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Collar: in THR

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  • stress transfer to femur is desirable because it provides physiologic stimulus for maintaining bone mass & preventing disuse osteoporosis;
  • if prosthesis has a collar that is seated on the cut surface of the neck or if there is a layer of cement in contact with the bone and undersurface of the collar, there will be axial loading of the bone;
  • although the role of a collar in preventing loosening of a cemented femoral component has not clearly established, any loading of proximal medial neck is likely to decrease bone resorption and thereby reduce stresses in the proximal cement;
  • collar also serves as a simple means of determining depth of insertion of femoral component, since vision is temporarily obscured by extrusion of the cement;

Collar in Austin Moore Prosthesis

  • Calcar Pivot: (distal toggle) is frequent complication of a non cemented collared prosthesis;
  • only true calcar support prosthesis is the old Moore implant, whose stem was too small to provide canal fill in most cases;
  • designed for patients w/ 1/2 to 3/4 inches of remaining femoral neck above the lesser trochanter;
  • collar of Austin Moore prosthesis is more transverse than that of the Thompson prosthesis, a fact that increases ability of neck to receive the compression stresses inserted on to it;
  • Moore initially desinged his prosthesis with fenestrations in the stem in an effort to induce "self locking" and bony ingrowth