Deep tissue injuries (DTI) in pressure ulcers have recently drawn much attention.^1–6 Experiments exploring how shear force and pressure over a bony prominence cause DTI in pressure ulcers⁷ have shown that pressure ulcer necrosis resulting from DTI appears commonly as a column with a rectangular-shaped cross section. DTI necrosis may also be shaped like an hourglass (hourglass-shaped necrosis). Necrosis of either shape may be either continuous or have a layer of relatively healthy tissue sandwiched between a superficial and deep layers of necrotic tissue (sandwich-shaped necrosis).

In order to understand this progression, definitions and mechanisms of occurrence of hourglass-shaped necrosis and sandwich-shaped necrosis are presented below. The progressive development for each is illustrated in Figure 1 and Figure 2. Hourglass-shaped necrosis is caused when pressure and shear force applied to the body surface causes damage to the soft tissue (Figure 1). These types of mechanical stress tend to be stronger with areas of bony prominence and thus produce more severe and wider damage to tissue nearest the bony prominence. On the other hand, while the skin surface has direct contact with pressure/shear force a shallow ulcer and sometimes a deep ulcer will develop in the upper layer of the tissue. Consequently, the total damage to the soft tissue is shown as two layers of necrosis, which are connected with a narrow area of necrosis in a cross section because such damage is less in the middle layer, which is influenced by the effects of strain rather than direct stress.
The defining conditions of hourglass necrosis are application of external forces (pressure/ shear force) that are applied to the body. The skin surface receives mild to moderate damage, but the soft tissue in the deep layer around a bony prominence is extensively and severely damaged. However, the soft tissue in the middle layer is not greatly affected. Consequently, the distribution of necrosis shows an hourglass shape in the soft tissue. Hourglass-shaped necrosis progresses from initial undermining of the pressure ulcer after the discharge of liquefied necrosis, defined as the initial or discharge phase of undermining.⁷

Sandwich-shaped necrosis occurs when initial damage to tissue in the middle layer between bone and skin in hourglass-shaped necrosis is small or negligible; the tissue remains healthy (Figures 2 and 3). Thereafter, the necrotic area in both the upper layer and deep layer are completely separated by an intermediate layer of nearly intact tissue, appearing as sandwiched or sandwich shaped. Damaged shallow ulcer necrotic tissue in the upper layer is liquefied and discharged early, while the necrosis in the deep layer remains as either slough necrosis or pus. The middle layer of relatively healthy tissue may temporarily hide this DTI.
The blood circulation through the necrotic deep tissue loses integrity and ischemia occurs in the middle layer of tissue during the healing process and then necrosis develops, which eventually leads to a perforation and connects the upper and lower necrotic areas that appeared previously as intact tissue. In other words, a shallow pressure ulcer in the upper layer separates from apparently healthy intermediate tissue, which then perforates, thus connecting to an originally present deep necrotic area, to ultimately become a DTI. Clinically this process appears initially as new necrosis in the central part of the wound surface. It then grows relatively slowly with a clear circumscribed border, developing and expanding in depth and area without additional pressure and shear forces around the wound.