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The Effect of Lower-Limb Coronal Alignment Correction and Component Positioning on Coronal Ankle Alignment After Total Knee Arthroplasty

This retrospective study of 134 knees demonstrates that the magnitude of coronal alignment correction and femoral component positioning, rather than tibial component alignment, are the primary determinants of postoperative changes in coronal ankle alignment following total knee arthroplasty.

Original authors: Mehmet Önüt, Sualp Turan

Published 2026-08-14

📖 6 min read🧠 Deep dive

CC BY 4.0

Original authors: Mehmet Önüt, Sualp Turan

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Body's Balancing Act: From Knee to Toe

Imagine your leg is a tall, wobbly tower of blocks. When you stand up, gravity pulls everything down, and your body has to work hard to keep that tower straight so you don't topple over. In a healthy person, the weight travels in a perfect, straight line from your hip, through the center of your knee, down to your ankle, and finally into the ground. But sometimes, arthritis (that stiff, painful wear-and-tear) hits the knee, and the tower starts to lean. In many cases, the knee leans inward, like a tower tilting toward the middle of the room. This is called "varus" deformity.

To fix this leaning tower, surgeons often perform a Total Knee Arthroplasty (TKA), which is a fancy way of saying they replace the worn-out knee joint with a new, artificial one. The goal is to straighten the leg back up. But here's the tricky part: your body is a connected system. If you suddenly straighten the knee, does the rest of the tower—the ankle and the foot—just stay exactly as they were? Or does the whole structure have to shift and adjust to find a new balance? This is the question that researchers are asking. They want to know if fixing the knee "breaks" the ankle's balance, or if the ankle can happily adapt to the new, straighter leg.

The Study: Straightening the Knee, Watching the Ankle

In this study, two researchers, Mehmet Önüt and Sualp Turan, decided to play detective with X-rays. They looked at 134 knees that had undergone surgery to replace a knee joint because of severe arthritis that had caused the leg to bow inward. They didn't just look at the knee; they took a long, full-body X-ray from the hip all the way down to the foot, both before the surgery and after. They were looking for clues about how the ankle changed when the knee was fixed.

Think of the leg like a long ruler. Before surgery, the ruler was bent. The surgeons used a specific method called "mechanical alignment" to replace the knee parts, aiming to make the ruler as straight as possible (close to 180 degrees). The researchers measured several angles to see what happened to the ankle. They measured the angle of the bottom of the shin bone (the "plafond") relative to the ground, the angle of the top of the foot bone (the "talus") relative to the ground, and how the two bones fit together inside the ankle joint.

What They Found: The Big Shift
The results were clear: when the knee was straightened, the ankle definitely moved.

The Knee: The angle of the leg (called the Hip-Knee-Ankle angle, or HKAA) went from being bent at an average of 167.87° to being much straighter at 176.16°. That's a correction of about 8.29°.
The Ankle's Ground Angle: The angle of the bottom of the shin bone relative to the ground dropped significantly, from 7.06° down to 3.25°. This means the ankle tilted back to become more parallel with the floor.
The Foot Bone: Similarly, the top of the foot bone also tilted back, dropping from 7.27° to 3.38°.
The Joint Gap: The angle between the shin and the foot bone (the talus-plafond angle) changed a tiny bit, from 2.99° to 2.65°.
The Shins: Interestingly, the angle of the lower part of the shin bone itself (the LDTA) didn't really change at all, staying around 88°.

The "How Much" vs. "How Straight" Mystery
The researchers wanted to know why the ankle changed. Was it because the final leg was perfectly straight? Or was it because of how much they had to bend it back to get there?

They discovered that the amount of correction was the real boss. If a patient had a very bent leg that needed a big fix (a correction of 6° or more), their ankle angle was very likely to change by 3° or more. It wasn't about the final result being perfect; it was about the journey of straightening. The more the knee was corrected, the more the ankle had to adjust its tilt relative to the ground.

The Sneaky Culprit: The Femur Part
Here is where it gets really interesting. The surgeons place two main pieces in the knee: one on the thigh bone (femur) and one on the shin bone (tibia). The researchers checked if these pieces were placed perfectly straight or if they were slightly off-center.

The Tibia (Shin) Part: It turned out that whether the shin piece was placed perfectly or slightly off didn't seem to have a consistent, predictable effect on the ankle. It was like a background character that didn't really drive the plot.
The Femur (Thigh) Part: This was the star of the show. If the thigh piece was placed even a little bit off-center (specifically, if it deviated by 2° or more from the perfect neutral spot), it was a strong warning sign. This small mistake was linked to a worsening of the angle between the shin and the foot bone (the talus-plafond relationship).

The Takeaway
So, what does this mean for the story of your leg? The study suggests that when surgeons fix a bent knee, they can't just look at the knee and say, "Good job, it's straight." They have to realize that the ankle is watching.

The biggest factor changing the ankle is simply how much the knee was bent to begin with and how much they had to straighten it. However, the researchers also found a specific rule of thumb: if the piece of metal on the thigh bone is placed even slightly crooked (more than 2° off), it might mess up the way the ankle bones fit together.

The study concludes that while fixing the knee helps the ankle find a new balance, the surgeon needs to be very careful with how they position the thigh part of the new knee. It's not just about the big picture of the leg; the tiny details of where the metal pieces go can ripple all the way down to the foot.

