Partial Knee Replacement: A Treatment Option in Unicompartmental Knee Arthritis

Orthopedic surgeons at world-renowned HSS perform more hip surgeries and knee replacements—including partial knee replacements—than any other hospital in the nation. As the scientific data demonstrates, patients do best when undergoing surgery at an institution where a high volume of procedures are performed by highly skilled surgeons.

At HSS, you will not only find the surgical volume and expertise that helps ensure a good outcome, but an institutional commitment to leading-edge technique in every aspect of treatment and recovery, including computer-assisted surgical techniques and non-narcotic pain control. If you or a loved one is considering partial knee replacement, look to a leader in the field: HSS.

Partial knee replacement: An overview

Over the course of their lifetimes, approximately one in five Americans will develop knee arthritis. Fortunately, a wide range of nonsurgical and surgical techniques are available to address the discomfort and disability that can accompany this condition. Partial knee replacement is a surgical treatment option that replaces (or resurfaces) only the damaged portion of the knee while conserving knee ligaments and unaffected cartilage.

“Patients with unicompartmental knee arthritis have cartilage degeneration in only one section or compartment of the knee. In cases where nonsurgical techniques do not provide sufficient symptom relief, surgeons can remove damaged cartilage and bone in the diseased area only, while preserving the ligaments that help support the knee joint,” explains Friedrich Boettner, MD, Attending Orthopedic Surgeon at HSS.

A prosthesis—which may also be called an implant—takes the place of the damaged area of the knee, leaving the other compartments intact. (Partial knee replacement surgery may also be referred to as partial knee resurfacing, unicondylar knee surgery, unicondylar knee replacement, or unicondylar knee arthroplasty.)

Over the past 15 years, improvements in surgical techniques and instrumentation have made partial knee replacement a viable option for a growing number of patients; in fact, recent data suggests that anywhere from 10 to 15% of all patients with osteoarthritis of the knee may be eligible for the procedure.

Understanding unicompartmental knee arthritis

Arthritis of the knee may occur in any one of the three compartments that make up the knee joint. The inner or medial compartment of the knee and the outer or lateral compartment of the knee are formed by the articulation (or joining) of the lowest part of the thighbone (femur) and the highest part of the shinbone (the tibia). The third compartment of the knee is formed by the kneecap (patella) and the front part of the femur. This is referred to as the patellofemoral joint.

The medial compartment is the most frequent site of osteoarthritis of the knee, with the disease occurring less frequently in the lateral compartment. (Arthritis that is confined to the patellofemoral compartment of the knee is rare.)


Medial compartment osteoarthritis prior to partial knee replacement Radiograph of Medial compartment osteoarthritis prior to partial knee replacement

Figures: Medial compartment osteoarthritis

Who can benefit from partial knee replacement

Partial knee replacement is appropriate for patients with arthritis that is confined to a single compartment of the knee and is generally restricted to patients who are not morbidly obese. The surgery is not appropriate for patients with marked stiffness in the knee or those with a significant angular deformity. Intact ligaments are generally a requirement for a partial knee replacement. Patients with rheumatoid arthritis are not candidates for partial knee solutions since inflammatory-type arthritis typically involves the entire joint.

Additional considerations are evaluated on a case-by-case basis with the surgeon and patient determining together whether partial knee replacement is the best treatment option. Selecting the right patient is considered one of the most important steps to ensuring a good functional outcome and longevity for a partial knee replacement.

What happens during partial knee replacement?

During partial knee replacement, the orthopedic surgeon makes a small incision to gain access to the affected compartment of the knee. He or she gently moves supporting structures of the knee out of the way and removes damaged cartilage and bone tissue from the surfaces of the tibia and the femur in the arthritic area. The surgeon then prepares these surfaces for insertion of the prosthesis components which are specifically sized to the patient’s joint. Cement is used to secure these components. All surrounding structures and tissues are restored to their anatomic position and the incision is closed.

Partial Knee Replacement animation
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Depending on which compartment is affected and whether the patient’s anterior cruciate ligament (ACL) is intact, the surgeon will use either a unicondylar fixed bearing knee replacement—the most commonly used prosthesis—or a mobile bearing unicondylar knee replacement. These prostheses are made of plastic and metal components.

Example of a fixed bearing partial knee replacement Radiograph example of a fixed bearing partial knee replacement

Figure: Example of a fixed bearing unicondylar knee replacement

Mobile bearing partial knee replacement, actual metal component Mobile bearing partial knee replacement model

Figure: Mobile bearing unicondylar knee replacement

Some surgeons at HSS apply computer-assisted techniques and robotic tools to partial knee replacement. “This technology allows the surgeon to create and manipulate a 3D image of the patient’s knee in advance of surgery, a process that allows for highly precise removal of damaged tissue and refined positioning of the prosthesis during the actual procedure,” says Andrew D. Pearle, MD, Attending Orthopedic Surgeon at HSS.

At HSS, patients undergoing partial knee replacement surgery are usually given regional anesthesia, which numbs the lower half of the body and allows the patient to remain awake during the procedure. Throughout partial knee replacement surgery, patients are carefully monitored to minimize the risk of complications.

Patients who have been properly screened for the procedure can expect to experience a low level of complications and a rapid recovery. However, as with other types of knee surgery, in a small percentage of cases, revision surgeries are required. Results obtained with revision surgery may not be as good as those achieved with primary surgery.

