By the end of World War II, the large number of veterans who suffered limb loss during combat inspired prosthetists to experiment with new materials and techniques to improve prosthetic comfort and function. In 1959, a symposium was held at the University of California Biomechanics Laboratory to promote the development of transtibial socket fitting. The result was the patellar tendon-bearing (PTB) socket design. This design has been used successfully over the past five decades to strategically load the limb in areas that are more pressure tolerant, namely the patellar tendon and medial tibial flare, and relieve the tissue over bony prominences like the tibial crest and head of the fibula. In most cases, this eliminated the need for proximal weight bearing.9

The main goal of the PTB socket design was to increase the surface area on the residuum that is available for weight bearing so as to eliminate the need for the knee joints and thigh corset. The PTB socket was described as “total contact,” which meant that there were supposed to be no voids or air pockets between the limb and socket. This design allowed weight bearing to occur in any area that capable of supporting a load. The term patellar tendon-bearing originates from the use of a patellar “bar” that is built into the socket at the level of the center of the patellar ligament, midway between the patella and the tibial tubercle (Figure 23-2). The socket is aligned in approximately 5 degrees of knee flexion, which allows the bar to act as a weight-bearing surface within the socket. The proximal trim line of the posterior wall is located just proximal to the patellar bar to stabilize the limb in the anteroposterior direction and to prevent the limb from sliding to far down into the socket. The posterior trim line should be lower on the medial side to accommodate the insertion of the medial hamstring during knee flexion.

The total surface-bearing (TSB) socket strives to further distribute the weight-bearing load over the entire surface of the limb, even in areas that had been traditionally considered to be pressure intolerant. Strategic compression of soft tissue and relief for bony prominences are the tools used to direct more force into areas of the limb that can tolerate it, and less force into areas that are prone to skin breakdown. The intent in designing a TSB socket is to distribute uniform pressure over the entire surface of the limb.10 It is expected however, that during a typical step while walking, the pressure in any given location will change from a negative pressure during swing phase to high pressure in stance that, if sustained, would cause tissue damage. Because the forces on the limb change quite dramatically throughout the gait cycle, that dynamic pattern must be anticipated so as to use those forces to design the reliefs for pressure-intolerant areas; larger forces means more tissue compression, which require larger reliefs. The other factor to consider is the density and structure of the tissues comprising the limb. Tissue properties vary widely, and there are temporal effects too; muscle tissue, for example, behaves one way while relaxed and very differently while contracting. Once they are accommodated, the relative locations of these tissues within the socket must be preserved; this not only provides a good environment for the tissues, but also allows accurate control of the prosthesis.

To fully accommodate the dynamic tissue loading that occurs in a prosthetic socket, the prosthetist must consider both the shear and the normal forces on the limb. Shear forces run parallel to the limb surface and are best mitigated through the use a socket interface. Interface materials, such as socks, sheaths, flexible liners, and gel liners, offer a continuum of shear reduction on the skin surface. The best materials to minimize shear are those found in gel liners. Normal forces are those that are applied perpendicular to the surface of the limb. The socket walls should be contoured according to the type of tissue in the area and the anticipated loading patterns. Because there is no way to reduce the force on the limb without restricting the inidividual’s activities, the best way to reduce pressure is to distribute the forces over as broad a surface as possible. The actual forces on the limb are a combination of shear and normal forces that occur together in various proportions.

