Patent Doctrine of Equivalents: When a Different Product Can Still Infringe
A product does not automatically avoid patent infringement merely because it falls outside the literal wording of a claim. Under the patent doctrine of equivalents, a court may treat an accused feature as equivalent to a claimed limitation when the difference is legally insubstantial.
That principle protects patent owners from copyists who make trivial substitutions. But it is a limited doctrine, not permission to rewrite claims after issuance. The analysis remains tied to individual claim limitations, the patent’s prosecution history, the prior art, and what the specification disclosed but did not claim.
For patent owners and companies planning a design-around, the real question is not simply whether two products look or operate similarly. It is whether each disputed limitation is present literally or through a legally permissible equivalent—and whether an estoppel or other claim-scope rule forecloses that argument.
Literal infringement comes first
Patent infringement under 35 U.S.C. § 271 begins with the asserted claims. A claim defines the protected combination through a series of limitations. To establish literal infringement, the patent owner generally must show that the accused product or process satisfies every limitation as properly construed.
This is why patent claim construction often controls an infringement case. If a claim requires a component in a specified location, a process performed in a specified order, or a material with a particular property, the accused technology may fall outside the literal claim even if it achieves a similar commercial result.
The doctrine of equivalents addresses a narrower problem: whether an accused feature that does not literally meet one limitation should nonetheless be treated as its equivalent. It does not eliminate the limitation or permit comparison only at the level of the overall product.
What the doctrine of equivalents asks
In Warner-Jenkinson Co. v. Hilton Davis Chemical Co., the Supreme Court confirmed the doctrine’s continuing role while requiring an element-by-element analysis. Equivalence must be examined for the particular limitation at issue, not by asking whether the accused product and patented invention are generally alike.
Courts commonly frame the merits in one of two related ways:
- Function-way-result: Does the accused element perform substantially the same function, in substantially the same way, to achieve substantially the same result as the claimed element?
- Insubstantial differences: Is the difference between the claimed limitation and the accused element insubstantial in the technological context?
Neither formulation is a mechanical shortcut. The appropriate inquiry depends on the technology and the claim language. Evidence may include the patent, prosecution record, technical documents, testing, inventor or engineer testimony, and qualified expert analysis. The relevant comparison is between the accused element and the claimed limitation—not between marketing descriptions or high-level product purposes.
The Federal Circuit reinforced that point in its August 4, 2026 decision in Scilex Pharmaceuticals Inc. v. Aveva Drug Delivery Systems, Inc.. The court described the doctrine as a limited exception to claim-defined exclusivity and reiterated that it is limitation-specific. There, the asserted claims involved a non-aqueous lidocaine patch with a specified two-component “dissolving agent.” The patent owners conceded no literal infringement and relied on equivalents. The Federal Circuit affirmed the judgment that the proposed generic patch did not infringe, concluding that the properly understood claimed agent had to dissolve the lidocaine as well as maintain it in a non-crystalline state.
An equivalent cannot erase a claim limitation
The all-elements rule prevents the doctrine from effectively deleting a limitation. If the proposed theory would make a meaningful claim requirement irrelevant, a court may conclude that no reasonable factfinder could find equivalence.
This issue is sometimes discussed as “claim vitiation.” Vitiation is not a separate formula under which courts compare labels such as opposite versus similar. It describes the conclusion that a particular equivalence theory is legally insufficient because it would eliminate the limitation rather than recognize an insubstantial variation.
For example, a patent owner cannot ordinarily convert a precise numerical, structural, or relational requirement into a limitless concept merely because the accused product serves the same broad purpose. The specification and prosecution history may show why the claimed boundary mattered. Conversely, a small physical or numerical difference is not automatically substantial. The answer depends on what the limitation means and how the accused feature works in context.
Prosecution history can surrender equivalents
The most important limit is often prosecution history estoppel. Patent applicants routinely amend claims or make arguments to overcome prior art and other patentability rejections. Those choices can narrow not only literal claim scope but also the territory later available under the doctrine of equivalents.
In Festo Corp. v. Shoketsu Kinzoku Kogyo Kabushiki Co., the Supreme Court held that a narrowing amendment made for a reason related to patentability creates a presumption that the applicant surrendered the territory between the original and amended limitation. The patent owner may rebut that presumption by showing one of the recognized circumstances, such as:
- the alleged equivalent was unforeseeable when the amendment was made;
- the reason for the amendment was only tangentially related to the alleged equivalent; or
- another reason shows that the applicant could not reasonably have described the substitute.
Those exceptions are demanding and fact-specific. The public is entitled to rely on the patent’s prosecution record, including what the applicant changed and why.
