Klenow, Tyler D. MSOP, CPO, CPT; Kahle, Jason T. MSMS, CPO, FAAOP; Fedel, Frank J. MS, CES; Ropp, Jeffrey BS, CP; Highsmith, M. Jason PhD, DPT, CP, FAAOP

The technological revolution has conincided with a myriad of innovations to the field of prosthetics including componentry, suspension methodology,¹ surgical procedures,² osseointegration,³ and prosthetic interface design.⁴ One area of particularly intense innovation is that of transfemoral (TF) interface design. A significant shift in standard of care was taken a few decades ago from the quadrilateral TF socket to ischial containment, which greatly improved prosthetic function.⁵ The field then experienced a quantum leap from ischial containment to brimless around the turn of the millenium, which decreased proximal trimlines while increasing comfort and range of motion (ROM).¹ Recent innovations in TF interface design include modular systems, instant user-controlled adjustment systems, and the tissue compression and release method of indirect-skeletal prosthetic anchoring.⁶

The lattermost innovation was brought about with the introduction of the High-Fidelity (HiFi) interface (Biodesigns, Inc, Westlake Village, CA, USA). The HiFi is said to increase the intimacy of the prosthetic interface by implementing a series of alternating zones of tissue compression and release to focus the attachment of the prosthetic interface to the residual long bone(s).⁶ This is accomplished through use of a novel casting jig called the HiFi imager (Figure 1). This stands in contrast to traditional socket designs, which are based on tissue containment and uniform tissue loading, in the case of hydrostatic socket designs,⁴ or selective loading of pressure-tolerant areas, in the case of ischial-ramus containment (IRC),⁵ quadrilateral,⁷ and patellar tendon-bearing designs.⁸ The HiFi system was originally developed as an upper-limb application but has been used regularly for persons with TF and knee-disarticulation amputation and has most recently been implemented for persons with transtibial (TT) amputation.

Figure 1

The indirect skeletal anchoring of the HiFi reportedly enables several benefits to traditional interface designs.⁶ One such reported benefit is increased proprioception of the prosthesis by the individual with amputation, called osseoperception.⁹ This phenomenon is caused by vibrations, which are attenuated in soft tissue in traditional interface designs, being transferred more to the skeletal system, thereby increasing proprioceptor and mechanoreceptor activity. As a subischial design, the HiFi also implements reduced trimlines that increases sitting comfort, hip ROM, and function similarly to other brimless designs.¹ Initial reporting also shows reduced pistoning and improved balance confidence and preference compared with IRC designs.^9,10 However, there has been no peer-reviewed work comparing the HiFi to traditional interface designs published to date. A additional benefit of the HiFi is a reported increase in adduction angle of the femur resulting in increased prosthetic control and balance.⁶ Comprehensive comparisons of interface designs must include both a biomechanical analysis of gait and a subjective opinion from the user as outcome measures to provide a balanced profile of the prosthetic interventions. Therefore, the purpose of this study is to measure the effects of the HiFi prosthetic interface system on temporal-spatial, kinetic, and kinematic parameters of gait, as well as perceived disability, compared with a traditional socket design in subject with TF amputation.

METHODS

SUBJECT RECRUITMENT

This study evaluates the first subject fit with the HiFi interface in Southeast Michigan. The subject in this case is a 65-year-old male with TF limb loss to his right lower limb sustained in a blast injury during his service in the US Army during the Vietnam War. He is 183.0-cm tall, weighs 104.1 kg, and ambulates at a K3 functional level confirmed by a score of 42 on the Amputee Mobility Predictor.¹¹ His prosthetic limb use reportedly exceeds 12 hours each day. The prosthesis consisted of an IRC socket, flexible interface rigid frame design, cushion liner, X2 microprocessor knee, and 1C63 foot utilizing the Harmony elevated vacuum system (Ottobock Healthcare, Duderstadt, Germany).

