To the Editor:
In the paper by Walter et al. on the subject of “squeaking hips”(1), the authors describe the subject as “poorly understood”. I would disagree with that assessment and point to the important and relevant history of investigations on the subject of joint lubrication.
It was a squeaky Judet prosthesis (an acrylic hemiarthroplasty articulating against bone introduced in 1946) that inspired the late Sir John Charnley to investigate the failure of these prostheses. MacConnail, a professor of anatomy at Cork University had postulated that synovial joints were lubricated hydro-dynamically (by a fluid film). Up to the 1980s, mechanical engineers held to this belief. Hydrodynamic lubrication works well in engine bearings, where there is a contained lubricant, and continuous high-speed rotation. A wedge of fluid lubricant is created by the rotating axel, which can support a heavy load – as soon as the rotation stops, the wedge of fluid collapses, and the opposing surfaces come into to direct contact.
Charnley’s research convinced him that this mechanism could not work in synovial joints where the movement is slow, intermittent and oscillating, and where “stick” does not occur. Charnley concluded that joint lubrication had to be by a boundary mechanism, but was unable to identify the lubricant. This was why Charnley experimented with polytetrafluorethylene, and later used a stainless steel on PE implants.
The late Dr Brian Hills, an Australian paediatric respirologist with a Cambridge degree in physical chemistry took a new approach. He proposed that surfactants, and in particular surface-active- phospholipids, bonded electrostatically to mesothelial surfaces and were the universal lubricating system in the body. Hills published multiple papers on this subject over almost 20 years (2-9).
When hard bearing surfaces move in relationship to each other lubrication may become a problem, e.g. chalk on a blackboard, the metal on metal Birmingham hip, or ceramic on ceramic. A “stick-slip” phenomenon causes vibrations, which may be audible as a squeak. It is odd that orthopaedic surgeons since Sir John have paid so little attention to joint lubrication.
The author did not receive any outside funding or grants in support of his research for or preparation of this work. Neither he nor a member of his immediate family received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, division, center, clinical practice, or other charitable or nonprofit organization with which the author, or a member of his immediate family, is affiliated or associated.
References
1.William L. Walter, Tim S. Waters, Mark Gillies, Shane Donohoo, Steven M. Kurtz, Amar S. Ranawat, William J. Hozack, and Michael A. Tuke
Squeaking Hips
J Bone Joint Surg Am 2008; 90: 102-11
2. Hills BA, Butler BD. Surfactants identified in synovial fluid and their ability to act as boundary lubricants. Annals Rheumat Dis. 1984; 43:641-648.
3. Hills BA. Oligolamellar nature of articular surface. J Rheumatol. 1990;17:349-355.
4. Hills BA. Synovial surfactant and the hydrophobic articular surface. J Rheumatol. 1996;23(Editorial):1323-5.
5. Hills BA, Monds MK. Enzymatic identification of the load bearing boundary lubricant in the joint. Br J Rheumatol. 1998;37:137-142.
6. Hills BA, Monds MK. Deficiency of lubricating surfactant lining the articular surfaces of replaced hips and knees. Br J Rheumatol. 1998;37:143-147.
7. Hills BA. Boundary lubrication in vivo. Proc Instn Mech Engrs. 2000;214H:83-94.
8. Purbach B, Hills BA, Wroblewski BM. Surface-active phospholipid in total hip arthroplasty. Clin Orthop Related Res. 2002;396:115-8.
9. Hills BA, Crawford RW. Normal and prosthetic synovial joints are lubricated by surface-active phospholipid. A hypothesis. J Arthroplasty. 2003;18:499-505.I
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