Joint replacements have been around long enough now to study patient results for a long time. Some studies show outcomes up to 10 or 15 years later. And it's true that researchers have noticed a link between number of operations done and results.
The surgeons who do the most number of same operations have the best results. There are fewer problems after the surgery and even fewer deaths. Surgeons and hospitals with low-volumes have longer hospital stays for patients compared to middle- or high-volume centers.
There are a few other factors to consider. For example, how do patients' pain levels compare from low-volume to high-volume centers? What is their function like after the operation? Are they more or less independent based on how often the surgeon does the procedure?
Likewise, what are their activities of daily living (ADLs) like? Can they return to their previous levels (and better)? What is the quality of life like for patients treated by low-, medium-, and high-volume surgeons and hospitals.
There is more than meets the eye on second look at this issue. We should get a "bigger picture" view as more studies are done on this topic.