Total knee replacement requires the surgeon to consider a number of factors when deciding on the type of total knee replacement for any particular patient.
Cemented or cementless?
The cement fixing the prosthesis to the bones of femur and tibia are an issue in knee replacement, because cement can eventually degrade, allowing the prosthesis to loosen. Manufacturers offer cemented and cementless options -
- Cemented - a better option when the patient has poor-quality bone, the stem of the prosthesis is fixed with quick-drying bone cement. Using cement is a better option for the older patient, but may need replacing within 10 years, so not a good option for the younger patient.
- Cementless ('press-fit') - relies upon a porous coating on the stem of the prosthesis itself into which new bone will grow and hold the prosthesis in the bone.
Fixed or mobile bearing?
The plastic component between femoral and tibial components is known as a 'bearing' -
- Fixed bearing - in a fixed bearing prosthesis there is no mobile plastic component, and the femoral component rolls directly on the tibial component. This is a cheaper option but has limitations - motion is more limited with poor rotation, it may wear more quickly and may be more prone to loosening.
- Mobile bearing - a mobile bearing prosthesis has a plastic (polyethylene) insert that can slide and rotate to some extent on the tibial component to allow the knee more movement as well as rotation. It requires that supporting muscles and ligaments are strong, so this is more suitable for the younger more active patient.
Cruciate retaining or posterior stabilised?
The surgeon needs to make choices for his patient of implant design from the various available options -
- Posterior-stabilised - where the PCL is not intact, a special design of 'post and cam' prevents the femur slipping forward on the tibia, while still allowing the femur to roll back when the knee bends. The 'post' is a plastic stop on the polyethylene component and the 'cam' is a bar across the gap in the femoral component.
- Cruciate-retaining - this design allows the patient to keep the PCL but the ACL is usually removed.
- Bicruciate-retaining - this design allows the patient to keep both cruciate ligaments.
Gender-specific
Small women need a special design of prosthesis to cater for the smaller bones and different hip angle than ordinary designs.
Custom implants in knee replacement
Some patients have issues that may lead the surgeon to contemplate a custom implant. The joint may be unusually small, unusually big or deformed in an unusual way.
After MRI or CT scanning, a 3-dimensional model of the knee can be produced, both digitally and printed in 3-D, allowing for manufacture of a prosthesis specifically tailored to the individual in question.
Prosthesis choice for total knee replacement
There are three kinds of prosthesis suitable for total knee replacement -
- non-constrained - where the patient's own ligaments and muscles continue to offer stability to the knee.
- semi-constrained - provide some stability for the knee, but the ligaments and muscles remain important for full stability.
- constrained - where all the stability is provided by the prosthesis itself.
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