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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 total knee replacement 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.

Knee Replacement “Clicking”

Following knee replacement surgery, 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.

Minimally Invasive Knee Replacement

Surgeons and the developers of knee implants have begun to market an alternative to standard knee replacement surgery that has been termed “Minimally Invasive Surgery” or “MIS” Knee Replacement. This should really be termed “Less Invasive” since a significant (although reduced) incision is still required, the entire joint and bones are still exposed and “standard” implants are still used. What makes it less invasive is the exposure that the surgeon chooses to work with. Often this means less visibility for the surgeon and a more difficult procedure.

The possible benefits to the patient are:

• The potential for less scaring
• Possibly more rapid recovery since the surrounding muscles and tissue are left more intact.

However, this procedure can be quite challenging for the surgeon that is less experienced or new to this type of operation. The surgeon must rely on landmarks which are less visible and must use smaller instrumentation to position and align the implant components. This is important to note because malalignment of implants is one of the key short-term failure mechanisms in Total Knee Replacement (although newer designs are more “forgiving” than older versions).

For more information on Minimally Invasive Knee Replacement, you can visit some of the company sponsored sites.

Stryker: http://www.miknee.com

Total Knee Replacement Surgery Overview

In general, Total Knee Replacement 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.

Knee Replacement FAQ

Q. What, exactly is “replaced” in knee replacement?

A. The worn ends of the femur ( thigh bone), tibia (shin) and patella (knee cap) are removed with a saw and replaced with metal and plastic. More on Knee Replacement Surgery

Q. What, is a Knee Replacement Implant made of?

A. Implants are typically made of a combination of Cobalt Chrome and/or Titanium metals and UHMWPE (ultra- high-molecular-weight polyethylene) plastic. More on Knee Implants.

Q. Are there different kinds of implants?

A. Yes! There are many types of implants that fit into broad categories related to their interaction with the remaining natural soft tissue. For example a CR or Cruciate Retaining knee implant is designed to work with the PCL (posterior cruciate ligament) intact. Other types of implants are designed to work without it. In addition, there are many manufacturers of implants which create several brands of knees in each of the broadcategories. Surgeons have many, many choices. More on Types of Knee Implants. Links to Implant Manufacturers

Q. How long will my Knee Replacement last?

A. There are many studies on this topic and the actual time an implant will survive before needing revision will vary on a case by case basis. Several studies demonstrate greater than 90% survivorship at 15 years (1)

Q. How are the implants held in place?

A. Total knee replacement implants are secured to the bone either with a special polymeric compound (“bone cement”) or are “press-fit” into place. Press fit implants have rough or porous under surfaces into which bone will grow. More on Knee Implants.

Q. What is a Uni (or Partial or Half-knee) Replacement?

A. A “Uni” or unicompartmental knee replacement replaces only the medial (inner) or lateral (outer) half of the knee joint (the tibio-femoral joint). The knee cap or patella is not replaced. More on Unicompartmental Knee Replacement

Q. What is MIS or Minimally Invasive Knee Replacement?

A. MIS or Minimally Invasive Knee Replacement is like standard knee replacement except that the incision is smaller, and care is taken not to resect the quadriceps muscle (also known as “quad sparing”). Also, the patella is not “everted” or “flipped” over This is all intended to result in less trauma and speed recovery. More on Minimally Invasive Knee Replacement.

Q. What about recovery, pain etc?

A. Every case is unique and this discussion is best left for a medical professional. You can read about the experience of other recipients at a very interesting discussion forum at Robin’s Total Knee Replacement.

References 1) Dixon MC, Brown RR, Parsch D, Scott RD. Modular fixed-bearing  total knee arthroplasty with retention of the posterior cruciate  ligament. A study of patients followed for a minimum of fifteen years. J Bone Joint Surg Am. 2005 Mar;87(3):598-603

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