Death and taxes, it is often said, are the only certainties in life. But what is also becoming increasingly certain is that, as people live longer, the demand for health services in this country will increase rapidly.

Investors have long recognized the opportunities presented by the graying of America and it is no coincidence that health-related stocks have been able to resist the recent market downturn better than most.

In the Washington area, companies in the health care field that have attracted investor interest include Manor Care Inc., which operates nursing homes; Mid-Atlantic Medical Services Inc., which operates MD-IPA, a health maintainence organization; and PHP Healthcare Corp., which operates clinics for the military.

Joining this list soon will be a new public company, Hanger Orthopedic Group Inc., of Bethesda. The company is planning to sell 2.25 million shares at from $8 to $10 a share to raise from $18 million to $22.5 million before commissions. The underwriter is PaineWebber Inc. of New York.

The company expects the stock to trade in the Nasdaq National Market System under the symbol, HNGR.

Hanger Orthopedic, as its name suggests, is in the business of providing people with both orthotic and prosthetic services.

Orthotics, the firm's prospectus notes, involves supplying various types of braces for people with back ailments and other physical disorders. Prosthetics involves providing patients with artificial limbs.

The orthotics and prosthetic field -- called O&P in the language of the industry -- is estimated to constitute a $700 million a year market. That estimate covers patient care services, manufacturing and distribution.

The company says it expects strong growth in orthopedics rehabilitation because of the growing number of elderly citizens, the increasing emphasis on physical fitness and the treatment and prevention of sports injuries.

Hanger Orthopedics, which calls itself "one of the nation's largest providers of patient care services" in this field, currently has 34 patient centers in 11 states, including Maryland and Virginia. The company also has facilities in the District.

The company's business strategy is to grow by acquisition.

The O&P industry, the company notes, is highly fragmented. There are 2,650 certified practitioners in the field and 1,170 O&P firms, most operating as small group practices.

The prospectus suggests that, in time, there will be fewer and fewer small firms, as they are merged into larger entities such as Hanger Orthopedic.

This is a trend long noticeable in many other business areas.

Indeed, Hanger Orthopedic is, itself, a firm created out of an amalgamation of 13 O&P companies, of which 11 have been acquired during last year and this year.

Although this has made the history of the current company quite complicated, the firm now bears the name derived from J.E. Hanger Inc. of Washington, which was founded in 1861 by a Civil War amputee.

The chairman of Hanger Orthopedic is Ronald J. Manganiello, 41, who was a senior vice president in the corporate finance division of Drexel Burnham Lambert from 1979 to 1986. The president is Ivan R. Sabel, 45, who is a certified O&P practitioner and was the president of Capital Orthopedics of Washington.

Once the offering is completed, voting control over the firm will rest with the venture capital firms that put up the money to get the new company rolling. Chemical Venture Capital Associates, allied with Chemical Banking Corp. of New York and Exeter Capital L.P., will have 45.1 percent of the stock. Officers and directors will have another 5.1 percent for a total of 50.2 percent, according to the plan in the prospectus.

All the shares are being sold by the company rather than by individuals, as is sometimes the case. But insiders who already hold stock will see the value of those shares appreciate substantially -- at least in the short run.

The money raised in the offering will be used, in large measure, to pay off the debt accumulated in the acquisition of the companies that now make up Hanger Orthopedic.

Because of the way the company has been put together, the financial information offered to potential shareholders is far more complicated than is usually found in a public offering document. What is clear is that the company was not profitable for the 10 months ending Dec. 31, 1989, or for the six months ending June 30, 1990.

However, through the magic of an accounting device called "pro forma" the company offers a hypothetical look at what its finances might have looked like if the acquisitions already had been in place and if the public offering already had been completed.

Under this scenario, for the year ending Dec. 31, 1989, the company would have earned $1 million (18 cents a share) on sales of $20.6 million. For the first six months of 1990, it would have earned $494,000 (eight cents) on sales of $10.6 million.

Even if the hypothetical were actual, it still would not necessarily become a crystal ball for an investor trying to glimpse of the future at Hanger Orthopedic. Only time will provide that information.

To a significant extent, the future at Hanger Orthopedic is linked to federal and state policies and guidelines in the health area. During 1989, about 30 percent of the company's net sales were derived from medicare, medicaid and the Department of Veterans Affairs, which set maximum reimbursement levels for O&P products and services. In the first half of 1990, the figure was 28.5 percent.

And no one can be sure whether those levels might be lowered in the future -- especially in an era of tight state and federal budgets.

In any event, the creation of Hanger Orthopedic will give Washington a major new company and a new name to add to The Washington Post 200, a listing of the biggest firms in this region. With $20 million in sales, Hanger Orthopedic would come onto the Post 200 list at around No. 70.

Speaking of health-related stocks, analyst Lawrence C. Marsh of Wheat, First Securities Inc. in Richmond, says he sees an important trend at Manor Care, the Silver Spring nursing home company.

"We believe that there are clear signs that the {profit} margin reduction that Manor Care has suffered through over the past three years has ended," he says.

"Specifically, the significant labor cost increases seen since 1988 will be almost impossible to duplicate, especially in a slow growth, recessionary environment."

Marsh also notes that Manor Care has spent almost $300 million to build new nursing home facilities during the past three years, which has cost the company 20 percent of its earnings per share. However, as that program nears completion, the new homes will be big contributors to the bottom line.

Marsh looks for profits to continue to rise. He believes Manor Care, which earned 69 cents a share in fiscal 1990, ending May 31, will earn 85 cents in 1991 and $1.15 in 1992. That would put the company's price-earnings ratio, which stood at 22.8 for fiscal 1990, at 18.5 for 1991 and 13.7 for 1992. The overall market PE these days is about 16.

Manor Care is one of the nation's largest providers of long-term health care services and owns and operates 163 nursing homes and retirement facilities. The company also has a Choice Hotels International subsidiary and owns or manages more than 2,000 hotels and resorts. However, business conditions in the hotel industry generally have been lackluster for some time.

With stocks prices depressed, stock buybacks continue. Legent Corp. of Vienna will buy 1 million shares of its own stock from time to time. It already has bought 800,000 shares on an April 1990 authorization to buy 1 million shares. Legent supplies computer software and services.

Similarly, Cadmus Communications Corp., a Richmond printing company, has boosted its buyback target to 200,000 shares. Since May 1989, the company has taken in 98,000 of its own shares.