Bone cement represents a two-component adhesive, which is formed by mixing a powder with a liquid within a short time before use. It is used to elastically anchor artificial endoprostheses to bone. After the implants are inserted, the artificial joints can immediately bear normal loads thanks to the properties of the bone cement.
What is bone cement?
Bone cement is an adhesive that can firmly and elastically bond artificial endoprostheses to the joint. It is a polymer of methyl methacrylate. Methyl methacrylate or PMMA is a widely used material also known as Plexiglas. PMMA combines two materials very firmly and is also very elastic at the same time. It is precisely these properties that predestine this adhesive for the stable bonding of components that are subject to constant mechanical stress. This applies in particular to artificial joints. Apart from the fatigue caused by the operation, the patient is able to bear a full load immediately after insertion of the implant, because the material has a high elasticity in addition to its high bonding capacity. However, changing the implant can be difficult because the bone cement is difficult to remove. Bone cement has been used successfully in the insertion of artificial joints since the middle of the 20th century. This applies to all joints such as the knee joint, hip joint, elbow joint or shoulder joints. Today, bone cement is routinely used because its handling in clinical practice is convenient and simple.
Forms, types and kinds
Bone cement is a uniform material that is a polymer of methyl methacrylate. It is formed by an exothermic polymerization reaction after mixing together two components called binder and hardener. These are a powder and a liquid. The liquid consists of a solution of the monomer, while the powder contains the activating substance. Polymerization takes place with the generation of heat. After mixing the two components, a doughy paste is initially formed, which transforms into an elastic glassy substance. This substance constitutes the actual bone cement. The only difference in the composition of the bone cement is the addition of antibiotics such as gentamicin to prevent local infections at the surgical site. The addition of antibiotics is adjusted individually. Furthermore, the bone cement also contains different proportions of so-called contrast agents in order to be able to visualize it in imaging procedures such as X-ray examinations. Among others, barium sulfate or zirconium dioxide are used as contrast agents.
Structure and mode of operation
During surgery, bone cement is mixed by mixing powder and liquid together. Heat is generated to form a doughy mass that is filled into the bones. All cavities are thereby mixed with this mass and sealed with it. The prosthesis is then carefully placed in this doughy substance. The viscosity increases progressively, and the cement mass hardens and forms a matrix. This permanently fixes the artificial joint. The cement is still flexible enough to ensure the mechanical load capacity of the prosthesis. The heat of reaction during cement formation can rise to 70 degrees Celsius. However, the organism can only tolerate a maximum temperature of 42 to 46 degrees Celsius. Above this temperature, denaturation of body protein occurs. To ensure this low temperature, the surgical procedure must be so precise that it is possible to apply very thin layers of bone cement. With a layer of less than five millimeters, heat dissipation is sufficient to spare the surrounding tissue due to the larger surface area. In addition, heat dissipation also occurs through the large surface area of the prosthesis and through the blood stream.
Medical and health benefits
The use of bone cement has proven to be very successful in clinical practice. Patients can be quickly remobilized after surgery. The prosthesis can be fully loaded quickly. The material is very stable and durable, so the long-term results are very good. The mechanical load-bearing capacity is also very high from the outset due to the elasticity of the bone cement.It is also advantageous that antibiotics can be added to the powder before mixing the components, which effectively prevent infections at the surgical site. After the operation, these active ingredients are slowly released and thus exert a local influence. The release is so small that local antibiotic effectiveness is ensured, but the entire organism is not burdened with antibiotics. Only in the case of a known allergy to antibiotics should joint surgery be performed without the use of bone cement. In rare cases, a drop in blood pressure and oxygen saturation may occur during surgery. A vasodilatory effect of the cement due to the formation of gases during polymerization is discussed for this. Overall, the use of bone cement is part of routine medical practice due to its high success rate. However, when a prosthesis needs to be replaced, bone cement often proves stubborn. If there is no infection, the cement does not need to be completely removed. However, in the case of infection, radical replacement of the bone cement is necessary. However, current evidence suggests that it is easier to remove cement than to replace cementless prostheses that are well ingrown into the bone bed.
