BONE CEMENT PROPERTIES
CONTENTS
Role of bone cement in orthopaedic surgery - 2007
1 Composition, structure, function
The ready bone cement is a compound consisting of 90 % of polymethylmetacrylate, (PMM), the rest are mainly crystals of barium sulfate or Zirconium oxide that make the resulting product radio-opaque).
Bone cement is used for fixation of the artificial joints to the skeleton. It acts, however, not as a glue, it acts as a filler. The familiar materials Plexiglas or Lucite consist of pure polymethylmetacrylate; Plexiglas is one of the strongest plastics.
Picture : X-ray picture of a cemented total hip. (click on the icon for a full size picture)
On the X-ray pictures one sees the bone cement as a white layer around the shadows of the total hip device.
In the upper part you may discern the plastic (polyethylene = not radio opaque) cup as a negative (gray) relief within a mass of white (radio opaque) bone cement.
In the lower part you see the most prominent object central stem of the femoral component. It is made from metal and thus most radio opaque.
Around the stem is a massive lump of bone cement, also radio opaque. The lump is quite irregular, it is concentrated to the upper part of the stem. The lower part of the stem is without cement mantle. Also there is no cement mantle on the outside of the stem.
The microscopic structure of bone cement is made by two substances glued together. One substance are the small particles of pre-polymerized PMMA (PolyMethylMetaAcrylate), so called "pearls. These pearls are supplied as a white powder. The other substance is a liquid monomer of MMA(MethylMetacrylate). Both substances are mixed together at the operation table with added catalyst that starts the polymerization of the monomer fluid.
The polymerizing fluid glues together the pearls into a firm, strong, but brittle mass.
Show Pictures: Structure of bone cement and bumper function
(click on the icon for a full size picture)
On the upper picture you see the microscopic structure of the polymerized bone cement. Basically, the bone cement consists of individual acrylic (polymethylmetacrylate) spheres "pearls" that are glued together and embedded by a net of the polymerized monomer. In an insert you see one such "pearl" with characteristic fluffy surface. The net of the polymerized monomer that keeps the pearls together has a honeycomb-like structure This structure of bone cement develops during the preparation of the bone cement at the operation table. The surgeon's assistant prepares the bone cement by mixing the pearls (the powder) with the monomer liquid. In this mixture the individual pearls are dispersed within the liquid and swell up.
When the liquid monomer polymerizes and the bone cement hardens, the individual pearls are entrapped and glued within a net of the polymerized monomer, but there is no chemical binding between the pearls and the polymerized monomer.
On the upper picture you see the honeycomb -like structure of the polymerized monomer that entraps the polymer pearls. You may see that there is a lot of space- bubbles- within the hardened bone cement, making the structure honeycomb-like. These small voids were formed by air bubbles entrapped during mixing of bone cement and by small rests of non polymerized monomer that eventually evaporated and left a free space.
The resulting net (honeycomb)-like structure gives the bone cement the ability to absorb downward (compression) loads. An important characteristics in an otherwise brittle material.
On the the lower pictures (schematic pictures) you see how the bone cement act mechanically as an shock absorber (bumper): it damps the blows generated by patient's activity so that they are not transmitted fully on the skeleton. The bone cement is displayed a net between the shaft of the total hip (inside the net) and the thighbone (red). You may observe three phases:
(1) The unloaded phase: the bone cement net is regular, not deformed.
( 2) Load applied by body weight impact on the total hip (its femoral component): the bone cement net within marrow cavity deforms elastically, but does not brake and stays in contact with both total hip and skeleton.
( 3 ) When the load ceases the bone cement's structure returns to its original regular form.
This structure also explain why it is easy to load the bone cement with antibiotics and why antibiotics elute so easy from the bone cement. The antibiotics (usually soluble powder) is hidden inside the polymer net.
This picture also explain why the rests of the liquid polymer may escape into the patient's circulation and cause problem (rarely). The not polymerized rests of monomer (resulting from bad mixing of bone cement components) are hidden inside the net and successively leak out.
In the ideal case the bone cement should make an even layer 2 to 5 millimeter thick between the skeleton and the total hip surface. The bone cement hardens (polymerizes, cures) within 7 -10 minutes. When it is hard the total hip device is solidly fixed to the skeleton. Actually, the fixation is best just after the finished operation. Theoretically, the patient can put the whole body weight on such total hip at the end of the operation.
Function of bone cement - filling the space between the skeleton and the total joint device.
