Holding stocks for years and years is one of the best ways to get massive returns. But when you're planning to buy for a long hold, you'll need to pick companies that are capable of persistently thriving in an ever-changing world. 

Today, I'll be discussing two growth stocks that are worth including in your retirement portfolio. While neither of these two companies make trendy or high-profile products, they won't have much trouble maintaining their sales volume over time. Nor has their competition managed to chip away at their market shares. Let's dive in.

Two dentists examine an image on a tablet while standing in an empty examination room.

Image source: Getty Images.

1. Abiomed

After suffering an injury or undergoing cardiac surgery, people's hearts need to rest to heal and recover. That's a bit of a problem, considering the constant need for regular blood circulation. So, Abiomed (NASDAQ:ABMD) makes tiny heart pumps which clinicians use to assist with circulation while a patient is in the hospital, giving their ticker a lighter workload and some R&R. Then, once their heart is back in good working order, the pump is removed and the patient's heart can (hopefully) operate without as much trouble.

Abiomed's line of Impella pumps has chalked up a handful of regulatory approvals in the last decade, and the company has consistently made improvements and new versions of the device as well. And, compared to older technologies, Impellas have been shown in a smattering of clinical studies to be superior for patients in terms of survival rate, recovery time, complication rate, and the average duration of their hospital stays. 

As if that wasn't enough, hospitals that use Impellas instead of the alternatives tend to save money as a result of the patients' shorter hospital stays and fewer complications. That means there are two strong incentives for healthcare systems to adopt the product. 

So, Abiomed has a life-saving and money-saving product, but does its financial performance stack up? Over the last decade, its quarterly earnings before interest, taxes, depreciation, and amortization (EBITDA) have grown by a colossal 3,860%, and its quarterly revenue expanded by nearly 757%. Plus, it has a scant $5.66 million in debt, which is practically negligible. 

In my view, there's nothing stopping Abiomed from continuing its successful streak for many years to come.

Charts showing upward trend in Abiomed's and Align's quarterly revenue and EBITDA since 2017.

ABMD Revenue (Quarterly) data by YCharts

2. Align Technology

If you've ever had braces, you know that the field of teeth straightening devices is begging for disruption. And that's exactly what Align Technology (NASDAQ:ALGN) is doing with its Invisalign brand of transparent teeth straighteners. Compared to braces, Align's devices are more aesthetically appealing and more comfortable, which makes it no surprise that the company has an 11% share of the market.

Of course, consistent spending on direct to consumer advertising and healthcare provider outreach are also part of the picture, but there's a lot more to this business than tooth straighteners and an effective marketing campaign.

Align's business model relies on dental professionals to buy cases of its straighteners and then fit them to patients' teeth with the help of the company's iTero scanning devices. Therefore, clinics need to pay the company for the purchase of these systems and services before they can start working with patients. And that's just one reason why the company's quarterly revenue increased steadily by 262.8% over the last five years.

Having your customers pay you for the privilege of finding opportunities to sell your product is a pretty impressive accomplishment for nearly any business. But getting them to want to do it is even better. Once dental offices have Align's scanners set up, they also get access to a suite of clinical imaging and computer aided design (CAD) dentistry software that can help them in a huge swath of different situations by providing accurate and 3D models of a patient's teeth. 

The ability to offer patients Invisalign straighteners is only one part of Align's appeal to clinicians. In terms of the stock's appeal to retirement investors, there are two things worth mentioning. First, the company's quarterly free cash flow (FCF) has grown by 738.8% in the last decade, indicating that its financial strength is steadily increasing over time. That's a good omen for Align's long-term viability, and it shows that management knows how to keep the number on Align's bottom line a positive one. 

The second thing is that demand for tooth straighteners isn't about to go anywhere. Braces have existed for a very long time despite their drawbacks, and the global market for straighteners is nowhere near being fully penetrated. As Align continues to grow into its markets, it won't need to change much to displace braces, and eventually, it could supplant them entirely.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.