MODERN CEMENT TECHNOLOGY IN ARTHROPLASTY
Bone cement used in various orthopedic trauma and arthroplasty surgeries is chemically Polymethylmethacrylate (PMMA). It has been commonly used by orthopaedic and dental surgeons from last 60 years and remains a key component of modern practice (1). Actually, Cement is misnomer because PMMA does not bind the implant with the bone but actually acts as space filler thus acting as a ‘grout’ (2). In this chapter, bone cement word is used interchangeably in place of PMMA. Presently, PMMA is used in variety of cases like in joints replacements, fracture fixation, tumour surgery, percutaneous vertebroplasty etc (3,4). The understanding of its properties has evolved and progressed alongside the advancement of the specialty, and has indirectly helped improve implant design and understanding of biomechanics. This area requires further research to explore more clinical applications and to lessen the side effects associated with its use.
2. Historical perspective
Polymethylmethacrylate was first unveiled by the chemical industry in 1843 and named ‘acide acrylique’ because of the acrid smell of the monomer (5). Otto Rohm and Kulzer were early researchers who worked extensively on the physical properties and uses of bone cement. Its early applications were in the field of dentistry (6). The first bone cement use in Orthopaedics was by the Scales and Herschell (7) and Judet and Judet (8) in 1945 and 1950 respectively, they used PMMA prostheses of the femoral head. Credit for major break thorough in use of PMMA in orthopaedics is given to the famous English surgeon, John Charnley, who in 1958, used it for total hip arthroplasty (9). He used it for securing fixation of acetabular cup and femoral component in THA. In the 1970’s, the U.S. Food and Drug Administration (FDA) approved bone cement for use in hip and knee prosthetic fixation. Since then, bone cement has become widely used for fixation of prostheses to living bone; the trends of bone cement usage have evolved (10).
3. PMMA constituents
PMMA is an acrylic polymer that is formed by mixing two sterile components (Table 1): a liquid MMA monomer and a powered MMA-styrene co-polymer. When the two components are mixed, the liquid monomer polymerizes around the pre polymerized powder particles to form hardened PMMA by breaking the covalent C=C bonds of monomer (Fig.1). In the process, heat is generated, due to an exothermic reaction of about 52 KJ/mole of monomer, equating to heat production of 1.4 to 1.7 108 J/m3 of cement (11). 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. An initiator, di-benzoyl peroxide (BPO), is added to the powder, and an accelerator, mostly N, Ndimethyl-p-toluidine (DmpT), is added to the liquid to encourage the polymer and monomer to polymerise at room temperature. In order to make the cement radiopaque, a contrast agent like zirconium dioxide (ZrO2) or barium sulphate (BaSO4) is added.
Fig.1 – Components of bone cement.
Commercial constituents of bone cement
Powder components Constituent Role
Co-polymers (e.g. MA-MMA) Alter physical properties of the cement
Barium sulphate or zirconium dioxide Radio-opacifiers
Antibiotics Antimicrobial prophylaxis
Dye (e.g. chlorophyll) Distinguish cement from bone
Monomer Methylmethacrylate monomer
N,N-dimethyl-p-toluidine (DMPT) Initiates cold curing of polymer
Benzoyl peroxide Reacts with DMPT to catalyse polymerisation
Hydroquinone Stabiliser preventing premature polymerisation
Dye (e.g. chlorophyll) Distinguish cement from bone
Antibiotic bone cement is found useful in delivering in antibiotics locally, which can be added to the powder component easily without adversely affecting cement’s mechanical property (12). This makes bone cement a modern drug delivery system that delivers the required drugs directly to the surgical site. Antibiotics which are heat stable and have longer duration of action like Gentamycin, Tobramycin, Erythromycin, Cefuroxime, Vancomycin, Colistin etc can be added. While heat labile drugs like penicillins, chloramphenicol and tetracycline are avoided. Some drugs combinations like Gentamycin with tobramycin are found to have synergistic action (13, 14). This property of cement to act as a carrier for antibiotics is useful in septic revisions where in spacers of cement act as mechanical blocks maintaining the soft tissue spaces in absence of hip and knee implants and simultaneously delivering antibiotics locally.
3.1 Usage and properties
Polymethylmethacrylate is a brittle, notch sensitive material. PMMA has modulus of elasticity (Young’s modulus) lower than that of surrounding cortical bone (10X) and that of metal stem (100X) thus acting as elastic inlayer between two stiff layer (15). It has the unique property of continued polymerisation in vivo but this is a prolonged process lasting for between 28 and 70 days (16). The long-term properties of bone cement, including fatigue behaviour, the visco-elastic properties of creep and stress relaxation which are both time and temperature dependent are central to the success of cemented hip replacement.
