Is SAPL the boundary lubricant in prosthetic joints: friction testing and surface rinsing

Abstract

The purpose of this study was to test whether surface-active phospholipid (SAPL) has the potential to decrease friction in the prosthetic joint by acting as a boundary lubricant. The specimens used were ultra high molecular weight polyethylene (UHMWPE) pins and highly polished (surface roughness of 0.01 to 0.02 μm) stainless steel plates (materials commonly used in prosthetic joints). A Hounsfield test rig was used to perform two sets of experiments. The first of these experiments used equine synovial fluid as the lubricant. It was found that the coefficient of friction (μ) was quite high, between 0.12 and 0.18. The second experiment was comparative testing using synthetic dipalmitoyl phosphatidylcholine (DPPC), calcium chloride (CaCl2), saline, and hyaluronic acid (HA) as the lubricants. It was found that when DPPC was used the friction was halved (μ changed from 0.12 to 0.05), suggesting that DPPC acts as an effective boundary lubricant. Reciprocating tests using a purpose built apparatus were also performed using DPPC and saline in order to assess the friction over an extended period of time. It was observed that the friction obtained was comparable to the Hounsfield friction tests using DPPC (coefficient of friction in the range of 0.05 to 0.07). Surface rinsings were performed on the specimen after soaking in equine synovial fluid to determine what had absorbed to the surfaces and the quantity that had absorbed. Only small amounts of DPPC and sphingomyelin, also a phospholipid, were found, due to the lack of SAPL in the synovial fluid, suggesting why such high values of friction prevailed in the first Hounsfield experiment. The above experiments strongly suggest that SAPL has the ability to decrease friction within the prosthetic joint by acting as a boundary lubricant.

Keywords:

• synovial fluid
• surfactant
• phospholipid
• friction testing
• surface rinsing

Additional information

Notes on contributors

R J Coller

Prof Brian A Hills [email protected] Professor Hills has a particularly interdisciplinary background, graduating in Chemistry at Cambridge, Engineering at London and Physiology (Ph.D. and D.Sc.) at Adelaide. His field of research is surface-active phospholipid and the many desirable properties which it can impart to a surface in vivo, ranging from a lubricant in the joints to an acid-barrier in the stomach and a release agent at many sites. Ten applications are currently undergoing clinical evaluation for their commercial potential. Prof. Hills has published four books and 350 papers, receiving his Sc.D. from Cambridge University. He is now Laboratory Head of the Golden Casket Paediatric Research Laboratory at the Mater Medical Research Institute and Honorary Professor in the Department of Medicine, University of Queensland.

D J Hargreaves

Prof Ross Crawford [email protected] Professor Crawford is the newly appointed Professor of Orthopaedic Research in the School of Mechanical, Manufacturing and Medical Engineering. He trained in medicine at the University of Queensland and then subsequently in orthopaedic surgery gaining Fellowship of the Australian College of Surgeons in 1995. Subsequently he has pursued a research career whilst maintaining an active clinical practice. He obtained his DPhil at Oxford University, UK, in failure of hip joint replacements. Professor Crawford has over twenty international publications relating to orthopaedic practice and implants. His research interests are in the clinical outcomes of joint replacement surgery, issues concerning the integration of implants and the reconstruction of damaged joints and musculo-skeletal structures.

B A Hills

Rebecca Coller [email protected] Rebecca Coller has a Bachelor of Medical Engineering from Queensland University of Technology (QUT). The study presented in this paper was based on a project undertaken while studying at QUT, the aim of which was to investigate the role of surface active phospholipid in artificial joint lubrication. Rebecca has also started a PhD focusing on the change in localised structural and biomechanical properties surrounding different classifications of osteoarthritis in the knee joint.

R Crawford

Assoc Prof Doug Hargreaves [email protected] Assoc Prof Hargreaves has a BEng (Mech) from Qld Institute of Technology, a MSc and a PhD in tribology from the University of Leeds UK. He is currently Acting Head of School of Mechanical, Manufacturing and Medical Engineering at Queensland University of Technology (QUT), Australia and is Program Leader for the Asset Maintenance and Management Research Group. He has been teaching at QUT for the past 15 years. His research interests includes the performance of environment friendly lubricants as well as engineering education. He has published extensively and consulted widely, both nationally and internationally. Assoc Prof Hargreaves was the President of the Queensland Division of the Instituion of Engineers, Australia in 2000.