Richard F. Edlich M.D., PhD
Distinguished Professor of Plastic Surgery and Professor of Biomedical Engineering
University of Virginia School of Medicine

Jarrett Arnette, B.S.
Research Project Coordinator

Thomas F. Henzey, III, B.S.
Educational Programmer

The occurrence of pressure ulcers in patients is very high in certain high-risk groups. These special high-risk groups include elderly patients, patients with spinal chord injuries, or any individual with an impaired ability to reposition. The prevalence of pressure ulcers in skilled care and nursing home facilities is approximately 23%.¹ A prevalence of pressure ulcers exists in many acute care facilities. Prevention of pressure ulcers is by far the best treatment of this condition, warranting certain interventions and preventive measures. Two major risk factors to be minimized are the exposure of skin to moisture and the possibility of friction and shear force injuries. A patient's skin may be exposed to a variety of substances that are moist: urine, stool, perspiration, or wound drainage. Although these substances may contain factors other than moisture that irritate the skin, moisture alone can predispose the skin to serious injury and subsequent pressure ulcers.^2,3 Underpads are often used to protect the skin of patients who are incontinent. These products effectively absorb moisture and present a quick-drying surface to the skin. Because these pads are designed to reduce injury attributed to the moisture associated with urinary and fecal incontinence, it is reasonable to assume that they would serve a similar function in those instances where the source of moisture is perspiration or wound drainage.

An underpad may also reduce the friction or shear forces between the skin and the underlying bed sheet. Friction injuries to the skin occur when a patient moves across a coarse surface such as bed linens. Most friction injuries can be avoided with appropriate techniques when patients are moved so that their skin is never dragged across the linens. Placement of an underpad whose backing has a low coefficient of friction beneath the patient is a reliable means of reducing these frictional forces, thereby minimizing the potential for injury.

The construction of an underpad should accomplish three goals. First, its backing should have a low coefficient of friction to prevent frictional skin injuries. Second, an inner absorbent core should rapidly contain moisture and disseminate it throughout the entire pad. Third, the core and coverstock should prevent wet-back, successfully working together to retain moisture and prevent wet-back or fluid return.

The purpose of this study was to determine the biomechanical performance of seven commercially available underpads and bed linens to reduce the development of pressure sores in patients who are at high risk. The three biomechanical performance parameters examined were coefficient of friction, absorbent capacity, and rewet. Each of these measurements of these performance parameters can be easily replicated in other laboratories. The results of these studies provide a scientific basis for selecting an underpad to prevent pressure ulcers in patients.

Basic Concepts. Underpads are absorbent sheet-like products that are placed on top of the bed or chair of the patient who is incontinent to contain urine for both comfort to the patient and the protection of the furniture and bedding. Their basic structure consists of three layers: a fluid-impermeable backing sheet against the bed, a fluid-pervious coverstock on the body-contacting surface, and an interposed absorbent core. The backing sheet is composed of a strong film of hydrophobic material, usually a polyolefin, rendering this side an effective moisture barrier and reinforcing the product as a whole. A low coefficient of friction is preferable to allow the underpad to shift easily with patient's movements.

The coverstock or top layer exists to contain and conceal the underlying absorbent core while also directly contacting the patient's body. A nonwoven fabric is used, and its characteristics are derived from three factors: the nature of the polymeric material, the method of fiber assembly, and the technique of web bonding. To perform as an effective coverstock, a lightweight, low-density material in the form of a smooth, soft web is suggested. This unique web must freely allow fluid to pass through its openings into the interior of the pad but then inhibit the reverse flow (rewet). A hydrophobic, nonabsorbent material such as polypropylene is preferred so that no liquid is attracted to this outer layer.

Two types of webs, carded and spunbonded, can serve as a coverstock. Carding is a process that separates staple fibers with fine wires or combs, aligning them essentially parallel to the machine direction. This process leads to a weakness in the cross direction. A carded web requires an adhesive, usually a cross-linked acrylic binder, to hold the web fibers together. In contrast, spunbonded webbing is a more complicated procedure but appears to be more effective. This process begins with polymer chips that are melted, extruded through spinnerets as infinitely long fibers, and then collected on a conveyor belt in random array. As the fibers cool web-bonding occurs. Finally, embossing may be done to the coverstock, in which a pattern is pressed into the sheet by passing it between heated rolls that have a raised design. This treatment adds a three-dimensional character to this layer, allowing an adequate separation between the body-contacting surface and the absorbent core beneath.

