Orthopaedic Bioengineering: Materials, Devices & Related Issues

Biomechanics�

• DaVinci (1452-1519): “Mechanical Science is the noblest and above all others the most useful, seeing that by means of it all animated bodieswhich have movements perform interactions …”
• Borelli (1608-1679): Founded “iatrophysics” - physics applied to human motion, wrote “De Motu Animalium” (On the Motion of Animals)
• Newton (1642-1727)
• Hooke (1635-1703)

Fig. 1 describes the conjunction of two

levers (or bones), IFS and HDR, at pivot

point C.

Fig. 2 shows how elastic bands

(muscles) attached externally to the

levers (at D and F) and to the pivot (B)

might bring the levers closer to each

other.

Fig. 3 shows the elastic bands attached

externally to the levers so that they can

be "expanded".

Fig. 4 is a sketch of a twin-lever system,

in which the levers are of unequal length.

Fig. 5&6 demonstrate the muscle and

bone configurations in two humans

carrying different loads.

Fig. 7&8 are studies of pulley

arrangements.

Fig. 9&10 demonstrate the actions of

muscles that enable a human to hold a

weight with an extended arm.

Mechanobiology�

• von Meyer, Culmann (1821-1881), Wolff (1836-1902)
• Roux (1850-1924) developed theories of cells, bone modeling, tissue differentiation and functional adaptation
• Thompson (1860-1948) “On Growth and Form”
• Pauwels (1895-1980) – tissue differentiation theory
• Carter – FEA applied to tissue differentiation theory
• Cowin – theory of adaptive elasticity
• Frost - mechanostat bone remodeling theory

Orthopaedics�

• Coined in 1741: ortho “to straighten” + paedeia “children”
• First known orthopod: Hippocrates (460-370 BC)
• Historically, nonsurgical treatment of: scoliosis, fractures, infections of bones and joints
• The branch of surgery that is concerned with the preservation and restoration of function of the skeleton, its articulations and associated structures

What is Orthopaedics?

Def’n: The surgical or manipulative treatment of disorders of the skeletal system (bones, cartilage & joints) & associated motor organs.

Synovial Joint Schematic

Types of Synovial Joints:

• Ball & Socket
• Hinge
• Saddle
• Gliding
• Pivot

Bone

• Major Characteristics:
• Hard, elastic & tough connective tissue
• Dynamic - Wolff’s Law
• Major Functions:
• Structural & protective components of internal organs
• Attachment sites for muscles

Cancellous

Cortical

Articular Cartilage

• Major Characteristics:
• Avascular, aneural, & alymphatic connective tissue
• Structurally heterogeneous
• Major Functions:
• Transfer & distribute forces between articulating bones
• Allow pain free mobility

Mechanical Properties of Natural Tissues

Tissue

Modulus of Elasticity (GPa)

Tensile

Strength

(MPa)

Compressive

Strength

(MPa)

Femur

17.2

121

167

Cervical

Vertebrae

0.23

3.1

10

Spongy

bone

0.09

1.2

1.9

Articular

Cartilage

0.016

15

What problems require joint�replacement?�

• Arthritic pain
• Cartilage damage
• Tumors
• Fractures

Clinical Arthritis & Its Treatment

• Characteristics:
• Loss of capacity for ease of articulation
• Loss of joint stability
• Pain!

​

• Current Treatment Methods:
• Oral Medication
• Intra-articular injections
• Biomechanical adjustments
• Arthroscopy
• Total Joint Replacement!!!

​

Transplantation�

Transplantation�

Transplantation�

What joints can be replaced with prosthetics?�

• Fingers
• Wrists
• Elbows
• Shoulders
• Temporalmandibular
• Spine
• Hips
• Knees
• Ankles
• Toes

Orthopaedic Surgical Procedures�

• 1827 Barton (UK) first subtrochanteric osteotomy to create a pseudoarthosis at the hip
• 1880- bone plates, screws and nails
• 1894 Gluck ivory femoral head replacement
• 1905 bone and ivory inserts for fracture stabilization
• 1938 Wiles metal-on-metal hip replacement
• 1946 Judet (F) acrylic head femoral head replacement
• 1956 McKee (UK) metal-on-metal femoral head
• 1963 Charnley (UK) classic total hip replacement design
• 1965 Mueller (CH) classic total hip replacement design

Judet Hip�

• Hemiarthroplasty
• Acrylic
• First implant: 1946

Orthopedic Implants�

• Hips 250,000/a in USA
• TJR 500,000/a in USA
• Hips 800,000/a in NA+EU
• Hips and Knees: 1.2 million/a Worldwide
• Ankles: <50,000 in USA

​

What are the goals of TJR?�

• Improve quality of life
• Reduce pain
• Restore function
• Movement
• Strength
• Last throughout life

How successful are prosthetic�joints?�

• Relieve pain
• Restore function
• Most (>90%) last for >10 years

What artificial biomaterials are most commonly used?�

• Metals
• Stainless Steel
• Cobalt Chrome
• Titanium Alloy
• Ceramics – Aluminum oxide
• Polymers
• Bone cement (PMMA)
• UHMWPE (polyethylene)

Properties of Orthopaedic Biomaterials

Material

Modulus of Elasticity (GPa)

Tensile

Strength

(MPa)

