What is my new knee made of ?
I am often asked what exactly you put inside when you replace a knee. In fact a person who is thinking about getting his/her knee replaced is curious to know what will be implanted inside his/her body.
It is important to go into history of knee replacement to put things into perspective and to know how far down the road we have come. Earliest operations for knee arthritis were done in 19th century and these included putting some tissue inside the knee joint between the two articulating surfaces. Another method was to cut different thickness of bones. In 1880,s a surgeon in Berlin tried putting implants made of ivory inside the knee and guess what material was used to fix ivory prosthesis. It was a mixture based on plaster of paris. Needless to say the results were poor.
In 1950,s McKeever used metal to make prosthesis..After going through many hits and misses knee replacement was done with use of plastic on the tibial side ( the leg side) and use of metal alloy on the femur( thigh bone) side of the joint and this bearing couple is in use till date. Earlier designs used full thickness of plastic but later on a base of metal was built into the system for better load distribution on the tibia side.
What revolutionized the joint replacement surgery was availability and use of certain type of plastic and bone cement. Ultra High Molecular Weight Polyethylene (UHMWPE) provides a bearing which has extremely low friction when it moves against metal alloy and this provides it the needed longevity. Many processes are now used to improve its wear. Bone cement is made of methyl methacrylate and it provides a firm grouting material to fix the implants to bone. Implants which do not need bone cement to fix to bone have been designed and used but at present these implants have inferior long term results compared to cemented ones.
Knee replacement is a surface replacement i.e. that diseased and degenerated joint surface is removed and a new artificial surface is put in place to allow painless motion. Bone cuts are made so that any alignment mismatch present in a degenerated knee is also corrected.
On thigh bone side alloys of chromium and cobalt are commonly used for cemented implants and titanium for uncemented ones. These are highly polished to decrease the friction. Recent introduction is use of oxidized Zirconium known as Oxinium for femoral implants. These have shown lesser friction and better wear with polyethylene in lab but whether they would prove to be successful in long term will be known only with time. Patellar component is also made of plastic.
Different designers, surgeons and engineers have introduced several different designs and different materials as well as different manufacturing processes to make these materials to improve the longevity of implanted knees. Hopefully advances in material science, design and surgical techniques will help surgeons achieve the long term aim of making knee replacement a once in a life time surgery even for patients who need it at younger age.
It is important to go into history of knee replacement to put things into perspective and to know how far down the road we have come. Earliest operations for knee arthritis were done in 19th century and these included putting some tissue inside the knee joint between the two articulating surfaces. Another method was to cut different thickness of bones. In 1880,s a surgeon in Berlin tried putting implants made of ivory inside the knee and guess what material was used to fix ivory prosthesis. It was a mixture based on plaster of paris. Needless to say the results were poor.
In 1950,s McKeever used metal to make prosthesis..After going through many hits and misses knee replacement was done with use of plastic on the tibial side ( the leg side) and use of metal alloy on the femur( thigh bone) side of the joint and this bearing couple is in use till date. Earlier designs used full thickness of plastic but later on a base of metal was built into the system for better load distribution on the tibia side.
What revolutionized the joint replacement surgery was availability and use of certain type of plastic and bone cement. Ultra High Molecular Weight Polyethylene (UHMWPE) provides a bearing which has extremely low friction when it moves against metal alloy and this provides it the needed longevity. Many processes are now used to improve its wear. Bone cement is made of methyl methacrylate and it provides a firm grouting material to fix the implants to bone. Implants which do not need bone cement to fix to bone have been designed and used but at present these implants have inferior long term results compared to cemented ones.
Knee replacement is a surface replacement i.e. that diseased and degenerated joint surface is removed and a new artificial surface is put in place to allow painless motion. Bone cuts are made so that any alignment mismatch present in a degenerated knee is also corrected.
On thigh bone side alloys of chromium and cobalt are commonly used for cemented implants and titanium for uncemented ones. These are highly polished to decrease the friction. Recent introduction is use of oxidized Zirconium known as Oxinium for femoral implants. These have shown lesser friction and better wear with polyethylene in lab but whether they would prove to be successful in long term will be known only with time. Patellar component is also made of plastic.
Different designers, surgeons and engineers have introduced several different designs and different materials as well as different manufacturing processes to make these materials to improve the longevity of implanted knees. Hopefully advances in material science, design and surgical techniques will help surgeons achieve the long term aim of making knee replacement a once in a life time surgery even for patients who need it at younger age.