Biomechanics ofTHR

Question 1

What assumptions are made in free body diagrams of hip?

Answer 1

• A single leg stance
• weight of leg is 1/6th of the body weight

Question 2

Draw a free body diagram of the hip?

Answer 2

• note abductor force - role to stabilise pelvis in single leg stance
• resolving the moments
• a= 0.15m, b=0.05m, BW 600N
• Fab xb= a x5/6 F_BW
• Fab x 0.05= 0.15 x 500
• FAB= 0.15X500/0.05
• = 1500N

Question 3

How is the joint reactio force calculated?

Answer 3

• From the vector triangle

Question 4

What is the major determinate of joint pathology?

Answer 4

• Joint reaction force
• ie the resultant of thee abdcutor force Fab and 5/6th body weight
• of these 2 factors teh abductor force predominates
• but a reduction in either will reduce the JRF

Question 5

What are the strategies to reduce JRF?

Answer 5

• Reduce body-weight moment
  
  • reduce body weight
  • decrease lever arm
    
    • medialize the axis of rotation
    • Trendenberg gait
• Help the adductors
  
  • provide additional moments
    
    • walking stick opposite hand
    • suitcase ipislateral hand
  • Increase abductor lever arm
    
    • increase offset
    • osteotomy
    • Lateral transfer of greater trochanter
    • varus angulation of stem in THR
  • Improve abductor line of function

Question 6

Describe how trendelenberg gait aids movement and reduce JRF?

Answer 6

• Pt sways body weight over, towards affected side. the lateral motion decreases the body weight moment arm and hence the JRF

Question 7

Can you ilustrate how a walking stick in the contralteral hand reduces JRF?

Answer 7

• resolving the moments clockwise
• (Fab x b ) + 0.5 x100 = a x 5/6Fwb
• Fab x 0.05 + 50= 500 x 0.15
• fab= 500 x0.15/ 0.05 -50
• fab = 500N a reduction of 67%

Question 8

What does a varus osteotomy do to JRF

Answer 8

• reduce it by displacement medially by
• increasing abductor lever arm
• more horizontal line of action of the abductors
• may increase congruency

Question 9

What is the main effect of a valgus osteotomy?

Answer 9

• make the capital drop osteophyte weightbearing so increasing the size and quality of the surface available for weight bering
• other effects include
• lengthening the abductor lever arm
• making the abductor action more horizontal

Question 10

What movement can occur in THR at low speed and with poor lubrication?

Answer 10

• Stick- slip
• aka stiction-friction
• this produces torque 40x higher than in unaffected arthroplasties
• may cause acetabular loosening
• a combination of thick PE and small head size diffuses load more than thin PE, leading to lower frictional tourque and decreased stress at the cup interface, with widening of stress dissipation within the bone 9 and therefore less loosening)

Question 11

Where is the acetabular component placed?

Answer 11

• restore anatomical location
• with a smaller head size effectively medialising the centre of rotation
• lab test shave demonstrated athat acetabular preparation that gives the strongest fixation in cemented cups is the use of three large keyholes with preservation of the subchondral plate

Question 12

Decribe the design of modern acetabular cups?

Answer 12

• UHMWE
• Flanges- to compress the cement
• Grooves to increase the surface area and improve bonding
• Pods to prevent bottoming out
• but deep grooves reduce PE thickness -> stress risers predisposing to creep, fatigue and failure

Question 14

What are the modes of femoral stem fixation?

Answer 14

1. Cementless
2. Cemented- all interfaces fixed- composite beam
3. Cemented- bone interface fixed , stem interface free of slip - taper slip

Question 15

What stresses do cementless stems produce?

Answer 15

• High shear stress
• low compression stress
• medium tensile stress

Question 16

What stresses do cemented all interface stems produce?

Answer 16

• High shear stress
• low compression stress
• medium tensile stress
• similar to uncemented stem

Question 17

What do cemented tapered stem stresses are produced?

Why is that?

Answer 17

• Low shear stress
• High compressive
• almost no tensile stresses

The ability of the bone cement to undergo _creep and stress relaxatio_n is primarily responsible for the conversion of tensile stress to compressive stress

Question 18

What are the adv of metal metal hip resurfacing?

Answer 18

• Bone conserving
• normal femoral loading
• avoids stress sheilding
• improved funcitonal results
• restores anatomy
• reduced risk of dislocation
• lubrication is likely to be fluid filmed

Question 19

What are the modes of failure of Femoral stem?

Answer 19

• Gruen et al
• type 1
  
  • 1a- pistoning within cement
  • 1b- pistoning stem within bone
• type 2
  
  • Medial midstem pivot
• Type 3
  
  • Calcar pivot
• Type 4
  
  • bending cantilever ( fatigue)