October 2005 • Vol. 1, No. 4
	Advancing Orthotic and Prosthetic Care Through Knowledge

Case Study: A New Semi-flexible Hip Disarticulation Socket

ABSTRACT

We present an alternate socket design for hip disarticulation prosthetic fitting, with increased patient comfort, acceptance, and satisfaction. The three main goals of our socket design are to provide comfort during sitting, minimize socket bulk, and improve suspension. The Glenrose socket design presented is made of three layers, with aggressive trimlines and an improved variable patient-controlled suspension method. We hope sharing our experiences encourages ongoing communication among practitioners to lead to further innovation and clinical refinement of HD sockets.

INTRODUCTION

A small percentage of lower extremity amputations occur at the hip disarticulation (HD) amputation level, with reported incidence from 1 to 2 percent.^1,2 It is estimated that 25 percent of these patients are fit with a prosthesis, and only a portion continue to use their prosthesis over time.³ However, it is important for clinicians to maintain expertise in fitting this level of amputation to ensure the greatest chance of success for patients with HD amputation who have functional prosthetic restoration as a goal. A general overview of HD prostheses was comprehensively reviewed in 2001.⁴ The purpose of this report is to present an alternate socket design that we have found to be of benefit for HD prosthetic fitting, with increased patient comfort, acceptance, and satisfaction.

The reasons often given for hip disarticulation prostheses being rejected relate to lack of socket comfort, particularly while sitting.³ The three main goals of our socket design are to provide comfort during sitting, minimize socket bulk, and improve suspension by giving control to the patient.

Comfort during Sitting
Given the nature of the field of lower extremity prosthetic restoration, many clinicians focus on walking ability as the primary measure of success. Sitting comfort is usually an afterthought, if considered at all. We acknowledge that HD prosthetic gait requires significantly greater energy expenditure (oxygen cost) compared to lower levels of amputation or normal gait, or even crutch walking without a prosthesis.⁵ Given this, it stands to reason that the HD patient may actually spend more time sitting than walking. Socket stability during gait is important, but sitting comfort takes priority to ensure patient acceptance.

Minimal Socket Bulk
HD prostheses are often rejected due to the significant bulk and high encompassing trimlines. The smaller and less bulky a socket, the greater the chance of acceptance. It is our experience that HD patients do not like to buy larger clothes to accommodate their prosthesis. Our goal is to have our patients wear the same clothes pre- and post-fitting.

Increased Suspension Controlled by the Patient
The more effective the suspension, the more efficient the prosthetic gait may be. A well-suspended socket will reduce energy loss by decreasing displacement between the residuum and the socket during pelvic tilt, and by minimizing relative prosthesis lengthening during swing phase. However, socket suspension that provides efficient gait may not be comfortable for long periods of sitting. Controlled variable suspension, as presented here, allows the patient to easily increase or decrease the tension over the iliac crest as desired, depending on the activity.

METHODS AND DESIGN

The Glenrose socket design is made of three layers and encompasses the affected side and iliac crest, reducing the band passing around the sound side between the iliac crest and the greater trochanter to 50 mm (Figure 1). The basic design is similar to the traditional Northwestern University Diagonal Socket.⁶ The major differences between our design and the Northwestern University design are in the aggressiveness of the trimlines and improved suspension method presented below.

Socket Layers
1. Semi-flexible butterfly (portion A of Figures 1-3).
The first layer is made of a soft thermoplastic elastomer. This layer provides a soft flexible transition on the medial wall, a high friction surface to aid suspension, and a flexible, forgiving trimline above the iliac crest. The butterfly shape allows movement of the iliac crest suspension without buckling of the material. This piece is difficult to fabricate and requires a skilled technician to accomplish.

Trimlines
The goal of this socket design is to minimize all socket trimlines until socket stability is beginning to be compromised. It is worth sacrificing some socket stability to gain sitting comfort. We expect the patient to have the ability to bend forward to touch his or her toes comfortably. Each layer is feathered back to provide a smooth transition and reduce bulk, particularly in the perineum and inferior to the ischium.

1. Inferiorly: The material directly below the ischial tuberosity is thinned out as much as possible. This includes altering the hip joint attachment plate if necessary. The pelvis should be as level as possible upon sitting.

2. Proximally: On the affected side, the proximal trim is just high enough over the iliac crest to provide the beginning of a reverse curve. We have found the usual standard of 50 mm proximal to the iliac crest4 consistently leads to pressure up into the inferior ribcage area on sitting.

3. Posteriorly: The proximal edge should be kept as low as possible to minimize bulk under clothing. The inferior edge must allow full contraction of the gluteus maximus on the sound side.

4. Anteriorly: The anterior closure points are tapered down to approximately 60 mm, depending on the size of the patient. This will reduce impingement on the abdomen during forward flexion. The semi-flexible portion over the affected side iliac crest will bend under pressure, allowing forward flexion. The socket does not encapsulate the iliac crest on the sound side.

Suspension
A 13 mm wide double thick strap provides suspension (Figure 1). The strap is attached to the posterior aspect of the semi rigid layer and passes anterior-proximal over the semiflexible portion above the iliac crest. The strap descends medially to the posterior surface of the anterior semi-rigid portion. The strap then passes through a cutout to the anterior surface and is reflected inferiorly and laterally to a 25 mm ratcheting buckle. The angle of pull will produce a medial and inferior force above the iliac crest. The ratcheting buckle allows the patient to control the amount of tension on the suspension.

