This week, the ratings of three medical devices stocks on Portfolio Grader are down. Each of these rates a “D” (“sell”) or “F” overall (“strong sell”).

This week, Mazor Robotics Ltd Sponsored ADR (MZOR) falls to a D (“sell”), worse than last week’s grade of C (“hold”). In Portfolio Grader’s specific subcategories of Earnings Momentum and Equity, MZOR also gets F’s. For more information, get Portfolio Grader’s complete analysis of MZOR stock.

Tandem Diabetes Care, Inc. (TNDM) ratings are on the decline this week as the company earns an F (“strong sell”). Last week, it received a D (“sell”). The stock gets F’s in Equity and Cash Flow. Shares of the stock have been changing hands at an unusually rapid pace, three times the rate of the week prior. To get an in-depth look at TNDM, get Portfolio Grader’s complete analysis of TNDM stock.

The rating of HeartWare International, Inc. (HTWR) declines this week from a C to a D. HeartWare International is developing a range of implantable mechanical circulatory assist devices or heart pumps, used for the treatment of congestive heart failure. The stock also gets an F in Equity. As of Dec. 22, 2014, 11% of outstanding HeartWare International, Inc. shares were held short. For more information, get Portfolio Grader’s complete analysis of HTWR stock.

Louis Navellier’s proprietary Portfolio Grader stock ranking system assesses roughly 5,000 companies every week based on a number of fundamental and quantitative measures. Stocks are given a letter grade based on their results — with A being “strong buy,” and F being “strong sell.” Explore the tool here.