Understanding Implants in Knee and Hip Replacement

An interview with Douglas E. Padgett, MD

Information about knee and hip replacements is more easily available than ever before. But direct-to-consumer advertising, online articles and other reports in the media may make it more difficult for people considering these surgeries to understand their options. Becoming familiar with the key considerations that go into selection of the implant that is used to replace the injured or damaged joint can make the process easier.

This article provides answers to many of the questions surgical patients may have about implants (also called prostheses); readers can access specific topics directly by clicking on the following headings:

Types of Knee Replacement

In the majority of total knee replacements, the surgeon places components of the implant (also called a prosthesis) on the joint surfaces of three bones that make up the knee joint: the femur, the long bone in the thigh, the tibia, the larger of the two bones in the lower leg and a patellar (kneecap) component, which glides on the femur, the bone that you feel at the front of the knee. The patellar component is sometimes not needed.

Although there are now many different models of knee implants available from a number of different manufacturers, there are two primary types – one that substitutes for the posterior cruciate ligament (PCL), a central ligament that stabilizes the knee joint, and one that spares it.

Anatomy of the knee joint
Figure 1: Anatomy of the knee


The choice between these two implants usually comes down to surgeon preference," says Dr. Padgett.. "At HSS, the majority of surgeons prefer an implant that replaces the PCL." (Find a knee replacement surgeon or hip replacement surgeon at HSS to suit your specific condition, location and insurance.)

In part, this preference is based on the pioneering work done at HSS by John Insall, MD, Chit Ranawat, MD and Peter Walker in 1973, the orthopedists and bioengineer who developed the total condylar prosthesis, designed to closely replicate the anatomy and functional characteristics of the knee joint. "This really marked the beginning of successful modern knee replacement," says Dr. Paddgett. Since that time, thousands of successful surgeries have been completed using more modern, refined versions of this implant. But research also suggests that equally successful outcomes are achieved with implants that spare the PCL.

"Prospective patients may also read or hear about fixed-bearing implants versus rotating platform implants, and wonder which one is preferable," Dr. Padgett notes. These terms refer to the portion of the replacement that is attached to the tibia.

When fixed-bearing implants are used, a metal component is inserted into the tibia and a polyethylene (plastic) tray is locked into place on top of it. [Figures 2a,b,c]

An image of a knee replacement implant from an article about Arthritis of the Knee from Hospital for Special Surgery An image of a knee replacement implant within the knee joint from an article about Arthitis of the Knee from Hospital for Special Surgery
Figures 2a and b: Implants for the knee

X-ray of a total knee arthroplasty from an article about Arthritis of the Knee from Hospital for Special Surgery
Figure 2c: X-ray of a total knee arthroplasty (total knee replacement)

In rotating platform implants, a similar metal implant is inserted into the tibia, but the polyethylene tray is placed on a circular stem that allows slight rotation of the tray on the metal tibial platform during knee motion. This mobile-bearing design may allow for slightly greater range of motion in the knee. [Figure 3]

Rotating platform knee replacement Rotating platform knee replacement
Rotating platform knee replacement
Figure 3: Example of a rotating platform knee replacement implant: front view, side view, and flexed

Rotating platform implants were originally designed with the younger, more active patient in mind. Some data suggests that there is less wear of the polyethylene component with this type of implant, but a clinical benefit has not been established. (This choice of fixed-bearing versus rotating platform implants is also relevant to people who are candidates for partial knee replacement, in which only one compartment of the knee is replaced.) [Figures 4 and 5]

Example of a fixed bearing partial knee replacement Radiograph example of a fixed bearing partial knee replacement
Figure 4: Example of a fixed bearing unicondylar knee replacement
Mobile bearing partial knee replacement, actual metal component Mobile bearing partial knee replacement model
Figure 5: Example of a mobile bearing unicondylar knee replacement

During knee replacement surgery these implant components are usually connected to the bone with cement, a technique that generally yields excellent results. However, the orthopedic community is studying "cementless" fixation of implants – a technique already in widespread use in hip replacement – in which the components have a trabecular metal surface (one that resembles a honeycomb). This porous design permits the growth of new bone tissue on the surface of the fixed metal components.

Knee Implant Materials, Shapes and Sizes

As described, knee implants generally include components made of metal and plastic. In order to ensure a smooth, gliding motion, and to avoid friction, metal surfaces never come into contact with one another. Metal components are usually made of cobalt chrome (which contains nickel) or titanium (which is known for its strength and lightness). Titanium-only implants are available for patients who are allergic to nickel, but it is more difficult to obtain the highly polished surface that facilitates smooth motion with titanium than it is with cobalt chrome.

