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Current Transfemoral Sockets

Schuch, C. Michael, CPO; Pritham, Charles H., CPO

Section Editor(s): Gottschalk, Frank MD

Clinical Orthopaedics and Related Research®: April 1999 - Volume 361 - Issue - p 48-54
Section I: Symposium: Amputation and New Prosthetic Devices
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Clinicians have two types of prosthetic sockets for patients with a transfemoral amputation: the quadrilateral socket introduced in the 1950s and the ischial containment socket introduced in the 1980s. For many years, the quadrilateral socket was acceptable to clinicians and patients alike. With the introduction of the ischial containment socket claims were made that the quadrilateral socket had been inadequate from its inception. It also was claimed that the ischial containment design was the answer to all problems that patients with transfemoral amputations may have. Unfortunately, there is only one scientific study that lends any support to claims of the ischial containment socket's superiority and its findings are limited. Still, clinicians must make recommendation and prescription decisions daily regarding what is best for their patients. There are two acceptable socket designs for patients with transfemoral amputations. Experience with both supports the concept that they have more similarities than differences and that each has a legitimate place in the treatment of patients with transfemoral amputations. A comprehensive understanding of each socket design and its biomechanical intentions is essential for successful clinical application and treatment of patients with amputations.

From the *Department of Prosthetics and Orthotics, Duke University Medical Center, Durham, NC.

Reprint requests to C. Michael Schuch, CPO, Department of Prosthetics and Orthotics, M04 Davison Building, Duke University Medical Center, Durham, NC 27710.

The basic goals for fitting and aligning prostheses for patients with a transfemoral amputation seem simple enough: comfort, function, stability, and cosmesis. Obtaining these goals is significantly more challenging than might be expected. One reason is the opinion that many practitioners have taken too many shortcuts in trying to treat the patient with a transfemoral amputation. It is disturbing to note the number of new practitioners who have never read the publications of Radcliffe, who is considered worldwide to be the founding expert of transfemoral (above knee) biomechanics and socket design.8 It is equally disturbing to consider the number of experienced practitioners who do not follow these time proven principles. Simple formulas for the quadrilateral socket design have been ignored, forgotten, or replaced with shortcuts. New transfemoral socket designs that incorporate the principle of ischial containment have been developed and Michael6 reports that "these new designs represent evolutionary rather than revolutionary advances". Pritham7 has stated his belief that the principle of ischial containment "is fully compatible with Radcliffe's biomechanical analysis of the function of the quadrilateral socket and that the varying socket configurations are not at odds, but rather, are separate but related entities in a continuum labeled above knee sockets".

The biomechanical requirements of the patient with a transfemoral amputation functioning with a prosthesis do not vary because of socket design; the outcomes may vary, but the needs remain the same. Suspension should not be influenced to any substantial degree by socket design. Alignment in the sagittal plane should be the same from socket to socket, with variations arising only from the stability afforded by different knee and foot components and/or the patient's desire for more or less alignment stability. There are no documented or stated sagittal plane alignment variations attributable to socket design. Coronal plane alignment might vary with mediolateral socket stability, but the variations ultimately are not substantial.

The goals of this paper are for the reader to: (1) recognize and acknowledge that there are two acceptable socket designs for transfemoral amputations; (2) understand the similarities and differences of these two designs; (3) understand the indications for each; and (4) be able to relate accepted transfemoral biomechanical principles to each.

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ASSESSMENT AND EVALUATION OF THE RESIDUAL LIMB RANGE OF MOTION

Careful measurement and evaluation of the patient's anatomy and kinesiology are essential for correct socket design and initial socket alignment. Important and less understood is the need for accurate measurement and evaluation of the range of motion of the residual limb in the sagittal and coronal planes, that is, flexion and adduction analysis. Proper planning and incorporation of these angular measurements into the socket and overall prosthesis design allows for certain biomechanical and alignment principles that are advantageous to the patient during the various phases of gait. It is the authors' contention that this is the most neglected area of evaluation and treatment of a patient with a transfemoral amputation.

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BIOMECHANICS

Four key phases of the gait cycle are considered when meeting the biomechanical objectives for a transfemoral prosthesis. The biomechanical objectives may be met or assisted by any or all of the following: socket design, prosthetic alignment, and component selection. The relevance of socket design to each of these four key phases of the gait cycle is discussed below.

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Heel Strike and Initial Stance Phase

For the patient with a transfemoral amputation, dissipation of forces at heel strike is not possible in the normal manner. Thus, heel strike and initial stance phase are a period of potential knee instability for these patients. The biomechanical requirement is that of knee stability, maintaining the prosthetic knee in complete extension, and overcoming the tendency of the shank to rotate forward and cause the knee to buckle. The transfemoral socket must be designed in a position of flexion at least 5° greater than the resting flexion angle of the patient's residual limb. This positions their hip extensors on stretch allowing them to more effectively control the knee flexion moment at heel strike.