Recovering from partial knee replacement surgery

Following surgery, most patients undergoing partial knee replacement can expect to spend one to two nights in the hospital. At HSS, most patients are able to walk with assistance, or independently, on the same day as their surgery. Typically, the patient is given a cane within a week of surgery to allow for increased independence and begins outpatient rehabilitation. Patients are often finished taking narcotic-type pain medication within four weeks post-surgery.

Partial knee replacement usually involves minimal blood loss and is associated with a low rate of complications; most patients can expect to be back to their daily activities within three to six weeks. Many patients find that after undergoing physical rehabilitation, they are able to return to sports such as golf, within six to ten weeks.

Setting the stage for a successful outcome: Surgery at HSS

Partial knee replacement is widely recognized as a technically demanding surgery. As demonstrated in the scientific literature, data shows that choosing an orthopedic surgeon and institution with extensive experience with this procedure can help ensure a good result. In fact, at high-volume institutions like HSS, in well-selected patients, surgeons achieve the same longevity for partial knee replacement as that reported for total knee replacement.

“It’s important to understand that partial knee replacement is a challenging procedure to perform,” says Michael M. Alexiades, MD, Attending Orthopedic Surgeon at HSS. “At HSS, we collect data on partial knee replacement patients on an ongoing basis. This allows us to continuously refine screening and surgical techniques to achieve predictable results and the best outcome possible.”

Frequently asked questions

Q: I have been told that I might benefit from partial knee resurfacing. Is this the same thing as partial knee replacement?
A: Yes. Partial knee replacement, partial knee resurfacing surgery, unicompartmental knee replacement, and unicondylar knee replacement all refer to the same procedure.

Q: Are there any age restrictions for partial knee surgery?
A: There are no strict recommendations, but in general, partial knee replacements are appropriate for patients over 40 years old who meet other eligibility criteria.

Q: What is the knee prosthesis made of?
A: Knee prostheses or implants are made of metal and plastic. These surfaces are designed to glide smoothly against one another just as cartilage does in a healthy knee.

Q: Will I be able to resume bicycle riding/tennis/skiing after my partial knee replacement?
A: Although not all patients are able to return to unrestricted sport, many patients are able to resume biking, tennis and skiing.

Q: What kind of complications can occur with partial knee surgery?
A: As with any joint replacement surgery, complications may include instability of the knee, loosening of the implant, infection, nerve injury and deep vein thrombosis. Generally, complications occur less frequently after partial knee replacement than they do following total knee replacement. Be sure to discuss any concerns you have regarding these or other issues with your surgeon.

Q: Is recovery from partial knee replacement painful?
A: All surgeries result in some pain. At HSS, we have robust systems and resources dedicated to address post-operative pain management. While pain varies by patient, typically patients experience less pain and stiffness following partial knee replacement than they do after total knee replacement. After partial knee replacement surgery, most patients receive 7-14 days of narcotic medications but HSS also offers a limited narcotic pathway for patients who do not tolerate narcotic medications well. In addition, clinicians and researchers at HSS are currently developing treatment protocols that limit the amount of pain medicine needed after partial knee replacement surgery.

Q: If I decide to have partial knee replacement, does that mean that I will no longer have arthritis of the knee?
A: Partial knee replacement will address the arthritis that is present in the particular compartment of the knee that is affected. However, there is no guarantee that arthritis will not develop elsewhere in the knee. Should this occur, surgical revision to a total knee replacement may be necessary.

Q: How long can I expect my partial knee replacement to last?
A: A well-done partial knee replacement in an appropriately selected patient can have a survival rate that is comparable to that achieved with a total knee replacement in the first decade following surgery. The literature suggests that after the second decade, the revision rate may be somewhat higher for partial knee surgery than for total knee surgery.

Additional reading

Total knee arthroplasty has higher postoperative morbidity than unicompartmental knee arthroplasty: a multicenter analysis. Brown NM, Sheth NP, Davis K, Berend ME, et al. J Arthroplasty 2012.

Relative risk of different operations for medial compartment osteoarthritis of the knee. Sikorski JM, Sikorska JZ. Orthopedics 2011 Dec 6;34(12):e847-54.

Participation in sporting activities following knee replacement: total versus unicompartmental. Hopper GP, Leach WJ. Knee Surg Sports Traumatol Arthrosc 2008 Oct;16(10):973-9.

Return to sports and recreational activity after unicompartmental knee arthroplasty. Naal FD, Fischer M, Preuss A, Goldhahn J, von Knoch F, Preiss S, et al. Am J Sports Med 2007 Oct;35(10):1688-95.

Unicompartmental or total knee replacement: the 15-year results of a prospective randomized controlled trial. Newman J, Pydisetty RV, Ackroyd C. J Bone Joint Surg Br 2009 Jan;91(1):52-7.


Headshot of Michael M. Alexiades, MD
Michael M. Alexiades, MD
Associate Attending Orthopedic Surgeon, Hospital for Special Surgery
Associate Professor of Clinical Orthopedic Surgery, Weill Cornell Medical College
Headshot of Friedrich Boettner, MD
Friedrich Boettner, MD
Attending Orthopedic Surgeon, Hospital for Special Surgery
Professor of Orthopedic Surgery, Weill Cornell Medical College
Headshot of Andrew D. Pearle, MD
Andrew D. Pearle, MD
Chief, Sports Medicine Institute, Hospital for Special Surgery
Attending Orthopedic Surgeon, Hospital for Special Surgery

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