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A Case Study Using Fluoroscope to Determine the Vital Elements of Transfemoral Interface Design

Jason T. Kahle, CPO, LPO

ABSTRACT

The purpose of this case study was to isolate and examine the key elements in a transfemoral interface and determine their impact on the femur and the pelvis using a dynamic medium. There are many variations and theories on the most functional interface design. This case study attempts to verify the significance of certain elements of the transfemoral interface. This case study utilizes static medium (radiograph) to confirm an ideal fit of the transfemoral prosthesis. Once this is achieved the vital elements are isolated and examined using the dynamic medium of the fluoroscope. It attempts to show the reaction of the femur, ischium, and the pelvis during the stages of gait. In examining the vital elements in a dynamic medium, and determining their importance, we can place more emphasis on those elements and de-emphasize the non-vital elements. This could potentially increase comfort and function.

Keywords: Adduction, ischium, femur, fluoroscope, radiograph, transfemoral interface

MATERIALS AND METHOD

She method used in this case study was to cast, fabricate, and fit a transfemoral interface and achieve ideal function and ambulation. The subject was a 35-year-old man. His amputation was from a MVA in 1994. A hand cast was used to achieve the following in the fit of the interface: ischial containment, a muscle contoured interface, suction suspension, flexible interface/rigid frame, 8 degrees of static socket adduction, total contact, and a gait free of deviations.

The same flexible interface/rigid frame was then duplicated four times. Each rigid frame was than modified by eliminating one element of the proximal contour/brim. There were four elements of the proximal brim that were modified and then observed to create four different variations of the full interface: anterior, posterior, lateral, and medial. The anterior wall was cut down to 12 centimeters distal to the height of the ischial tuberosity. The posterior wall was cut down to be 4 centimeters proximal to the distal end of the femur. The lateral wall was cut down to the level of the iscial tuberosity. Finally the medial wall was cut down to be in contact with, but not to go proximal or medial to, the iscial tuberosity. The four modified interfaces, and one full interface, were than observed using the fluoroscope.

RESULTS

FULL SOCKET

The static alignment of the prosthesis was confirmed with radiograph. The ischium was well contained in the interface (dispelling any notion that you cannot achieve ischial containment with a hand cast.¹ In the full socket, the femur was held in 2-3 degrees of adduction during the static radiographs (Figure 1A and Figure 1B ), despite 8 degrees of socket adduction. However, during weight bearing the femur achieved no closer than 2-3 degrees of adduction (Figure 2A ). It varied from 3 degrees of adduction to 4-5 degrees of abduction from weight (Figure 2A ) to non-weight (Figure 3A ) bearing. The elements seemed to achieve their perspective functions when working collectively in the full interface. The sagittal view of the pelvis confirmed that the ischium was held into the ischial containment compartment in weight (Figure 4A ) and non-weight (Figure 4B ) bearing. It also shows how the ischium migrates anterior during non-weight bearing (Figure 4B ). However, it does stay within the ischial containment compartment. The medial element contained the ischium within the containment area in weight (Figure 2A and Figure 2B ) and non-weight (Figure 3A ) bearing. The posterior wall maintained position of the ischium and prevented posterior migration. The lateral wall seemed to assist in maintaining an adduction angle close to that of the static radiograph. No significant pelvic shifting was observed.

ANTERIOR WALL

In the sagittal view of the full interface the ischial tuberosity was contained within the ischial containment compartment during weight and non-weight bearing. When the anterior wall was cut away the ischial tuberosity migrated anterior causing it to fall outside of the containment area (Figure 4C ) only during hip extension (or heel off). In the coronal view of the full socket, the ischial tuberosity does stay within the containment area, both during weight bearing (Figure 2A ) and non-weight bearing (Figure 3A ). Cutting the anterior wall causes the ischial tuberosity to migrate more distally and anteriorly (Figure 2C ) and Figure 4C ) in both). In non-weight bearing the position of the ischial tuberosity (Figure 3B ) appears to be the same as in the position of the ischial tuberosity in the full interface (Figure 3A ). Cutting the anterior wall did not significantly improve the comfort of the patient, however, did increase flexibility in hip flexion.

POSTERIOR ELEMENT

Removing the posterior element did not seem to affect the femur or the pelvis. The position of the ischium seems to be in the same place in swing and stance (Figure 2D and Figure 3C ) when compared to the full interface (Figure 2A and Figure 3A ). The most significant change was the increase in comfort. The patient remarked that it was the most comfortable of the five interface designs.

LATERAL ELEMENT

The lateral element was removed to the level of the ischium. During stance phase the ischium seemed to achieve the same containment in this interface (Figure 2E ) as in the full interface (Figure 2B ). There was no notable difference in medial or lateral shift of the pelvis in weight bearing. Removing the lateral wall did not significantly change the position of the femur during weight bearing (Figure 2F ). However, removing the lateral element did effect a change during swing phase. This change enabled the femur to achieve a more abducted position during swing phase (Figure 3D ) than that of the full socket during swing (Figure 3A ).

ISCHIAL CONTAINMENT COMPARTMENT

The removal of this element did increase pelvic shift in weight bearing (Figure 2G ), however, not significantly or consistently when compared to the full interface (Figure 2A and Figure 2B ). The ischium seemed to maintain the same position in swing phase in the removal of the ischial containment element (Figure 3E ) when compared to the full interface ( Figure 3A ). The position of the femur was not significantly or consistently effected by the removal of the ischial containment element. Once again, comfort was increased when this element was removed.

