© 2011 Wichtig Editore - ISSN 1120-7000 Hip Int 2011; ( : 02 ) 238 - 242 22 238 DOI: 10.5301/HIP.2011.6517 INTRODUCTION Bone cement, or polymethyl methacrylate (PMMA), was first used medically in the 1940s to close defects in the skull and later as an artificial femoral head in 1945 by Scales and Herschell (1). The use of bone cement as a grout was first described in 1953 (2). Bone cement acting to secure the femoral and acetabular components of total hip arthroplasty was pioneered by Sir John Charnley in the late 1950s (3). It has subsequently been used successfully in arthroplasty of other joints and it is evident that it acts as a grout rather than an adhesive. Bone cement, as commonly used, is a polymer of methyl methacrylate (C 5 O 2 H 8 ), constituted by mixing powdered polymer of polymethylmethacrylate to the liquid monomer methylmethacrylate. An exothermic free-radical polymeri- sation reaction then creates solid bone cement. The effect of isotonic saline on the strength of bone cement during the polymerisation period Christopher J. Pearce 1 , Shelain Patel 2 , Shaun A. Sexton 3 , Alexander Reeves 4 , Hamidreza Khairandish 5 , Arshad Khaleel 6 1 Hillingdon Hospital and Mount Vernon Hospital, Middlesex - UK 2 University College Hospital, London - UK 3 Royal Cornwall Hospitals NHS Trust, Cornwall - UK 4 Alex Reeves Engineering Ltd, Gloucestershire - UK 5 Kettering General Hospital, Kettering, Northamptonshire - UK 6 Ashford and St Peter’s Hospitals NHS Trust - UK ABSTRACT: It is common practice to irrigate the wound in hip and knee replacement surgery during the working and setting phases of cement polymerisation. We sought to establish whether the addition of normal saline during the various stages of polymerisation would affect bone cement strength. Ce- ment specimens were moulded to the dimensions defined in ISO 5833: 2002 and tested in an electro- mechanical test machine with a calibrated 50 kN load cell at a compression rate of 24 mm/min. The results suggest that the compressive strength of bone cement is not affected by the presence of saline following the initiation of polymerisation. However, saline added to the mix prior to the initiation of polymerisation increased its porosity and significantly decreased its strength. This may explain why vacuum mixing is important as it removes water vapour. KEY WORDS: Polymethyl methacrylate, Irrigation, Saline solution, Compressive strength Accepted: February 15, 2011 ORIGINAL ARTICLE In addition to the primary monomer and polymer, propri- etary bone cement also contains butylmethacrylate, di- methyl-paratoluidine, hydroquinone and benzoylperoxide which are the binding agent, activator, inhibitor and initia- tor respectively (4). In addition, barium sulphate or zirco- nium dioxide used in bone cement preparations render it radio-opaque and thus allow it to be easily seen on plain radiographs (5). Another constituent is chlorophyll, a green dye which helps distinguish bone from cement intra-oper- atively, which is especially useful in revision surgery. The visco-elastic nature of bone cement has facilitated its use in total hip arthroplasty (THA). In vivo, it can be thought of as elastic interlayer between two stiff sheets, since its modulus of elasticity is lower than both the surrounding cortical bone and metal (6). Clinically this means that creep (continuous deformation under constant load) occurs; a property which has been exploited by collarless, highly