Casting and check socket
1. Casting: The key to success with this design is a weight-bearing cast and a weight-bearing check socket, which is then re-casted under weight bearing. If a non-weight-bearing casting method is used, the socket will not suspend well. When the patient weight bears in the socket, the tissue beneath the ischial tuberosity will compress. This causes the superior aspect of the socket (the suspension portion above the iliac crest) to migrate proximally, according to the amount of tissue compression. The result will be socket pistoning and discomfort on the inferior margin of the ribs.

The lower one-third of the residuum is initially casted with or without weight bearing, depending on the patient build. This section is reinforced with fiberglass casting tape and allowed to harden. This is to prevent distortion during the next weight-bearing stage. The upper two-thirds of the cast is then taken with full weight on the lower one-third. The goal is to get an accurate and molded distance between the iliac crest and the compressed tissue inferior to the ischium. The positive model is then modified according to the practitioner’s preference.

2. Check Socket: The check socket has two functions. The first is to assess the comfort and volume of the socket. The second is to provide a custom casting device for the definitive cast.

The check socket is fabricated in two pieces (Figure 4). The lower portion is fabricated in glycol-modified polyethylene terephthalate (PETG; portion E of Figure 4). This portion is fit and adjusted to provide comfort in the groin under full weight bearing. Volume is also assessed and adjusted if necessary.

The upper portion is fabricated in polypropylene or polyethylene, 4 to 5 mm thick (portion D of Figure 4). The portion above the iliac crest is pulled in tight with hook and loop straps. Care is taken to ensure the iliac crest is positioned underneath this portion. The lateral extension is marked where the upper portion overlaps the lower section, as the point of attachment. Both sections are removed so the upper portion can be trimmed and attached to the lower portion. The joined sections are fit back on the patient and reapplied very tightly.

The fit and adjusted check socket is now a custom weight bearing casting brim. With the patient weight bearing hard and the check socket fitting very tightly, plaster is wrapped around the socket. Once the plaster is cured, alignment lines in the anterior-posterior and medial-lateral plane are marked under weight bearing using a laser line (Figure 5). The cast is then filled and the positive model modified according to the practitioner’s experience.

The double casting process evolved to avoid significant socket gapping. This can occur laterally between the inner flexible layer and the two outer layers when the suspension is tightened in the definitive socket. The double cast creates the gap during the second cast, so it can then be rectified during modification.

Fabrication
It is not the intention of this paper to present detailed instruction for fabrication. The basic procedure would be evident to a skilled technician. The definitive socket is made of three distinct layers. The fabrication of each layer varies according to the size of the patient, the activity level, the materials and the type of hip joint being used.

LOW PROFILE OPTION

There is a variation of the Glenrose Design referred to as the Low Profile Glenrose Design (Figure 6). In this socket, suspension and medial-lateral stability are compromised for cosmesis.

The adjustable suspension and the semi-rigid layer are left out. The result is an ultralow profile socket that is more cosmetically acceptable under tight clothing. The rigid laminate layer extends further laterally past the midpoint to make up for some of the loss of stability from the absence of the semi-rigid layer. This socket is not as stable, but this is a compromise—some patients prefer to have less socket bulk under tight clothing.

CONCLUSION

This paper has three goals. The first is to share our experiences, which we feel have led to improved patient acceptance as we pushed the limits of socket design. We have found that our fitting success increased as we made sockets more comfortable to sit in and less obtrusive to the eye.

The second goal is to challenge some traditional beliefs about HD socket design. These sockets do not have to be large, bulky, or restrictive. A socket that is adequate for walking but flexible, low, and adjustable is possible. The patient, not the socket, should determine how he or she will sit in a chair.

The third goal is to generate discussion among interested practitioners. We hope that this discussion will lead to further innovation and clinical refinement of HD sockets.

ACKNOWLEDGMENTS

We acknowledge the Glenrose Rehabilitation Hospital and the Glenrose Prosthetic Department staff for their ongoing support, particularly Warren Matthews for his technical innovation and contributions.

REFERENCES

1. Kaye HW, Newman JD. Relative incidences of new amputations: statistical comparisons of 6,000 new amputees. Orthot Prosthet 1975:29:3-16.

2. Dillingham TR, Pezzin LE, Mackenzie EJ. Limb amputation and limb deficiency – epidemiology and recent trends in the United States. South Med J 2002;95:875-883.

3. Shurr DG, Cook TM, Buckwalter JA, Cooper RR. Hip disarticulation: a prosthetic followup. Orthot Prosthet 1983;37:50-57.

4. Stark G. Overview of hip disarticulation prostheses. J Prosthet Orthot 2001;13:50-53.

5. Nowrozzi F, Salvanelli ML. Energy expenditure in hip disarticulation and hemipelvectomy amputees. Arch Phys Med Rehabil 1983;64:300-303.

6. McLaurin CA, Hampton F. Diagonal Type Socket for Hip Disarticulation Amputees. Chicago (IL): Northwestern University Prosthetic Research Center; 1962. Contract No. V1005M 1079. Sponsored by the US Veterans Administration.

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