The prosthetic sock is a traditional socket insert design for transtibial prostheses (Figure 2). Socks were originally used as the primary interface between the skin and the prosthetic socket and can provide volume management and cushioning. Prosthetic socks (alternatively known as stump socks) are now manufactured from wool and synthetic blends. Prosthetic socks come in various thicknesses typically referred to as the ply of the sock (eg, 1-ply, 3-ply, etc.). However, it should be noted that adding sock ply does not always increase sock thickness in an equal amount. For example, a 1-ply cotton has a measured thickness of ˜0.45 mm and a 5-ply cotton sock has a measured thickness of ˜1.48 mm,⁷ much less than five 1-ply socks stacked together (˜2.25 m). This is because the number of ply refers to the number of yarns that were woven together to create the sock and this is different than adding those same yarns on top of each other.

In some instances, the individual may need to increase or decrease sock thickness to accommodate residual limb volume fluctuations that occur throughout the day. Generally, if the patient experiences pain on the distal portion of their residuum, and/or their patella has sunken into the socket, it is an indication that they need to add a prosthetic socket to compensate for volume loss. Conversely, if they are having difficulty donning the prosthesis, and/or feel general tightness around the residuum and/or the femoral condyles, the patient should remove prosthetic socks. Prosthetic socks provide an accessible way to compensate for volume fluctuations, but there are strategies to maintain volume that do not require the use of socks. More recent research has suggested that an effective strategy to regain volume loss is to doff the prosthesis when the person will be sitting for 30 minutes.^8,9 Over longer periods of time (weeks to months), a prosthesis user may start using progressively thicker socks to achieve a comfortable fit as their residual limb continues to atrophy. The use of prosthetic socks to accommodate for residual limb volume loss will eventually compromise socket fit as residual limb volume loss is disproportional at the distal region relative to the proximal region of the limb where much of the underlying tissue is tibia and femur bone. A total volume loss of approximately 10% represents a general guideline for the point where socket replacement should be
considered versus continued addition of prosthetic socks.¹⁰ This represents a sock thickness of greater than 2.1 mm, or a sheath count of 10.⁷ A prosthesis may be designed to be used with thicker socks to provide extra cushioning and potentially compensate for swelling if they are undergoing dialysis treatments. However, the use of prosthetic socks for cushioning is an older practice that has been increasingly replaced by newer materials with improved cushioning and greater comfort.^11,12

Thermoformable foam inserts (Figure 3) generally offer increased cushioning compared with prosthetic socks. Foam inserts are composed of various closed-cell foams (such as pelite and kemblo) designed to increase cushioning and prosthetic control.¹³ The foam form replicates the shape of the residual limb on the inside and the prosthetic socket on the outside, thereby providing an intimate fit. An advantage of foam inserts is the ability to vary insert thickness to provide additional cushioning where needed, creating a more uniform outer shape. Socks can only facilitate global changes in limb volume, whereas foam inserts allow localized changes to accommodate load or relieve targeted areas of the limb as needed. In some cases, foam inserts can be designed to provide anatomic suspension (discussed later in the chapter). In addition, foam inserts do not typically provide as much insulation as newer socket interface materials (discussed in the next section) and thus may be cooler for the patient. Also, foam inserts may be an alternative for prosthetic users with sensitive skin who could not tolerate newer socket interface materials without irritation. Finally, the low initial cost of a foam insert may seem advantageous, but damage or excessive wear may require the need to manufacture an entirely new socket as foam inserts are difficult to retrofit.

