Ensnarement is a powerful defense in any patent case where the doctrine of equivalents is at issue. It's time for defendants to "discover" this under-utilized defense. Despite litigating numerous doctrine of equivalents cases, I've only litigated one case where an ensnarement defense was vigorously asserted.
Ensnarement, seminally set forth in Wilson Sporting Goods, prevents a patentee from claiming a range of equivalents that "ensnares" the prior art. To make that determination, the original claim is redrafted as a hypothetical claim by replacing certain claim terms with the asserted equivalents. As a result, the hypothetical claim literally encompasses the claimed product. In DePuy v. Medtronic, e.g., the claim term "spherically-shaped portion" was replaced with "conically-shaped portion" in the hypothetical claim because the accused product included a conically-shaped portion. The relevant question is then whether the hypothetical claim is allowable over the asserted prior art. If the hypothetical claim is not allowable, then there is no infringement because the asserted range of equivalents "ensnares" the prior art.
Two features of ensnarement make the defense particularly powerful. First, ensnarement is a question of law. In DePuy, the district court held a separate bench trial on the issue of ensnarement after the jury trial (See DePuy, 526 F.Supp.2d 126). The Federal Circuit affirmed the propriety of the court's ensnarement proceeding, holding that ensnarement is a question of law like any other limitation on the doctrine of equivalents. Second, the defendant merely bears the burden of producing prior art to challenge the hypothetical claim; the patentee bears the burden of establishing that the hypothetical claim is patentable. This is in stark contrast to the clear and convincing standard that a defendant must meet to establish invalidity.
Ensnarement, then, effectively allows a defendant two chances to win with its invalidity arguments. A defendant should request a "DePuy Hearing" after the trial if there is a finding of infringement under the doctrine of equivalents. At that hearing, defendant can re-raise its same basic invalidity arguments because ensnarement and invalidity have significantly different burdens and ask different questions (patentability of the hypothetical claim vs. validity of the original claim). The second bite at the apple could prove invaluable. As an example, a court is unlikely to grant defendant's JMOL on anticipation or obviousness if the question is close. With an ensnarement defense, though, the "close call" should go to the defendant, and "close calls," of course, are far easier to assert post-KSR. Further, from the Court's perspective, a finding of ensnarement may be a better option because it is more likely to withstand appeal than a grant of JMOL on a jury verdict of validity. From a defendant's perspective, whether the patent is invalid or the prior art ensnared is typically of no significant consequence. Thus, in any case where infringement is based on the doctrine of equivalents, defendant should assert an ensnarement defense and request a "DePuy Hearing" on the issue.