When it comes to growth stocks that tend to keep growing year after year, Intuitive Surgical (ISRG 0.59%) is near the top of the list. Since its surgical robots hit the market in the year 2000, its shares are up by more than 14,250%, obliterating the market's return of 340% in the same period.

It probably won't climb that much over the next 23 years or so, but it could still be an excellent investment for growth. Here are three numbers that explain why that's the case. 

It's a recurring-revenue machine 

Intuitive Surgical's recurring-revenue flywheel is one of the strongest drivers of its present and future growth. In short, via its razor-and-blade business model, it sells da Vinci-branded robotic-surgical suites to customers, who then also need to pay for a handful of different add-on products and services on a regular basis to continue using their robotics. The more customers use their systems, the more they need to pay. And the more da Vinci units are installed in hospitals worldwide, the bigger the company's recurring income becomes.

In the second quarter, Intuitive reported revenue of $1.8 billion. Of that sum, $1.1 billion was from sales of instruments and accessories for customers to use with their da Vinci units. But the more important figure for growth is that management estimates that 79% of its revenue is recurring in nature thanks to those instruments and accessories, not to mention other recurring sources like maintenance packages, training courses for surgeons, replacement parts, and software subscriptions. As it develops more accessories and similar products, that proportion could grow even further. 

Being able to count on most of its sales recurring each year is a very bullish factor. As customers accumulate more and more add-ons for their surgical suites, and their surgeons get trained on more and more operations to perform with them, the cost of switching to a rival system increases. And locked-in customers tend to support larger top lines over time too. 

It practically prints cash too 

Intuitive is also likely to continue growing because it's a business that generates a ton of cash. Its trailing-12-month free cash flow (FCF) is nearly $1.2 billion, up 61% from 10 years ago. Since it doesn't pay a dividend, all of that cash flow can be stashed away for a rainy day, used to acquire promising smaller companies, or funneled into share repurchases to boost the share price. And it does all three of those activities quite frequently. 

Its cash holdings were just under $6 billion as of Q2, and over the last 12 months, it bought back more than $2 billion of its shares. In 2020, it bought Orpheus Medical, a private surgical informatics platform developer, for an undisclosed sum. That year, it also launched a $100 million venture capital fund for minimally invasive care, which has since invested in at least eight companies. Each of those could one day provide the technologies or know-how to expand the capabilities of Intuitive's products or develop new ones, spurring further growth. 

There's no debt weighing it down

Debt puts a damper on the ability of a company to grow. Intuitive has zero debt, which means that every dollar of its earnings can be reinvested in itself for growth. Given what we've already discussed regarding its output of cash, the situation is unlikely to change. 

But if it does, it'll probably be due to a big strategic-acquisition play costing many billions. Such a play would almost certainly lead to more growth too, even if it wasn't immediate. Therefore, investors should consider the company's total lack of debt to represent a powerful option to borrow big if the right opportunity presents itself.

And so optionality is yet one more reason why Intuitive Surgical is a top-notch growth stock that's likely to keep expanding over time.