EasyFix TTA system: Preoperative Planning and Execution
1. Obtaining an Adequate Radiograph
With the patient in general anaesthesia, images are taken in a mediolateral projection. The stifle is held in extension at an approximate angle of 135 degrees, without rotation.
A well positioned image. The femoral condyles are superimposed at about 135 degrees of extension.
The tibia is not rotated, the fibula is seen separately at the level of the tibial crest.
The tibial crest is well defined, with the caudal cortices of the medial and lateral tibial condyles in line, and close to one another.
Errors of Positioning
The femoral condyles align correctly but the fibula cannot be visualised.
The caudal cortices of the medial and lateral tibial condyles are seen separately (with the medial one more caudally) .
Inadvertent inward rotation of the tibia.
The fibula is seen entirely separately from the tibia and the appearance of the tibial crest is unusual.
Inadvertent outward rotation of the tibia.
A severe positional error. The caudal portions of the femoral condyles are not supported by the tibia.
The eminences are abnormally close to the insertion site of the long digital extensor muscle.
The tibia is cranially displaced (i.e. in cranial drawer position).
A hyperextended stifle with the patella located proximally to its groove and a slack patellar tendon.
As this does not occur during ambulation, such a positioning is unfit for the planning of a TTA.
2. Planning the TTA
It is adviseable to calculate the planned degree of advancement by both the classical tibial plateau (TP), and the recently described common tangent (CT) methods.
In most cases the TP method results in a higher degree of advancement. If not extremely high, it is best to go by this.
Whichever method is used, however, the distances measured on the radiograph need to be converted to the actual measure of advancement.
Determining the factor of correction
Almost all radiographic images are magnified. In order to quantify the degree of magnification, a metallic object of a predetermined size is to be placed next to the patient parallel with the film, positioned to be in line with the thickest part of the stifle.
The percentage of the difference in the length of the actual object and its radiographic image, is the degree of magnification.
Thus, if a 10 cm ruler or K-wire is seen as a 11.2 cm long metallic object in the image, the degree of magnification is 12 percent. As the radiographic images for the planning of TTA procedures are taken in standard positioning, it is not necessary to recalculate this in every case. As an option, the measure of advancement (in millimetres) may be subsequently modified by the calculated percentage of magnification. Should this be less than 10 percent though, the difference may probably be safely ignored.
Determining the magnification of the radiographic image
The Tibial Plateau (TP) Method
The goal of TTA surgery was originally defined as the cranial advancement of the tibial tuberosity (TT) so far as to render the patellar tendon (PT) perpendicular to the tibial plateau.
Therefore, a line perpendicular to the tibial plateau from the insertion site of the PT on the patella is drawn, and the distance between this line and the TT is taken.
1. the most cranial and most caudal points of the tibial plateau are marked and connected with a line (TP line).
2. a line perpendicular to the TP line is drawn from the most cranial point of the insertion site of the patellar ligament on the patella. The distance of this line from the TT is the planned degree of advancement, which can be modified by the degree of magnification, if necessary.
The Common Tangent (CT) Method
According to this alternative definition, the goal of TTA surgery is to advance the distal insertion site of the PT (i.e. the TT) cranially until it is perpendicular to the common tangent (CT).
1. three points are designated on the perimeter of the femoral condyles through which a circle is drawn. Its center is marked. If the two condyles do not coincide, circles are drawn around each, and the two centers are then connected.
The midpoint of the line that results is used as a center for further planning.
2. a somewhat larger circle can be drawn around the tibial condyles so that it contacts the caudal edge of the medial tibial condyle, and passes just underneath the intercondylar eminences following the curved line of the joint. The two centers are connected. A line perpendicular to this is drawn, which is the Common Tangent (CT).
3. A line perpendicular to the CT is drawn from the most cranial point of the patellar insertion site of the patellar ligament. The distance of this line from the TT is the desired measure of advancement. This can be modified by the degree of magnification, if necessary.
3. Choosing the Cage
TTA cages come in the following sizes: 6, 9, 12 and 15 millimetres. The choice of the right cage is based on the planning described above. Correction in size resulting from radiographic image magnification is considered. As a main principle, using too large a cage is a lesser mistake, thus it is advisable to round off to higher values.
As the TT is pushed forward around a fixed center of rotation, advancement also results in a slight downward displacement. This is another factor that favors the use of a larger cage when in doubt.
4. Planning the Osteotomy
1. marking the distal end of the osteotomy (cortex; C)
Five to fifteen millimetres distally from the prominent distal tip of the tibial crest the width of the cortex (C) is measured. The point just caudal to the inner line of the cortex is marked.
