Polymethylmethacrylate (PMMA), or bone cement, is a polymer that was first created in 1928. It is the substance also known as “Plexiglass”, and has been used for everything from a component of bullet-proof glass to guitar bodies. It has been used as a grout for implant fixation since the first orthopaedic use reported by Charnley in 1958. Its use as a grout allows for a tight fit between implants and bone, which in turn minimises any focal stresses. Its composition is of a liquid comprised of monomer, accelerator and stabiliser, and a powder comprised polymer, initiator, radio-opacifier and antibiotics. The polymerisation reaction occurs faster at higher temperatures, is exothermic, and reaches up to mid-80 degrees within the body. Thermal injury to surrounding tissues is limited by having a limited volume of cement mantle around any implant.
Cementing techniques
Cementation techniques have evolved over time. While these evolutionary changes have occurred gradually, in retrospect they have been classified into multiple generations:
– First generation: minimal canal preparation with cancellous bone left in situ. Canal irrigation and suction followed by digital application of cement. Pressurisation was implemented towards the end of this stage. Implant positioning is performed manually.
– Second generation: Thorough canal preparation with removal of cancellous bone out to the endosteal surface; use of a distal cement restrictor; use of pulse lavage, packing and drying of the medullary canal; and use of a cement gun to deliver the cement in a retrograde manner. The implant is positioned manually.
– Third generation: This features vacuum-centrifugation cement mixing to reduce porosity of the cement; adrenaline-soaked gauze swabs to further restrict blood contamination of the prepared canal;
– Fourth generation: This has been described as being defined by the use of distal and proximal centralisers for the implant.
It is apparent that most surgeons will use a combination of various elements of these, based on availability of equipment, presence of centralisers with chosen implants, personal preference for various aspects, and patient-specific factors such as completing a quick operation in high-risk individuals. It is also apparent that these classifications are designed purely around the femoral component of hip arthroplasty, and do not take into account other uses of bone cement.
The overall objective of this evolution is towards a homogenous cement mantle around the implant, with the implant relatively centred for a symmetrical mantle. The cement should have minimal and consistent porosity throughout, minimal laminations and striations with blood and fat, and optimal osseo-integration through thorough mechanical interlock with the prepared surface and residual cancellous bone.
Bone Cement Implantation Syndrome
Bone Cement Implantation Syndrome (BCIS) is an uncommon complication which may be seen during arthroplasty cementation. Its incidence is generally low, but inconsistently reported based on varying definitions, varying reporting and rates which differ with different operations. BCIS is classified into three levels of severity:
– Grade 1: Moderate hypoxia (SpO2 <94%) or hypotension (decrease in SBP of >20%)
– Grade 2: severe hypoxia (SpO2 <88%), hypotension (a decrease in SBP >40%), or unexpected loss of consciousness
– Grade 3: cardiovascular collapse requiring CPR.
One study reported incidence and mortality rates among cemented hemiarthroplasty patients as:
– Grade 1: 21% incidence, 9.3% early mortality
– Grade 2: 5.1% incidence, 35% early mortality
– Grade 3: 1.7% incidence, 88% early mortality
Underlying pathophysiology of BCIS is still being debated and elaborated. The most accepted hypothesis is a multi-factorial process including:
– embolisation: an embolic shower comprising fat, marrow, cement, bone fragments, air and platelet aggregates is seen, at time of cementation, pressurisation and implantation. This has been demonstrated post-mortem in lungs of arthroplasty patients who died perioperatively, however degree of BCIS has not been shown to correlate strongly with magnitude of this shower.
– inflammatory response: complement activation occurs in response to the embolic shower, as well as potentially in response to direct effects of bone cement or monomers. This produces effects within the lung and systemic vascular beds. There has been demonstrated increased levels of complement activation in patients undergoing cemented arthoplasty vs uncemented.
– Histamine release: the pathological responses seen between anaphylaxis and BCIS are similar, and increased circulating histamine levels have been demonstrated in patients undergoing cemented arthroplasty. Histamine blockade has shown some limited protection in one study.
– Individual variation: the above factors then interact with the individual patient’s underlying co-morbidities (cardiac, respiratory, immune, etc) and produce the dynamic final result
Techniques for avoiding BCIS are limited. The few techniques that have been shown to work include:
– Medullary lavage: pulse lavage prior to cementation reduces the amount of material which is available for embolisation, and this has been confirmed as leading to a smaller embolic shower.
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Antibiotic cement
Antibiotic-loaded cement is routinely used both in primary and revision arthroplasty. Antibiotics with proven use include vancomycin, gentamicin, tobramycin, colistin, erythromycin and cefuroxime. The basic requirement is for an antibiotic which is heat-stable and should last well within the cement over time. Dosage of up to 2g of antibiotic per packet of cement powder has been shown to not affect structural integrity of the final cement, however over 2g does cause weakening of the final product.
Antibiotic-loaded cement is routinely used as part of the Masquelet procedure for bone defects, cement spacers for two-stage arthroplasty revision. Routine use of antibiotic-loaded bone cement in primary arthroplasty is more controversial, although a growing body of meta-analyses and other literature is supporting the use of prophylactic-dose antibiotics in cement in primary arthroplasty, including in relatively low-risk patients.
Author Contributions
Sean Griffiths, WH Resident, 2020
References
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- Hines, C. B. (2018). Understanding Bone Cement Implantation Syndrome. AANA journal, 86(6).
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- Sebastian, Sujeesh, et al. “Antibiotic containing bone cement in prevention of hip and knee prosthetic joint infections: A systematic review and meta-analysis.” Journal of Orthopaedic Translation (2020).
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