Abstract
There exists a lot literature referring to the cementing technique of hip replacements, but when talking about longevity of knee prostheses only seldom the cementing technique is mentioned even though 90% of the knees are cemented. Especially the tibial component, that has to cope with different forces such as pressure, rotation, tilt and sliding, is said to last longer when cemented.
Cementing Technique: There are many aspects that need to be thought of when cementing knee prostheses:
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The preparation of the bone: The preparation of the surface of the bone is of great importance, as the-bond of the cement with the bone is by the shape of the surfaces and not by a chemical reaction. A good penetration of the cement into the cancellous-bone enlarges the connecting surface and optimizes the power transmission. The pulse-lavage is the most effective to open the spongy bone. Sclerotic bone needs to be penetrated.
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Selection of cement: PMMA-cement (Polymeth-ylmetacrylat) is used with proven effectiveness since 1958 (Charnley). Very Similar to the well known cement Palacos (BiometMerck) is the new SmartSet GHV (DePuy) but it provides a longer time for processing, which is useful when cementing all components in one go. Mixing and hardening time are therefore shorter.
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Mixing of the cement: Mixing is mostly done manually even though it is known that the quality of the cement is minor than with a vacuum system. The advances of such a system are better microporosity, no air bubbles, and safety for the staff, who breathe less fumes.
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Application of cement – viscosity: The cement can be applied to the prosthesis or directly onto the bone. If the implant is precoated, the viscosity of the cement should be low to achieve better joint. The bone should in any case be dry to avoid mixing with blood.
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Pressure: During implantation a short high pressure is of importance for the depth of penetration. Some implants have an edge to guarantee better distribution of the pressure. During the hardening of the cement the pressure has to b ekept at a certain level as the volume of the cement changes a bit during the polimerisation.
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Hardening: The pressure needs to be controlled avoiding small movements. When cementing all components at once the ligaments have to be balanced, otherwise unnoticed deviations might occur. The leg should not be hyperextended to avoid tilting of the components.
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Temperature of polimerisation: The temperature can be reduced in vivo by cooling of the bone or the cement and by good spongy bone that transports the temperature away. If the cement penetrates more than 5 mm or its homogeneous thickness is more than 3mm osteonecrosis is likely to occur.
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Thickness of cement layer: Several authors and the finite element measurements found out, that acement layer from 2–5 mm ensures good stability for the tibial component. Cementing the shaft does not lead to significant better results but may lead to atrophy of the bone underneath the tibial plateau. Femoral components show good results also uncemented.
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Excess of cement: Cement that juts out must be removed especially in the dorsal parts, where an impingement can be produced. Bits in the soft tissue must also be removed with care. Cement should not touch the polyethylene during the whole procedure.
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Antibiotics: The quality and longevity of the cement is reduced by adding antibiotics because of resulting higher porosity. A special management for risk patientsis necessary.
When cementing knee prostheses one should give high attention to the cementing technique as especially a good anchorage of the tibial component will lead to longevity of the implant.
Correspondence should be addressed to Richard Komistek, PhD, International Society for Technology in Arthroplasty, PO Box 6564, Auburn, CA 95604, USA. E-mail: ista@pacbell.net
