We’re not getting any younger. That axiom can be said of people in the United States and just about any other major economic power. And as any physician will tell you, aging takes a toll on our spines. Back pain problems tend to increase as we slow down, as bones weaken and years of stress take their toll. But if you talk to anyone who has had had back surgery, they’ll tell you it’s a brutal experience. Surgeons take hours to carefully navigate around nerves and other sensitive areas.

In response, San Diego-based Nuvasive (Nasdaq: NUVA) has come with a set of products to make back surgery a far less onerous experience. Hundreds of new doctors are being trained on the company’s equipment every year, which is fueling white-hot sales growth for the company. And we may only be in the middle innings.

In conventional surgery, access to the spine is achieved by a large incision to the front or the back of the body. Nuvasive’s gear allows doctors to make a less invasive incision in the side of the body. The company’s platform is called Maximum Access Spine (MAS), which provides surgeons with more than 50 tools to operate more delicately and quickly. The platform’s visualization systems avoid nerve damage, access devices reduce trauma and cut operating times by half and systems help patients retain a wide range of motion. In addition, patient recovery times are faster, hospital stays are shorter and the body suffers less blood loss and trauma.

All of those advantages have made the MAS an easy sell for doctors and patients, enabling Nuvasive to steadily take market share. Sales have risen at least +48% every year for the last seven years, from less than $50 million in 2004 to $370 million last year. The company believes it can approach $500 million in sales this year and approach $1 billion in sales within the next five years. (The entire spinal surgery market stood at $6.8 billion in 2009, and is growing at a double-digit clip, according to the North American Spine Society).

The bullish forecast stems from a host of factors. For starters, the company is still poised for further market share gains with the MAS platform in the United States, as 400 new doctors are trained on the system each year. Only 10% of all back surgeons have been trained on the platform thus far. To train those physicians, the company’s exclusive sales force is expanding from 300 to 500. The overall market opportunity should steadily expand as 13,000 Americans turn 60 every day for the next 20 years.

Nuvasive recently made a pair of acquisitions to bolster its position in bone graft regeneration and in the field of cervical disc replacement. The company is also rolling out new products targeting specific back ailments like deformity and scoliosis. Lastly, the company is just getting underway in the untapped international market. International sales accounted for just 3% of revenue in 2009, but with new offices opened in Germany, Australia, the U.K. and elsewhere, that figure should rise to 10% to 15% within a few years.

Shares of Nuvasive swooned this past winter when major insurers classified the company’s platform as “experimental” and thus not eligible for reimbursement. But in early March, Aetna (NYSE: AET) and United HealthCare (NYSE: UNH) relented and began authorizing full reimbursement for most procedures. That helped push shares back up, and the stock could gain another lift if Cigna (NYSE: CI) and Humana (NYSE: HUM) also remove the “experimental” designation. The decision should be a no-brainer, as Nuvasive’s MAS platform is actually more cost-effective than other approaches when hospital stays and other factors are accounted for.

As is the case with many high-growth stocks, shares look expensive based on current operating metrics, but attractive based on longer-term metrics. For example, the P/E ratio on likely 2010 profits is about 35. But assuming the company can build revenue to about $700 million by 2012,  profits would likely exceed $3 a share, implying a much more reasonable forward P/E of 14. Of course, the days of +50% top-line growth are well behind, but +20% annual sales growth and +30% to +40% annual profit growth appear quite feasible during the next three to five years. In that light, a P/E of 20 on projected 2012 profits yields a $60 target — nearly +50% above current levels.