Because only 2% of all amputations are at the hip disarticulation level, 1 the average prosthetist may not be able develop a consistent prosthetic fitting protocol with regard to evaluation, impression taking, modification, component selection, and alignment. As with other proximal amputation levels, hip disarticulation prostheses have a higher rejection rate because of weight, increased energy requirements, and proximal interface enclosure. This has been addressed with the standard use of endoskeletal components, softer interface materials, and more dynamic designs. The energy requirements for the hip disarticulation amputee have been estimated to be as much as 200% greater than those for normal human ambulation.2
Successful fitting of the hip disarticulation prosthesis hinges on the evaluation of balance, lower abdominal tissue condition, and pelvic lordosis. Balance is needed to successfully put on and ambulate with the limb with minimal assistive aids. If this is not present, the client may not be considered a good prosthetic candidate. 2 A properly fitting interface is also critical because it must provide geometries for comfortable axial support, ambulation, and suspension while keeping size to a minimum. Abdominal condition must be evaluated for volume reduction and shaping to create a volumetrically tight and accurate anteroposterior (AP) fit between the sacrum and lower abdomen. 3 Pelvic lordosis must also be evaluated because this is the main biomechanic work source during gait for maintaining knee stability and initiating knee flexion. To be successful in gait, the amputee should be able to demonstrate active pelvic lordosis using the muscles of the lower back and abdomen. The most successful amputees are able to maintain a low weight so that the maximum lordotic range of motion may be captured. The presence of any redundant or fleshy tissue should be noted because it must be contained and shaped to create reaction surfaces for proper prosthetic function ( Figure 1 ). Tight mediolateral measurements are also necessary to preserve ML stability during gait. ML measurements should be recorded over the iliac crests and between the trochanter and the iliac crests3 ( Figure 2 ). The bony anatomy should also be evaluated with respect to ischial load bearing and the presence of a femoral head. Bony prominences of the pubis and iliac crests should also be noted for relief within the interface. Any gluteal tissue available should be utilized for axial loading.
There are two main methods of taking impressions for the hip disarticulation interface: forming blocks 3 and total suspension casting. 4 When using forming blocks, the iliac crests, costal margin, pubis, femoral head or joint, and ischium are marked. Plaster is wrapped over the lower limb from the perineum to 2 inches proximal to the iliac crests, and reinforcing splints are added to the ischial area. The iliac crests are modified using plaster rope, surgical tubing, or radiator hose to forcefully compress the tissue circumferentially and downward. In one method, the plaster rope is squeezed by being twisted around a dowel or hammer handle to attain good loading. The rope should be flattened in the sacral area to avoid localized pressure over the spinous processes. 4 The patient is then seated on a flat surface and 45° forming blocks are placed anteriorly and posteriorly to create the AP reaction surfaces necessary for ambulation. The posterior block is placed to load the gluteus and provide some relief for the ischium. The anterior block is placed to help form the geometry for the hip attachment plate with 5° of external rotation. 3 The posterior forming block provides the counterpressure needed to maintain contact with the anterior block ( Figure 3 ). Forming blocks work well for thinner clients with good muscle tone. For those who have soft abdominal tissue, the blocks may tend to distort the mold by expanding the ML.
Total suspension casting is another method used to contain the soft tissue and achieve a better volumetric loading within the interface. With this approach, the casting garment is suspended from the ceiling using a mechanical winch device. When the support height has been adjusted to the point where the iliac crests are even, the same landmarks are indicated and the plaster splints are added. The iliac crests are modified with the same roping technique, and the ischium is cupped in the palm of the hand ( Figure 4 ). The disadvantage of this technique is that the anterior surface is not clearly defined. Some prosthetists use both casting methods by containing the tissue in suspension and then using forming blocks to place the hip joint.
Often the initial cast that is taken does not reduce the volume adequately to create a tight interface. For this reason, the client should recline in the cast and the anterior panel should be cut. The anterior section is squeezed together, lapping the anterior panels. The location is marked and the cast is removed. Before the mold is filled, the cast is again squeezed to this point and secured to eliminate extra interface volume and maintain circumferential tension. 5 Many hip disarticulation interface shapes will fit the client comfortably, but they may not provide adequate suspension or lordotic capture. Substantial modification is necessary to achieve comfort and function. After the cast is removed, the landmarks are remarked. Plaster is removed from the anterior portion of the mold, avoiding the pubis. When the abdomen is more pendulous, flattening is adequate; thinner individuals may have a slight concavity to load the abdomen. The posterior sacral area is also flattened along the lower back to maintain tight AP pressure. To ensure a tight fit, one half inch of material should be removed between the trochanter and iliac crest to avoid the anterior superior iliac spine (ASIS) and the iliac crests from the measured ML. The superior iliac crest modifications made with the plaster rope are then deepened by one quarter to one half inch even to the crests. 3 Some loading of the gluteus is also advisable to avoid too much ischial pressure. Some prosthetists have suggested that medial ischial pressure is advantageous but achieves little biomechanically without a distal reaction point. Cupping of the ischium seems to be the most comfortable geometry. Relief may have to be added to the pubis, iliac crests, and ASIS. The socket may be made with a side-opening or anterior-opening configuration, although the latter predominates because it is easier to put on and remove. The trimlines are approximately 2 inches proximal to the iliac crests and through the perineum. A small suspension band is cut to provide relief on the contralateral ASIS.
