Conventional Knee Replacement Surgery
When a knee replacement is performed, small portions of the bone and cartilage on the end of the thigh bone (femur) and top of the shin bone (tibia) are cut and removed.
In the 30 year history of conventional knee replacement (before custom fit computer technology), all of the shaping and cutting measurements and decisions were done in the operating room. The goal has been to place every knee on the same “mechanical axis” or alignment, no matter what that patient’s preoperative natural alignment happened to be. This is discussed further below.
Surgeons use many different instruments to do the actual measuring. Both the femur and the tibia have six different “freedom of motion” calculations to be made during surgery. These include the amount of bone to cut, the angle of the cuts in several different planes and several rotational calculations. Not only is this difficult to do, but many of the guidelines that surgeons have used to try to make these determinations have recently been shown to be quite unreliable.
So not only was this a difficult task, even if the guidelines were reliable, but you can imagine the imprecision in many cases with instruments and guides that have been questioned recently for their accuracy. Computer navigation technology is being used in some large centers; but it can only accurately measure one or two of these motion planes. Custom Fit Total Knee guides measure all six.
Measuring and Cutting Bone
In most conventional knee replacement operations, the surgeon uses a long metal rod, inserted into the femur (thigh bone) to help with the alignment of the femoral implant. The insertion of this rod has been associated with some increased risk of lung problems after surgery. With Custom Fit Knee Replacement surgery, this rod is no longer needed.
Modern knee replacement surgery has had a high success rate, but patients sometimes do not regain the expected range of motion, or say that their knee feels “tight” or unnatural after surgery. There are three major reasons for this:
Challenge of “Balancing” Soft Tissue
In conventional knee replacements, after cutting bone and implanting the new artificial knee, the surgeon must release or balance the soft tissues (ligaments) surrounding the knee. This balancing act challenges the skills of even the most experienced surgeon. One of the major ligaments, the posterior cruciate ligament, is traditionally removed during knee replacement surgery due to balancing difficulty. If these knee ligaments are not properly balanced, the knee may feel too tight or in most instances, too loose or unstable. Instability is the number one reason for having to re-operate and correct a knee replacement. With Custom Fit Total Knee Replacement (CFTKR) surgery, ligament balancing is no longer necessary, the posterior cruciate ligament is saved and the knee is much more stable than it has traditionally been.
Alignment Challenges
All conventional knee replacements have been performed in order to restore an alignment in the leg known as the "mechanical axis”. Surgeons developed this concept many years ago and it has been the basis for knee reconstruction ever since. The concept is that the femur should be angled on the tibia at approximately a 5 degree angle. Nearly all knee replacements try to recreate this ideal “proper” pre-arthritic mechanical axis alignment. Yet recent studies show that only 1 out of every 65 normal (non-arthritic) legs actually conform to this “ideal.” Every individual and every individual knee is to varying degrees, unique.
Rotational Challenges
The other challenge the surgeon faces with proper alignment has to do with rotation. Unless the femur and tibia are aligned in proper rotation, the foot may also be improperly rotated. This could lead to less range of motion and pain. In conventional knee replacement, surgeons use two little bumps (epicondyles) on the inside and outside of the end of the femur as anatomic landmarks. They calculate an imaginary line passing between the epicondyle bumps. Orthopaedic surgeons have been taught that when we draw a line through the bone, connecting those two bumps, that we can use that line to judge the rotation of the knee. Once again, recent studies show that this transepicondylar axis is as inaccurate as the mechanical axis discussed above. With CFTKR we no longer have to rely on this inaccurate measuring system, as the computer model finds a much more accurate “rotational axis” which is the basis for proper knee function.
Conventional Knee Replacement Surgery – The Bottom Line
Just as every person is an individual, every knee is too. As it stands now, surgeons have to use our best judgment with our most important slopes, angles and rotations. Moreover, the two anatomic alignment calculations we have been taught to use for this over the years have now been found to be inaccurate for many knee surgery candidates. Modern knee replacement surgery has enjoyed a high success rate, but patients sometimes do not regain the expected range of motion. Or they say that their knee feels “tight” or unnatural after surgery. But what does that mean?
Improper alignment is like setting a door with hinges that aren’t properly placed. The door will still function as a door. It may swing a good bit in the middle, but not open or close all the way. Or, it might jam or feel tight. Much the same is true with knees. If we do not restore the knee to its previous alignment, how can we expect it to function in the best possible manner?
Remember, a few degrees can make a huge difference. Precise alignment of the bone surgical cuts and balancing of ligaments are the keys to successful knee replacement. CFTKR surgery is the only technology to address these issues accurately and anatomically.
Other Useful Links
