Overview
Unicompartmental knee arthroplasty (UKA) replaces the articular surface of one femoral condyle and the adjacent tibial plateau, sparing the cruciate ligaments, the opposite tibiofemoral compartment and the patellofemoral joint. This is purported to give more normal kinematics and easy revision. It is increasingly used, often with minimally invasive and outpatient techniques. Ten-year survivorship of modern designs ranges from 82% to 98%; a series of over 500 medial Oxford mobile-bearing UKAs reported 94% at 10 years and 91% at 20 years. Most studies still show slightly worse survivorship than TKA, though better than revision TKA. (Campbell)
Indications (Campbell)
Elderly, thin patients with unicompartmental disease who would otherwise undergo TKA: shorter rehabilitation, greater average range of motion, preserved cruciate proprioception (a more natural-feeling knee), less blood loss and outpatient feasibility. A 98% 10-year survivorship has been reported with a cemented design under stringent selection. Do not perform UKA in an elderly patient with arthritis in more than one compartment unless TKA is medically contraindicated.
Younger patients with isolated medial disease, as a "first arthroplasty" instead of high tibial osteotomy. Evidence is limited; one study reported 92% 11-year survivorship under age 60, with progression in the unresurfaced compartment in another 22%.
Key selection criteria: intact ACL, unicompartmental arthritis, passively correctable deformity and reasonable body weight. Preexisting patellofemoral disease did not harm 10-year outcomes in one review.
Contraindications (Campbell)
Inflammatory arthritis.
Flexion contracture of 15 degrees or more.
Preoperative arc of motion under 90 degrees.
Angular deformity more than 10 degrees from the mechanical axis in varus knees, or more than 5 degrees in valgus knees.
Significant cartilage erosion in the weight-bearing area of the opposite compartment.
ACL deficiency.
Obesity (relative); some reports show higher early revision with high BMI.
Results and Caveats (Campbell)
Only about 6% of patients needing arthroplasty may have none of the contraindications. Success depends on technique and volume: community-hospital 10-year survivorship was 89% versus 95% for TKA at a large reconstructive center. Registry patient-reported outcomes are similar to TKA, but some registries show higher short- and midterm revision.
A prospective multicenter study of robotic-arm-assisted fixed-bearing medial UKA reported 91.7% 10-year survivorship; the commonest reasons for revision were unexplained pain and aseptic loosening.
Revision of contemporary UKA rarely needs structural graft, and results approach primary TKA, although revision of failed UKA was slightly worse than conversion of a prior HTO in one matched review. Mobile-bearing UKA may need more medial augments at conversion.
Fixed- and mobile-bearing designs both require strict equalization of flexion and extension gaps; mobile bearings risk "spit-out" (dislocation).
Technique 8.12: Fixed-Bearing Unicondylar Knee Arthroplasty (Campbell)
Approach. Make a longitudinal incision along the medial or lateral border of the patellar tendon, depending on the compartment. A medial approach can be used for a lateral UKA but requires more extensive exposure for patellar eversion or subluxation; a minimally invasive lateral UKA requires a lateral approach.
Keep the capsular incision below the vastus medialis or lateralis. Lever the patella aside with a Hohmann retractor in flexion to expose the condyle.
Medial exposure: incise the coronary ligament, remove the anterior horn of the medial meniscus and raise the anteromedial tibial periosteal sleeve. Lateral exposure: raise the anterolateral sleeve to the medial edge of Gerdy's tubercle.
Confirm candidacy by inspecting the two compartments to be preserved.
Remove osteophytes before bone cuts: peripheral femoral and tibial osteophytes (their removal should be enough to balance the arthritic compartment, letting a varus knee correct passively) and intercondylar osteophytes that can impinge on and damage the cruciates. Needing more extensive soft-tissue release suggests inadequate bone resection or a deformity too severe for UKA.
Tibial cut first (most fixed-bearing systems): use an extramedullary guide aligned with the center of the ankle, reproduce the native posterior slope with about a 2-mm deep resection (or as the system specifies). For onlay tibial implants, complete the sagittal cut with a reciprocating saw just beside the tibial eminence.
Flexion gap: with the knee flexed, check with a spacer block that the gap accommodates the thinnest tibial component (usually about 8 mm).
Extension gap: in full extension, use a spacer block to determine the distal femoral resection that balances flexion and extension gaps, then cut with the implant-specific jig.
Size the femur and make the posterior condylar and chamfer cuts.
Excise the remaining meniscus and clear loose bodies from the posterior recess.
Trial: the knee should be stable in extension and flexion with full motion, without overstuffing (which overloads the opposite compartment) or excessive laxity.
Complete implant-specific preparation and cement the components, removing all excess or loose cement from the posterior tibia and posterior femoral condyle.
Close in layers.
Postoperative Care (Campbell)
Compressive dressing; a knee immobilizer only until quadriceps control is adequate. Begin range-of-motion exercises early (CPM is no longer routine), encourage extension with the foot on a pillow and flexion by dangling. Home program plus outpatient physical therapy 1 to 2 times weekly for 6 to 8 weeks, emphasizing quadriceps strengthening and gait training. Same-day discharge criteria include safe walker ambulation, straight leg raise, controlled pain, voiding and tolerating diet.
Complications (Campbell)
Progression of arthritis in the unresurfaced compartments, aseptic loosening, unexplained pain, polyethylene wear, bearing dislocation in mobile-bearing designs, and retained posterior cement.
References
Ford MC, Holland CT, Mihalko WM. Arthroplasty of the knee. In: Azar FM, Sawyer JR, Throckmorton TW, eds. Campbell's Operative Orthopaedics. 15th ed. Philadelphia: Elsevier; 2026. Chapter 8.