Cemented Fixation

Chapter 2 of 3

Cemented Fixation

Total joint implants are often fixed in place with bone cement and, for many patients, the only viable option is a cemented implant.

In this type of fixation, the bone cement is applied to the prepared bone, the implant, or both. After proper impaction of the prosthesis, the cement quickly hardens, securing the prosthesis in position.

Initially, bone cement is pliable and can be formed by the surgeon into the desired shape. It can be forced into the spaces within cancellous bone to create a mechanical interlock when it hardens. By filling a cavity or applying bone cement to a bone surface, an implant can be pressed against the cement to create a custom shape that hardens to fit the implant contours, and the shape of the patient’s bone. This creates a strong mechanical interlock between the implant and the bone.

Bone cement consists of poly (methyl methacrylate) (PMMA). It is usually supplied as a powder component and a liquid component that are mixed at the time of use. The components interact chemically in a process called polymerization and go through several phases of hardening, eventually forming a hard acrylic plastic chemically similar

to Plexiglas material. Because polymerization occurs without adding heat or pressure, the bone cement used in orthopaedics today is considered to be self-curing. The chemical reaction is also considered to be exothermic, i.e., it gives off heat during the polymerization process.

This section explains how bone cement works and describes the components of bone cement, the polymerization process, and the characteristics and uses of bone cement in orthopaedic surgery.

Fixation Cemented Fixation

CONFIDENTIAL. Do not reproduce. Training purposes/internal use only. 14

Bone cement is not a glue. Most glues bond by chemical or molecular action between the glue and the surfaces being joined. With bone cement, the integrity of the fixation is not achieved by a chemical or molecular bond, but by the processes of static friction and mechanical interlock.

Static friction is a force created between two contacting surfaces that tends to resist relative motion between the two surfaces (Fig. 4). The frictional force is proportional to the perpendicular force that one object exerts on the surface of another object. Therefore, when two objectsare pressed tightly together, the frictional force increases with the perpendicular force. This is why a heavy truck is typically less likely to slide on an ice-covered highway than is a light car. The weight of the vehicle is the perpendicular force, and because the truck weighs more than the car, the frictional force is greater for the truck.

Mechanical interlock occurs as the cement penetrates into the pores of the adjacent cancellous bone and the surface irregularities in the prosthesis material (Fig. 5). Think of bone cement as a grout, similar to the mortar used between bricks. Mortar holds the bricks in place, but it does not glue them together.

How Bone Cement Works

Fig. 5: Mechanical interlock occurs as cement penetrates into the pores of the bone.

Fig. 4: Static friction occurs between two static contacting objects.



Bone cement must achieve a congruent fit to both the bone and prosthesis surfaces. To further facilitate fixation, some implant surfaces are designed to provide interdigitation of the cement. For example, they may have a roughened or macrotextured surface to allow the bone cement to flow into the spaces created by these surface textures, thus creating a mechanical interlock (Fig. 6).

When properly used, bone cement can create reliable fixation of an implant. Once it has hardened, movement between the bone, cement, and prosthesis is largely eliminated. Since motion (or micromotion) is one chief reason for implant failure, mechanical interlock is of particular importance to the surgeon. Bone cement, therefore, holds the prosthesis in place and helps to transmit forces from the prosthesis to the surrounding bone.

Although in a cemented arthroplasty, bone cement is considered to be part of the implant, it is the bone cement that contacts the bone, providing a buffer between the bone and prosthesis. But it is important to realize that, when bone cement is used, there are two interfaces; one is the interface between the prosthesis and the cement, and the other is the interface between the cement and the bone (Fig. 7).

Fig. 6: Mechanical interlock occurs as cement flows into macrotextured surface.

Fig. 7: Cement interfaces



Fig. 8: Microscopic irregularities in implant/ cement interface

Historically, bone cement has been considered the weak link in joint arthroplasty. Tiny bubbles and gaps in the implant/cement or cement/bone interface have led to cement breakdown and implant failure. Electron micrographs show that bone cement normally forms a slightly irregular layer over the material of the prosthesis (Fig. 8). These irregularities or gaps are mostly caused by entrapped air, and become the starting points for cracks. More recent techniques in cement preparation have reduced cement gaps and improved the success rate of cemented arthroplasty.

Uses Of Bone Cement

The first known use of a cement-type material in a prosthetic joint replacement dates back to 1890. A German physician named Themistocles Gluck implanted knee and other prostheses made of ivory, using plaster of Paris or stone putty, which is a mixture of rosin, pumice stone, and gypsum.

In modern joint arthroplasty, acrylic bone cement was one of the first types of fixation used. At the request of Sir John Charnley, it was developed by Dennis Smith, a dental materials scientist at the University of Manchester in England. A similar cement was already being used successfully in dentistry. In 1951, Charnley first used the new cement to implant hip prostheses. In the late 1960s, Howmedica became the first company in the United States to manufacture acrylic bone cement. Since then, it has been used extensively and today acrylic bone cement remains a common fixation method for some joint implants.



