“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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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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