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Midstance

Saunders et al10 described the six determinants of gait that provide efficiency in locomotion. The hip abductors, primarily the gluteus medius, maintain pelvic control during midstance through eccentric and isometric contraction. This determinant of gait allows for a narrow based gait and upper trunk control, and minimizes excursion of the center of gravity. However, in the case of the patient with a transfemoral amputation, the femur is not part of a structure that terminates in a foot firmly planted on the ground. The residual femur is only a lever of less than half of the normal length of the entire lower leg and floats in a mass of muscle, tissue, and fluid. As a result, in midstance, the residual femur tends to displace laterally in the mass of residual muscle and tissue, making it difficult to maintain horizontal stability of the pelvis and trunk. Effective pelvis and trunk stabilization and the resultant narrow based gait only can be achieved in a transfemoral prosthesis by providing adequate lateral support to the entire femoral shaft, maintaining normal or maximum possible adduction of the femur, and distributing the proximal medial component of the resulting midstance force couple evenly against the adductor muscle mass of the upper thigh or, as is the case of the ischial containment socket, against the medial aspect of the ischium and ramus (Fig 1). This is accomplished by appropriate socket design and alignment.

Fig 1

Fig 1

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Heel Off and Terminal Stance Phase

Effective heel off and terminal stance propulsion are provided in a transfemoral prosthesis via properly assessed hip flexion analysis and proper initial hip flexion designed in the socket. By designing and aligning the socket in a position of initial flexion, the patient is allowed to more easily reach the 15° of extension required on the prosthetic side to provide a normal stride length on the sound side.

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Swing Phase

Swing phase tracking addresses the smoothness of the pathway of the prosthetic limb during the swing phase of the gait cycle. Goals are lack of vertical displacement of the prosthesis with respect to the residual limb, and lack of deviation in the sagittal plane as the prosthetic limb advances forward during the swing phase. Problems regarding vertical displacement are the result of poor suspension and resulting piston action. Deviations in the sagittal plane include whips during the swing phase, which may be caused by improper socket shape. Socket design geometry and volume are critical for smooth swing phase tracking.

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AMPUTATION TECHNIQUE AND SOCKET DESIGN

In their study published in 1989, Gottschalk et al3 reported: "We believe that socket configuration does not influence the position of the residual femur and that a proper surgical procedure for above knee (transfemoral) amputations is essential for functional restoration and success of prosthetic fitting". This point is reiterated in their most recent paper.4 The present authors do not disagree with the premise of Gottschalk et al; in fact, it would be ideal if all transfemoral amputations were performed by surgeons with such a sure knowledge of, and passion for, surgical biomechanics. However, this is not the case. Transfemoral amputation is morbid and mortal surgery, and more frequently than not is performed by surgeons other than orthopaedists, who are much more concerned with life saving measures than postamputation rehabilitation potential.13 The authors contend that even in the best transfemoral amputations, proper detail to socket design and biomechanics is crucial to transfemoral outcomes.

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SOCKET ANALYSES

Similarities and Differences

General Overview

Similarities between the quadrilateral and ischial containment sockets are biomechanical principles, socket volume, distal socket design, and socket alignment in initial flexion and adduction. There is one striking difference between the sockets, which is direct ischial bearing in the quadrilateral socket, as opposed to ischial containment in that so named socket design. As a result, the contours of the proximal brim of the ischial containment socket are different from the proximal brim contours of the quadrilateral socket.

Ischial containment may be defined as a proximal extension of the posteromedial brim of the socket so that it bears against the pelvis, specifically the medial aspects of the ischial tuberosity and the ramus of the ischium (Fig 2). Ischial bearing has been accepted as direct inferior support to the ischial tuberosity by means of a flat, relatively horizontal surface of the posterior brim that provides vertically directed upward forces. This is opposed to the rather oblique and sloping contour of the proximal most posteromedial brim of the ischial containment socket. Distal to this oblique and sloping contour of the ischial containment socket, at a point approximately 3 or 4 cm below the edge of the brim, contouring beneath the ischial tuberosity may occur and thereby provide the same measure of ischial bearing as is present in the quadrilateral socket9 (Fig 3).

Fig 2

Fig 2

Fig 3

Fig 3

Distribution of the proximal and medial concentration of forces to the ischium helps reduce trauma to the soft tissue of the perineum. Obtaining rotational stability in the quadrilateral socket is dependent on muscle channel contours and generally is unsuccessful in an unstable residual limb. Rotational forces are better controlled by purchase against the medial border of the ischium and balanced by the accurate diameter between the medial ischial surface and the greater trochanter.

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Anteroposterior and Mediolateral Diameters

The anteroposterior (AP) dimension of the quadrilateral socket will be smaller than that of an ischial containment socket, because the ischium rests proximal and posterior to the inner AP dimension of the quadrilateral socket. Conversely, the ischium is within the ischial containment socket, necessitating a greater inner AP diameter.