Components Of Bone Cement

Because cement was the first successful fixation method, it has the longest history of success. Many of the prosthetic designs for cement have been in use for years and many have low failure rates. Cemented applications tend to be used in three type of patients.

• Older patients

• Patients with poor bone stock

• Relatively sedentary patients

Bone cement is also used as a supplement to spinal fixation, and as a filler for pathologic fractures.

In orthopaedics, bone cement is supplied in kit form. It consists of a finely ground polymer powder and a liquid monomer that are mixed in the operating room. When the two are mixed, a chemical process called polymerization begins that binds the molecules together in long chains. When polymerization is complete, the cement is a homogenous mass. One reason that bone cement must begin as separate powder and liquid components is so it can be handled in a viscous or doughy state.

Polymer Powder

The first major component of bone cement is poly (methyl methacrylate) (Fig. 9), which is derived from a fully formed polymer such as a block of Plexiglas material that has been reduced to a powder. The preformed powder particles are 5 to 130 microns in diameter, and consist of small PMMA chains with two or more links. The polymer powder is typically contained in plastic pouches (Fig. 10).

Fig. 9: Chemical structure of PMMA Fig. 10: The polymer powder is contained in a pouch, while the liquid monomer is contained in a brown

glass ampule.



Copolymers

Most of the bone cement used today is known as copolymer bone cement because it includes a copolymer of PMMA. A copolymer is a polymer that contains two or more different chemical subunits (monomers). It is part of the powder component, and is added during synthesis of the powder. The copolymer that is most commonly used in bone cement is styrene. Thus, the powder component is known as poly [(methyl methacrylate)-co-styrene]. Copolymer bone cement has been shown to increase fatigue strength and reduce the inherent brittleness of PMMA cement, while maintaining favorable handling characteristics.

Powder Component Additives

In the formulation of the powder component of bone cement, two substances may be added.

Barium Sulfate: This is a radiopaque substance that is often added to the powder to allow the cement to show up on a radiograph. Without such a material, x-rays pass through the cement as they do through water or air, preventing the cement from appearing on a radiograph. Barium sulfate is evenly dispersed throughout, and typically makes up about 10% of the powder component. Because the barium sulfate particles are somewhat sticky, they typically do not settle out. Although barium sulfate is the most common, some manufacturers use zirconium oxide as a radiopaque agent.

Benzoyl Peroxide: Benzoyl peroxide is an organic compound found in the powder component. It serves as an initiator to begin the polymerization process when it combines with the DMPT in the liquid component. The correct combination of these materials helps control the setting time for the bone cement.

Liquid Monomer

The liquid portion of bone cement largely consists of a chemical called methyl methacrylate, which is a monomer, a nonpolymerized molecule (Fig. 11). This is what gives bone cement its strong, pungent odor when it is mixed and as it cures. Under appropriate conditions, the monomer molecules can be connected to form polymer chains. Because methyl methacrylate is a highly volatile liquid, other substances are added to stabilize it. These ingredients are discussed later in this section. The liquid is typically contained in an ampule (See Fig. 10) made of brown glass to minimize the monomer’s exposure to light, which can initiate premature polymerization. The container is opened by breaking off the neck of the ampule.

Fig. 11: Chemical structure of methyl methacrylate monomer



Liquid Component Additives

In the formulation of the liquid component of bone cement, two chemicals are typically added.

Hydroquinone: Hydroquinone acts as a stabilizer and keeps the monomer from polymerizing before it is mixed with the powder. Such premature polymerization may be initiated by physical causes such as heat or light, or by chemical reagents. Essentially, hydroquinone increases the shelf life of the monomer.

DMPT (n,n-dimethyl-p-toluidine): When mixed with the powder component, DMPT acts as an accelerator. Together with benzoyl peroxide, an additive in the powder component, DMPT helps promote curing at body temperature. The amount of DMPT added affects the setting time of the polymer.

Other Bone Cement Additives

Other additives may include antibiotics and coloring agents.

Antibiotics: Antibiotics may be added to bone cement during the manufacturing process to reduce the risk of postoperative infection or to combat any unidentified preoperative infection. Sometimes, surgeons add an antibiotic to bone cement at the time of use in surgery; however, such action has not been cleared by the FDA.

Coloring Agents: Currently the only coloring agent approved for use with bone cement is chlorophyll.

Component Ratios

The ratio of polymer powder to liquid monomer is typically about two-to-one. This ratio is important because it affects the handling characteristics and other properties of the cement. For example, by increasing the amount of monomer relative to the amount of polymer powder, the heat given off during polymerization is increased, the setting time is prolonged, and the tissues are exposed to the potentially noxious effects of more free monomer. These characteristics and effects will be discussed later.

Polymerization

Polymerization is a process where small chemical compounds called monomers join to form high molecular weight chains called polymers. This occurs when the ingredients of bone cement are combined. The polymerization process is sometimes referred to as setting, hardening, or curing.

When the two components are completely mixed, the powder is said to be wetted out. At first, the cement is runny and sticky. As polymerization takes place, the cement becomes doughy, and then begins to harden into a brittle, low-density solid. At room temperature, bone cement hardens in approximately 12-20 minutes, but hardening time varies according to a variety of factors, including the cement manufacturer, mixing time, ambient temperature, humidity, and the amount of manipulation and compressing during insertion.

