Patent infringement is determined by comparing the patented "claims" against the product that might be infringing. The patent claims are the part of the granted patent which describe the "metes and bounds" of the patent protection. In other words, they describe the outer limits or boundaries of the patented invention.
This means that there is not a quantitative amount or rule that can used to determine if there is patent infringement.
Therefore, whether there is patent infringement cannot be answered by determining how similar is the patented invention and the supposedly infringing product. Likewise, one cannot avoid patent infringement by making one's product 1% different or 99% different.
Patent owners often express the concern that if someone else changes the patented product by just a small change, that person can avoid patent infringement. While that may be true in some instances, the law attempts to avoid an injustice by applying the doctrine of equivalents.
The doctrine of equivalents is said to prevent the "unscrupulous copyist from making unimportant and insubstantial changes and substitutions in the the patent, which, though adding nothing, would be enough to take the copies matter outside the claim, and hence outside the law."
Under the doctrine of equivalents, an accused product that does not literally infringe the specific patent claim terms may still be infringing if there is equivalence between the accused product and the patent claim terms. The accused product must have elements identical or equivalent to each element of at least one patent claim.
But there is a limitation to the doctrine of equivalents. A patent may describe a feature of the invention, but not include it in the patent claims. In that situation, the law says the feature not in the claims has been dedicated to the public for use. And those dedicated features cannot serve as a basis for infringement by equivalence.