An improved delivery system for an implantable medical device includes a retention sheath having a proximal end, a distal end, and an inner lumen extending from the proximal end to the distal end. The implantable medical device is disposed within the inner lumen of the retention sheath, which restrains the implantable medical device. A plurality of substantially incompressible rings are disposed within the inner lumen of the retention sheath in a stacked co-axial configuration that extends from the proximal end of the retention sheath in a pre-deployment position to a proximal end of the implantable medical device. Each ring in the plurality of separate rings abuts at least a portion of an adjacent ring. The plurality of separate rings is configured to prevent the implantable medical device from moving toward a proximal end of the retention sheath when the retention sheath is moved from the pre-deployment position to a deployment position.