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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MIS Knee Replacement May be Linked to Early Failure?

A high incidence of failures performed using a Minimal Incision Surgery (MIS) technique was noted in a paper presented at the 2009 annual meeting of the American Academy of Orthopaedic Surgeons (AAOS). 1

In total knee replacement, the worn ends of the bones that make up the knee joint are resurfaced with metal and plastic implants. The surgeon must carefully align the implants relative to the bones and must restore the proper alignment of the bones to one another. If proper alignment is not achieved, excessive wear and premature loosening of the implants may result.

In “Minimal Incision” or “Minimally Invasive” Surgery, exposure of the surgical site is limited. Often the landmarks used in standard total knee replacement surgery may be obscured. The surgeon must rely on experience and sophisticated instruments to properly align the components. Although MIS knee replacement has grown in popularity in recent years, the possibility of issues arising due to the limited exposure has remained a primary concern.

In their paper, Dr. Derek Miller et. al. examined a series of revision cases performed by five surgeons at three different centers between 2004 and 2006. Excluding revisions due to infection and re-revisions, 236 first time revisions were recorded. Of those, 43 were originally performed via MIS, and 193 were revisions of implants first implanted using a “standard” knee replacement technique. In comparing the two groups of revisions, the findings were dramatic. On average, the time to revision was 14.8 months in the MIS group compared to 80 months for the standard group. Thirty-seven percent of the MIS group were failures less than 12 months after surgery compared to only 5% in the standard group. More than eight out of ten of the MIS revisions were in total knee replacement patients that had their implant in for less than two years.

Although MIS knee replacement may lead to quicker recovery and less scarring, this study suggests that the long-term performance of these procedures may be compromised. It remains to be seen whether continued advances in the technique and surgeon experience can counteract this trend.

1 Miller DW, Barrack RL, Barnes CL, Clohisy, JC, Maloney WJ, “Minimal Incision Surgery As A Risk Factor For Early Failure Of Total Knee Arthroplasty?” 2009 AAOS Annual Meeting, Podium No. 272

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Knee Replacement Allergies – Nothing to Sneeze At?

Knee Replacement Allergies – Nothing to Sneeze At?

Each year, hundreds of thousands of knee replacements are performed. In knee replacement, the worn ends of the bones that make up the knee joint are replaced with metal and plastic implants. In most cases, results are excellent, however in rare cases, an immune response may arise and can become problematic. That is, the patient may suffer from an allergy or hypersensitivity to the metallic components of the implant. Symptoms may include warmth or swelling at the knee, a skin rash and even loosening of the implant.1

This phenomenon is so rare, that its very existence has been debated. But at this year’s annual meeting of the American Academy of Orthopaedic Surgeons, a paper was presented documenting what appear to be allergic reactions to knee replacements.2 Dr. Alvin Ong et. al. recorded 19 cases from 2005-2008 where total knee replacement failures were attributed to metal sensitivity. The authors ruled out all other possible causes of implant failure and then performed a special test, the lymphocyte transformation test (LTT) which looks at a patients sensitivity to certain metals. Seventeen of the nineteen patients with failed knee replacements were tested. Ten patients recorded high sensitivity while the remaining seven indicated mild sensitivity.

Following the sensitivity testing, 16 of the patients had a second procedure where the traditional metal was removed and replaced with a non-metallic implant (the ceramic Oxinium implant from manufacturer Smith and Nephew). Following this knee replacement, the authors reported good results and elimination of the inflammation that was present with the traditional metal implant.

So although an allergic reaction to metal knee replacement implants may be rare, it is not unheard of. Unfortunately, diagnostic tests like LTT are relatively expensive and persons testing positive are not always symptomatic.1 In their article, Dr. Ong et. al. suggest that surgeons be mindful of the possibility of metal hypersensitivity especially when failures of implants are noted and cannot be attributed to other factors.

1 Rabin SI, Graf CN, Hopkinson, WJ, Hallab NJ “Immune Response to Implants.” emedicine.medscape.com. 08 Mar. 2009. <http://emedicine.medscape.com/article/1230696-overview>

2 Jafari SM, Della Valle CJ, Orozco F, Ong AC. “Metal Hypersensitivity Following Total Knee Arthroplasty: A Real Phenomenon?” 2009 AAOS Annual Meeting, Poster No. P134

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“Total Knee Replacement: Get it While You Can”

Each year, the American Academy of Orthopaedic Surgeons (AAOS) holds its annual meeting. This year, two presentations highlighted a pending “perfect storm”: the demand for (total knee arthroplasty) will increase dramatically while the supply of Orthopaedic Surgeons trained to do them will decrease sharply.

In a paper entitled, “Joint Replacement Access in 2016: A Supply Side Crisis,” 1 Dr. Thomas K Fehring et. al. stated that “demand for arthroplasty is expected to double in 10 years.” By 2016, this translates into an annual demand of 1,046,000 knee replacement procedures. At the same time, the expected supply of Orthopaedic surgeons is expected to fall such that if they were to work at current rates, they will only be able to perform 287,759 knee replacements, leaving many “waiting in pain”.

In a related presentation, “National Projections of Younger Patient Demand for Primary and Revision Joint Replacement,” 2 Dr. Steven Kurtz, et. al. explained the “demand side” of this equation. Overall demand for knee replacement is growing, but not just because the population is aging (i.e. a greater number of people over 65), but also because the demand for knee replacement in folks under 65 is growing as well. That is, it will become increasingly more common for folks under 65 to be total knee replacement recipients.

Hip replacement was also highlighted in both studies; however the shortfall is not estimated to be as severe. Ultimately, both papers warn that something must be done to alleviate this pending crisis.

References:

1 Odum S.M., Iorio R., Fehring T.K. “Joint Replacement Access in 2016: A Supply Side Crisis” 2009 AAOS Annual Meeting”, Poster Presentation P043

2 Kurtz S., Lau E., Ong K., Kelly, M.P., Bozic K.J. “National Projections of Younger Patient Demand for Primary and Revision Joint Replacement”, 2009 AAOS Annual Meeting, Podium No. 183

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