Endoskeletal components have become standard for hip disarticulation prostheses because they are lightweight and have adjustable stability. However, special considerations for component selection should be made to optimize function. Dynamic response feet are commonly chosen for their lightweight design. Because of the slowed gait of the hip disarticulation patient, only in the more active patient can true dynamic responsiveness be observed. An inexpensive solid ankle cushioned heel foot with a soft heel cushion can also be used to increase knee stability at heel strike by shifting the reaction line anteriorly. Although single-axis and multiaxial feet may be used to increase stability, they add substantial weight to the distal end of the limb.
Single-axis knees are the predominant choice for hip disarticulation because they are lightweight and friction control is adequate for clients with a single cadence speed. Stance control knees should not be chosen because there is a downward thrust during active pelvis lordosis anteriorly to initiate knee flexion. This engages the stance control brake and impedes normal knee flexion. Because stability is provided with alignment of the hip and knee, added stability features are not necessary unless the client is more active. These clients may require the stability feature for uneven terrain; in this case, the stability feature is adjusted to minimally hinder knee flexion. Polycentric knees are often chosen for active users who can tolerate the increased weight and desire the variable stability and shin-shortening features. Shin shortening is attractive to the hip disarticulation prosthetic wearer because there is no hip flexion and little knee flexion at midswing to help shorten the anatomic leg.
The hip disarticulation prosthesis often feels "long" and must be 6 to 12 mm shorter than the anatomic leg. Hydraulic control is not often used because the hip disarticulation amputee does not generate the gait speeds needed to justify the increased weight, but it may help initiate hip flexion during swing. Originally, the hip joint was mounted laterally, near the anatomic hip joint center. The joint was locked for standing and walking and unlocked for sitting. 6 In 1954, Ian McLaurin introduced the Canadian hip disarticulation configuration, which is standard today. It uses an anteriorly mounted hip joint that is held stable during stance using biomechanical stability of a posteriorly placed reaction line. 2,5 Currently there are a variety of hip joints that differ with regard to attachment and function. The older, endoskeletal designs have double anterior and posterior free motion hinges that offer simple hip extension (with rubber bands) or a manual hip lock. These are relatively lightweight and offer transverse rotation and sagittal plane adjustment but have distal components that can make sitting a problem. Newer, modular systems are more integrated and have attachments that make sitting easier. These attachments are usually mounted anterior to a flat or dished plate and have an internal adjustable spring hip extension assist. A slight amount of abduction/adduction adjustment is available along with and flexion/extension adjustment. This common design is also offered in a pediatric size. 3
Different pylons and attachment components are also available. Twenty-two-mm pediatric sizes are available along with the more common 30-mm pylon, and 34-mm pylons are available for heavier loads. It is important to include an angled tube clamp above the knee to receive the upper femoral pylon, because this usually has a considerable anterior angle. Carbon composite strut systems have been introduced that offer more dynamic motion and shock attenuation during stance. The strut flexes when loaded and releases its force at the beginning of swing to increase hip and knee angular acceleration, which can help speed the step. 7 The disadvantage of this system is that the spring is lessened as the strut is shortened, and too soft a spring may dampen the initial anterior lordotic movement. In addition, special attachments for the hip and knee are required to accommodate this system.
The hip disarticulation interface must serve three purposes: ML support, comfortable suspension over the iliac crests, and surfaces for lordotic action. Laminated interfaces can be fabricated with varying degrees of stiffness of the plastic for proximal flexibility and rigidity at the attachments. Also, it is much easier to achieve an accurate interior surface because the attachment plate is laminated within the socket shell in a one-step process. New thermoplastic materials have been developed that greatly increase socket comfort, especially over the iliac crests. The main difficulty with thermoforming comes in the fabrication of the inner attachment. Normally, a soft anterior pad is placed on the mold before the flexible inner liner is formed. A build-up is then made for the attachment, and the frame is laminated or made of stiffer thermoplastic. A plastic or foam cover for the hip joint can then be made and glued to conceal the hip joint. Kempfer8 suggests using the anterior panel cut from a polyethylene check socket. Although thermoforming offers a variety of materials and construction ease, it is thicker and the hip plate attachment is not as integrated as it is in lamination.
Stability of the hip disarticulation prosthesis relies primarily on the alignment of the prosthesis. In the bench alignment, a line is projected from the hip center through the knee center. This line should fall 25 to 50 mm behind the heel of the shoe ( Figure 5 ). In the frontal plane, the hip joint should be placed 10 mm lateral to the frontal one-quarter mark with 5° to 10° of external rotation to match anatomic lower limb rotation 4 ( Figure 6 ). The hip joint placement is established sagittally with the forming blocks. The knee center and midfoot are placed in relationship to a plum line from the bisection of the interface based on their design. For example, a single-axis knee is placed 15 mm posterior and a dynamic response midfoot is placed 20 mm anterior to the bisection. 4 The length of the prosthesis is 12 mm shorter than the sound side, so that the foot can clear the floor during midswing. During dynamic alignment, the hip joint should be adjusted so that it is not engaged before midstance during forward pelvic lordosis. 4 As a note, the cosmetic cover may have to be preformed to lessen the amount of tension, which could cause premature hip flexion. 4
GERALD STARK, BSME, CP, FAAOP, is Director of Product Development and Education, Fillauer, Inc., Chattanooga, TN. A selected presentation from the American Academy of Orthotists and Prosthetists Annual Meeting and Scientific Symposium held in Dallas, Texas, March 7 through 10, 2001. Gerald Stark, BSME, CP, FAAOP, Fillauer, Inc., 2710 Amnicola Highway, Chattanooga, TN 37407. Phone: (800) 251-6398, ext. 256; Fax: (423) 624-1402; E-mail: firstname.lastname@example.org