The Polymerization Process

The polymerization process requires free radicals, which are very unstable and highly reactive chemicals. A suitable source of free radicals is necessary to initiate and continue the polymerization process.



When the powder and liquid components of bone cement are mixed, the activator (DMPT) in the liquid causes the initiator (benzoyl peroxide) in the powder to decompose rapidly, creating free radicals. The free radicals cause the methyl methacrylate molecules of the monomer to bind together to form long, intertwined poly (methyl methacrylate) chains. Once this polymerization process is initiated, the process itself generates the free radicals necessary to continue. The process continues until most of the monomer has polymerized, although some evaporates and escapes as a gas.

As the monomer is mixed with the powder, it fills in the spaces between the prepolymerized powder particles to create a matrix for the particles (Fig. 12). But as the monomer begins to penetrate into the particles, it partially dissolves them (Fig. 13), allowing the newly formed polymer chains to intertwine and, in some cases, bond with the prepolymerized powder (Fig. 14). This results in a homogenous mass of PMMA (Fig. 15). The boundaries between the prepolymerized powder particles and the newly polymerized material are no longer distinguishable. When polymerization is complete, the former powder particles make up about 70% of the mass of cured bone cement.

Fig. 12: Immediately after mixing, the polymer powder particles can be seen in the monomer

matrix (The black particles are barium sulfate).

Fig. 13: The monomer begins to penetrate the powder particles, which softens them.

Fig. 14: As polymer chains intertwine and bond, the boundaries between the powder particles

and monomer matrix become less distinct.

Fig. 15: When polymerization is complete, the bone cement mass becomes homogenous with no boundaries between the powder particles and the

newly polymerized material.



Heat Generation

The polymerization process is exothermic, which means the bone cement produces heat as it hardens. The more polymerization that occurs when cement is mixed, the more heat that is generated. The fact that much of the cement mass consists of prepolymerized particles helps reduce the amount of heat generated.

Characteristics Of Bone Cement

Selecting an appropriate bone cement requires that the surgeon consider the handling characteristics and physical properties of the cement.

Handling Characteristics

Handling characteristics, which include viscosity, working time, and setting time, are very important to bone cement. These characteristics must be predictable and reliable so the surgeon can accurately plan the implantation of a prosthesis.

Viscosity

Viscosity refers to the consistency or flow characteristics of a material. High viscosity materials are more resistant to flow, while low viscosity materials are less resistant to flow (Fig. 16).

Depending on the specific type of cement and the manufacturing process used for the components, the mixed bone cement may initially be more or less runny in consistency. Regular bone cement (high viscosity) is initially less runny in consistency, while low viscosity bone cement is initially more runny. The viscosity of both will continue to increase until the cement hardens into a rigid solid. Typically, the viscosity of bone cement increases slowly during mixing, then more rapidly after mixing.

Fig.16: Viscosity



After mixing, the viscosity of bone cement begins to increase; that is, it begins to become more doughy. The period from the completion of mixing to the onset of the dough state is called the low viscosity time. During the low viscosity time, the cement can be injected, but it is too runny and sticky to be worked by hand. Surgeon preference plays a role in deciding when to inject the bone cement. Some surgeons prefer to inject the bone cement in a lower viscosity state.

When the mixed cement has formed a “skin” and no longer sticks to a gloved finger, it is said to have reached the dough state. This typically occurs between three and four and a half minutes after the components are first combined, depending on the temperature, humidity, and mixing technique used. In the dough state, the cement is more workable, like modeling clay, and can be digitally applied, or possibly injected in the early stages of the dough state.

Working Time

The working time is the actual time the surgeon has to use the cement while it is still pliable. It is the sum of the low viscosity time and dough time. The dough time is the period from the onset of the dough state to the point where the cement is too hard to work. Keep in mind that if the surgeon is applying the cement by hand, he must wait until the cement has reached the dough state. The working time must be sufficient to allow the surgeon to apply the cement and seat the prosthesis without rushing.

Setting Time

The setting time is the period from the onset of mixing until the cement has completely hardened. The setting time is dependent on temperature, humidity, storage conditions, and mixing method and technique. The setting time must be long enough to provide enough working time, but short enough to minimize the possibility of implant movement during the curing process.

Fig. 17: Summary of polymerization times



Factors Affecting Handling Characteristics

The handling characteristics and properties of bone cement are significantly affected by operating room conditions such as temperature and humidity, as well as cement manufacturing factors and mixing techniques.

Temperature

Temperature is one of the most important factors in determining the handling characteristics of bone cement. Temperature affects the viscosity and setting time of bone cement. The higher the temperature, the faster the reaction and the shorter the setting time. Chilling bone cement increases its setting time and allows viscosity to remain low long enough for the cement to be injected. If the temperature of the cement rises during mixing, however, the setting time, and hence the working time, is reduced. A rule of thumb is that for every 2° Fahrenheit difference in temperature, the setting time will change by about one minute.

