Unicompartmental Knee Replacement (UKR): Patient Selection, Anatomy, & Biomechanics Guide
Key Takeaway
An ideal candidate for Unicompartmental Knee Replacement (UKR) has isolated, end-stage osteoarthritis in one knee compartment. Crucially, they must possess intact anterior and posterior cruciate ligaments, stable collateral ligaments, and a healthy patellofemoral joint. Minimal fixed flexion deformity and a passively correctable varus/valgus alignment are also essential for successful outcomes.
A 62-year-old patient presents with a 2-year history of localized medial knee pain. Clinically, they have full extension and 120 degrees of flexion. The knee is stable to stress testing, and the Lachman test is negative. You are presented with the following radiograph. How do you assess this patient for a Unicompartmental Knee Replacement (UKR), and what are your primary "red flags" for contraindication?
Candidate: I would confirm the diagnosis of isolated medial compartment osteoarthritis. I would examine the patient to ensure the deformity is passively correctable and that the ACL is intact. My assessment includes full-length standing films to check the mechanical axis. Contraindications would include inflammatory arthritis, significant patellofemoral disease, or ligamentous instability.
Candidates often focus solely on the radiographs. They fail to mention the "non-negotiable" clinical requirements, such as the integrity of the ACL or the specific degree of fixed flexion deformity (<15°). They also often neglect to mention the necessity of checking the patellofemoral joint on skyline views, which is a major source of early UKR failure.
I would approach this by confirming three pillars: Clinical, Radiographic, and Functional.
1. Clinical: Confirm isolated medial compartment pain, intact ACL/PCL, and no significant patellofemoral symptoms (grind test/skyline views).
2. Radiographic: Use weight-bearing AP and lateral views to confirm K-L grade III/IV in the medial compartment, while ensuring the lateral and patellofemoral compartments are preserved. A long-leg standing film is mandatory to quantify deformity.
3. Absolute Contraindications: Inflammatory arthropathy, active or prior infection, and significant ligamentous deficiency (specifically the ACL). Relative Contraindications: Fixed flexion >15°, varus deformity >15°, and BMI >35-40 kg/m².
You have decided to proceed with a medial UKR. During the trial reduction, you notice the knee feels "tight" in flexion but balanced in extension. How do you systematically address this?
Candidate: I would check if I have removed enough posterior femoral condyle bone. If the flexion gap is tight, I might need to perform an additional posterior femoral cut or check for posterior osteophytes that may be impinging on the PCL.
The candidate ignores the impact of the tibial cut. A common mistake is to "just increase the extension gap" by cutting more tibia, which will cause mid-flexion instability. A high-scoring candidate must prioritize balancing the gaps symmetrically.
I would evaluate the Flexion-Extension Gap relationship:
If the flexion gap is tight: 1. Check for posterior osteophytes (common culprit). 2. Ensure I have not undersized the femoral component. 3. Consider an additional posterior femoral condylar resection. 4. Critically, ensure I have not over-resected the tibia, which would have required a thicker bearing that now impinges in flexion. The goal is a rectangular gap; I would avoid cutting more tibia to fix a flexion-specific tightness, as this leads to global instability.
