Total knee arthroplasty (TKA) replaces all three compartments of the knee joint with prosthetic components. Unicompartmental knee arthroplasty (UKA) replaces only the compartment that is diseased — typically the medial (inner) compartment — leaving the remaining knee structures intact. The choice between them depends on the distribution of disease, ligament integrity, and patient factors that determine whether UKA anatomy is appropriate.
Anatomy of the Knee: Three Compartments
The knee joint consists of three distinct compartments:
- Medial compartment: between the inner aspects of the femur (thigh bone) and tibia (shin bone)
- Lateral compartment: between the outer aspects of femur and tibia
- Patellofemoral compartment: between the patella (kneecap) and the anterior femur
Osteoarthritis, the most common indication for knee arthroplasty, does not necessarily affect all three compartments equally. In many patients, wear is concentrated in the medial compartment — which bears the larger share of load during walking — while lateral and patellofemoral cartilage remains intact. UKA was developed to address this scenario.
Total Knee Arthroplasty (TKA)
What is Replaced
TKA resects the ends of the femur, tibia, and in most cases the posterior surface of the patella. Metal components are cemented or press-fit onto the cut bone surfaces:
- A femoral component caps the distal femur
- A tibial component (metal baseplate with a polyethylene insert) resurfaces the proximal tibia
- A patellar component resurfaces the kneecap (some surgeons omit this depending on technique and patient factors)
A polyethylene bearing surface sits between the metal femoral and tibial components, providing the articulating surface on which the knee moves.
What TKA Does to Ligaments
TKA removes both cruciate ligaments in most designs, relying on the implant’s geometry and the collateral ligaments for stability. Posterior-stabilized (PS) designs include a central post-cam mechanism that substitutes for the posterior cruciate ligament. Some designs (cruciate-retaining, CR) preserve the posterior cruciate.
Indications for TKA
TKA is indicated when arthritis affects multiple compartments of the knee, when ligament structures are compromised, or when varus or valgus deformity exceeds the range correctable with a partial resurfacing. Inflammatory arthritis (rheumatoid, psoriatic) that tends to affect the whole joint is also a common TKA indication.
Unicompartmental Knee Arthroplasty (UKA)
What is Replaced
UKA replaces only the compartment with disease. In medial UKA (the most common form), the medial femoral condyle and medial tibial plateau are resurfaced with smaller components than those used in TKA. The lateral compartment, patellofemoral compartment, anterior cruciate ligament (ACL), and posterior cruciate ligament are left intact.
The smaller implant preserves more bone stock, which simplifies potential revision surgery later.
Patient Selection for UKA
UKA requires careful patient selection because the procedure depends on preserved anatomy:
- Disease isolated to one compartment. Clinical examination and imaging must confirm that the other compartments retain functional cartilage.
- Intact anterior cruciate ligament. The ACL is critical for UKA stability — its absence makes UKA a poor choice.
- Correctable deformity. Varus (bow-legged) deformity is common in medial compartment arthritis but must be passively correctable to near-neutral alignment, indicating that the cartilage loss rather than bony change is driving the deformity.
- Adequate range of motion. Flexion contracture greater than 10–15 degrees and flexion less than 90–100 degrees may be relative contraindications depending on surgeon judgment.
BMI and activity level were historically considered in UKA selection, but contemporary evidence does not consistently support restricting UKA on BMI grounds alone.
Recovery Comparison
UKA is generally associated with faster initial recovery than TKA: smaller incision, less bone removed, less blood loss, and preservation of normal ligament proprioception. Patients after UKA often describe a more natural-feeling knee, attributed to preservation of cruciate ligaments and normal compartment anatomy.
TKA recovery is longer — typically several months to functional recovery — but outcomes at two-year follow-up are high for both procedures when patients are appropriately selected.
Long-term revision rates differ: published registry data indicate that UKA has higher revision rates than TKA in most national joint registries. However, UKA revisions are technically simpler than TKA revisions, the difference in revision rate narrows with careful patient selection, and the preserved bone stock from UKA is an advantage if revision is required.
The Role of Robotic Assistance
Both TKA and UKA benefit from robotic assistance through more precise bone cutting and implant positioning. In knee arthroplasty, implant alignment affects load distribution across the bearing surface and long-term wear. Small alignment errors — a few degrees of malalignment in femoral or tibial component position — have measurable effects on implant function.
Robotic systems for knee arthroplasty fall into two categories:
Active systems — robots that autonomously make bone cuts according to a pre-operative plan, with the surgeon supervising. The TiRobot II from Tinavi Medical is an example of a navigation-integrated orthopedic robotic system used in Chinese clinical practice.
Semi-active (haptic boundary) systems — the surgeon holds the cutting tool and the robot provides tactile resistance when the surgeon moves toward the boundary of the planned resection zone. This allows the surgeon to feel the planned limits without the robot executing cuts autonomously.
Hurwa and Cornerstone Robotics are among Chinese companies developing robotic platforms for knee arthroplasty, reflecting significant domestic investment in this space.
UKA Precision and Robotics
Robotic assistance has particular relevance for UKA. Because UKA involves smaller bone resections and relies on preserved anatomy, precise bone cuts matter more than in TKA where more of the joint is replaced. Malalignment of a UKA tibial component by a few degrees affects load distribution more acutely than an equivalent error in TKA.
Published comparative studies have generally shown that robotic UKA produces implant positioning closer to the pre-operative plan than manual UKA, though the correlation between improved positioning accuracy and improved clinical outcomes remains an active research question.
Frequently Asked Questions
Can a UKA be converted to TKA if it fails?
Yes. This is one of the strategic advantages of UKA: because it preserves bone stock, conversion to TKA at the time of revision typically uses primary TKA implants rather than the more complex revision-specific implants often needed when a TKA fails. This simplifies and de-risks revision surgery.
Is UKA appropriate for both medial and lateral compartment disease?
Medial UKA is far more common than lateral UKA because medial compartment arthritis is the predominant pattern. Lateral UKA is technically more demanding due to different anatomical geometry and is less commonly performed. Patellofemoral arthroplasty is a third form of unicompartmental replacement, addressing isolated patellofemoral arthritis.
What imaging is required before deciding between TKA and UKA?
Weight-bearing full-leg X-rays assess deformity and overall alignment. Knee X-rays in multiple projections assess compartment-specific joint space narrowing. MRI can assess ligament integrity and cartilage quality in compartments not clearly evaluable on X-ray. Clinical examination — assessing range of motion, ligament stability, and correctable deformity — is essential and cannot be replaced by imaging alone.
How does robotic assistance affect the surgeon’s role?
Robotic assistance does not remove the surgeon from the procedure — it changes the nature of their interaction with the patient. The surgeon sets the pre-operative plan, confirms intraoperative registration, directs the procedure, and retains responsibility for all decisions. The robot improves the physical accuracy of execution within the surgeon’s plan.
What is the learning curve for robotic knee arthroplasty?
Studies on learning curve for robotic knee arthroplasty suggest that surgeons can achieve proficiency in robotic technique within 10–30 cases, with outcomes stabilizing after this period. The learning curve for the robotic technique is separate from — and typically shorter than — the learning curve for the underlying arthroplasty procedure itself.