The absorbent core exists as a bulky, loosely formed web known as a batt that is composed of very short fibers in random array. Absorbency is highly dependent on the number of free fiber ends available to attract and retain fluid; thus the loose fibers of the batt, or the fluff, effectively serve this goal. The fibers are cellulosic in nature, and delineated, bleached wood pulp is most commonly used. In addition to absorbency an effective wicking mechanism is desirable, that is the pad should rapidly direct fluids away from the initial wetting site to more remote areas for storage, minimizing local saturation and maximizing the pad's capacity. Unfortunately the large interfiber spaces of the fluff, though excellent for absorbency, have a poor wicking capability. Decreasing the pore size between fibers improves the capillary action that promotes fluid movement. Therefore compression of certain areas of the batt, successfully provides a means of fluid wicking. Typically a diamond pattern is pressed into the batt, oriented to direct fluid horizontally away from the patient's body. Such compacted regions also add a degree of stability to the otherwise flimsy batt. A second means of support is often included too; at least one tissue layer of denser cellulosic wood pulp surrounds the batt, maintaining its integrity. These reinforcement sheets may even be heat-bonded directly to the batt with the compressed diamond pattern.

A recent advance in absorbent products has been the addition of a superabsorbent polymer, or SAP, to the fluff that significantly increases the total amount of fluid that can be absorbed. The polymer is a hydrocolloid material, typically a cross-linked polyacrylate, that is embedded within the batt as a powder. Upon the addition of a liquid, it converts to a gel particle swollen with fluid but retains its original shape and remains undissolved. Currently used SAPs can absorb up to 70 times their own original weight in urine and swell to an average particle size 1 to 2 mm.

The final underpad product is completed by a hot melt construction. The polyolefin backing is folded up and over all three layers and heat-sealed to maintain its shape and integrity of the complete product. This rectangular product is known as an underpad. Two variations of this final construction step exist. One is to include adhesive strips in the four corners of the backing to keep the underpad in place. A second innovation is to extend the backing on two sides so these flaps can be tucked under the sides of the mattress to prevent the pad from slipping. (This type of pad is referred to as a drawsheet).

This study examined and evaluated seven individual underpads, each is a variation of the basic three-layered structure outlined above - a coverstock, an absorbent core, and a backing (Table 1).

Underpads. A first type of hospital underpad (Ni-Med, Division of Oak Medical Industries, LLC., Farmington, MO.) measures 48.4 cm x 77.0 cm and weighs 43 grams. Its three layers consist of a carded web of polypropylene, a thin inconsistent cellulose batt, and a polyethylene film backing.

Depends® underpads (Kimberly-Clark Corporation, Neenah, WI.) measures 58.4 cm x 91.4 cm and weighs 78 gm. A spunbonded polypropylene web with an embossing pattern serves as a coverstock. The cellulose fluff is held between two tissue-paper layers, with the underlying tissue paper heat-sealed to the polyethylene backing. A diamond pattern thermally pressed into these core layers.

Secure®underpad (G. Hirsch & Co., Inc., Burlingame, CA.) is 58.4 cm x 91.4 cm and weighs 65.7 gm. A spunbonded polypropylene web forms the coverstock. The absorbent core consists of a cellulose batt between two tissue layers heat-pressed together with a diamond pattern. The moisture barrier backing is a polyethylene film.

Attends® underpad (Procter and Gamble Company, Cincinnati, OH) is 58.4 cm x 91.4 cm and weighs 53.9 gm. Its patented coverstock is a nonfibrous, hydrophobic film of polyethylene containing tapered capillaries or orifices.⁴ The small apertures serve as one-way valves for fluid movement and as a means of spatially lifting the corner from the absorbent core. A tissue-paper layer and a cellulose batt lie beneath the coverstock. The backing is a polyethylene film.

