THR

Question 1

What do biomechanics of TH arthroplasty depend on ?

Answer 1

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

Question 2

What are the designs include?

Answer 2

• 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

Question 3

Describe the different types of press fit stems?

Answer 3

• 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

Question 4

Describe some complications of press fit stems?

Answer 4

• 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

Question 5

Describe the types of cemented femoral stem?

Answer 5

• 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

Question 6

What is the unqiue complications of cemented femoral stems?

Answer 6

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

Question 7

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

Answer 7

• 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

Question 8

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

Answer 8

• 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

Question 9

What are the CI for MOM implants?

Answer 9

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

Question 10

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

Answer 10

Adv

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

Dis

• 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

Question 11

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

Answer 11

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

Question 12

Is Titanium on PE a good bearing surface?

Answer 12

• No rarely used due to high wear rates

Question 13

Describe the placement of acetabular screws in THR?

Answer 13

• 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

Question 14

Why is the posterior-superior zone safe?

Answer 14

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

Question 15

Why is the posterior-inferior zone caution?

Answer 15

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

Question 16

Why is the anterior- inferior zone danger zone?

Answer 16

• Obturator nerve, artery and vein at risk
• “O’

Question 17

Why is the anterior-superior zone death zone?

Answer 17

• External iliac vessels at risk

Question 18

What is prothestic impingement?

Answer 18

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

Question 19

What are the causes of impingement?

Answer 19

• 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

Question 20

What is the disadv of the

anterior approach

anterolateral approach

lateral approach

posterior approach to the hip

Answer 20

• 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

Question 21

What are the symptoms of ilipoas impingment?

Answer 21

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

Question 22

How can ilipoas impingment be further investigated?

Answer 22

• 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

Question 23

describe the position of the acetabular cup?

Answer 23

• inclination 35-45 degrees
• anterversion 15 degrees