MCQs-Hip recon 3
Volume of wear = K x load x distance Where K = constant for materials
– the highest K is with metal on metal; lowest K is with ceramic on poly; in between is metal on poly.
– notice that DISTANCE is an important factor – hence, the bigger heads have more volumetric wear because with each movement, there is more distance covered.
– the best treatment for staph epi is vancomycin
– the hardest bug to eradicate in THA is pseudomonas; it may in fact be staph epi, but in general, the gram negatives are hardest to get rid of.
– the most common bug in an indolent infection 6 months postop THA is going to be staph epi; in a fulminant infection, it is most likely staph aureus.
– in doing a THA after a patient has a Salter osteotomy – beware the lack of posterior wall. Their acetabulum has now been retroverted, so you don’t want to put your component in the same alignment as the native acetabulum. Note that when you put your cup in with adequate anteversion, because the acetabulum is so retroverted, there may be quite abit of anterior lip overhanging – you must cut this out or you’ll impinge.
– there are many factors to consider in doing THA in dysplastics
– Femur: short neck, small canal, straight canal, trochanter way around the back, significant anteversion, need for shortening osteotomy
– Acetabulum: antero/superior bone defect, high riding, need to find native floor
– the anticoagulated patient who develops a wound hematoma and complete femoral nerve palsy – reverse the anticoagulation and consider what to do next. Many questions suggest that you can just observe. Some authors suggest that you should decompress this. Hard to know what to do. I think I’m gonna go in and decompress this, or get radiology to put in a drain.
– to decrease the stress on a prosthetic stem, you can increase the cross sectional area of the stem. ???
– Stress = My/I where M=bending moment, y= linear distance from neutral axis, and I = moment of inertia. The moment of inertia is significantly increased (r to the 4th power) by increasing the radius, and hence the stress would decrease.
– from Campbell’s: “increasing the modulus of elasticity, the stem length, and the cross-sectional area of the stem increases the stress in the stem, but decreases the stress n the cement and proximal third of the femur” I think he means that you increase the stiffness of the stem and thus potentiate stress shielding.
– remember: modulus of elasticity is a material property, not a structural property. Modulus does not change with thickness, length, cross-sectional area, etc. The overall stiffness of an implant may change though, but this is not the same as the modulus. Note that viscoelastic materials do change with time, and so their modulus would change a little over time (eg. Polyethylene)
– lucency in zones 1 and 7 represent loosening in a PCA component where the beads are all up at the top (in zones 1 and 7)
– if you see a patient who dislocates her THA during the transfer but you get it reduced in the PAR – treat the patient with bed rest and an abduction pillow if you are happy with the components otherwise.
– if you get huge bleeding after putting a retractor around the anterior column – you’ve hit the femoral artery.
– AVN in adults is mostly likely to be associated with injury to the lateral epiphyseal artery
– AVN typically involves the anterosuperior aspect of the head.
– Alcaptonuria is not a cause of AVN of the femoral head. Alcaptonuria is the excretion of black urine and is seen in patients with Ochranosis, a deficiency of the enzyme to break down homongentisic acid. AVN is seen with Gaucher’s, sickle cel, Caisson’s disease, and dislocation
– the appearance of AVN on MRI – would see a double line sign on T2; on T1 the dead area is DARK, on T2 the dead area is BRIGHT.
“The earliest finding in AVN is a single-density line (low signal) on T1 that presumably represents the separation of normal and ischemic bone. On T2 images, a second, high signal