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Bilateral Kinematic and Kinetic Data of Two Prosthetic Designs: A Case Study

Hank White, MSPT
Keith VandenBrink, MD
Sam Augsburger, MSME
Tim Cupp, PT
Wayne Cottle, BOC, PT
Chester Tylkowski, MD

ABSTRACT

This article discusses a comparison of ankle kinematic and kinetic data and the energy efficiency of two different prosthetic designs for a 16-year-old girl with bilateral transtibial amputations. Kinematic and kinetic data were obtained by having the patient walk at a normal, self-selected walking speed and wear the old prosthetic and then the new prosthetic. With the new prosthetic design, we noted that the patient displayed significantly greater sagittal plane ankle motion and ankle power, resulting in a higher ratio of energy efficiency. Because the patient has bilateral below-knee amputations, the changes noted cannot be attributed to an uninvolved extremity's compensating for an involved extremity.

Keywords:Bilateral transtibial amputee, energy efficiency of prosthetic designs, kinematic and kinetic data

The patient in this case study has a primary diagnosis of bilateral transtibial amputation caused by a meningococcemial infection. Meningococcemia is an acute infection caused by Neisseria meningitidis, a gram-negative diplococci bacteria.¹ This acute bacterial infection can cause fulminant meningococcemia, the most dramatic form of infection. Symptoms include shock, acute vasculitis, intravascular coagulation, hemorrhage, and tissue necrosis.¹

At 13 years of age, the patient was diagnosed with meningococcemia, an acute bacterial infection that resulted in vasculitis, hemorrhage, and resultant tissue necrosis that necessitated amputation. She initially underwent bilateral partial foot amputations, but eventually was converted to bilateral transtibial amputations. Two months after the patient's amputations, she was fitted with temporary prostheses. With the temporary prostheses, she developed skin ulcers on the distal portions of both her residual limbs.

For the next 2 years, the patient wore bilateral titanium endoskeletal patellar tendon-bearing (PTB) prosthetic sockets with pelite insert, sleeve suspension, and solid ankle cushioned heel (SACH) prosthetic feet (Otto Bock Orthopedic Industry, Inc., Minneapolis, MN) without any incidence of ulceration. New prostheses of the same design were made as needed to accommodate her growth and to allow for proper alignment and comfortable fit. Development, fitting, and adjustments of the bilateral prosthesis were performed per our normal clinical procedure by a certified prosthetist with a referral from a physician. A physical therapist trained the patient in the use of each new prosthesis for a 1- to 2-week period. Training activities included maintaining balance, performing proprioception tasks, walking on level and unlevel surfaces, walking up and down flights of stairs, and running.

The clinical decision was made to change the type of prosthetic foot and suspension so that the patient could try to return to a competitive level of athletics. The patient believed that she was unable to participate in sports with the old prosthetic design. She was fitted with bilateral test sockets that used a 3-S suspension (Fillauer, Inc., Chattanooga, TN) and Flex-Foot (Flex-Foot, Inc., Laguna Hills, CA). A certified prosthetist conducted the fitting and assessment of the bilateral prosthesis according to our normal clinical procedure. Training of the patient with this device was similar to that described previously. At the time of delivery of the new prostheses, a decision was made to study the different results achieved for the patient with the different prosthetic designs.

This patient presented a unique opportunity to compare the sagittal plane kinetic and kinematic data and energy efficiency differences because of a bilateral change in prosthetic design. Previous studies^{2,3,4,5,6,7,8,9} comparing kinematic and kinetic data changes in amputee gait have been limited in that only unilateral amputees were examined. The purpose of the current case presentation was to analyze the differences in prosthetic energy efficiency, ankle kinematic data, and kinetic data in a bilateral transtibial amputee while she wore an old prosthetic design and a new prosthetic design. The old prosthetic design consisted of PTB sockets, supracondylar suspensions, and SACH foot prostheses. The new prosthetic design consisted of 3-S suspensions and Flex-Foot prostheses. Because the patient had bilateral amputations, the changes noted were due to her body's adaptation to the devices and to the prosthetic component changes.

Methodology

A 16-year-old female patient participated in this study. Before her amputations, she had been active and had participated in recreational and scholastic sports. After her amputations, she returned to a high level of activity. She is considered a community ambulator who wears her prostheses without complaint throughout the day.

At the time of the study, the patient's physical examination revealed equal residual 13-cm limb lengths, as measured from the inferior pole of the patella to the end of residuum. The patient demonstrated 5/5 strength upon manual muscle testing of bilateral hip and knee musculature. Passive range of motion testing of both hips and knees showed no contractures. We assessed her protective sensation using a 10-gram monofilament and found it to be intact in both lower extremities.

Temporospatial, kinematic, and kinetic data were collected while the patient was wearing her old prostheses, and these data were collected again 6 months later while she was wearing the prostheses with the new design. All studies were performed by the same examiner and with the patient wearing the same footwear.

