I read somewhere recently that it takes anywhere from 500,000 to 1,000,000 cycles to wear in a resurfaced hip. This is known from bench testing that companies and other folks do to test for patterns of wear on the metal surfaces. There is a lot of science as to the metallurgy of hip implants but a constant is that they have carbide blocks which strengthens the metal but which also has abrasive properties. Setting aside whether the metal is heat treated, as cast, forged, etc., there is a period of time where the carbide blocks will wear down and smooth out through repetitive cycling. During that period there is sometimes a rise in the metal ions in the body, which are excreted in the urine, and then the "shedding" drops dramatically to very low levels. I was told by my Dr. that I will have my metal ion levels tested at the 2 year mark when this "running in" state has been completed. So I got to thinking about what those repetitive cycles mean in every day life. Here's some real simple math but remember that this all can vary on how active or inactive you are and how diligent you might be to doing regular exercise.
So.....there's 5280ft in a mile. There's 3ft in a yard and a typical stride is 3ft. That makes 1760 yards in a mile or strides. If you walk 2 miles per day that's 3520 strides. 500,000 cycles divided by 3520 strides per day equals 142 days. This equates to 4.66 months. 1,000,000 cycles for run in would then take 284 days or 9.33 months.
This all kind of makes sense to me now about that magic 6 month mark so many Dr's adhere to. With the rehab period after surgery not having us walking that far out of the gate, 6 months for 500,000 wear in cycles is very realistic, and certainly 1,000,000 by the one year mark is also. This does not even take into account all the extra strides you do for work and just every day activities or the extra mileage you may put in on walks. That's why my Dr. said that for the first 6 months walking is the best exercise for your new hip.
The other cool thing is that although our hips are referred to as MoM, or Metal on Metal, it is actually MoFoM, or Metal on Fluid on Metal. There is a cushion of liquid called synovial fluid which lubricates our joints and helps reduce wear produced by friction of dry bone on dry cartilage. As the hip heals and synovial fluid flows into the breached joint capsule, the purposely designed space between the femur head component and the metal acetabular cup, fills with this slightly viscous fluid. Interestingly, the fluid is comprised of proteins which in fact adhere to the Cobalt Chromium metal to aid in it's lubriciousness (fancy word for slicker than snot!!). I'm not sure what aids us in having more synovial fluid in our joints but could that be the reason we wore out our cartilage when other just as active people have not? We don't produce as much Joint Juice? I do know it's been suggested that we stay hydrated....makes sense there.
Happy Wearing In Hippys......