Well, there are various facets to this:
1. metallurgy: we know a lot more about the materials used in implants now, cobalt chrome, stainless steel, titanium, ceramic, polyethylenes and bone cement. We also know a lot more about the interaction of bearings of same or different materials and the impact on the human body such as metal on plastic or ceramic on ceramic.
2. implant manufacture: has increased in sophistication beyond all recognition to what it was 40 years ago! It's very sophisticated and improving all the time. The sciences involved here are bio-mechanics, bio-engineering, implant manufacture and all the other allied sciences like metallergy and plastics.
3. surgical skills: surgeons have a lot more information, improvements, research and training into the technique of joint replacement. They go into every little detail these days, bearings, stress shielding, cement security, implant durability, etc., etc., where years ago they just talked about JR in the general!
However, not all surgeons avail themselves of this information and training, so when you are making you selection, you might ask them how often they attend symposia and when they last attended one. Most all of the surgeons who tell people to wait as long as possible or that you are "too young" are ones who are not keeping themselves up to date and are still working on old opinions.
It's because of all this that the age limit is gradually coming down. When I was first trained, you had to be over 70 to have a THR! 69 was just not considered so tough luck! There was a lot of nervousness about how long these new flangled things would last. But now (Oct 09) we know there are people who've had their joints in for 40 years and even longer. Remember, that means they had their JR done in 1969 when the science was barely out of its infancy!
[Edit 4th Feb 2012]
In recent times there have been a lot more records of hip and knee patients still using their original devices after 40+ years. At a surgeon's conference I attended in London on Jan 2011, surgeons there were confidently talking about the 'forever' hip and when challenged from the floor on whether this would this apply to adults as young as 40+. The answer came, why not! Now this does not preclude incidents such as loosening, infection, metal-on-metal issues, malalignment, peri-prosthetic fracture and prosthetic fracture, but even so, the future looks pretty bright in regards to this particular topic.
[Edit 26th July 2013]
Commentary about longevity in TKR "How Long Do Knee Replacements Last?"
[Edit February 14, 2014] to add
Does your joint replacement have a warranty?
[Edit June 20, 2015] to add
Around November 2014, I was in conversation with some hip surgeons about longevity of the replacement and one, who was recently retired, told how he had just revised the hip of a lady whose hip he replaced in 1967 making it 48 years old! He added that the following week he was doing a revision on a male patient whose primary he did two years before that!
You might also like to read this Hips that have lasted 41, 40 and 32 years
note 13 Jun 2012
nearly 2.9 million joint replacement procedures take place worldwide, annually – including more than 1.4 million hips, 1.1 million knees and more than 100,000 shoulder replacements.
Over 231,000 hip replacements occur annually in the US alone.
ORTHOPAEDIC INDUSTRY ANNUAL REPORT®: Focus ... - Orthoworld
1. metallurgy: we know a lot more about the materials used in implants now, cobalt chrome, stainless steel, titanium, ceramic, polyethylenes and bone cement. We also know a lot more about the interaction of bearings of same or different materials and the impact on the human body such as metal on plastic or ceramic on ceramic.
2. implant manufacture: has increased in sophistication beyond all recognition to what it was 40 years ago! It's very sophisticated and improving all the time. The sciences involved here are bio-mechanics, bio-engineering, implant manufacture and all the other allied sciences like metallergy and plastics.
3. surgical skills: surgeons have a lot more information, improvements, research and training into the technique of joint replacement. They go into every little detail these days, bearings, stress shielding, cement security, implant durability, etc., etc., where years ago they just talked about JR in the general!
However, not all surgeons avail themselves of this information and training, so when you are making you selection, you might ask them how often they attend symposia and when they last attended one. Most all of the surgeons who tell people to wait as long as possible or that you are "too young" are ones who are not keeping themselves up to date and are still working on old opinions.
It's because of all this that the age limit is gradually coming down. When I was first trained, you had to be over 70 to have a THR! 69 was just not considered so tough luck! There was a lot of nervousness about how long these new flangled things would last. But now (Oct 09) we know there are people who've had their joints in for 40 years and even longer. Remember, that means they had their JR done in 1969 when the science was barely out of its infancy!
[Edit 4th Feb 2012]
In recent times there have been a lot more records of hip and knee patients still using their original devices after 40+ years. At a surgeon's conference I attended in London on Jan 2011, surgeons there were confidently talking about the 'forever' hip and when challenged from the floor on whether this would this apply to adults as young as 40+. The answer came, why not! Now this does not preclude incidents such as loosening, infection, metal-on-metal issues, malalignment, peri-prosthetic fracture and prosthetic fracture, but even so, the future looks pretty bright in regards to this particular topic.
[Edit 26th July 2013]
Commentary about longevity in TKR "How Long Do Knee Replacements Last?"
[Edit February 14, 2014] to add
Does your joint replacement have a warranty?
[Edit June 20, 2015] to add
Around November 2014, I was in conversation with some hip surgeons about longevity of the replacement and one, who was recently retired, told how he had just revised the hip of a lady whose hip he replaced in 1967 making it 48 years old! He added that the following week he was doing a revision on a male patient whose primary he did two years before that!
You might also like to read this Hips that have lasted 41, 40 and 32 years
note 13 Jun 2012
nearly 2.9 million joint replacement procedures take place worldwide, annually – including more than 1.4 million hips, 1.1 million knees and more than 100,000 shoulder replacements.
Over 231,000 hip replacements occur annually in the US alone.
ORTHOPAEDIC INDUSTRY ANNUAL REPORT®: Focus ... - Orthoworld