The largest joint in the body, the knee is made up of the lower end of the thighbone (femur), the upper end of the shinbone (tibia), and the kneecap (patella). The ends of these bones, where they touch, are covered with articular cartilage, a smooth substance that protects the bones and enables them to move easily. Between the femur and tibia are C-shaped wedges, called menisci, that act as shock absorbers to cushion the joint. Large ligaments holding the femur and tibia together provide stability and long thigh muscles give the knee strength. All remaining surfaces of the knee are covered by a thin lining, or synovial membrane, that releases fluid to lubricate the cartilage, reducing friction to nearly zero in a healthy knee. Normally, these components work together in harmony, but when disrupted by disease or injury the result can be pain, muscle weakness and reduced function.
Unicompartmental (or partial) knee replacement is an option for a small percentage of patients with osteoarthritis of the knee that is limited to a single compartment of the knee. During this procedure, only the damaged compartment is replaced with metal and plastic, while the healthy cartilage and bone in the rest of the knee is left alone.
Quicker recovery, less pain after surgery and less blood loss are among the advantages of unicompartmental knee replacement. Because the bone, cartilage and ligaments in the healthy parts of the knee are left intact, most patients report a more natural feeling and better flexibility than with a total knee replacement. For the vast majority of patients that qualify as ideal candidates, unicompartmental knee replacements have consistently demonstrated excellent medium- and long-term results in both younger and older patients.
Compared with total knee replacement, a partial knee replacement offers slightly less predictable pain relief and the potential need for more surgery. For example, if arthritis develops in the parts of the knee that were not replaced, a total knee replacement may be necessary in the future. However, this problem usually can be avoided with careful patient selection.
Your doctor may recommend surgery if: your knee is having a significantly negative impact on your quality of your and ability to perform normal activities; you have advanced osteoarthritis of the knee; and have exhausted nonsurgical treatment options. You may be a candidate for partial knee replacement if your arthritis is limited to only one compartment of your knee. However, you may not be an ideal candidate for this option if you have inflammatory arthritis, significant knee stiffness, deformity, or ligament damage. Your orthopaedic surgeon will perform a complete examination to determine if this procedure is appropriate for you.
Your orthopaedic surgeon will closely examine your knee, take a thorough medical history and order X-rays and possibly an MRI in order to thoroughly evaluate the condition of your knee and recommend the best treatment. If your pain is located almost entirely on either the inside or outside portion of your knee, you may be a candidate for a partial knee replacement. However, if you have pain in front under the kneecap, pain throughout the entire knee, if your knee is too stiff, or if ligaments in your knee feel weak or torn, you may be better qualified for a total knee replacement.
After admission into the hospital, you will be evaluated by a member of the anesthesia team. The most common types of anesthesia are general anesthesia (you are put to sleep) or spinal anesthesia (you are awake but your body is numb from the waist down). Your orthopaedic surgeon and your anesthesia team, with your input, will determine the best option for you. Your surgeon will see you before the surgery and sign your knee to verify the surgical site.
A partial knee replacement operation typically lasts between 1 and 2 hours. During surgery, your surgeon will make an incision at the front of your knee and explore the three compartments to verify that your cartilage damage is, in fact, limited to one compartment and that your ligaments are intact. If your surgeon feels your knee is not suitable for partial knee replacement, a total knee replacement may be performed instead. This contingency plan will be discussed with you before the operation to make sure that you have agreed to this strategy.
If your knee is suitable, your surgeon will continue the partial knee replacement, removing the cartilage from the damaged compartment of your knee and capping the ends of the femur and tibia with metal coverings that are usually held in place by cementing them to the bone. A plastic insert is placed between the two metal components to allow for a smooth gliding surface. Following surgery, you will be taken to the recovery room, where nurses will closely monitor you as you recover from the anesthesia. Once you're awake, you will be taken to your hospital room.
A partial knee replacement is done through a smaller, less invasive incision, typically resulting in shorter hospitalization and faster rehabilitation and return to normal activities. Patients usually experience less postoperative pain and swelling and have easier rehabilitation than those undergoing total knee replacement. In most cases, patients go home the day after the operation.
You will begin putting weight on your knee immediately after surgery. A walker, cane or crutches may be needed for a few days or weeks until you become comfortable enough to walk without assistance. Your doctor will discuss with you what precautions can be taken to avoid problems and complications after surgery, and your physical therapist will help you with specific exercises to maintain your range of motion and restore your strength. You will continue to see your orthopaedic surgeon for follow-up visits at regular intervals and will most likely resume your regular daily activities approximately 6 weeks after surgery.
Your knee implant may activate metal detectors required for security in airports and some buildings. Tell the security agent about your partial knee replacement if the alarm is activated.
As with any surgical procedure, there are risks involved with partial knee replacement. Your orthopaedic surgeon will discuss these with you and take specific measures to help avoid potential problems. Although rare, the most common complications include: infection, blood clots in leg veins or pelvis, bearing dislocation, stiffness, progressive arthritis in unreplaced portions of the knee, loosening and wear of implant, nerve and blood vessel injury, bleeding, risks of anesthesia, and in a small number of patients, continued pain following surgery. Your surgeon will prescribe antibiotics prior to surgery to prevent infection and blood thinners after surgery to prevent blood clots.
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