The Federal Circuit’s May 13, 2026 decision in Actelion Pharmaceuticals Ltd. v. Mylan Pharmaceuticals Inc. illustrates the point. The applicant had narrowed a pH limitation from greater than 12 to 13 or higher after an obviousness rejection and an examiner’s indication that the narrower range showed unexpected results. The court held that prosecution history estoppel barred Actelion from using equivalents to recover the surrendered range. It rejected the asserted tangentiality exception because the pH threshold was central to the objective reason for the amendment.
Prosecution history estoppel can also arise from claim cancellation in the right circumstances. In Colibri Heart Valve LLC v. Medtronic CoreValve, LLC, the Federal Circuit held that cancellation of a closely related claim communicated a substantive narrowing that barred the asserted equivalent. The court explained that the applicant could have pursued the desired territory through a continuation application, subject to the usual patentability and written-description requirements.
Disclosed but unclaimed alternatives may be dedicated to the public
A separate limitation is the disclosure-dedication rule. When a patent specifically discloses an alternative but does not claim it, the patent owner may be unable to recapture that alternative through the doctrine of equivalents.
Actelion applied this rule as an independent ground for rejecting the equivalents theory. The Federal Circuit explained that a disclosed alternative need not be mutually exclusive with what was claimed. The question is whether the disclosure was specific enough for a person skilled in the art to identify the subject matter that the drafter disclosed but left unclaimed.
This rule has practical consequences during prosecution. A specification should describe useful alternatives and fallback positions, but the claim strategy must also account for commercially important variants. Disclosure alone does not guarantee enforceable coverage. Tucker Law’s patent application practice addresses claim layering, continuation planning, and specification support before those choices become litigation issues.
Prior art sets an outer boundary
The doctrine cannot give a patent owner coverage that could not lawfully have been obtained over the prior art. In an “ensnarement” analysis, a court may consider a hypothetical claim broad enough to cover the accused equivalent and ask whether that claim would be patentable over the prior art.
This creates an important asymmetry. A feature may be technologically close to a claimed element, yet the available range of equivalents may be narrow because earlier patents, publications, products, or public uses occupy the surrounding territory. Patent owners should evaluate validity and infringement together. Accused infringers should not assume that a failed literal-infringement argument ends the analysis, but they should test whether the asserted equivalent would improperly reach the prior art.
What patent owners should develop as evidence
A credible equivalents case begins with a limitation-by-limitation claim chart. For each nonliteral limitation, the patent owner should identify the precise accused feature, select the technically appropriate equivalence framework, and support each part of the comparison with admissible evidence.
General assertions that two products perform the same overall job are vulnerable. So are expert opinions that state a conclusion without explaining the technology, the claimed requirement, and the accused substitute. The Federal Circuit’s 2026 decisions continue to show that claim meaning, prosecution statements, and the quality of the technical proof can end an equivalents theory before a jury reaches it.
Damages planning should begin at the same time. If the doctrine reaches only particular configurations, versions, or uses, that scope may affect the accused-sales base and the period of recoverable damages. Tucker Law’s patent litigation practice evaluates infringement, validity, evidence, and remedies as connected parts of the case.
What a defensible design-around requires
A design-around should be evaluated against the claims, not merely against the patent’s drawings or a competitor’s commercial product. Changing a component’s name, supplier, shape, or implementation may avoid literal language yet still leave an equivalents dispute if the substitute performs the same claimed role in essentially the same way.
A stronger analysis documents why the change is meaningful. Relevant considerations may include a different operating principle, architecture, sequence, material property, data flow, or technical tradeoff. Engineers and counsel should also review the prosecution history for surrendered territory, the specification for unclaimed alternatives, and the prior art for limits on the patent owner’s permissible reach.
Timing matters. A contemporaneous design record is generally more useful than a litigation-created explanation. Product requirements, test results, engineering decisions, and alternative-use evidence can show what changed and why. Licensing may be appropriate when technical or commercial constraints make the remaining risk unacceptable; Tucker Law’s patent licensing practice can help define scope, fields of use, releases, and future-product coverage.
The practical line between fair scope and rewritten claims
The doctrine of equivalents preserves meaningful patent protection when an accused infringer substitutes an insubstantially different element. It does not allow a patent owner to ignore claim language, reverse positions taken before the USPTO, reclaim specifically unclaimed alternatives, or reach the prior art.
That balance makes the doctrine highly dependent on the record. The strongest analysis combines careful claim construction, technical evidence, prosecution-history review, and a disciplined assessment of each disputed limitation.
If your company is evaluating a competitor’s product, responding to a patent accusation, or planning a design-around, contact Tucker Law. Tucker Law can assess literal infringement, potential equivalents, prosecution-history limits, invalidity issues, and practical business options. For out-of-state counsel handling a Florida patent case, the firm also serves as Florida patent litigation local counsel.
This article provides general information and is not legal advice. Patent infringement and the available range of equivalents depend on the particular claims, specification, prosecution history, accused technology, prior art, and procedural record.