DISCUSSION

ANTERIOR WALL

The job of the anterior wall is to control rotation. This process is twofold: using the soft anatomy and contours. Secondly, this is achieved skeletally by applying a posterior directed counterforce to the ischial tuberosity. This works in conjunction with the posterior wall. It helps lock the tuberosity into the interface and wedges the skeleton into place in the interface.^2,3

This was difficult to observe in the medium of a fluoroscope. Essentially, it would be necessary to observe this with a medium that could view the skeleton in the transverse plane. In addition, there was a significant amount of contralateral interference with the fluoroscope imaging. However, we can view whether the ischium is held onto the shelf in the perspective of a linear sagittal observation. Using the fluoroscope it cannot be concluded if the anterior wall does in fact help control rotation, but we can conclude that if the anterior wall does not hold the ischial tuberosity on the shelf, then it will compromise the skeletal control that could be achieved.

POSTERIOR WALL

The job of the posterior wall is to support the soft gluteal tissue, and to provide a counterforce to the anterior wall (which helps control rotation).^2,3 It is hard to state any skeletal function of the posterior wall other than it unites the medial and lateral walls (in conjunction with the anterior wall). This provides the frame with structural integrity. If the integrity of the medial wall is compromised, then this could cause insufficient support of the pelvis. In turn, the ischium could fall into the interface and possibly cause pelvic shifting, rotation, and discomfort. Consideration had to be given to this particular interface so as to not compromise the integrity of the interface.

The other important function of the posterior wall is to provide a linear anteriorly-directed counterforce to the anterior wall. This helps hold the ischial tuberosity on the shelf and prevent a posterior shift in the tuberosity. If the ischial tuberosity shifts posterior and out of the containment compartment, it could cause skeletal shifting, rotation of the interface, and loss of stability. The only apparent function was soft tissue support. Comfort was greatly improved when the posterior element was removed. In addition, cosmetics were improved by creating a more uniform gluteal profile.

LATERAL WALL

The job of the proximal lateral wall is to provide a medial directed counterforce to the medial wall. It can help control rotation using the contour of the soft tissue. In terms of skeletal alignment it assists in holding the femur in adduction.^2,3 The lateral wall can also help minimize abduction by applying a counterforce proximal to the greater trochanter. Skeletally it is important to establish femur adduction and consequently pelvic stability. The lateral wall may play the biggest role in effecting the pelvis and femoral adduction, especially during nonweight bearing and swing phase.

MEDIAL WALL

This is the aspect of the transfemoral interface that has seen the most emphasis over the last two decades. The basic gist is that the medial wall, and more specifically the ischial containment area, assist in maintaining femoral adduction, prevent lateral shift of the pelvis, and create pelvic stability. In achieving this, a more normal alignment can be maintained. Subsequently, a smoother gait pattern can emerge.^2,4 In short, ischium containment plays a dramatic role in establishing skeletal alignment and stability.

The fluoroscope showed that the absence of ischial containment made no significant impact on the shift of the pelvis. There was no loss of stability noted, and the femur angle was not compromised by the absence of the bony lock. Although this is an isolated case, this is not the first such study to report that socket configuration does not affect the position of the femur.⁵ This case study reports that ischial containment seemed to have no affect on the pelvis or the femur from weight to non-weight bearing.

CONCLUSION

The evidence points in the direction of placing less importance on the elements of interface design and more on the amputee's limitations. Closer attention needs to be made to surgical biomechanics, more specifically, myodesis and fixing the femur in a more functional adducted, or over-adducted position.⁵ If the interface interrupts the delicate balance of coordination between the adductors and abductors, it could cause more of a biomechanical compromise. In other words, using the interface to adduct the femur may place the abductors in a biomechanical advantageous position, but then it also may place the already compromised adductors at a gross disadvantage. Design needs to gravitate back toward comfort and to placing emphasis on the elements that actually make a difference.

The femur angle and pelvic stability were unchanged in weight bearing when the "vital"; elements were removed. Alignment should be determined more by the amputee's given limitations, function, and outcome,⁶ and less by a standard alignment based on interface design and the assumed resultant forces of the interface. The assumptions of CAT-CAM, NSNA, and other IRC interfaces need to be more formally scrutinized as to the actual scientific validity of those designs. If these designs do not affect femur angle or pelvic stability, then their presumptions could potentially be detrimental.

It is dangerous to draw gross conclusions from a case study. This particular case study is meant to help pursue the aspects of interface design that will actually make a difference to transfemoral interface design and ultimately the amputee.

ACKNOWLEDGMENTS

The author would like to thank Jim Russ for imparting his wisdom to the author and all who he has come into contact with--that there is a necessity to raise the standards to a higher level of professionalism. The author would also like to thank Hans Schaepper, for always being a role model of that necessity, and his Mom and Dad.

JASON T. KAHLE, CPO, LPO, is the Director of Prosthetics for Westcoast Brace and Limb, Tampa, Florida. Correspondence to: Jason T. Kahle, CPO, LPO, 907 Bruce Street, Tampa, Florida 33606; E-mail: jtkahle@earthlink.net.

References:

1. Pritham C. Workshop on Teaching Materials for Above-Knee Socket Variants. J Prosthet Orthot 1989;1:50-67.
2. Sabolich J. Contoured aAdducted Trochanteric-Controlled Al-lignment Method (CAT-CAM): Introduction and Basic Principles. Clin Prosthet Orthot 1985;9:15-26.
3. Sabolich/NovaCare. NovaCare Training Program - the Sabolich Socket; [training manual], 1995.
4. Long IA. Normal Shape-Normal Alignment (NSNA) Above-Knee Prosthesis. Clin Prosthet Orthot 1985;9:9-14.
5. Gottschalk F, Kourosh S, Stills M, McClellan B, Roberts J. Does Socket Configuration Influence the Position of the Femur in Above-Knee Amputation? J Prosthet Orthot 1990;2:94-102.
6. Breakey J. Theory of Integrated Balance: The Lower Limb Amputee. J Prosthet Orthot 1998;10:42-4.