Ischial Containment Socket

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Ischial Containment Socket

The term “ischial containment” is rather self-descriptive. It describes several similar concepts in socket design in which the ischium (and in some cases the ischial ramus) are enclosed inside the socket.

Objectives that would ideally be achieved in the ischial containment socket:

  1. Maintenance of normal femoral adduction and narrow-based gait during ambulation.
  2. Enclosure of the ischial tuberosity and ramus, to varying extents, in the socket medially and posteriorly so that forces involved in maintenance of mediolateral stability are borne by the bones of the pelvis medially and not just by the soft tissues distal to the pelvis, that is to say, creation of a “bony lock.”
  3. Maximal effort to distribute forces along the shaft of the femur.
  4. A decreased emphasis on a narrow AP diameter between the adductor longus-Scarpa’s triangle and ischium for the maintenance of ischial-gluteal weight bearing.
  5. Total contact.
  6. Utilization of suction socket suspension whenever possible.

The physical and functional characteristics of this socket will be described within the perspectives of comfortable weight bearing, stance-phase stability, and normal swing phase.

Weight bearing in the ischial containment socket is focused primarily through the medial aspect of the ischium and the ischial ramus. The socket encompasses both the ischial tuberosity and the ramus; the specific contour depends on the musculature, soft tissue, and skeletal structure of the amputee. As opposed to the quadrilateral socket, in which the proximal contours are affected primarily by muscular variation, proximal contours of the ischial containment socket are affected by differences in pelvic skeletal anatomy.

Of particular importance are the variations in the position of the ischium with respect to the trochanter; in females, the is-chia are positioned more laterally, or closer to the trochanter, to allow for childbearing. The posterior brim of the socket is proximal to and tightly posterior to the ischium. Countersupport, intended to keep the ischium and ramus solidly against the medio-posterior aspect of the socket, is produced in three ways. First, the “skeletal mediolateral” dimension, the distance between the medial aspect of the ischium and the inferolateral edge of the trochanter, must be carefully designed into the socket. Second, countersupport occurs through the “distal mediolateral” dimension, a soft-tissue measurement that reflects the diameter of the residual limb 1 to 2 in. distal to the skeletal mediolateral dimension.

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The third form of counterpressure, most important in females because of their pelvic anatomy, is anterolateral counterpressure from the trochanter anteriorly to the tensor fasciae latae. Additional weight-bearing support is thought to be provided by the gluteal musculature and the lateral aspect of the femur distal to the trochanter, as well as from pressures distributed as evenly as possible over the entire surface of the residual limb. It should be noted that significantly more residual limb surface and volume is contained within the ischial containment socket as compared with the quadrilateral socket. Therefore, identical residual limbs have greater force distribution and hence lower pressures with an ischial containment design.

It has been hypothesized that the quadrilateral socket is displaced laterally during midstance and thus results in a shearing force on the perineal tissues. Secondarily, femoral abduction may occur and decrease the effectiveness of the gluteus medius. The solution provided by the ischial containment socket is to extend the medial brim of the socket upward until pressure is brought to bear against the ramus. The resulting “bony lock” between the ischium, trochanter, and laterodistal aspect of the femur provides a much more stable mechanism for acceptance of perineal biomechanical forces. Two clinical results are increased comfort in the groin and better control of the pelvis and trunk.

Stance stability may be enhanced by extensive contouring posterior to the femoral shaft; this allows more effective transmission of the movements of the femur to the prosthesis.

Swing-phase suspension is critical and is usually achieved by suction. As with the quadrilateral socket, proper contours allow for smooth swing-phase tracking. Rotational control is provided by the proxiomedial brim and its bony lock against the ischium, the shape and channels of the anterior wall, and the post-trochanteric contour of the lateral wall seen in transverse view. Socket rotation control for very fleshy residual limbs with poor muscle tone is best achieved with an ischial containment socket.