This article is from the Recreational Figure Skating FAQ, by Karen Bryden with numerous contributions by others.
Landing your knee when you fall, although quite painful, has often no
worse consequences than a bruise. If you hit your knee, get off the
ice, put ice on the knee and put your foot up. You want to apply the
ice for 20 minutes and be sure and keep a piece of cloth between the
ice and the skin to prevent frostbite.
In some rare cases, a hard blow to the knee can be a cause of
misalignment of the knee cap, which in turn can lead to chronic knee
pain by wearing down of the cartilage. This condition is known as
"chondromalacia patellae". Most often, the misalignment of the knee is
caused by an strength unbalance between the inner and outer thigh
muscles. The hip configuration can also contribute to this problem
(and it is a reason why it affects women more frequently than men).
The best cure and prevention is off-ice exercises which strengthen the
leg muscles, particularly the inner quads.
If you twist the knee (for instance on a bad jump landing or spin
entry) you can hurt the knee ligaments. Frequently the damage is to
the medial collateral ligament (MCL). A sprain or rupture of this
ligament is characterized by pain on the inner side of the knee and
possibly, a feeling of instability on the knee. Although complete
rupture of the ligament could keep you off the ice for weeks, this
type of injury usually heals well with a combination or rest and
physiotherapy. On the other hand, damage to the Anterior Cruciate
Ligament (ACL) (at the front of the knee below the knee cap) often
requires surgery. A tear of the ACL can sometimes be recognized by the
knee "giving out" when putting weight on it. Your doctor may order a
MRI scan to confirm the diagnostic and rule out cartilage damage.
Paradoxically, it is probably easier to sprain your knee practicing
jumps off-ice than on the ice, because on an unchecked landing your
foot is more likely to stick to the floor while you upper body
continues to rotate, putting lots of torque on the knee. To avoid
this, never "stick" a landing on the floor, but do a little hop as
soon as you feel your toes touching the floor; even better, land on