A fail-safe attachment is provided for coupling an exoskeletal prosthesis to a patient limb having a bone anchored mounting post with an external fixation structure (1). The fail-safe attachment comprises a first component (2) adapted for connection to the fixation structure, and a second component (3) adapted for connection to the prosthesis. A safety release mechanism is provided normally retaining the prosthesis in a rigid substantially motion-free interconnected relation to the fixation structure, said safety release mechanism responding to a force overload condition to permit relative movement between the prosthesis and the fixation structure. The safety release mechanism comprises a bending force release mechanism responsive to a bending force overload condition to permit the separation of the second component (3) relative to said first component (2). The bending force release mechanism comprises a plurality of spring loaded jaws (37) adapted to engage a complementary portion (12) of the other component, the jaws (37) being pivotable in an over-centre spring configuration between a first position in which they engage the complementary portion (12) to secure the first and second components one to the other, and a second position in which the first and second components are able to separate.