Humidity

Humidity is another factor that affects setting time. Higher humidity speeds up the reaction and reduces the setting time. Conversely, decreased humidity increases the setting time. Humidity affects setting time to a lesser degree than temperature.

Storage Conditions

Bone cement should be kept in a cool, dark environment. Heat degrades the benzoyl peroxide in bone cement, which is a catalyst for the reaction. This degradation increases the setting time of the cement, making it difficult to accurately predict the working time.

Mixing Method

The way bone cement is mixed can also affect its handling characteristics. Because oxygen slows the polymerization process, removing air will cause the cement to set faster compared with open bowl mixing where the cement is exposed to air. Mixing the cement too fast may also speed up the reaction as it imparts more energy into the mix in the form of heat.

Expiration Date

The label on a package of bone cement includes an expiration date. Bone cement that has passed its expiration date may have an unpredictably longer setting time and reduced strength as benzoyl peroxide degrades over time. For these reasons, bone cement should not be used past its expiration date.

Additives or Contamination

Physicians are cautioned against adding materials like antibiotics or methylene blue. Adding such materials, whether intentionally or unintentionally, including antibiotics, methylene blue, water, saline, or blood, alters the setting behavior of bone cement. Extra care must be taken to clean the bone thoroughly before bone cement is applied so blood and other contaminants are not inadvertently mixed with the cement.



Composition

The composition of bone cement is a significant factor in determining its handling characteristics as well as its physical properties. Specific factors include:

• Molecular weight of the polymer (the length of the polymer chains)

• Texture of the powder (the size of the powder particles)

• Proportion of activator and initiator

• Proportions of liquid and powder

Physical Properties

Physical properties are important in determining the function and integrity of bone cement. Like bone, PMMA bone cement provides excellent resistance to compressive force. In fact, bone cement has a compressive strength equal to or just slightly less than that of cortical bone. However, bone cement is not as strong as bone when exposed to shear, tensile, torsional, or bending stress. Thus, to maximize effectiveness, bone cement should be used in situations that primarily produce compressive forces and minimize other forces.

Most of the mechanical properties of bone cement, including tensile strength and hardness, are dependent on the molecular weight of the cement. This means that the chains created during polymerization must be long enough so that the weight of all the atoms in each chain meets or exceeds a specific minimum.

In addition to composition, the strength of bone cement can be affected by many factors, including environmental factors during the polymerization process, and the mixing and delivery techniques used.

When polymerized at a temperature of 98.6° Fahrenheit, i.e., the temperature of the human body, bone cement achieves about 90% of its strength in four hours, and its ultimate strength in 24 hours.

Porosity

Porosity refers to small voids within the cured cement (Fig. 18), and can significantly affect the physical properties of bone cement. Typically, bone cement has a porosity of 3% to 11%. Each void in the cement mass is a stress raiser (sometimes referred to as stress riser) that affects the strength of the cured cement.

Fig. 18: Micrograph showing voids in bone cement



Much of this porosity results from hot spots that are created during the polymerization process. Because bone cement is a poor conductor of heat, these hot spots can exceed the boiling temperature of the methyl methacrylate monomer. The gases generated from the boiling monomer create small spherical voids within the cement mass.

Other ways that voids or pores are created include:

• Insufficient penetration of the liquid monomer into the particles of the polymer powder.

• The entrapment of air in the cement during mixing or implantation.

• The expansion of the cement mass during mixing, and the subsequent contraction during polymerization. Because it has partially cured, the cement mass cannot fully contract and internal pores are created.

The porosity of bone cement can be minimized by using proper cementing techniques. These techniques will be discussed in a later section of this module.

Bone Cement Issues

Optimal use of bone cement requires that many factors be considered. Appropriate patient selection with regard to age and pre-existing condition is important. The risk of cement loosening or fracture increases with length of service. This is an important consideration for younger patients who are more likely to have longer lives and therefore require effective long-term arthroplasties. Patients with extreme bone deficiencies and those with cardiopulmonary irregularities may not be candidates for cemented implants.

Adherence to sterile techniques and personnel well trained in storing, mixing, inserting, and disposing of bone cement are crucial for effective bone cement use. Furthermore, proper preparation of the bone bed and correct implant placement will increase the likelihood of implant success. Finally, postoperative care and patient education can affect long-term outcomes.

One of the most common ways to detect problems with bone cement after joint surgery is the appearance of radiolucent lines at the cement/bone interface (Fig. 19). These lines mean that the interface may have been compromised in some way. For example, they may indicate bone necrosis that may be associated with the direct thermal effect of polymerization, the chemical action of the methacrylate monomer, or the bone preparation techniques. The lines may also indicate blood or other debris at the interface, which may result from improper bone preparation techniques. Or, they may indicate fibrous tissue growth at the interface. More importantly, they may indicate a breakdown of the cement mantle at the interface.



Fig. 19: Radiolucent lines indicate a compromised cement/bone interface.

Biological Issues

The biological issues associated with bone cement include heat formation, hypotension, and the possible effects of pressurization.

