Preoperative Calculations

 

Goals

 

Medial OA

 

Slight overcorrection into valgus

- anatomical axis 8 - 10^o valgus

- mechanical axis 3 - 5^o valgus

- Fujisawa point / 62% lateral tibial plateau / lateral tibial spine

 

 

Kujisawa point

 

Medial cartilage restoration procedures

 

Correct to normal alignment

- anatomical axis 5 - 7^o

- mechanical axis 180^o

- lateral 30 - 40% of the lateral plateau

 

1. Calculate Desired Angle of Correction on Long Leg Views

 

A.  Simple Calculations

 

Measure anatomic femoral tibial angle

 

Medial OA

- overcorrect to anatomical axis valgus 8^o

 

Cartilage restoration

- correct to anatomical axis 5 - 7^o 

- mechanical axis of 180^o

 

Example of medial OA

- anatomical angle 1^o valgus

- correct to 8^o valgus

- need to correct 7^o

 

Example of medial cartilage procedure

- anatomical angle 0^o

- correct to 5^o valgus

- need to correct 5^o

 

B.  Miniaci method

 

 

Line A: Desired mechanical axis

- center of the hip to new desired weight bearing axis on lateral tibial plateau

- Fujisawa point (62% from the edge of the medial tibial plateau)

- slightly lateral to the lateral tibial spine

- extend this line to the level of the ankle

 

Line B: Center of ankle to pivot point on the proximal lateral tibia

 

Line C: Lateral pivot point to meet Line A

 

The angle between B and C is the angle of the desired correction

 

C. Fujisawa point / Weight bearing line method

 

 

Fujisawa point

- 62.5% from medial edge medial tibial plateau

- just lateral to lateral tibial spine

 

Line A: Center femoral head to Fujisawa point to level ankle

Line B: Center ankle joint to Fujisawa point

 

The angle between these lines is the angle of the desired correction

 

2. Calculation of Wedge

 

A.  Rule of Thumb Method

 

1mm opening = 1° correction

- i.e. 10^o correction, 10 mm open / closing

 

Based upon average tibial width of 56 mm 

- average tibial width at the flare is 75mm 

- tends to under correct

 

 

B.  Trigonometry Calculation

 

Tangent angle formula

Tan Angle = opposite/adjacent

 

Opposite = adjacent x Tan angle

 

Example of 5 degree correction for medial cartilage procedure

- Opening height medially = tibial width X Tan angle correction required

- Wedge = 85 mm x tan 5 = 85 x 0.87 = 7 mm

 

C. Intra-operative check

 

Use diathermy lead / drop rod

- center femoral head to center on talus

- ensure passes over lateral tibial spine / Fujisawa point