Legendairy Milk Inserts are compatible with both the Silicone Collection Cups and most traditional 24/25 mm plastic breast shields aka flanges. The insert is very soft and pliable — when inserting it into a hard plastic shield, applying a dab of Pumping Spray or coconut oil to the hard plastic shield may help the insert slide in more easily. This product may be covered by your FSA/HSA account.