First let me apologise for the lack of posts recently…things have been a bit hectic in the past couple of months. We now have over 1200 users accessing the site via the iPhone App which is fantastic. The key with these things though is user interaction, so if you have any comments or queries regarding this or other posts, don’t hesitate to feedback.
This series is drawn from a presentation I prepared and presented at last weekend’s TTA training course at Zurich Vet School and at the Kyon Symposium 2012 again in Zurich. The point of this series is to explore the variables regarding post operative TTA radiographs and using our planning to control these, thereby getting a more consistent piece of surgery with more consistent outcome for our patients and clients. TTA (when performed with Kyon implants) is a procedure which forgives the surgeon for a reasonable amount of deviation from the ideal post operative appearance in most cases. It is for precisely this reason that we as surgeons need to be more diligent in terms of our planning. This sounds like a contradiction but if we consider that the ideal outcome will be derived from the ideal modification of the geometry of the tibia, when we miss the mark, we may not give our patients all of the potential benefits that have been shown to be associated with Tibial Tuberosity Advancement (TTA) such as normalisation of intra articular pressure profile and therefore the long term benefits that may be associated with this unique attribute (progression of DJD, Incidence of Late Meniscal Injury etc). So the surgeon “gets away with it” but our patients may not!
Goals of Planning
There are four main goals in our TTA planning:
- Determining Optimal Plate Size.
- Determining Optimal Plate Position.
- Determining Osteotomy Position and Orientation.
- Determining the required amount of advancement to give a 90 degree relationship between the Straight Patellar Tendon and the Tibial Tuberosity.
This post will deal with the key to establishing these measurements/assessments which is optimal positioning for radiography.
Basis for Precise Planning
It is not possible to achieve optimal planning from suboptimal radiographs. The key variables are:
- Stifle Angle.
- Rotation (Femur and/or Tibia).
- The presence of femorotibial subluxation.
These must be controlled prior to engaging in planning for TTA.
1. Stifle Angle
There has been a bit of confusion here. The key is to remember that TTA is designed to provide femorotibial stability in stance phase. For this reason, all of the templating should be performed with the stifle in the normal standing angle on the mediolateral projection. This will vary from breed to breed and patient to patient. Previously people suggested that the stifle should be at 135 degrees extension, this should not be used for the reasons just stated. It can be difficult to assess the normal standing angle in cruciate deficient patients as, if the condition is unilateral, the contra lateral limb is likely being held in slight hyperextension to offload the affected limb, if it is bilateral, the patient’s pathological standing angle will be altered too. For this reason we should not simply rely on the person positioning the patient but go to the patient and palpate the Straight Patellar Tendon (SPT), if this feels to have low tone, then we are in an overextended position for that patient if it is very taut then we are likely in hyper flexion, we should use our experience in this assessment. If we see the following appearance on the radiograph then we are definitely hyper extended:
Note the wavy appearance of the Straight Patellar Tendon indicating Hyper Extension.
This abnormality is easily identifiable and the stifle should be re positioned to eliminate this appearance. Measuring advancement from hyper extended radiographs will underestimate the required amount of advancement and therefore influence post operative stability and all that may result from this.
Measuring from radiographs with rotated femur and/or tibia will seriously affect our measurement in terms of plate, advancement required and osteotomy position and orientation. We should reposition the patient until we have the ideal. Whilst there are adjustments we can make in our templating in radiographs with a rotated femur, it will always be more precise if our templating is from perfect mediolateral projection with femoral condyles superimposed. We will return to the issue of femoral rotation when we look at assessment of advancement. For now we will focus on Tibial rotation.
Given that the Cranial Cruciate Ligament (CCL) limits internal tibial rotation, pathology of the CCL will result in increased internal tibial rotation and therefore this is the most common problem encountered. We first need to be able to recognise when this has happened, and this is not as easy as determining femoral rotation. The key areas to look at are:
- The caudal cortices of the Tibia.
- The relationship between the Tibia and the Fibula.
- The definition of the distal aspect of the Tibial Crest.
The images below show Perfect unrotated, internally rotated and external rotated isolated tibias:
The Tibia is perfectly positioned. The asymmetry of the caudal tibial cortices is minimal.
The Tibia is externally rotated and the medial cortex of the tibia is in a cranial position.
The Tibia has been internally rotated and the lateral cortex is now positioned cranially.
The degree of rotation above is not exaggerated beyond that which we may commonly encounter in our radiography pre surgery. The following image shows the change in relationship between the Tibia and Fibula associated with rotation of the tibia.
Left: Normal, Middle Internal Tibial Rotation, Right External Tibial Rotation
The Image below shows the impact on the definition of the Tibial Crest of the same types of rotation (this can also be seen above). The effect of this will vary between patients but in general if it is difficult to identify the distal aspect of the crest on the radiograph, go to the patient and palpate this area. If you can feel it distinctly in the patient, you should be able to see it on the radiograph!
3. Femorotibial Subluxation
If the tibia is cranially displaced (due to CCL pathology) on our planning radiograph, this will have a variable impact on our estimation of required advancement as the Tibial Tuberosity is already advanced. We need to identify landmarks that may indicate that this is present. These are:
- The relationship between the Tibial Eminence and the intercondylar region of the femur.
- The relationship between the caudal aspect of the tibial condyles and the femoral condyles.
- The relationship between the Origin of the Long Digital Extensor Tendon and the Sulcus for this tendon on the tibia.
The image below shows these points. If we identify subluxation, we need to correct it before proceeding to templating otherwise we will undersize the cage and fail to achieve sufficient advancement. Note also that the tibia and femur are rotated on the radiograph below as identified by a lack of superimposition of the femoral condyles, indistinct fibula, indistinct distal aspect of the crest and increased distance between the tibial cortices.
If we avoid these three potential pitfalls, we will be more likely to achieve reliable measurements in our planning that will guide our surgical execution and determine our surgical outcome. We need to make precise measurements from well positioned radiographs in order to deliver the perfect opportunity for perfect outcome.
Next… Plate Size and Position.