To analyze the effects of various amount of peripheral defect of the acetabulum in terms of the stability of the acetabular cup with screw fixation.

The porous coated acetabular cup of Trilogy was press-fitted with 3 screw fixations after 2 mm undersized reaming into the distal part of 100 bovine femurs, but with insufficient peripheral coverage of 0%, 5%, 10%, 15% and 20% respectively. The micromotions of the acetabular cups were measured under the axial loads of 50, 100, 150, 200, 250, and 300 Kp, and torques were measured at 1° to 5° of cup rotation under applied rotational force in 10 specimens in each of five groups.

The micromotion between the cup and bone interface increased proportionally with peripheral coverage insufficiency and the torque values in the five groups increased in proportion to the rotation degree and were inversely proportinal peripheral coverage insufficiency.

15% or less coverage insufficiency of the acetabular cup may be acceptable as the average micromotion is less than 150micrometa and compatible with biologic fixations.