eprintid: 219
rev_number: 15
eprint_status: archive
userid: 7
dir: disk0/00/00/02/19
datestamp: 2009-05-05 14:51:39
lastmod: 2015-05-29 19:49:42
status_changed: 2009-05-05 14:51:39
type: report
metadata_visibility: show
item_issues_count: 0
creators_name: Hjorth, P.G.
creators_name: King, J.
creators_name: Korobeinikov, A.
creators_name: Mason, J.
creators_name: McKee, S.
creators_name: Wilson, S.
corp_creators: C. Birkinshaw
corp_creators: S. Flanagan
title: Stryker Osteonics: Prosthetic Knee Joint
ispublished: pub
subjects: materials
subjects: medicine
studygroups: esgi62
companyname: University of Limerick
full_text_status: public
abstract: We examine, within a simple bearing model of a knee joint that only consideres pure sliding, the effect of the presence of a small vertical hole in the load area on the fluid film properties. The calculations indicate that fluid is entrapped in such a hole, which, for constant load, causes a smaller minimal film separation of the two surfaces.
This will lower the horizontal friction, but may also bring about surface contact in high load situations.
problem_statement: The human knee is a hinge-type joint comprising two major bones, the femur and the tibia. The bone is end capped with a soft compliant substance cartilage, which acts as a low friction bearing. Lubrication is added by the cartilage synovial fluid, which ensures that the bearing works with full fluid film lubrication.
As the knee articulates, fluid is drawn through the joint space, ensuring that the two opposing cartilage surfaces do not come into contact. The load applied to the knee depends upon the activity, and can be as much as 8 times body weight. The cartilage also acts as a shock absorber, dampening the excessive loads applied and protecting the bones from shock. Synovial fluid is non-Newtonian, and when the joint is stationary, the fluid is almost gel-like. The cartilage is porous, and never dries out in the healthy knee.

In the diseased knee cartilage breaks down, and the articulation tends to a bone-on-bone bearing. This is painful, and can be operated on to replace the diseased joint with a prosthetic joint.
The material properties of the prosthetic joint do not simulate cartilage in general. However, a new type of material is being considered polyurethane (PU), which simulates cartilage more closely than the conventional materials used in joint replacement. The system being considered is a PU tibial bearing articulating against a metal femoral bearing.The PU is not porous, but can develop full fluid film lubrication if the conditions are correct. Many mathematical approaches have been adopted to explore the nature and tribology of lubrication between poroelastic surfaces. The entire lubrication process is complex and involves the simultaneous solution of the equations of fluid dynamics with the equations of Hertian contact mechanics.
In the joints manufactured, it is necessary to have a soft, compliant PU as the bearing material. Under load, this material is easily deformable, and as the load increases so the contact area increases, thereby lowering the contact stress. The soft PU bearing is supported by a harder PU backing.
The interface between the two materials is formed by fusion bonding, and is consequently an inherent weakness in the system. The weakness is most noticeable at the edges; when the material is loaded and the soft PU deforms, the displaced material exerts a shear force at the interface.

The Study Group was asked
(1) To investigate the inclusion of holes/channels in the loaded area of bearing to determine whether these features can: entrap 
uid and thus simulate a porous surface, and can release fluid when the bearing is articulated to maximise the development of a fluid film.
(2) To investigate if holes around the periphery of the bearing can reduce the shear stress at the interface.
date: 2008
citation:   Hjorth, P.G. and King, J. and Korobeinikov, A. and Mason, J. and McKee, S. and Wilson, S.  (2008) Stryker Osteonics: Prosthetic Knee Joint.  [Study Group Report]     
document_url: http://miis.maths.ox.ac.uk/miis/219/1/Stryker_Report.pdf