Is Knee Replacement in Your Future?

Most knee replacements are performed for the treatment of where the smooth cartilage in the knee joint breaks down resulting in pain, stiffness and swelling. The x-ray below is of a knee where osteoarthritis is present.  Note the close proximity of the upper (femur) and lower (tibia) bones.  In this knee the cartilage, which is not visible on an x-ray has worn away.  If this had been a healthy knee, there would be a distinct gap between the two bones.

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Osteoarthritis is usually progressive.  It often begins mildly, but over time, wear in the cartilage starts to result in an uneven distribution of weight across the knee joint; often with more damage observed on one side versus the other.  This in turn causes even more force to pass through the affected side, resulting in even faster degeneration of the joint.

This localized damage is most commonly found on the inside or “medial” part of the knee (), but it can also occur on the “lateral” or outside part of the knee (lateral compartment).  When it occurs on the medial side, the patient may have a varus or “bow-legged” deformity.  When lateral osteoarthritis is present, a distinct “knock-kneed” or valgus deformity is observed.

These deformities are usually corrected at the time of .

It is interesting to consider what, if anything, may initiate this cycle of uneven wear, worsening load distribution, and further wear.  Is it possible that some folks may be predisposed to this condition due to their bony anatomy?  A presentation at this years AAOS meeting looked to answer this very question. 1 The researchers examined the anatomy in patients receiving knee replacements whose pre-operative varus and valgus deformities were significant.  In those patients, they found that key functional axes in the knee were not aligned normally.  Specifically, they found that the alignment of the “mechanical axis” of the lower limb to the in the knee was not normal. To understand these axes and their observed relationship, a review of basic knee anatomy is required.

In the normal lower limb, the mechanical axis is defined as an imaginary line originating at the center of the femoral head (the “ball” on the “ball-and-socket hip joint) passing through the center of the knee and ending at the center of the ankle.  When the hip, knee and ankle line up in this way, the lower limb and knee joint alignment are considered “normal”.  In a varus or valgus knee, the center of the knee is no longer on this line and is pushed “outward” in varus and “inward”  in valgus.

The epicondylar axis is an imaginary line connecting the femoral (thigh-bone) origins of the medial collateral and lateral collateral ligaments (MCL and LCL).  One can think of these ligaments as the “ropes” or “chains holding a swing.  In this analogy, the epicondylar axis is like the top bar of the swing.  The tibia or thigh bone is the swing which rotates about the bar (epicondylar axis) during knee flexion and extension.

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In their study, the authors observed that in patients with severe varus or valgus deformities, the epicondylar axis was “misaligned” relative to the mechanical axis in the same direction (albeit smaller in magnitude) as in the knee joint deformity.  For example, if a patient had severe osteoarthritis in the medial or inside part of the knee, a significant bow-legged or varus deformity was observed and the epicondylar axis was “tilted” in the same direction.  In other words, the inherent bony anatomy appeared to predispose certain patients to uneven loading across the knee joint (a “crooked swing”), which, in turn appears to have made them more susceptible to the resulting osteoarthritic deformity.  The results were not statistically significant, but a noticeable trend was observed.  So to answer the question “is knee replacement in your future?” one may not have to look much further than one’s own bony anatomy.

1. Beyers-Thering MT, Krackow KA, Mihalko WM. “Relationship of the Femoral Epicondylar Axis to the Mechanical Axis in Deformed Osteoarthritic Knees.” 2009 AAOS Annual Meeting, Poster No. P201

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What’s the “Best” Total Knee Replacement?

(TKR) is an operation where the worn ends of the bones that make up the knee joint are resurfaced with metal and plastic implants.  Surgeons have many options to choose from.  Not only do several manufactures make different brands of knee replacements, but within each brand are different types.

For example, in a “PCL sacrificing” knee replacement, the PCL (posterior cruciate ligament) in the knee is removed and its function is replaced by the special geometry of the implant.  There are also a group of implants known as “mobile bearing” or rotating platform” where one of the components (the tibial insert) actually moves or rotates relative to the metal tibial implant affixed to the tibia (shin bone).

Given the myriad of implants available, it is natural to ask, “Which total knee replacement is best?”  Unfortunately, the answer to that question is not straightforward.  While each manufacture will tout the benefits of their particular design, long-term follow-up of most contemporary implants yields similar results.  In reality, it is probably more important for perspective patients to seek out a good, experienced surgeon, rather than a particular product.

This is because factors such as proper alignment of the implant components and restoration of appropriate soft tissue tension (things which the surgeon directly controls) are more likely to influence the outcome of a total knee replacement than the particular brand of implant.

At the end of the day, perspective knee replacement recipients should seek out a doctor they are comfortable with and one that has a lot of experience with a particular implant system.  Experience is important because the technique of total knee replacement is rather involved.   The surgeon must utilize a series of complex instruments to perform the operation.  Often, these instruments are “implant specific” so surgeons will usually become loyal to a single brand allowing them to develop familiarity and expertise with a particular system.

So the answer to the question “what is best” may simply be the one which your experienced surgeon has chosen to utilize.

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