Visual Element: A side-by-side anatomical comparison of a Medial UKA (preserving ACL/PCL/Lateral Compartment) versus a TKA (sacrificing ACL and often PCL), highlighting the ligaments preserved.

The Great Divide: Partial or Total?

In the world of knee arthroplasty, few topics elicit as much debate as the Unicompartmental Knee Arthroplasty (UKA). For some, it is the perfect operation—restoring native kinematics and offering a "forgotten knee." For others, it is a "temporary" operation with an unacceptably high revision rate compared to the reliable Total Knee Arthroplasty (TKA).

This article cuts through the dogma to present the current evidence, indications, and surgical philosophies defining this choice in 2025.

The Philosophy of Conservation

The TKA Philosophy: "The knee is worn out. Resurface everything. Realign to mechanical neutral. Substitute the ligaments." It is a reconstruction.

The UKA Philosophy: "Only the medial tire is bald. Replace the tire, keep the car." It is a resurfacing.

• Preservation: ACL and PCL are kept.
• Proprioception: Native mechanoreceptors in the cruciate ligaments remain.
• Bone Stock: Minimal bone is removed, making future revision easier (in theory).

The Evidence: Outcomes vs. Survivorship

Here lies the paradox of UKA.

1. Functional Outcomes (UKA Wins)

Multiple studies (including the TOPKAT trial) show that UKA patients have:

• Better Range of Motion (ROM).
• Higher PROMs (Patient Reported Outcome Measures).
• "Natural Feeling" knee (patients often forget they had surgery).
• Lower risk of peri-operative complications (VTE, Infection, MI/Stroke) - The risk of death after TKA is 3-4x higher than UKA.

2. Survivorship (TKA Wins)

Registry data (AOANJRR, UK NJR) consistently shows UKA has a higher revision rate.

• TKA: ~5% revision at 10 years.
• UKA: ~10-12% revision at 10 years.

Why?

1. Lower Threshold: It is easier to revise a painful UKA to a TKA than a painful TKA to a Revision TKA. Surgeons (and patients) pull the trigger earlier.
2. Progression of Disease: Arthritis develops in the lateral compartment (see "Overcorrection" below).
3. Technical Error: UKA is less forgiving of malalignment.

Patient Selection: The Pendulum Swings

In 1989, Kozinn and Scott published strict criteria that limited UKA to the "perfect" patient (Old, Sedentary, Thin). Today, we know these were too restrictive. The ideal candidate has changed.

Modern Indications

• Anteromedial OA: Bone-on-bone medial disease with preserved posterior medial cartilage (functional ACL).
• Intact ACL: Essential. Without an ACL, the femur subluxes posteriorly, wearing out the back of the implant.
• Correctable Deformity: The varus must be passively correctable.
• Asymptomatic PFJ: Patellofemoral arthritis is not a contraindication if it is not the primary source of pain.

Clinical Pearl: Age is just a number. A 50-year-old active patient may actually benefit more from a UKA (bone conservation, high function) than a TKA, even if they eventually need a revision at age 70.

The "Surgical Sin": Overcorrection

The most common cause of failure in UKA is surgeon error, specifically Overcorrection.

• Native Anatomy: Most varus knees were always in varus (constitutional varus).
• The Mistake: The surgeon puts in a thick poly to make the leg straight (neutral).
• The Consequence: This "stuffs" the medial side, lifting the lateral femoral condyle off the tibia (opening the lateral side). The patient walks, the lateral side slams down, and rapid lateral progression of arthritis occurs.
• The Rule: Leave them in Varus. Aim to restore pre-disease alignment, which is usually 2-3 degrees of varus. Under-correction is forgiven; over-correction is fatal to the implant.

The Revision Myth

"Don't worry, if it fails, it's just a primary TKA." False. Revising a UKA to a TKA is not always a simple primary.

• Bone Loss: Removing the UKA often leaves defects requiring augments or stems.
• Stiffness: The revision knee is often stiffer than a primary TKA.
• Outcomes: Outcomes of UKA-to-TKA are generally worse than Primary TKA, but better than TKA-to-Revision.

Conclusion

The UKA is a scalpel, not a hammer. In the right hands and the right patient, it offers a superior knee to a TKA. However, it demands respect for soft tissue balancing and strict adherence to indications.

The Verdict:

• For the surgeon: Do enough of them to be good (high volume surgeons have lower revision rates).
• For the patient: Understand the trade-off. "Better knee, higher risk of second surgery."

References

1. Beard DJ, et al. "The clinical and cost-effectiveness of total versus partial knee replacement in patients with medial compartment osteoarthritis (TOPKAT): 5-year outcomes of a randomised controlled trial." The Lancet. 2019.
2. Liddle AD, et al. "Optimal usage of unicompartmental knee arthroplasty." Bone & Joint Journal. 2015.
3. Kennedy JA, et al. "The effect of age and gender on the revision rate of unicompartmental knee arthroplasty." J Arthroplasty. 2020.