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"Total hips are now so improved that they will last 30 years."
Actually, in young patients (<50) implant survivorship is approximately 83% at 10 years and 50% at 20 years. Based on this ….it is hard to say: “your implant will last 30 years”. Most clinical data is derived from older folks (mean age 70), where implant survivorship is 95% at 10 years and the patients are unlikely to live 30 years.
If you have a series of patients who are mostly old and have a few younger ones sprinkled in, it is inaccurate to assume that the overall results apply equally to the younger subset. The only exception is ceramic-ceramic THR where a few expert surgeons have shown similar implant survivorship to HRA. But this does not apply to ceramic-polyethylene implants. Extrapolating results achieved in older patients to younger patients is inaccurate.
Extrapolating benchtop laboratory wear testing to clinical durability predictions is likewise inaccurate. It is true that idealized testing of wear in the laboratory would predict extremely long lifespans for ceramic on vitamin E crosslinked polyethylene bearings.
But in real life, it is impossible to achieve “ideal” implant positions. Also, wear testing machines don’t accurately mimic complex patient motions and loading. Furthermore, idealized testing cannot predict breakage due to edge loading and other activity-related factors. Finally, there are far more failure mechanisms than just wear which are not accounted for. Clinical studies account for all failure modes.
The only valid data is actual clinical data in younger patients. Few surgeons have it, and the registries show fairly poor “average surgeon” outcomes. It is fair to say that giving a 30-year lifetime estimate for THR is engaging in wishful thinking.