An expanding and collapsing apparatus, comprising: a plurality of elements (12, 52, 302) assembled together to form a ring structure (54, 304) around a longitudinal axis, wherein the ring structure (54, 304) is configured to be moved between an expanded condition and a collapsed condition by movement of the plurality of elements (12, 52, 302), wherein the plurality of elements comprises: a plurality of support elements, each support element having a first end and a second end, wherein the plurality of support elements are configured to move between the expanded condition and the collapsed condition by movement of the first end in an axial direction, and by movement of the second end in at least a radial dimension; a plurality of ring elements configured to be moved between the expanded and collapsed conditions by sliding with respect to one another in a direction tangential to a circle concentric with the ring structure; and a retaining ring configured to couple to the plurality of support elements; wherein a first end of each support element of the plurality of support elements is coupled to the retaining ring via a hinge connection and a second end of each support element of the plurality of support elements is coupled to an adjacent ring element of the plurality of ring elements via another hinge connection, characterised in that each support element of the plurality of support elements includes one or more male interlocks extending from a first surface of the support element, and one or more female interlocks extending into a second surface of the support element and each male interlock of the one or more male interlocks are configured to mate with a corresponding female interlock of the one or more female interlocks of an adjacent support element to guide movement of the support element relative to the adjacent support element. a plurality of elements (12, 52, 302) assembled together to form a ring structure (54, 304) around a longitudinal axis, wherein the ring structure (54, 304) is configured to be moved between an expanded condition and a collapsed condition by movement of the plurality of elements (12, 52, 302), wherein the plurality of elements comprises: a plurality of support elements, each support element having a first end and a second end, wherein the plurality of support elements are configured to move between the expanded condition and the collapsed condition by movement of the first end in an axial direction, and by movement of the second end in at least a radial dimension; a plurality of ring elements configured to be moved between the expanded and collapsed conditions by sliding with respect to one another in a direction tangential to a circle concentric with the ring structure; and a retaining ring configured to couple to the plurality of support elements; wherein a first end of each support element of the plurality of support elements is coupled to the retaining ring via a hinge connection and a second end of each support element of the plurality of support elements is coupled to an adjacent ring element of the plurality of ring elements via another hinge connection, a plurality of support elements, each support element having a first end and a second end, wherein the plurality of support elements are configured to move between the expanded condition and the collapsed condition by movement of the first end in an axial direction, and by movement of the second end in at least a radial dimension; a plurality of ring elements configured to be moved between the expanded and collapsed conditions by sliding with respect to one another in a direction tangential to a circle concentric with the ring structure; and a retaining ring configured to couple to the plurality of support elements; wherein a first end of each support element of the plurality of support elements is coupled to the retaining ring via a hinge connection and a second end of each support element of the plurality of support elements is coupled to an adjacent ring element of the plurality of ring elements via another hinge connection, characterised in that each support element of the plurality of support elements includes one or more male interlocks extending from a first surface of the support element, and one or more female interlocks extending into a second surface of the support element and each male interlock of the one or more male interlocks are configured to mate with a corresponding female interlock of the one or more female interlocks of an adjacent support element to guide movement of the support element relative to the adjacent support element.