Creep. This is the deformation of a material under constant load. Under constant load a material capable of creep will deform by an amount dependent on the size of the load and the length of time it is applied. The rate of loading is also important, where visco-elastic materials demonstrate a higher Young’s modulus at higher loading rates (15).
Stress relaxation. This is the time-dependent change in stress within a material under constant strain. The force needed to maintain a set deformation will reduce with time if stress relaxation occurs.
Studies have shown that subsidence of a stem within the cement mantle by the process of creep protects the vital bone/cement interface and hence the replacement overall (17).
3.2 Curing process
The curing process is divided into 4 stages:
b) Sticky/ waiting
The viscosity of all cements increases during polymerisation as the polymer chains lengthen. Studies have shown that high viscosity cements result in better prosthetic fixation, as compared to low viscosity cements (18). Manufacturers can alter the viscosity of cement by changing the molecular weight, by using co-polymers, and by varying the methods of sterilisation (11). On the basis of these phases bone cements are described in 3 types-
• Low. These have a long waiting phase of three minutes, also known as a sticky phase. The viscosity rapidly increases during the working phase and the hardening phase is one to two minutes long.
• Medium. There is a long waiting phase of three minutes, but during the working phase, the viscosity only increases slowly. Hardening takes place between one minute 30 seconds to two minutes 30 seconds.
• High. A short waiting/sticky phase is followed by a long working phase. The viscosity remains constant until the end of the working phase. The hardening phase lasts between one minute 30 seconds and two minutes.
However, the rates of curing are very sensitive to environmental factors (19). Low ambient temperatures during storing and mixing, and high humidity both prolong setting time (20).
4. Methods of Mixing
Bone cement components are mixed manually in bowl but this technique lacks reproducibility and produces cement with uncontrollable porosity. Also it results in noxious fumes, which creates serious safety concerns (Fig.2).
Fig.2 – Manual mixing in a bowl.
A vibrating mixing technique was introduced in hopes of improving bone cement properties. The results, however, were not encouraging.
In this technique, cement was first mixed manually and then subjected to centrifugation to eliminate any air inclusions introduced during mixing. The method succeeded in reducing porosity but procedures varied significantly depending on the type of centrifugation and cement used.
4.4 Vacuum Mixing
In most operating rooms today, bone cement is mixed under a vacuum, which results in a low porosity cement with increased strength and resistance to cement fatigue and creep (21). The cement is mixed in a syringe, bowl, or cartridge. All of these systems consist of an enclosed chamber connected to a vacuum source (eg, wall suction or a dedicated vacuum pump). All ingredients are added and mixed while the system is closed.
4.5 Cartridge mixing and delivery
The latest advancement in bone cement mixing technique is a simple, universal power mixer that quickly mixes and then mechanically injects all types of bone cement. This type of device reduces mix times, as it requires fewer steps to load, mix, and transfer the cement. The rotary hand piece reduces variability, which results in consistent mix times; a built-in charcoal filter reduces harmful fumes.
5. Methods of Application
The original method for cement application was hand packing. In THA, femoral canal was used to be packed with cement by pressing with fingers or thumb; this pressurization forces the cement into the bone interstices. While in TKA, hand packing is still commonly preferred because the surfaces are readily visible, thus making the application by hand feasible.
Syringes are used to apply, or inject, the cement in femoral canal especially in THA. Bone cement is inserted in retrograde manner (Fig.3). Appropriate pressure is maintained throughout hardening phase.
Fig.3 – Syringe filled with bone cement.
5.3 Gun pressurisation
Many studies state that higher pressurization results in greater penetration within bone, improved bone cement interface and increased fatigue strength of the cement. This gun system allows the surgeon to force more cement into the interstices of the bone via higher pressurization; delivering more cement without overflow. But its use is associated with increased incidences of cement embolism and mortality (22).
5.4 Stem centralizer
Stem centraliser helps in attaining neutral implant position and ensures a homogeneous cement mantle between bone and prosthesis (23). The distal centralizers usually are pyramidal shape. They are attached to a corresponding tunnel at the tip of the prosthesis. Nowadays proximal centralisers are also available which improve overall prosthesis alignment and even cement layering. It has its own disadvantages like void around centraliser, failure of interface and impingement over cortex (24).