DATA COLLECTION

The study protocol was approved by the Institutional Review Board of Eastern Michigan University. The subject signed an informed consent before testing. All data were collected at the Motion Analysis Laboratory at Eastern Michigan University (Ypsilanti, MI). The laboratory includes an 8-camera Vicon Motion Capture system (Oxford, United Kingdom) and an 8.5-m elevated platform with two embedded AMTI force plates (Watertown, MA, USA). Analysis of gait in a human subject using this system allows the interface conditions to be tested with the criterion standard of biomechanical analysis. After collection of anthropometric data, the subject was fit with 37 reflective markers placed on anatomical landmarks in accordance with the plug-in gait model.¹² Markers were placed on the prosthesis on the lateral aspect of the knee axis, lateral malleolus on the footshell, on the dorsal aspect of the shoe at the second digit of the foot shell, and on the apex of the heel of the shoe in a plane with the toe marker parallel to the ground. Additional segment markers were placed at the proximal third of the lateral aspect of the thigh and shank using a laser line between the greater trochanter and knee marker and the knee and ankle markers, respectively. The subject then completed a static calibration pose and performed dynamic walking trials at his preferred self-selected gait velocity (SSGV) until 20 trials were recorded for each interface condition. Trajectory and kinematic data were recorded at 120 Hz, and kinetic data were recorded at 960 Hz. Desired variables were extracted from collected data using MATLAB R2013a (MATHWORKS, Natick, MA, USA). Data were low-pass filtered using a fourth-order Butterworth filter with a 15-Hz cutoff frequency before statistical analysis.

All socket fittings and adjustments were performed by the study prosthetist (J.R.) who is certified by the American Board for Certification in Orthotics, Prosthetics, and Pedorthics, and is trained to fit the HiFi system. The subject was tested first using his IRC socket, which was fabricated according to Sabolich.⁷ The subject had a 3-month history of wear with this socket, all without any reported fit issues. The IRC socket was well-fitting at the time of initial testing as determined by the study prosthetist. After this initial testing with the IRC, the subject was fit with the HiFi Interface, which utilizes the imager casting jig in fabrication, as described by Alley et al.⁶ The subject was then given a 30-day accommodation period with the HiFi and returned to the laboratory to repeat the testing protocol with the HiFi in a second session. Apart from the interface design, all components, suspension, and footwear were kept consistent between testing sessions.

In addition to 3D motion analysis, the subject was administered the Oswestry Low Back Pain Disability Questionnaire v2.0 and Western Ontario and McMaster University Osteoarthritis index (WOMAC) at initial testing and again at HiFi testing. These questionnaires are validated assessments of condition-specific disability and were administered by the study prosthetist.^13,14

STATISTICAL ANALYSIS

Data were analyzed using SPSS 23.0 statistical software (IBM Corporation, Armonk, NY, USA). Symmetry ratios (SRs) were calculated by dividing a prosthetic-side variable by the corresponding variable result on the sound side. A one-way analysis of variance (ANOVA) was used to determine differences in results between conditions. Fischer's least significant difference (LSD) test was used post hoc to determine statistical significance. The level of significance for all tests was set at P ≤ 0.05.

RESULTS

Temporal-spatial results are shown in Table 1. SSGV increased from 0.93 to 0.97 m/s with use of the HiFi, which was found to be statistically significant. Sound-side step length also increased significantly (0.52 to 0.55 m) with use of the HiFi. Stride time decreased with HiFi use (1.21 to 1.18 seconds). Improved symmetry (Table 2) was noted in step length, width, and center of mass (CoM) deviation in gait; however, only the step length symmetry (SR = 0.86 to 0.93) calculation was found to be statistically significant.

