TTO vs. TTAR for the Treatment of Cranial Cruciate Ligament Disease
Cranial cruciate ligament (CCL) disease is the most common cause of lameness in canines. Partial and complete cranial cruciate ligament rupture leads to stifle instability (i.e., craniocaudal motion of the tibia relative to the femur and internal rotation of the tibia). This stifle instability results in the progressive development of osteoarthrosis of the affected joint.
Tibiofemoral shear forces are generated during weight bearing (stance phase) and are transmitted across the stifle joint. Cranial tibial thrust is that component of the tibiofemoral shear force which results in cranial movement of the tibia relative to the femur. The cranial cruciate ligament provides a passive restraint against this cranial tibial thrust. The caudal horn of the medial meniscus is also a passive restraint against cranial tibial thrust. Active forces acting on the stifle joint include the stifle extensors (quadriceps femoris muscles), stifle flexors (semimembranous, semitendinous, and biceps femoris muscles), and the extensors of the hock (gastrocnemius muscles). Cranial tibial thrust is a force created by weight bearing and muscular contraction (stifle and hock extensors) resulting in compression of the tibial plateau against the femoral condyles. In the active model of the stifle, this force is counteracted by the passive restraints (CCL and caudal horn of the medial meniscus) and the active restraints (flexor muscles of the stifle).
Slocum proposed that the tibiofemoral shear force was parallel to the functional axis of the tibia and that cranial tibial thrust would be neutralized if the tibial plateau was perpendicular to the functional axis of the tibia (a tibial plateau angle [TPA] = 0°). The TPLO technique involves a radial osteotomy of the proximal tibia and rotation of the proximal fragment (tibial plateau) along this osteotomy to reduce the tibial plateau angle to zero. It was then determined that a TPA = 0° resulted in a caudal tibial thrust during weight bearing and potential overload of the caudal cruciate ligament. Current clinical practice is to reduce the TPA to 5–7°.
Further biomechanical investigations (Tepic, Montavon) proposed that the tibiofemoral shear force was in fact parallel to the patella ligament. This resulted in the development of the tibial tuberosity advancement technique (TTA). The TTA procedure involves a single osteotomy of the tibial tuberosity, which is then advanced a predetermined distance and stabilized with a specially designed implant. By advancing the tibial tuberosity (and insertion of the patella ligament), the tibial plateau is made perpendicular to the patella ligament.
The triple tibial osteotomy was developed (Bruce WJ) and combines features of the TTA and the cranial closing wedge osteotomy. The TTA allows for reduction of the tibial plateau slope to a point where it is perpendicular to the patella ligament. The TTA achieves this by some rotation of the tibial slope as well as some cranial advancement of the tibial tuberosity. This is achieved by a series of three tibial osteotomies. There is limited specialized equipment required to perform the procedure (wedgie, saw guide, and measuring device). All the tibial plateau leveling techniques appear to have similar results, complication rates, and outcomes.
Undiagnosed and untreated medial meniscus tears are a major reason for continued postoperative lameness and poor results. The medial meniscus is prone to injury due to the fact it is a passive stabilizer of cranial tibial thrust and is attached to the medial collateral ligament. This attachment to the medial collateral ligament prevents the caudal horn of the meniscus from being mobile. In a CCL-deficient stifle joint, the tibia moves cranially relative to the femur. The caudal pole of the medial meniscus acts as a "wedge" and counteracts cranial tibial thrust. Due to its attachment, the caudal horn of the medial meniscus may become trapped between the tibial plateau and the femoral condyles. The menisci are important structures of the stifle joint in that they improve congruency between the femoral condyles and the tibial plateau, facilitate load transmission of forces across the joint and act as a passive restraint to cranial tibial thrust. Partial meniscectomy and medial meniscal release reduce the meniscus's functionality, therefore resulting in overload of the articular cartilage and subchondral bone, the result being the development of osteoarthrosis. Medial meniscus release (MNR) remains a controversial topic. All menisci must be carefully examined at the time of surgery; this should include palpation with a small right-angled probe. Arthroscopy may allow more accurate examination of the caudal pole of the medial meniscus.
