PELVIC STRESSES INVITRO .1. MALSIZING OF ENDOPROSTHESES

Seven embalmed human cadaveric hemi-pelves have been stripped of all soft tissues except the articular cartilage and instrumented with 25 strain-guage rosettes on both the medial and lateral surfaces. Loads up to 2. 5 kN were applied, without simulated muscle forces, to mimic the line-of-action of the resultant joint-force in a single-legged stance. In this parametric study, endoprostheses were studied at the 'correct' size and in sizes of 1 and 2 mm above and below the correct size. The correct size of endoprosthesis produced similar periacetabular stress data to those obtained with the normal femoral head; however, significant changes in stress on the medial and lateral aspects of the ilium suggested the correctly-sized spherical endoprosthesis had a different moment arm than its anatomical counterpart. Endoprostheses of 1 mm greater than the correct size caused major increases in the magnitude of both periacetabular and medial-wall stresses, while prosthes of plus 2 mm created dramatic and significant increases in magnitudes of these stresses. Endoprostheses smaller than the correct size caused only small but significant changes in the pattern of stresses up to plus 2 mm malsizing and, in particular, were associated with the disappearance of all periacetabular tensile stresses.