5.5 Cement restrictors
To get good cement bone interdigitation, high pressure must be attained within femoral canal. To achieve that intramedullary canal plugs are now routinely used in cemented THA. Also it prevents in excess penetration of cement distally. It is placed about 2 cm distal to tip of the stem. Restricters are made up of either plastic or bone. They help in achieving greater penetration and enhanced prosthetic stability.
6. Bone bed preparation
Bone cavity is reamed up to appropriate size to incorporate best size implant along with even cement layer between bone and prosthesis. Preoperative templates are helpful in determining size of reaming, as over reaming hampers endosteal blood supply and could lead to thining of cortical bone.
Fine brushes are available for cleaning blood and tissue debris in acetabulum and femoral canal. Persistence of these debris may lead to overall decrease in bone cement’s effective strength.
6.3 Pulse lavage
Pulse lavage is the high pressure lavage system use to remove bone debris and cement particles (Fig. 4). They clean the blood film over the bone, thus increases the mechanical strength of cement bone interface. Its use has also been found to reduce chances of fat emboli, asceptic loosening and physiological derangements (25). It is also use in debridement of infected cases as it breakes biofilm and decrease bacterial load (26).
Fig.4 – Tibia preparation, holes made on sclerotic medial condyle for better bone cement interdigitation.
Fig.5 – Anchorage holes in the acetabulum to enhance cement interigitation.
6.4 Anchorage holes in the acetabulum
In cemented acetabulum cases, after reaming of acetabular cup anchorage holes are either punched or drilled holed (Fig.5). They increase the surface area for better cement bone fixation.
7. Evolution of cementing techniques
With the advent of better understanding and newer technology cementing technique has undergone various changes, not only in cement mixing but also bone bed preparation and cement delivery.
According to evolution, advancements in cementing are classified in first, second, third, fourth generation.
7.1 First generation
This was the earliest technique in which minimal bone bed preparation is done with brushing, canal is left open. Cement is mixed in a bowl and introduced by hand with digital pressurisation.
7.2. Second generation
In this technique, bone bed is prepared with pulsatile irrigation, then packed and dried. Cement is mix open manually and with help of cement gun, inserted in femoral canal by retrograde fashion. Cement restrictors are also used prior to cement insertion. Further improvement lead to the development of third generation cementing techniques.
7.3 Third generation
In this generation, bone bed is prepared with same technique. Change lies in cement mixing technique, under vacuum condition centrifugation is used to mix bone cement. Cement is inserted in femoral canal via retrograde manner. Femoral and acetabular pressurisers (Fig.6) are used to pressurise cement for better micro interlocking.
Fig.6 – Acetabular Pressuriser for better micro interelocking.
7.4 Fourth generation
It is the latest recommendation. In this, distal and proximal centralisers are use to ensure an even cement mantle.
Swedish registry data (Fig.7) supports that with advancement in cementing technology and use of a distal cement restrictor, pulsatile lavage, cement gun and a proximal centralisers reduce the risk for revision by approximately 20% each [27,28]
Fig.7 – Observed implant survival with different cementing techniques. Note significant improvement of implant survival with better cementing techniques in 3 cohorts. Green; modern (n=27,842), Red; early (n=19,100), Blue; old (n=20,404). [27,28]
8. Caution and adverse effects (29,30,31)
• Transitory decreased blood pressure
• Post operative thrombophlebitis
• Bladder fistula
• Short-term cardiac conduction irregularities
• Intestinal obstruction because of adhesions and stricture of the ileum from the heat released during the exothermic polymerization
• Loosening or displacement of the prosthesis
• Superficial or deep wound infection
• Pain and/or loss of function
• Trochanteric bursitis
• Heterotopic new bone formation
• Trochanteric separation
• Delayed sciatic nerve entrapment from extrusion of the bone cement
• Local neuropathy
• Local vascular erosion and occlusion
8.3 Cautions for nursing personnel
• MMA fumes, which are emitted during preparation of PMMA bone cement, have been shown to have toxic side effects ranging from allergic reactions to neurological disorders
• Skin contact with the liquid monomer can cause contact dermatitis and hypersensitivity Reactions
• Eye contact with the liquid can be quite serious, causing considerable irritation or burns to the eyes.
Bone cement implantation syndrome (BCIS) (22) is a poorly understood, rare, and potentially fatal complication occurring in patients undergoing cemented orthopaedic surgeries especially hip arthroplasties. It can occur within minutes of the procedure intraoperatively or may also be seen in the postoperative period in a milder form causing hypoxia and confusion. It is characterized by a number of clinical features that may include hypoxia, hypotension, cardiac arrhythmias, increased pulmonary vascular resistance (PVR), and cardiac arrest. It usually occurs at one of the five stages in the surgical procedure; femoral reaming, acetabular or femoral cement implantation, insertion of the prosthesis, or joint reduction.
