Unicompartmental Knee Replacement


A painful knee can severely affect your ability to lead a full, active life. Over the last 25 years, major advancements in artificial knee replacement have improved the outcome of the surgery greatly. One of the more recent advances in knee replacement surgery is the unicompartmental knee replacement (also known as a unicondylar knee replacement). This type of knee replacement is less invasive than a full knee replacement. The operation is designed to replace only the portions of the joint that are most damaged by arthritis. This can have significant advantages, especially in younger patients who may need to have a second artificial knee replacement as the first one begins to wear out. Removing less bone during the initial operation makes it much easier to perform a revision artificial knee replacement later in life.

This guide will help you understand:

  • What your surgeon hopes to achieve
  • What happens during the procedure
  • What to expect after your operation


The knee joint is formed where the femur (thighbone) meets the tibia (shinbone). A smooth cushion of articular cartilage covers the end surfaces of both of these bones so that they slide against one another smoothly. The articular cartilage is kept slippery by joint fluid made by the synovial membrane (joint lining). The fluid is contained in a soft tissue enclosure around the knee joint called the joint capsule.

The patella, or kneecap, is the movable bone on the front of the knee. It is wrapped inside a tendon that connects the large muscles on the front of the thigh, the quadriceps muscles, to the lower leg bone. The surface on the back of the patella is covered with articular cartilage. It glides within a groove on the front of the femur.

There are two femoral condyles in each knee. The medial femoral condyle (the one closest to the other knee) and the lateral femoral condyle (on the outside half of the knee joint).

Related Document: A Patient’s Guide to Knee Anatomy


The main reason for replacing any arthritic joint with an artificial joint is to stop the bones from rubbing against each other. This rubbing causes pain. Replacing the painful and arthritic joint with an artificial joint gives the joint a new surface, which moves smoothly and without causing pain. The goal is to help people return to many of their activities with less pain and with greater freedom of movement.


The decision to proceed with surgery should be made jointly by you and your surgeon. The decision should only be made after you feel that you understand as much as possible about the procedure.

Once you decide to proceed with surgery, several things may need to be done. Your orthopedic surgeon may suggest a complete physical examination by your regular doctor. This is to ensure that you are in the best possible condition to undergo the operation. You may also need to spend time with the physical therapist who will be managing your rehabilitation after the surgery. Your therapist will begin the teaching process before surgery to make sure you are ready for rehabilitation afterward.

One purpose of the preoperative visit is to record a baseline of information. This includes measurements of your current pain levels, functional abilities, the presence of swelling, and the available movement and strength of each knee.

A second purpose of the preoperative therapy visit is to prepare you for your upcoming surgery. You will practice some of the exercises used just after surgery. You will also be trained in the use of either a walker or crutches. Whether the surgeon uses a cemented or uncemented artificial knee will determine how much weight you will initially apply through your foot while walking. Finally, an assessment will be made of any needs you will have at home once you’re released from the hospital.

Surgical Procedure

Before we describe the procedure, let’s look first at the unicompartmental artificial knee itself.

There are two major types of artificial knee replacements:

  • Cemented prosthesis
  • Uncemented prosthesis

Both are still widely used. In many cases, a combination of the two types is used. The decision to use a cemented or uncemented artificial knee is usually made by the surgeon based on your age, your lifestyle, and the surgeon’s experience.

Each prosthesis is made up of two main parts.

The tibial component (bottom portion) replaces the top surface of the lower bone, the tibia. The femoral component (top portion) replaces the bottom surface of the upper bone (the femoral condyle).

The femoral component is made of metal. The tibial component is usually made of two parts: a metal tray that is attached directly to the bone, and a plastic spacer that provides the slick surface. The plastic used is so tough and slick that you could ice skate on a sheet of it without much damage to the material.

A cemented prosthesis is held in place by a type of epoxy cement that attaches the metal to the bone. An uncemented prosthesis has a fine mesh of holes on the surface that allows bone to grow into the mesh and attach the prosthesis to the bone.