Heat Formation

As you have read, heat is released as bone cement polymerizes. Experimentally, temperatures of 150° Fahrenheit have been reported at the center of a polymerizing mass of PMMA, raising concern that the elevated temperature could cause bone damage (necrosis). While this is theoretically possible, such necrosis rarely, if ever, occurs. Studies have indicated that surgical trauma is probably more significant than heat necrosis caused by the cement.



Three factors reduce the risk of bone damage from heat.

• The elevated temperatures reported are found deep within a mass of cement. In practice, cement forms a thin layer over the medullary surface, so heat is less likely to be concentrated. Furthermore, the temperature rise is not as great on the surface of the cement mass, which might be in contact with living tissue.

• The metal of the implant itself acts as a heat sink, absorbing heat and removing it from the area.

• Blood flowing through living bone removes heat from the site, further reducing the risk of bone damage.

Hypotension

There is some concern that the use of PMMA bone cement may cause hypotension (low blood pressure) in some patients. However, while small quantities of methyl methacrylate and other chemicals are released into the blood during polymerization, there is no direct evidence that these chemicals have any deleterious effects. In fact, hypotension often occurs before these chemicals appear in the blood.

Pressurization Effects

In rare cases, cardiac failure has occurred during surgery when PMMA bone cement was being used. In most of these cases, fat embolism is believed to be the cause. Fat and other medullary contents may be forced into the vascular system when intramedullary pressure is increased during pressurization of the cement, or during insertion of the implant.

Fig. 20: Radiograph shows good mechanical interlock between cement and bone.



Mechanical Issues

Mechanical issues include interfacial strength, fatigue life, trapped air, and cement mantle thickness.

Interfacial Strength

As you have already learned, the strength of the interfaces between the cement and bone, and the cement and implant are very important in bone cement fixation. The more continuous the implant/ cement and cement/bone interfaces, the less likely the cement is to crack and the implant to loosen.

When a good mechanical interlock has been achieved at the cement/bone interface, a radiograph shows a fuzzy, indistinct border between the cement and bone (Fig. 20). To achieve this interlock, the surgeon must open up the trabecular bone and be sure that the bone cement penetrates into the trabecular spaces after clearing the prepared cavity or bone surface of any loose or foreign material, including blood and bone cement fragments.

Proper cement preparation and delivery techniques can also help optimize interfacial strength at both interfaces. Implant surface treatments can help optimize the strength of the implant/cement interface. These techniques and treatments will be discussed in later sections of this module.

Fatigue Life

Cement can fail over a period of time due to fatigue. In fact, the primary in vivo failure mechanism of bone cement is fatigue. Fatigue life is a function of the inherent strength of the cement as a material, as well as the way the cement is mixed and delivered. Fatigue life is particularly important when bone cement is used in a younger, more active, patient.

Trapped Air

Mixing cement in an open mixing bowl traps air in the mixture. The increased porosity resulting from air bubbles can become the starting point for crack formations in the cement (Fig. 21). This can affect both the interfacial strength and the fatigue life. If proper cementing techniques are followed, the porosity caused by trapped air can be minimized. These techniques are discussed in the next section.

Fig. 21: Air bubbles in cement can result in crack formation.



Fig. 22: Uniform cement mantle in hip prosthesis

Cement Mantle Thickness

The thickness and uniformity of the cement mantle are critical for optimal cement/bone interface strength (Fig. 22). A mantle that is too thick or too thin can compromise the integrity of the fixation. It can also affect the final physical properties of the cured cement. For example, a thicker cement mantle will generate more heat as it cures, which potentially increases the porosity of the cement and reduces its strength. While polymerizing, a lump of cement the size of a golf ball will become too hot to hold in the hand. However, in joint arthroplasty, the cement mantle is much thinner than a golf ball and, therefore, the cement maintains a much lower temperature. The implant design itself plays an important role in maintaining the uniformity of the cement mantle. Further discussion of this topic is found in specific implant training modules.

Other Product-related Issues

In addition to biological and mechanical issues, surgeons and O.R. staff are concerned about inconsistent working and setting times, lot-to-lot variability in the formulation of bone cement, and the effort and attention required in mixing cement components.

Cementing Technique

Although bone cement fixation has improved since it was first introduced, the cement itself has seen few significant changes. Advancements to bone cement fixation have been accomplished mainly by improving the techniques used in preparing the bone surfaces, and in the preparation and delivery of the cement.

In meeting the fixation goal of minimizing gaps between the bone and prosthesis, cemented fixation is somewhat more forgiving than cementless fixation methods. Precise removal of bone is important, but not as critical when implanting a cemented prosthesis because, within reason, the cement will fill any space that is created by the removal of bone. On the other hand, it is important to create a cement mantle that will best resist the stresses that an implant and bone cement must endure. A cement mantle that is too thick or too thin can lead to fracture of the cement mantle.



As you read earlier, one of the most common causes of cement failure is porosity in the cement. Another common cause of cemented implant failure is gaps at the implant/cement interface (Fig. 23). These gaps may be the starting points for the implant separating from the cement. To eliminate or minimize such gaps and improve the contact between implant and cement, several techniques are used, including texturing implant surfaces and reducing or eliminating air trapped in cement.

