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Acrylic materials used for orthotics

Discussion in 'Biomechanics, Sports and Foot orthoses' started by rdp1210, Jun 5, 2013.

  1. rdp1210

    rdp1210 Active Member

    Members do not see these Ads. Sign Up.
    I have in my hand, three acrylic materials that have been used for making orthotics over the last 60 years. They are: 1) First Generation Rohadur (light amber color; 2) Second Generation Rohadur (red); 3) Polydor (currently on market).

    I am looking for individuals that have experience prescribing and working with all three materials. If you have, would you please contact me (either personally or on this forum) and let me know:
    1) Which one of these three materials was the easiest to work with?
    2) Which one did patients have the fewest problems with?
    3) If you had a choice of the three today, which one would you choose and why?

    I realize that you're going to have to be older than Simon Spooner to be able to answer these questions. If you know someone not on this forum who has had such experience, please refer them to me.

    Daryl Phillips
  2. Believe it or not Daryl, I have worked with acrylics as well as doped cellulose. Does that make me old? The question is why would you want to now? Wasn't Rohadur carcinogenic as well as brittle?
  3. Oh, and by the way :welcome: For those that don't know Daryl, he's a very smart biomechanical podiatrist; one of the sharpest minds in the business, in my opinion.:drinks
  4. rdp1210

    rdp1210 Active Member

    To say the truth, Simon, I think I've tried all the plastics and polyolefins and I find myself going back to acrylic. It seems that it has the most linear response and is closer to be isotropic. Of course I'm using this for people who have normal soft tissue mass and density on the bottom of the feet. I do know that you have to make the current polydor acrylic about 1mm thicker than you did the rohadur.

    When I was in podiatry school, Gen II rohadur had just replaced Gen I, and everyone was lamenting how much inferior Gen II was to Gen I.

    BTW: I wish I deserved the flattery, but maybe you're just trying to encourage me to do something more that write to podiatry forums.
  5. Why might these material properties be significant in foot orthoses? To be honest, I don't use any acrylics these days. BTW, my old boss always reckoned gen 1 was better than gen 2 and he was a "lab man".
  6. rdp1210

    rdp1210 Active Member

    You and I have discussed this before. I can't tell you I have a lot more information than I did a year ago in Orlando. I have found the anisotropic materials to be fickle friends. I know that they are torsionally less stiff than in the fiber direction. I have not been able to predict what they will do. That is because I have no idea about the frontal plane torsional moments across the MTJ and MCJs that any foot is subjected to. I know that Howard Hillstrom started some work on this a few years ago, but I don't think he finished and published anything about it.

    Also as you know, there is NO orthotic that has the NWB shape when you stand on it. I do have pictures of my own foot standing on my 5mm thick acrylic orthotics both from the medial and lateral sides to demonstrate this. Because the lateral side usually has a greater radius of curvature, it is more flexible than the medial side. Therefore, standard Root orthotics allow some degree of midfoot (either MTJ or other joint) inversion of the forefoot against the rearfoot. I believe that acrylics may have less of this tendency than polyolefins. Haven't proved it yet. Did an interesting demonstration a few years ago with Doug Richie on stage, in which we showed that making the lateral column of the orthotic thicker than the medial column markedly decreased the postural sway of a person standing on their orthotic. So we do have an n=1 study so far.

    I know I'm going to get lots of posts that say, "I get the same results with material XYZ as you do with acrylic." I realize that I'm speaking purly from a clinical impression side, not a research side.

    BTW - can you ask your former boss why Gen I was better than Gen II rohadur? Does he have any experience with polydor?

    Take care,
  7. Phil Rees

    Phil Rees Active Member

    Daryl, if its of any help, I've worked in orthotic manufacturing for quite some time and made my first Rohdur foot orthosis in Oct 1979 and continued to use it right up untill it was withdrawn by Rohm Chemicals in the late 80's.
    I still have a 6x3 sheet in my garage
  8. Daryl:

    Good to finally get you over here on Podiatry Arena.:welcome: Looking forward to many good discussions with you.

    I used quite a bit of the second generation Rohadur early in my career and I think it was a good material but is not as good as polypropylene for orthoses, in my opinion. Rohadur was easier to grind and maintained its shape well over the years. However, Rohadur broke frequently (whereas polypropylene orthoses rarely break) and the rearfoot posts on polypropylene orthoses are far superior when they are fused to the polypropylene shell when compared to any Rohadur-shelled orthosis.

