Gait Assessment in the Transtibial Mercer Universal Prosthesis Designed with a “Neutral Alignment” Socket

Abstract

Standardizing socket design and maintaining a default socket alignment in transtibial prostheses are innovations that aim to simplify fitting procedures and reduce prosthetic service costs, particularly in low-income countries. Objectives: This study evaluated the Mercer Universal Prosthesis (MUP) with a standardized “neutral alignment” against custom-made conventional prostheses (CVPs). Methods: Twenty transtibial amputees (n = 20) completed gait assessments using their CVP and immediately after fitting with an MUP. Temporal–spatial and sagittal plane kinematics (hip, knee, and ankle angles) were analyzed, along with a gait symmetry index. Results: the MUP group reported a significant difference between the prosthetic and the intact limb for both hip and knee kinematics (p < 0.05), but there was no change in the CVP group. When compared with the sound limb in the MUP group, post hoc analysis showed that both hip flexion and the hip range of motion (ROM) in the MUP limb significantly increased by 5.7° and 7.3° (p = 0.002 and p < 0.001, respectively). Spatial and temporal gait parameters were comparable between the MUP and CVP groups, and gait symmetry showed no significant differences. The CVP showed greater symmetry in terms of hip (19%, p = 0.012) and knee flexion (8%, p = 0.026) compared to the MUP, while the MUP had higher plantarflexion symmetry (24.4%, p = 0.013). Conclusions: Immediately post fitting, MUP improved joint mobility in the prosthetic limb, potentially enhancing kinematics. While short-term benefits are evident, further research is needed to assess long-term gait adaptation and quality of life impacts.

1. Introduction

In their 2021 study published in Prosthetics and Orthotics International, McDonald et al. estimated that 57.7 million people globally were living with traumatic limb amputation in 2017, with lower limb amputations typically accounting for 75–85% of these cases based on broader epidemiological patterns. This suggests that approximately 43–49 million people are living with traumatic lower limb amputations worldwide [1]. According to the World Health Organization (WHO), approximately 40 million amputees reside in developing countries, yet only 5% have access to prosthetic care [2,3]. This stark disparity underscores a critical global health challenge. The demand for prosthetic supplies is projected to increase as a result of vascular complications, diabetics, and traumatic accidents in these regions [4]. However, the availability of prosthetic healthcare resources is severely limited, particularly in underserved areas, complicating the acquisition and fitting of necessary prosthetic devices for amputees [3]. The high costs associated with prosthetics, coupled with the need for specialized expertise and the scarcity of trained technicians, further exacerbates this issue.

Thermoplastic polypropylene (PP) has been widely adopted for socket fabrication, as standardized by the International Committee of the Red Cross (ICRC). This low-cost material helps mitigate the expenses of prosthetic services in low-income countries [5,6]. In an extensive review of lower limb prosthetic technologies in developing countries, Andrysek et al. in 2010 emphasized that the primary goals of the International Society of Prosthetics and Orthotics (ISPO) should focus not only on maintaining sound biomechanical principles for amputees, but also on keeping prosthetic services economically affordable [2]. Establishing training programs to produce qualified personnel is essential to improving prosthetic care in developing countries. Despite these initiatives, significant barriers remain, particularly the high costs associated with skilled prosthetists and trained orthopedic technologists, which continue to limit access for many underserved populations.

In the context of reducing the cost of transtibial prosthetic (TP) services, this paper explores a feasible solution aimed at addressing the cost of prosthetic personnel. This solution involves the standardization of the size and volume of transtibial sockets, and the maintenance of a “neutral” socket alignment in the design and clinical fitting. This concept, known as Mercer Universal Prostheses (MUP^®), presents a promising approach to enhancing the accessibility and affordability of prosthetic care [7,8]. In 2018, Vo et al. suggested that the MUP concept addresses the urgent need for cost-effective prosthetic solutions without compromising quality or functionality [7]. Vo et al.’s paper described the technical aspects, clinical implementation, and potential impact of the MUP, emphasizing its benefits and outlining future research directions.