Table 1

Table 2

Ground reaction force (GRF) results are shown in Table 3. Significant results on the prosthetic side include a decrease in mean sagittal (14.14 to −5.31 N) and peak vertical GRF (1207.73 to 1147.02 N) with use of the HiFi. An increase in sagittal plane GRF range (133.89 to 225.11 N) and peak posterior-directed GRF (−45.09 to −88.96 N) are also shown with HiFi use. Significant results on the sound side include decreases in frontal plane GRF range (108.71 to 95.32 N) and peak posterior-directed GRF (−173.97 to −184.43 N) with HiFi use. The peak medial (−34.92 to −25.11 N), peak anterior-directed (74.52 to 113.89 N), and peak vertical GRF (1151.08 to 1209.83 N) all increased with use of the HiFi. Sagittal GRF mean (−24.64 to −13.11 N) and range (196.90 to 261.83 N) also increased with use of the HiFi.

Table 3

Kinematic results are shown in Table 4. Significant results for the prosthetic side include reduction in sagittal hip ROM (59.07° to 51.46°) as well as maximum, minimum, and mean flexion for both stance (50.73° to 31.31°) (−2.83° to −15.58°) (16.73° to 1.20°) and swing (56.23° to 35.88°) (12.06° to −1.69°) (45.42 to 26.49°) phases of gait with use of the HiFi. Prosthetic-side hip frontal plane ROM (16.31° to 12.89°) was reduced in the HiFi condition. Maximum, minimum, and mean hip abduction in stance (15.19° to 0.77°) (5.75° to −8.30°) (9.43° to −3.56°) and swing (22.05° to 4.59°) (11.23° to −3.74°) (17.39° to 0.71°) were also reduced on the prosthetic side with use of the HiFi. Similar reductions were found in the sound-side hip.

Table 4

The Oswestry v2.0 found the subject perceived reduction in general pain intensity and perceived disability in personal care, walking, sitting, standing, sleeping, social life, and traveling with the HiFi as compared with the IRC condition, an 8-point reduction of 10 total questions. These results indicate a 46% disability rating with the baseline IRC and 30% disability rating with the HiFi, which corresponds in a reduction in disability category from severe to moderate.¹⁴ The results of the WOMAC showed reductions in all pain and stiffness categories as well as reduction in difficulty of 11 of 17 total physical activity questions. The total reduction was 19 points from 59 with the IRC to 40 with the HiFi, representing a total disability reduction of 19.8%.

DISCUSSION

This study analyzed the effects of the HiFi Interface System on biomechanical parameters of gait and perceived disability compared with a baseline IRC condition in a single elderly veteran subject with history of TF amputation. Temporal-spatial, kinematic, and GRF were analyzed with a 3D motion capture system with integrated force plates, and disability was analyzed using two valid questionnaires commonly used in rehabilitation settings. The first finding was an increase in SSGV. This increase was statistically significant and indicates potential increased stability in gait.¹⁵ Another marker of increased stability is the trend toward symmetry noted in step length, step width, and CoM lateral deviation with use of the HiFi. This trend is corroborated by slight increase on the prosthetic side and slight decrease on the sound side of peak lateral GRF resulting in increased symmetry there. Gait symmetry has been linked to increased stability and increased gait efficiency in the past.¹⁶

Reduction of lateral CoM deviation is shown with the HiFi compared with the IRC baseline condition in this case. Although it did not reach a level of statistical significance, reduction in lateral CoM deviation during gait has been shown to increase gait stability.^15,17 Similar reductions in the range of frontal plane GRF on the sound side were found, including reduced magnitudes to both the medial and lateral peaks. These changes, along with the increase in lateral GRF peak of the prosthetic side, indicate more prosthetic control and a reduction in the “fall and catch” gait pattern from prosthesis to sound limb displayed by many persons with unilateral TF amputation. This finding is consistent with the reduction in bilateral hip stance and swing abduction noted in the HiFi condition. Excessive hip abduction indicates reduced balance and balance confidence as they are strategies to reduce lateral falling and increase minimum toe clearance with circumduction.^18,19 This study shows reduction of hip abduction to a mean adducted position with the HiFi, perhaps indicating the increased control and proprioception this interface is said to provide may reduce falling and/or fear of falling in this elderly subject.²⁰ Previous work by Gottschalk et al.⁵ concluded socket design does not control position of the femur. However, that work compared only IRC and quadrilateral socket designs. The change from mean abducted position to adducted position in this subject suggests socket design can control femur position and the HiFi does. Further research with larger samples will be needed to confirm this finding.

