THR stability and dislocation

Question 1

What is the incidence of THR dislocation in primary, revision hips?

Answer 1

• Primary THR 1-2%
• Revision 5-7%
• highest incidence found in >80 years for failed femoral neck orif converted to THR

Question 2

What are the 4 important variables that determine stability of THR?

Answer 2

• Component design
• Component alignement
• Soft tissue tensioning
• Soft tissue function

Question 3

How does component design affect stability of THR?

Answer 3

• Arc range before impingement -> dislocation
• factors include
  
  1. Head neck ratio ( diameter of head/Diam of neck)
     
     • Most important
     • best to maximise
     • reduced by
       
       • skirted femoral heads - early hip impingment-> dislocation
       • acetabular hoods- decrease primary arc
       • constrained acetabular liners- sig decrease in primary arc
  2. Excursion distance
     
     • distance head must travel to dislocate after primary impingement
     • 1/2 diameter of femoral head
     • larger heads have higher head/neck ratios and larger excursion distance
  3. Leaverage range
     
     • measure by excursion distance

Question 4

How does component alignement affect stability of THR?

Answer 4

1. Centre of rotation
   
   • THR primary arc is smaller cf THR ( due to smaller prothetic head size)
   • so THR arc range centred on antomical arc range
   • if not centred -> unstabilty due to leavering
   • neck length long enough so GT doesn’t impinge on acetabulum
2. Cup alignement
   
   • avoid retroversion->post dislocation
   • avoid anteversion-> ant dislocation
   • Vertical cup- post sup dislocaiton
   • horizontal cup- inferior dislocation
3. Stem alignment

Question 5

What are the ideal positions for the cup and stem alignement in THR?

Answer 5

• Acetabular cup
  
  • 15-30^o anteversion
  • 35-45^o coronal tilt
• Stem
  
  • 10-15^o anteversion

Question 6

what is key in soft tissue tensioning which affect stability of THR?

Answer 6

1. Abductor complex ( Glut medius and medinimus)
   
   • restoring tension of abdcutors is key to soft tissue tensioning by
   • Head offset
   • Neck length
     
     • if short- use long prosthetic neck
     • skirt on long neck would decrease primary arc of motion and increase risk of dislocation
     • if neck-shaft angle < than native hip then
       
       • excessive increase in offset-> lateral hip pain and trochnateric bursitis
   • normal hip centre of rotation
     
     • prevents blatant creation of unstable zone
2. Trochanteric deficiency escape
   
   • occurs when GT pulls away from prox femur
     
     • due to poor fixation of GT after revision THR/ trauma/osteolysis
     • -> deficient abductor complex
     • Increase risk of dislocation

Question 7

How is femoral offset decreased?

Answer 7

• weak abductor complex
• increased joint reaction force
• decreased lever arm
• Trendelenberg sign
• Gluteus medius lurch
• increased risk for dislocation

Question 8

How is trochanteric deficiency/escape tx?

Answer 8

• Maximise femoral /head ratio
• resect GT fragment to prevent impingement/ Levering
• Contrainsed acetabular liner as last resort

Question 9

How important is soft tissue function to THR stability

Answer 9

• Critical
  
  • require coordinated vector forces ( muscle firing) to stablise THR
  • both thry abductor complex and other vector forces that stabilise the hip
  • 3 main factors controlling proper soft tissue function
    
    • CNS
      
      • stroke/cerebellar dysfunction, dementia, parkinsons. MS, myeolpathy , delirum-> affect balance/coordination
    • PNS
      
      • spinal stenosis ( glut medius L5) , peripheral neuropathy, radiculopathy, paralysis/paresis
    • Local soft tissue integrity
      
      • trauma- soft tissue loss
      • deconditioning
        
        • age/poor health
      • irradiation
      • osteolysis
      • collagen abnormalities
      • myopthy
      • malignancy
      • infection

Question 10

What is the epidemiology of hip dislocation?

Answer 10

• 2nd most common cause for revision surgery of THR
  incidence 1-3%
• 70% occur within first month
• 75-90% posterior
• Mechanism
  
  • anterior= extension, ER
  • posterior= Flexion, IR
• Risk factors
  
  • prior hip surgery
  • **Female **
  • >70-80 years
  • Posterior surgical approach
    
    • repairing capsule and reconstructing ER brings dislocation close to anterior approach
  • ​Malposition of components
    
    • ideal cup- 10^o anterversion, 40^o coronal tilt
  • Spastic neuromuscular disease ( parkinson’s)
  • Inflammatory arthritis
  • ETOH/ Drug abuse
  • AVN
  • Acute fracture
  • decreased offset ( decreased tissue tension/stability)
  • Decreased femoral head to neck ratio

Question 11

What is the hx from the pt regarding dislocation?

Answer 11

• Activity that puts hip out- Hip flexion, adduction, IR
  
  • shoe tying
  • sitting in low seat or toilet

Question 12

What is the tx of THR dislocation?

Answer 12

• Non operative
  
  • Closed reduction and immobilisation
  • 2/3rd of early dislocation tx with closed reduction adn immobilsation
• Operative
  
  1. Revision THR
     
     • 2 or more dislocation with evidence of
     • polyethylene wear
     • malalignment
       
       • vertical acetabular compenent may need revision
       • also acetabular retroversion
     • Hardware failure
  2. ​Conversion to Hemiarthroplasty with larger femoral head
     
     • ​for soft tissue deficiency or dysfunction
     • CI if acetabular bone compromised
  3. Resection arthroplasty
     
     • when all options exhausted
     • sig bone loss and soft tissue deficiency
     • psychiatric pts

Question 13

What are the aims in revision THR surgery?

Answer 13

• Realign components
• Head enlargement- optimise
• Trochanteric osteotomy and advancement
  
  • places abductor complex under tension with increases hip compression forces
• Conversion to a constrained acetabular compoenent
• when
  
  • recurrent instability with a well postioned actetabular component due to soft tissue deficiency or dysfunciton
  • must have compliant pt otherwise will fail by fx of cup or loosening from the pelvis

Question 14

What does femoral subsidance have on the function of a THR?

Answer 14

• Femoral stem subsidence effectively decreases the neck length of the prosthesis resulting in a lax abductor complex
• -> increase in the joint reactive force
• This decrease in leg length can also lead to increased hip instability.
• Illustration A shows a free body diagram of the hip joint. The magnitude of the joint reaction force depends critically on the ratio of (d1:d2). As d2 decreases due to less offset, such as in this question, and body weight remains the same, the joint reaction forces increase.