Reasons for Hip Replacement

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Reasons for Hip Replacement

• Osteoarthritis (OA)
• Trauma and post-traumatic arthritis
• Congenital deformities
• Bone tumors
• Avascular necrosis

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Before Modern Total Hip Arthroplasty

• To treat hip fracture
• Resect (remove) the remainder of the fractured
  femoral neck.
• Displace the greater trochanter distally.
• Place the femoral shaft into the acetabulum.
• Variations
• Place the femoral head on the end of the femoral
  shaft before inserting into the acetabulum.
• Interpose a portion of the abductor muscle to
  form an articulating surface
• Results were variable. (Steinberg and
  Steinberg, 2000)

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Before Modern Total Hip Arthroplasty

• Cup Arthroplasty
• Interposition of a cup between the the acetabulum
  and femur
• First cup made of glass brittleness led to
  frequent fracture
• Vitallium
• Double-cup arthroplasty
• One cup cemented over the femoral head
• One cup cemented into the acetabulum.

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Sir John Charnley

• The Father of Modern Total Hip Replacement
• Contributions
• Low-friction arthroplasty, where a metal head
  articulates against a plastic socket
• Introduced PMMA bone cement for fixation
• Introduced high-density polyethylene as a bearing
  surface
• Many advances in biomaterials, surface
  replacement, biomechanics, operating techniques
  and operating instruments
• Knighted in 1977 by Queen Elizabeth

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The Hip Joint
Femoral head
Femoral neck
Greater trochanter
Acetabulum
Acetabulum
Femoral shaft
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Osteoarthritis

• Radiographic evidence Narrowing of the joint
  space
• The joint space on an x-ray is cartilage
• Appears as a space because it is less dense than
  bone
• Progressive narrowing of the joint space means
  loss of cartilage on the articulating surfaces.

Singer, 2008
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Loading of the femoral component
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Shear
Bending
Compression
Torsion
Approximate region of maximum bending
Compression
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The HGP stem (courtesy of Zimmer)
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Bone cement

• Bone cement is a grout not a glue fixation is
  achieved by mechanical interlock rather than
  adhesion. (Learmonth et al., 2007)
• Two interfaces
• Bone-cement (the foundation for durable fixation)
• Cement-implant
• Improvements in bone cement application
• Pressurization forces cement into the bone
• Cleaning of the bone surface
• Retrograde insertion
• Mixed and cured in the operating room

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Pros Cons of Bone Cement

• Exothermic reaction can cause tissue damage
• Cement mantle can fracture and result in
  corrosion
• Allows for a milder transition in material
  properties since the modulus of elasticity is
  between that of the implant and bone
• Allows for early mobility after surgery
• Decreases bone resorption associated with reduced
  loading (Wolffs Law.)

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Uncemented THA

• Development driven by early failure of cemented
  stems
• Believed to be a result of cement disease
  (osteolysis)
• Fixation is via bony ingrowth (biologic fixation)
• Hydroxyapetite (HA) and tricalcium phosphate
  (TCP) coatings can encourage ingrowth
• Uncemented stems have been more succesful than
  uncemented acetabular cups.
• Preferred over cemented THA, despite similar
  success rates (Learmonth et al., 2007)

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Biologic Fixation

• Requires sufficient pore size in the coating
• Must restrict micromotion between the implant and
  the bone during progression of bone ingrowth
• Must have an initial good fit (not too much space
  between the implant and the bone that should grow
  into it.)
• Revisions are difficult. A lot of bone may be
  removed with the implant.

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Modularity Separate stem and head

• Advantages of Modularity

• Disadvantages of Modularity

• More room in the surgical field
• Surgeon performs final assembly
• Uses trial components to test sizes
• Can have a cobalt-chrome head (good wear
  resistance) and titanium alloy stem (better
  elastic modulus)

• Potential for disassembly
• Can have motion between components
• Fretting, corrosion
• Metal combination
• Potential for galvanic corrosion

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Material combinations Metal on PE

• Osteolysis initially attributed to cement
  particles
• Actually due to a local reaction to polyethylene
  particles.
• Annual wear of conventional polyethylene
  0.2mm/year
• Cross-linked polyethylene has much better wear
  resistance lt0.001mm/million cycles

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Material combinations Metal on Metal

• Initial introduction 1960s
• Many early failures
• Annual wear 0.004 mm/million cycles
• Can use a large femoral head diameter
• Reduces the risk of impingement and provides more
  stability
• Concern Release of metal ions
• Can be detected systematically
• No long-term adverse affects have been reported

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Ceramics

• Low co-efficient of friction
• Scratch-resistant
• No ion release
• Low wear lt0.001mm/million cycles
• Potential for brittle fracture

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TOTAL HIP ARTHROPLASTY
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Qaem,s General Hospital
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Abduction Pillow
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