Thomas H. Lee Partners is joining Linden Capital Partners as an equal investor in orthodontics chain Smile Doctors, at a $2.4 billion valuation, sources tell Axios. Look ahead: The two private equity firms next year plan to run a secondaries process that would move Smile Brands into a continuation fund, one of the sources said, providing them with more time and capital to manage, and ride, Smile Doctors' growth. By the numbers: The partial sale was valued off of about $160 million to $165 million of EBITDA, implying a mid-teens multiple, sources say.