The data collection system was a HiRes Expert Vision System (Motion Analysis Corporation, Santa Rosa, CA), including six HiRes Pulnix cameras (PULNiX America, Inc., Sunnyvale, CA), and two AMTI force plates (Advanced Management Technology, Inc., Arlington, VA). The Cleveland Clinic Marker set was used in conjunction with OrthoTrak 2.5 (Motion Analysis Corp., Santa Rosa, CA) to reduce the data and plot kinematics and kinetics. Data were collected at 60 Hz for 4 seconds. Kinetic data were normalized to the patient's body weight. The raw data were filtered using a Butterworth filter at 6 Hz. The patient walked along a 10-meter runway. Three to four strides consisting of two consecutive, clean-force plate strikes were collected with the patient wearing each prosthetic design. Kinematic and kinetic data were normalized to 60% stance and 40% swing, with kinetic data also being normalized and averaged for body weight, with standard deviations calculated. The kinematic and kinetic data for three to four strides were found to be replicable (as evident by the small standard deviation); therefore, the averages of each condition were used for comparison.

Statistical comparisons were performed using a time series point-by-point paired t-test (p < 0.05) to compare each prosthetic design's effect on the patient's kinematic and kinetic data. A paired t-test was also used to compare the temporospatial changes observed with each prosthetic design.

The kinetic data for each prosthetic walking condition was compared by using Winter's² description and definitions of joint moments and powers at the ankles (P = M × w). The power (P) is positive if the moment of force (M) and angular velocity (w) have the same polarity. For a transtibial amputee, negative ankle power would represent energy absorption by the prostheses and residual limb. Positive ankle power would represent energy returned by the prostheses and residual limb.

The A1 power phase is the power absorbed as the leg rotates forward over the foot during a single-limb stance. The A2 power phase is the power generated during push-off as the foot plantarflexes before toe-off (Figure 1 ).

The energy ratio of each prosthetic design was calculated by dividing the energy released (positive ankle power) by the energy stored (negative ankle power).^3,6,8 The energy ratio is calculated as the positive area of the ankle power curve divided by the negative area of the ankle power curve.

Results

The patient's height increased by 5 cm (because of the change in prosthesetic design), and her weight increased by 1.4 kg (because of the change in prosthetic design and body weight increase) during the 6-month testing period. Temporospatial data with the new prosthesetic design indicated the following increases as compared with the old prosthetic design:

1. A 15% increase in velocity, from 122cm/sec to 140 cm/sec (±2.27).
2. A 6% increase in cadence, from 107 steps/min to 113 steps/min (±1.83).
3. A 10% increase in stride length, from 136 cm to 149 cm (±2.47).
4. A 15% increase in right-step length, from 66 cm to 78 cm (±2.23).
5. No increase in left-step length 71 cm (±2.00).

The new prosthetic design demonstrated a threefold higher ratio of ankle energy generation to energy absorption, as compared with the old prosthetic design (Table 1 ). Sagittal plane kinematic data showed total excursion of the ankle throughout the gait cycle to be 14° for the old prostheses and 30° for the new ones (Figure 2 ).

Kinetic data revealed that the timing of the A1 power absorption during a single-limb stance was similar when comparing the two prosthesis designs. A delay in timing of the A2 peak power generation during double-limb support was noted in the new prostheses during walking trials, as compared with the old prostheses during walking trials. A significant increase in the magnitude of both A1 and A2 was noted with the new prosthetic design, as compared with the old prosthetic design (Figure 1 ).

Discussion

Effects of temporospatial, kinematic, and kinetic data and energy return changes due to changes in prosthetic foot design have been reported previously. The results have been mixed with regard to which changes in these parameters occur.^{2,3,4,5,6,7,10,11,12} Some authors speculate that the differences stem from the patient's ability to compensate for the change in prosthetic design by relying on the uninvolved side of the body.^3,4,6,10 Because our patient has bilateral transtibial amputations of equal length, we had the opportunity to study these parameters in a unique way.

This study demonstrated an increase in stride length that could be due in part to the patient's 5-cm increase in height caused by the new prosthetic design. The patient's stride length and cadence increases resulted in a 15% increase in walking velocity. Previous studies^3,4 have reported no increase in walking speed. the 5-cm increase in height does not, however, explain the increase in right-step length and the absence of increase in left-step length. The increase in ankle motion noted in the sagittal plane kinematic data may be attributable to the increase in velocity and stride length.

In people without disabilities and in the sound side of unilateral amputees, the energy ratio has been found to be greater than ^1,3,6,8 The ratios of energy generation to energy absorption in our single-patient study showed approximately a threefold increase in efficiency, from 0.22 to 0.62 (Table 1 ). The ratio change does not in itself indicate energy generation or absorption amplitude or threshold changes per say, but it does indicate a change in the system's efficiency or the system's ability to return stored energy. Even though we changed both the prosthetic suspension and foot design, the threefold increase in the ratio of energy generation to energy absorption is consistent with data reported previously^3,6,8 in studies in which only the prosthetic foot design was changed. The change in suspension does not seem to have affected the energy ratio of the prosthetic feet.

