They result from life’s little mishaps or more serious events such as major accidents or surgeries, but most of us carry at least a scar or two with us. If they’re small and don’t cause discomfort, we probably don’t think about them much — but nonetheless, scar tissue is very different from healthy tissue, not just in physical appearance, but in its structure and capabilities.

The Physiology of Healing

Once a wound closes, it’s easy to think the affected area is good as new, as healthy as it was pre-injury. But W. Darren Hammond, MPT, CWS, senior director of The ROHO Institute for Education, explains that’s not the case.

“When someone has any injury — it could be a laceration, it could be a surgical scar, it could be you just bump your knee — all the tissues go through this physiology of normal healing,” he says. “When you have an open wound, people talk about two different ways that people will heal.”

Those two paths are primary intent (sometimes called primary closure) and secondary intent.

“Primary closure,” Hammond says, “is basically when you have a surgical incision. You bring the edges together and you keep them there, whether it’s with staples or sutures or Steri-Strips. There’s also something called delayed primary, when [physicians] have to wait a couple days before the swelling goes down or the infection is resolved, and then they stitch it up.

“And then you have secondary intent. That’s how a lot of pressure ulcers heal, and that is just by the normal healing process. The area of defect, the open wound, just starts to get smaller and smaller. So both ways provide you a scar. With primary intent that is a surgical incision, the scar is actually very linear and the scar is almost the same size as the initial wound, almost the same size as that surgical incision. With secondary intent, you typically have a lot more scarring because you have a larger defect. The wound gets smaller and smaller, and we have something called wound contraction — the wound literally contracts, and that’s why we have a scar getting smaller and smaller. With secondary healing, typically you have a big wound, but you have a scar that’s a lot smaller than the initial wound, and the scarring is more diffuse. It’s not very linear, and it covers a lot more surface area traditionally.”

How Scars Are Formed

While we typically think of healing as replacing damaged tissue with healthy tissue, Hammond says the body doesn’t form tissue that was identical to what was originally lost.

“Whether it’s a surgical scar or any defect, even internally, if you sprain your ankle and you tear your ligament, the body doesn’t go through a healing process to lay down the same tissue that was there,” Hammond notes. “So if we injure a muscle, if we injure a ligament, the body puts out a collagen-based matrix kind of material. That literally is what scar tissue is: It’s more of a collagen-based substrate that takes up the defect and kind of bridges a gap of that injury. That’s how scar tissue is different than the normal tissue that was there.

“Because it’s a collagen matrix, it sometimes doesn’t have the same amount of blood vessels, it doesn’t have the same properties. For instance, with muscle — [scar tissue] doesn’t have any of those contractile elements. Same thing with the skin — it doesn’t really have all the necessary layers. In skin, you have layers with hair follicles and sweat glands. When you have scar tissue, you don’t have those layers of skin that have those dermal appendages, those different physiological structures, any longer. Scars are different than the normal tissue. They’re made up of kind of a collagen that bridges the gap and fills the space in, but it is not a like for like.”

Hammond notes that scar tissue is inferior to healthy tissue in many significant ways. It’s stiffer than healthy skin, muscle and fat. It’s less pliable and less able to withstand being stretched or deformed without being damaged — a factor that raises the risk of future breakdown.

And scar tissue remains vulnerable to all those factors that make healthy skin and tissue more susceptible to breakdown. Poor nutrition, frequent and persistent moisture, heat, smoking and unrelieved pressure can all threaten healthy skin and tissue; they are even a greater threat to scar tissue.

“[Those factors] are going to increase their risk even more because scar tissue is not as resilient,” Hammond says. “It’s not as strong, it’s not as able to withstand all those forces.”