a "second opinion" in my office. In many cases, particularly in young patients who have high functional demands and expectations, an MRI may be ordered which reveals a complete tear of the rotator cuff. Because there is data that suggests that small asymptomatic tears will become larger in 30-50% of patients within a few years, and because there is the possibility that these tears may retract with time, or that the cuff muscle may atrophy or be infiltrated with fat--both of which may prevent repair or decrease the success of repair-- a tear may be regarded as an absolute indication for repair. However, there are many patients who simply don't have pain or weakness, and are looking for the okay not to have surgery.
The issue is whether nonoperative treatment of an asymptomatic rotator cuff tear burns any bridges. First----there is recent literature by Yamaguchi et al in the Journal of Bone and Joint Surgery (2010;92:2623-2633), which suggests that pain development in shoulders with an asymptomatic rotator cuff tear is associated with an increase in tear size, and that larger tears are more likely to develop pain in the short term than smaller tears. I have attached this article below. Yamaguchi has previously published regarding the natural history of asymptomatic tears--and has shown, as I mentioned above, that tear size seems to increase over time.
So--if you truly have no pain, and for that matter weakness or dysfunction-----though it may be "safest" to have a repair--so long as the tear is small, it's perfectly reasonable to "not have surgery" and follow it for progression. Based on the above, I often advise that one regard the development of symptoms as a reasonable proxy for progression. If and when a small tear becomes painful---then it may be time for reassessment and repair. I have attached a Commentary article below as well for your consideration.
However, because of the impact of time on size progression, and the observation that chronic tears may lead to nerve dysfunction and secondary muscular changes---and because this can decrease repair success rates, when a tear is large, ie involving both the infraspinatus and the supraspinatus, (albeit minimally painful) or small AND painful, I advise surgical repair.
