When it comes to organ transplants, medical science recently passed an important milestone but still has a considerable distance to go.

The latest breakthrough came to light at a medical conference in Pittsburgh this week. The conference was told that doctors in West Germany and France have performed the first known operations in which one liver is split in two and transplanted into two patients. Because the liver is the only organ in the body that can regenerate itself, it grows naturally along with the body after being reduced in size.Though the risk is greater than ordinary liver transplants and the new procedure should be considered a last resort, it still can increase the number of liver donations. Unhappily, the supply does not always match the demand. In the United States alone, 450 people are awaiting liver transplants. About half of them are children.

Consequently, the new operation calls attention to the continuing need for liver donors. For that matter, plenty of other organ donors are urgently needed, too.

Though a recent Gallup Poll found that 70 percent of Americans would be willing to donate their organs, only 5 percent to 10 percent of all the potential organs are made available. It seems that only 20 percent of the people who die in this country meet the medical standards for donating their organs for some other person's use.

Moreover, while 20 percent of the people carry cards saying they are willing to donate organs, the cards are often lost. As a result, less than 5 percent of all transplant organs are obtained through the cards.

One of these days, America may want to follow the example of Europe, where it is assumed that a person is willing to donate his organs unless he has specified otherwise. But then even European hospitals are often reluctant to remove organs without the family's explicit approval.

In any event, while medical science can take a well-deserved bow for its latest advances, they are still no substitute for more organ donations.