A support apparatus implantable within a biological lumen, comprising: a plurality of elongated strut members interlinked by a plurality of swivel joints forming a support structure, wherein the swivel joints cooperate with the strut members to adjustably define the shape of the support structure between a compressed orientation and an expanded orientation; wherein the plurality of elongated strut members comprises: a plurality of radially inner strut members (11-2, 11-4); and a plurality of radially outer strut members (11-1, 11-3) interlinked to the radially inner strut members by the plurality of swivel joints to define the support structure; wherein each radially inner strut member is interlinked to three radially outer strut members and none of the radially inner strut members is interlinked to another radially inner strut member; wherein each radially outer strut member is interlinked to three radially inner strut members and none of the radially outer strut members is interlinked to another radially outer strut member; wherein the plurality of swivel joints comprise middle swivel joints (15-1, 15-7) connecting the radially outer strut members (11-1, 11-3) to the radially inner strut members to form scissor mechanisms; and wherein the strut members (11-1, 11-2, 11-3, 11-4) are arranged as a serial chain of linked scissor mechanisms in which the last scissor mechanism in the chain is joined to the first scissor mechanism in the chain. a plurality of elongated strut members interlinked by a plurality of swivel joints forming a support structure, wherein the swivel joints cooperate with the strut members to adjustably define the shape of the support structure between a compressed orientation and an expanded orientation; wherein the plurality of elongated strut members comprises: a plurality of radially inner strut members (11-2, 11-4); and a plurality of radially outer strut members (11-1, 11-3) interlinked to the radially inner strut members by the plurality of swivel joints to define the support structure; wherein each radially inner strut member is interlinked to three radially outer strut members and none of the radially inner strut members is interlinked to another radially inner strut member; wherein each radially outer strut member is interlinked to three radially inner strut members and none of the radially outer strut members is interlinked to another radially outer strut member; wherein the plurality of swivel joints comprise middle swivel joints (15-1, 15-7) connecting the radially outer strut members (11-1, 11-3) to the radially inner strut members to form scissor mechanisms; and wherein the strut members (11-1, 11-2, 11-3, 11-4) are arranged as a serial chain of linked scissor mechanisms in which the last scissor mechanism in the chain is joined to the first scissor mechanism in the chain. a plurality of radially inner strut members (11-2, 11-4); and a plurality of radially outer strut members (11-1, 11-3) interlinked to the radially inner strut members by the plurality of swivel joints to define the support structure; wherein each radially inner strut member is interlinked to three radially outer strut members and none of the radially inner strut members is interlinked to another radially inner strut member; wherein each radially outer strut member is interlinked to three radially inner strut members and none of the radially outer strut members is interlinked to another radially outer strut member; wherein the plurality of swivel joints comprise middle swivel joints (15-1, 15-7) connecting the radially outer strut members (11-1, 11-3) to the radially inner strut members to form scissor mechanisms; and wherein the strut members (11-1, 11-2, 11-3, 11-4) are arranged as a serial chain of linked scissor mechanisms in which the last scissor mechanism in the chain is joined to the first scissor mechanism in the chain.