What is a medial retinaculum? Explaining Kristaps Porzingis' injury as Celtics hope to get big man back

Boston Celtics big man Kristaps Porzingis may be heading straight back to the sidelines. Late in the team's Game 2 win over the Dallas Mavericks in the NBA Finals on Sunday, Porzingis suffered a "torn medial retinaculum, allowing dislocation of the posterior tibialis tendon" in his left leg and is considered "day-to-day" moving forward. This new issue, which Celtics coach Joe Mazzulla called a "serious injury," is unrelated to the calf strain that kept Porzingis out for over a month …