The prosthetic gel liner (Figure 4) represented a technologic advancement beginning in the early 1980s.¹¹ For prosthetic liners, the term gel is synonymous with silicone, thermoplastic elastomer, and polyurethane. Gel liners have higher coefficients of friction compared with foam inserts, allowing them to protect the limb differently.¹³ Specifically, the compliance of the gel liner allows it to assume the shape of the residual limb, creating a sealed layer intended to minimize movement between the gel and the skin. The gel liner now acts as an additional dermal layer, protecting the biological skin from breakdown, irritation, and abrasion.¹⁴ This additional protection also comes with the cost of thermally insulating the residuum.¹⁵

Sanders et al¹⁶ reported that silicone, thermoplastic elastomer, and urethane all have advantages and disadvantages. Thermoplastic elastomer tends to be the softest gel, behaving most similarly to soft tissue, whereas silicone and urethane tend to be stiffer under compression. When dry, none of the materials have a coefficient of friction low enough to induce slipping against the skin. Shear stiffness is similar to compression stiffness; thermoplastic elastomer is the softest, silicone is the stiffest, and urethane is similar to silicone. Using these results, the authors suggested that thermoplastic elastomer liners may be best suited for patients with bony anatomy and limited soft tissue in the residual limb. Patients with abundant soft tissue may benefit more from the stability provided by the increased stiffness of silicone or urethane. However, material properties testing does not always translate perfectly to human outcomes, and recommendations provided based on material properties should be cautiously considered.¹²

Gel liners are available in different thicknesses and profiles (such as uniform thickness or tapered from thicker to thinner more proximally). Gel liners are manufactured in numerous sizes to fit various common residual limb sizes and shapes. Liners that are custom-made based on either a scan or mold of the residual limb are also possible for those with uniquely shaped residual limbs or requiring additional cushioning in specific areas. These liners can have a fabric covering, which eases sliding of the residual limb into a socket and provides controlled stretching and stability to the entire liner. The fabric can be manufactured to reduce longitudinal stretch while allowing circumferential stretch. The liners can also have a rigid, threaded nut attached
to the distal end to allow pin-locking suspension.

A disadvantage of gel liners is thermal insulation, which has the highest insulative properties of any of the materials between the skin and the outside air.^15,17 In the presence of increased limb temperature, patients may experience excessive perspiration, dermatitis, or skin maceration. Liners made from silicone gel have the greatest measured thermal conductivity (TC)¹⁵ and may be best suited for transferring heat away from the residual limb, yet it remains unclear if the difference in TC among liners is enough to elicit a noticeable change with the user.¹⁸ To address this problem, researchers are now focusing on the TC of materials used for liners and sockets to help draw heat away from the residual limb. Phase-changing materials capable of absorbing and storing body heat have been tested with transtibial prosthesis users while exercising and demonstrated only a 0.2°C improvement with a large amount of variability (˜2°C) compared with a placebo liner.¹⁹ A change of 1° to 2°C may be necessary for the intervention to be clinically relevant,¹⁸ meaning that the change of only 0.2°C in the group mean with large variability indicates a clinically relevant benefit to some, but not all, individuals.¹⁹ A larger clinical trial is underway that may enable recommendations as to the efficacy of using phase-change materials for prosthetic liners.²⁰ Another creative method to minimize skin issues related to gel liners is to perforate the liner with small holes.²¹ The evidence to support this use is positive yet limited to clinical case studies and a small trial using patient-reported outcome measures.^21,22 There are several active research projects currently ongoing in this area that may yield broader conclusions. Until then, caution should be used when contemplating prescription of a perforated liner. For example, it is not clear how well the liners can be cleaned to remove sweat and microbes in the perforations, the optimal size and number of perforations per square inch is not established (too large a perforation may expose the skin directly to a negative-pressure environment), and the effect on objective physiological measures of limb health is not available. Ultimately, individuals may reject gel liners because of increased limb temperature and skin problems associated with gel liners. However, the advantages of cushioning, the ability to conform to bony prominences, and the facilitation of suspension within some systems typically outweigh the disadvantages of gel liners.

A waist belt and fork strap suspension (Figure 5) involves a belt worn around the waist with an elastic strap coming down the anterior portion of the thigh and connected to an inverted Y-strap via a buckle. The inverted Y-strap is then connected to either side of the socket and holds the prosthesis up when the foot is off the ground. The advantages of this suspension method are its simplicity and low production cost. However, the waist belt can be uncomfortable, and because the elastic strap crosses the hip and knee joints, it may not always maintain adequate tension in different limb configurations or through different motions. Therefore, its use has largely been confined to postoperative and early-stage prosthetic fittings.