TuckableTM drawsheet (Paper Pack Products (Wyant Healthcare), Summerville, N.J.) has a weight of 162.5 gm and an absorbent surface size of 68.6 cm x 91.4 cm. Extending from two opposing sides are 16.5-inch polyethylene flaps that tuck under each side of the mattress.⁵ Its four-layered structure begins with an embossed coverstock of spunbonded polypropylene that is thermally fixed to the underlying tissue layer. The cellulose fluff is heat-bonded to the polyethylene backing.

Tranquility® underpad (Principal Business Enterprises, Dunbridge, OH) measures 53.3 cm x 91.4 cm and weighs 139.4 gm. The coverstock is a spunbonded polypropylene web with a unique layering process. The fiber collecting rolls move very quickly, creating a web with near-parallel fibers that is very strong in the horizontal direction. Its advanced embossing process further reinforces the coverstock in this left-to-right orientation. The second layer, a tissue paper, is much thicker and stiffer that those of the other pads, helping to prevent rewet. A large amount of cellulose fluff serves as the absorbent core, and placing it between two tissue layers and thermally bonding them together develop its high integrity. Homogeneously mixed within the fluff is a superabsorbent polymer, probably a polyacrylate. Tranquility® is the only underpad studied that contains SAP. The polyethylene backing serves as the moisture barrier, with added adhesive slips on each corner to hold its position on the bed.

PrivaTM underpad ( Med-I-Pant, Inc, Montreal, Canada) was the only reusable underpad studied, measuring 43.2 cm X 70.0 cm and weighing 209.5 gm. A ribbed knit coverstock of 65% polyester, 35% cotton overlies the 100% polyester fiberfill that provides the durability to be reusable. This filling is thermally bonded to a nonwoven batt of 60% polyester, 40% cotton that has been tightly compressed to serve as the absorbent core. The backing is formed by a fabric of 100% polyester wrapped knit that has been bonded to a ployvinylchloride film.

Biomechanical Performance. Three major performance parameters of each underpad were studied: coefficient of friction of the backing, product absorbency, and coverstock rewet. The coefficient of friction between two surfaces was determined with an incline plane. Coefficient of friction is directly related to the shear forces developed between two sliding surfaces. The shear forces of two types of bed sheets - a 100% cotton sheet and a 50% cotton, 50% polyester sheet - were first compared with the use of a painted steel surface as the control. The tests were done with both dry and wet sheets. The sheet was tightly wrapped around the incline plane, and a 2.73 kg painted steel weight measuring 23.5 cm x 10.2 cm x 1.9 cm was placed on the plane. The plane was slowly raised until the weight first moved, and the angle was measured with an inclinometer. The tangent of this angle is the characteristic coefficient of friction between the two surfaces. To compare the shear forces of the sheets with that of a prototypical underpad backing, the incline plane was covered with a polyolefin film (TuckablesTM) and tested with the standard metal surface. Last, the backing of each individual underpad was tested on each of the two sheets. The incline plane was wrapped in the bed sheeting and the weight wrapped with the underpad that was being studied. All samples were tested 10 times and the results recorded as the means and standard deviation of the data. The statistical significance of the data was determined by Student's test.

The total absorbent capacity is defined as the maximum amount of liquid held by the product, which includes liquid absorbed by all the materials and that in the void space.⁶ A comparative absorbent capacity test was used, and samples included all layers of the product intact, not individual components. Cellulose fibers have a high atmospheric moisture regain, that is, they contain approximately 8% H₂O when equilibrated with air. Thus each sample was initially heated in a Stabil-Therm oven (Blue M Electric Company, Blue Islands, Ill.) at 212° F for 4 hours to establish its bone-dry weight. Each dry sample was then immersed in distilled H₂O and saturated, drained, and weighed to give a value of the total absorbent capacity. The data were normalized by the mass of the total sample to provide an absorbency scale for comparison. The grams H₂O per gram sample is the conventional means of reporting absorbent efficiency, referred to as its absorbent capacity index.⁷

Rewet is an indication of the liquid forced back through the coverstock when a saturated underpad is placed under load or pressure. Several factors influence the extent of wet-back: the amount and type of nonwoven web used as the coverstock, the presence of SAP in the absorbent core, and the use of an intermediate layer between the cover and the core. Rewet of each underpad was compared by saturating an area with 20 ml distilled H₂O, after which a blotter paper was placed with pressure onto the coverstock to note the extent of rewet. Each underpad's rewet was graded on an arbitrary scale of 0 to 5, with 0 being no obvious rewet and 5 being excessive rewet or moisture return.

