In total knee arthroplasty, does using a tibial component smaller than the native tibial plateau cause adverse outcomes such as uncovered bone, stress concentration, early loosening, or ligament imbalance?

Undersizing the Tibial Component in TKA: Risks and Considerations

Yes, using a tibial component smaller than the native tibia does cause harm—specifically increased tibial bone resorption, stress concentration, and theoretically higher risk of loosening—though the clinical significance depends on the degree of undersizing.

The Core Problem: Underhang Creates Biomechanical Consequences

When you undersize the tibial component, you create "underhang"—exposed bone at the periphery of the tibial plateau that lacks direct implant coverage. This matters because:

Bone Resorption Risk

Tibial component underhang directly correlates with progressive tibial bone resorption at the bone-implant interface 1. In a study of 134 TKAs, medial tibial resorption was significantly greater when medial underhang was present (p = 0.006), and posterior underhang was the strongest predictor of posterior tibial resorption (p = 0.001) 1. This bone loss occurs at the uncovered edges where stress shielding and altered load distribution trigger remodeling.

Stress Concentration and Fracture Risk

The biomechanical penalty varies by procedure type. In unicompartmental knee arthroplasty (UKA), undersizing dramatically increases fracture risk—size AA components showed significantly lower maximum load to failure (1039 N vs 1140 N, p = 0.031) and fewer cycles to failure compared to properly sized components 2. The strain patterns created wedge-shaped stress distributions that matched clinically observed fracture lines.

However, in cementless TKA, the story is more nuanced. Finite element analysis of 12 patients showed that undersizing by one or two sizes to gain 5° of external rotation increased the area at risk of bone collapse in only 8 of 12 patients, with no statistical difference in collapse risk between optimally sized (4.5%-17.9%) versus undersized implants (5.2%-16.4%, p = 0.24) 3. Paradoxically, four patients actually had decreased collapse risk with undersizing.

The Clinical Reality: Balancing Coverage vs. Rotation

Here's the surgical dilemma: maximizing tibial coverage often forces excessive internal rotation of the component, while optimal rotational alignment may require accepting some underhang. The evidence suggests:

For Standard TKA

• Aim for complete medial edge coverage—this is where bone resorption risk is highest 1
• Minor underhang (<3 mm) appears biomechanically tolerable in cementless designs 3
• The posterior edge is particularly vulnerable to resorption with underhang 1
• Avoid compensating with excessive cement at the keel—thick distal cement mantles independently increase lateral tibial resorption (p = 0.027) 1

For UKA

• Never undersize—the fracture risk is unacceptable 2
• Even one size down significantly compromises load-bearing capacity
• If you're between sizes, choose the larger component 4

What About Overhang?

You're right to worry about overhang, but the threshold matters. In UKA, major overhang (≥3 mm) severely compromises outcomes with 10-point worse Oxford Knee Scores and significantly worse pain at 5 years (p = 0.001) 4. However, minor overhang (<3 mm) showed no difference compared to underhang groups 4.

The Loosening Connection

While the ACR guidelines identify aseptic loosening as a major cause of late TKA failure (>2 years), resulting from mechanical stresses, osteolysis, or poor bone stock 5, the direct link between undersizing and clinical loosening remains theoretical rather than proven. The progressive bone resorption caused by underhang 1 creates the substrate for eventual loosening, but the AAOS guidelines note strong evidence that tibial component style (all-polyethylene vs. modular) and fixation method (cemented vs. cementless) show no difference in revision rates 6.

Practical Algorithm

When selecting tibial component size:

1. Prioritize medial and posterior edge coverage to prevent bone resorption
2. Accept minor underhang (<3 mm) if necessary for proper rotational alignment in standard TKA
3. Never undersize in UKA—fracture risk outweighs rotational concerns
4. Avoid overhang ≥3 mm at all costs—worse than minor underhang
5. Optimize cement technique—avoid thick distal keel mantles that increase resorption risk

Key Caveats

The finite element analysis showing minimal impact of undersizing 3 requires validation and only addresses immediate postoperative biomechanics, not long-term bone remodeling. The progressive bone resorption documented with underhang 1 suggests cumulative harm over years that may not manifest in short-term studies. When in doubt between sizes, err toward complete coverage rather than accepting significant underhang, particularly on the medial and posterior edges where resorption risk is highest.