How the research started
(00:21) “patients asked me very simple questions that I couldn’t answer, like why does my tendon hurt? why especially in the morning, why is my tendon so thick? and … despite that I saw these patients almost every day, I didn’t really have good answers to them.”
(01:55) “he had done a pilot experiment in the superficial digital flexor tendon of an [anesthetized] horse, a tendon that is somewhat comparable to the human Achilles tendon. And I remember him showing me a video where the pressure inside the tendon increase dramatically during ankle [dorsiflexion]. And that’s caught my attention. So until then I mainly thought that tendons were exposed to tensile load, but that experiment showed me that loading also generates a substantial pressure increase within the tendon itself.”
How fluid and pressure behave inside the tendon
(10:00) “And it’s assumed that that increase in pressure is helping driving fluid out of the tendon during loading. And that’s a process known as fluid exudation. For instance, after running a marathon, the Achilles tendon has been shown to become around 13% thinner, and that’s the change. That is largely attributed to fluid loss from the tendon.”
(10:45) “So the Achilles tendon is quite unique because it’s composed of three subtendons originating from the [soleus] and the medial and the lateral gastrocnemius. And these subtendons they twist around each other approximately 90 degrees before inserting into the heel bone. And this torsion can also generate additional internal compression during loading, a bit like wringing out a towel, potentially pushing more fluid out of the tendon.”
(13:40) “So those [GAGs] they are negatively charged, sugar [chains], they are bounded to proteoglycans, and you can see them as tiny water magnets. and because of their negative charge, they attract and [retain] a lot of water. … We know that in tendinopathy, also in the mid portion, we see a similar fibrocartilage-like adaptation. The cells become rounder, more type 2 and type [3] collagen, and importantly, for our hypothesis also, the [GAG] content increases four to five folds.”
(16:50) “So if [GAGs] attract more water and the tendon starts to swell, the [epitenon] might restrict how much the tendon can expand. And Jake, there’s where your [SpongeBob] analogy works really well. So imagine the tendon as a sponge taking up more water. That sponge is sitting within a quite confined space. So at some point more swelling could translate into higher pressures within the tendon.”
(18:40) “these [GAGs] they not only attract water, but they also retain it, and this can reduce the tissue permeability. So if you can go back to the [SpongeBob] analogy, so the pathological sponge is not only wetter, it’s also harder to squeeze the fluid out of the structure.”
Why tendons hurt in the morning
(24:33) “that’s something that we think happens during the night, for example, and may also explain like the morning pain, the morning stiffness you have. If these [GAGs] stay in place in your tendon and your tendon is completely relaxed, then your tendon can again be overhydrated in the morning, and then the first steps that you take in the morning can be quite painful because with the extra fluid that you have in your tendon.”
(1:04:45) “That’s why I also advise for patients with morning pain, morning stiffness, to instead of walking to perform first of all isometric exercises against the wall for example, submaximal isometrics long enough at least eight seconds to potentially reduce the fluids content and with that also the pressure and the pain during the first steps in the morning.”
What the cadaver studies showed
(26:40) “So the deep insertional tendon layers consistently showed much higher pressures than the superficial layers. And that’s interesting because insertional [Achilles tendinopathy] also seems to develop primarily in these deep tendon layers nearby the [calcaneus]. For example, during a straight knee [calf] stretch at 15 degrees of [dorsiflexion], the pressure at the deep insertion layers was about three times higher than in the superficial insertion layers.”
(30:50) “for the insertion … we were quite sure that this would occur. We were quite … surprised that also at the mid portion the pressure increased exponentially. … the pressures were I think ten to twelve times bigger at the mid portion than the proximal portion. So that means that torsion does indeed have quite a big impact on the pressure within your tendon.”
(34:40) “So at 15 degrees of [dorsiflexion], the type 2 tendons had about 80% higher [intratendinous] pressures than type 1 tendons … we just found that torsion is not only beneficial for tensile strength, but greater torsion also comes with more internal compression. and it’s also I think clinical relevant because it also means that during loading the same external load that you apply on your patient does not always create the same internal load in every person because of that torsion that [varies] between people.”
Pressure and blood flow in living tendons
(38:10) “At rest the pressure was already high, so 44 millimeters of mercury, which was much higher than we expected. Normally in muscles it’s lower than 15, so it was way higher than what we expected. And as we increased the [plantarflexion] force the pressure increased almost linearly. So at only 50 newtons it already reached around 100 millimeters of mercury.”
(40:15) “so this means that even at low levels of contraction the pressure inside your tendon increases that high that it collapse the blood vessels. So … basically in a healthy tendon this means that during normal loading the pressure rises, the blood flow decreases, and during unloading the pressure falls down in the tendon and you get reperfuse.”
(45:15) “it was a basketball player. I think he had a game in Turkey. he went by plane the same day and when he came back he had like a massive Achilles [tendinopathy]. it’s speculative but I think it can be explained by the fact that … when you take a plane for two or three hours into a static [dorsiflexion] position, you don’t have the normal blood reperfusion. That is necessary for the recuperation”
How to load the tendon
(1:01:30) “so the slow heavy condition, so three seconds eccentrics into maximal [dorsiflexion] produced the greatest response. so tendon thickness decreased by almost nine percent, compared with only three to four percent in the other conditions. … So again the wet sponge analogy works really well here. So if you squeeze a wet sponge harder, you create a stronger driving force to push the water out. But if you squeeze it slowly, you also give the water more time to escape.”
(1:14:30) “it was not no compression, it was low compression … Once you go back into the fields when you perform your sports, you need a proper amount of dorsiflexion. If during the [rehab] you never put your ankle into dorsiflexion and your tendon has not been exposed to compressive load, and then suddenly you go back in the field and it’s exposed to massive amounts of compression, you get a flare-up of course of your tendon again.”
(1:17:50) “So what you could do there is just load the tendon heavy but really slowly, use [isometrics] in the morning. for the isotonics really heavy enough. It’s a common mistake that patients don’t load the tendon heavy enough, but also at the same time slow. so at least three seconds down, for example, with an eccentric [heel] drop program.”
Flare-ups and rest
(1:19:15) “I think in the first stage if your tendon is really swollen and it’s only a couple of days or weeks, just rest, rest, rest. you don’t have to load your tendon especially not fast movements because that can induce the [vicious] cycle I was talking about earlier. If you want to load your tendon, do it with isometrics [or] heavy slow.”
(1:22:17) “… exactly the same thing that I explained, that they just have … flare up, so more swelling, more maybe [GAG] synthesis, and those [GAGs] will reduce again if you give them a proper amount of rest for a couple of days. And then it’s important not to load the tendon in a reactive stage or flare up because that can … get things worse. But don’t say your patient that they lost everything that they built up for the last months because … I don’t think it’s a fact.”
On injections
(40:25) “it’s an argument I think to be cautious with intratendinous injections, particularly in an early stage with a swollen tendon, because the resting pressure is already high. If you then further inject it with additional fluids, it could potentially hinder or induce even more hypoxia or ischemia.”
(52:53) “First of all, I’m not a fan of injecting things in or around your tendons, so I would never do that like in an early stage [tendinopathy].”
(1:20:00) “before I used PRP well as [a trainee], you just do what is asked you to do. So I injected … tendinopathies and I mean it was really hard to get something in into that tendon because it was already swollen, so I pushed a massive amount of … PRP into a really swollen tendon, I will never do that again anymore.”
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GhenTendon: https://www.ugent.be/ge/hsr/en/research/orthopaedics-and-traumatology/ghentendon