Artificial Hip Implants 2011

Transcription

1 A Novel Elastic Squeeze Film Total Hip Replacement Stephen Boedo Department of Mechanical Engineering Rochester Institute of Technology Rochester, NY John F. Booker Sibley School of Mechanical and Aerospace Engineering Cornell University Ithaca, NY

2 Martini and Bartholomew (2000))

3 Types of Artificial Hip Joints R 1 = mm R 1 = 25 mm (THR) (HRR) Mattei (2011)

4 Market Trends Total revenue generated by sales of hip implants in the United States in 2011 was approximately $2.8 billion, representing a 3.5 percent increase over Revenue in the U.S. hip implants market is projected to grow at a compound annual growth rate (CAGR) of 3.9 percent from 2011 to 2016, reaching $3.3 billion in In 2011, the percentage revenue contributions of different hip implant segments were: primary hip implants 56.3 percent; partial hip implants 32.2 percent; revision hip implants 11.5 percent. Competitors generating the highest revenue in the U.S. hip implants market in 2011 were Zimmer, DePuy, and Stryker. Together, they contributed 67.6 percent of the total market revenue. Frost and Sullivan (2012)

5 Market Trends Procedures in the U.S. hip implants market are projected to grow at a CAGR of 2.7 percent from 2011 to 2016, reaching 537,423 in Technology advancement and an aging population are two main contributors to the overall market growth. Price is a major competitive factor for hip implant manufacturers. Prices will remain relatively stable from 2011 to As all market participants offer similar product lines in terms of technology and categories, it is important for companies to focus on new product launches and product differentiation to increase their market share. Frost and Sullivan (2012)

6 Market Trends The FDA received about 11,000 reports of defective hip failures by September 2011 some independent studies showed that metal-on-metal hips failed three times more than other hips. Metal-on-metal artificial hips include models manufactured by DePuy (Johnson & Johnson), Biomet, Stryker, and Zimmer. Though these numbers may seem gloomy for manufacturers, metal-onmetal hips will regain credibility once companies are able to show data that illustrate the advantages of metal-on-metal designs. Increased advances in biomaterials will encourage smaller companies to compete in niche sections of the market Frost and Sullivan (2012)

7 Metal-on-Plastic THR

8 Osteolysis (Metal-on-Plastic)

9 Osteolysis (Metal-on-Plastic) Abu-Amer et al. (2007)

10 Metallic Wear Particles from Metal-on-Metal THR 100 nm Firkins et al. (2001)

11 Ceramic-on-Ceramic THR Issues New York Times (2008)

12 Why is wear an issue in THR? Hip joint kinematics and load history inadequate mechanisms driving lubrication Gait cycle load does not reverse direction squeeze film action absent Gait cycle ball angular velocity is low wedge film action limited Spherical joint geometry Spherical ball and cup point contact vs. line contact (journal bearings) Large radial clearance

13 MoM Lubrication Analysis - Uniform Clearance Study 14 mm ball/cup Radial clearance = 30 μm (uniform) Viscosity = mpa-s Min film thickness = nm Max film pressure = MPa Wang and Jin (2008) Cup inclination angle unimportant

14 MoM Lubrication Analysis Nonuniform Clearance Study 14 mm ball Nominal radial clearance = 30 μm Peak ellipticity = 6 μm Viscosity = mpa-s Min film thickness = 10 nm (spherical cup and ball) = 15 nm (best case) Max film pressure = 55 MPa (spherical cup and ball) = 45 MPa (best case) Wang et al. (2009)

15 MoM Lubrication Analysis Nonuniform Clearance Study Alpharabola MoM Hip Joint R 1 = 14 mm Fitted bearing (zero clearance) Viscosity = 2 mpa-s Max film pressure = 55 MPa Min film thickness = 60 nm (best case) Meng et al. (2011)

16 Ball-on-Plate Testing with Bovine Serum 40 nm protein deposition layer on ceramic ball surface same order as calculated minimum film thickness values! hydrocarbon oil bovine serum film thickness measurements suggests a completely different lubrication mechanism for current artificial hip joints Myant et al. (2012)

17 We propose a new artificial hip joint design that: Enhances film thickness (well above protein boundary layer) Allows for larger design clearances Employs rigid surface assumptions in the design process (independent of material, elastic effects as a bonus)

18 start of stance phase Consider a hypothetical mechanical spring inserted between ball and cup spring load magnitude is on the order of the swing phase load ball and cup separated at start of stance phase synovial fluid external load

19 Stance Phase most of external load carried by squeeze film action of lubricant external load greater than spring load normal approach of ball and cup Design does not rely on wedge film action external load stance phase progresses

20 End of Stance Phase all of external load carried by spring External load equal to spring load end of normal approach of ball and cup external load end of stance phase

21 Swing Phase Spring load greater than external load end of normal separation of ball and cup cavitation of synovial fluid swing phase progresses

22 Swing Phase Spring load greater than external load normal separation of ball and cup cavitation region collapse (refilling) swing phase progresses

23 End of Swing Phase Spring load greater than external load normal separation of ball and cup ends complete film reformation

24 shell ball cup Current MoM hip joint

25 Proposed New Artificial Hip Joint

26 Cup Design Features (spring elements)

27 { Z Ellipticity and Nominal Clearance Definitions n Z cup δ ϴ r 2 r 2 h e R 1 Y α R 2 Y ball C = r 2 - R 1 h = C - e n h = R 2 - R 1 + δ cos 2 θ - e n r 2 = R 2 + δ cos 2 θ h = C 0 + δ cos 2 θ - e n Nominal clearance Ellipticity

28 Z ISO Duty Cycle F Z e Y ω Y

29 Effect of Ellipticity on Minimum Film Thickness History (Stance Phase) R 1 = 16 mm, C 0 = 30 μm Initially concentric ball and cup conventional designs

30 Effect of Ellipticity on Maximum Film Pressure History (Stance Phase) R 1 = 16 mm, C 0 = 30 μm conventional designs

31 Film Pressure Distribution at Peak Load C 0 = 30 μm δ = 30 μm

32 Film Pressure Distribution at Peak Load C 0 = 30 μm δ = 40 μm

33 Film Pressure Distribution at Peak Load C 0 = 30 μm δ = 50 μm

34 Film Pressure Distribution at Peak Load C 0 = 30 μm δ = 50 μm

35 Cup Optimization Study R 1 = 16 mm Minimum film thickness (nm) conventional designs

36 Cup Optimization Study R 1 = 16 mm Maximum film pressure (MPa) conventional designs

37 Cup Optimization Study R 1 = 25 mm (HRR)

38 Cup Optimization Study R 1 = 25 mm (HRR)

39 Conclusions A novel design approach for artificial hip joints exploits squeeze-film action to yield substantially thicker lubricant films and smaller lubricant film pressures compared with conventional designs. Optimal squeeze-film bearing performance during the stance-phase portion of the gait cycle is accomplished though ellipsoidal cup geometry with ellipticity specifications which result in line contact in the limit of ball-cup relative motion along the load line. Low squeeze-film pressures and large film thicknesses produced in the optimal cup designs should not result in significant elastic deformation of the cup regardless of material choice. Thus, a UHMWPE cup with either a metal or ceramic ball is a plausible material combination for the proposed design. Low squeeze-film pressures and large film thickness are predicted assuming rigid ball and cup surfaces; effects of elasticity of the rigid cup surface should yield even thicker films and lower film pressures.