Primary vs. Secondary Healing

00:01 Let's remodel this scar.

00:04 So those of you who may have had a surgical procedure, know that very early on of the procedure is kind of a red looking, angry thing.

00:15 And over time, that scar gets smaller and smaller, and more white.

00:21 Why does that happen? So again, we're looking at overlapping effects.

00:27 And slightly different than what we've seen previously, but inflammation, acute and chronic, kind of occurs over a period of two to seven days after injury.

00:36 Granulation tissue overlapping five to 10 days.

00:40 And it kind of peaks in that period of time.

00:43 And then the dotted line that's going up, that's collagen accumulation.

00:49 And there will be remodeling.

00:51 So the cells that are in there, that are making the matrix are actually as it turns out, modified smooth muscle cells.

01:00 They are fibroblast like, but smooth muscle like.

01:03 And over time, they can contract, and we get a smaller wound.

01:09 And over time, we also release matrix metalloproteinases, and don't release tissue inhibitors and metalloproteinases.

01:18 And we get some restructuring of that matrix.

01:22 And over a period of weeks to months, that wound will contract.

01:27 This has important ramifications not only in terms of making that scar on your abdomen smaller and less conspicuous, but let's say, I have a total burn on my entire hand, and it's scars.

01:39 And then if I don't have regular physical therapy, that wound will contract.

01:45 And instead of having a hand, I'll have kind of a claw.

01:49 So wound contraction actually has ramifications.

01:52 As we'll see, when we talk about the complications associated with wound healing.

02:02 Then this is now wound healing kind of one on one.

02:06 This is what we worry about a kind of the surgical level.

02:11 So surgeons talk about healing by first intention and healing by secondary intention.

02:18 Healing by first intention or by primary intent.

02:21 We have a nice kind of circumscribed injury, such as an incision that we cut into the tissue, and then we sew back together.

02:31 So we approximate the edges.

02:33 Clean, not infected, all of that.

02:36 But the injury that's going on there and then typical wound healing process will lead us to have all the usual things.

02:44 We will get inflammatory mediators to get elaborated, we'll get a little bit of angiogenesis, we will grow into that area.

02:52 So over time, we'll get granulation tissue right on schedule, seven to 10 days.

02:56 And then in that area of injury, we will regenerate mostly, but we will lay down a little bit of scar.

03:03 Okay, that's healing by first intent, where we don't have big areas of damage.

03:09 And it's just following the usual processes that we talked about.

03:12 And then there's healing by second intent.

03:15 And this happens when we don't want to necessarily close it up.

03:19 This will actually happen quite frequently.

03:22 You'll see it on the wards when a patient has a large decubitus ulcer, for example.

03:27 Where the tissue has not healed, and there's a big area of ulceration in the skin.

03:31 You don't want to close that up because there will be bacteria and other things in there that will get infected.

03:37 So what you do is every day, as a medical student, go in, pack the wound with sterile gauze, sometimes containing antibiotics.

03:45 And then the next day, you come along, take out the gauze and debris the wound and you'll see granulation tissue in there, and slowly but surely, that big ulcer will eventually close in and you'll get skin over the surface of that.

03:59 That's healing by second intent. And we do it all the time.

04:02 But it's a much bigger process with much more inflammation, much more injury, and greater angiogenesis, and we count on eventually that getting smaller, and smaller, and smaller because of wound contraction.

04:14 So, here we have this big open area of ulceration, decubitus ulcer with a big scab in it, which is just coagulated blood products.

04:25 And we are now recruiting because of the injury, all the macrophages that are going to drive angiogenesis as you see them in the new capillaries.

04:33 And you're going to begin the process of filling this in slowly but surely.

04:38 And over time, you will have a big area of fibrosis that will eventually over time contract.

04:44 Note to, on this last piece.

04:48 See in the middle one where there was a new capillary sprout, that was necessary to maintain this highly metabolic scar activity? Once we have scar, scar is actually kind of low metabolic activity.

05:00 It doesn't take much blood supply to maintain a scar because it's mostly extracellular matrix.

05:07 So over time, not only does that wound contract and gets smaller because of the myofibroblast, but it also blanches.

05:16 We don't need as much vasculature, so that remodels and that goes away.

05:20 And that's why a wound over a longer period of time starts out very pink because of neovascularization, and it ends up very white because we don't need much blood vessel supply to keep that scar alive.

05:31 So, long term scars, get wider and wider, paler and paler.

05:37 And they get smaller and smaller, up to a point.

05:41 Primary versus secondary healing.

05:43 So primary healing, there's mild inflammatory infiltrate because there's not much damage.

05:47 In secondary healing, healing by secondary intent, more intense inflammation because we have these big areas of injury.

05:55 There is less granulation tissue, in the primary intent.

05:58 There is greater volume of granulation tissue and therefore a greater amount of resulting scar.

06:04 And in the primary wound, there is not much significant wound contraction.

06:08 You really haven't had much damage, but there may be significant wound contraction that occurs in healing by secondary intent.

06:17 So with that, we've kind of walked all the way through, from acute to chronic inflammation, to angiogenesis, to wound healing, and we are ready to talk about the complications of wound healing.