Bone Cement
Properties
During polymerization, cement properties critical for operating procedures, such as viscosity change, setting time, cement temperature, mechanical strength, shrinkage and residual monomer, are determined. These properties will influence cement handling, penetration and interaction with the prosthesis.
Viscosity
Mixing together the powder and the liquid components
marks the start of the polymerization process. During the
reaction, the cement viscosity increases, slowly at first, then
later more rapidly. Clinical experience has
shown that high viscosity cements produce better clinical
results, as compared to low viscosity cements.5,6
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Viscosity affects the following:
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Bone cements may be divided into two kinds: low viscosity and high viscosity
Low viscosity: These cements have a long-lasting liquid, or mixing phase, which makes for a short working phase.4 As a consequence, application of low viscosity cements requires strict adherence to application times.
High viscosity: These cements have a short mixing phase and loose their stickiness quickly. This makes for a longer working phase, giving the surgeon more time for application.
Charnley THAs
Revision due to stem loosening
The Norwegian Arthroplasty Register shows that high viscosity bone cements yield better long-term results than low viscosity cements6.
As a practical consequence the cement as well as the implant should not be applied too fast so that the cement can have the time to flow and fill the trabecular structure, leading to good fixation of the cement.
Temperatures
Temperature affects mixing time, delivery of the cement, prosthesis insertion, and other aspects of the cementing process. It is therefore very important to control the temperature of the bone cement and the OR.
To achieve optimal cement properties, it is important to adhere to the time schedules indicating the correlation of temperature to handling time. These time schedules are usually included in the instructions for the bone cement.
High viscosity cements are sometimes pre-chilled for use with mixing systems for easier mixing and prolonged working phase. This will also increase the setting time. The relative humidity might also influence the handling properties. That is the reason why the the working time and setting time of the cement might vary in winter and summer.
Mechanical properties
The bone cement is subjected to high mechanical stress in the body. In vivo, the biomechanical situation is rather complex, involving different types of loading (bending, compression, shear), which must be tested. The international standard ISO 5833 describes the methods for determining compressive strength, bending strength and bending modulus.7
As the cemented implant is subjected to not only static load but also dynamically alternating loads, the fatigue properties of the cement affect survival of the implant.
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