Clinicians judged the underpad performance, in part, by the security of attachment of the underpad to either the bed or chair. Clinicians positioned the respective underpads on either the bed or chair before the patient was positioned on the underpads. When the patient was removed from the underpad, the clinician determined if the underpad remained in place.

RESULTS

The two types of sheeting had a small but significant difference in their coefficients of friction with the standard metal surface (p < 0.05). The coefficient of friction of the 100% cotton sheet was 10.7% greater than that of the 50% cotton, 50% polyester sheet. However, wetting the sheets had a dramatic effect. The coefficients of friction of the wet sheets were increased almost twofold over those of the comparable fry sheets (Figure 1).

The prototypical polyolefin underpad backing, when tested with the standard metal surface, had a significantly lower coefficient of friction than all dry and wet sheets (p < 0.05) (Figure 2).

When examining the different underpad backings, the type of bed sheet had no consistent effect on the coefficient of friction. The coefficient of friction of TuckablesTM, Depends®, Attends®, Tranquility®, and Ni-Med® were significantly greater on the cotton sheet than on the blended sheet (p < 0.05). In contrast, the blended sheet had a higher coefficient of friction with Secure®, and PrivaTM than did the cotton sheet.

All the polyolefin backings demonstrated relatively low coefficients of friction with bedsheets. The polyethylene backing of TuckablesTM was the lowest. The other five disposable backing were remarkably similar, showing low coefficients of friction. PrivaTM was the only underpad that possessed a significantly high coefficient of friction with bed sheets.

The absorbent capacity results showed Tranquility® to be the most absorbent (Figure 3). The absorbency of PrivaTM was significantly lower than the other products, being more than four times less absorbent than the disposable underpads. When examining absorbency results it is important to note that the whole product weight factors into the total absorbent capacity of each underpad, and this computation must be done to achieve true representation of a products efficacy.

The rewet results showed Tranquility® to be the only underpad with no wet-back, with no fluid returning through the coverstock. The other rewet values were as follows: Attends®, 1; PrivaTM, 2; TuckablesTM, 3; Depends®, 3; Secure®, 4;Ni-Med®, 5. The carded coverstock web of Ni-Med® allowed the most wet-back. In contrast, Attends® coverstock, with its tapered capillaries as one-way valves, was very effective. Although exhibiting a low rewet, PrivaTM required a longer time for the fluid to sink into the pad. The liquid had a tendency to roll off the coverstock, but once in the pad it did not return.

The clinical performance of the underpad was judged by clinicians by the security of the attachment of the underpad to either the bed or chair. The TuckableTM underpad was the only underpad that remained securely positioned on the bed after patient removal. Its polyethylene flaps that tuck under each side of the mattress held the underpad securely in place. While the adhesive strips of the Tranquility® remained attached to the sheets, movement of the sheet would reposition the underpad. Its small size also covered a limited surface area of the bed, making the mattress susceptible to fluid drainage. In contrast, the adhesive strips of the Tranquility® underpads positioned them securely to the chair after patient repositioning. The TuckableTM underpad did not remain securely positioned on the chair, unless its polyethylene flaps were tucked beneath the chair cushion. It is important to emphasize that all other underpads could not be reliably positioned on either beds or chairs.

DISCUSSION

Many studies have examined the effects of underpads on the skin condition of incontinent adults or infants. However, only few studies involved some form of control or comparison group and included testing for statistical significance. Four studies conveyed sufficient information to determine that the investigators were comparing cloth products with products designed to absorb moisture and present a quick-drying interface with the skin.^8-11 In all four of these studies, investigators reported either a significant improvement in skin condition or a significantly lower occurrence of skin rashes for subjects who used products specifically designed to absorb moisture when compared with subjects who used products made of cloth.