Accurate radiographs of the affected stifle are essential in order to accurately plan the surgery for both TTO and TTA Rapid (TTAr). An extended mediolateral view (stifle at 135°) is required with the femoral condyles superimposed.
Measurements for the TTO procedure include the length of the patella ligament and the angle between the tibial plateau and the patella ligament. The length of the patella ligament equals the length of the tibial crest osteotomy, while the angle between the tibial plateau and the patella ligament is known as the correction angle. The angle is inserted into a formula to obtain the wedge angle.
For the TTAr procedure, one needs to calculate the tibial tuberosity advancement required. This is done using either the common tangent method or the "Ness" method. One also requires the measurement for the position of the Maquet hole and the thickness of the cortex at this level.
Triple Tibial Osteotomy (TTO)
Triple tibial osteotomy surgery involves a straight incomplete osteotomy of the tibial tuberosity. A wedge-shaped ostectomy is then performed caudal to the tibial tuberosity osteotomy (i.e., a wedge-shaped piece of bone is removed from the tibial shaft). The ostectomy is then reduced and stabilized with a clover-shaped bone plate with three screws in the proximal fragment and three to four screws distally. This results in the tibial plateau being reduced and the tibial tuberosity being advanced.
Indications for the TTO include partial and complete CCL rupture, especially in large-breed dogs, individuals with pathological tibial plateau angles, and revision of failed extracapsular stabilizations. Triple tibial osteotomy allows for correction of other abnormalities simultaneously (i.e., patella luxations, genu varum/valgum, and tibial torsional deformities).
Tibial Tuberosity Advancement Rapid (TTAr)
Tibial tuberosity advancement rapid involves a single straight incomplete osteotomy of the tibial tuberosity. The distal end of the osteotomy is the Maquet hole. The osteotomy is then progressively levered open until the required advancement is achieved and then the appropriate sized cage is inserted and stabilized with screws to the tibial tuberosity and the tibial diaphysis.
Indications for the TTAr are similar to those for the TTO. Correction of genu valgum/varum is not possible with the TTAr. Dogs with very steep tibial plateau angles may also be better treated with a TTO as the advancement of the tibial tuberosity with the TTAr may be too large. The largest cage available for the TTAr is a 15-mm cage.
The overall complication rate of the TTO procedure is comparable to that of the TPLO and TTA surgeries. Most of the complications seen are relatively minor and easily corrected. The most common complications intraoperatively include fracture through the distal end of the tibial crest osteotomy, fracture through the caudal tibial hinge/cortex, haemorrhage from the popliteal artery, and uncommonly intra-articular screw placement. Postoperative complications include tibial crest fracture, infection, late meniscal tears, and undiagnosed meniscal tears. Tibial crest fractures are best stabilized with a single K-wire and a figure-of-eight tension band. The incidence of implant failure (plate and screws) is very low.
The complication rate with TTAr is similar to that of the TTO. Most complications seen are minor complications not requiring further surgery. Complications seen included fissure formation distally from the Maquet hole and fracture of the tibial tuberosity distally. Management for these complications is not necessary unless there is displacement of the tibial tuberosity.
Tibial tuberosity advancement rapid surgery is less invasive than TTO surgery, as no dissection is required around the caudal aspect of the tibial. This also reduces the risk of damage to the popliteal artery.
Tibial plateau leveling provides dynamic stabilization of the stifle joint during the stance phase of weight bearing. There are numerous techniques available to achieve tibial plateau leveling. Evidence-based surgery does not indicate any one technique to be superior to another at this point in time. The advantages of the TTO technique are that it is a relatively simple surgery to perform, requires limited specialized equipment, has a low complication rate, and has comparable results/outcomes and complication rates to the TPLO and TTA. Tibial tuberosity advancement rapid surgery is a simple and effective method of treating cranial cruciate ligament disease. The advantage of TTAr surgery is the reduced surgical time compared to a TTO surgery. Further objective research studies are required (force plate analysis, etc.). The overall complication rates with both surgeries are acceptable and the outcomes are good clinically.