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recent court decision clarified the patent defense of “ensnarement,” and warns of this potential trap for patent owners. 

At trial, construing the scope of a patent claim presents a dilemma for the patent owner. Interpret the claim too narrowly, and the infringer may escape liability by showing that his product or method falls outside its scope. Interpret the claim too broadly, and it may be invalidated by the prior art, and again the infringer will escape.

One way of broadening a claim is by the “doctrine of equivalents,” which permits a finding of infringement in some cases even if the accused device does not include all of the features required by the patent claim. Specifically, infringement may be found if the missing feature has been replaced in the device with an equivalent feature, that is, one that performs substantially the same function, in substantially the same way, to achieve substantially the same result. 

A potential trap awaits the patent owner, however: Stretch the claim too far, and it will encompass - or be “ensnared” by - the prior art. The June 2009 decision in DePuy Spine, Inc. v. Medtronic Sofamor Danek, Inc. illustrates the patent owner's dilemma and clarifies the law of ensnarement. 

DePuy has exclusive rights in a patent covering spinal support hardware used in back surgery. The patent claim in suit required that one piece of the hardware include a “spherically-shaped” cavity.

The Court of Appeals for the Federal Circuit had previously ruled that the accused Medtronic product did not literally infringe the patent claim because its spinal support hardware did not include the spherically-shaped cavity that the claim requires. DePuy, however, asserted that Medtronic's conically-shaped cavity was the equivalent of the claim's spherically-shaped cavity, and pursued an infringement case based on the doctrine of equivalents. 

In response, Medtronic raised an “ensnarement” defense. The court explained that ensnarement “limits the scope of equivalency that a patentee is allowed to assert” by barring the patentee “from asserting a scope of equivalency that would encompass, or ‘ensnare,’ the prior art.”

In laying out this defense, Medtronic hypothesized a claim identical to the DePuy claim, but with the “spherically-shaped” cavity replaced with a conically-shaped cavity - essentially the interpretation that DePuy proffered under the doctrine of equivalents. Medtronic then presented two prior art patents, and argued that the hypothetical claim would be invalid as obvious under 35 U.S.C. §103 in view of those patents.   

This put DePuy on the defensive. Patents are normally presumed valid, but the ensnarement defense shifts the burden to the patentee to prove that the patent is valid in the face of the asserted prior art. 

The trial court summarized the parties’ burdens as follows: “The burden of producing evidence of prior art to challenge a hypothetical claim rests with an accused infringer, but the burden of proving patentability of the hypothetical claim rests with the patentee.” The appeals court explained the defense: “if such a claim would be unpatentable under 35 U.S.C. §§ 102 or 103, then the patentee has overreached, and the accused device is noninfringing as a matter of law.” 

Medtronic argued that it should be allowed to have the jury determine whether the hypothetical claim would be valid. The law on this point was not well developed, but the Federal Circuit disagreed with Medtronic, ruling that the question of ensnarement is to be determined by the court, not the jury.  

Against that background, the court employed traditional obviousness analysis to evaluate  the prior art asserted by Medtronic. Ultimately, DePuy carried its burden by convincing the court that the two prior patents could not be combined to teach the hypothetical conically-shaped cavity. The expanded claim was valid, and the ensnarement defense had failed.   

A jury had previously determined that, absent a successful defense, the Medtronic product infringed the DePuy claim under the doctrine of equivalents, and awarded DePuy over $149 million in lost profit damages.  Because the appellate court ruled against Medtronic's ensnarement defense, that award was affirmed.

When considering a doctrine of equivalents case, patent owners should bear in mind the limiting effect, and burdens, of a potential ensnarement defense.  As the DePuy court confirmed, the ensnarement defense requires the patentee to convince the court that the expanded patent claim is valid over prior art asserted by the accused infringer, and failure to carry that burden will let the infringer escape.

Still, the challenge of overcoming a potential defense of ensnarement should not deter a determined patent owner for whom the doctrine of equivalents is the best option for proving infringement.