The Supreme Court's Warner-Jenkinson Decision
In Warner-Jenkinson, the Supreme Court stated that the doctrine of equivalents, "when applied broadly, conflicts with the definitional and public-notice functions of the statutory claiming requirement." 1997 U.S. LEXIS 1476, *20. The Court cabined the doctrine by requiring that it be applied "to individual elements of a claim, not to the invention as a whole." Id. Evidence of known interchangeability by a person of ordinary skill in the art is relevant to this determination. The Court cautioned, however, that "[i]t is important to ensure that the application of the doctrine, even as to an individual element, is not allowed such broad play as to effectively eliminate that element in its entirety." Id.
The Court further limited the doctrine by announcing a new rule creating a presumption of prosecution history estoppel with respect to subject matter surrendered during prosecution of the patent. Under the new prosecution history estoppel rule, a patent owner can rebut the presumption by demonstrating that a claim was amended for reasons unrelated to patentability. The Supreme Court did not address whether amendments to secure "patentability" narrowly include only avoidance of prior art, or more broadly encompass other concerns such as non-enablement or utility, for example, and if so, under what circumstances. Nor did the Supreme Court state whether the presumption applies only to amendments, as opposed to interpretive statements made during prosecution.
The Warner-Jenkinson decision also left for the Federal Circuit to resolve the question of what evidence is admissible to rebut the presumption. Prior to Warner-Jenkinson, the Federal Circuit was moving in the direction of limiting evidence to the file wrapper itself. However, the majority appears to invite use of extrinsic evidence. The Court stated:
Whether a reason in fact exists, but simply was not adequately developed, we cannot say. On remand, the Federal Circuit can consider whether reasons for that portion of the amendment were offered or not and whether further opportunity to establish such reasons would be proper.
Warner-Jenkinson, 1997 U.S. LEXIS 1476, *30. The concurrence by Justice Ginsburg recommends this approach as a matter of fairness to the patent owner, at least with respect to applications that predate the Warner-Jenkinson decision. Id. at 41-2.
The Federal Circuit's Opinion on Remand
The Federal Circuit's opinion on remand adopts, if somewhat tentatively, Justice Ginsburg's approach to the admissibility of extrinsic evidence regarding prosecution history estoppel. Specifically, the opinion cites Justice Ginsburg's concurrence, noting that the prosecution history itself may not always be adequate to show the reasons for a claim amendment, since "express recitations may not exist " in applications prosecuted prior to the Supreme Court decision." The Court stated:
" we conclude that where the prosecution history is silent or unclear the district court should give a patentee the opportunity to establish the reason, if any, for a claim change. " We hesitate to specify the procedures that the district court can employ to answer the question posed by the newly created presumption of prosecution history estoppel. The better course is to allow the district court to use its discretion to decide whether hearings are necessary or whether the issue can adequately be determined on a written record.
Consistent with this framework, the Federal Circuit denied Warner-Jenkinson's motion, which sought to bind Hilton Davis to "admissions" made during the appellate process to the effect that there was no reason for the disputed claim amendment.  The Court took the position that Hilton Davis's statements could easily reflect either an inadequately developed record, or alternatively, a lack of reason for the claim amendment in the prosecution history. Thus, the Court remanded the case to give Hilton Davis an opportunity to augment the record "to show the reason for the claim amendment based on other facts that may be available" (emphasis added).
In remanding the case, the Court declined to state what reasons would be sufficient to overcome the presumption, or what factors should be considered in making this determination. By way of guidance, the Court stated simply "[w]hat reason is sufficient depends on the particular facts of the case," citing to its prior opinion in Pall Corp. v. Micron Separations, Inc., 66 F.3d 1211, 1219 (Fed. Cir. 1995). In Pall Corp., the Federal Circuit held that amendments to avoid prior art create a presumptive estoppel, whereas the estoppel effect of other amendments that arguably relate to "patentability," for example, non-enablement considerations, should be decided on a case-by-case basis.
Furthermore, the opinion on remand avoids addressing whether the newly-created presumption applies only to claim amendments. There is some suggestion that the presumption might apply more broadly to include "interpretive assertions." In particular, with respect to applications that post-date the Warner-Jenkinson decision, the Court admonishes applicants and the PTO to include in the file wrapper "express statements of their reasons for requiring or making claim changes or interpretive assertions." Whether the Federal Circuit will apply the presumption broadly in future cases remains to be seen. For now, there is uncertainty whether applicants who fail to articulate the basis for interpretive statements or other positions will later find themselves estopped from asserting the doctrine of equivalents in subsequent patent litigation.
The Federal Court's admonishment to include "express statements" in the prosecution history also suggests that, in cases involving applications made after the Warner-Jenkinson decision, extrinsic evidence may be allowed less frequently, if at all, to rebut the presumption of prosecution history estoppel. In these cases, the fairness considerations raised by Justice Ginsburg's concurrence are substantially diminished. Still, the opinion is not definitive on this point, stating "we expect that the PTO and applicants henceforth usually will include"" (emphasis added). For now, it is worth noting that the Federal Circuit has not announced a bright line rule. Even with respect to applications that predate the Warner-Jenkinson decision, the opinion on remand leaves it to the district courts to determine whether to admit extrinsic evidence.
The only guidance the Federal Circuit offers the district courts in adjudicating the presumption of prosecution history estoppel is to follow the Supreme Court's exhortation to "consider carefully the importance of public notice and reliance on the prosecution history, as well as the need for fairness to the patentee." How these interests will be balanced in resolving the above questions remains to be seen.
Because of the generality of this article, the information provided herein may not be applicable in all situations and should not be acted upon without specific legal advice based on particular situations.
 The Court also affirmed the jury's finding of equivalence between a pH of 5 and the claimed pH of "approximately 6." The Court noted that evidence in the record, consisting of testimony from the inventor and an expert witness, reflected that the claimed process would operate at a pH of 5.0, and concluded "[t]he jury's finding that the accused process with a pH of 5.0 is equivalent to the claimed process with a lower limit of approximately 6.0 does not therefore vitiate the claim limitation."