Stainless Steel

190

480

CoCrMo

200

650

Ti6Al4V

110

860

Cortical Bone

10-20

100-200

UHMWPE

2.2

30

Cancellous Bone

10-20

0.2-0.5

What are key design criteria for artificial biomaterials?�

• Restore mechanical function
• Biocompatibility
• Chemically Stable

Ceramics�

• Issues
• Hard and biocompatible but brittle

Polymers�

• Bone cement (PMMA)
• Exothermic
• Toxic effects of monomer
• Shrinkage with polymerization
• Grout, not adhesive
• UHMWPE (polyethylene)
• Lipid absorption
• Wear particles
• Low coefficient of friction and creep resistant

Polymers in Orthopedics�

• Total Hip and Knee Replacements:
• 1953 acrylic cement, polymethyl methacrylate PMMA
• 1958 Polytetrafluorethylene PTFE
• 1961 High Density Polyethylene HDPE
• 1962 UHMWPE, γ (25-40 kGy) irradiated in air
• 1970s Carbon Fiber Reinforced UHMWPE
• 1980s UHMWPE, γ irradiated in ethylene oxide
• 1980s Silane, Cross-Linked HDPE
• 1986 Hylamer, Extended Chain Recrystalized UHMWPE
• 1998 Highly Cross-Linked (50-105 kGy) and thermally treated (110-150°C) PE

Polymers in Orthopedics�

• Swanson Finger
• Wright Medical Technology
• Silicone and Titanium
• First implant: 1969

Polymers in Orthopedics�

• Isoelastic Robert Mathys Hip
• Synthes, Bettlach, CH
• Polyacetal resin with metal core
• First Implant 1973

What are overall design requirements for prosthetic joints?�

• Geometric requirements for fit and stability
• Geometric requirements for function
• Ease and reliability of implantation
• Minimize bone loss
• Minimize trauma to surrounding tissue
• Minimize changes in bone loading
• Durability
• Cost

TJR Design Process

• Geometry
• Strength
• Modularity
• Kinematics
• Wear
• Stiffness
• Fixation

Design Analysis�

• Geometry

• Strength

• Modularity

• Kinematics

• Wear

• Stiffness

• Fixation

Design Analysis�

• Geometry

• Strength

• Modularity

• Kinematics

• Wear

• Stiffness

• Fixation

Design Analysis�

• Geometry

• Strength

• Modularity

• Kinematics

• Wear

• Stiffness

• Fixation

Design Analysis�

• Geometry

• Strength

• Modularity

• Kinematics

• Wear

• Stiffness

• Fixation

Design Analysis�

• Geometry

• Strength

• Modularity

• Kinematics

• Wear

• Stiffness

• Fixation

Design Analysis�

• Geometry

• Strength

• Modularity

• Kinematics

• Wear

• Stiffness

• Fixation

Design Analysis�

• Geometry

• Strength

• Modularity

• Kinematics

• Wear

• Stiffness

• Fixation

Design Analysis�

• Geometry

• Strength

• Modularity

• Kinematics

• Wear

• Stiffness

• Fixation

Design Analysis�

• Geometry

• Strength

• Modularity

• Kinematics

• Wear

• Stiffness

• Fixation

Design Analysis�

• Geometry

• Strength

• Modularity

• Kinematics

• Wear

• Stiffness

• Fixation

Factors Contributing to Wear

Dumbleton, 1991

Results of Particulate Wear Debris

Wear Debris

Osteolysis

Loosening

REVISION

LaBerge et al., 1992

Shanbhag et al., 1995

What is Tribology?

“The science of interacting surfaces in relative motion including its related practices”

Dept. of Education & Science, 1966

Wear Mechanisms

Design Analysis�

• Geometry

• Strength

• Modularity

• Kinematics

• Wear

• Stiffness

• Fixation

Design Analysis�

• Stiffness

​

Design Analysis�

• Geometry

• Strength

• Modularity

• Kinematics

• Wear

• Stiffness

• Fixation

What are the main problems and�limitations?�

• Loosening
• Wear debris
• Infection
• Bone loss
• Finite life (wear out faster in larger and/or active patient)
• Revisions are less successful and do not last as long
• Young patients would need many over normal life span

Engineering Considerations for THR

• Restrictions to Motion:
• Ratio of ball diameter/neck diameter
• Neck length
• Orientation of components in bone
• Stresses on femoral stem:
• E ratio
• ↓ force on femoral head
• Wear & Friction:
• Surface finish
• Tolerance (manufacturing, choice of components)

Major Problems Associated with THR

• Loosening
• Wear of UHMWPE
• Stem fracture
• Protrusion/Dislocation
• Fatigue fracture of cement mantle
• Infection

“Tools” of the Trade

• Material Parameters:
• Chemistry, Composition
• Operational Parameters:
• Load, Motion, Temperature, Duration
• Tribocontact Conditions:
• Contact mechanics, Lubrication mode, Surface topography
• Mechanical Parameters:
• Modulus, Hardness, etc.
• Friction & Wear Parameters:
• Friction coefficient, Wear factor, Wear mechanism

Tribological Assessment

Experimental vs Clinical Data

Influential Parameters:

• Loading & Loading Pattern
• Lubricant/Lubrication
• Fixation
• Positioning
• Temperature & Time

What future improvements can�come from BME?�

• Tissue engineering to regenerate or heal pathological tissues
• Improvements in biomaterials (osteoconductive and/or osteoinductive, wear, load-sharing stiffness)
• Improvements in surgical instrumentation
• Improvements in: geometry, stiffness, strength, kinematics, wear, fixation

Classification of Fractures

• Transverse (tension/bending force)
• Spiral (torsional force)
• Comminuted
• Greenstick (common in children)
• Oblique (axial compressive force)

Fracture Fixation

• Goal:
• Mend fractured or diseased bone
• Mechanism:
• Casting
• Internal
• External