BONE CEMENT

ARJUN K GOPI2ND M.Sc BPS

INTRODUCTION

Bone cement chemically is nothing more than Plexiglas (i.e. poly methyl methacrylate or PMMA).

PMMA was used clinically for the first time in the 1940s in plastic surgery to close gaps in the skull.

The bone cement fills the free space between the prosthesis and the bone and plays the important role of an elastic zone.

COMPONENTS

Bone cements consist of two primary components: A powder consisting of copolymers based on the substance poly methyl methacrylate (PMMA), and a liquid monomer, MMA. These two components are mixed at an approximate ratio of 2:1 to form a poly methyl methacrylate cement.

Exposure to light or high temperatures can cause premature polymerization of the liquid component. Hydroquinone therefore is added as a stabiliser or inhibitor to prevent premature polymerization

In order to make the cement radiopaque, a contrast agent is added. Commercially available cements use either zirconium dioxide (ZrO2) or barium sulphate (BaSO4). Zirconium dioxide is one hundred times less soluble than barium sulphate and has less effect on the mechanical properties of the cement.

During the exothermic free-radical polymerization process, the cement heats up. This polymerization heat reaches temperatures of around 82–86 °C in the body. The cause of the low polymerization temperature in the body is the relatively thin cement coating, which should not exceed 5 mm, and the temperature dissipation via the large prosthesis surface and the flow of blood.

POLYMERIZATIONWhen the polymer powder and monomer liquid meet, the

polymerization process starts. During polymerization of the monomer, the original polymer beads of the powder are bonded into a dough-like mass. The mass hardens approximately 7-15 minutes after the start of mixing, depending on temperature.

The polymerisation process can be divided into four different phases: mixing, waiting, working and hardening phase.

• MIXING PHASEIn the mixing phase, the cement should be mixed homogeneously, minimising the number of pores. Vacuum

• WAITING PHASEDuring this phase, the cements achieve a suitable viscosity for delivery of bone cement. The cement is still a sticky dough.

• WORKING PHASEThe working phase is the period during which the cement and the implant can be introduced with ease. The cement must not be sticky, and its viscosity should be suitable for application. If viscosity is too low, the cement may not be able to withstand the bleeding pressure and prevent blood from entering the cement.

• HARDENING PHASEDuring this phase, the cement hardens and sets completely. Hardening is influenced by the cement temperature, the OR temperature as well as the body

PROPERTIES

• VISCOSITYDuring the reaction, the cement viscosity increases, slowly

at first, thenlater more rapidly.

Viscosity affects the following:

• Mixing behaviour• Penetration into

cancellous bone• Resistance against

bleeding• Insertion of the implant

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. 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.

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.

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.

METHODS OF APPLICATIONS

DIGITALAll antibiotic bone cements can be applied digitally. The cement is mixed thoroughly but carefully to minimize the entrapment of air. Once dough is formed the surgeon should wait until the cement no longer adheres to the glove and the surface has become dull as opposed to shiny. The cement can then be taken into gloved hands and kneaded thoroughly. It is vital that premature insertion of cement is avoided as this may lead to a drop in the patient's blood pressure. Importantly, this stage will occur at different times for different cement types.The time of cement application and prosthesis insertion is at the discretion of the surgeon and will depend upon the surgical procedure used. In general, implant insertion should be delayed until the cement has developed a sufficient degree of viscosity to resist excessive displacement by the implant. However, implant insertion should not be delayed such that there is a risk that the

Syringe application

Antibiotic bone cements may be applied using a suitable cement gun and syringe. The surgeon should use their experience to judge when the cement has reached an appropriate viscosity to be extruded. This will not occur until after the cement has formed dough. A small amount of cement should be extruded from the syringe and visually assessed to ensure that the surface of the cement appears dull and excessive flow under gravity has ceased.

Prior to extrusion, it is recommended that a cement restrictor be inserted, at the required depth into, the prepared bone cavity. Introduction of bone cement into the prepared cavity should be carried out in a retrograde fashion. Once the cavity is filled it is advisable that adequate pressurization is applied

APPLICATION AREAS

Application Areas include information on hip, knee and spine replacement.

APPLICATION OF BONE CEMENT IN TOTAL HIP REPLACEMENT:-

FEMUR BONE FIXATION

Bone cement80 grams of cement is normally sufficient for stem fixation.

Cement mixingThe cement is mixed and collected in the cartridge under vacuum. The cartridge is then positioned in the cement gun

Pulse lavageMake a final pulse lavage before injecting the cement.

Cement deliveryUse a long nozzle in order to reach the distal femoral plug.

Inject the cement in retrograde fashion, letting the cement gun work its own way out of the femur.

Delivery of cement should never be done when the cement is in low viscosity stage. Normally wait approximately 3 minutes (depending on type of cement used) after start of mixing.

Apply the proximal seal and pressurize the cement.

Distal centralizerApply the distal centralizer to the stem. The centralizer should be used to avoid malposition of the stem in both planes.

Introduction of the stemGently introduce the stem and hold in place until the cement has polymerized.

Even cement mantleWith the stem in final position, you should have a 2 - 3 mm cement mantle around the stem and approximately 10 mm between the tip of the stem and the plug.

This will yield optimal stress distribution.

CAUTION AND ADVERSE EFFECTS

• Hypotensive episodes and cardiac arrest have been reported during cement insertion.

• Pressurization and thorough cleaning of the bone with expulsion of bone marrow has been associated with the occurrence of pulmonary embolisms, and this risk has been found to be increased in patients with highly osteoporotic bone and patients diagnosed with femoral neck fracture.

• Reaming of the marrow cavity can have similar effects on mean arterial pressure as the introduction of the bone cement. Marrow cavities should be vented when the cement is introduced digitally.

• The premature insertion of bone cement may lead to a drop in blood pressure, which has been linked to the availability of methyl methacrylate at the surface of the product,although

• This drop in blood pressure, on top of hypotension induced either accidentally or intentionally, can lead to cardiac arrhythmias or to an ischaemic myocardium.

• However, according to a report, the possible risk of death associated with the use of cemented implant is confined to early postoperative and perioperative period.

• The hypotensive effects of methyl methacrylate are potentiated if the patient is suffering from hypovolaemia.

The most frequent adverse reactions reported with acrylic bone cements are:• Transitory fall in blood pressure.• Elevated serum gamma-glutamyl-transpeptidase (GGTP) upto10 days post-operation.• Thrombophlebitis.• Loosening or displacement of the prosthesis.• Superficial or deep wound infection.• Trochanteric bursitis.• Short-term cardiac conduction irregularities.• Heterotopic new bone formation.• Trochanteric separation.

DRAWBACKS OF BONE CEMENT

• One of the major drawbacks of bone cement in joint replacement is cement fragmentation and foreign body reaction to wear debris, resulting in prosthetic loosening and periprosthetic osteolysis.

• The production of wear particles from roughened metallic surfaces and from the PMMA cement promotes local inflammatory activity, resulting in chronic complications to hip replacements.

• Histologically, a layer of synovial like cells which line the bone cement interface supported by a stroma containing macrophages and wear particles, has been described in loose prostheses.

• A third of dense fibrous tissue contains polymethyl

• Activated macrophages express cytokines including interleukin-1, interleukin-6 and tumour necrosis factor alpha, which mediate periprosthetic osteolysis.

• Bone cement generates heat as it cures and contracts and later expands due to water absorption.

• It is neither osteoinductive nor osteoconductiveand does not remodel.

• The monomer is toxic and there is a potential for allergic reactions to cement constituents.