Festo Corp. v. Shoketsu Kinzoku Kogyo Kabushiki Co., 535 U.S. 722 (2002)

The Doctrine of Equivalents says equivalents of the claimed elements in a patent claim can also cause infringement.  Judge Learned Hand said that courts "resort to the 'doctrine of equivalents' 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) One of the limits on the Doctrine of Equivalents is prosecution history estoppel. Prosecution history estoppel is the presumption that any narrowing limitation added to the claims during prosecution bars the doctrine of equivalents in defining scope of the amended claims.

After Festo began marketing its patented device, Shoketsu marketed a device that allegedly infringed Festo's patented device.  Festo charged infringement under the doctrine of equivalents.  Festo's claim had been amended during prosecution to fix a 35 U.S.C. §112 problem, and Shoketsu claimed that prosecution history estoppel should bar Festo from asserting equivalents.  The court decided that the prosecution history estoppel presumption is rebuttable where:

• the amendment cannot reasonably be viewed as surrendering a particular equivalent;

• the equivalent may have been unforeseeable at the time of the application;

• the rationale underlying the amendment may bear no more than a tangential relation to the equivalent in question; 

• or there may be some other reason suggesting that the patentee could not reasonably be expected to have described the insubstantial substitute in question.