"Because the 584 was ammended, does that mean that infringers are off the hook for infringements prior to the changes? "

This is acomplicated issue...Good luck

In 2002, the Supreme Court handed down a ruling that makes it harder for patent owners who amended their patent claims to assert that others are infringing their patent. In essence, patent owners can use the doctrine of equivalents only if they can show that their amendment did not "surrender" the equivalents at issue.

Here are the details. In 2000, a federal appeals court ruled that a patent owner could not assert any element of a patent claim in an infringement lawsuit if that element was amended (including voluntary amendments) during the patent prosecution process. In other words, the appeals court ruling, if upheld, would have barred the use of the doctrine of equivalents for any amended patent claim. In 2002, the U.S. Supreme Court struck down this absolute bar to the doctrine of equivalents and replaced it with a less arbitrary standard. Under the Supreme Court's standard, all amended claims are presumed to be narrowed so as to bar the doctrine of equivalents. But this presumption can be rebutted if a patent owner can demonstrate that the amendment did not surrender the equivalent at issue. In that case, the patent owner can use the doctrine of equivalents. In summary, patent owners who amended their claims prior to or after the Supreme Court's decision can still use the doctrine of equivalents if they can overcome the presumption that the amendment surrendered the equivalents at issue. Festo Corp. v. Shoketsu Kinzoku Kabushiki Co. Ltd., 535 U.S. 722 (2002).

If a case is brought to enforce a patent on a method or process invention, the defendant can escape liability if the defendant was using the invention more than one year prior to the patent application date.

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The doctrine of equivalents is a legal rule in most of the world's patent systems that allows a court to hold a party liable for patent infringement even though the infringing device or process does not fall within the literal scope of a patent claim, but nevertheless is equivalent to the claimed invention. U.S. judge Learned Hand has described its purpose as being "to temper unsparing logic and prevent an infringer from stealing the benefit of the invention". Royal Typewriter Co. v. Remington Rand, Inc., 168 F.2d 691, 692 (2d Cir. 1948).

The goal of the doctrine of equivalents is to provide patent owners with fair protection for their patents. Historically, courts took a literal approach to patent interpretation, based on established principles of legal interpretation. However, by the 18th and 19th centuries, this had come to be seen as unduly limiting on the scope of protection afforded a patent-holder, especially as patent applicants are often required to describe new technology for which an adequate vocabulary has not yet been developed. In response to this, the English courts developed a so-called 'pith and marrow' approach, which tried to distinguish between the essential and non-essential features of a patent claim when deciding infringement cases. At the same time, courts in other countries, notably the United States, developed slightly different approaches to claim interpretation, of which the 'doctrine of equivalents' is perhaps the most famous. The equivalents doctrine takes a more holistic approach when comparing the patented invention with an alleged infringing device than did the 'pith and marrow' approach.

Attempts are ongoing at harmonizing the different approaches internationally (see below); however, progress is slow due to the long history of patent law in developed nations. The doctrine has been criticized as unduly vague, to the extent that it injects uncertainty and unpredictability to a patent system.

http://smallbusiness.findlaw.com/pat...