Introduction

Around the knee joint, a 1° correction can be approximated as a 1 mm gap correction.

Since even a small angular discrepancy can significantly impact outcomes, precise preoperative planning is crucial for successful corrective osteotomy.

There are several methods for preoperative planning in HTO, including the Miniaci method, the Dugdale method, and direct planning using Photoshop.

This article is an instruction for my residents, focuses on the Miniaci method, which is widely used in clinical practice.

1. Ensure Accurate X-ray (Bell-Thompson view)

Before planning, it is essential to confirm that the Bell-Thompson view is properly taken and the anteroposterior (AP) alignment is correct.

1) The patella should be facing forward.

2) The X-ray should be taken with the knee fully extended. Flexion may lead to misinterpretation of lower limb alignment.

3) Consider exceptional cases:

- In patients with PF OA or patella subluxation, a slight lateral displacement may still indicate a true AP view.

- Patients with abnormal femoral anteversion or tibial torsion require a comprehensive evaluation to determine the true AP alignment.

2. Draw the Weight-Bearing Line (WBL)

1) draw a line from the hip center to the ankle center. This line indicates the load distribution through the knee. (alternative of HKA axis)

Above image is a Bell-Thompson view of a 48-year-old male patient with a full-thickness chondral defect and bone edema in the weight-bearing area of medial femoral condyle.

By drawing the line, significant degree of genu varum deformity was observed. Understanding the weight-bearing distribution is essential for planning correction.

3. Evaluate MPTA and LDFA

1) measure the medial proximal tibial angle (MPTA) and mechanical lateral distal femoral angle (mLDFA), to determine whether the varus deformity originates from the femur or tibia.

The normal range for both MPTA and mLDFA is approximately 86 to 88 degrees.
If an abnormality is identified in MPTA, it indicates that the tibia requires correction, making it a good candidate for HTO.
If an abnormality is identified in mLDFA, it indicates that the femur requires correction, making it a good candidate for DFO.

2) Knee and ankle joint line obliquity (JLO) also matters. It is important that the predicted postoperative knee JLO be less than 95°. Otherwise, a clinically inferior outcome is likely.

4. Determine the Target Point (Fujisawa Point)

1) The correction target of the WBL can be customized based on the cartilage status of medial and lateral femoral condyle. Usually, three common correction targets are considered:

Center of the tibial plateau
Lateral spine of tibia
The Fujisawa point (62.5% from the medial edge)

For the 48-year-old patient mentioned earlier, who was scheduled for simultaneous autologous osteochondral transplantation (OATS), the correction was aimed at the Fujisawa point to shift the load more laterally.

To find the Fujisawa point:

Measure the total tibial plateau width (e.g., 72.51 mm).
Multiply by 0.625 (e.g., 45.31 mm).
Mark the Fujisawa point accordingly

5. Draw the New Weight-Bearing Line

After marking the Fujisawa point, draw a new weight-bearing line from the hip center through the Fujisawa point to the ankle level.

This line (indicated by the blue arrows) represents the corrected lower limb alignment postoperatively.

6. Determine the Hinge Point

During surgery, the osteotomy is performed obliquely from the medial side, leaving a hinge to maintain stability.

Common hinge point references include:

Tip of the fibular head – a widely used landmark.
Midpoint of the lateral mass – another reliable reference.
Ideal hinge point that reduce the risk of hinge fracture is 7-8mm below the lateral tibial plateau.

In the given patient case, the hinge point was set between the tip of the fibular head and the midpoint of the lateral mass, ensuring a balanced approach.

7. Measure the Original Ankle Center - Hinge point Distance

To ensure bone length remains unchanged postoperatively:

Draw a line from the hinge point to the original ankle center.
Measure this distance (e.g., 317.73 mm), which will be used for the next step.

8. Determine the New Ankle Center, and the correction angle.

1) Find the new ankle center along the corrected weight-bearing line.

Ensure the distance from the hinge point remains 317.73 mm (equal to the original bone length).
Mark this point on the X-ray using PACS.

2) Measure the Osteotomy Angle

The angle between the original and new ankle center lines determines the correction angle (e.g., 11.4 degrees).
This angle guides the amount of medial opening during osteotomy.

9. Calculate the Osteotomy Gap

Using the measured correction angle:

1) Determine osteotomy depth – Distance from the osteotomy site to the hinge point (e.g., 60.08 mm after magnification correction).

2) Determine osteotomy gap – The medial opening required for correction (e.g., 11.63 mm after correction for magnification).

To improve accuracy:

Use an X-ray with a radio-opaque ruler to measure magnification.
Adjust PACS measurements accordingly.

10. Postoperative Evaluation

After surgery, evaluate alignment via full-leg X-ray to confirm the correction was achieved as planned. Proper postoperative assessment provides valuable feedback for future cases.

The case patient underwent surgery as planned, demonstrating accurate correction of lower limb alignment.

Conclusion

Proper preoperative planning using the Miniaci method is essential for successful HTO.

By ensuring accurate imaging, identifying deformity origins, and calculating precise correction angles, optimal outcomes can be achieved.

For best results, continuous postoperative evaluation and refinement of surgical techniques are recommended.

Hyo Yeol Lee, MD., PhD.

Department of Orthopaedic Surgery (Knee Surgery and Sports Medicine)

Chungbuk National University Hospital

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Preoperative Planning for High Tibial Osteotomy (HTO) Using the Miniaci Method