It is important to note that the key feature evaluated was whether the product was specifically designed to absorb moisture to present a quick-drying surface to the skin. While these careful, scientific studies confirmed that moisture predisposes to dermatitis, our studies demonstrated that moisture also markedly enhanced the frictional forces of bedsheets, thereby predisposing to skin ulcerations. The coefficients of friction of either wet cotton or wet cotton/polyester sheets were two times greater than those of comparable dry sheets. Because these wet sheets have such high coefficients of friction, hospital personnel must replace wet sheets with dry sheets before patient transfer. The use of an underpad between the patient's skin and the bed linen is always a reliable technique to ensure dryness and minimize the frictional forces between the skin and the bed sheeting. The disposable underpads tested in this study had polyolefin backings that exhibited low coefficients of friction and were effective barriers to moisture transmission. In contrast, the reusable underpad displayed a coefficient of friction that was nearly 50% greater than those of the disposable products.

The Tranquility® underpad had three structural components that provided optimum product performance by enhancing absorbency and inhibiting rewet. It was the only underpad studied that possessed SAP, which provided an absorbent capacity far greater than other underpads. This SAP irreversibly held the moisture to prevent fluid wetback. Second, its thick intermediate tissue layer between the coverstock and core limited the moisture to the pad's interior, inhibiting its return to the outer surface. Finally, because its coverstock was spunbounded web rather than a carded one, its moisture containment capacity was further enhanced.

The adhesive-tape backing of the Tranquility® underpad is one additional favorable feature. Once the patient is positioned comfortably on the product, its adhesive strips can be secured to the chair, maintaining its position on the chair after patient repositioning. These unique features of the Tranquility® underpad provide a scientific basis for its selection for patients seated on chairs who are prone to developing pressure ulcers (Fig. 4 and 5).

Our clinical studies, however, demonstrate that the Tranquility® underpad is not a reliable underpad for use on beds. While its adhesive strips remain securely attached to the bed linen, repositioning of the bed linen caused by patient movement will move the underpad. In addition, the small size of the Tranquility® underpad covers only a limited bed surface area. In contrast, the Tuckable underpad can be securely positioned on the bed surface by tucking its polyolefin flaps under the mattress. Moreover, the Tuckable underpad exhibits excellent absorbent capacity, considerable resistance to rewet (Fig. 6 and 7).

It is important to emphasize that there are some patients who exhibit considerable fluid discharge from either diarrhea, urinary incontinence, or wound drainage. In these patients, commercially available underpads cannot contain the fluid drainage. In such circumstances, we apply the inexpensive Komko underpad on top of the recommended products. These inexpensive underpads can be changed frequently, removing wound drainage and debris.

CONCLUSION The purpose of this study is to identify a scientific basis for selecting underpads to reduce the incidence of pressure ulcers. The biomechanical performance of eight commercially available underpads and linens was examined, using three reproducible parameters, to include: coefficient of friction, absorbent capacity, and rewet. Because wetting either cotton or cotton/polyester bed sheets markedly increases their coefficients of friction, underpads should be used routinely to protect the skin against friction forces. One disposable underpad, Tranquility®, is ideally suited to protect the skin of the high risk patient seated in a chair. It has a polyolefin backing with a low coefficient of friction that serves as an effective barrier to moisture transmission. In addition, it is the only underpad studied that contains a super absorbent polymer that provides a far superior absorbent capacity and minimizes rewet. Wet-back is further inhibited by its thick intermediate tissue layer and its spunbonded polypropylene coverstock. Its adhesive slips on each corner of the pad attach it securely to the sides of a chair, but not to bed linen. In addition, its small size does not allow it to be easily used on beds. In contrast, the Tuckable™ drawsheet is ideally suited as a bed underpad. Extending from the two opposing sides of the underpad are polyethlyne flaps that tuck under each side of the mattress. Its absorbent capacity is exceeded only by the Tranquility® product. In addition, its rewet value is exceeded only by three commercially available products, designed for chairs rather then beds. On the basis of this extensive scientific evaluation, the Tranquilty® underpads are recommended for chairs, while TuckableTM underpads are preferred for beds.

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