The Operation

To begin the procedure, the surgeon makes an incision on the front of the knee to allow access to the joint. Several different approaches can be used to make the incision depending on whether the outer half (the lateral compartment) or the inner half (the medial compartment) is being replaced. The incisions used to perform the unicompartmental knee replacement are much smaller than those used to perform a traditional artificial knee replacement. For this reason, this surgery is sometimes referred to as minimally invasive.

Once the knee joint is opened, a special positioning device (a cutting guide) is placed on the end of the femur. This cutting guide is used to ensure that the bone is cut in the proper alignment to the leg’s original angles, even if arthritis has made you bowlegged or knock-kneed. With the help of the cutting guide, the surgeon cuts several pieces of bone from the end of the femur. The artificial knee will replace these worn surfaces with a metal surface.

Next, the surface of the tibia is prepared. Another type of cutting guide is used to cut the tibia in the correct alignment.

The metal femoral component is then placed on the femur. In the uncemented prosthesis, the metal piece is held snugly onto the femur because the femur is tapered to accurately match the shape of the prosthesis. The metal component is pushed onto the end of the femur and held in place by friction. In the cemented variety, epoxy cement is used to attach the metal prosthesis to the bone.

The metal tray that holds the plastic spacer is then attached to the top of the tibia. This metal tray is either cemented into place or held with screws if the component is of the uncemented variety. The screws are primarily used to hold the tibial tray in place until the bone grows into the porous coating. The screws remain in place and are not removed.

The plastic spacer is then attached to the metal tray of the tibial component. If this component should wear out while the rest of the artificial knee is sound, it can be replaced. The replacement procedure is sometimes called a retread.

Finally, the soft tissues are sewn back together, and staples are used to hold the skin incision together.


As with all major surgical procedures, complications can occur. This document doesn’t provide a complete list of the possible complications, but it does highlight some of the most common problems. Some of the most common complications following artificial knee replacement are

  • Anesthesia complications
  • Thrombophlebitis
  • Infection
  • Stiffness
  • Loosening

Anesthesia Complications

Most surgical procedures require that some type of anesthesia be done before surgery. A very small number of patients have problems with anesthesia. These problems can be reactions to the drugs used, problems related to other medical complications, and problems due to the anesthesia. Be sure to discuss the risks and your concerns with your anesthesiologist.


Thrombophlebitis, sometimes called deep venous thrombosis (DVT), can occur after any operation but is more likely to occur following surgery on the hip, pelvis, or knee. DVT occurs when blood clots form in the large veins of the leg. This may cause the leg to swell and become warm to the touch and painful. If the blood clots in the veins break apart, they can travel to the lung, where they lodge in the capillaries and cut off the blood supply to a portion of the lung. This is called a pulmonary embolism. (Pulmonary means lung, and embolism refers to a fragment of something traveling through the vascular system.) Most surgeons take preventing DVT very seriously. There are many ways to reduce the risk of DVT, but probably the most effective is getting you moving as soon as possible after surgery. Two other commonly used preventative measures include

  • Pressure stockings to keep the blood in the legs moving
  • Medications that thin the blood and prevent blood clots from forming


Infection can be a very serious complication following an artificial joint surgery. The chance of getting an infection following an artificial knee replacement is probably around one percent. Some infections may show up very early, even before you leave the hospital. Others may not become apparent for months, or even years, after the operation. Infection can spread into the artificial joint from other infected areas.


In some cases, the ability to bend the knee does not return to normal after knee replacement surgery. Dr. Kiritsis takes care to be sure that your knee is able to be fully bent and straightened after the surgery is complete. In addition, he makes certain that there is equal tension on all the ligaments and soft tissues once the surgery is completed.

It is the patient’s responsibility to keep the motion that was given to him at the time of surgery. To be able to use the leg effectively to rise from a chair, the knee must bend at least 90 degrees. A desirable range of motion is greater than 110 degrees.

Sometimes extra scar tissue develops after surgery and can lead to an increasingly stiff knee. If this occurs, Dr. Kiritsis may recommend taking you back to the operating room, placing you under anesthesia once again, and manipulating the knee to regain motion. Basically, this allows the surgeon to break up and stretch the scar tissue without you feeling it. The goal is to increase the motion in the knee without injuring the joint.


