THR Flashcards
Prosthesis design THR technique
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
- cemented
-
Bearing surfaces
- polyethylene
- metal
- ceramic
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
-
tapered stem
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
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
-
Colbat-chrome
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
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
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?
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
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
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
Why is the posterior-superior zone safe?
- Only risk is the scaitic nerve , superior gluteal nerve ad vessels if the hip centre is elevated
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
Why is the anterior- inferior zone danger zone?
- Obturator nerve, artery and vein at risk
- “O’
Why is the anterior-superior zone death zone?
- External iliac vessels at risk
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
describe the position of the acetabular cup?
- inclination 35-45 degrees
- anterversion 15 degrees
