Knee

Unicompartmental Knee Arthritis / Partial Knee Replacement

Not all knee arthritis requires a total knee replacement. When cartilage loss is confined to a single compartment (most often the medial (inner) side of the knee), a unicompartmental knee arthroplasty (UKA), commonly called a partial knee replacement, can restore pain-free function while preserving healthy tissue, the native cruciate ligaments, and proprioceptive feedback. For carefully selected patients, partial knee replacement offers faster recovery, a more natural-feeling knee, and excellent long-term survivorship. At Maryland Orthopedic Specialists, our Adult Reconstruction team is experienced in both the rigorous patient selection and the technical execution that make UKA a successful, durable procedure.

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What is unicompartmental knee arthritis / partial knee replacement?

Unicompartmental knee arthritis is osteoarthritis limited to just one of the knee's three compartments, most often the inner (medial) side. When only one compartment is worn, a partial knee replacement can resurface it while preserving healthy bone and ligaments, often allowing a faster recovery than total knee replacement.

The knee is divided into three compartments: the medial (inner) tibiofemoral compartment, the lateral (outer) tibiofemoral compartment, and the patellofemoral compartment. Osteoarthritis can affect one, two, or all three compartments.

Isolated medial compartment arthritis is the most common pattern. Cartilage on the medial femoral condyle and medial tibial plateau wears away, causing bone-on-bone contact, pain with weight-bearing, and a progressive varus (bowlegged) deformity. The lateral compartment and patellofemoral joint remain healthy, and the anterior cruciate ligament (ACL) is intact. These are the essential features distinguishing a UKA candidate from a total knee replacement candidate.

Medial UKA replaces only the damaged surfaces: a femoral component caps the medial condyle, and a tibial component resurfaces the medial plateau, with a polyethylene bearing (fixed or mobile) between them. The lateral compartment, patellofemoral joint, ACL, and PCL are left entirely intact.

Symptoms — do you recognize these?

Patients with isolated medial compartment arthritis typically experience:

Pain localized to the inner aspect of the knee
Weight-bearing pain that worsens with prolonged walking or stair climbing
Morning stiffness that improves within 30 minutes of activity
A gradual bowlegged (varus) appearance to the knee
Knee swelling after activity
Night pain in advanced disease
Preserved range of motion compared with more advanced tricompartmental disease

Call us at (301) 515-0900. Same-day appointments available at our Bethesda and Germantown locations.

How we diagnose it

Accurate compartment localization is essential before planning partial knee replacement:

Weight-bearing AP, lateral, and Rosenberg views (45° flexion PA): Confirm medial joint-space narrowing and document lateral compartment preservation. The Rosenberg view is particularly sensitive for cartilage loss in the posterior tibiofemoral joint.
Long-leg alignment radiograph (hip-to-ankle): Measures the mechanical axis of the limb. Moderate varus deformity is acceptable for UKA; severe fixed deformity indicates total knee replacement.
Patellofemoral radiograph (sunrise view): Documents patellofemoral compartment status.
MRI: Evaluates ACL integrity, cartilage quality across compartments, and meniscal status, all essential to UKA candidacy.
Physical examination: Full range of motion assessment, ligament stability testing, compartmental tenderness, deformity quantification.

Treatment options

For patients with isolated single-compartment knee arthritis who meet selection criteria, unicompartmental knee arthroplasty offers faster recovery and a more natural feel than total knee replacement, with excellent long-term outcomes.

Non-Surgical

Initial treatment includes weight optimization, physical therapy targeting quadriceps and hip strengthening, activity modification, NSAIDs, acetaminophen, an unloader knee brace, and intra-articular corticosteroid or hyaluronic acid injections.

Patient Selection Criteria for UKA

Strict adherence to selection criteria is the key to excellent UKA outcomes. Ideal candidates have arthritis isolated to the medial tibiofemoral compartment, an intact ACL (essential for UKA kinematics), a well-preserved lateral compartment with minimal articular cartilage changes, flexion contracture less than 15 degrees, flexion greater than 90 degrees, correctable (not fixed) varus deformity, absence of inflammatory arthritis, and body weight and activity level within implant design parameters. Patients who do not meet these criteria are better served by total knee replacement.

Conversion to Total Knee Replacement

When UKA fails, whether from bearing wear, loosening, progression of arthritis in other compartments, or unexplained pain, conversion to total knee replacement is the standard salvage. Outcomes of conversion TKA are generally good, though the technical considerations related to the prior UKA are involved. Meticulous initial patient selection and surgical technique minimize the need for revision.

