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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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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The Parts of a Total Knee Replacement

The knee joint involves three bones; the femur, the tibia and the patella. Total Knee Replacement, each one of these bones is resurfaced with a separate component. To “resurface” the bone the surgeon uses a saw guide and small power saw to trim off the outer worn surface of each bone in the exact shape of the inside of the corresponding implant.

The resected bone can be several millimeters thick (about 1/4-1/3 inch).

The separate components work together to form the complete prosthetic device. In some cases the surgeon may choose not to replace the patella and let your native patella mate with the other artificial components (this is quite common in Europe, less common in the US).

The Femoral Component

Typically this is a Cobalt-Chrome metal component. The highly polished outer surface serves as the end of the femur, the other side is either “cemented” or “press-fit” onto the prepared bone surface.

Most implants in the US today are cemented. A polymer based compound is mixed in the OR forming a putty which hardens. The surgeon puts the putty in between the implant and bone to secure it.

In the “press-fit” variety, a roughened surface on the inside of the component has a porous 3-D structure designed to promote bony ingrowth.

There are pros and cons to each type of fixation. As mentioned, the cemented version is more common, but both have well established clinical histories

The Tibial Component

The tibial component is usually a two-piece metal and plastic implant although there are all plastic versions available.

The metallic portion of the implant can be titanium or cobalt-chrome and it is fixed to the tibial bone in much the same way as the femoral implant.

The plastic component is known as a “tibial bearing” or “tibial spacer” or simply “tibial insert.” It is a plastic component made of a special grade polyethylene (UHMWPE). Manufacturers have begun to “treat” the plastic with gamma radiation to improve wear properties.

The Patellar Component

The patellar component is typically an all plastic component that is fixed to the cut surface of the underside of the patella
or knee cap.

The component is secured with bone cement and articulates or “joins up” with the femoral component, as it would with the normal knee.

Because the patella moves up and down across the surface of the femoral component, the relative alignment of the two components is critical. Patella problems are among the most common complications following total knee replacement, although the overall complication rate is small.

Primary Knee Replacement

Primary Knee Replacement is, as the term suggests, the name given to a family of knee implants designed to be used as a first knee replacement. This type of knee replacement allows the remaining soft-tissue to provide some of the constraint to the knee joint and is usually implanted in patients who have not had a previous Total Knee. The term constraint, does not imply limiting  movement, rather it simply allows some of the normal stabilizers in the knee (namely the collateral ligaments, and sometimes the PCL) to still perform their function.

Revision Knee Replacement

Revision Knee Replacement is designed to replace a worn-out and/or loosened Primary Knee Replacement. This type of knee replacement has more intrinsic constraint and can “play the role” of some of the natural stabilizers which may be absent or no longer functioning. In some cases, a revision knee implant may be used as a patient’s first knee implant if the disease is advanced enough.

PS Knee Implants

  • PS, Posteriorly Stabilized, or PCL Substituting Knee Implants are designed to be implanted with the Posterior Cruciate Ligament or PCL removed. In this type of knee replacement, a protrusion on the tibial insert is designed to mate with a specially shaped bar (cam) on the femoral component. This “cam and post” interaction substitutes for the normal function of the PCL by:
    • Limiting anterior or “frontward” movement of thefemur (thigh) relative to the tibia (shin).
    • Promoting posterior or backward movement of the femur relative to the tibia as you bend or flex your knee. This movement is necessary to allow for deep flexion of the knee.
  • Studies have shown that PS knees tend to be more predictable and provide for slightly higher degrees of flexion compared to CR knees (see CR section). This is because the function of the knee is dependent on a fixed metal and plastic mechanism rather than the native PCL whose function and balance can be variable in a diseased knee.
  • For some photos and description of a PS knee you can visit:
    http://www.stryker. com/jointreplacements/sites/scorpioknee/scorpiops.php

CR Knee Implants

  • CR or Cruciate Retaining Knee Implants are designed to be implanted with an intact Posterior Cruciate Ligament or PCL. Often this ligament is still functioning to some degree in patients with advanced osteoarthritis. In this type of knee replacement, the PCL is allowed to act as the primary stabilizer limiting anterior or “frontward” movement of the femur (thigh) relative to the tibia (shin). This is the primary function of the PCL in the normal knee. Another function of the PCL is to promote posterior or backward movement of the femur relative to the tibia as you bend or flex your knee. This movement is necessary to allow for deep flexion of the knee.
  • Studies have shown that CR knees tend to have less predictable and slightly lower degrees of flexion compared to PS knees (see PS section). This is because the function of the knee is dependent on the native PCL whose integrity and “health,” if you will, can be somewhat variable. That being said, there are studies that have shown that the performance of CR knees and PS knees are comparable. If you are so inclined, you can view abstracts of publications at:http://www.ncbi.nlm.nih.gov/entrez/query.fcgi
  • For some photos of CR knee implants you can visit:http://www.stryker.com/jointreplacements/sites/scorpioknee/scorpiocr.php

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Total Knee Replacement Surgery Overview

In general, is a very safe and effective procedure. However, it is a major operation that is quite invasive.