2. marking the proximal end of the osteotomy (G)
The most proximal point of osteotomy is the tubercle of Gerdy, which is the cranial rim of the groove for the proximal tendon of the long digital extensor muscle. On the radiograph it is seen as a prominence cranially to the small groove that is just cranial to the tibial condyles.
3. marking the distance between the line of osteotomy and the tibial tuberosity (TT-O)
4. collecting data
Relevant data are collected and should be readily available in the operating theatre
Osteotomy line to Tibial Tuberosity (TT-O)
Width of the cranial tibial Cortex (C)
Tubercle of Gerdy to the TT-O (GTTO)
Size of cage
5. The Procedure: a Step by Step Guide
1. Cleaning up
The first step of all CCL surgeries regardless of the type of procedure is cleaning up, i.e. the examination of the joint cavity, with particular attention to the menisci and the excision of the remnants of the ruptured ligament. If meniscal injury is detected, only the damaged parts are to be removed.
2. Surgical approach for TTA
The skin is incised starting from 1 to 2 cm proximal to the tibial tuberosity half to one cm caudomedially to the patellar tendon (parapatellar incision) and (further) on to 1 to 2 cm distal to the most distal point of the tibial crest. A slight undermining of the skin reveals the very proximal site of insertion of the patellar ligament (i.e. the tibial tuberosity, TT). At the distal end of the incision the most distal point of the tibial crest is identified, the cranial aspect of which is freed of its cutaneous attachments in a 1 to 2 cm long line. Care should be taken to leave the rest of the insertion site of the patellar tendon intact.
3. The osteotomy
Half to one cm distally to the most distal end of the tibial crest the width of the cortex (C) is marked with the Z-Device for Drillhole Aiming, resulting in a drill position just caudally to the cortex. (I.e. a thinner cortex dictates the use of a smaller gauge drill bit.) A hole is drilled.
It is important to keep the drillbit at a right angle to the sagittal plane of the shaft of the tibia, and not the tibial crest which normally bends laterally. The drill In the picture is pointed a little too cranially.
A drill bit is then passed through the hole to secure the saw guide to the tibia. Using the Z-device, the TT-O distance is marked on the tibia (see text above for TT-O definition). The saw guide is positioned to coincide with the TT-O mark, and a 1.5mm drill hole is created through it. The drill bit is left in place to keep the saw guide in position. It is advisable to cut down the soft tissue through the saw guide before the osteotomy is made with an oscillating saw. Cooling of the saw during cutting is important.
When the saw blade is well past the cortex, the saw guide is removed, the line of the osteotomy is extended distally through the first drillhole and proximally through the bone with caution to avoid injury to the patellar tendon.
4.Inserting and securing the cage
The key point of the procedure is the careful advancement of the osteotomised tibial crest. The blade of a bone bender is placed in the gap parallelly with the cut edge, then turned 90 degrees downward. If significant resistance is met, the completeness of the osteotomy should be reassessed. Additionally, an approximately 10 mm long incision of the retinaculum on either side may be performed for better mobilisation. The position may be maintained with another similar instrument at the distal end of the osteotomy until the cage is inserted.
The cage should be situated approximately 3mm under the tibial plateau at caudal osteotomy wall. If the desired degree of advancement is slightly more than has resulted, the cage may be inserted a little more distally. This may be the case when the measured distance is somewhat larger than the cage used, but not large enough for a larger cage.
The smaller bone bender is 6mm in width. This helps measure the distance of the cage distally to the plateau.
A cage placed too distally may result in the subsequent fracture and detachment of the tibial crest. The cage should not be positioned distally to the tibial tuberosity. The G/TT value helps assess this.
The cranial ears of the cage are bent slightly inward while the caudal ones slightly outward with the cage ear bender. The cage is fitted in position. A correctly placed cage is 3 to 5mm distally to the level of the tibial plateau and the ears are fully in contact with the bone. The bone bender is removed from the distal end of the osteotomy. A bone reposition forceps may be used to assist the compression of the cage to the caudal osteotomy wall. The last step is the securing of the cage by screws. 2mm screws are used in a diverging fashion, pointing away from the osteotomy line. When the screws are fastened, the wound is sutured in layers.
5. Management of Potential Complications
1. The most common intraoperative complication is the fracture and detachment of the tibial tuberosity at the distal end of the osteotomy. As soft tissue is handled gently during the procedure, the cage is fixed by four screws and the patellar tendon extends further distally from the distal tip of the tibial crest, usually no further action is taken. If a full detachment is present in the distal osteotomy line, a small gauge K-wire and/or tension band wiring can be used to stabilise the osteotomy.
2. The ear of the cage may break while contouring. If the distal contact is strong enough, one screw may be sufficient. If not, change the cage and use it for another procedure.