Ambulation is a dynamic event in which the forces on the limb are continually changing; for this reason the prosthetic socket must be designed to function under a variety of loading patterns. The socket must be designed and fitted under physiologic conditions that match that of the intended use. Soft-tissue compression will vary with load; the socket contours must reflect the anticipated load so as to prevent excessive loading on bony prominences. Throughout the gait cycle the forces and moments on the socket and limb change continuously. There is a flexion moment during loading response, a varus moment throughout midstance, an extension moment in terminal stance, and a flexion moment again in preswing (Figure 23-3). The forces on the limb range from a compressive force of 1.2 times body weight in stance, to a distractive force slightly higher than the weight of the prosthesis in swing phase.¹¹ A well-fitting prosthesis must provide tolerable pressure distribution in all of those varied loading conditions. Soft tissue, muscle tissue, and bone contours must each be accounted for in a specific way to achieve a good fit. Soft tissue can tolerate moderate compression so the prosthetist will precompress that tissue in the socket. Muscles can tolerate mild compression but should be encouraged to contract with each step so less precompression should be applied. The shape of muscle tissue changes when it contracts. Flexible materials can be used over muscle bellies that allow for the geometric variability. Finally, bony prominences must be given extra volume within the socket so that when the tissue around them compresses during loading, the pressure will not exceed the tolerable limit.

The load-bearing capabilities of the limb can also be affected by the surgical technique used for the amputation. The Ertl procedure, named after Dr. Janos Ertl Sr., is a surgical procedure that involves creating a bone bridge between the distal end of the tibia and fibula, as shown in Figure 23-4 (see Chapter 19 for more detail). The goal of this procedure is to create a tougher, more force-tolerant limb. One problem that this technique aims to solve is nerve impingement. Transtibial amputees are prone to nerve compression between the long bones of the lower leg.¹² Forces within the socket push the tibia and fibula together and compress anything in between. If the tibial nerve is trapped between the bones, pain can result. By fusing the bones together at the distal end, the relative motion between them is minimized thereby protecting the soft tissues that are located between them. Many individuals who have had this type of surgical procedure can bear weight directly on the distal end of their limb. This end-bearing capability allows the prosthetist to distribute the person’s weight differently and potentially provide a prosthesis that does not extend as far proximally; this can increase comfort over standard weight-bearing areas and increase the range of knee flexion available to the individual. However, the increased surgical time and subsequent increase in infection risk are often cited as reasons to forgo the Ertl procedure surgical technique.¹³

Early prostheses were made from hard materials like wood, which did not offer much cushioning. Persons with amputation used layers of cotton or wool socks to provide a soft interface between their limb and the hard socket. There are several advantages to this system: the socket is relatively thin, so the it is easily concealed under clothing; a clean sock can be used each day, or changed times throughout the day if needed; the number and ply of socks can be adjusted to accommodate for limb volume fluctuation during the day; and the socket itself is very durable. Because there are no compressible surfaces, the fit is reliable; it will not become “packed down” in high pressure areas. It is nonporous, easy to clean, and relatively maintenance free. It also does a fair job of eliminating shear as the coefficient of friction between the socks and socket is relatively low compared to that between the socks and skin.14 This type of socket is most challenging to fit and is not recommended for mature limbs that have lost much of their soft-tissue protection over bony prominences. It is also more difficult to adjust than other socket styles.

Prosthetic socks can be made from various combinations of cotton, nylon, wool, Lycra, polyester, and spandex. Some manufacturers have recently started using silver fibers in their socks as well to enhance the antimicrobial properties of their socks and sheaths (Figure 23-5). The prosthetic sock provides shock absorption, decreases the shear forces on the limb, and wicks away moisture. To further decrease friction, a nylon sheath is often recommended as the initial layer and the thicker socks would be donned over the sheath. The sock also provides the individual with a method to control socket fit; as the residual limb matures and shrinks, additional sock ply may be required to restore the fit and comfort of the socket. Prosthetic socks come in various ply thicknesses for convenience to the prosthetic wearer; for example, a person can wear one five-ply sock rather than having to don five individual single-ply socks. New prosthetic wearers are typically provided an assortment of one-, three-, and five-ply socks from which they can select. The socks can be layers one on top of the other to achieve the appropriate number of ply.