Measurement of the mediolateral diameter is referenced differently by various practitioners. There is no consensus as to the best anatomic landmarks. The term narrow mediolateral socket is misleading because this dimension varies between males and females because of anatomic differences between the male and female pelvis5 (Fig 4).

Fig 4

Fig 4

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INDICATIONS, CONTRAINDICATIONS, AND RECOMMENDATIONS FOR SOCKET DESIGN

The indications and contraindications for transfemoral socket design were considered by a panel of experts in 1987.1,11,12 A summary of their conclusions follow. There were no specific contraindications reported for any socket design. Some advocated not attempting to change successful quadrilateral socket users to the newer ischial containment socket design. Successful fitting of quadrilateral sockets is more likely to be attained on long, firm residual limbs with intact adductor musculature, whereas ischial containment sockets are more successful than quadrilateral sockets on short, fleshy, unstable residual limbs. Ischial containment sockets are the better recommendation for high activity sports participation and running. Finally, there was lack of agreement on the best recommendation for patients with bilateral transfemoral amputations.

It has been the authors' experience that ischial containment sockets are preferred by most patients with bilateral transfemoral amputations. Correctly designed quadrilateral sockets function fine in the authors' experience when used on longer amputations that reveal principles of surgical biomechanics.3,4 The more of the adductor magnus intact, the more successful the quadrilateral fit. Quadrilateral sockets are recommended for the geriatric or debilitated patient. These patients ambulate with canes and walkers for assistance, thereby canceling out or greatly decreasing the typical medial lateral pelvic and trunk stability demands of midstance.

For the very short, fleshy, and rotationally unstable residual limb, ischial containment sockets have provided success where quadrilateral sockets have failed previously. In addition to enhanced stability and perineal comfort for these patients with transfemoral amputations, the additional socket volume afforded by the higher brims enhances the ability to obtain and maintain suction suspension.

Flexible brim sockets are recommended for use with ischial containment sockets. The socket frame or retainer should provide rigid support in the posteromedial corner of the proximal brim, under and beside the ischium; however, the remainder of the proximal frame can be lowered below the level of the flexible socket brim. Use of thermoplastic flexible sockets is common practice.

Discussion has focused on the transfemoral socket designs of quadrilateral and ischial containment, including similarities and differences, indications and recommendations. The authors strongly propose the case for recognizing and clinically using both socket designs, depending on the individual patient and his or her prognosis, level of amputation, and residual limb characteristics. A comprehensive understanding of each socket design and their biomechanical intentions is essential for successful clinical application and treatment of patients with amputations.

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References

1. Donovan RG, Pritham CH, Wilson AB (eds): Report of ISPO Workshops: International Workshop on Above-Knee Fitting and Alignment. Glasgow, International Society for Prosthetics and Orthotics 31-37, Appendices A-F, 1987.
2. Gailey RS, Lawrence D, Burditt C, et al: The cat-cam socket and quadrilateral socket: A comparison of energy cost during ambulation. Prosthet Orthot Int 17:95-100, 1993.
3. Gottschalk FA, Kourosh S, Stills M, McClellan B, Roberts J: Does socket configuration influence the position of the femur in above-knee amputation? J Prosthet Orthot 2:94-102, 1989.
4. Gottschalk FA, Stills M: The biomechanics of transfemoral amputation. Prosthet Orthot Int 18:12-17, 1994.
5. Hoyt CP, Littig D, Lundt J, Staats TB: The UCLA Cat-Cam Above-Knee Prosthesis. Los Angeles, University of California, Los Angeles Prosthetics Education and Research Program 14-49, 72, 1987.
6. Michael JW: Current concepts in above-knee socket design. Instr Course Lect 39:373-378, 1990.
7. Pritham CH: Biomechanics and shape of the above-knee socket considered in light of the ischial containment concept. Prosthet Orthot Int 14:9-21, 1990.
8. Radcliffe CW: Functional considerations in the fitting of above-knee prostheses. Artif Limbs 2:35-60, 1955.
9. Sabolich J: Contoured adducted trochanteric - controlled alignment method (cat-cam): Introduction and basic principles. Clin Prosthet Orthot 9:15-26, 1985.
10. Saunders JB, Inman VT, Eberhart HD: The major determinants in normal and pathologic gait. J Bone Joint Surg 35A:543-558, 1953.
11. Schuch CM: Report from international workshop on above-knee fitting and alignment techniques. Clin Prosthet Orthot 12:81-98, 1988.
12. Schuch CM: Modern above-knee fitting practice. Prosthet Orthot Int 12:77-90, 1988.
13. Schuch CM: Above-Knee Amputation: Literature Review and Prosthetic Experience. In Murdoch G, Jacobs NA, Wilson Jr AB (eds). Report of ISPO Consensus Conference on Amputation Surgery. Copenhagen, International Society for Prosthetics and Orthotics 66-68, 1992.
© 1999 Lippincott Williams & Wilkins, Inc.