Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Implantable prosthesis – Bone
Reexamination Certificate
1998-05-01
2001-04-24
Milano, Michael J. (Department: 3738)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Implantable prosthesis
Bone
Reexamination Certificate
active
06221108
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a process for improving start up and steady rate friction of soft/compliant polyurethanes and to polyurethane elements which have been treated by the process. An important potential application for such treated material is in compliant layer bearings, for example, those used in artificial joint replacements, although it also has other applications.
2. Description of the Prior Art
Typically these artificial joint replacement bearings are relatively recent and rely on the use of soft/compliant polyurethanes (or other elastomeric materials) to improve the lubrication mechanism compared with convention artificial joints. The concept of using a compliant polyurethane layer on the bearing surface of an artificial joint replacement is inspired by the natural synovial joint which has such a compliant layer, the articular cartilage. A combination of lubrication mechanisms have been proposed for synovial joints which result in fluid film lubrication, where the joint surfaces are completely separated by a thin film of lubricant. Currently most artificial joints are based upon metal on ultra high molecular weight polyethylene (UHMWPE), ceramic on UHMWPE or metal on metal material couples. These bearings are far less compliant than the natural joint and hence operate in a mixed lubrication regime, with partial contact of the two bearing surfaces. This leads to higher friction, and wear of the bearing surfaces of conventional joints. In contrast, compliant bearings operate with fluid film lubrication and extremely low friction, and hence potentially negligible wear and a long implant life.
The design parameters for the construction of compliant layer bearings are known. Polyurethane elastomers have been the materials of choice for these bearings. Under conditions of cyclic loading and motion typical of the major load bearing joints in the human body there is much experimental evidence that this type of bearing will operate with extremely low friction, typical of fluid film lubrication. In contrast, Caravia et al (L. Caravia, D. Dowson and Fisher J., Start up and steady rate friction of thin polyurethane layers. Wear, (1993), 160, 191-197) cites that the use of compliant polyurethane layers in this application can result in unacceptably high friction values under conditions which combine heavy loading and low sliding velocities, i.e., at the onset of motion (start up friction).
SUMMARY OF THE INVENTION
The purpose of this invention is to address the important area of improving start up friction while maintaining effective fluid film lubrication during normal cyclic loading and motion.
The following definitions are used herein:
Ringers Solution: This is a material which comprises a sterile solution of sodium chloride, potassium chloride and calcium chloride in water. It contains in each 100 ml. not less than 323.0 mg and not more than 354.0 mg of sodium, not less than 14.9 mg and not more than 16.5 mg of potassium, not less than 8.20 mg and not more than 9.80 mg of calcium; and not less than 523.0 mg and not more than 580.0 mg of chloride.
Coefficient of friction (&mgr;): The ratio of tangential frictional force to the normal load for a plane surface.
Friction factor (f): The ratio of the product of frictional torque (T) and cup radius to the normal load for the cupped geometry considered in this study, during steady state motion.
Frictional torque (T): The torque required to resist rotation of the complaint layered cup about an axis perpendicular to the axis of loading, under the normal load and motion conditions.
Start up friction factor (f
s
): Friction factor (f) at the onset of motion.
According to the present invention a process for improving start up and steady rate friction in an aqueous lubricant of soft/compliant polyurethanes includes treating a polyurethane element in Ringers solution, an aqueous solution of phosphate buffered saline or de-ionized water.
Preferably the process includes treating the element in a heated bath containing the respective aqueous solution at a constant temperature between 30° C. and 65° C., for example, good results have been obtained by treating the element at a temperature of 37° C. for 96 hours.
The invention also includes a polyurethane element which has been treated by the process set forth. The element can be or form part of a surgical or medical device which contacts body tissue and fluids, for example a prosthetic device, a stent, a catheter or an angio plastic balloon. The devices referred to above all operate with an aqueous lubricant provided by the synovial fluid in the human body.
As mentioned above, material treated by the process can be used as a bearing surface and thus a prosthetic device according to the invention can have a bearing surface at least part of which is formed from a polyurethane element which has been treated by the method set forth above.
The prosthetic device can have a first bearing surface formed from the treated material and a second cooperating surface formed from for example, metal or ceramic. The metal can be cobalt chrome steel, or similar metal or alloy used in implantable medical devices, and it can be provided with a diamond-like carbon (dlc) coating.
A diamond-like carbon coated metal prosthesis is disclosed in U.S. Pat. No. 5,593,452, the teachings of which are incorporated herein by reference.
When applied to, for example, a hip joint, the first treated surface can be provided on an acetabular cup and the second cooperating surface can be the cooperating ball head of the implant.
These and other objects and advantages of the present invention will become apparent from the following description of the accompanying drawings, which disclose several embodiments of the invention. It is to be understood that the drawings are to be used for the purposes of illustration only and not as a definition of the invention.
REFERENCES:
patent: 4888024 (1989-12-01), Powlan
patent: 5047054 (1991-09-01), Vijayan et al.
patent: 5067964 (1991-11-01), Richmond et al.
patent: 5593452 (1997-01-01), Higham et al.
patent: 5759205 (1998-07-01), Valentini
patent: 5879404 (1999-03-01), Bateman et al.
J. Blamey et al.: “Soft Layered Prosthesis For Arthritic Hip Joints: A Study of Materials Degradation”. Journal of Biomedical Engineering, vol. 13, No. 1, Jan., 1991 (1991-01), pp. 180-184, XP000863669.*
F. Quigley et al.: “The Dynamic Mechanical Behaviour of Selected Medical Grade Polyurethanes” Key Engineering Materials, vol. 118-119, pp. 313-320. XP000863670 CH. The whole document.
J. Blamey et al.: “Soft Layered Prosthesis For Arthritic Hip Joints: A Study of Materials Degradation” Journal of Biomedical Engineering, vol. 13, No. 1, Jan., 1991 (1991-01), pp. 180-184, XP000863669. The whole document.
Habal, Muntaz B. et al.: New Finger Joint 1-6 Implantable Prosthesis in an Ex-vivo Model: Biostereometric Studies Polymeric Materials Science and Engineering, vol. 53, 1985, pp. 775-777, XP0000863672. The whole document.
McCarthy S J et al.: “In-vivo degradation of polyurethanes: transmission—FTIR microscopic characterization of polyurethanes sectioned by cryomicrotomy” Biomaterials, GB Elsevier Science Publishers BV., Barking, vol. 18, No. 21, pp. 1387-1409. XP00402619, ISSN: 0142-9612. Abstract.
Howmedica International Inc.
Lerner David Littenberg Krumholz & Mentlik LLP
Milano Michael J.
Phan Hieu
LandOfFree
Process for improving the friction rate of soft/compliant... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Process for improving the friction rate of soft/compliant..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Process for improving the friction rate of soft/compliant... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2471116