No clinical trials have been reported in which no prophylaxis or
aspirin was compared to low molecular weight heparin or warfarin as
prophylaxis against fatal pulmonary embolism after THR or TKR.
Consequently, Dr. Lotke’s hypothesis has never been adequately tested.
Because such a trial would require a very large sample size, the total
deep vein thrombosis prevalence as determined by mandatory venography
has been used as a surrogate outcome measure. Among control or
placebo patients undergoing THR or TKR, the total deep vein
thrombosis prevalence is 51% and 61%, respectively.1
Among THR and TKR patients receiving aspirin prophylaxis, the total
deep vein thrombosis prevalence for control and placebo patients
is 52% and 74%, respectively, providing relative risk reductions of
essentially zero for each type of surgery. In contrast, low molecular
weight heparin or warfarin prophylaxis provide relative risk reductions
of 71% and 57%, respectively, for THR and 49% and 20%,
respectively, for TKR. Whereas most of these thrombi are small,
asymptomatic, and confined to the deep veins of the calf, in the
absence of prophylaxis, between 23% and 36% of THR patients, and
between 9% and 20% of TKR patients, have proximal deep vein
thrombosis, which imparts a substantially greater risk for symptomatic
venous thromboembolism and fatal pulmonary embolism. Although isolated
calf vein thrombosis can propagate to involve the proximal deep veins,
most such thrombi resolve without apparent clinical sequelae.
Consequently, using the prevalence of total deep vein thrombosis (as
determined by mandatory venography at the time of hospital discharge)
as the primary outcome measure tends to magnify the severity of the
problem. Nevertheless, this outcome measure remains appropriate for
comparing the efficacy of different prophylaxis methods.