Increased proprioception could be one explanation for the decreased hip flexion found in the HiFi condition. This presumed increased awareness of limb position could increase the subject's accuracy of the relationship between the bottom of the foot and ground, thus reducing the need to create additional toe clearance with a compensatory steppage pattern or the previously mentioned circumductory pattern.²¹ The reduction of compensatory gait strategies implemented with the HiFi compared with the IRC condition may result in a reduced energy expenditure of walking with the HiFi, which has been shown to lead to increased walking velocities, balance confidence, and participation in social activities.²²

Increased prosthetic control with use of the HiFi may be a factor in the sagittal GRF changes noted in the results of this report. The prosthetic side showed a decrease in anterior peak GRF, or braking force, with the HiFi even though the SSGV had increased in this condition compared with the IRC baseline. In addition, the magnitude of the posterior-directed GRF of the prosthetic side increased in the HiFi condition indicating increased prosthetic loading and potential for energy return.²³ This increased loading is reflected in the mean sagittal GRF, which switches from anterior-directed in the baseline IRC condition to posterior-directed with the HiFi condition. This indicates the foot is loading and returning energy more than it is braking, a pattern implemented by able-bodied subjects and by this subject's sound side in both conditions. These results may be explained by a phenomenon that occurs with the vertical GRF patterns of the two conditions. The second hump of the classic “double hump” of the vertical GRF graph (Figure 2) is somewhat flattened in the IRC condition, whereas it is a much more smooth form in the representative sample of the HiFi condition. As the second hump of the graph is the “propulsive hump,” this pattern would indicate a smoother loading pattern of the foot in the HiFi condition compared with the IRC condition.²⁴ It may be that the alleged more intimate fit of the HiFi interface may lead to less motion of the socket on the residual limb (RL). This would result in less force being absorbed by the RL tissue and instead transferred to the foot for increased loading and subsequent energy return.

Figure 2

The claimed benefits of the HiFi Interface System and supporting biomechanical changes previously discussed may have led to reduction in perceived disability noted by results of the Oswestry and WOMAC questionnaires. The Oswestry is a valid and rigorous measure of condition-specific disability and, as a result of the 30-day wear period with the HiFi, 1-point reductions in 8 of the 10 disability questions were shown. A 6-point change was previously found to be clinically significant in a rehabilitation setting.²⁵ This change signifies a reduction in the subject's disability category from severe to moderate.¹⁴ Similar incremental, but sweeping, reduction in disability was found in the WOMAC results with this subject. All pain and stiffness scores reduced with HiFi use, and a majority of the questions regarding difficulty of physical activity were also reduced. In total, this subject's WOMAC disability score was reduced 19 points with use of the HiFi, eclipsing the score of 9-point reduction, which was previously found to be clinically important in a population with hip osteoarthritis and 8-point reduction found in a similar population with knee osteoarthritis.²⁶

LIMITATIONS

Limitations to this study include the use of one subject as opposed to a larger sample, which limits the generalizability of the findings of this case. The HiFi is a recent interface technology, however, and abundance of subjects initializing fitting is limited. These initial results do indicate larger studies, possibly utilizing multiple sites in recruitment, and are warranted in the future.

CONCLUSIONS

The results of this study indicate the HiFi Interface System may present some biomechanical advantages to traditional IRC socket designs, which may allow for increased stability in patients utilizing a TF prosthesis. Accumulation of these biomechanical advantages led to a reduction in perceived disability in two measures used in this case. Additional research is needed to confirm the results of this case study in a larger population.

REFERENCES

KEY INDEXING TERMS: prosthesis; artificial limb; amputation; limb loss; above knee; biomechanics; HiFi