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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Arthritis, Osteoarthritis and Osteoporosis….What’s the Difference anyway?

In this post we will provide some simple definitions for similar sounding terms that are related to the knee joint and overall bone/joint health.

Arthritis

The term simply means an inflammation (-itis) of a joint (arthro-). There are over 100 kinds of arthritis. A joint is an area in the body where two bones meet. In the knee, there are actually three bones which come together. There is the mating of the femur (thigh bone) and tibia (shin bone) forming the tibio-femoral joint and the mating of the femur with the patella (knee cap) forming the patella-femoral joint. Either one or both of these joints can be inflamed and, therefore, be arthritic.

Osteoarthritis

is a specific type of arthritis caused by the breakdown and possibly eventual loss of articular cartilage. Articular cartilage is a special kind of cartilage that covers the ends of the bones where they meet or articulate. In the knee, articular cartilage is found at the mating ends of the femur, tibia and patella. When this cartilage begins to wear down (which can be caused by a number of things such as age and obesity), pain, stiffness, and swelling (arthritis), can occur. This type of arthritis is also called degenerative arthritis, and as the name suggests, usually worsens over time. If it becomes severe enough, Total Knee Replacement may be indicated. For more information, see our section on Osteoarthritis.

Osteoporosis

Unlike arthritis and osteoarthritis, is not related to the joints or joint degradation. It is a change in the actual structure of bone itself. Our bone is a living, dynamic tissue that changes over time. It has a complex structure specifically designed to serve its weight bearing function. Within a long bone like the femur, for example, there are two types of bony structures. Cancellous or spongy bone is found in the interior near the ends of the bones. This type of bone is porous, hence the name. This porous interior is covered by a layer of dense bone known as cortical or compact bone. This bone also forms the hallow shaft of long bones. The picture below shows a cut away view of bone and it various structures.

illu_compact_spongy_bone

In osteoporosis, the major impact on bone is that the porosity of the spongy bone increases. That is, the pores or holes in the “sponge” get bigger. This leads to an overall weakening of the bone and can result in fractures. Common sites for such fractures are the vertebrae, hips and forearm. For more information on Osteoporosis, visit our dedicated section on that topic.

Hopefully these descriptions will clarify some confusion related to these similar terms and conditions.

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Is One Really Better? Fixed vs. Mobile Bearing Knee Replacement

In our recent post “What’s the ‘Best’ Total Knee Replacement?” it was suggested that more often than not, it “may simply be the one which your experienced surgeon has chosen to utilize”. This sentiment was echoed in a recent publication. 1

In the January 2007 Journal of Orthopedic Surgery and Research article, the authors reviewed two major types of total knee replacements: fixed bearing and mobile bearing designs. The theoretical advantages of the mobile bearing design are discussed and the review is broken down into two main sections, a “Biomechanical Review” and a “Clinical Review” where the results of the two types of designs are compared. (See our section on Mobile Bearing Knee Replacement Here, for a brief description of this type of implant).

In the “Biomechanical Review”, laboratory testing of relative wear rates of the plastic components in each type of design is discussed. Also, studies of knee motion comparing mobile and fixed bearings implanted in both cadaver experiments and in actual patients are summarized. Here, the mobile bearing design seems to have less wear and more natural motion.

In the “Clinical Review”, the results of some earlier clinical studies are examined. In these studies, increased wear rate and subsequent osteolysis (see our section on Implant Wear) in the mobile bearing design was observed. At first, this seems to contradict the previously discussed lab studies on wear (where the mobile bearing was superior). However, as the article goes on to explain the increased wear and osteolysis can be attributed to the presence of the additional wear surface on the underside of the mobile bearing as well as to size of the wear particles. Mobile bearings tend to produce smaller particles which actually result in a greater biologic response and osteolysis. Also, the possibility of tibial insert dislocation or “spinout” in mobile bearings is discussed and some observed cases of insert dislocation are shown.

