The Basics of Total Knee Replacement Surgery

A total knee replacement replaces your diseased knee joint and eliminates the damaged bearing surfaces that are causing you pain. The design of the implant offers you renewed stability and minimizes the wear process. Overall there are three benefits from your knee replacement:

Elimination of pain
Improved motion
Minimum wear and tear

Total knee replacement offers the greatest quality of life improvement of all operations. It has one of the highest success rates and one of the best outcomes.

Knee Replacement Components

The lower part of the replacement knee joint is comprised of a flat metal plate and stem that your surgeon will implant in the tibial bone, as shown in the movie above. This tibial tray can be either cobalt chrome alloy or titanium alloy. It can be fixed by either cement or bone “ingrowth”. Next, a polyethylene insert is clipped into the tibial tray to serve as the new knee bearing surface. The upper part of the replacement knee joint consists of a contoured metal shield that fits around the lower end of the thigh bone (femur). The inner surface can be fixed to the cut bone surfaces by the surgeon’s choice of bone ingrowth or bone cement. The outer surface of the contoured metal shield is shaped to allow the knee cap (patella) to slide up and down in its groove. The surgeon may choose to retain the natural knee cap or re-surface it. In this case a polyethylene button will be cemented in place.

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Total Knee Replacement Implants
A diagnosis of advanced osteoarthritis of the knee will indicate the need for total replacement of the knee joint. Based upon your age and lifestyle, there are several design options to choose from that will help you return to an active enjoyable life.

Fixed Bearing

The polyethylene cushion may be part of the platform (fixed) or free to move on a metal base (mobile). Most people get a fixed-bearing prosthesis that reduces knee pain dramatically and may last for many years. If you are younger, more active and/or overweight, sometimes a doctor may recommend a mobile-bearing or a rotating platform knee replacement designed for potentially longer performance with less wear. Doctors also consider gender, occupation, disability level, pain intensity, interference with lifestyle and other medical conditions in selecting the appropriate prosthesis.

Medial Pivot

The Medial-Pivot knee replicates the rotating, twisting, bending, flexion, and stability of your natural knee, so it feels more like your natural knee. A normal knee actually pivots on its medial (inner side) condyle. When the knee flexes, the lateral (outer side) side rolls back while the medial side rotates in one place. This design "stays put" or is more stabile during normal knee motion as opposed to sliding forward slightly.

Rotating Platform and Mobile-Bearing

Knee implants imitate the motion of your natural knee. In the Rotating Mobile Knee prosthesis, the polyethylene insert can rotate slightly around a conical post. It also slides back and forth on the tibial tray. This allows the knee to twist and turn (called rotation), as well as move back and forth, flexing and extending (called articulation). The insert absorbs forces across a larger contact surface, helping reduce the amount of wear to the bearing and loosening in places where the implant attaches to bone. However, compared with Fixed-Bearing designs, Mobile-Bearing knee implants are less forgiving of imbalance in soft tissues. They may increase the chance of dislocation and may cost more than Fixed-Bearing implants.

Posterior Cruciate Ligament(PCL)-Retaining or Substituting

The Posterior Cruciate Ligament is one of the major ligaments in the knee. It provides support and stable movement of the knee. In total knee replacement surgery, the PCL can be kept or removed and this choice depends on the condition of the PCL, the type of knee implant or the type of surgery the surgeon likes to do. Each of these designs has advantages and disadvantages. Surgeon preference depends on his or her training and the clinical situation.

PCL-Retaining
In PCL-Retaining designs, rearward movement of the tibia is resisted by an intact PCL, which creates stability.

PCL-Substituting

PCL-Substituting knees (also called posterior stabilized knees) have a raised sloping surface or a polyethylene post that compensates for the missing PCL to give your knee more stability.

Special Need Knee Implants

Narrow Femur vs. Normal Femur

Narrow Femur Vs Normal Femur Special Need Narrow Femur

People with smaller skeletons present challenges that require the adoption of implants constructed for their special needs. Research shows that regular-sized implants may overhang on the bone and lead to soft tissue interference or mid-flexion instability in those people with narrow femurs.

Implant manufacturers are now introducing special need implants for smaller than average patients. These new implants (represented in teal in the image to the right) are designed to accommodate those male or female femora with smaller than average dimensions. This helps ensure the special need patient will receive the best implant fit possible.

Custom Fit Through Graduated Sizing

An important issue for knee implant design is how the kneecap moves against the new femoral component. Knee implants are designed with a groove, or track, that the kneecap rides in as the leg straightens and bends. How this track is designed is important because it partially determines how mobile the knee is and, to a certain extent, the level of comfort the implant provides.

Implant manufacturers who offer a broad range of implant sizes will best be able to fit special need patients. This is important because implants that are too wide can hang over the bone and irritate the soft tissues in the knee.