Knee replacement surgery has not been in widespread use quite as long as total hip replacement surgery, but it is a close second, with knee replacement designs continually evolving over the past 3 decades. One reason that hip replacements predated knee replacements is the increased complexity of the knee joint; the human knee is a significantly more complicated structure than the hip, and the biomechanics are correspondingly complex.. The earliest designs treated the knee as a hinge (and some salvage-type prostheses used for tumors or other large surgeries still work in that manner), but it turns out the knee is actually more complicated than that, resembling a four-bar linkage with some additional complexities such as the screw-home mechanism and femoral rollback.
As with hip replacements, there are numerous different designs and variations on knee replacement prostheses. This chapter discusses the most common features of the principal types of prostheses. Materials are also discussed.
Decision To Select The Implant
There are many factors that the surgeon must consider when choosing an implant. Firstly, there are many different options today, including cemented versus noncemented components, cruciate retaining and posterior stabilized designs, mobile bearing knees, partial and total replacements, and several different choices for bearing surfaces. Secondly, there are advantages and disadvantages to each of these choices, and the rest of this chapter presents the most common pros and cons of each design choice. Thirdly, it is a fact of life that cost is increasingly becoming a factor in this aspect of the surgery, and in many places around the country, surgeons are at least somewhat limited by hospitals (and in some regions, limited quite severely) to what choices of implants can be considered. In some cases, the newest and best prostheses may cost more than the hospital will be reimbursed by insurance or Medicare, and the hospital may have policies in place to limit the use of such implants.
In general, operating rooms may often allow orthopaedic surgeons who perform a large volume of joint replacement surgeries more latitude because of the numbers of patients in which the hospital does not take financial losses. Surgeons performing a small number of joint replacements each year may not have that flexibility at the community hospital. We have been fortunate in our institutions to have the ability to choose from a wide variety of implants for our patients.
Partial (Unicondylar and Patellofemoral) and Total Knee Replacements
As discussed in previous chapters, some patients are candidates for partial knee replacements, while others need a total knee replacement. The most common types of partial knee replacements replace the inside (medial) side of both the tibia and femur surfaces. These types of replacements, called unicondylar knee replacements, typically have metal surfaces (usually titanium or cobalt chrome) that are fitted to the tibia and femur surfaces with cement, and some sort of polyethylene (plastic) bushing fits between them to decrease friction. Less commonly, a patellofemoral replacement may replace only the undersurface of the knee cap (patella) and the groove that it travels in.
A total knee replacement is so named because it completely replaces the contact surfaces over the ends of the femur and tibia. This also typically has some sort of low friction polyethylene bushing placed in between. There is considerable variety among total knee replacement designs.
Cemented And Noncemented Knees
In the early days, all knee replacements were secured into the bone with cement. This is still the case for most knee replacements, for mechanical reasons that are discussed in the knee arthroplasty section. The cement itself has changed very little, and polymethylmethacrylate (PMMA) cement is utilized in all sorts of orthopaedic applications where cement may be needed. In actuality, it functions more as a grout than true cement, filling in the porous spaces around a prosthesis.
The cement offers the advantage of initial strength. At the time the patient leaves the operating room, it is as strong as it will ever be. This is also the downside, given that cemented components eventually loosen like a cobblestone in a cemented walkway. Although noncemented prostheses are now outperforming many cemented prostheses in hip replacement surgeries, this has not proven to be the case with knee replacements. Noncemented knee replacements are "press-fit" over the ends of the bones, and the bone grows into the porous surfaces on the backsides of these devices. While somewhat more popular in the 1990's, noncemented knee replacement designs are not currently favored by most surgeons because of early loosening that was seen in these devices, and good results have been consistently seen with cemented designs (often lasting 15 to 25 years).
Another advantage of cement is the the ability to mix antibiotics into it, like vancomycin or gentamicin. These antibiotics are released by the cement over a period of months. Sometimes antibiotic-impregnated cement beads or spacers are placed temporarily into a joint in order to fight infection for weeks or months.
Introduction to Bearing Surfaces
The ends of the femur and tibia are replaced with artificial components, much like capping a tooth in dentistry. Most of the time the endpieces themselves are made from metal, typically cobalt chrome or titanium (and in the case of the tibial component, occasionally completely from polyethylene).
The bearing surface, however, can be made from a different material than the endpiece components. This allows the surgeon to have some flexibility in using the best material for the bone interface (such as titanium, which is structurally strong but does not work well as a bearing) and the best material for the bearing itself (such as cobalt chrome or zirconium oxide). There are a number of different materials now used for the bearings, but the most common are metal on plastic (a cobalt chrome femoral surface on a plastic spacer, which can be exchanged years later if it wears out), hybrid material on plastic (such as using a zirconium oxide ceramic surface, such as OxiniumTM, that moves on a plastic spacer), or using an all-polyethylene (plastic) tibial component.
Cruciate Retaining vs. Posterior Stabilized
Most patients are surprised to learn that the majority of the ligaments in the knee are retained after knee replacement surgery. While there are some designs (such as constrained designs used in revision surgeries) that do assume the function of the body's ligaments, for most primary (first-time) knee replacements it is advantageous to keep many of the patient's ligaments to make the knee function more anatomically and to keep it stable.
Nearly all knee replacement designs do assume that the anterior cruciate ligament (ACL) is no longer viable, as it often is torn or absent by the time knee replacement surgery is needed, but most also assume that the medial and lateral collateral ligaments (MCL and LCL, the ligaments that keep the knee from moving side to side) are still intact and retained.