    I never used Polydur since polypropylene seemed to me to be a great replacement material compared to any of more brittle acrylics. I now use polypropylene and high-density polyethylene foam (Plastazote #3) for all my orthosis shells.

    Daryl....since you like Rohadur so much...do you also only listen to vinyl records and not digital music?:rolleyes:

    And for all those following along, like Simon said, Daryl is one of the most knowledgeable biomechanically-minded podiatrists that I know and has written quite a few papers that are worth looking at, for those interested in biomechanics.

    Really good to have you here, Daryl!
  9. Lab Guy

    Lab Guy Well-Known Member


    Hi Daryl,

    It was good discussing biomechanics with you in Atlantic City recently. Glad you are here.

    I know many people love polypropylene but my personal favorite material is JMS 500, a high molecular weight high density polyethylene. It's not rigid but patients seem to tolerate it much better when your using minimal cast fill and inverted pouring position. It is more expensive than polypro. You can buy it at JMS plastics.

    I will be in Orlando later this summer and hopefully we can have dinner. Hope all is well.

  10. Bruce Williams

    Bruce Williams Well-Known Member

    When you say you "made the lateral column of the orthotic thicker than the medial column"' does this mean thicker cuing calipers, or did you make the lateral column taller? And, do you know for sure that if you only made it thicker that you did not also make it taller or increase the lateral column posting height? I think that needs some " 'splaining" :confused:
  11. rdp1210

    rdp1210 Active Member

    Thank you for those who have replied so far.

    Kevin, I too went through the polypropylene phase and I am now swinging back to acrylic in the last couple of years. Now we may be treating two different populations. You are in private practice, probably treating high number of athletic patients. On the other hand, my population base is probably more sedentary, and more overweight.

    Some questions:
    1) I know that you you have said in the past that you use some sort of prestressed polypropylene. Where do you get it? How much do you know about its molecular structure compared with standard polypro. Where can you send me to find out more about basic material properties of varioius polypros?
    2) What is the thickest polypro that you use? What formula do you use to calculate the polypro thickness you use?
    3) Are you using milled polypro?

    Now I had this great idea about using milled polypro - suppose to retain its shape better, etc. With due respect, I went to a well recognized lab that utilized the Sharp-Shape system, and I was totally disappointed - the lateral column was mfg thinner than the medial column of the orthotic, and the lab said there was nothing they could do about it, that Alex Chang would charge them more to program the system so that the lateral column was the same thickness as the medial column. Went to a company that uses Delcam - the medial and lateral columns were the same thickness, but the orthotic was too thin overall, and they didn't seem to be too responsive about my concerns about material stiffness.

    Bruce - not sure what the term "lateral column posting height" means. You'll have to show me next time we're together, or send me a drawing. To be precise, here Doug and I were on the podium, with an Fmat, Doug demonstrating postural stability (you've all probably seen him do it). All I did was put a little bit of felt under the lateral column of the orthotic, between the floor and the polypropylene, and voila' there was almost no movement of the CoP as the patient stood on one foot on top of the orthotic. Like I said, anecdotal, but it was dramatic, and it went with what I predicted should happen based on theory. The idea here was that I made it so that the orthotic lost less of its "neutral shape" when he stood on it -- it pronated the forefoot to the rearfoot more.

    Finally, Kevin, I prefer talk radio. And one advantage of vinyl recordings, they're harder to pirate.

    Looking forward to hearing from more of you.
    Best wishes,
  12. Phil Wells

    Phil Wells Active Member

    Hi Daryl

    I would like to add my 2 'penneth' from a CAD designers perspective.
    The idea of making the shell flex, or not, at specific points is fairly easy to do with the right software. The ability to engineer clinically relevant anistrophic or isotrophic properties into a shell is only limited by the clinicians imagination. This can be done by altering the thickness of the shell, adding torsion bars to the underside of the shell or cutting out vents or apertures in the shell to weaken it at specific loads - bit of guess work required there.
    However when I have offered this to fellow podiatrists they have usually stated that it is simply too difficult to define the design and I tend to agree. A couple of guys do use this approach successfully - I won't name them as they are members of this site and may want to speak for themselves.
    I do know that Doc Simon may be able to do this with FEA software but there are again limitations - the surface profile of the orthotic and its loading characteristics are able to be modeled but GRF/GRV, shoe orthotic intercation etc make it difficult to fully predict orthotic loading for every individual.