Socket alignment plays a critical role in various aspects of prosthetic fitting for transtibial amputees, such as comfort, balance, and stability, irrespective of the prosthetic socket fit and components chosen [9] [10]. Conventional prosthesis (CVP) alignment involves three stages: bench, static, and dynamic alignment. Bench and static alignment entail setting the prosthetic socket in a default orientation (e.g., +5° flexion in the sagittal plane and +5° in the frontal plane, Figure 1) while setting the prosthetic foot at a 5–7° external rotation [11,12,13,14]. By contrast, dynamic alignment (DA) allows for further adjustments of up to ±10° in both the frontal and sagittal planes; this process is time-intensive and requires expertise [15]. Unfortunately, dynamic alignment can be biased, as prosthetists must accommodate patients’ perceptions of socket pressure, often resulting in alignment that prioritizes patient’s comfort over gait function and long-term joint health [9,11,12,15,16]. This patient-driven DA may lead to a misaligned prosthesis, deviating from the clinician’s intended alignment [13,15]. Misalignment can cause imbalanced loading between the intact and affected limbs, which may result in joint degradation or osteoarthritis (OA), and eventually reduce patients’ quality of life as the OA disease progresses in the long term [15,16].

Despite clinical efforts to optimize dynamic alignment (DA) for gait symmetry, studies on transtibial gait with socket DAs have not shown significant changes in gait symmetry when DA is adjusted within the clinically accepted 10-degree (i.e., +/−5 degree) tolerance [9,12,14,15,16]. If symmetry remains unchanged within this DA tolerance, and if comfort is not lessened, adopting a standard alignment, as with the MUP “neutral” alignment concept, could benefit current and future amputees. This approach not only reduces the cost of prosthetic services in developing countries, but also potentially eliminates the need for the DA process in current fitting procedures, simplifying the fitting process, reducing fitting time, and significantly lowering prosthetic costs.

In contrast with the CVP, the MUP employs a patella weight-bearing socket design with a default axial alignment. Consequently, the MUP endeavors to maintain alignment with the femoral and tibial bone longitudinal axes (see Figure 1). The longitudinal axis of the socket aligns with the pylon axis in relation to the prosthetic foot, and this neutral alignment remains constant throughout prosthetic fitting. The MUP-fitting procedures are standardized, providing a practical method for training local technicians within a short period (i.e., 3–4 weeks) so that they can successfully perform fittings. Efforts to reduce personnel costs for prosthetic fitting have demonstrated the potential and competency of the MUP concept compared to conventional custom-made prosthetic devices. Additionally, MUP socket technology, pre-made using low-cost PP, further contributes to cost reduction [7,8,17].

MUPs for transtibial amputees, costing only USD 150, have been fitted for over 18,000 amputees in Vietnam and Cambodia since 2009 [7,8]. These countries have the highest number of amputees per capita due to landmines and explosive devices remaining from past conflicts [17]. Since 2009, MUP technology has been sponsored and distributed free of charge in several rural regions of Vietnam (e.g., Ben Tre, An Giang, Dong Thap, Kien Giang, Quang Tri, Thai Nguyen) and Cambodia (Preah Vihear) through Mercer On Mission (MOM), a service-learning program operated by Mercer University in Macon, Georgia, USA [7]. The MOM program incorporates service learning and cultural exchange for motivated undergraduate and graduate students, regardless of their backgrounds and majors. The MUP concept of universal design and standard alignment has proven effective and transferable to train Mercer students.