Based on the successful use of ceramic components in hip replacement, one manufacturer has developed a ceramic implant for the knee. It will be some time, however, until there is data to indicate whether this will offer any advantage in the knee, which is a very different kind of joint.

The patient's size, weight, and gender also have a bearing on implant selection. While once there were only a few sizes available, surgeons now have a wider selection to choose from. This includes a gender-specific "woman's knee", which is designed to accommodate a woman's narrower bone structure, especially on the femur. This development reflects the trend toward customizing implants to achieve the best results possible, in terms of comfort, function, and longevity.

Prospective patients may find that orthopedists in some hospitals use only one or two different implant systems, whereas, orthopedists at HSS have access to the systems of all the major orthopaedic implant manufacturers.

Types of Hip Replacement

Total hip replacement surgery – in which the damaged joint is replaced with an implant – is associated with a high degree of successful and predictable outcomes.

Orthopedic surgeons may approach the surgery in one of three different ways:

  • the direct anterior approach in which the incision is made on the front of the hip; this approach may be less disruptive to the muscles and soft tissues surrounding the hip joint, however, surgeons who employ this approach usually use a custom operating table and fluoroscopy (a type of X-ray) to guide placement of the implant during surgery; fluoroscopy is not routinely used with the other two surgical approaches
  • the anterolateral approach in which the incision is made on the side of the hip, toward the front of the body
  • the posterolateral approach in which the incision is made on the side of the hip toward the back of the body

Much of the important early work in the development of successful hip implants took place in the late 1960s and early 1970s, including that of British surgeon John Charnley, MD and introduced at HSS by Philip D. Wilson, Jr. MD.

Hip Implant Materials and Fixation

The majority of people who undergo hip replacement receive a traditional hip arthroplasty in which the surgeon uses a stemmed device and prosthetic head to replace the upper part of the femur (the head and neck of the bone) and a hemispherical shaped cup to replace the acetabulum. [Figure 6]

However, some patients may be candidates for hip resurfacing in which the head and neck of the femur are not removed. In this procedure, the surgeon resurfaces or sculpts the femoral head to accept a metal cap with a short stem. Hip resurfacing is usually most successful in male patients under the age of 55, who are larger in stature There is little data to support functional benefit of one type of hip replacement over the other, although if revision surgery is needed, this may be easier after hip resurfacing. [Figure 6]

Illustrations and X-ray imaging of a tradtional hip replaement implant at left and a hip resurfacing implant at right.
Figure 6: Illustrations and X-ray images of a traditional hip replacement versus a hip resurfacing implant
(Hip resurfacing images courtesy of Smith and Nephew)


Hip implants come in two primary types: the traditional single-piece implants and modular models, in which the stem and head of the implant portion that is placed in the femur can be matched independently. Although the single piece implants provide a good fit for many patients, "modular devices were developed to improve the fit of the implant to the patient's specific anatomy," explains Dr. Padgett. However, he adds, some modular implants have recently been found to be associated with problems related to the linkage between the various parts.

Some degree of corrosion and fretting has been seen with these metal on metal components, a process that can result in the creation of metallic debris that is destructive to the soft tissue surrounding the joint. As a result, some of these implants have been recalled.

Implants may be made of a variety of materials including metal (usually titanium), ceramic or polyethylene (a type of hard plastic). Ongoing research and enabling technology will determine new directions in materials used in hip replacement surgery. At present, bearing surfaces – where the femoral component of the implant meets the acetabulum – may combine in three different ways:

  • Metal or ceramic on polyethylene, in which the metal or ceramic head meets a medical-grade polyethylene socket or acetabulum in the pelvis. This combination is now generally regarded to be the "gold standard" for hip replacement. The polyethylene components currently in use are significantly more wear-resistant and resistant to degradation than earlier generations of these plastics.
  • Metal-on-metal, in which a large metal ball at the top of the femur articulates with a metal socket or acetabulum in the pelvis. This combination was originally developed to offer the benefits of increased stability – related to the size of the head – and the ability to create a very thin, but durable metal shell. Owing to the creation of metal fragments or metallic debris that is destructive to the soft tissue surrounding the joint, use of some of these total hip replacement implants has been discontinued in the United States. Metal-on-metal bearing surfaces continue to be used for hip resurfacing.
  • Ceramic on ceramic, in which both components are made of the same material; this combination has been shown to have good durability, but there is a risk of the ceramic breaking and ceramic on ceramic hips can produce an audible squeak.

Placement of the hip implant components involves an additional consideration: how the implants are bonded to the bones. This can be accomplished either through the use of acrylic cement or with uncemented fixation, in which the surface of the implant is composed of a porous, honeycomb-like surface that allows for the in-growth of new bone tissue to help hold the component in place.

