ISCHIAL-RAMAL SOCKET FUNDAMENTALS

Although many prosthetists recognize the biomechanical benefits of using "Narrow M-L" interface designs, many practitioners have difficulty achieving this fit. This stems from popularity of the quadrilateral interface, that had the benefit of well-documented teaching methods and standard fitting brims. Ischial containment methods, somewhat less defined, employ unique hand casting techniques and modifications. These methods often represent a departure from the widely known quad interface methods and present the prosthetist with new challenges and fitting issues.

This brief modification guide is a summary of the work developed by Jack Uellendahl, CPO, Mark Edwards, CP, and Elaine Uellendahl, CP, at the Rehabilitation Institute of Chicago and Northwestern University's Prosthetic-Orthotic Center to teach prosthetic students. The method integrates a variety of modifications from the aforementioned techniques. These principles have also been adopted by NAPOE to help educate new students because of the special emphasis placed on simplicity and repeatability. Northwestern University also offers this course for certified practitioners as well.

Charles Radcliffe, PhD, introduced the quadrilateral socket (quad socket) that emphasized gluteal weight loading, ischial support, and anterior rotational control. His work first established many of the standards in transfemoral modification and alignment. Ivan Long, CP, noticed that it was difficult to maintain the femur in the proper amount of adduction since the quad socket relied upon compression of the adductor muscles. He found that medial ischial pressure combined with femoral loading helped limit the amount of lateral shift and maintained femoral adduction better. John Sabolich, CPO, advocated more aggressive ischial-ramal containment methods in combination with geometry that utilized the soft tissue anatomy for an even greater amount of control. These dramatic modifications were enabled by his use of highly flexible interface materials and construction methods. Also notable in socket design, Robin Redhead suggested that axial support could be achieved by supporting the volume of the limb hydrostatically as a "bag of fluid." This method of support has been more predominantly accepted as a way to augment loading in combination with gluteal or ischial loading.

Modification Goals

All interface designs adhere to five (5) main biomechanic goals: axial support, frontal stability, rotational control, sagittal control, and medial brim comfort.

Axial Support

The gluteus is identified by the quad socket as an excellent source of axial loading since it can act as a relatively pressure tolerant load bearing area. Ischial support is not nearly as comfortable, but transfers loading directly to the pelvic frame. Medial pressure to the ischium is more important for biomechanical reasons discussed later. Loading just below the ischium in the sub ischial triangle utilizes the adductors to increase loading comfort of the ischium by providing greater surface area. Hydrostatic support greatly adds to axial support by utilizing the tissue of the limb itself as a fixed volume. However, it is difficult to utilize as the primary source of axialsupport as first intended. Global compression is achieved through elongation of the soft tissue during casting and the application of standard tension values.

Frontal Stability & Containment

Frontal Stability refers to the support of the femur in an adducted attitude to minimize c.g. movement and energy consumption while providing a cosmetic narrow base of support. This is the main biomechanic advantage of ischial-ramal containment. The quad socket depended on pressure applied on the adductors by the medial brim to maintain lateral wall contact. Patients with fleshy residuums or soft musculature provided little opposition to a lateral shift. A two-point pressure system is used in ischial-ramal containment interfaces which applies force medial to the ischium and distolateral along the femur. The sub-ischial triangle (defined by the semitendinosis, gracilis, and pubic ramus) along with the medial wall angle provides the necessary soft tissue counterpressure, keeping the femur against the lateral wall distal to the brim. A general flattening along the femur and distal end relief must be created for femoral loading comfort. Some have implied that a third point of pressure is applied proximal to the trochanter to lock in the residuum. While this may serve a kinesthetic purpose for volumetric containment, the proximo-lateral wall above the trochanter migrates laterally during stance, negating its biomechanic relevance.

Rotational Control

One of the great advantages of the quad socket was its ability to control rotation by applying pressure anteriorly and accommodating limb musculature, namely the rectus femoris and gluteus maximus. Unfortunately, these features were removed with the first ischial containment methods and rotation problems re-emerged. The NU-RIC method reapplies these valuable rotational controls.

Medial Brim Comfort

Medial brim comfort is as important as any of the biomechanical goals since discomfort can manifest itself as gait deviations. Adductor longus relief and ramal exit are areas of concern especially with aggressive ischial containment.

Sagittal Control

The interface must also provide a surface for the femur to act against to provide stability at heel strike. This is provided by the posterior-lateral wall angle. It must be remembered to pre-flex the socket an additional 5° over the hip flexion contracture (if any) to place the hip extensors on stretch and enable the amputee to take a normal stride length.

Socket Interface Geometry

The quadrilateral interface has been successfully described by defining the biomechanical objectives and subsequent geometry of each wall. The ischial ramal containment interface may also be described in such a way by addressing each quadrant of the interface.

