Adult Reconstruction Flashcards
What is femoral stress shielding?
Proximal femoral bone loss observed over time in the setting of a well fixed implant.
What is spot welding?
New endosteal bone growth contacting the porous coating of the implant. Usually seen w/extensively coated stems. Signs of well-fixed cementless component.
What is the main contributor to proximal femoral stress shielding?
Stem stiffness (modulus mismatch). Remember Hoek’s law that 2 springs next to each other. More force transmitted through stiffer spring. Thus the stiffer femoral stem takes on more stress and force and wolfe’s law shows us the result. (bone loss with less stress)
What type of loading is usually seen with extensively coated femoral stems?
Distal bone loading. (more proximal stress shielding)
What type of loading is seen with proximal pourous coating?
Proximal bone loading in metaphysis and proximal diaphysis
What is the major factor contributing to femoral stem stiffness?
Stem diameter. The stiffness approximates the radius to the 4th power. Small changes in radius cause exponential increase in stiffness
What is the most common initial presentation in a THA that is failing?
Start-up pain. Groin pain may be loos cup, thigh pain loose femoral stem. Must R/O infection
When replacing acetabular cup on revision case, what is ideal position for lowest joint reaction force?
Low and inside. (inf and medial)
What is the safe zone for acetabular cup screws?
The posterior superior zone. Structures at risk here are sciatic n, sup gluteal n and vessels. This is the recommended zone
What structures are at risk if acetabular cup screws are placed in anterior superior quadrant?
Zone of death! Risk laceration to external iliac artery and veins. Avoid this area.
Name the 4 quadrants for screw placement for acetabular cup screws and risks in each.
- Anterior/Sup: Zone of death. Ext iliac art and veins
- Posterior/Sup: Safe zone. Sciatic n, sup glu art and n
- Posterior/Inf: Safe if screws
- Anterior/Inf: Obturator n and vessels. Avoid screws
What is the significane of bone defects (cavitary or segmental) on femoral sided lesions?
Revision femoral stem must bypass the most distal cortical defect by a minimum of 2 cortical diameters.
What is the main culprit in osteolysis?
PE wear. From 2 sources.
- PE bearing wear: the head/cup articulation
- Backside wear: PE insert rubs against metal shell
What type of wear is the most important process that generates sub-micron sized particles?
Adhesive wear
What is adhesive wear?
Microscopic sticking of the PE to the implant which causes pulling off of the submicron beads from the PE
What is abrasive wear?
Cheese grater effect. A rough femoral head caues mechanical scratching of the PE.
What is 3rd body wear?
Particles within the joint space that cause abrasion. Some particles could be from cement, metal debris from cup or stem, metal debris from corrosis, hydroxyappetite debris etc.
What is the effective joint space?
It is any contiguous area around the joint where the implant touches the bone. Incues area around cup, stem and screws. Osteolysis can occur anywhere in this space
What is the main determinant of the number of PE particles produced from wear?
Volumetric wear. Head size is most important factor in predicting amount of particles.
What is the eqution for volumetric wear?
V= 3.14r2w (r2 = r squared)
- r= radius of head
- w = linear wear = distance the head has penetrated into cup
Smaller head has more linear wear and less volumetric, fails from wear through PE
Larger head is opposite and has more stability. Fails from osteolysis. 28mm head is standard
What rate of linear wear is associated w/ osteolysis?
>.1mm/yr
Osteolytic lesions that appear in first 2-3 yrs of prosthesis often signify what?
Infection. Osteolysis usually not present for>10yrs
What is the hallmark finding is osteolysis?
Endosteal scalloping
What is the mechanical axis of the femur?
A line drawn from the center of femoral head to the medial tibial spine and from there through the center of the ankle joint