"On the cup portion of the implant, uncemented fixation is clearly superior," Dr. Padgett says. "However, cemented fixation may be preferable in older patients or others with compromised bone quality." With regard to fixation of the stem, Dr. Padgett notes, available data shows no advantage of one type of fixation over the other, and the decision to use one over the other is left to the surgeon's discretion.

Is Newer Better?

Knee implants
Because the essential design of the knee implants introduced in the mid-1970s has resulted in successful outcomes in a significant number of patients, it's not surprising that newer models introduced over the years have involved modest refinements rather than radically different designs. While marketing campaigns may seek to persuade consumers that "newer is better", it is important to bear in mind that it will take time to determine whether modifications to previous implant systems actually fulfill their promise and whether they offer any advantage over existing systems.

One new technology that may enhance the precision of knee implant placement has also received some coverage in the lay media. Many orthopedic surgeons now use custom cutting blocks – models based on the patient's specific anatomy – to refine precision and accuracy during surgery. The cutting block is created based on information obtained with preoperative MRI. The goal of this technology is to allow the surgeon to more precisely plan where surgical cuts in the bone will be made and to preserve as much of the patient's anatomy as possible. Further long term follow up will be necessary to determine if there is benefit in using custom cutting blocks.

Hip and Knee Implants
With a history of excellent surgical technique and outcomes, the focus of new development in this area is on the use of enabling technology to improve preparation and component positioning in hip and knee replacement surgery. These tools include navigation devices that provide three-dimensional spatial orientation and robotics which can also provide tactile feedback during surgery. "The role of these emerging technologies is still evolving," Dr. Padgett notes.

Choosing a Surgeon

As with any orthopedic surgery, people contemplating knee or hip replacement are advised to seek out a surgeon whom they trust, who does a high volume of these procedures – and an institution with a reputation for excellence in the field. Surgeons who focus on a given surgery or technique are most likely to have predictably successful outcomes. Single specialty centers of excellence, such as HSS, also have outstanding anesthesiologists, nurses and rehabilitation therapists who collaborate to achieve the best possible outcomes of care.

At specialized institutions like HSS, surgeons may also serve as consultants in the development of implants. These surgeons are particularly practiced in the use of implants they have helped to develop. All doctors and surgeons at Hospital for Special Surgery disclose such professional affiliations, and this information can be found on each physician's HSS profile web page.

When to Schedule Surgery
Timing of surgery is another important consideration in predicting successful outcomes. It is generally advised that patients should consider surgery once the disability and pain in the knee or hip is affecting their quality of life and they have tried all other nonsurgical means available to alleviate symptoms.

Prospective patients should also take into account the importance of having surgery while they are otherwise in good health. In the past, some middle-aged patients have elected to wait for a joint replacement, because of their concern about the longevity of the new joint; that is, that a second joint replacement surgery might eventually be required. However, not only does the data indicate that knee and hip replacements are lasting as long as twenty-five or even thirty years, but delaying surgery may result in the surrounding muscles becoming deconditioned owing to reduced function. In turn, this may make recovery from surgery more difficult.

Older individuals, including those in their 80s and 90s may also want to consider the benefits of scheduling surgery sooner versus "living with" disability. While the ability to tolerate surgery and recovery, as well as the existence of coexisting medical conditions must be taken into account, recent data shows that joint replacement that helps to preserve function can have an overall positive effect on the health of older patients. Those who are able to be more active have a reduced risk – when compared to their more sedentary counterparts – of various medical conditions including pulmonary embolism, deep vein thrombosis and pneumonia.

"Knee and hip replacement surgery improves the quality of life of thousands of patients each year. However, both advertising claims and misinformation on the Internet can lead to confusion about which implant works best or even an exaggerated sense of risk associated with the surgery. In many cases, a discussion with your surgeon can clarify implant options and establish realistic expectations about hip and knee replacement surgery.

As with any surgery there is always some risk involved. There have been a few knee and hip implant systems that have not worked well," Dr. Padgett says, "and isolated recalls of specific models have occurred. But people should be aware of the shared commitment on the part of industry, hospitals, and surgeons to the welfare of the patient and to addressing any problems with implants that do occur."

One method to monitor performance of hip and knee implants is through a patient registry. The orthopedic surgeons at HSS are dedicated to following the long-term patient centered outcomes of joint replacement surgery in our Total Joint Prospective Clinical Outcomes Research Registry. Using this registry, long term implant performance may be followed and hopefully surgeons can identify as early as possible any poor performing implants.


Douglas E. Padgett, MD
Associate Surgeon-in-Chief and Deputy Medical Director, Hospital for Special Surgery
Attending Orthopedic Surgeon, Hospital for Special Surgery

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