Surgical Procedure

Partial Knee Replacement (Unicompartmental)

Resurfacing of only the damaged compartment of the knee (medial, lateral, or patellofemoral) while preserving the intact ligaments and articular cartilage in the remaining healthy compartments.

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Recovery & rehabilitation

UKA recovery is notably faster than total knee replacement:

Same-day or next-day discharge for most patients
Immediate full weight-bearing with a walker or cane
Walking independently: typically by week 2
Return to driving: 2–4 weeks (left knee earlier)
Return to light work and activities of daily living: 3–6 weeks
Full recovery and return to recreational activity: 6–12 weeks

Formal outpatient physical therapy focuses on range of motion restoration, quadriceps strengthening, and gait normalization.

Survivorship: Large registry studies demonstrate medial UKA survivorship of approximately 90–93% at 10 years. The Oxford Knee Score and functional outcomes in well-selected patients are equivalent to or superior to TKA for isolated medial OA.

Frequently Asked Questions

How do I know if I'm a candidate for a partial rather than total knee replacement?

Candidacy is determined by the compartmental extent of arthritis (confirmed on imaging), ACL integrity, range of motion, and deformity. Our surgeons perform a thorough evaluation including weight-bearing X-rays and, often, MRI before recommending UKA.

Does partial knee replacement last as long as total knee replacement?

Registry data support comparable survivorship at 10 years for well-selected UKA patients compared with TKA. Long-term (15–20 year) survivorship is somewhat lower due to progression of arthritis in remaining compartments, which is expected in patients living for many more years.

What happens if the arthritis spreads to other compartments?

If arthritis progresses in the lateral or patellofemoral compartment and pain recurs, conversion to total knee replacement is straightforward and reliable. Bone preservation from the original UKA is an advantage in this scenario.

Is the partial knee replacement less painful after surgery?

Most patients experience significantly less post-operative pain than patients undergoing total knee replacement. The smaller incision, less bone removal, and preservation of native anatomy contribute to a more comfortable early recovery.

Can I return to sports after a partial knee replacement?

Yes. UKA patients typically return to golf, cycling, swimming, hiking, and doubles tennis. High-impact running and contact sports are generally discouraged to protect longevity.

Meet the specialists

Christopher S. Raffo, MD

Orthopedic Surgery · Sports Medicine · Knee & Shoulder Arthroscopy · Joint Replacement

Meet Dr. Raffo →

James S. Gardiner, MD

Orthopedic Surgery · Sports Medicine · Knee & Shoulder Arthroscopy · Knee Replacement

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Brian McCormick, MD

Adult Reconstruction · Hip and Knee Replacement

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Related conditions

Total Knee Replacement Osteonecrosis of the Knee

Medically reviewed by Christopher S. Raffo, MD

Last reviewed May 1, 2026

References

Kozinn SC, Scott R. Unicondylar knee arthroplasty. J Bone Joint Surg Am. 1989;71(1):145–150. https://doi.org/10.2106/00004623-198971010-00022
Liddle AD, Judge A, Pandit H, Murray DW. Adverse outcomes after total and unicompartmental knee replacement in 101,330 matched patients: a study of data from the National Joint Registry for England and Wales. Lancet. 2014;384(9952):1437–1445. https://doi.org/10.1016/S0140-6736(14)60947-7
Pearle AD, O'Loughlin PF, Kendoff DO. Robot-assisted unicompartmental knee arthroplasty. J Arthroplasty. 2010;25(2):230–237. https://doi.org/10.1016/j.arth.2008.09.024
Weston-Simons JS, Pandit H, Kendrick BJ, et al. The management of patients with bilateral compartment wear undergoing joint replacement surgery: an analysis of 544 consecutive UKAs. Bone Joint J. 2012;94-B(9):1195–1200. https://doi.org/10.1302/0301-620X.94B9.29069
Newman J, Pydisetty RV, Ackroyd C. Unicompartmental or total knee replacement: the 15-year results of a prospective randomised controlled trial. J Bone Joint Surg Br. 2009;91(1):52–57. https://doi.org/10.1302/0301-620X.91B1.20899
American Academy of Orthopaedic Surgeons. Unicompartmental Knee Replacement. OrthoInfo. https://orthoinfo.aaos.org/en/treatment/unicompartmental-knee-replacement/