The outline below is indicative of the “standard” approach. A minimally invasive technique is similar, but the incision and disruption to surrounding soft-tissue is reduced. An incision is made a few inches above the knee cap and extends just below the knee into the shin. The outer capsule of the joint is opened and the bones of the knee are inspected. In one type of procedure, both the ACL and PCL are removed (if present). The ACL (anterior cruciate ligament) and the PCL (posterior cruciate ligament) are the primary front-to-back stabilizers of the knee joint. In patients with advanced disease, these ligaments are compromised or absent. (See section on different types of implants)

The link below is an excellent interactive overview of knee surgery-you get to be the surgeon! (it is animated so you do not have to worry if you are a bit squeamish). Having developed both implants and instrumentation, I was amazed at how “realistic” this is. They have some nice photos of actual surgery as well for those of you that are really curious.

http://www.edheads.org/activities/knee/swf/surgery.htm

Femoral Preparation in Total Knee Replacement

A small hole is drilled in the end of the femur (at about the center of the knee joint.

A long rod is placed in the intramedullary canal of the femur (the hollow portion of the bone where the marrow is).

A cutting guide is placed on the rod and rotated so that it sets up a slot to make the femoral distal cut. This is a cut that created a plane that is roughly parallel to the floor if you were in a standing position. This cut is made such that it is perpendicular to a line connecting the center of the hip and the center of the ankle. This line is the mechanical axis of the lower extremity and it is important that the implant is aligned properly with respect to this. In doing so, the load distribution across the implant is optimized and the longevity of the implant maximized.

Next, a cutting guide is placed on the plane of the distal cut, it is aligned in rotation and 4 more cuts are made. The rotational alignment here (which is called internal/external rotation) is important in balancing the soft tissue and aligning the patella track of the implant. The patella track is literally a track or groove in the implant in which the patella moves during knee motion. Incorrect alignment of this track can lead to pain and/or accelerated wear of the patella component.

Once these cuts are made, the preparation is basically finished. Some implants have small fixation pegs on the distal surface so two holes are sometimes drilled for these. In the case of a PS implant (see PS implant post), a “box” is cut out from roughly the center of the prepared femur to make space for the cam/post mechanism of the PS implant The outline below is indicative of the “standard” approach. A minimally invasive technique is similar, but the incision and disruption to surrounding soft-tissue is reduced.

Tibial Preparation in Total Knee Replacement

Typically, the top surface of the tibia is resected creating a planar surface that is perpendicular to the shaft of the tibia. In a “PCL retaining” procedure, a “bony island” is preserved around the insertion site of the Posterior Cruciate Ligament in the tibia. In a “PCL sacrificing” procedure, the PCL and the insertion area are removed.

  • About 8mm of bone is removed as measured off of the “good” side of the joint surface of the tibia known as the tibial plateau. In the arthritic knee, there is usually uneven wear of the joint surface. The joint basically collapses on the diseased side. The resection level is typically a couple of millimeters below the surface of the worn side of the joint, or 8mm off of the “good” side. Creating a flat tibial surface therefore requires a wedge shaped piece of bone to be removed.
  • Once the resection is made, trial components are placed in the joint. With the femoral and tibial prep complete (and often the patella prep as well), and a trial femur and tibia implant in place, the surgeon basically moves and tenses the joint through a range of motion. The surgeon can use tibial insert trials (also called tibial bearings or tibial spacers) of various thicknesses until the correct stability is achieved. The tibial insert trial corresponds to the tibial insert implant that will eventually sit in between the metal femoral and tibial implant components.
  • Often, the surgeon must make surgical corrections of certain soft-tissue structures to help align and stabilize the knee.
  • Once trialing is complete, the appropriate “keel” preparation is made. The keel is a fin or cross shaped protrusion on the implant that helps it stay fixed and in the bone. This can be punched, chiseled, broached or cut into the tibia and a set of instruments is used so that the resulting shape matches the implant. Here, the surgeon must take care in aligning the keel preparation so that the correct rotation of the implant about the axis of the tibia is achieved. Correct rotational alignment of the implant promotes better distribution of load across the implant and better motion of the knee joint.
  • Next, the “tibial baseplate” portion of the implant is simply pressed or cemented in place.
  • A plastic tibial insert is then snapped into the tibial baseplate (see the Implant section for more details). The insert comes in the various thicknesses that correspond to the trial used previously.

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Normal Knee Anatomy

  • The is the mating and movement of three bones; the femur or thigh bone, the tibia or shin bone and the patella or knee-cap. The end of the femur rides on the top surface of the tibia and the patella moves within a groove on the femur.
  • The bones are joined together by ligaments and tendons. These soft-tissue structures guide the movement of the bones and provide the stability needed for normal knee motion. Unlike the ball and socket hip joint, the bones are not interconnected; it is the soft-tissue around the bones which holds the joint in place. This is why one of the key factors in a successful total knee replacement is surgical management and later rehabilitation of the remaining soft-tissue (i.e. muscles, ligaments and tendons).

  • In a normal knee, the tendons attach the muscles to the bones and the ligaments attach the bones to one another. During the knee replacement operation, one or more of the ligaments is removed. Often the normal function of one or more of the ligaments is severely compromised due to the deterioration in the knee joint. One of the main jobs of the knee implants is to recreate the normal function of these removed ligaments.
Knee Joint Anatomy
  • Between the femur and tibia is a soft-tissue pad known as the meniscus. This pad acts as a “shock absorber” helping to evenly distribute the loads or forces between the bones of the knee. The meniscus also provides additional stability during motion.
  • The link below provides a more in-depth review of basic knee anatomy: http://www.kneepaininfo.com/kneeanatomy.html

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