BLOG: 3D patellofemoral joint imaging provides new path for better surgical planning
Optimizing surgical intervention for a patient with recurrent patella instability can be challenging, particularly when considering the possibility of adding an osteotomy. Accuracy is paramount.
Surgeons have become accustomed to using 2D criteria to try to understand very complex 3D and 4D geometry. The tibial tubercle-trochlear groove (TT-TG) distance is frequently used to evaluate laterality of the tibial tubercle when considering whether to move a tibial tubercle medially or anteromedially, but Johannes M. Sieberer, MSc, MS, and colleagues have demonstrated again recently that the TT-TG metric is prone to error, raising further concerns about using this single metric for surgical decision-making about moving a tibial tubercle. Adam B. Yanke, MD, and colleagues have noted also inconsistent inter-rater reliability regarding trochlea classification in 2D, again demonstrating a need for more accurate imaging in patellofemoral surgery decision-making. Fortunately, 3D imaging offers this accuracy using new metrics and multi-directional visibility.
Many authors, including Andrew J. Cosgarea, MD; David R. Diduch, MD; Seth L. Sherman, MD; Miho J. Tanaka, MD, PhD; Yukiyoshi Toritsuka, MD, PhD; Yanke and others have emphasized and employed 3D imaging to help with patellofemoral surgical planning. Fortunately, 3D imaging is now readily available allowing surgeons to have a more accurate, comprehensive and holistic understanding of the complex patellofemoral joint for surgical planning.
3D planning for patella instability surgery is inevitably more accurate. Try these simple steps:
- With 3D CT at 0° and 20 of knee flexion, one can evaluate the entire joint from every vantage point by simply spinning the 3D knee on a computer screen. This overview alone is valuable and removes the need to attempt an understanding of complex patellofemoral geometry with 2D images. One can truly get a holistic view of the entire joint configuration and structure from every angle.
- With a good look at these images, one gets a sense of how far lateral the patella is in extension and how far the patella must go to engage the femoral trochlea by 20° knee flexion. One can appreciate the patella entry zone into the trochlea, which is readily identified between the proximal medial and lateral trochlea ridge ends. The center of this entry zone, we call the entry point(EP), for ease of making measurements (Figure 1). This alone is a big step forward compared with a TT-TG measurement, which crosses the very mobile tibio-femoral joint, alone. Visual 3D planning uses fixed proximal metrics.
One can excise the patella with a scalpel tool to study the whole trochlea path, leave some or all of the patella to see how it lines up in flexion. Viewing the patellofemoral joint in 3D is powerful. Spin it around on the screen, look at it face on, from the side, from above or from below to gain understanding of how far lateral the patella goes upon knee extension. This enables reliable decision-making. The further laterally or proximally (alta) the patella starts, the more one may consider an osteotomy, usually using a tibial tubercle transfer, to better align the patella with its trochlea path, aiming to get it to the deeper trochlea sooner thereby securing stability.
If one sees a proximal spur, the surgeon may want to anteriorize the patella to bypass the spur and/or recess the spur. If the patella is proximal (alta), one can see, measure and determine how much to distalize and/or anteriorize and/or medialize to position a patella to avoid the spur or whether to resect some or all of the spur.
While visual 3D planning alone is more accurate and easier to understand than traditional 2D methods, we recommend some simple metrics when considering a tibial tubercle transfer osteotomy: The fixed proximal metrics (which are not affected by tibial rotation) are the entry point-trochlear groove (EP-TG) angle (Figure 2), the entry point-transition point (EP-TP) angle (Figure 3) and the Caton-Deschamps ratio for alta and supratrochlear spur height. EP-TG angle of greater than 30°, EP-TP angle of greater than 40°, Caton-Deschamps ratio of greater than 1.3 and TT-TG distance of greater than 15 mm suggest tibial tubercle transfer can substantially improve understanding of when to add a tibial tubercle transfer, or rotational osteotomy.
A supratrochlear spur of greater than 3mm indicates adding anteriorization to bypass the spur in early knee flexion.
As the proximal trochlea becomes flatter, and particularly with convexity and and/or lateralized entry point, the need to assist patella entry to the trochlea in a stable manner by adding tibial tubercle transfer increases in order to achieve permanent stability and reduce risk of eventual arthritis. 3D heat mapping of contours can be helpful also in understanding whole trochlea contours.
3D patellofemoral joint imaging using computerized tomography provides a new, relatively inexpensive, widely accessible path to better patellofemoral surgical planning.
To learn more about the use of 3D to assess patellofemoral instability, please visit https://patellofemoral.org/.
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