Six years after my total knee replacement, the implant came loose and had to be replaced. The surgeon told me the first joint implant was turned in a few degrees too many and that's what caused the problem. How often does this happen?

Alignment is one of the most difficult aspects of knee joint replacements for the surgeon. Instruments used to make the bone cuts are based on average bone shape. But bone size and shape can vary from patient to patient.

The surgeon can't always tell when a joint with a new implant is still slightly flexed at the end of the operation. In fact it can be bent as much as 10 degrees when fitting the implant in but look straight. There is also a tendency to internally rotate the upper (femoral) half of the implant. The result is often loosening and failure of the prosthesis.

Scientists are working with surgeons to limit and eventually eliminate this problem. The use of 3-D computer programs before surgery may help surgeons plan ahead. Plotting out when and where to make bone cuts and forming a template of the patient's joint may help improve accuracy of alignment.