Cementing techniques have evolved over many years. The current process is referred to as third generation cementing technique. It may also be called modern cementing technique. This advanced technique helps minimize porosity of the cement, achieve a more uniform distribution of cement, achieve enhanced apposition to bone, and improve interface strength.

Third Generation Cementing Technique

Third generation cementing technique refers to a multiple-task procedure to optimize the fixation of bone cement at both the bone and implant interfaces. The actual bone cement used, while important, is only a part of modern cementing technique.

Third generation cementing technique involves the key steps in achieving successful fixation with bone cement, which include:

• Bone Preparation

• Cement Mixing and Porosity Reduction

• Cement Delivery and Pressurization

Fig. 23: Gaps at the implant/cement interface



Bone Preparation

As with any type of fixation, inadequate joint preparation can lead to implant loosening. For cemented fixation, the specific preparation of the bone bed differs according to the joint, and will be discussed in more detail in the specific joint training modules. However, the general objectives are the same for all joints. First, the bone must be accurately cut and, if necessary, broached. Then the cut surfaces or walls of the medullary canal must be thoroughly cleaned and dried. It is important that no blood, soft tissue, or debris of any kind be between the cement and the bone, or between the cement and the prosthesis. Because cancellous bone is more porous than cortical bone, it provides a better bed for mechanical interlock of the cement.

Special powered instruments are used to irrigate and clean bone (Fig. 24). This process may be referred to as irrigation, debridement, or lavage, and involves spraying a normal saline solution under pressure into the bone while a vacuum simultaneously removes the solution and anydebris that it has forced from the pores of the bone. The bone can then be dried with a dry sponge or sponge soaked in an epinephrine solution to help control bleeding. This process of irrigation and cleaning is also important in that it helps minimize the possibility that any intramedullary contents are introduced into the bloodstream during the injection of cement or the insertion of the implant into the prepared medullary canal.

In cemented total hip arthroplasty, the medullary canal is typically blocked with a medullary canal plug (Fig. 25). This occludes the femoral canal distal to the implant to prevent cement from being pushed too deeply into the canal. Occluding the canal is necessary to achieve cement pressurization, providing greater penetration of the cement into the trabecular spaces and improving interlock of the cement with bone.

Fig. 24: The Zimmer PulsaVac® Plus Wound Debridement System can be used to clean the bone in

preparation for cement.

Fig. 25: Medullary canal plug in total hip arthroplasty



Cement Mixing and Porosity Reduction

Mixing bone cement is the process of combining the liquid monomer with the polymer powder. Major concerns in mixing bone cement include thoroughly mixing exact proportions of cement components, sterilizing mixing instruments, ease of use, porosity reduction, and removing monomer fumes.

It is important to use the entire quantity of components supplied in the package as any changes in the proportions of liquid monomer to polymer used will alter the setting time and working time of the cement. Also, to help guarantee exact, reproducible results, it is important that the cement be completely mixed until it has a uniform consistency with no dry powder remaining. When the components have been thoroughly mixed, the powder is said to be wetted out.

Because polymerization begins at the onset of mixing the components, the mixing process must be done quickly. The period from the initial combination of monomer and powder until the mixture is homogenous and becomes fluid is called mix time. This is the time it takes to pour and completely mix the components. It usually takes about 45 to 90 seconds, depending on the temperature, relative humidity, and mixing method used.

Porosity reduction is an important consideration with bone cement, and can be addressed in the mixing process by using a vacuum mixing technique. It can also be addressed after mixing by performing an additional step called centrifugation. Fewer gaps or voids in the cement result in greater fatigue strength, and stronger implant/cement and cement/bone interfaces.

Vacuum Mixing: As you read earlier, mixing cement in an open container traps air in the mixture, increasing the porosity of the cement and possibly creating a starting point for crack formation. Proper mixing techniques can minimize trapped air. The most common way to accomplish this is by mixing the cement components in a vacuum. The vacuum mixing process is facilitated by special mixing bowls that are attached to vacuum hoses.

Remember, however, that mixing cement in a vacuum decreases the setting time of the cement. This is because the vacuum reduces the amount of air, and hence, oxygen in contact with the cement, which allows more free radicals to react with the monomer, thereby increasing the reaction rate and decreasing the setting time.

Centrifugation: Another way to reduce voids in mixed cement is through a process called centrifugation. Essentially, this involves compacting the cement to remove trapped air. While the cement is in its low viscosity state, it is placed in a centrifuge which spins at a very high speed. The resulting centrifugal force compresses the cement, forcing the air out. Both vacuum mixing and centrifugation have been shown to significantly reduce the porosity of bone cement and eliminate large voids that compromise the strength of the cement. Although centrifugation was once common, with the advent of vacuum mixing systems, it is seldom used today. Bone cement manufacturers advise against centrifugation because it may interfere with the uniform dispersion of barium sulfate.