    I do think that Simon's idea of an open source library of virtual orthotic shells combined with FEA software would allow us to model orthoses more along the lines of what we need.


  13. Most manufacturers should be able to provide you with the technical data sheets for their materials. I use these to input material characteristics into FEA models.

    I'm all ears when it comes to peoples opinions on "formulas to select material thickness"...

    You got to remember too that when you vacuum form you are not going to end up with uniform thickness in the finished shell and it certainly will not be the same thickness as the un-molded sheet.
  14. rdp1210

    rdp1210 Active Member

    I realize that polypro surpasses its Tm when heated, whereas acrylic does not. That indeed does mean that you may not get an even thickness of the polypro when it's vacuum molded.

    What has your FEA models told you about the difference in torsional stiffness of polypro vs. acrylic?

  15. Non-stress relieved polypropylene (PP) is what we use. My understanding is that it maintains it's shape better over time than stress relieved PP. However, that is about all I know about it. However, I now have PP orthoses from Precision Intricast that I made 20 years ago that are still being worn by my patients so, in most cases, it is pretty durable stuff. I often tell my patients that their foot will likely change in shape more than the PP orthosis will in 10 years.

    I also have been using milled PP for about the past 8 years and have found it to be very similar in material characteristics to vacuum-formed PP, though, theoretically, since it is not heated, it should be even more durable and have even better shape retention characteristics over time than vacuum-formed PP. I haven't used Rohadur or any other acrylic since Rohadur was taken off the market and quit using graphite laminates over 20 years ago. I never used any of the TL materials.

    As far as material thickness, here are the parameters I consider:

    1. Body weight.
    2. Medial arch height of foot.
    3. Specific activity (sports or nonsports) that the orthosis is primarily intended for.
    4. Special design criteria necessitated by patient's diagnosis.

    Most common thicknesses of PP? 4 mm - 5 mm

    When I was lecturing in Australia about 5 years ago, I remember a lab that was already varying medial and lateral arch orthosis with ribbing and grooves in their CAD-CAM system. It certainly seemed like a logical thing to do with this type of technology. However, most podiatrists probably would find this level of customization too much work for their practices since it's hard enough to get them to even draw heel bisections on their casts.

    The demonstration that you describe is something I have been using clinically for the past few decades and have lectured on regarding the lateral balancing effects that can be achieved with foot orthoses. I commonly add lateral column padding plantar to the lateral arch (Rich Blake calls this the Denton modification) in patients that exhibit lateral instability, have peroneal tendinopathy or have other symptoms related to inadequate subtalar joint (STJ) pronation moment being generated by the orthosis.

    In essence, such a modification increases the stiffness of the lateral longitudinal arch of the orthosis which increases the orthosis reaction force lateral to the STJ axis so that the supination moments from Achilles tendon tension force are effectively counterbalanced by the external STJ pronation moment from the stiffened lateral orthosis arch. Similar effects can also be generated by the use of reverse Morton's extensions, or valgus forefoot extensions added to the orthosis since these modifications all increase the reaction forces lateral to the STJ axis, increasing the magnitudes of external STJ pronation moment from the orthosis during weightbearing activities, including unipedal balance.

    Here is an illustration from one of my lectures where I describe this modification specifically for the treatment of peroneal tendinopathy.

    Good discussion!:drinks
  16. The acrylic will still be differentially pulled and compressed to greater or lesser extent when it is formed over the positive model, Daryl. I do not believe that acrylic maintains a completely uniform thickness following vacuum forming.

    Define "torsional stiffness"? Torsional where? FEA models give you stresses and deformations between the nodes in the mesh. Depending on how fine the mesh you employ will determine the node density. What you end up with are deformation and stress plots which look similar to pressure maps from f-scan. Generally, given identical loading, the deformation patterns are determined by the surface topography of the device and the patterns will be similar across materials (unless you start mashing the device with excessive loads); deformation magnitudes are determined by the material properties. To illustrate this see the attached two images: 2 orthoses identical geometry, identical loading pattern, one is modelled from polypropylene copolymer and the other from high density polyethylene. Note how the distribution of the relative deformations is identical, but the exact magnitude is different between materials. In other words, if we modelled two identical devices and loaded them with the same loading pattern, but one was made from acrylic and the other from polyprop, the deformation distribution would be identical, but the magnitudes should differ.

