Thomas H. Lee Partners (THL) has completed a strategic investment in Smile Doctors, according to a news release.

Smile Doctors, based in Georgetown, Texas, is an orthodontics-focused dental support organization. The company reports operating more than 295 locations in 25 states.

THL, based in Boston, is a private equity firm investing in middle market growth companies in three sectors, including healthcare. Founded in 1974, the firm typically targets companies with enterprise values between $250 million and $2.5 billion.

THL is investing in Smile Doctors alongside existing sponsor Linden Capital Partners in an equal partnership.

Linden, based in Chicago, is a private equity firm focused exclusively on the healthcare industry. Founded in 2004, the firm pursues acquisition opportunities across the healthcare industry, including services, products and distribution companies, and prefers to make more substantial investments from a dollars perspective.

Terms of the investment were not disclosed.