Pre-operative Planning in High Tibial Osteotomy (HTO): A Step-by-Step Approach

High Tibial Osteotomy (HTO) is a realignment procedure that demands precise pre-operative planning to ensure successful mechanical axis correction and optimal patient outcomes.

A minor error in angle calculation or hinge placement can result in under-correction, over-correction, or compromised fixation. Hence, understanding biomechanical axes, radiologic landmarks, and mathematical planning tools is crucial.

This article breaks down the 5 key steps of osteotomy planning, supported by classic radiographic tools and validated trigonometric charts.

📌 Step 1: Define the Malalignment

The first goal is to evaluate the mechanical axis deviation — most commonly varus in medial compartment osteoarthritis.

• The Mechanical Axis (Mikulicz Line) extends from the center of the femoral head to the center of the ankle joint.
• In a normally aligned limb, this line passes through the center of the knee (i.e., 50% of the tibial width from medial to lateral).

🔎 Interpretation:

• If the axis falls medially (e.g., at 20–30%), it indicates a varus deformity.
• This line is drawn using long-leg standing radiographs, with values expressed in percentage of tibial plateau width (0% = medial edge, 100% = lateral edge).

📌 Step 2: Locate the Deformity

Determine whether the deformity lies in the:

• Femur (Distal Femur)
• Tibia (Proximal Tibia) – most common in HTO candidates
• Or both (combined deformities)

You assess this by measuring:

• mLDFA (mechanical lateral distal femoral angle): Normal ≈ 87° ± 3°
• mMPTA (mechanical medial proximal tibial angle): Normal ≈ 87° ± 3°

If mMPTA is decreased (e.g., < 84°), the deformity is tibial and HTO is indicated.

📌 Step 3: Define the Target Axis

Objective:

Determine the desired weight-bearing line (WBL) post-correction.

Fujisawa Point (62.5%):

• The ideal WBL should pass through 62.5% of the tibial plateau width (lateral to the midline).
• This ensures load redistribution from the damaged medial compartment to the healthier lateral side.

How to Mark It:

• Divide the tibial plateau into 10 equal parts (0% = medial edge, 100% = lateral edge).
• The target WBL should cross at 62.5% (slightly lateral to the centre).

✅ Step 4: Choose Osteotomy Type and Hinge Point

Objective:

Choose between opening vs. closing wedge and define the hinge location.

A. Osteotomy Type Selection

B. Hinge Point Location

• Opening Wedge: The lateral hinge is 1 cm below the joint line (must remain intact for stability).
• Closing Wedge: The lateral hinge is at the cortex, and a wedge is removed medially.

✅ Step 5: Measure the Correction Angle (Miniaci Method)

Objective:

Calculate the wedge size needed to achieve the desired correction.

Miniaci Method Steps:

• Draw Pre-Op MA Line: From femoral head → ankle center (passes medial to knee).
• Draw Post-Op Target MA Line: From femoral head → Fujisawa point (62.5%).
• Find Intersection Point (CORA):
• The apex of deformity is where the pre-op and post-op MA lines intersect (Center of Rotation of Angulation, CORA).
• Measure Correction Angle:
• The angle between the two MA lines at the CORA = required correction angle (θ).
• Calculate Wedge Size:
• Opening Wedge: Use trigonometry (wedge height ≈ tan(θ) × osteotomy width).

🔚 In a Nutshell

High Tibial Osteotomy (HTO) is not just a bridge to knee replacement — it’s a proactive approach to preserve the joint, optimize biomechanics, and extend active life in younger, high-demand patients. With accurate pre-operative planning and precise execution, HTO transforms the way we manage unicompartmental knee arthritis. A well-aligned joint today can mean a well-preserved knee tomorrow.

References:
HTO Planning: How to do it

: https://www.slideshare.net/slideshow/high-tibial-osteotomy-when-and-how/234823470

Art of Realignment Surgery

: https://orthopaedicprinciples.com/2019/05/art-of-realignment-surgery/