Studies of hundreds of knee replacements show this can and does happen ... even in patients with no risk factors or deformities. Ligament damage isn't rare but it's uncommon. It can happen to anyone but it does tend to occur in patients who are very overweight.
The surgeon usually repairs the surgically torn ligament. The surgeon may use an implant that is designed for use with a torn or absent ligament. Rotating-hinge knees are favored when the patient has severe joint instability. The doctor's task is to balance the ligaments, stabilize the joint, and restore motion.