    As an educational point for those following: what does this tell us?

    Attached Files:

  17. I want to know how I can make my patient's orthoses also have pretty colors like yours, Dr. Spooner?:rolleyes:
  18. rdp1210

    rdp1210 Active Member

    Thanks, Kevin.

    I didn't realize that you use the lateral column reinforcer as well. I have been using for quite a few years, w/o realizing that you were doing so as well. I would say that I'm doing it on at least 1/3 of my patients. One of the things my father did for many years, long before I joined him in 1979, was that he would skive away the lateral plantar prominence of the abductor digiti quinti muscle from the cuboid to just proximal to the 5th metatarsal head. It took me years to finally realize that this was his way of reinforcing the lateral column, i.e. he knew that the lateral column would flex more than the medial column, so he just made it higher so that it would flex to the point that the MTJ and 4th-5th MCJs would still be pronated on the frontal plane.
    I've tried to analyze why some people are in love with the Sole-Support orthotics. I know that he is depending on moderate to high orthotic flexibility properties to accomodate for the high degree of MTJ plantarflexion. He'd never get away with using acrylic. So far I've examined two people who are wearing and love their SoleSupports and both of these people had almost no sagittal plane motion in any motion of their MTJs.

    Reply to Simon - torsional stiffness is the frontal plane stiffness of the forefoot inverting/everting against the rearfoot. The big problem with the TL products is they have much lower torsional stiffness than bending stiffness in the sagittal plane.

    Best wishes,
  19. Isn't the increase in postural stability observed in the demo simply an example of increasing the base of support? If we take the horizontal distance between the centre of pressure of left foot (A) and the centre of pressure of the right foot (B) to be the base of support for the triangle formed between these two points and the centre of mass (C), making a triangle in the frontal plane ABC, then increasing the stiffness of the orthoses in the lateral arch should tend to pull the centre of pressure laterally under each foot, resulting in an increase in the distance between A and B and thus increasing stability of C above AB.
  20. Use one of these, Prof. Kirby.;)

    Attached Files:

  21. rdp1210

    rdp1210 Active Member

    Simon - we're talking about CoP when the patient is standing flamingo (i.e. unipedal)
  22. I've not modelled any composite materials, I assume it is due to the direction weave of the fibre cloth.
  23. I believe it is not only an increase in "base of support" but probably more importantly an increase in external subtalar joint (STJ) pronation moments being applied to the plantar foot to counterbalance the internal STJ supination moments from the gastrocnemius-soleus activity (and resultant Achilles tendon tension) that is required to maintain the center of mass (CoM) of the body anterior to the ankle joint axis during unipedal or bipedal standing.

    In other words, since Achilles tendon tension is required to maintain the CoM anterior to the ankle joint axis, and since Achilles tendon tension produces an internal STJ supination moment, a more stable rotational equilibrium position of the STJ will be achieved only when sufficient external STJ pronation moment is applied by the orthosis to properly counterbalance the internal STJ supination moments from Achilles tendon tension.
  24. Agreed. I erroneously thought that Daryl was talking about bipedal stance. It's the old contralateral wedging of the beer bottle trick.
  25. But in heating it, then pulling it over the positive model you will be differential re-stressing the material and altering it's point to point thickness from it's sheet form, as you will an acrylic sheet too. It's unavoidable in vacuum forming.
  26. rdp1210

    rdp1210 Active Member

    So the question, Kevin, is this: Is it only about STJ axis theory, or does the pronation moment across the MTJ and MCJs also contribute to the decrease in variation of the CoP?

    Remember that pressure against the metatarsal heads can only produce a moment around the STJ axis when there is stability of the joints between the forefoot and the rearfoot.

    This brings us back to the original Root postulate that continues to dog everyone, from Schuster to Wernick, to Glaser, and everyone else: Is the foot in its optimal standing state when a person stands with the MTJ at its pronation EROM? And since we can discuss ad-infinatum the definition of the EROM, I will make it this, the EROM that Mert Root could produce with his RA hands.

