Bone cement has been used very successfully to make artificial joints (hip joints, knee joints, shoulders and elbow joints) for more than half a century. The artificial joint (referred to as the prosthesis) is anchored with cement bone. Cement bone fills free space between prosthesis and bone and plays an important role of the elastic zone. This is necessary because the human hip is acted upon by about 10-12 times body weight and therefore bone cement must absorb forces acting on the hip to ensure that the artificial implants remain in place for the long term.
Bone cement is chemically nothing more than Plexiglas (ie polymethyl methacrylate or PMMA). PMMA was used clinically for the first time in 1940 in plastic surgery to close the gap in the skull. Comprehensive clinical tests of bone cement compatibility with the body are performed prior to use in surgery. The excellent tissue compatibility of PMMA allows bone cement to be used for head prosthesis removal in the 1950s.
Currently several million of this type of procedure are performed every year worldwide and over half of them regularly use bone cement - and the proportion increases. Bone cement is considered a reliable hook material with ease of use in clinical practice and in particular because of its proven survival rate with cemented prosthesis. Hip and knee exclusions for artificial joint replacements such as those in Sweden and Norway clearly demonstrate the advantages of cemented cementing. A similar register for endoprosthesis was introduced in Germany in 2010.
Video Bone cement
Composition
Bone cement is provided as a two-component material. The cement bone consists of a powder (ie, pre-polymerization PMMA and/or PMMA or MMA co-polymer beads and/or amorphous powder, radio-opacifer, initiator) and liquid (MMA monomer, stabilizer, inhibitor). The two components are mixed and the free radical polymerization takes place from the monomer when the initiator is mixed with the accelerator. The viscosity of bone cement changes over time from dilute liquids to a dough such as a state that can be applied safely and eventually harden into a solid hardening material. The prescribed time can be adjusted to help doctors safely apply bone cement into the bone base for either anchor metal or plastic prosthetic device to bone or used alone on the spine to treat osteoporotic compression fractures.
During the exothermic free-radical polymerization process, the cement heats up. This polymerization heat reaches a temperature of about 82-86 Â ° C in the body. This temperature is higher than the critical level for protein denaturation in the body. The cause of the low polymerization temperature in the body is a relatively thin layer of cement, which does not exceed 5 mm, and temperature dissipation through the surface of the large prosthesis and blood flow.
The individual components of bone cement are also known in the field of dental fillers. Acrylic based plastic is also used in this application. While individual components are not always completely safe as pharmaceutical additives and active substances per se, such as bone cement, individual substances are altered or completely enclosed in a cement matrix during the polymerization phase from increased viscosity to curing. From current knowledge, dried bone cement can now be classified as safe, as demonstrated at the beginning of studies on the compatibility with the body performed in the 1950s.
More recently bone cement has been used in the spine in either vertebroplasty or kyphoplasty procedures. The composition of this type of cement is largely based on calcium phosphate and newer magnesium phosphate. A new biodegradable, non-exothermic, and self-adjustable orthopedic cement composition based on amorphous magnesium phosphate (AMP) was developed. The occurrence of undesirable exothermic reactions is avoided by using AMP as a solid precursor
Maps Bone cement
Important information for bone cement use
What is called bone cement implantation syndrome (BCIS) is described in the literature. For a long time it was believed that completely unconverted monomers released from bone cement were the cause of circulatory and embolic reactions. However, it is now known that these monomers (residual monomers) are metabolized by the respiratory chain and broken down into carbon dioxide and water and excreted. Embolism can always occur during the anchorage of the artificial joint when the material is inserted into the previously cleansed thigh bone. The result is an increase in intramedullary pressure, potentially pushing fat into the circulation.
If the patient is known to have an allergy to bone cement constituents, according to current knowledge bone cement should not be used to anchor the prosthesis. Anchorage without cement - the placement of cement-free implants - is the alternative.
New bone cement formulations require characterization in accordance with ASTM F451. This standard describes test methods for assessing cure rates, residual monomers, mechanical strength, benzoyl peroxide concentrations, and heat evolution during healing.
Revision
Revision is the replacement of the prosthesis. This means that the previously planted prosthesis in the body is removed and replaced with a new prosthesis. Compared with earlier surgical revisions it is often more complicated and more difficult, because each revision involves the loss of healthy bone substance. Revision operations are also more expensive for satisfactory results. Therefore, the most important goal is to avoid revisions by using good surgical procedures and using products with good long-term outcomes.
Unfortunately, it is not always possible to avoid revisions. There are also different reasons for the revision and there is a difference between septic or aseptic revisions. If necessary to replace the implant without infection confirmation - for example, aseptic - the cement does not need to be completely removed. However, if the implant has relaxed due to septic reasons, the cement must be completely removed to clear the infection. In the current state of knowledge, it is easier to remove the cement than to release the free-cement prosthesis from the bone site. Finally it is important for the stability of the revised prosthesis to detect the possibility of loosening the initial initial implants in order to maintain as much of the healthy bones as possible.
A prosthesis fitted with cement bone offers very high primary stability combined with rapid patient remobilization. The cemented prosthesis can be loaded completely immediately after surgery because PMMA gets most of its strength in 24 hours. The rehabilitation required is relatively simple for patients who have cemented the seedling prosthesis. The joints can be loaded again immediately after surgery, but the use of crutches is still needed for a reasonable period of time for safety reasons.
Bone cement has proven to be very useful because specific active substances, such as antibiotics, can be added to the powdered component. The active substance is released locally after the installation of a new joint arrangement, which is around a new prosthesis and has been confirmed to reduce the danger of infection. Antibiotics work against the bacteria exactly where they are needed in open wounds without bending the body in general to unnecessarily high levels of antibiotics. This makes bone cement a modern drug delivery system that sends the necessary medications directly to the surgical site. An important factor is that there is not much active substance in the cement matrix but how much of the active ingredient is released locally. Too many active substances in bone cement will be detrimental, since the mechanical stability of the prosthesis remains attenuated by the high proportion of the active substance in the cement. Levels of locally active substances from industrial-made cement formed by the use of bone cement containing the active ingredient are approximate (assuming that there is no discrepancy) and significantly below the clinical routine dose for single systemic injection.
See also
- Osteoplasty - the use of bone cement to reduce pain
External links
- An application note explaining how to measure the residual monomers in bone cement
- Presentation on bone cement rheology
References
Source of the article : Wikipedia
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