Cement Delivery and Pressurization

Cement is either applied to the bone surface or prosthesis surface by hand after it has reached the dough state (Fig. 26), or it is injected onto the bone or into a medullary canal by a device called a cement gun or injector (Fig. 27). Cement is typically injected while it is still in its low viscosity state, but it can also be injected during the early stages of the dough state. The cement gun has a variety of attachments that allow cement to be delivered at specific rates and in specific volumes.

Cement that is injected into a medullary canal can be pressurized as it is injected. Attachments that provide a tight seal between the bone and the injector nozzle of the cement gun allow the

cement to be forced into the canal under pressure. Because each joint has a different shape, these pressurizing and sealing attachments vary according to the joint for which they are designed.

The advantages of cement pressurization include:

• Maximizing intrusion of the cement into the cancellous bone bed for a better interlock

• Removing any remaining trapped air in the cement mixture

• Raising the boiling point of the monomer and thereby helping to decrease the porosity caused by hot spots as polymerization takes place

• Helping to prevent voids that result from insufficient penetration of the monomer into the polymer beads

Fig. 26: Cement that has reached the dough state can be applied by hand

Fig. 27: Injecting cement



Implant Insertion Techniques

Some of the porosity found at the implant/cement interface is created when air is pushed into the cement when the implant is inserted. It is important that implants be inserted slowly and at a consistent rate.

Cement Handling Precautions

When mixing bone cement, there are also other precautions that have to be considered relative to the safety of the surgical team and O.R. staff. Fumes generated during polymerization can be noxious.

Eye irritation is a potential problem from methyl methacrylate monomer fumes. To reduce the risk of discomfort and possible injury, contact lenses should not be worn if exposure to the fumes is possible. The monomer fumes can contaminate, attack, and destroy soft contact lenses.

Although no effects on a developing fetus have been reported from methyl methacrylate fumes, current evidence is insufficient to support the safety of fume exposure during pregnancy. To eliminate the risk, any contact with methyl methacrylate and its fumes during pregnancy should be avoided.

Methyl methacrylate can irritate and cause a skin reaction in sensitive people. To reduce the risk, exposure of bare skin to the monomer should be minimized. Gloves should be worn by the circulating nurse when presenting the ampule to the sterile field in case the ampule should break and spill the contents.

The highest risk of exposure occurs during mixing. A vacuum mixing system and evacuation system removes fumes from the operating theater before they are released, minimizing vapor exposure. These systems are discussed in detail in later sections of this program.

Additional precautions include:

• Use only full packages of monomer and polymer. Even if this means mixing more than is needed, do not estimate proportions for partial batches.

• Do not spill the liquid monomer or polymer powder. If either spills, start over with new packages.

• Do not use the powder component from a package that contains a broken monomer ampule. The monomer fumes may penetrate the polymer powder pouch and react with the benzoyl peroxide component. This may result in unpredictable working and setting characteristics of the mixed cement.



Cemented Implant Fixation Surfaces

Cemented components are available with a variety of surface characteristics (Fig. 28). Some are essentially smooth, a result of polishing the surface, while others are textured to enhance the interdigitation of the cement into the implant surface. Some cemented implants may be coated with PMMA to provide a cement-to-cement bond when implanted.

Polished Surfaces

Cemented implants may have polished surfaces, which are smooth surfaces created by buffing the implant. Depending on the amount of polishing and the grit of the polishing medium, this could create a surface that is anywhere from a satin finish to a high-gloss, mirror-like finish.

Most cemented hip implants have polished surfaces. One reason is to minimize cement debris that may be created by micromotion if the implant should become loose. In a loose hip stem, micromotion of a roughened surface against the cement may abrade the cement mantle, creating a sand-paper-like effect. The resulting particles of cement wear debris may contribute to osteolysis and further loss of fixation.

Another reason a cemented hip implant may have a polished surface is to allow the stem to firmly seat itself within the cement mantle. Even after the cement has completely cured, the stem is allowed to subside slightly to achieve a strong, self-locking wedge fit that enhances stability and fixation.

Finally, because cement is relatively weak when exposed to shear stress, a polished surface may be intended to minimize the frictional forces that contribute to shear stress.

Fig. 28: Surface finishes of cemented hip implants



Textured Surfaces

Some cemented implant surfaces may be roughened or textured to allow more interdigitation of the cement and provide a better mechanical interlock between the cement and the implant. This increases the shear strength of the implant/cement interface. The texture is considered to be either microtextured or macrotextured.

Grit-blasted Microtextured Surface

One way to achieve a roughened surface is through a grit-blasting process where an abrasive grit is introduced into a stream of compressed air directed at the implant surface. As the blast media impacts the implant surface, it creates small depressions in the surface, making it feel rough. The type and number of depressions determine the roughness of the surface texture. Specific factors that affect the texture include the implant material, the size (grit) and shape of the blast media, the velocity and pressure of the blast media, and the type of blast gun and nozzle used. The resulting roughness is typically measured in microns or micro inches and referred to as the Ra factor. The Ra factor may be provided as a range representing the difference between a typical peak and valley of the surface texture.

The roughness of a grit-blasted microtextured surface can vary from a light matte finish to a heavier matte finish. Typically, a glass bead medium is used to achieve the lighter finish, while an oxide medium is used to achieve the heavier finish. In orthopaedics, the most common blast medium is aluminum oxide.