Under ISPO certification, VIETCOT (Vietnamese Training Centre for Orthopedic Technologies) and ICRC operate clinics, train prosthetic technologists, and provide prosthetic healthcare in Vietnam and Cambodia. However, demand for low-cost prosthetic devices continues to exceed supply and remains a challenge due to a lack of trained technologists. The MUPs with “neutral alignment” were developed as a standardized alternative to conventional custom-made prostheses (CVP), aiming to provide a cost-effective and accessible solution for transtibial amputees, particularly in resource-limited countries. To achieve ISPO or ICRC recognition and certification for MUP technology, extensive research is necessary to demonstrate the biomechanical effectiveness of MUPs in amputees. This paper aims to quantitatively compare the gait characteristics of transtibial amputees using both conventional custom-made prostheses (CVP) and MUPs. It is hypothesized that temporal–spatial and kinematic gait parameters will differ between the sound limb and prosthetic limb in transtibial amputees using both conventional custom-made (CVP) and Mercer Universal (MUP) prostheses, and there will be small differences between MUP and CVP within intact and prosthetic limbs. In addition, gait symmetry is expected to show small differences between MUP and CVP for both kinematics and temporal–spatial parameters, suggesting that MUPs could perform similarly to the CVP.

2. Materials and Methods

2.1. Participants

For this study, 20 Vietnamese transtibial amputees (19 males and 1 female; mean age: 60 ± 8 years) were recruited for this study through the Mercer On Mission (MOM) Vietnam prosthetic program at Mercer University (see

Table 1. Participant demographics and prosthetic feet mass. In the study setting, participants used a solid SACH foot design for the current conventional device, while the Mercer Universal Prostheses utilized the patented C-shaped design (US patent number: US20110320010A1).

Sex	19 Male/1 Female
Age	60 ± 8 year
Height	1.60 ± 0.07 m
Prosthetic Mass	Conventional Prosthesis (CVP)	1.55 ± 0.35 kg
	Mercer Universal Prosthesis (MUP)	1.31 ± 0.16 kg

: Participant demographics, prosthetic feet mass). This sample size was informed by previous research on socket perturbations in the DA process, where studies typically included 9 to 14 participants [11,12,13,14,16], such as that by Pinzur et al. (1995), who examined prosthetic alignment in 14 experienced transtibial prosthetic users through a U.S. Veterans Affairs study. Pinzur’s team found significant differences in stance phase duration and peak vertical ground reaction force when comparing misaligned prosthetic sockets (adjusted to 10 degrees of varus, valgus, flexion, or extension) to a neutral baseline alignment. Building on this precedent, a sample size of 20 participants (n = 20) was considered appropriate for this study. Inclusion criteria required participants to (1) have used a prosthetic device with a SACH foot design for over one year, (2) not use any assistive devices for walking (i.e., classified as prosthetic K-2 and K-3 levels), (3) have a residual limb length between 12 and 15 cm, and (4) have no major complications, infected wounds, or localized pain on the residual limb. Each patient arrived with their existing CVP, which was custom-made and fitted using technology from ICRC (Figure 2). Participants received both written and oral information regarding experimental procedures and potential risks before providing their informed consent.

2.2. Gait Assessment Protocol

Prior to commencing gait assessment, all subjects were fitted with the Mercer Universal Prosthesis (MUP), and they were instructed by the in-house physician to obtain a quality-of-life assessment using the SF-36 medical survey. The transtibial MUP configuration employs a “patella socket weight bearing” principle. The transtibial MUP socket is prefabricated from a standardized mold that has been refined through extensive field development in Vietnam since 2009. Available in predetermined sizes (small, medium, and large) and lengths (short, medium, and long), these sockets are manufactured using injection molding technology with cost-effective polypropylene plastic. A socket size closely matching the patient’s residual limb dimensions is selected during fitting. Additionally, a prefabricated polyurethane liner is inserted into the prosthetic socket to furnish a softer, more accommodating inner lining for the residual limb. Expert prosthetic fitters from MOM Vietnam (Mercer University, GA, USA) performed a sequence of adjustments during socket fitting to ensure patient comfort. These adjustments may entail thickening specific areas of the prosthesis for enhanced padding, thermally “bubbling” and expanding the rigid wall of the prosthesis to alleviate pressure on pressure-sensitive regions of bony prominence on the residual limb, and widening the socket with a posterior V-cut for more substantial adjustments to the rigid socket wall. Unlike the CVP, which relies on individualized socket fabrication and DA adjustments that can introduce variability in limb length and gait performance, the MUP incorporates a universal design with controlled fitting to maintain consistent limb length and neutral alignment, neglecting DA adjustments. Thus, the entire MUP-fitting process was completed within 3–5 h.

