A boss for a pressure vessel has a flange. The flange includes an interior keyway having an inner sidewall and a plurality of bores disposed on the inner sidewall. A pressure vessel includes a main body section and an end section. The end section includes a boss and the boss includes a flange. The flange includes an interior keyway having an inner sidewall and a plurality of bores disposed on the inner sidewall. In another aspect, a pressure vessel includes an outer shell, and inner liner and a boss including a flange with an exterior side and an interior side. The liner is mechanically integrated with the flange via a plurality of anchors, each anchor contacting the flange only on the interior side. A method of forming a pressure vessel includes providing a boss having a flange. The flange has an interior keyway having an inner sidewall and a plurality of bores disposed on the inner sidewall. The method comprises allowing a fluid polymer material to flow into the interior keyway and into the plurality of bores.