The U.S. Patent and Trademark Office’s recent confirmation of the validity of a patent that netted i4i Limited Partnership a $290 million infringement judgment against Microsoft Corp. means the U.S. Supreme Court is likely Microsoft’s last hope to overturn the judgment.

On Tuesday, i4i announced that the PTO affirmed the patentability of all the claims in its patent for processing and storing information about electronic documents’ structure. In its lawsuit, i4i claimed that Microsoft Word 2007 infringed that patent, and Microsoft had requested a re-examination in the hopes that the PTO would declare the patent invalid. The PTO has yet to issue a formal certificate confirming the patent’s validity, but the agency informed Canadian software company i4i of its notice of intent to issue an ex parte re-examination certificate on April 28.