Prosecution history estoppel does not always complete bar the inventor from asserting infringement against any equivalent to the narrowed element

An amendment to a patent application that narrowed a patent claim to comply with the Patent Act does not in all cases create an absolute bar to equivalents for the particular claim limitation that was narrowed by the amendment. 

The patent laws reward innovation with a temporary monopoly and like any property right, its boundaries should be clear. But a patent and thus disclosing the invention to the public, rather than exploit it in secret bears the risk that others will devote their efforts toward exploiting the limits of the patent’s language.
The language in the patent claims may not capture every nuance of the invention or describe with complete precision the range of its novelty. If patents were always interpreted by their literal terms, their value would be greatly diminished. Unimportant and insubstantial substitutes for certain elements could defeat the patent, and its value to inventors could be destroyed by simple acts of copying. For this reason, the clearest rule of patent interpretation, literalism, may conserve judicial resources but is not necessarily the most efficient rule. The scope of a patent is not limited to its literal terms but instead embraces all equivalents to the claims described. See Winans v. Denmead, 15 How. 330, 347 (1854).

Estoppel arises when amendment is made to secure the patent and the amendment narrows the patent’s scope
If a §112 amendment is truly cosmetic, then it would not narrow the patent’s scope or raise an estoppel. On the other hand, if a §112 amendment is necessary and narrows the patent’s scope–even if only for the purpose of better description–estoppel may apply. A patentee who narrows a claim as a condition for obtaining a patent disavows his claim to the broader subject matter, whether the amendment was made to avoid the prior art or to comply with §112. We must regard the patentee as having conceded an inability to claim the broader subject matter or at least as having abandoned his right to appeal a rejection. In either case estoppel may apply.

Prosecution history estoppel does not always complete bar the inventor from asserting infringement against any equivalent to the narrowed element
Though prosecution history estoppel can bar challenges to a wide range of equivalents, its reach requires an examination of the subject matter surrendered by the narrowing amendment. The complete bar avoids this inquiry by establishing a per se rule; but that approach is inconsistent with the purpose of applying the estoppel in the first place–to hold the inventor to the representations made during the application process and to the inferences that may reasonably be drawn from the amendment.