2.3. Instrumentation and Gait Analysis

Within a week after fitting the MUP device, all participants were asked to return to conduct a 3D gait assessment. The 3D gait data were collected using a Noraxon Ultium™ Portable Lab system (Noraxon Inc., Scottsdale, AZ, USA), which is a system that has been validated against the “golden method” optical motion capture (OMC) method in the existing literature founded in Berner and Park’s studies [18,19] for accurately measuring kinematic joint angles in the sagittal plane, including the hip, knee, and ankle. Notably, this gait assessment method did not incorporate ground reaction forces (GRFs) due to the mobile setup conducted outside of a laboratory environment in Vietnam. Additionally, surface electromyography (EMG) data were collected for the knee flexor and extensor muscles of both the prosthetic and intact limbs in participants wearing the MUP and CVP; however, EMG analysis will be addressed in a separate study evaluating the MUP’s performance over a one-year longitudinal assessment. The Ultium Motion system involves the use of 8 IMU sensors for the bilateral lower extremities, including the lower thoracic, pelvis, bilateral thighs, shanks, and foot, recorded at 200 Hz (Figure 3). Each subject walked at self-selected speed across a 12 m walkway in two prosthesis conditions, namely wearing the CVP and wearing the MUP, and the order was randomized among subjects.

Joint angles were calculated using an IMU-based body model in MyoResearch 3.18. Advanced functional walking calibration in this software, employing accelerometer-based techniques to correct course misalignments, was routinely applied for each subject to mitigate magnetic distortion prior to recording. Furthermore, the software utilized a Kalman filter to optimize IMU-based data [18]. Real-time course stabilization and correction were applied to remove low-frequency sensor drift and stabilize course angles. The joint angle decomposition sequences in this software followed the recommendations of the International Society of Biomechanics (ISB) [19,20]. The lower-extremity sagittal joint angles analyzed in MR 3.18 were exported to MATLAB (R2021a, MathWorks, Natick, MA, USA).

2.4. Data Processing

Kinematic data from subjects were batch-analyzed using a developed pipeline in MATLAB (R2021a) (as shown in Figure 4). For each stride, the local max, min and range of motion (ROM) (degrees) for the sagittal joint angles (i.e., hip, knee and ankle) were computed. Kinematic data were averaged separately for each subject’s intact and prosthetic limb. Spatial–temporal outcome measures were also extracted, including gait speed (m/s), cadence (step/min), stance and swing duration (sec), stride length (m), and step length (m).

2.5. Statistical Analysis

All mean values of the processed kinematics and spatial–temporal gait for the 20 subjects’ intact and prosthetic limb were compared between the CVP and MUP groups using JASP software [23]. Pairwise t-tests (p < 0.05) were used to identify between-group differences in demographic and anthropometric data.

Repeated measures ANOVA with 2 factors (Limb * Device), using the subject as a random variable, were used to determine limb effects (intact, prosthetic) and device effects (CVP, MUP) on temporal–spatial and sagittal kinematic measures within the TTA population. When significant Limb * Device interactions were detected, pairwise post hoc analyses were conducted using a Bonferroni–Holm adjusted significance (α) value set to 0.05/4 = 0.0167 [24].

To test the hypothesis of gait symmetry of the TTA wearing CVPs and MUPs, all kinematics and temporal–spatial gait parameters were normalized using a gait symmetry index (GSI) [25,26,27]. Paired t-tests were used to compare SI of the temporal–spatial and kinematic data between subjects’ intact and prosthetic limbs for MUP and CVP groups, and calculated Cohen’s d value with benchmarks of 0.2 (small), 0.5 (medium), and 0.8 (large) for effect size magnitude. The GSI score was calculated using the formula below:

$$\mathrm{G}\mathrm{S}\mathrm{I}=\left({\displaystyle \frac{X_{prosthetic}-X_{intact}}{0.5\left(X_{prosthetic}+\mathrm{ }{X}_{intact}\right)}}\right)\mathrm{ }\times \mathrm{ }100\mathrm{\%}$$

where

X_Prosthetic is the value of the gait parameter for the prosthetic limb.