The major reason that artificial joints eventually fail continues to be a process of loosening where the metal or cement meets the bone. Great advances have been made in extending how long an artificial joint will last, but most will eventually loosen and require revision.

Hopefully, you can expect at least 15 years of service from an artificial knee, but in some cases, the knee will loosen earlier than that. A loose prosthesis is a problem because it usually causes pain. Once the pain becomes unbearable, another operation will probably be required to revise the knee replacement.

Related Document: A Patient’s Guide to Revision Arthroplasty of the Knee

After Surgery

You will have physical therapy treatments twice each day as long as you are in the hospital. Therapy treatments will address the range of motion in the knee. Gentle movement will be used to help you bend and straighten the knee. Your leg may be elevated to help drain extra fluid in the leg. It is important that you continue to perform the exercises given to you by your therapist and Dr. Kiritsis while you are in the hospital.

Your therapist will also go over exercises to help improve knee mobility and to start exercising the thigh and hip muscles. Ankle movements are used to help pump swelling out of the leg and to prevent the possibility of a blood clot.

Most patients are able to go home after spending two days in the hospital. You’ll be on your way home when you can demonstrate a safe ability to get in and out of bed, walk up to 75 feet with your crutches or walker, go up and down stairs safely, and access the bathroom. It is also important that you regain a good muscle contraction of the quadriceps muscle and that you gain an improved knee range of motion.

Dr. Kiritsis recommends regular checkups after your artificial joint replacement. How often you need to be seen varies from every six months to every five years, according to your situation and what your surgeon recommends. You should always consult Dr. Kiritsis if you begin to have pain in your artificial joint, or if you begin to suspect something is not working correctly.

Most patients who have an artificial joint will have episodes of pain, but when you have a period that lasts longer than a couple of weeks you should consult your surgeon. Dr. Kiritsis will examine your knee in search of reasons for the pain. X-rays may be taken of your knee to compare with X-rays taken earlier to see whether the artificial joint shows any evidence of loosening.


Once discharged from the hospital, you will begin your outpatient therapy. Your therapist may use heat, ice, or electrical stimulation to reduce any remaining swelling or pain.

You should continue to use your walker or crutches as instructed. Our goal is for you to discontinue these devices as soon as you are safe. We want you to resume your normal life as quickly as possible.

Your therapist may use hands-on stretches for improving the range of motion. Strength exercises address key muscle groups including the buttock, hip, thigh, and calf muscles. Endurance can be achieved through stationary biking, lap swimming, and using an upper body ergometer (upper cycle).

Therapists sometimes treat their patients in a pool. Exercising in a swimming pool puts less stress on the knee joint, and the buoyancy lets you move and exercise easier. Once you’ve gotten your pool exercises down and the other parts of your rehab program advanced, you may be instructed in an independent program.

Finally, a select group of exercises can be used to simulate day-to-day activities, such as going up and down steps, squatting, rising on your toes, and bending down. Specific exercises may then be chosen to simulate work or hobby demands.

Many patients have less pain and better mobility after having knee replacement surgery. Your therapist will work with you to help keep your knee joint healthy for as long as possible. This may require that you adjust your activity choices to keep from putting too much strain on your new knee joint. Heavy sports that require running, jumping, quick stopping or starting, and cutting are discouraged. Cycling, swimming, and level walking are encouraged, as are low-impact sports like golfing or bowling.

The therapist’s goal is to help you improve your knee range of motion, maximize strength, and improve your ability to do your activities. When you are well under way, regular visits to the therapist’s office will end. The therapist will continue to be a resource, but you will be in charge of doing your exercises as part of an ongoing home program.

A portrait of Doctor Paul Kiritsis, MD smiling.

Paul Kiritsis, MD

Dr. Kiritsis, a Richmond native, is one of a select number of Orthopedic Surgeons in the Richmond area to hold a second subspecialty board certification in Sports Medicine.

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