The surgeon uses the doughy bone cement when it is no longer sticky. The mass cannot glue to the skeleton or to the total hip /joint device. The goal is to press the doughy bone cement into all small openings and voids in the spongy skeleton and fill all hollows on uneven surfaces. The bone cement shall adhere closely to the surface of the total joint.
See Picture: Bone cement- grout not glue
The upper picture shows two bone cement sprouts that penetrated the beams of spongious bone. When severeal such sprouts mix with the beams the whole construction makes a strong fixation between the bone cement and the spongious bone. The lower picture shows the surface of a bone cement layer removed at revision operation of failed cup component. The surface of the cement layer is uneven, but there are no more prominent parts of bone cement that could penetrate deeper into the skeleton.
The lower figure (at 900 x magnification) shows a pearl with damaged layer of MMA polymer that once covered the whole pearl. Nice documentation of the fact that in the body the cement successively breaks down and forms small fragments.
The ability to penetrate deeper into the skeleton depends on the viscosity of the bone cement. More liquid products penetrate easily the skeleton, more viscous products stay at the surface. Studies, however, demonstrated that use of low viscosity cements in surgery of total hips produced more failures than use of conventional doughy products. (See history of bone cement)
Preparation of bone cement at the operation table
The mechanical characteristics of bone cement, however, are inferior to the characteristics of Plexiglas thanks to the admixed products (Barium sulfate crystals) and thanks to another polymerization procedure. The Plexiglas is polymerized under ideal conditions at the factory premises from the pure monomer substance at high temperatures whereas the bone cement is polymerized in the patient's body during the operation from a "dough" prepared by mixing two substances at low (body temperature).
(See also the chapter Bone cement history for more details)
The surgeon's assistant prepares the bone cement directly at the operation table by mixing manually a white polymer powder of PMM (PolyMethylMetacrylate) with a nasty smelling monomer fluid. The resulting product is a doughy white mass which polymerizes to a hard and brittle substance within ten minutes. The polymerization is accompanied by development of heat so that the surface temperature of a massive ball of polymerizing bone cement reaches temporarily 60 to 100 degrees Celsius.
A thin layer of polymerizing bone cement is cooled by the mass of the total joint on one side and by the skeleton itself which is permeated with blood 37 degr C warm on the other side. These conditions make that the surface temperature of bone cement layer, fortunately, never reaches these high temperatures because the skeleton's cells cannot survive temperatures over 47 degr C longer time.
When the surgeon presses the doughy bone cement into the prepared cavity in the bone, small quantities of monomer fluid are still present in the product. The toxic monomer fluid may leak into the circulation and cause sudden blood pressure fall during the cementing of the total hip device.
The fully polymerized bone cement also contains air bubbles which were entrapped in the product during the mixing procedure. These air bubbles diminish the strength of the polymerized bone cement. Manufacturers thus developed vacuum mixing systems that decrease the amount of air bubbles in the ready bone cement. The vacuum system also suctions out the vapors of the loose monomer hich remained after imperfect mixing of the substances.
See Picture: bone cement mixing in a bowl
(click on the icon for a full size picture)
The monomer liquid evaporates even at room temperature, so the manufacturers developed clever small mixing apparatuses that are closed. These apparatuses suction continually the noxious vapors and absorb them into an active carbon filter so that the monomer does not leak into the operation room atmosphere. Moreover, constant suction also keeps low air pressure and diminishes the number of air bubbles that are admixed into the product by stirring and mixing the powder with the liquid. On this picture you see the mixing apparatus which is used for preparation of bone cement in lump form for cementing the cup component. In the bottom of this apparatus is included the active carbon filter that absorbs the evaporating fumes of the monomer. The continuous suction is done by connecting the apparatus to vacuum pump. The apparatus is disposable.
In the upper right part of the picture you see the constituents of the bone cement: the paper bag contains the polymer powder and the brown glass ampoule contains the volatile monomer. (Howmedica)
The assistant takes away the upper part (the lock with the handle) off and empties the content of the white paper bag (polymer powder) into the bowl inside the apparatus. Then the assistant adds the liquid monomer from the glass ampoule , puts the lock on and mixes the content of the bowl. The vacuum pump is on. The mixing is done by rotating the white handle. After some 1 -2 minutes of mixing the two constituent there forms a lump of dough like mass of bone cement. When it is no longer sticky it can be placed into the hole in the hip socket for cementing the cup component.
For fixation of the shaft by modern cementing technique the surgeons uses a cement gun.
