Dr. Christian Christensen describes a large diameter metal-on-metal hip replacement.
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A large diameter metal-metal hip replacement is one where the femoral head that goes on top of the stem is really of an anatomic size instead of the more standard sizes that were used five and ten years ago that were normally 28 and 32 mm. These head sizes are now 46, 50, 54 mm in diameter and that is a benefit for two reasons. One, it improves the range of motion that’s allowable in the hip before the neck of the implant hits the edge of the socket. Then if it does hit the edge of the socket, the distance that the neck has to wedge the ball out is much greater. It’s really half the diameter of the head before the head actually comes all the way out. So previously with a 28 mm head, the head only had to be displayed 14, just over 14 mm before the ball would slide out. Now if you have got a 50 mm diameter socket, 50 mm diameter head, you’ve got to go 25 mm, a little over 25 mm in order to get enough displacement to actually dislocate the ball. So, that small improvement has made a world of difference and reduced dislocation rates to, you know, 1 in 500 whereas they used to be anywhere from 1 to 5 out of a 100 hip replacements.