A principal difference is whether or not the knee replacement keeps or replaces the posterior cruciate ligament (PCL). This ligament is normally responsible for keeping the knee from sliding backwards under the thigh, particularly when going up or down stairs and inclines. A cruciate retaining design assumes that the PCL is left in place, and it is thought to offer some advantages in reproducing the knee's natural motion (particularly femoral rollback). A posterior stabilized design uses a plastic post in the center of the knee to replace the PCL, which may be needed if the posterior cruciate ligament is loose, torn, or absent.
In some cases, it may be desirable to replace the posterior cruciate ligament if the patient has an inflammatory arthritis (such as rheumatoid arthritis) in which the PCL may be expected to deteriorate over time. However, a posterior stabilized design does have the disadvantages of requiring more bone removal from the femur and uncommonly resulting in the knee "jumping the post," in which the femur may jump over the post (or cam) and essentially dislocate, requiring a visit to the hospital to relocate the knee.
Some surgeons routinely use one type of design or the other, and although there are advantages and disadvantages, both types in general have very good outcomes and few mechanical problems. In our practice, we generally only use the posterior stabilized design in patients who have a torn or absent PCL or have an underlying disease process that may predispose to ruptures of the PCL in the future.
Mobile Bearing Knees
Mobile bearing knees have been used for a number of years, but are not used in the majority of knee replacements. These designs employ a slightly different design philosophy, allowing the plastic (polyethylene) bearing surface to spin. The theoretical advantage of this is to separate the motions for flexion and extension from rotation, possibly improving bearing wear and biomechanics.
Over time, however, these advantages have not been clearly identified in most studies, and the majority of orthopaedic surgeons performing joint replacements find there is little difference in the long term performance of these designs from standard designs. However, mobile bearing knees can on infrequent occasions "spin out," similar to a dislocation. For this reason, we typically do not employ them in our own practice, although it should be noted that surgeons in some centers have reported good results with these types of designs.
Constrained Knees, Augmentations, And Stems
Revision knee replacement surgeries are significantly more complex than first time knee replacements. Besides requiring removal of scar tissue, previous components, and any remaining cement, these procedures often have special requirements, such as dealing with knee instability.
Revision knee replacement designs may include a number of features not seen in most primary (first time) replacement prostheses, including "constrained" knee designs. These designs make up for a lack of knee stability by having matching curved surfaces that do not allow much give outside of normal flexion and extension. In cases of severe instability, such as patients facing revision surgery after multiple previous surgeries or large excisions (such as with tumor surgery), a hinged prosthesis may actually be used. These do not recreate the motion of the knee as well as those designs used for primary surgeries, but they can be used to treat difficult problems with knee instability.
Some patients have large cysts present (seen at the time of surgery or on their x-rays) that may be large enough to plan on grafting and filling at the time of surgery. Some bones have cysts (hollow areas) or thinned areas of weakened bone that will need to be examined and possibly grafted. If areas of bone are significantly weakened, then struts of graft bone may need to be wired or cabled around the shaft for additional support.
Sometimes the underlying bone may not be as strong as it needs to be, or in some cases, large defects (e.g., holes) may exist from previous surgeries, infections, or trauma. These can be filled in with metal augmentations, such as metal wedges that fit under the joint surfaces to fill and "shim" the missing area. Sometimes long stems may be needed, similar to the keel under a sailboat, that anchor the knee replacements into the femur and/or tibia. These are commonly used in revision surgeries.
Other Factors - Geometry and Gender Specific Knee Replacements
This chapter is by no means an exhaustive list of the different factors used in implant selection, but it acquaints you with the major design differences and the general benefits and drawbacks of each.
The surgeon also considers a number of other factors in implant selection as well. The geometry of the femur varies considerably from patient to patient; some patients have a narrow shape to the femur and others are broad. There are prostheses that fit each of these and other geometries.
Some orthopaedic implant companies have recently introduced "gender specific" knees to try to address geometry differences seen between men and women. This topic is quite controversial in contemporary orthopaedic surgery at the time of this writing; while most surgeons acknowledge that there are general variations in the shapes of the bones between men and women, many surgeons view these new labels (and corresponding large national advertising campaigns) as a marketing "gimmick" to try to drive patients (consumers) to request the use of various manufacturers' implants. As a result, many patients are learning about these offerings through television and magazine advertisements as companies try to increase their market share. Some companies are marketing "knee replacements for women," and not surprisingly, women make up the largest percentage of patients undergoing knee replacement surgery.
Many surgeons, including myself, view the "new" concept of "gender specific" knee replacements with some curiosity. Even between members of the same sex, there are tremendous variations in the size and shape of the implants needed; for this reason, it is more accurate to say that knee replacements need to be "patient specific." In fact, this is the case with most of the modern knee systems, in which large differences in size and shape can be accommodated by selecting the right implant for the right patient. Men may have a small femur and women may have a broad femur, and vice-versa. It is more important to get it right for the individual, by taking the time to carefully measure and template x-rays before surgery and to make careful measurements and implant selection during surgery, rather than to rely on a shape and size that is estimated to work better for one gender over the other.
In the end, there is a long list of factors that can play a role in the implant selection, but your surgeon is trained to consider the pros and cons of each and uses these factors to reach a decision for which implant to use. An educated patient is the best patient, but consider that there is not that much information conveyed in a 30 second television commercial urging you to ask your surgeon to use a particular type of implant. However, your surgeon has spent years studying the different types of implants. If you have questions about why he believes a particular implant is best for you, then you should certainly ask your surgeon.
Please remember the information on this site is for educational purposes only and should not be used to make a decision on a condition or a procedure. All decisions should be made in conjunction with your surgeon and your primary care provider.