Technical Summary: The Effect of Lower-Limb Coronal Alignment Correction and Component Positioning on Coronal Ankle Alignment After Total Knee Arthroplasty

Problem Statement
Total knee arthroplasty (TKA) is the standard treatment for advanced varus gonarthrosis, aiming to restore lower-extremity alignment. While the primary focus is often the hip-knee-ankle angle (HKAA), chronic varus deformity can induce compensatory changes in the distal tibia, ankle, and hindfoot. A critical gap in the current literature is the insufficient clarification of whether postoperative changes in coronal ankle alignment are driven primarily by the magnitude of the global mechanical axis correction (HKAA) or by the specific coronal positioning of the femoral and tibial components. Existing studies have not simultaneously evaluated these factors within a single analytical model for mechanically aligned TKA, leaving uncertainty about whether achieving a neutral global axis is sufficient to restore ankle congruity or if component-level deviations play a distinct role.

Methodology
This retrospective radiographic study analyzed 134 knees from patients undergoing primary TKA for varus gonarthrosis between March 2019 and December 2023. All patients received a posterior-stabilized (PS) knee system (Vanguard, Zimmer Biomet).

Data Collection: Preoperative and postoperative weight-bearing anteroposterior long-leg radiographs were evaluated.
Measurements: The study quantified the Hip-Knee-Ankle Angle (HKAA), femoral and tibial component angles, and four specific coronal ankle parameters: ground-plafond angle, ground-talus angle, lateral distal tibial angle (LDTA), and talus-plafond angle.
Definitions: Component deviations were calculated relative to a neutral 90° angle. HKAA correction magnitude was defined as the difference between preoperative and postoperative values.
Statistical Analysis: Non-parametric tests (Wilcoxon, Mann-Whitney U, Kruskal-Wallis) were used for group comparisons. Spearman correlation, multivariable linear regression, and Receiver Operating Characteristic (ROC) curve analyses were employed to identify independent predictors and clinically meaningful threshold values for ankle alignment changes. Intraobserver reliability was confirmed with an Intraclass Correlation Coefficient (ICC) > 0.90.

Key Results

Global Alignment Changes: Post-TKA, the mean HKAA improved from 167.87° to 176.16° (mean correction 8.29°). Significant reductions were observed in the ground-plafond angle (7.06° to 3.25°), ground-talus angle (7.27° to 3.38°), and talus-plafond angle (2.99° to 2.65°). The LDTA showed no significant change.
Predictors of Ground-Plafond Change: The magnitude of HKAA correction was the strongest independent predictor of changes in the ground-plafond angle ( $\beta$ =-0.355; $p<0.001$ ; $R^2$ =0.328). A threshold analysis revealed that a $\ge6^\circ$ HKAA correction predicts a $\ge3^\circ$ change in the ground-plafond angle (AUC=0.778).
Predictors of Talus-Plafond Change: While HKAA correction influenced global orientation, the absolute deviation of the femoral component was a significant factor for intra-articular congruity. A femoral component deviation of $\ge2^\circ$ from neutral predicted deterioration in the talus-plafond relationship (AUC=0.725).
Tibial Component Influence: Unlike the femoral component, tibial component deviation did not demonstrate a consistent or independent effect on ankle alignment changes in correlation, regression, or ROC analyses.
Group Comparisons: Patients with HKAA within $\pm3^\circ$ of neutral showed similar ground-plafond improvements to outliers, but the talus-plafond difference was significantly better in the neutral group. Similarly, femoral component alignment within $\pm2^\circ$ resulted in significantly better talus-plafond outcomes compared to deviations outside this range.

Significance and Claims
The study concludes that the response of the coronal ankle alignment after mechanically aligned TKA is a multilevel process influenced by two distinct factors:

Magnitude of Correction: The change in the ground-plafond angle (distal tibial orientation relative to the ground) is primarily determined by the magnitude of the HKAA correction from the preoperative deformity, rather than the final postoperative axis value alone.
Component Positioning: The coronal position of the femoral component is a critical determinant of the talus-plafond relationship (intra-articular tibiotalar congruity). Specifically, femoral component deviations exceeding $\pm2^\circ$ are associated with malalignment at the tibiotalar joint.

The authors assert that assessing the global HKAA alone is insufficient to explain ankle-level responses. Surgical planning must consider both the magnitude of the mechanical axis correction and the precise coronal positioning of the femoral component to ensure optimal coronal ankle congruity. The study does not claim that tibial component positioning has a similar independent impact on these specific ankle parameters.

Limitations
The authors note the study's retrospective nature, the lack of standardized follow-up timing, the reliance on 2D radiographs without 3D imaging or hindfoot views, and the absence of clinical outcome scores (pain/function). Additionally, the identified thresholds ( $\ge6^\circ$ HKAA correction and $\ge2^\circ$ femoral deviation) require validation in larger, independent cohorts.

The Body's Balancing Act: From Knee to Toe

The Study: Straightening the Knee, Watching the Ankle

Technical Summary: The Effect of Lower-Limb Coronal Alignment Correction and Component Positioning on Coronal Ankle Alignment After Total Knee Arthroplasty

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