Now that we've flipped the calendar to 2022, we're going to look at the performances of the pure-play 3D printing stocks in 2021. Past performance is not necessarily indicative of future results. But considering a stock's past performance can be helpful in making investment decisions. Sometimes the same or similar catalysts still exist that drove a stock up or down in the past. Last year, the 3D printing stock group included one big winner -- 3D Systems (NYSE: DDD) -- and several big losers, with Desktop Metal (NYSE: DM) the worst performer. Image source: Getty Images. How the 3D printing stocks stacked up in 2021 Companies had to be pure plays to make this list. Moreover, their stocks had to be small caps or larger (market caps of at least $300 million), listed on a major U.S. exchange, and publicly traded during the entire year. Company Market Cap Wall Street's Projected 5-Year Annualized EPS Growth 2021 Return (Decline) 3-Year Return 3D Systems $2.8 billion 30% 106% 112% Stratasys (NASDAQ: SSYS) $1.6 billion 33% 18.2% 36% Materialise (NASDAQ: MTLS) $1.4 billion 63% (56%) 19.2% Nano Dimension (NASDAQ: NNDM) $975 million N/A (58.2%) (65.8%) Desktop Metal $1.5 billion N/A (71.2%) N/A S&P 500 -- -- 28.7% 100% Data sources: YCharts and Yahoo! Finance. EPS = earnings per share. There is one fewer stock on this chart than in similar articles in past years. The ExOne Company, which focuses on industrial metal 3D printing, was acquired by Desktop Metal in November 2021. Why did 3D Systems stock outperform? Along with other 3D printing companies, 3D Systems' business took a big hit during the earlier stages of the pandemic. More specifically, its industrial vertical was significantly hurt by the crisis because many industrial companies either shut down entirely or scaled back operations after the global crisis exploded in early 2020. The company's healthcare business held up quite well. 3D Systems stock's strong performance in 2021 can probably be primarily attributed to company results that rebounded earlier and more robustly from the pandemic hit than others in its industry. In the first nine months (or three quarters) of 2021, 3D Systems' revenue jumped 21% year over year to $464.8 million, even as the company sold off some noncore assets in 2021. It posted adjusted net income of $45.1 million, or $0.36 per share, compared with an adjusted net loss of $23.7 million, or $0.20 per share, in the year-ago period. And it generated cash from operations of $62.7 million, compared with using cash of $32.7 million running its operations in the first nine months of the prior year. Why was Desktop Metal stock such a big loser? One reason Desktop Metal stock performed so poorly last year wasn't within the company's control: Investors were overly enthusiastic and drove a surge in the price of the stock following its debut on the New York Stock Exchange in December 2021. In other words, the stock price got way ahead of where it should have been. This dynamic has been common for new listings, especially those that used the special-purpose acquisition company (SPAC) route. Several other probable reasons for the stock's poor showing can be summed up by this snippet from my mid-November 2021 article following the company's release of its third-quarter results: "The reasons for caution include the lack of transparency about organic revenue growth, the scaling back of full-year guidance, and the type of strategy [growth by acquisitions] the company's pursuing." One other likely reason for some investors' skepticism is the company's delay in releasing its highly touted Production System P-50. The targeted launch date for this product has been pushed back several times over the past few years. What did The Motley Fool do to help its followers avoid taking a big beating on this stock in 2021? For my part, in a March 2021 article following the company's release of its full-year 2020 results, I wrote: "Desktop Metal stock is speculative. ... [The company's] revenue decline of 38% year over year underscores that caution is warranted." Indeed, the stock has plummeted 74% since that time. Investors should remain cautious on Desktop Metal stock but keep an open mind, as it's extremely early in the game. 10 stocks we like better than 3D SystemsWhen our award-winning analyst team has a stock tip, it can pay to listen. After all, the newsletter they have run for over a decade, Motley Fool Stock Advisor, has tripled the market.* They just revealed what they believe are the ten best stocks for investors to buy right now... and 3D Systems wasn't one of them! That's right -- they think these 10 stocks are even better buys. See the 10 stocks *Stock Advisor returns as of December 16, 2021 Beth McKenna has no position in any of the stocks mentioned. The Motley Fool recommends 3D Systems. The Motley Fool has a disclosure policy.Source