What feature of a skin injury (for example, depth of laceration?) determines whether scars will form as hypertrophic or not? For this question, I am differentiating hypertrophic from keloids, which is a different process. An example is below of the same individual with both regular and hypertrophic scars.
It will most likely depend on the location of the injury if there's tension (joint areas) and not the depth, size or type of injury.
"There is evidence suggesting that increased mechanical tension can initiate hypertrophic scars formation.Based on this hypothesis, it makes sense to minimize mechanical forces after surgery. Surgical excision scars should be positioned along, rather than across, relaxed skin tension lines whenever possible"
And also "Inflammation is also known to contribute to hypertrophic scarring, and every attempt to minimize the inflammatory response should be made by ascertaining clean surgery and good wound care to prevent infection thereafter. Using inert suture materials is also important in this context"
Update on hypertrophic scar treatment Felipe Bettini Rabello, Cleyton Dias Souza, and Jayme Adriano Farina Júnior.
Another factor that comes into play is related to the individual, and not just the location, type, and severity of the injury. For example, some connective tissue disorders, such as Ehlers Danlos Syndrome (particularly classical type), can make you higher risk for hypertrophic, atrophic, and keloid scarring. Depending on the severity of skin involvement, even relatively minor injuries can result in such types of scarring.