So at this point in the article, the advantage of one type of design over the other is not very clear. The “Clinical Review” continues on with a review of long term clinical results. Unfortunately, that section concludes with the statement: “So far, the theoretical advantages for mobile bearing design to provide long-term durability have not been demonstrated by any outcome study.” Long term performance simply does not help in differentiating the two designs, with both performing similarly. At the end of the day, the study concludes, (as suggested in the previous post) “For the experienced surgeon, one familiar surgical protocol and instrumentation is suggested rather than implant design, either in fixed bearing or mobile bearing.”

1. Huang CH, Liau JJ, Cheng CK: Fixed or Mobile-bearing Total Knee Arthroplasty. J of Ortho Surg and Res 2007, 2:1

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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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New Total Knee Replacements Specially Designed for Female Anatomy

Recently, two major orthopaedic implant manufacturers, (Stryker Corp.  and Zimmer Holdings Inc.) announced the availability of implants specifically designed for women.  We all know that men and women are different, but how does this relate to knee implants?

The key difference is the relative size of the end of the femur or thigh bone.  For a given front-to-back or anterior-posterior (AP) width, the femurs of females tend to be narrower in the medial-lateral (ML) or side-to-side direction compared to males.  An implant which is proportioned based on male anatomy would, therefore, be too wide on a similarly sized female.  The result is that when a surgeon sizes the implant based on the AP width of the femur, the implant could “overhang” the narrower female bone and cause irritation of the surrounding soft-tissue.  Faced with this situation, orthopaedic surgeons are sometimes forced to “downsize” the femoral component during total knee replacement surgery.  In downsizing the femoral component, the doctor chooses a smaller implant, so that the implant does not overhang the sides of the bone.

There are, however, some potential disadvantages to downsizing depending upon the type of instrumentation system the surgeon is using during the knee replacement procedure.  If the surgeon is using an implant system where the femoral component is placed at a particular distance relative to the anterior surface of the femur (anterior referencing), excessive laxity in flexion can occur with downsizing.  Alternatively, if the doctor is using a system which references the posterior aspect of the femur, the resulting femoral preparation could “notch” or undercut the anterior aspect of the femur.  This could, in turn, lead to fracture of the femur.

Recognizing that neither of the above scenarios is desirable, the makers of implants specifically designed for females have sought to remedy this by making the implants more narrow in the ML direction.  On a male knee, the implant could “underhang” leaving some distal femoral bone exposed, but this is generally not a problem.

In the company announcements, both manufacturers noted that their new “Female” knee replacements are designed to address the more narrow female femur.  Zimmer also noted that differences extend beyond femur width to things such as the angle of the femur in relation to the tibia.  It will be interesting to see how these new implants fair clinically and whether the potential advantages are realized.

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Knee Replacement “Clicking”

Following , patients sometimes report hearing clicks or a clicking sound during certain activities or at a certain point or points during their gait cycle.  In most cases, this sound is believed to be benign and is not associated with pain or other adverse consequences.

So what causes this clicking?

To answer this question, it is helpful to understand a little bit about the nature of total knee replacement.  In knee replacement, the ends of the bones that make up the knee joint are “resurfaced” with metal and plastic components.  The surgeon uses specialized instrumentation and surgical technique to properly align the implants to the bones and the bones to one another.

This alignment is not straightforward because in the replaced knee joint, as in the normal knee, the bones are not “mechanically interconnected”; rather the joint is constrained by the remaining soft tissue (ligaments, muscles/tendons) and the conformity of the implant components.  During the knee replacement operation, the surgeon works to optimize range of motion and joint stability through proper alignment and sizing of the implant.

The resulting tension in the replaced joint can, therefore, vary slightly from patient to patient and can be different within the range of motion of a single patient.  That is, some patients may have “tighter” knees or “looser” knees or a single knee can be “tight” in flexion, but “loose” in extension and vice versa, but in general, some degree of laxity is desirable to allow for adequate motion.

Because there is almost always some laxity in a replaced knee, clicking can sometimes be heard as a result of contact between the metal and plastic components during activity.  Often it is heard during a transition from low or non-weight bearing to weight bearing.  For example, a patient may hear the click while walking as the leg comes out of swing phase and makes contact at heal strike.  For the most part, the clicking is usually not associated with any adverse conditions.

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