Closed cell foam, used because they do not absorb moisture, can be molded over a model of the limb to create a soft insert. This insert lines the entire socket and terminates just proximal to the socket trim lines (Figure 23-6). For increased protection, a distal end-pad, which is an extra layer of soft material at the bottom of the insert, can be used to cushion the distal end of the tibia. Soft inserts provide an extra layer of cushioning that is needed for more mature limbs that lack adequate soft-tissue thickness. They can be worn over a nylon sheath, which is a very thin nylon stocking similar to women’s stocking, or over any number of sock ply. Wearing the insert directly over the skin without a sock may lead to excessive shear and skin breakdown because of the relative motion between the limb and insert. Single durometer inserts provide a uniform compression profile whereas multidurometer inserts, made from layers of different materials with varied properties, can take advantage of the force-altering characteristics of each layer. For example, a material that has high plastic deformation might offer good shock absorption but would wear out very quickly if used alone. Mating that material with one that has low compression resistance would prevent some of the plastic deformation and extend the useful life of the insert. Soft inserts that can deform during the donning process can be used to accommodate anatomic irregularities that would not be able to slide directly into a rigid socket. For example, an insert for a limb with a bulbous distal end can be made thicker in the narrow area above the bulge so that the diameter of the finished socket would not impede donning. Another example is the wedge needed for supracondylar suspension; this wedge can be integrated as part of the soft insert to facilitate ease of donning.

When the limb is amputated at or below the ankle, the resulting long residual limb present an interesting challenge to the prosthetist. The proximal trim lines of the prosthesis can be lowered to a more distal position on the limb because there is a long lever arm for prosthetic control during ambulation. However the distal residual limbs is larger in diameter than they are in the more proximally because the malleoli are still present. The prosthetist can accommodate for a larger distal size by creating a removable wall in the socket that is replaced after the prosthesis is donned, by using a specially designed soft liner, or by creating an expandable wall socket. The expandable wall socket is made from an elasticized material that stretches enough for the individual to push his or her limb through in weight bearing, and tightens up over the malleoli to provide suspension. This socket is too flexible to attach a foot to, so a rigid frame is made over the flexible socket with a small space between them in which the expansion can occur. This is a self-suspending socket that can be very comfortable for the person. It is difficult to fabricate this style socket, and it is even more difficult to make adjustments to the fit once it is fabricated. More information on these designs can be found in Chapter 22.

The term gel liner is loosely used in the field to describe a liner that is made from a material that exhibits gel-like properties. There are three basic varieties of these liners: (a) silicone elastomers, which are highly cross-linked at the molecular level; (b) silicone gels that have a relatively low amount of crosslinking; and (c) urethanes. The properties of these materials vary and are relevant to the prosthetist and person with amputation because they directly affect the forces that are transmitted through them to the residual limb. Of particular interest are certain properties of gel liners including: coefficient of friction, compressive stiffness, and shear stiffness. Silicone gels have the lowest compressive and shear stiffness values; this makes them useful in reducing compressive loading and eliminating shear forces on the limb. Lower shear stiffness would be beneficial for a bony limb, but might compromise stability by creating excessive motion on a limb that has more biological soft tissue. Silicone elastomers present the highest compressive stiffness values, so they are best suited to supporting loading without deformation. Elastomers would be beneficial for use on a limb that has a high proportion of soft tissue. Urethanes show the highest coefficient of friction with skin, a property that is useful for preventing localized skin tension and shear.15 Understanding these properties, allows the prosthetist to choose a material that is complementary to the socket design and effectively leverages the force transmission properties of the material against the soft-tissue characteristics of the limb.

Gel liners are a key component of TSB sockets. Gel liners are designed to be worn directly on the skin or over a thin liner referred to as “liner liners.” Liner liners are thin nylon sheaths with silver fibers that are meant to be worn between the skin and gel liner to prevent skin irritation caused by the warm moist environment of the gel. Although great effort is made to eliminate relative motion between the limb and socket, a small amount of motion is unavoidable. Gel liners have a high-friction inner surface, where it is in contact with the limb, and a low friction outer surface where it meets the socket. This encourages whatever small amount of motion is present to occur on the outer surface of the liner and minimizes motion at the liner–skin interface. The colloidal nature of the gel absorbs the shear that is not dissipated by the liner–socket interface so that only a small percentage reaches the skin (Figure 23-7).

