After analyzing the evolution of bundled payment programs, Citi analyst Amit Hazan says his Sell ratings on both Zimmer Biomet (ZBH) and Stryker (SYK) replace Buy-rated Intuitive Surgical (ISRG) as his highest conviction medical device sector call. About 5% of total hips and knees procedure volumes in the U.S. will be at risk of not occurring by 2017, the analyst tells investors in a research note. U.S volumes will start to consolidate towards higher volume centers in each of the next several years, driving average selling prices lower, Hazan contends. He raised his downside risk for Zimmer and Stryker and lowered his price target for Zimmer to $91 from $92. The analyst keeps an $88 price target for Stryker.