TTA & TTA-2 Principles

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BIOMECHANICAL RATIONALE:

The TTA technique was developed on the basis of several observations and simple biomechanical analysis of the canine knee joint:

  • Total joint force in the stifle is approximately parallel to the patellar ligament (a point of departure from Slocum, who 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 ( ~135° stifle angle) 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 (advancing the patellar ligament).

EXECUTION:

The TTA involves an osteotomy of the non-weight bearing portion of the tibia. The patellar ligament is aligned perpendicularly to the common tangent of the femorotibial joint, eliminating cranial tibial thrust. This new alignment eliminates the need for the CrCL and results in a stable joint.

  • 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, the 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;

  • The open osteotomy, distal to the cage, is grafted with autologous cancellous bone or other graft material such as allograft or hydroxyapatite (HA) to accelerate healing.

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TTA vs. TPLO

Since its clinical introduction, TTA has been mostly compared with the Tibial Plateau Leveling Osteotomy (TPLO). The January 2009 issue of Veterinary Surgery gives the most in-depth comparison of the two techniques in a review article: Boudrieau, RJ – Tibial Plateau Leveling Osteotomy or Tibial Tuberosity Advancement? Vet Surg 38: 1-22, 2009.
Some of the key differences are:

  • 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 lengthening the “lever arm” to the patellar tendon.
  • TTA does not change the geometry of the joint;
  • By logical extension, but without clinical evidence at this time, TTA may reduce the development of osteoarthritis[1][2];
  • TTA is less invasive, surgically simpler (but not simple);
  • Angular corrections are not possible with TTA, but TTA is less prone to unintended angulations.
  • TTA restores femorotibial contact patterns similar to those obtained before CrCL transections, while TPLO does not.
  • TTA takes into account the stabilizing force of the quadriceps muscle which is ignored in the TPLO biomechanical rationale.

Advancement of the patellar ligament insertion point on the tibia increases the lever arm of the dominant active force at the stifle, leading to a general reduction of all reaction forces, including those between the patella, femur, and the tibia.

In contrast, TPLO causes a cranial shift of the femur, i.e. closer to the insertion point of the patellar ligament. This shift reduces the lever arm and thus increased force is required to extend the leg. Inflammation of the patellar ligament, frequently seen in TPLO, may be an indication of an increased force required to keep the joint balanced. Increased patellar ligament force, increases joint reaction forces and elevates the risk of cartilage degeneration. In view of these arguments and the apparent lack of patellar ligament inflammation in TTA, progression of joint disease may be slower than reported in TPLO.[2]

A more obvious, though generally overlooked, difference is that TPLO intervention places the stifle into increased flexion by the amount of the plateau rotation, i.e. by some 20 degrees. Joint congruity at near full flexion must be compromised, not the least in the region of the caudal meniscus. Whether the meniscus is released at surgery, as instructed by Slocum, or through the wear and tear of its use, reduced fluid sealing function is bound to lead to cartilage degradation.

TTA does not effect joint congruency. However, it does increase the loading of the caudal cruciate, although to a lesser extent than TPLO. This is partially offset by general reduction of internal joint reactions due to the now longer patellar ligament lever arm.

On balance, TTA is expected to provide a further improvement in long-term joint function, although to date there is no definitive data proving this.

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Notes:
[1] – Boudrieau, RJ – Tibial Plateau Leveling Osteotomy or Tibial Tuberosity Advancement?
VetSurg 38: 1-22, 2009 (p.8)
[2] – Kim SE et al. – Effect of Tibial Tuberosity Advancement on Femorotibial Contact Mechanics and Stifle Kinematics
Vet Surg 38: 33-39, 2009