An Improved Downhill Skiing Prosthesis

Claude Levesque, C.P.(c)
Christiane Gauthier-Gagnon, M.Sc.(A)

Introduction

Downhill skiing is an activity accessible to most lower extremity amputees. The below-knee amputee usually skis with a prosthesis and can achieve the proficiency of an able-bodied skier. In fact, some below-knee amputee ski enthusiasts reach high levels of performance and manage intermediate and expert slopes.

Since the early 60s, great advances have been made in the design and alignment of the below-knee skiing prosthesis, enabling the amputee to ski with maximum comfort and function. The skiing prosthesis is aligned as for walking, but the socket is flexed about 120 to 150 and moved forward linearly so that the center of gravity of the skier falls ahead of the ball of the foot.^1,2 The ankle is dorsiflexed by 23° to 25° to fit the design of modern ski boots. This alignment and fit enable the amputee to squat for more effective control over moguls and in turns, and for faster descents. The overall length of the prosthesis is reduced according to the length of the intact limb when the ski boot is worn.

When managing bumps at fairly high speeds, considerable stress is put on the residual limb as the skier jumps up and rebounds on his skis. In order to minimize piston action, special attention is given to the suspension of the skiing prosthesis. In most instances, either a removable medial wedge, a supra-condylar cuff or a rubber sleeve suspension is selected to ensure limb-socket contact.^1,3A waistbelt and fork strap help to eliminate piston action and mitigate the weight of the prosthesis. A silicone gel insert improves residual limb comfort and protects bony areas. Further comfort can be obtained through the use of a thigh lacer. With this type of suspension, body weight can be distributed over the thigh as well as over the end of the residual limb.² Hence greater protection is given to the residual limb and the eventuality of skin breakdown is reduced.

However, the need for improvement was pointed out by a unilateral below-knee amputee skier participating in the amputee program at the Montreal Rehabilitation Institute. The complaint was that with a conventionally aligned exoskeletal skiing prosthesis equipped with a thigh lacer, soft insert, and Greissinger foot, piston action was still present, and the limb-socket reaction time was inadequate when dealing with moguls (Figure 1) . Moreover, even though a four-way ankle joint did allow greater flexibility in different planes, the natural knee varus and valgus in the frontal plane were restricted by the lateral bars of the thigh corset.

When downhill skiing, the coordinated flexion and extension movements at the hip and knee allow moguls to be absorbed smoothly. On the other hand, managing turns and side steps require lateral shifting of the body over the skis and edging of the skis to prevent side slipping. This is made possible by the medio-lateral mobility available at the ankle, knee, and hip in conjunction with rotation of the limb under a relaxed and balanced upper body.^4,5,6 Even though the foot is fixed in the ski boot, some movement is present at the ankle joint as the skier's weight is shifted from side to side. The skier uses the sensation of pressure on the inside of the upper boot to readjust the edging on the inner or outer border of the ski. At the knee, varus and valgus occurs, to a small extent, as the weight-bearing line is transferred medially or laterally to the center of the knee joint. This medio-lateral mobility is important for the skier because the position of the knees adjusts the edge-set of the skis in the snow. These ankle and knee movements have been shown to be used to a greater extent by champions than by intermediate skiers .⁴ Consequently, performance may be limited for the competitive amputee skier who is fitted with a thigh lacer and lateral bars which restrict knee-motion in the frontal plane.

Fabrication

The improved downhill skiing prosthesis includes a conventional socket and soft insert, an exoskeletal shank, and a Greissinger foot. The thigh corset suspension is modified in order to restore full knee function without compromising the advantage of the increased weight-bearing surface area of the thigh lacer. A rubber sleeve and an inverted figure-eight supra-patellar cuff are added to the prosthesis for an improved limb-socket relationship. A conventional waistbelt and fork strap are also provided (Figure 2) .

Shank and Foot

The lower part of the skiing prosthesis consists of an exoskeletal shank attached to a Greissinger foot (Otto Bock #1A12) with a forward inclination of approximately 23° to 25°. The flexibility of the Greissinger foot, particularly in the sagittal plane, produces a smooth knee motion and enables the skier to bend more deeply over his skis, hence improving control at higher skiing speeds. From the lower third of the socket to the base of the foot, the shank is reinforced with two layers of carbon fiber (IPOS #09505) to prevent breakage due to the extra stress imposed when the skier leans over on the tip or the tail of the skis to compensate for different types of terrain.

Socket and Alignment

A conventional polyester resin socket is fabricated. The posterior wall is lowered by 0.95 cm (3/8") to avoid excessive pressure in the popliteal area and to allow for extra knee flexion and stability as the center of gravity of the body is lowered; thus, higher skiing speeds can be reached. The socket is flexed by 10° to 12° and, as reported in previous publications, ^1,2,3 it is moved forward so that the brim of the anterior wall falls 2.54cm (1") ahead of the ball of the foot. A PE-LITE® insert is used to offer extra cushioning and protection to the residual limb, against the unusual residual limb-socket interactions which are likely to occur during descents on rough terrain.

