Calculate Desired Correction on Long Leg Views
1. Simple Calculations
Measure anatomic femoral tibial angle
- normally 5-8o
Calculate correction desired
- need to overcorrect to 8o femoro-tibial alignment
- or correct to mechanical axis of 180o
2. Miniacci method
A. Draw the predicted mechanical axis of the affected limb on the full-length radiograph
- connecting a point from the center of the hip to
- a point of new desired weight bearing axis on lateral tibial plateau
- one third of the way from the center of the knee to the lateral margin
- this is line A
- extend this line to the level of the ankle
B. Make a second line (line B)
- from the point on the proximal medial tibia that will be the pivot point of the osteotomy
- to a point in the center of the ankle
C. Draw a third line (line C)
- from the medial pivot point to the mechanical axis of the ankle
Angle (X)
- between the second and third lines (lines B and C)
- this is the angle of the wedge to be resected
Must then use trigonometry to calculate wedge height or rule of thumb
Calculation of Wedge (Closing Wedge)
1. Rule of Thumb Method
Bauer 1969
- estimates 1mm resected wedge = 1° correction
- this relies on average tibial width of 56mm
- average tibial width at the flare is 75.2mm
- Bauer's estimation tends to under correct
Example
- 12o correction, 12 mm wedge laterally
2. Trigonometry Calculation
Tan Angle = opposite/adjacent
Wedge height medially = tibial width X Tan angle correction required
Intra-operative Checks
Use diathermy lead
- centred on femoral head
- centred on talus
- held straight
- ensure passes over lateral tibial plateau