After Sientra (US:SIEN) announced last week that the FDA had approved the company’s tissue expander, the shares’ put/call ratio remains very favorable this week, and the number of purchases of SIEN stock options held by investors surged on June 12, data from Fintel shows.

However, the name’s put/call ratio is significantly less bullish than it was last week.

Sientra specializes in making breast implants. The company’s tissue expander, called the AlloX2 Pro, is now the only such product approved by the FDA “for exposure to magnetic resonance imaging (MRI), an important screening tool for breast reconstruction patients,” the company explained in a June 8 press release.

Only 6% of women with silicone breast implants followed the previous U.S. Food and Drug Administration (FDA) recommendation for regular MRI screening, according to a study last year in the official medical journal of the American Society of Plastic Surgeons (ASPS).

Also noteworthy is that many institutional investors sold all of their shares of the small-cap name ($20.6 million) last quarter, while the short sellers appear to be gravitating toward SIEN this week.

Put/Call Ratios Euphoria Fades

On June 13, the overall put/call ratio of SIEN stock was 0.29, down from 0.42 a day earlier. Both ratios are positive indicators for the shares as they show that significantly more bullish call options have been bought than bearish put options.

On June 9, in the wake of the company’s announcement of the FDA’s news on the tissue expander, its put-call ratio was a more bullish 0.1, although the number of options and the ratio was little changed from the levels that had been in place since May 22.

Starting on June 12, however, the number of options greatly increased, constituting unusual options activity. Specifically, there were 1,309 open put options on that day, up from just 99 on the previous trading day, June 9. And on June 12, the number of open call options soared to 3,085 up from 963 on June 9.

There was little change in the number of open options, however, from June 12 to June 13, as there were 3,079 open call options and 886 open put options on June 13.

Institutions Dumped Stock in Q1

Data compiled by Fintel shows that many institutional investors sold all of their shares of Sientra stock in Q1. It’s highly unusual to see so many institutions completely eliminate their stakes in a firm.

Among the major institutional investors that sold their entire stakes in SIEN last quarter were Citadel Advisors, the large hedge fund controlled by the famous investor, Ken Griffin, JPMorgan, Blackstone Group, Royal Bank of Canada, Wells Fargo, Bank of America, and BNP Paribas, a major French bank.

Also unloading all of their shares during the period were UBS, investment bank Oppenheimer, and Met Life.

Short Sellers Moving In

On June 13, 38.5% of the off-exchange trading volume of SIEN stock was “marked as short sales,” Finra data showed. The same metric came in at 48.5% on June 12 and 555 on June 9.

However, the interest rate that short sellers pay to borrow the stock was nearly 60% on the morning of June 14, up from 58% on June 13. On June 12, the fee was 25%, versus 14.4% on June 8.

Fintel gives SIEN stock a very high Short Squeeze Score of 93.96, ranking it 12 in the metric out of 4,620 names. The score suggests that SIEN could undergo a short squeeze soon.

This article originally appeared on Fintel

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