Polymethylmethacrylate

Bone Cement

Reviewed by

Dr Saeed Asadollahi
MBBS
Unaccredited Orthopaedic Registrar

1-Polymer powder
- (More variable in different brands)
- Copolymer 83% to 99%
- Radiopacifier
- Initiator
- Antibiotic
2-Monomer liquid
- (Constant in different brands)
- MMA 97% to 99%
- Accelator
- Stabiliser

Reaction: Combining powder and liquid monomer initiates an exothermic reaction.
In vivo temperatures are reported to be between 40° and 56°C.
Free radical polymerisation from the
monomer methyl methacrylate

Endogenous factors:
- component variations
- formulation ratio
- molecular weight
- physical size of specimen
Exogenous factors:
- entrapped air
- handling and mixing times
- water and body fluid
- temperature
- sterilisation

Entrapped Air:
- ↑ porosity → ↓ fatigue life of cement
- ↑ handling time → ↑ porosity → ↓fatigue life of cement
- vacuum mixing/ centrifugation → ↑ fatigue life of cement
Water and body fluid / moisture
- ↑ water content → ↓ in fatigue life and tensile strength
- Incorporation of blood into cement → ↓ compressive strength by 8% to 16%.
- (↓ peak temperature by circulating blood)
- Relative humidity > 40% → ↓ working time
Ambient Temperature
- ↑ temp → ↑ polymerisation rate → ↓ working and setting times
Sterilization
- Radiation
  - The most prevalent
  - ↓molecular weight of PMMA → ↓ fatigue life
- Ethylene oxide
  - More time-consuming and expensive alternative
    - No effect on molecular weight of PMMA

Arthroplasty
- provides stability
- PMMA has no adhesive properties to implants on a molecular level.
- The quality of apposition between the implant-cement and bone-cement interfaces → the longevity of a cemented prosthesis.
- Implant fixation within bone cement
  - Enhancing fixation minimising motion
  - Minimising cement abrasion in presence of motion
  - Sharp edged” and undercutting geometry presented highest gap/crack rates.
- Prosthesis design and its affect of cement
  - Anatomically shaped prosthesis →Uniform cement mantle thickness
  - Collared stem →↓ tensile strength on proximal cement mantle
  - Low elasticity modulus → ↑ stress imparted on mantle
Infection
- Antibiotic loaded cement
  - Treatment of active infection
  - Prophylactic use
  - Role in maintaining soft tissue tension
- Antibiotic beads
  - advantage: large surface area
  - disadvantage: difficulty in removal, soft tissue intrusion
- How long is the duration of antibiotic elution?
  - Several months
- What the chance of systemic toxicity given the high dose of local antibiotic?
  - Rarely happens
Spine
- Vertebral augmentation (vertebroplasty, kyphoplasty)
- Aim: to restore the compressive strength and stiffness of the involved vertebra
- Advantage: rapid cure to mechanically sound state → immediate mobilisation

Cardiopulmonary complications
- Reported in hip arthroplasty and vertebral augmentation
- Likely mechanism: embolisation of marrow debris and neurogenic reflex

Composition and properties of PMMA has not been completely understood.
Understanding of the exogenous factors affecting the mechanical properties of the cement is important in its application.
PMMA plays a significant role in orthopaedics.
PMMA application in treatment of infection is evolving.

Todd Jaeblon, Department of Orthopaedics, St. Vincent Mercy MC, Toledo, OH, J Am Acad Orthop Surg2010;18:297-305
Caroline Duval-Terrié and Laurent Lebrun. Polymerization and Characterization of PMMA. Polymer Chemistry Laboratory Experiments for Undergraduate Students. J. Chem. Educ., 2006, 83 (3), p 443
Gladius Lewis. Properties of Antibiotic-Loaded Acrylic Bone Cements for Use in Cemented Arthroplasties: A State-of-the-Art Review. J Biomed Mater Res Part B: Appl Biomater. 2009 89B: 558–574
Macaulay W, Digiovanni CW, Restrepo A, et al. Differences in bone-cement porosity by vacuum mixing, centrifugation, and hand mixing. J Arthroplasty. 2002 Aug;17(5):569-75.
Crowninshield R: Femoral hip implant fixation within bone cement. Operative Techniques in Orthopaedics 2001;11: 296-299.
Gravius S, Wirtz DC, Siebert CH, et al. In vitro interface and cement mantle analysis of different femur stem designs. J Biomech 2008;41:2021-2028.
Miller MA, Race A, Gupta S, Higham P, Clarke MT, Mann KA: The role of cement viscosity on cement-bone apposition and strength: An in vitro model with medullary bleeding. J Arthroplasty 2007;22:109-116.