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The use of birth control pills appeared to boost the risk of blood clots in women undergoing simple knee arthroscopy and anterior cruciate ligament (ACL) reconstruction, a new study found.

The risk of post-surgery venous thromboembolism (VTE) more than doubled in this population (OR 2.1, P<0.001), and obesity and smoking increased the risk even more (adjusted ORs 3.05 and 4.25, respectively, P<0.001), reported Harris Slone, MD, of Medical University of South Carolina in Charleston, and colleagues in a poster presented at the American Academy of Orthopaedic Surgeons annual meeting.

The findings were previously published in the journal earlier this year.

The researchers conducted their study after seeing several young, healthy women develop clots either after orthopedic surgery or between the time of an injury and the planned surgery, Slone told 乌鸦传媒. "We know that the estrogen and progesterone in combination oral contraception pills increase the risk of clots," he said. "And we know that clots are problematic following orthopedic surgery, especially surgery around the pelvis, hip, and knee. But there has been little research to date quantifying the risk of oral contraception plus orthopedic surgery."

Miho J. Tanaka, MD, director of the Women's Sports Medicine Program at Massachusetts General Hospital in Boston, told 乌鸦传媒 that several risk factors should be taken into account, such as smoking, obesity, lack of mobility, and genetic predisposition. "All of these should be factored in, in addition to the type and duration of oral contraceptive pills, in their relationship to the development of VTE," said Tanaka, who wasn't involved in the study.

Slone and colleagues used a large healthcare database to track 64,165 female patients ages 16 to 40 who underwent simple knee arthroscopy and ACL reconstruction from 2010 to 2015. They focused on post-surgery blood clots -- both deep vein thrombosis (DVT) and/or pulmonary embolism -- that occurred within 90 days of the procedures.

They found that the risk of VTE increased with oral contraception compared with no oral contraception in all patients (OR 2.13, 95% CI 1.83-2.48, P<0.001), for those who underwent ACL reconstruction (OR 2.34, 95% CI 1.79-3.07, P<0.001), and for those who underwent non-ACL knee procedures (OR 2.02, 95% CI 1.69-2.44, P<0.001).

The risk of postoperative clots increased even more -- beyond the cumulative effect -- if patients had another risk factor in addition to use of birth control pills.

The overall risk of clots was 0.95% in patients without obesity and oral contraception use. This risk rose to 1.62% in those who were obese (adjusted OR 1.35, 95% CI 1.01-1.82, P=0.41), to 1.72% in those who used oral contraception (adjusted OR 2.12, 95% CI 1.80-2.48, P<0.001), and to 3.13% in those who had both risk factors (adjusted OR 3.05, 95% CI 1.99-4.65, P<0.001).

Among smokers, the risk of clots was 1.24% (adjusted OR 1.00, 95% CI 0.668-1.50, P=0.998), which grew to 4.04% when these patients also used oral contraceptives (adjusted OR 4.25, 95% CI 2.41-7.50, P<0.001).

"These findings suggest that patients on perioperative OCPs [oral contraceptive pills] who are undergoing these procedures should be counseled regarding their increased risk of VTE," the study authors noted in their poster. "Patients may benefit from either stopping their OCPs one month prior to surgery or consider postoperative anticoagulation when perioperative OCP use and/or other risk factors are identified."

Birth control pills seem to boost the risk of clots in the lower extremities following orthopedic procedures, but not the upper extremities, Austin Stone, MD, PhD, of the University of Kentucky in Louisville, told 乌鸦传媒. His own found no link between oral contraceptive use and VTE after arthroscopic shoulder surgery.

The reason for the difference in risk between upper and lower extremity surgeries isn't clear, and doesn't appear to be related to the wider use of tourniquets in surgeries in the lower extremities, said Stone, who wasn't involved in this research.

The causes of the excess risk with oral contraception are also unclear, he added, although it is well known that birth control pills can cause a hypercoagulable state. "This effect is normally negligible, but becomes relevant in the setting of lower-extremity orthopedic surgery. Several additional factors could be at play, as the study highlights, such as smoking and obesity in the setting of birth control use," he noted.

Stone said that he would counsel patients to avoid oral contraceptives before surgery if possible, since "the risk of DVT is significant." Still, he added, "we need to determine who faces the highest risk and when is the optimal time to stop oral contraceptive pills."

He recalled treating a young patient with no risk factors other than oral contraceptive use who developed a postoperative DVT despite chemical prophylaxis after a knee arthroscopy and partial meniscectomy. "Fortunately, she was OK, but the effects can be devastating," he said.

Going forward, Tanaka said, it would be helpful to understand which types of oral contraceptives pose the highest risk, since many vary in dosage and the types of hormones included.

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