Response to the Letter to the Editor Regarding TTA

Comments on the “Effect of Tibial Tuberosity Advancement on Femorotibial Shear in Cranial Cruciate-Deficient Stifles. An In Vitro Study”

I appreciate the invitation from the Editor to answer the questions posed by the Author of the Letter to the Editor: Comments on the ‘Effects of tibial tuberosity advancement on the femoral shear in cranial cruciate-deficient stifles. An in vitro study’.

A careful reading of the Comments does not reveal any questions that I was supposed to address. Instead, there are assertions and accusations, even serious ones claiming scientific misconduct.

Since I was asked to respond to non-existing questions, I will make my task easier by attempting to extract some from the Comments.

Was the experiment in publication set up to be successful? [1]

The answer is ‘Yes’. The experiment was not set up to reveal the direction of the joint force but to demonstrate the effects on the joint stability assuming that the joint force is nearly parallel to the patellar ligament. This was clearly stated in the publication.[1] The Author, whether by omission or intent, fails to quote other early, relevant publications on the subject from the team at the University of Zurich, led by the late Prof. Pierre Montavon.[2] [3] [4] The support for the assumption of joint force being nearly parallel to the patellar ligament was given by Tepic et al.[5]

Do we know what the femorotibial joint force is?

The answer is ‘No’. I have done my graduate research at the laboratory of the late Prof. R.W. Mann, MIT, in which the estimation of joint forces was a subject of research for decades. I was surrounded by and interacted with graduate students and academic staff working hard to improve the knowledge of the musculoskeletal system and the forces controlling it. Two fundamental problems make the task extremely hard: (i) redundancy and (ii) co-contraction (attention to both brought by Kipfer et al).[1] Resolving redundancy always calls for optimization via a cost function and even in the seventies the work at our lab at MIT included a cost function based on the total expenditure of chemical energy by the muscles. Co-contraction is even harder to study because it does not affect the kinetics of locomotion, which is in most cases the starting point of joint forces analysis. Seminal studies of Emilio Bizzi and Neville Hogan at MIT, including the experimental work with monkeys, led me to include in my presentations a highly simplified but useful rule of thumb to place the joint force between the major muscle groups of agonists and antagonists which surgeons can appreciate from the basic anatomy and radiographs and put to good use in their practice.[6]

What is the rationale for TTA? TPLO?

With full understanding and disclosure, in presenting the rationale for TTA I have left out the antagonists (flexors) in stating that the joint force is nearly parallel to the patellar ligament (tendon). The action of the quads is essential to maintain the stifle extended yet it has been ignored in the rationale for TPLO. Even a cursory look at the history of cruciate surgery should teach us how lucky all of us have been, including – and especially – the dogs treated by TPLO. The presentation of the rationale for TPLO by Barclay Slocum is based on the ‘functional axis’ of the tibia. There is no such axis and basing TPLO or any other intervention on it is fundamentally wrong. ‘Functional axis’ would exist only if there was no force acting on the tibia other than the two joint reaction forces. That is not the case. Quads are pulling hard on the tibial tuberosity and hamstrings are there too. So how come TPLO works clinically? Barclay Slocum made two errors which luckily cancelled each other. One was the assumption that femorotibial joint force acts along the ‘functional axis’ and the other was that the tibial condyles are flat, whereas there are convex. I can recommend to anyone interested in the ‘luck’ aspect of cruciate surgery the last presentation I gave on the subject at the 2019 Kyon Symposium in Boston available on YouTube. Other than demonstrating the ‘luck’ aspect of TPLO, the presentation demonstrates how to plan for TPLO (and TTA) to avoid relying on ‘luck’.

Are in vitro studies of any use?

At the WVOC 2018, Barcelona, meeting I presented a review of in vitro studies related to cruciate ligament surgery. Kyon supported no less than 20 such studies only two of which got published. All of those studies contributed to improvements in the surgical technique of TTA and all were presented by Kyon instructors at the courses and congresses, most of them video recorded and freely available. I find it somewhat amusing that the Author considers the 2008 publication so irritating but also fundamental to the clinical practice of TTA. The in vitro studies that Kyon has supported have given us confidence that TTA is sound. Not perfect, but sound.

Is Nisell misquoted in?

The Author starts his critique by stating that Nisell did not show the knee joint force to be nearly parallel to the patellar ligament in humans.[7] We did not quote him for that but for his studies that showed the change of the patellar ligament inclination to the tibia for different angles of flexion extension. We have done those measurements in our studies, on the dog stifle, and found that the ‘crossover point’ does not differ much.[2] [3] It was in publications, as well as given by Kipfer et al, that we considered Nisell's work worthy of reference.[1] [2] [3] [4]

Is TTA based on the three-force rigid body mechanics?

The answer is ‘Yes’. It was presented by Tepic et al and in numerous presentations and publications that followed.[5] And yes, it is the subject of a high school course on physics. That is exactly where the statement of the stifle joint reaction being nearly parallel to the patellar ligament comes from. I wonder if the Author has seen this anywhere – ubiquitously presented by us related to Kyon, but also by many others.

Was publication an obstacle to further studies? [1]

Hard to judge, but many studies followed. There were several key in vitro studies on TPLO and TTA where the pull by quads was represented by a ‘cosmetic’ spring (just to keep the patella in the groove). The most recent studies that I was involved in, now more than five years ago, addressed the role of co-contraction. All of what we have learned is in the public domain. I would be happy to see further work done on TTA. The Author, unfortunately, does not offer any suggestion or any indication of what this may involve. The sketches in the Letter are less than convincing and leave me in doubt about how this is going to clarify the ‘mystery’ of TTA clinical success.

Publikationsverlauf

Artikel online veröffentlicht:
24. Februar 2025

• References

• 1 Kipfer NM, Tepic S, Damur DM, Guerrero T, Hässig M, Montavon PM. Effect of tibial tuberosity advancement on femorotibial shear in cranial cruciate-deficient stifles. An in vitro study. Vet Comp Orthop Traumatol 2008; 21 (05) 385-390
• 2 Dennler R, Kipfer NM, Tepic S, Hassig M, Montavon PM. Inclination of the patellar ligament in relation to flexion angle in stifle joints of dogs without degenerative joint disease. Am J Vet Res 2006; 67 (11) 1849-1854
• 3 Schwandt CS, Bohorquez-Vanelli A, Tepic S. et al. Angle between the patellar ligament and tibial plateau in dogs with partial rupture of the cranial cruciate ligament. Am J Vet Res 2006; 67 (11) 1855-1860
• 4 Voss K, Damur DM, Guerrero T, Haessig M, Montavon PM. Force plate gait analysis to assess limb function after tibial tuberosity advancement in dogs with cranial cruciate ligament disease. Vet Comp Orthop Traumatol 2008; 21 (03) 243-249
• 5 Tepic S, Damur DM, Montavon PM. Biomechanics of the stifle joint. Abstracts of the 1^st World Orthopaedic Veterinary Congress ESVOT-VOS; September 5–8, Munich, Germany. Accessed 2002 at: https://www.cabidigitallibrary.org/doi/pdf/10.5555/20193219072
• 6 Bizzi E, Chapple W, Hogan N. Mechanical properties of muscles: implications for motor control. Trends Neurosci 1982; 5: 395-398
• 7 Nisell R. Mechanics of the knee. a study of joint and muscle load with clinical applications. Acta Orthop Scand Suppl 1985; 216 (Suppl.): 1-42