  27. Bruce Williams

    Bruce Williams Well-Known Member

    Kevin and Daryl;

    I appreciate what you have both described re: the use of felt under the lateral column and the poron or eva modification that Kevin's illustration showed so well.
    Now that I understand a littel better what you were doing I see that there would be no increase in the height of the lateral column but instead a potential decrease in the lateral arch collapsing b/c of the material under that column.

    I would say that I am not certain that the change in the deformation of the lateral column is what likely added stability to Doug Richie's balance test on the F-mat. I would suspect that it is more likely that if the fel came into contact with both the mat and the orthotic that it changed the affect of the orthotic on the CoM of the body and this added stability that way instead. It most definitley could be both, but I would think it more likely a combination of the two.

    my 2 cents.
  28. Should have known old Mert would rear his head pretty soon with you here, Daryl ;) On an incline, out in my garden, down the beach? In which environment are you hoping to define such an optimal position? And in whom: me with my de-cruciated knee or in my 3 year old nephew? Lets not start getting overly simplistic by pretending that there is one all governing optimal position for all people in all environments, it's just naive. Standing in the position postulated by Root?- which people rarely do unless they are in a podiatrists office or on the parade ground- go to your local mall and observe how people naturally stand on a hard, flat surface- bet it's not how they stand in front of you in your office. Are those people in the mall all barefoot or are they wearing shoes of varying construction? Moreover, lets not divert this interesting thread.
  29. rdp1210

    rdp1210 Active Member

    Simon, I didn't think you were correct because polypropylene is vacuum formed above its Tm and acrylic below, so I checked out what you said on two orthotics I've got sitting on my desk right now, one polypro and one acrylic, both formed to the same cast. I have a digital caliper that gives measurements to the nearest 0.1 mm. Looks like the variance of thickness of the poly pro over its surface is from 3.9 - 4.2 mm. The variance of thickness of the acrylic over its surface was identical, 3.9 - 4.2 mm. Now of course I have to question the accuracy of my measurements with this simple tool because I cannot guarantee that the caliper plane of measurement was absolutely perpendicular to the normal plane at all points.

    I'll look more into this subject.
  30. What, you didn't trust me? ;oP I think it's partly due to the fact that the sheets aren't uniform to begin with and partly that when you lower the membrane on the former you begin to compress and pull the material sitting on the highest points of the positive before the lower areas come into contact with the membrane and before the vacuum takes hold. Out of interest, did the variance in thickness match on a point by point basis between the two shells of different materials, Daryl? As an aside, we try to avoid abbreviations such as "Tm" here without giving their expanded form- not everyone will understand your abbreviations on this world wide forum.
  31. rdp1210

    rdp1210 Active Member

    This is not a defense of Mert -- I just didn't want to get bogged down into the definition of the EROM of the MTJ, so I used him because he couldn't produce very much pronatory moment around the MTJ axes. However the stiffness needs of an orthotic do go back to the Root Postulate. (I call it the Root Postulate because I can find no one in the literature that makes such a statement -- everything else Mert may have said, I can find an earlier reference for). If you tell me that you are still taking NWB casts, then I will say that you are still a believer in that postulate. If you are now doing partial WB casts, then I will say that you have discarded the postulate. And if you discard the postulate, then you have to have a new postulate to tell us how a vertical force against the metatarsal heads can produce a moment around the STJ. Now the Root postulate is expanded by the Sarrafian twisted plate paradigm, which Clint Jones seems to have even some additional insight with his wring theory.

    So put aside Mert :deadhorse:. The question had nothing to do with him. What I asked is whether the MTJ or other lesser tarsal joints also contributed to the improved stability? Yes I will always agree :D that the lack of variation in the CoP has to have equilibrium around the STJ axis, however the Root Postulate is what allows that equilibrium to occur. I should probably start another thread about midfoot stability as I've deviated too far from the original subject.

    Hopefully I'll have more real data for you in the near future about orthotic stiffness of acrylic vs. polypro.

    Best wishes,
  32. Go ahead, I should be happy to contribute my thoughts within such a thread and provide research references that question the "Root postulate".

    Look forward to it.
  33. Daryl:

    Regardless of what Mert Root believed and also what he did and did not say, here is what I believe.

