Spotting trends before they happen can make you rich. While figuring out what technology will become popular can be difficult, there's one trend that's been predicted for more than 40 years.

Baby boomers are getting old. The oldest are entering their mid-sixties and there's a 20-year wave behind them. If there's one thing that old people do consistently, it's get sick.

Brittle bones
Osteoporosis, a weakening of the bone, affects 10 million Americans. While the market is huge -- and it'll only get larger as the population ages -- it's also pretty crowded.

Drug

Sold by

2008 sales (in millions)

Evista

Eli Lilly (NYSE:LLY)

$1,076

Forteo

Eli Lilly

$779

Fosamax franchise

Merck (NYSE:MRK)

$1,600

Boniva

GlaxoSmithKline

$438

Reclast

Novartis

$254

Source: Company press releases.

One company to keep an eye on in this space is Amgen (NASDAQ:AMGN). Denosumab looked good in clinical trials for osteoporosis and will come up for a Food and Drug Administration advisory panel review next month. More importantly, while the drug may have lots of competition in the osteoporosis space, it seems to work better than the current treatment for bone damage due to cancer.

Pump up the pressure
The market for high blood pressure -- or hypertension, as the doctors like to call it -- is more crowded than osteoporosis. When Forest Labs got its high blood pressure medication, Bystolic, approved in 2007, it was the 19th beta blocker to hit the market. 19th!

It's a huge market -- 73 million Americans have hypertension -- but, with that many competitors, breaking in is difficult. Plus, many of the hypertension drugs are so old that they already have generic competition, making entering the market with a new branded drug extremely difficult.

Sore joints
Nearly 70% of people over the age 70 have detectable evidence of osteoarthritis, although only half of those develop symptoms. The treatment for osteoarthritis starts with pain relievers like Johnson & Johnson's (NYSE:JNJ) Tylenol up to the stronger stuff like Pfizer's (NYSE:PFE) Celebrex. While pain relievers get most of the patients, they're not necessarily the best play on the aging population, since they're used for plenty of other issues.

A stricter take on osteoarthritis, specifically of the knee, are viscosupplementation -- supplementing the joint with viscous fluids. Smith & Nephew's Supartz, sanofi-aventis' (NYSE:SNY) Hyalgan, Johnson & Johnson's Orthovisc and others are injected when oral medications aren’t enough to control the pain. The viscosupplementation drug that probably has the best potential for taking off is recently approved Synvisc-One from Genzyme, because it only requires one injection to provide six months of relief -- the original version of Synvisc required three injections.

The next step down the arthritis pathway is joint replacements. Stryker (NYSE:SYK) and Zimmer Holdings are good plays on joint replacements -- from hips to knees, they've got the elderly covered.

The big unknown
Clearly more people are going to need medical services than ever before. The big question is whether the government will be willing to pay for it. There's a push right now to decrease medical costs, which should worry anyone investing in health-care companies.

The trick, as I see it, is to invest in innovative companies. The government many not be willing to pay for a me-too drug when there's a cheap generic that can do as good a job. But constituents will demand that Medicare pay for truly novel therapies that extend people's lives.

Extended Foolishness:

Pfizer and Stryker are Motley Fool Inside Value selections. Johnson & Johnson is a Motley Fool Income Investor recommendation. Try any of our Foolish newsletters today, free for 30 days.

Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. The Fool owns shares of Stryker and has a disclosure policy.