Suspension

Fit a rubber sleeve (Otto Bock #99B4) on the proximal third of the socket (Figure 3) . To increase weight-bearing surface area, use a conventional thigh corset with stainless steel bars. To provide freedom of motion at the knee joint, use two four-way joints (ACME #3J-7704-00). Lower the lateral and medial walls of the socket by 1.27 cm (1/ 2") in order to achieve optimum congruency between the mechanical and anatomical knee axes and minimize the relative limbsocket displacement as knee flexion and extension occur. Screw the distal ends of the bars to a 5.0 cm² (2"²) metal plate incorporated into the medial and lateral walls of the socket. Hence, once the screws are loosened, the thigh corset and side bars can be pivoted forward for proper fit of the rubber sleeve (Figure 4) . As illustrated by the dotted lines in Figure 3 , carve a groove in the lateral and medial walls of the prosthesis so that when the side bars are brought up along the vertical axis of the prosthesis and the distal screws tightened, the side bars are encased in the walls of the prosthesis and secured safely to restrict any unwanted displacement of the bars. Use a 0.95 cm wide (3/ 8") and 0.30 cm thick (1/8") stainless steel band to link the side bars together at the level of the patellar tendon groove. Fix the Y-strap to the band (Figure 3) . Affix a gear- type hose clamp (Spae Haur #S-104) on the posterior aspect of the side bars. Cut it into two parts for installation. Screw the screw end of the clamp to the lateral bar and the other to the medial bar. Once closed and tightened, the clamp safely secures the bars on the proximal third of the prosthesis. At the point of attachment of the anterior band and hose clamp, curve the side bars inward to reduce width. To protect clothing and the rubber sleeve against tearing, glue a felt pad on the inside and outside shank portions of the side bars, hose clamp, and metal band.

Add an inverted supra-patellar figure-eight strap to further improve limb-socket contact on bumps and moguls, especially at high speeds. Use a 2.54 cm (1") Dacron strap (Knit-Rite #6W7314-10). Attachments are located on the proximal third of the posterior wall of the socket, 3.0 cm (1 1/4") distal and posterior to the extremity of the side bars. Cross the strap over the back of the knee, under the medial side bar, over the patella, and under the lateral side bar, and fasten it with Velcro® onto the medial side of the prosthesis to facilitate donning. Fit the medial attachment with an 0-ring. The purpose of the inverted figure-eight strap is best illustrated in Figure 1 .

Practical Applications

The improved downhill skiing prosthesis was clinically tested with a 27-year-old below-knee amputee. It improved his downhill skiing capabilities; he could attempt more challenging descents, try downhill racing, and finally participate in a number of local and international ski competitions for the handicapped. The increased mobility in the frontal plane was shown to facilitate edging of the skis. The combination of thigh corset suspension, rubber sleeve, and supra-patellar figure-eight strap considerably reduced piston action between the residual limb and the socket. Piston action was diminished to a point that the skier could hardly feel it. Hence, jumps could be attempted and moguls managed with minimal discomfort. One major advantage of this combined suspension device is that its use is not restricted to downhill skiing. It can be used in the practice of a wide variety of sports where limb-socket reaction time, as well as full knee function, need to be improved: volleyball, basketball, squash, racquetball, etc. In these other cases, the prosthesis is aligned for gait with the appropriate type of shoe.

Future Plans

Some modifications are still needed. The rubber sleeve could be replaced by an elastic sleeve which is warmer during outdoor winter sports and absorbs perspiration during indoor sports. Finally, the possibility of incorporating a Flex-Foot or a Seattle foot to the downhill skiing prosthesis should be investigated, as it might help in absorbing rough terrain and unweighting the skis when steering over bumps and initiating turns at fairly high speeds.

Conclusion

A new suspension was designed for the below-knee skiing prosthesis with a thigh lacer and Greissinger foot. The improved downhill skiing prosthesis provides mobility in the frontal and sagittal plane and maximizes limb-socket contact when skiing over rough terrain. It consists of a conventionally aligned below-knee skiing prosthesis to which two multi-axial joints are added to the side bars of the thigh corset to restore mobility of the knee in the frontal plane. A rubber sleeve is fitted over the proximal brim of the prosthesis. The lower portion of the side bars is screwed to a metal plate incorporated into the lateral and medial wall of the prosthesis, which allows the thigh corset to swing forward, thereby facilitating adjustment of the rubber sleeve. The side bars are stabilized on the proximal portion of the socket with a gear-type hose clamp and an anterior metal band. Closer limb-socket contact is provided for absorbing moguls through the use of an inverted figure-eight supra-patellar strap.

Claude Lévesque, C.P.(C), is with the Institut de réadaptation de Montréal, 6300, Avenue Darlington, Montréal, Québec, H35 2J4, Canada.

Christiane Gauthier-Gagnon, M.Sc.(A), is with Ecole de réadaptation, Université de Montréal, Case postale 6128, Succursale A, Montréal, Québec, H3C 3J7, Canada.

References:

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