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 — 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.
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.
Frequently Asked Questions
How do I know if I'm a candidate for a partial rather than total knee replacement?
Does partial knee replacement last as long as total knee replacement?
What happens if the arthritis spreads to other compartments?
Is the partial knee replacement less painful after surgery?
Can I return to sports after a partial knee replacement?
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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/