Investors might be tempted to give Cooper (NYSE:COO) the evil eye because the stock of this contact lens maker has been nearly cut in half this year. The culprit isn't anything especially new in the world of medical devices: Competition has eroded a fair bit of growth from this once-consistent above-average grower.

Even though estimates here had been taken down already, the company still posted a slight miss for the fourth quarter. Sales came in basically as expected, though the reported growth of 69% is a little misleading because much of that came from acquisitions. On an organic basis, growth was in the mid single digits.

One of the big issues with Cooper these days is that competition has chewed up its growth in the spherical lens business. While the market continues to grow at a high single-digit rate, the company's sales fell 7% in the quarter and 3% for the fiscal year. The trouble is that competitors like Johnson & Johnson (NYSE:JNJ), Bausch & Lomb (NYSE:BOL), and Novartis' (NYSE:NVS) CIBA are all offering silicon hydrogel lenses -- one of the newest technologies -- and Cooper does not.

Making matters worse, there are fears that the company's toric lens business could also be at risk of similar market share erosion. Because toric lenses make up about one-third of the company's contact lens revenue and about half of the reported growth in the last fiscal year, that's a meaningful threat.

While it's the No. 3 player in the space (with a market share in the high teens), Cooper isn't just taking this lying down. The company has a host of new products slated to be introduced in the next two years, including an expected launch of a silicon hydrogel sphere lens late next year. What's more, the company is clearly targeting Japan as a potential growth market, and that could help counterbalance present weakness in the Americas.

Valuation is tricky here. If you believe management's guidance, the stock looks potentially interesting on an earnings-based approach. Unfortunately, high capital expenditures tied to increasing capacity make the cash flow picture a little murkier. What's more, the company has also recently made some dilutive acquisitions for its surgery business in an effort to better compete with the likes of J&J, Tyco (NYSE:TYC), and BostonScientific (NYSE:BSX).

Before the recent market-share problems, this was a company with a good record of above-market growth and respectable returns on invested capital. That has all been thrown into question now. Should the company manage to rebuild its market share and quiet some of the concerns about competition, it could prove to be a good turnaround story. That said, this is a stock that could require an above-average level of patience and tolerance for risk in the meantime.

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Fool contributor Stephen Simpson owns shares of Johnson & Johnson, but has no financial interest in any other stocks mentioned (that means he's neither long nor short the shares).