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Flashcards in THR Deck (23):

What do biomechanics of TH arthroplasty depend on ?

  • Prothesis design
  • bearing surface and lubrication characteristics
  • fixation method


What are the designs include?

  • Femoral componenent
    • cemented
    • uncemented
      • tapered stem
      • extensively porous coated stem vs proximal
      • modular stem
  • Acetabular 
    • cemented
      • polyethylene
      • metal
    • Press- fit ( uncemented)
      • metal
  • Bearing surfaces
    • polyethylene
    • metal 
    • ceramic

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Describe the different types of press fit stems?

  • Rely on bioloigical fixation
  • compression hoop stresses provide initial stability
  • types
    • tapered stem
      • most proximally porous coated that taper distally- see pic- Tri-lock depuy
    • extensively coated stem
      • porous coating extends into  the diaphysis for distal engagement
    • Modular stems
      • distal stems and proximal body can be mixed-matched

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Describe some complications of press fit stems?

  • Intra-op fx
    • > in press fit stems
    • due to underreaming
  • Loosening
    • high loosening rate when used in irradiated bone ( due to lack of ingrowth)
  • Junctional corrosion
    • seen in modular stems

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Describe the types of cemented femoral stem?

  • Rely on cement fixation
    • cement is a grout that provides initial and long term stability
    • limited remodelling potential
    • preferred for irradiated bone due to bone's limited ability for ingrowth
  • composite
    • Colbat-chrome
      • most common
      • reduces cement stresses
    • ​Ti- may be prone to micromotion & debonding


What is the unqiue complications of cemented femoral stems?

  • Stem breakage
    • cemented stems are smaller than press fit stems  so unable to tolerate as much cantilever bending

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What are the benefits and disadv of metal on PE bearing surface?

  • metal colbat chrome femoral head on polyethylene acetabular liner
  • ADV
    • longest track record of bearing surfaces
    • lowest cost
    • most modularity
  • DIS
    • higher wear and osteolysis cf MoM & ceramics
    • Smaller head cf MoM-> > risk of impingement


What are the benefits/disadvantages of metal on metal bearing surface?

  • Adv
    • better wear properties than Metal on PE
    • lower linear wear rate
    • decreased volume of particles
    • greater head allows for increased rom before impingement
  • Dis
    • more expensive than Metal onPE
    • Increased metal ions in serum and urine 5-10x
      • serum levels highest at 12-24 months
      • correlates with initial wear in or run phase of increased particle generation but then followed by steady state phase of decreased particle generation
    • no proven cancer risk
    • May form pseudotumours

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What are the CI for MOM implants?

  • Pregnant women or child bearing age
  • Renal disease
  • Metal hypersensitivity due to metal ions


What are the benefits/disadv of ceramic on ceramic bearing surface?


  • Best wear properties of all bearing surfaces
  • Lowest coefficient of friction of all bearing surfaces
  • inert particles
    • no concerns for cancer risk


  • More expensive than M on PE
  • worse mechanical properties- Brittle
  • squeaking
    • increased risk with
    • edge loading
    • impingement and acetabular malposition
    • third body wear
    • loss of fluid film lubrication
    • thin, flexible titanium stem
  • Less modularity with fewer neck length options
  • Stripe wear


What is stripe wear seen in ceramic on ceramic bearing surfaces?

  • Caused by contact between the femoral head and rim of cup during partial subluxation
  • results in crescent shaped line on femoral head

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Is Titanium on PE a good bearing surface?

  • No rarely used due to high wear rates


Describe the placement of acetabular screws in THR?

  • 4 quadrants
  • draw line from ASIS to centre of acetabulum
  • second line is perpendicular to first
  • Posterior- Superior = SAFE ZONE
    • Posterior -inferior- Caution zone
    • Anterior- inferior- danger zone
    • Anterior- superior - death zone

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Why is the posterior-superior zone safe?

  • Only risk is the scaitic nerve , superior gluteal nerve ad vessels if the hip centre is elevated 

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Why is the posterior-inferior zone caution?

  • if screw exceeds 20mm the following structures are at risk
  • sciatic nerve
  • inferior gluteal nerve and vessels
  • internal pudendal nerve and vessels

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Why is the anterior- inferior zone danger zone?

  • Obturator nerve, artery and vein at risk
  • "O'

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Why is the anterior-superior zone death zone?

  • External iliac vessels at risk

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What is prothestic impingement?

  • Leaving the anterior rim of acetabular component proud above the native acetabulum may result in anterior iliopsoas tendon impingement


What are the causes of impingement?

  • Medialisation and raising the acetabulum cup centre of rotation will increase the risk of bone on bone impingement by decreasing femoral offset
  • Lateralisation of cup will increase metal femoral neck on metal acetabulum impingment
  • Femoral head to neck ratio <2:1 will increase impingement
    • femoral skirts
    • small femoral heads


What is the disadv of the 

anterior approach

anterolateral approach

lateral approach

posterior approach to the hip

  • Anterior - Smith peterson
    • difficult visualisation to the femur
  • Anterolateral ( watson- jones)
    • possible abductor weakness
    • adv- post op hip precautions limit extension/ER - to prevent ant dislocation
  • Lateral ( hardinge)
    • risk of trendelenberg lurch due to abductor weakness
  • Posterior
    • can be extended to perioprosth
    • post op hip precautions limit flexion and IR to prevent post dislocations


What are the symptoms of ilipoas impingment?

  • Pain in groin
  • Pain with passive extension
  • snapping pain in groin


How can ilipoas impingment be further investigated?

  •  Cross-table lateral imaging and CT scan can be used to evaluate for protrusion of the anterior rim of the acetabular cup causing impingement with the tendon.
  • The diagnosis is confirmed by relief of pain with anesthetic injection of the tendon sheath

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describe the position of the acetabular cup?

  • inclination 35-45 degrees
  • anterversion 15 degrees