    Ground reaction force (GRF) acting on the plantar metatarsal heads will cause multiple mechanical effects across multiple joint axes at the instant of GRF application. For example, GRF acting plantar to the 4th and 5th metatarsal heads will nearly simultaneously cause an increase in dorsiflexion moment at the 4th and 5th metatarsal-cuboid joint, a dorsiflexion moment at the calcaneo-cuboid joint, a pronation moment at the subtalar joint (STJ) axis (assuming the GRF vector's line of action is lateral to the STJ axis) and an ankle joint dorsiflexion moment. Since a force cannot exist against a body without a reaction force, then we must assume that if the ground can apply a reaction force against the metatarsal head, then all these moments at the more proximal joints occur nearly simultaneously since the masses and moments of inertia of these foot bones are negligible relative to the large magnitudes of GRF acting on the plantar foot.

    By the way, Daryl, the midtarsal joint should not be described as having as having a pronation-supination axis since it is not either a uniaxial or biaxial joint, contrary to what Mert Root lectured on and wrote in his books for a few decades.

    You will need to start a new thread here Daryl since you are starting to get Simon and myself very interested in these discussions....by the way, Simon has been in hibernation recently here on Podiatry Arena and I want you to know that it took someone like you to get him out of his long springtime snooze....:rolleyes:
  34. Bruce Williams

    Bruce Williams Well-Known Member

    I respect your response but the same can be said of Kevin's across the board descriptor of the STJ axis. In whose foot, with what amount of AJ rom, with or without anterior FF equinus, etc.
    I understand what Daryl is saying and it is a valid argument and has not been sufficiently addressed or studied for that matter.
    Ultimately medially posting the heel or arch will only work appropriately once the lateral column stops dorsiflexing. Also the medial column will not achieve optimal DFion stiffness, IMHO, in this same instance.
    The midfoot / forefoot response is every bit if not more important to rotatiional moments about the STJ than how or where the heel is posted.
    They both have affects that come into play in a specific timely matter. If that timing is off, then compensations, detrimental or not, will occur.
  35. Bruce Williams

    Bruce Williams Well-Known Member


    I cast partial WB using my AMFIT digitizer. You are correct, I cannot "lock" the lateral column with this technique. Instead I use a lateral FF wedge under the 5th metatarsal to compensate. I can usually get the height of this wedge guaged by assessing the amount of DFion of the 5th metatarsal head vs the 3rd metatarsal head. 0-3mm of wedging in a 5th met that does not elevate or is plantarflexed to the 3rd metahead and 3-5mm and then 5-8mm as there is more DFion motion.
    When I use poly devices from AMFIT I have a little more lee way on downsizing the wedge height b/c of the stiffness of the poly vs the EVA I usually use.

    So, have I effectively discarded the root postulate or have I created a viable workaround? You tell me?:dizzy:

  36. Bruce,

    I understand what Daryl is saying, I just think it's for another thread. This is a good thread on material properties- lets keep it that way. When Grace was a baby she had a toy train with the carriages connected by string- i could pick up the lead carriage without the other cars being picked up until enough tension was generated in the string connecting it to the next carriage. Like I said, lets talk about it, but in a new thread. I've argued this analogy before with Kevin and others.

    Kevin, regarding my spring hiatus: I was sick and tired of certain individuals ruining every thread. It's great to have some new people here who I know that I can debate with, disagree with, learn from and yet still remain courteous to and be good friends with when we come together at conference. :drinks Moreover, my sister underwent a kidney transplant 6 weeks ago and Grace is ripping up the South West (SW) regional BMX series (currently ranked 1st SW and 5th Nationally), so I've had my mind on other things.
  37. Hopefully your sister is doing well, Simon. Good to have you back in the mix...I've missed you here.:drinks
  38. Bruce Williams

    Bruce Williams Well-Known Member

    here is to wishing your sister to a full and successful recovery!
  39. Thanks for your kind wishes. She's doing well- much better than she was on dialysis. Anyway, back to my academic interests- how do foot orthoses work? I really must write that paper this summer:eek:.
  40. Bruce Williams

    Bruce Williams Well-Known Member


    If you would ever like a sagittal plane "assistance" with that I would be willing to help. Not that you really need it, but offering just the same.:drinks


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