8.5 Measures to Reduce the Risk of BCIS
Various measures have been described to reduce the risk of BCIS that may be implemented by the surgeon or anaesthetic like as follows –
● using invasive hemodynamic monitoring when pre-existing cardiopulmonary problems exist and during cementing
● maintaining a high level of arterial oxygenation and increasing inspired oxygen concentration by administering 100% oxygen during the procedure
● decreasing the concentration of a volatile agent (when using general anesthesia) prior to insertion of the prosthesis
● maintaining normovolemia intraoperatively, especially at the time of cementing and insertion of the prosthesis
● placing a venting hole into the femur, especially if using long-stem prosthesis
● avoiding bilateral hip replacements with cemented prostheses if cardiopulmonary dysfunction is present
● using a non-cemented prosthesis, especially if the patient’s mean arterial pressure decreases 20% to 30% below baseline during canal reaming or plugging
● performing thorough, pulsatile, high-pressure, high-volume lavage and brushing followed by drying of the intramedullary canal of the femoral shaft
● using a cement restrictor combined with other methods to reduce intramedullary pressures
● using a low viscosity cement
● mixing the bone cement in a vacuum
● working the cement before insertion to remove volatile vasodilator compounds
● using cement gun to apply the cement under sustained low pressure
● using retrograde cement gun technique for cement insertion
● using a vacuum tube along the linea aspera to drain the proximal femur, which reduces high intramedullary pressure during cement and prosthesis insertion
● introducing the prosthesis stem slowly into the cemented femoral canal, thereby reducing pressurization
9.0 Key points for Knee Arthroplasty
• Keep the OT temperature optimum (16-18o Celsius) during cementing.
• Multiple drill holes on sclerotic part of condyles for better bone cement integration (Fig.4).
• Properly wash the condyle surface with pulse lavage to remove any bone debris and blood clot.
• Before application of cement, properly dry the surface with clean mop.
• Press cement with finger tip for deeper penetration (Fig.8).
Fig.8 – Cement pressed with finger tip for deeper penetration.
• Cement is applied on both bone surface and component (Fig.9).
Fig.9 – Cement is applied over under surface of implant.
• After cement application compress antero-posteriorly on both condyles since after alignment and balance, third most common factor in determining TKR success is cementing especially on posterior condyle as loosening most commonly occur at this site.
• Hammer the component in posterior to anterior direction, to prevent femoral component in flexion.
• After reduction, keep limb static until cement sets.
• Always check intercondylar box for extra cement before cement sets.
• In unicondylar knee arthroplasty, high viscosity palacos cement is preferred over others, as it has lower revision rate.
10.0 Key points for Hip Arthroplasty
• Good entry point is essential in predicting stem position later on.
• Optimum reaming of femoral canal and cup is essential for ensuring good bone cement interlocking.
• Remove all femoroacetabular osteophytes to prevent impingement.
• Clean canal and acetabulum cup with pulse lavage thoroughly to remove any bone debris and blood clot (Fig.10).
Fig.10 – Reamed and dried femoral canal, ready for cementation.
• Dry the canal with dry pack.
• Put distal cement restrictor in canal, 2cm from stem tip.
• A thin suction tube is kept in femoral canal to drain out extra fluid and to decrease intra marrow pressure. It is removed after pressurising bone cement.
• Apply cement in distal to proximal direction.
• On pressurising cement in acetabulum, prevent inferior blob (Fig.11).
• On putting implant cup, first tamp superiorly to well position cup inferiorly.
• A positive sign of good pressurization of cement is marrow extrusion in the greater trochanter (the so-called sweating trochanter sign) (32).
• Keep counter pressure on cement extruding out on femoral stem insertion as it provide additional cement pressurisation.
• Femoral stem centraliser is essential for well placed stem in canal.
• Keep a sustained pressure over implant, until cement sets.
Fig.11 – Uniformly compressed cement after pressurisation.
PMMA (Bone Cement) has played a pivotal role in modern day arthroplasty and has improved results and long term outcomes. The development of cement technology and types has progressed along with newer advances in implant design and surfaces. Though uncemented fixation is gradually taking over cemented fixation in hip and likely to overtake cemented fixation in the knee in coming years, cement would be written in golden letters in history of arthroplasty.
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