Ambulatory Veterinary Surgery

Rationale for TTA

• Total joint force in the stifle is approximately parallel to the patellar ligament (a point of departure from Slocum, who has maintained that it was parallel to the functional axis of the tibia);

• If the angle between the patellar ligament and the common-tangent at the tibio-femoral point of contact, call it alpha, is 90 deg, neither of the cruciates is loaded;

• In the canine stifle, alpha is 90 deg at 110 deg of flexion – call this a cross-over flexion point; in full extension alpha is approx. 105 deg; in full flexion it is approx. 80 deg;

• With the stifle in extension with respect to the cross-over point, the load is on the cranial cruciate ligament; with the stifle flexed past the cross-over point, the load is on the caudal cruciate ligament;

• With the cranial cruciate ligament gone, the stifle can be stabilized by shifting the cross-over point to the full extension;

• This can be done by either TPLO (turning the plateau), or by TTA (turning the patellar ligament);

Execution of TTA

• The required advancement of the patellar ligament insertion at the tibial tuberosity is measured from a radiograph of the stifle in extension;

• With a frontal plane osteotomy, tibial tuberosity is advanced and held in position by:

(1) A cage transferring the compression component of the patellar ligament force from the tuberosity to the proximal tibia;

(2) A tension band plate transferring the patellar ligament force to the proximal diaphysis of the tibia;

• Open wedge osteotomy is grafted by cancellous bone to accelerate healing;

TTA vs. TPLO

• TTA moves the joint force to meet the tibial plateau; TPLO moves the plateau to meet the joint force;

• TPLO increases internal joint forces; TTA reduces them;

• By logical extension, but without clinical evidence at this time, TTA may show less long-term joint degeneration;

• TTA is less versatile (angular corrections impossible), but also less prone to unintended angulations.

TTA is less attractive in high tibial slope angles and heavy weight dogs

http://www.kyon.ch/principles_tta.htm