The SACH prosthetic foot is often used for transtibial prostheses. The cushion heel is compressed at initial contact and acts as a shock absorber during weight acceptance. The rigid internal keel allows for forward progression of the tibia over the foot. The Flex-Foot internal keel is fabricated from a carbon-graphite composite leaf spring. The leaf spring compresses as the tibia rotates over the fixed foot and partially returns energy at toe-off.

The PTB prosthetic socket, made of a rigid material, often is used for transtibial prostheses. The socket is designed to allow for the majority of weightbearing on the anterior portion of the patellar tendon between the patella and tibial tubercle. The supracondylar portion of the hard shell contains the medial and lateral femoral condyles and provides suspension and rotational control. The 3-S silicone suction socket design provides suspension through the distal one-third of the sleeve and the pin, with the majority of the weightbearing occurring in the same area as it does in the PTB socket.

Sagittal plane kinematic data revealed total ankle motion changing from 14 degrees to 30 degrees (Figure 2 ), which is similar to previous findings.⁴ A limitation exists in the motion analysis data, however, by virtue of the link segment model. This link segment model assumes that the foot is a rigid body with the point of rotation at the ankle joint, which is not true for a prosthetic foot or for a normal foot while the person walks barefoot or with an orthosis. The SACH foot's motion occurs at the soft heel as it compresses during weight acceptance. The Flex-Foot deformation occurs at the midfoot during a single-limb stance. The motion that occurs at these different points other than the identified ankle joint could affect the degree of ankle motion measured.

Previous studies^2,6,7 have reported no significant difference in the amount of energy released during the A2 power generation phase when comparing the Flex-Foot with the SACH foot. Our single-patient study demonstrated statistically significant increases in the amount of A1 power absorption and A2 power generation. Besides the change in prosthetic design (both foot and socket), these increases were due in part to the increases in the patient's weight (1.4kg) and walking speed. Both of these factors could cause an increase in the A2 power generation phase.

Our study is limited by our single-patient design. The long period of time between data collection phases may have allowed other variables to occur; however, manual muscle testing and passive range of motion evaluations demonstrated no changes. Therefore, the change in prosthetic suspension and prosthetic foot and the patient's ability to modify her gait must be considered the cause when assessing the changes in the kinematic data, kinetic data, and the energy efficiency.

Conclusion

Because this is a single-patient pilot study, generalization of our findings requires further research. Based on the data from this patient's study, we draw the following conclusions:

1. We observed a significant increase in ankle motion throughout the gait cycle with the new prostheses as compared with the old prostheses.
2. The new prosthetic design led to sagittal plane kinetic data showing a significant increase in ankle power absorption (A1) during single-limb stance as the shank rotated over the fixed foot and a significant increase in ankle power generation (A2) at push-off.
3. Previous authors have speculated that the increase in the prosthetic ankle motion and A2 power generation may be due to the ability of the person's uninvolved limb to compensate for the limitations of the prostheses. Because our patient has bilateral transtibial amputations of equal length, this argument cannot be made. The increases in bilateral ankle motion and power were due to the patient's ability to adapt her gait pattern in response to the prostheses. Both of the patient's lower extremities compensated similarly with the different prosthetic designs.
4. The energy efficiency of the new prosthetic design was approximately three times greater than that of the old prosthetic design.

Clinically, it often is reported that a change in prosthetic design does not change an individual's gait pattern. Often the individual who wears a prosthesis reports liking the way the new prosthesis feels. Perhaps the changes in energy efficiency and the kinetic data obtained through motion analysis are a way to document objectively what the patient feels. Continued research is needed to better understand and explain the kinetic effects of different prosthetic feet and suspensions.

Acknowledgments

The authors acknowledge and give a special thanks to Donna Oeffinger, MS, and Bobbie Edester, for their assistance with this study. We also acknowledge the time and effort of the patient and her family, without whom the study could not have been performed.

Copyright ©2000 American Academy of Orthotists and Prosthetists.

Hank White, MSPT, is a physical therapist in the Motion Analysis Laboratory and Rehabilitation Department at Shriners Hospital for Children, Lexington, KY.Hank White, MSPT, Shriners Hospital for Children, 1900 Richmond Road, Lexington, KY 40502.

Keith VandenBrink, MD, at the time of the study was the assistant chief of staff at Shriners Hospital for Children, Lexington, KY.

Sam Augsburger, MSME, is the director of the Motion Analysis Laboratory at Shriners Hospital for Children, Lexington, KY.

Tim Cupp, PT, is the director of Rehabilitation Services at Shriners Hospital for Children, Lexington, KY.

Wayne Cottle, BOC, PT, is a prosthetist at Shriners Hospital for Children, Lexington, KY.

Chester Tylkowski, MD, is the chief of staff at Shriners Hospital for Children, Lexington, KY.

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