See Picture: bone cement mixing for a cement gun
(click on the icon for a full size picture)
Preparation of bone cement for the cementing of the shaft component is again done in a closed system as follows; The assistant empties the polymer powder and the monomer liquid into a cylindrical container that is again fully closed afterward. (upper picture). There are containers with different volumes, revision operation with cementing the shaft in an eroded bone cavity may need twice as much bone cement as the uncomplicated total hip replacement. The container has a vacuum suction with an active carbon filter too. The mixing is done with a handle on the upper end of the container. When the mixing is finished, the whole cylinder is placed in a special cement gun. The surgeon then presses the still doughy bone cement into the marrow hole of the thigh bone. Usually, there is also a continuous vacuum suction tube placed inside the marrow hole that continuously keep low pressure inside the hole and thus helps the filling of the hole with bone cement. (Third generation cementing technique).
2 Antibiotic loaded bone cement
Most surgeons use bone cements with admixed prophylactic antibiotics. After the operation, the antibiotics leak from the bone cement into the tissues around the total hip. The local concentration of antibiotics around the cemented total hip prosthesis is sufficient to kill the bacteria left in the operative wound. On the other hand, the quantity of antibiotics that come into the circulation is low, so that the risk for general allergic reaction against the antibiotics is low.
Available statistics show that antibiotic loaded bone cement in combination with systemic antibiotics is the best prophylaxis against postoperative infection. Addition of antibiotics to bone cement does not change the mechanical characteristic of the product.
The critics argue that
antibiotics in the commercial bone cement are not specifically targeted at individual bacteria that just prevail at the hospital
that antibiotics in the bone cement may produce increased bacterial resistance
that the use of antibiotics in this way is not economical because the majority of antibiotics remain inside the bone cement
Drawbacks
Many commercial formulations of bone cement are now available on the market. These products differ in chemical composition and physical properties as well as in the mechanical strength and endurance of the product Statistics also demonstrated that total hip replacements done with certain bone cement products (low viscosity bone cement, Boneloc cement) have had increased rates of failures ( Thanner 1995) See also History of bone cement.
The bone cement as prepared by the surgeon at the operation table is a material with many drawbacks.
- it is mechanically weak because it has entrapped impurities such as air and blood,
- it is brittle, it has low endurance limit and is prone to fatigue failure.
- it spawns small particles from its surface containing hard crystals of Barium sulfate which scratch and damage the fine joint surfaces of the artificial joint.
- small cement particles may cause osteolysis - "bone dissolving disease"
- it has very large surface which may support colonization of bacteria and development of postoperative infections
- it may cause allergy and anaphylactic reaction during the operation
On the positive side:
The bone cement has a very long (>35 years) track record, none of the cementless competitors have as long track record.
The surgeons are used to work with the bone cement
The operation technique with bone cement is more forgiving
3 Allergy to bone cement and plastics:
There are only few reports about a delayed allergy to bone cement or some of its constituents in patients operated on with cemented total hips.There are papers describing failures of cemented total hips caused by allergy to these substances. (Haddad FS et al: Hypersensitivity in aseptic loosening of total hip replacement. The role of constituents of bone cement. J Bone Joint Surg-Br 1996: 78-B: 546-9).
One paper reported sensitivity to polymethylmetacrylate in 50% of 26 patients with aseptic loosening has been reported (Gill-Albarova J et al.: “ Lymphocyte response to polymethylmetacrylate in loose total hip prostheses The Journal of Bone and Joint Surg-Br 1992; 74-B: 825 -30 ).
This, today 15 years old paper has nice x-ray pictures of a typical osteolysis; today probably the failure of these total hips would be diagnosed as a typical osteolysis caused by polyethylene debris; perhaps nobody would come on the idea to test these patients for hypersensitivity to bone cement.
A similar sensitivity to polymeric materials among patients with a well functioning implant has not been demonstrated to my knowledge. However the prevalence of sensitivity to bone cement with a failing implant has been reported:
Dentists who used the same substance as bone cement have had problems with the dental cements. The substance caused allergic mouth sores and other allergic problems because of leaking some of these compounds such as benzoyl peroxide.
Sensitivity to polymeric -polyethylene materials in patients with well- functioning artificial joints has not been demonstrated as yet. (Hallab 2001)
For history of the use of bone cement see History bone cement
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Gill-Albarova J et al.: J Bone Joint Surg - Br 1992; 74-B: 825-30
Hallab N et al. J Bone Joint Surg 2001-Am; 83-A: 428 -36.
Thanner J et al.: Acta Orthop Scand 1995; 66: 207
Webb JC et al.: The role of polymethykmetacrykate bone cement in modeern orthopaedic surgery. J Bone Joint Surg-Br 2007; 89-B: 851-7
Uppdated July 2007
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