Artificial Hip Implants 2011

A Novel Elastic Squeeze Film Total Hip Replacement

Stephen Boedo

Department of Mechanical Engineering

Rochester Institute of Technology

Rochester, NY 14623

[email protected]

John F. Booker

Sibley School of Mechanical and Aerospace Engineering

Cornell University

Ithaca, NY 14853

[email protected]

(THR) (HRR) Mattei (2011)

R₁ = 14 – 16 mm R₁ = 25 mm

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

2010.

•

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 2016.

•

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.

•

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 2016.

•

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 2016.

•

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)

Frost and Sullivan (2012)

•

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-on-metal 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

Osteolysis (Metal-on-Plastic)

Abu-Amer et al. (2007)

Firkins et al. (2001)

Metallic Wear Particles from Metal-on-Metal THR

Ceramic-on-Ceramic THR Issues

Why is wear an issue in THR?

•

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

Hip joint kinematics and load history –

14 mm ball/cup

Radial clearance = 30 μm (uniform)

Viscosity = 1 – 2.5 mPa-s

Wang and Jin (2008)

Min film thickness = 15-25 nm Max film pressure = 55-60 MPa

Cup inclination angle unimportant

14 mm ball

Nominal radial clearance = 30 μm Peak ellipticity = 6 μm

Viscosity = 1 – 2.5 mPa-s

Wang et al. (2009)

MoM Lubrication Analysis – Nonuniform Clearance Study

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)

“Alpharabola” MoM Hip Joint

R₁ = 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)

40 nm protein deposition layer on ceramic ball surface

same order as calculated minimum film thickness values!

suggests a completely different lubrication mechanism for current artificial hip joints

Ball-on-Plate Testing with Bovine Serum

hydrocarbon oil bovine serum

Myant et al. (2012) film thickness measurements

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

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

start of stance phase

Stance Phase

external load greater than spring load

normal approach of ball and cup

external load

most of external load carried by squeeze film action of lubricant

stance phase progresses

Design does not rely on wedge film action

End of Stance

Phase

External load equal to spring load

end of normal approach of ball and cup

external load

end of stance phase

all of external load carried by spring

swing phase progresses

Swing Phase

Spring load greater than external load

end of normal separation of ball and cup

cavitation of synovial fluid

Swing Phase

Spring load greater than external load

normal separation of ball and cup swing phase progresses cavitation region collapse (refilling)

End of Swing

Phase

Spring load greater than external load

normal separation of ball and

cup ends complete film

Current

MoM

hip joint

ball

cup

shell

Cup Design Features

R

r

Y

Z

δ

ϴ

α

r

Y

Z

R

e

r

= R

+ δ cos

_θ

C = r

- R

h = C

-

e

•

n

n

h

h = R

- R

+ δ cos

_θ

_-

_e

_•

_n

h =

C

+

{

δ

cos

_θ

_-

_e

_•

_n

Ellipticity and

Nominal Clearance

Definitions

cup

ball

Y

Z

e

F

ω

Initially

concentric

ball and cup

Effect of Ellipticity on Minimum Film Thickness History

(Stance Phase) R

= 16 mm, C

= 30 μm

Effect of Ellipticity on Maximum Film Pressure History

(Stance Phase) R

= 16 mm, C

= 30 μm

C

= 30 μm

δ = 30 μm

C

= 30 μm

δ = 40 μm

C

= 30 μm

δ = 50 μm

C

= 30 μm

δ = 50 μm

Cup Optimization Study R

₁

= 16 mm

conventional designs

conventional designs

Maximum film pressure (MPa)

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.