MCQs-Hip recon 2
epiphyseal vessels are from the extrasynovial ascending cervical vessels become the key vessels. The best guess is then the medial epiphyseal vessels. Probably the least important of all is the ligamentum teres.
– the most common cause of death after total hip or knee replacement is pulmonary embolism. It is the most common cause of death occurring within 3 months of surgery, and is responsible for more than 50% of postoperative mortality after total hip. MI and CHF are the 2nd and 3rd most common.
– for total hip, the fatal PE rate is about 2% with no prophylaxis
– for total knee, the fatal PE rate is about .3%
– if the acetabulum is placed in neutral version, the hip is most likely to dislocate posteriorly. Optimum anteversion is about 10-20 degrees
– the results of cemented and cementless femoral components are no different in short term followup (3-5 years)
– best pore size for cementless femoral components is 200-500 microns (interestingly different from total knees – 150-400)
– theoretically, increasing the head size from 22 to 32 increases the stability by allowing greater range of motion before impingement of the neck on the cup. An increase in the head:neck width ratio decreases the chance of impingement, but increasing the size of the head also increases volumetric wear. (Decreasing size increases the linear wear) The best compromise is probably 26-28 mm.
– the most common cause of long-term failure in cemented total hip arthroplasty is the cup loosening
– intraoperative hypotension and hypoxia during a cemented total hip replacement is probably due to fat emboli during pressurization of cement. The monomer can have a direct effect on the heart, but its peak levels don’t occur for 3 minutes after cementing.
– risk of heterotopic ossification post-op hip replacement: DISH, hypertrophic OA, and ank spond, post-traumatic OA, revision surgery, Paget’s disease, and significant surgical stripping of soft tissues. Immobilization does not appear to be a risk factor. Nor does rheumatoid arthritis
– current methods of reducing HO include Indocid 75 mg po OD for 6 weeks, or single dose XRT of 700 rads sometime within 72 hours of surgery.
– risk factors for hip dislocation include:
– posterior approach
– faulty positioning of components
– trochanteric escape or rupture/weakness of abductors
– leg length too short
– impingement – either neck against cup or femur against soft tissues or pelvis
– neuromuscular disease – Parkinsons, stroke, confusion
– age, height, weight, or preop diagnosis do not seem to be causative.
– for converting a hip arthrodesis to arthroplasty, it is best if there has been no surgical trauma to the area – ie a spontaneous fusion is best. On the other end of the spectrum, the multiply operated on hip that eventually gets fused is the worst candidate. Their soft tissues and abductor muscles are probably toast, and it will be very hard to keep them stable.
– conversion of hip arthrodesis to arthroplasty has the worst results in patients with multiple previous surgeries, and age under 50. Length of time that it has been fused is not a prognostic factor.
– with regards to acetabular wear, wear is maximal right away as things get “worked in”; it then decreases with time. Wear is felt to be about 0.1 mm per year. Poly particles are a significant factor in wear.
– Charnley, using x-rays, reported a wear rate of 0.15 mm per year for cups. But in the lab, true wear is much less. Current understanding explains the difference – a large proportion of the shape change in polyethylene is a result of CREEP rather than wear. Acetabular wear is primarily abrasive,and the most crucial factor affecting the true wear rate of polyethylene cups under weightbearing conditions is the molecular weight of the material. In contrast, in the total knee where surface contours differ significantly, local contact stress and subsurface fatigue wear become the dominant mechanism of poly degradation.