Anterior Medial Quadrant

Adductor relief is important for socket comfort. With the Ischial containment design, the anterior medial radius is more acute due to the medial wall angle. Also there is a tendency to rope or deform the adductor longus area when taking the impression. Many times relief must be added to prevent discomfort or the occurrence of adductor rolls. If relief is too rounded, loss of suction or adductor rolls may result. Scarpa's Triangle, formed during casting, is placed to apply pressure against the femoral bundle and provides rotational control. The femoral triangle also functions to help keep the ischium in position, but is not as critical in providing counterpressure as in the quad socket. This is primarily done by the anterior lateral quadrant.

Anterior-Lateral Quadrant

The anterior lateral quadrant or rectus channel accommodates the anterior musculature including the rectus femoris and the tensor fascia lata proximally. It is important to evaluate the depth of this channel when the patient holds their limb internally rotated. Most amputees externally rotate their limb at rest (during casting) and internally rotate within the interface during gait. This modification is critical in providing counterpressure to maintain ischial contact with the medial wall. If it is too loose the ischium may slip off and lateral rotation at heel strike may result. When too tight the ischium may ride above the brim edge medially because of excessive pressure.

Posterior-Lateral Quadrant

The posterior lateral quadrant proximally cups around trochanter to provide a cosmetic transition and kinesthetic volume control. Distally it has a posterior lateral flattening to act as a surface for the femur to act when maintaining knee stability at heel strike. The posterior brim is 1" proximal to ischial level to provide gluteal support. It is necessary to make this a relatively tight radius rather than a rounded flair to provide tissue compression. The posterior-lateral wall provides a surface for the femur to work against, but also creates a gluteal channel proximally to accommodate movement. Many current methods use a variety of other anatomic soft tissue modifications, which also help maintain intimate fit and rotation.

Posterior Medial Quadrant

The medial wall of the interface is defined by the patient's pelvic geometry and it is usually internally rotated off of the line of progression. A perineal splint should be used at the time of casting to not only get a good impression of the ischium but the ramus as well. Wider hips as in females will result in a greater medial angle. This medial angle loads the adductors especially important during early stance phase to maintain lateral contact. Many "hybrid" designs create a medial wall that is parallel to the line of progression as in the quad socket. This results in a loss of adductor pressure and ischial contact which in turn causes lateral gapping. The medial brim also plays a part in rotational control. If it is too loose there may be a lack of contact resulting in whips in swing phase or discomfort during stance.

Ichial-Ramal Containment

The amount of ischial containment is defined by pelvic width and limb length. There are really two types of containment: anterior ramal containment and proximal ischial containment. In more narrow hips as in males more of the ramus will be contained. Patients with a narrow pelvis may tolerate 1" of anterior containment while wider hips may tolerate only ½". The precise location of the exit of ramus from the interface is crucial to socket comfort. It is imperative to mark the ramus and evaluate its position when using the evaluation interface. Many times the ramus is relieved distally when pressure posteriorly is really the culprit. The brim angle and flaires should be adjusted to match the ramus for medial brim comfort. Proximal ischial containment is determined by limb length. Short residuums may tolerate more aggressive trimlines up to 2-1/4" proximal, while long limbs may prefer only ¾" of containment.

Sub-Ischial Triangle

The subischial triangle serves three purposes: augmenting adductor loading, providing a additional ischial support, and helping to maintaining femoral contact with the lateral wall. This retroverted triangle is bounded by the gracilis, pubic ramus, and semitendinosis with its deepest point 1 ½"-2" distal of ischium. The depth of the modification 1/4"-3/4" depends on musculature firmness. Some amputees may have difficulty tolerating this modification especially in a more aggressive ischial containment design.

1. Northwestern University Transfemoral Fitting Manual
   Uellendahl, J, Edwards, M., Uellendahl, E." Ischial Containment Above-Knee Prosthesis". Northwestern University Prosthetic Certification Program Manual, 1998.

2. "Funtional Considerations in the Fitting of Above-Knee Prostheses"
   Radcliffe, C.W., Artifical Limbs 1:35-60, 1955.

3. "The Knud Jansen Lecture: Above-Knee Prosthetics"
   Radcliffe, C.W., Prosthetics and Orthotics International. 1977, 1:146-160

4. "Contoured Adducted Trochanteric-Controlled Alignment Method (CAT-CAM): Introduction and Basic Principles"
   Sabolich, J. Clinical Proshthetics and Orthotics. 9, 1985, pp.15-26.

5. "Normal Shape ,Normal Alignment (NSNA) Above Knee Prosthesis" Long, I. Clincial Prosthetics and Orthotics. 9, 1985, pp.9-14

6. "Workshop on Teaching Materials for Above-Knee Socket Variants"
   Pritham, Charles H. Journal of Prosthetics and Orthotics, Vol. 1, Num. 1, pp. 50-67

About the Author

Gerald Stark, Jr., BSME, CP, CPed, is the Director of Product Development and Education at Fillauer, Inc., in Chattanooga, Tennessee. He has an extensive background in both prosthetics and mechanical engineering and previously served as a Contributing Editor of the O&P Almanac.

Stark is a former instructor in Prosthetic Education at Northwestern University Prosthetics Orthotic Center.