Medical device start-up Hansen Medical (NASDAQ:HNSN) perennially gets compared to Motley Fool Rule Breakers pick Intuitive Surgical (NASDAQ:ISRG) because they have the same founder, Frederic Moll, and are both involved in robotic medical devices.

However, Hansen's direct competitor right now is Stereotaxis (NASDAQ:STXS), which also sells a robotic catheter system. Hansen has been a public company since November 2006. Its lead product is a robotic catheter system named Sensei that was approved for marketing in the European Union and the U.S. in May of this year.

The basic idea behind the Sensei System is that it allows surgeons to manipulate a catheter via remote control and by looking at a monitor like in a video game. This gives surgeons much more control over a snake-shaped catheter compared to a non-robotic catheter that can flail around uncontrolled like a hose inside a patient's body.

To see if Hansen's future path will follow that of Intuitive Surgical and whether it will outperform Stereotaxis, let's look at how many Intuitive Surgical da Vinci systems and Stereotaxis systems have been sold in the years since they came to market.

First Year

Second Year

Third Year

Fourth Year

da Vinci surgical systems sold

12

28

49

60

Stereotaxis systems sold and delivered

12-30*

13

23

**

*Stereotaxis doesn't break out sales between first full year 2004 and initial part year 2003.
**Only marketed since 2003

Obviously, much more than just the number of units sold matters for both Intuitive and Stereotaxis. The price of the unit and revenue from recurring service and parts also are important pieces of the equation.

Nonetheless, we can check back on this chart when Hansen reports its financials over the coming quarters to see how fast penetration of its Sensei device is going in comparison to these two other systems. Hansen has already reported that during the second quarter, it recognized revenue on the sale of four Sensei Systems -- three in the European marketplace and one to a customer in the U.S.

Waiting a few quarters to jump on the Hansen bandwagon could mean a few percentage points of lost investment gains, but it also reduces much risk in case Sensei turns out not to be the king of robotic catheter systems.

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Fool contributor Brian Lawler does not own shares of any company mentioned in this article. The Fool has a minimally invasive robotic disclosure policy.