The United States Court of Appeals for the Federal Circuit recently held that the ensnarement defense is a legal limitation on the doctrine of equivalents to be decided by the court, not a jury. DePuy Spine, Inc. v. Medtronic Sofamor Danek, Inc., 567 F.3d 1314 (Fed. Cir. 2009). The ensnarement defense prevents a patentee from asserting a scope of equivalency that would “ensnare” the prior art. In DePuy, the district court took the question of ensnarement away from the jury and thereafter denied the defense in a bench trial conducted after the jury found Medtronic had infringed DePuy Spine’s patent under the doctrine of equivalents. Medtronic challenged the district court’s denial of its ensnarement defense and argued that it was entitled to present the defense to the jury.

In upholding the district court decision, the Federal Circuit relied on Warner-Jenkinson Co. v. Hilton Davis Chemical Co., 520 U.S. 17 (1997), in which the United States Supreme Court recognized “various legal limitations” on the doctrine of equivalents which are to be decided by the court, namely the “all elements rule” and prosecution history estoppel. Although the Supreme Court did not include the ensnarement defense in the legal limitations described in that case, the Federal Circuit noted that it has consistently treated the ensnarement defense as one of those legal limitations. In an earlier case, the Federal Circuit described both prosecution history estoppel and ensnarement as “two policy oriented limitations” on the doctrine of equivalents which are to be decided as questions of law.

Procedurally, the ensnarement defense operates in the same way as prosecution history estoppel – after the jury has found equivalence for each element of a claim, the ensnarement defense can limit the scope of equivalency the patentee may assert. The burden is on the patentee to establish that the asserted scope of equivalency will not ensnare the prior art. As with prosecution history estoppel, factual issues underlying the legal question can be determined by the court on the basis of expert testimony or other extrinsic evidence regarding: “(1) the scope and content of the prior art; (2) the differences between the prior art and the claimed invention; (3) the level of ordinary skill in the art; and (4) any relevant secondary considerations.”

Based on the Federal Circuit’s decision, defendants should raise any ensnarement defense in pretrial motions to limit the scope of equivalency asserted by the patentee before the case reaches the jury.