A cranial anchor system (100, 300) configured to attach to an opening in a cranium, the system comprising: an anchor member (200, 400) that is elongate along a first axis, the anchor member (200, 400) including: a middle anchor portion (204, 404) defining a middle anchor recess that extends about the first axis; and a lower anchor portion (206, 406) coupled to and disposed in a downward direction relative to the middle anchor portion (204, 404) along the first axis, the lower anchor portion (206, 406) including: a plurality of contact members (208, 408) that together define a lower anchor recess, each of the plurality of contact members (208, 408) having: a leg that extends at least partially in the downward direction, a tab (216, 416) connected to the leg, the tab (216, 416) extending at least partially in a radial direction, the radial direction being perpendicular to the first axis, and a rib (214, 414) disposed between the leg and the tab (216, 416); an outer sheath (104. 304) that is elongate along a second axis and defines a first channel, the outer sheath (104, 304) including a lower outer sheath portion configured to be at least partially disposed within the middle anchor recess and the lower anchor recess such that the lower outer sheath portion is rotatable within the middle anchor recess and the lower anchor recess, the lower outer sheath being configured to engage the plurality of contact members such that a force applied to the lower outer sheath in the downward direction causes the plurality of contact members to move in the radial direction; and an inner sheath (106, 306) disposed within the first channel and slidable relative to the outer sheath (104, 304) along the second axis, the inner sheath (106, 306) defining a second channel that is elongate along the second axis. an anchor member (200, 400) that is elongate along a first axis, the anchor member (200, 400) including: a middle anchor portion (204, 404) defining a middle anchor recess that extends about the first axis; and a lower anchor portion (206, 406) coupled to and disposed in a downward direction relative to the middle anchor portion (204, 404) along the first axis, the lower anchor portion (206, 406) including: a plurality of contact members (208, 408) that together define a lower anchor recess, each of the plurality of contact members (208, 408) having: a leg that extends at least partially in the downward direction, a tab (216, 416) connected to the leg, the tab (216, 416) extending at least partially in a radial direction, the radial direction being perpendicular to the first axis, and a rib (214, 414) disposed between the leg and the tab (216, 416); a middle anchor portion (204, 404) defining a middle anchor recess that extends about the first axis; and a lower anchor portion (206, 406) coupled to and disposed in a downward direction relative to the middle anchor portion (204, 404) along the first axis, the lower anchor portion (206, 406) including: a plurality of contact members (208, 408) that together define a lower anchor recess, each of the plurality of contact members (208, 408) having: a leg that extends at least partially in the downward direction, a tab (216, 416) connected to the leg, the tab (216, 416) extending at least partially in a radial direction, the radial direction being perpendicular to the first axis, and a rib (214, 414) disposed between the leg and the tab (216, 416); a leg that extends at least partially in the downward direction, a tab (216, 416) connected to the leg, the tab (216, 416) extending at least partially in a radial direction, the radial direction being perpendicular to the first axis, and a rib (214, 414) disposed between the leg and the tab (216, 416); an outer sheath (104. 304) that is elongate along a second axis and defines a first channel, the outer sheath (104, 304) including a lower outer sheath portion configured to be at least partially disposed within the middle anchor recess and the lower anchor recess such that the lower outer sheath portion is rotatable within the middle anchor recess and the lower anchor recess, the lower outer sheath being configured to engage the plurality of contact members such that a force applied to the lower outer sheath in the downward direction causes the plurality of contact members to move in the radial direction; and an inner sheath (106, 306) disposed within the first channel and slidable relative to the outer sheath (104, 304) along the second axis, the inner sheath (106, 306) defining a second channel that is elongate along the second axis.