When Celtics point guard Rajon Rondo went down with a season-ending torn anterior cruciate ligament suffered on Jan. 25 in Atlanta, Celtics fans cringed.

The hand-wringing intensified for the Green faithful when Leandro Barbosa (traded Thursday to Washington) went down with a similar ACL injury on Feb. 11 at Charlotte.

Then, the very next day, when former Everett star and Kentucky standout Nerlens Noel hurt his knee in a game against Florida, it seemed as if an epidemic of ACL injuries was spreading through the sports world like a wildfire.

However, this sudden rash of injuries touching so close to home is nothing new in the sports arena. Perhaps these latest examples are more high-profile instances of the injury, but, according to the American Academy of Orthopaedic Surgeons, approximately 100,000 to 300,000 people hurt their ACL in the United States every year. The injury can happen to professional athletes and weekend warriors alike, experts say.

“It’s just the force and direction of the impact of injury that causes the tear,” said physical therapist Cynthia Berger, clinical supervisor for the Outpatient Physical Therapy Center at New England Baptist Hospital. “With rigorous sports, ACL injuries are very common.”

What is an ACL?

Basically, the ACL connects bone to bone, and provides stability and coordinates rotation for the knee, said Dr. John Richmond, chairman of the department of orthopedics at New England Baptist Hospital.