A waist belt connected by an elastic strap to the thigh corset was used to suspend early transtibial sockets. Waist belts encircle the pelvis between the iliac crests and the greater trochanters. These adjustable belts have buckles on the anterior aspect that mated with an inverted Y-strap that is attached to the socket; this allows them to be donned separately and then joined together. Because this system crosses the hip and knee joints, flexion and extension of these joints must be accommodated by an elastic component. Further accommodation of knee flexion is accomplished by the inverted Y-strap (Figure 23-8). The Y-strap is fitted over the patella so that the two arms of the Y move posteriorly during knee flexion to reduce elongation of the elastic strap. The person can adjust tension in the strap based upon individual comfort. Pistoning in the socket is controllable with enough tension in the elastic. Tension in the strap decreases with hip flexion so that the strap has slack while the person is seated. Hip extension produces tension in the strap and will aid in limb advancement as it assists hip flexion in preswing.

The joints and corset feature (first discussed in the section on Socket Design) provides suspension as well as a weight-bearing element if the thigh corset is properly fitted over the femoral condyles. A skillfully molded corset can gain purchase over the smaller circumference of the thigh just proximal to the knee joint. The stiff leather corset is fabricated with either straps or laces that can be tightened as the wearer dons the prosthesis. This permits the limb to pass through the corset and then be held securely in position once the corset is tightened. The knee joints, which are typically made from steel, provide a secure connection to the socket. When the condyles are prominent, this can serve as the primary means of suspension and a waist belt is not needed. As the prosthetic knee joints are positioned slightly posterior to the anatomical knee joint center, tension in the cuff to decreases over the condyles as knee flexes, thereby enhancing sitting comfort (Figure 23-9). The joints and corset system can also include a posterior check strap that limits full knee extension. This can be used to eliminate the terminal impact at the end of swing phase, which can be audible, and to prevent excessive wear on the prosthetic knee joints. The thigh cuff allows for full functional range of knee flexion but will cause binding in the popliteal fossa when the knee is flexed beyond approximately 110 degrees. Joints and corset may be the suspension of choice for persons with ligamentous instability of the knee.

A cuff strap is a flexible leather cuff that attaches to the medial and lateral walls of the socket at the same point that orthotic knee joints would attach, that is, just posterior and proximal to anatomic knee center (Figure 23-10). The cuff has an adjustable strap that completely encircles the thigh just proximal to the patella. After the person dons the socket, the cuff is secured in place so that prosthesis will hang from the cuff while standing and walking. The anatomic structures that provide the suspension are the patella and the femoral condyles. To create a strong hold, the medial and lateral walls of the socket need to be lower than the standard height; because this reduces mediolateral stability, cuff strap suspension it is not a good choice for short residual limbs. An elastic component may be added to the strap over the patella to increase sitting comfort. This system is simple, quick to fabricate, and provides a secure suspension for the prosthesis while accommodating an unencumbered angle of knee flexion. The cuff does not provide any weight-bearing or mediolateral stability. Cuff strap suspension may be problematic for Persons with much muscle or adipose tissue arounp the lower thigh

This type of socket can be difficult to don because the width of the proximal opening is smaller than the width of the condyles. This problem can be addresed in two ways: either by making the medial wall detachable, or by including the supracondylar wedge in a soft insert. The first method uses a steel bar that is formed into the prosthesis. The entire medial wall of the prosthesis, along with the steel bar, can be removed for donning. Once the limb is in the socket, the bar slides back into a channel in the distal portion of the socket and locks into position with a ball detent (Figure 23-11). The second method uses a flexible liner that has a wedge built into it proximal to medial condyle. The rigid socket is fabricated over the liner such that the medial–lateral dimension of the proximal end of the socket is equal to the widest dimension of the knee. This allows donning of the flexible liner first, then with slight compression of the liner, the limb and liner together slide into the socket and are locked in pace through pressure and friction (Figure 23-12).