Corundumization: One specific type of aluminum oxide blasting is called corundumization. A corundumized surface (Fig. 29) is achieved by blasting the implant with a relatively fine-grit blast media consisting of corundum, a type of aluminum oxide that is the same hard mineral that makes up rubies and sapphires. This mineral is second in hardness only to diamonds.

Fig. 29: Corundumized surface



Fig. 30: Macrotextured surface

Macrotextured Surfaces

Macrotexturing describes a heavily textured surface (Fig. 30). The texture may be in the form of small ridges, grooves, or other relatively large indentations that are clearly visible and provide an opportunity for a mechanical interlock of the cement onto the implant. Like microtexturing, macrotexturing is intended to increase the shear strength at the implant/cement interface.

Generally, there are three ways to achieve a macrotextured surface.

• By blasting the surface with very heavy blast media (16-grit or larger)

• By forging the texture into the surface when the implant is manufactured

• By machining the texture into the surface after the implant is shaped

Effects of Surface Texturing on Material Properties

Any process that creates a roughened surface on a metal implant affects the material properties of the implant. Typically, any interruption in the surface of a material potentially reduces the fatigue resistance of the implant. This weakening is referred to as notching. The notching effect can be created by something that indents the material, as occurs when blasting a material, or by something that is metallurgically bonded to the material, such as a porous surface. (The notching effect of porous surfaces will be discussed in a later section.)

Think of a notch as being a V-shaped depression in the surface of a material (Fig. 31), although a notch can be an indentation of any shape. At the point of the V, a stress raiser exists that can develop into a crack that may eventually cause the material to fail. This is the notching effect.

Fig. 31: Notching effect



When blasting a surface for the purpose of enhancing fixation, efforts are made to minimize the notching effect. One way to do this is to use the appropriate blast media and technique for that particular implant design and material. Another way is to avoid blasting those areas of the implant that undergo the greatest stress.

Surprisingly, it may also be possible to enhance the fatigue resistance of an implant by blasting the surface. The theory is that the depressions in the material surface will create residual compressive stress. In essence, this imparts energy into the surface that will help improve the fatigue resistance of the implant. This is often done in certain high- stress areas of a cobalt chromium implant. The texture created in this area is different from that used to enhance fixation. There is a fine balance between the degree and type of texturing that may cause notching and that which may improve the fatigue resistance.

Texturing to improve fatigue resistance may be accomplished with a method called shot-peening. This involves blasting the surface using high- carbon stainless steel shot as the blast medium. This produces a mottled surface with fewer irregularities because the shot tends to be rounded and more symmetrical than aluminum oxide grit. A zirconium oxide grit may also be used. Enhancing fatigue resistance through surface texturing will be discussed in more detail in the “Materials and Manufacturing Methods” training module.

Other Potential Effects of Surface Texturing

One other important issue relates to the potential effect of using a cemented hip stem that has a textured surface. While the interdigitation of the cement into the roughened surface generally may enhance fixation, there is some concern that the benefit may be overshadowed by the undesirable impact of the roughened surface should the implant become loose. As you read earlier, in a loose hip stem, micromotion of the roughened surface against the cement may abrade the cement mantle, creating a sandpaper-like effect. The resulting particles of cement wear debris may contribute to osteolysis and further loss of fixation.

PMMA Precoating

PMMA precoating is an implant coating technology that enhances the fixation of a cemented implant. Precoating produces a polymer film that locks mechanically onto the prosthesis surface that interfaces with the cement. According to the American Academy of Orthopaedic Surgeons (AAOS), polished implant surfaces have an interfacial shear strength of 0.5 MPa, a typical blasted surface has a shear strength of about 7 MPa, and a precoated surface has a shear strength of more than 2,000 MPa. The benefits of precoating can be obtained on both cobalt- chromium and titanium alloys.

Precoating the implant surface involves roughening and cleaning the surface, followed by the application of a layer of PMMA, which is the same polymer that makes up most bone cement. The only difference is that the PMMA used in precoating is pure; it does not contain barium sulfate.



The intimate metal/polymer interface is formed in a controlled manufacturing environment, creating a stronger bond than can be accomplished in a clinical setting. This is because the PMMA is applied to the implant in a form that allows it to thoroughly interdigitate into the surface of the implant (Fig. 32). Furthermore, there is no opportunity for contamination by blood or other body tissues.

When a precoated prosthesis is implanted with bone cement, the layer of polymer is highly compatible with the polymerizing bone cement. As the bone cement polymerizes, residual monomer partially dissolves the precoat layer, forming a homogenous, fully integrated bond between the bone cement and the precoat layer (See Fig. 32). In essence, a cement-to-cement bond is created, and voids at the interface are greatly reduced.

Compared to noncoated implants, precoated implants have been shown to have better tensile strength, as well as better shear strength. Laboratory tests have shown that precoated Zimmer implants result in a implant/cement interface that is twice as strong as a non-precoated implant.

Fig. 32: Normal and PMMA precoated implant/cement interfaces

Cemented Fixation

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