X_intact is the value of the gait parameter for the intact limb.

• The GSI score is used to quantify similarity of movements between limbs, where

  ❖

  A GSI of 0% indicates perfect symmetry (i.e., no difference between prosthetic and intact limbs).

  ❖

  A GSI > 0% indicates asymmetry, where $X_{prosthetic}>X_{intact}$.

  ❖

  A GSI < 0% indicates asymmetry, where $X_{prosthetic}<X_{intact}$.

3. Results

3.1. Demographics and Temporal–Spatial Parameters

On average, the MUP group (1.31 ± 0.16 kg) was significantly lighter than the CVP group (1.55 ± 0.35 kg, Table 1, p = 0.01). Spatiotemporal results detected significant differences in stance and swing time, for both MUPs and CVPs, and between intact and prosthetic limbs. In the MUP group, the prosthetic limb’s stance time was reduced by 50 ms (p < 0.001), and the CVP’s by 67 ms (p < 0.001), both compared to their respective intact limbs. This consistent reduction across both devices suggests an inherent asymmetry in gait dynamics, where amputees spend less time bearing weight on the prosthetic limb, likely reflecting a compensatory strategy to minimize discomfort, instability, or perceived weakness in the prosthetic side. However, no differences were detected in speed, stride length, step length, or stride time (

Table 2. Spatiotemporal results for transtibial amputees walking with conventional prosthesis (CVP) and Mercer Universal Prosthesis (MUP).

	Descriptive Statistics: Mean (Std)				Two-Factor Repeated ANOVA
	MUP		CVP		p-Value
	Prosthetic	Intact	Prosthetic	Intact	Device * Limb	Device	Limb
Stride Time (s)	1.26 (0.15)	1.26 (0.15)	1.24 (0.13)	1.24 (0.13)	0.171	0.086	0.641
Stance Time (s)	0.65 (0.12)	0.61 (0.05)	0.61 (0.09)	0.63 (0.06)	0.287	<0.001 **	<0.001 **
Swing Time (s)	0.71 (0.14)	0.55 (0.03)	0.67 (0.12)	0.57 (0.03)	0.785	0.003 **	<0.001 **
Speed (m/s)	0.68 (0.19)	0.69 (0.20)	0.72 (0.13)	0.72 (0.13)	0.590	0.378	0.276
Stride Length (m)	0.86 (0.21)	0.86 (0.20)	0.87 (0.10)	0.89 (0.10)	0.419	0.696	0.348
Step Length (m)	0.44 (0.12)	0.45 (0.14)	0.42 (0.10)	0.48 (0.09)	0.120	0.752	0.265

Note: * = p < 0.05 and ** = p < 0.001.

).

3.2. Kinematics

Regarding main effects, the device factor revealed significant differences between the MUP and CVP devices in several kinematic outcomes: hip extension (p = 0.013), ankle plantarflexion (p = 0.011), hip flexion (p = 0.039), knee flexion (p = 0.016), and knee ROM (p = 0.024) (see

Table 3. Kinematic results for transtibial amputees walking with conventional (CVP) and Mercer Universal (MUP) prostheses.

	Descriptive Statistics: Mean (Std)				Two-Factor Repeated ANOVA
	MUP		CVP		p-Value
	Prosthetic	Intact	Prosthetic	Intact	Device * Limb	Device	Limb
Hip Extension (°)	−18.6 (4.1)	−17.1(3.)	−16.1 (3.5)	−14.8 (4.7)	0.786	0.013 *	0.092
Knee Extension (°)	−0.4 (6.2)	−1.4(4.5)	−3.2 (5.8)	−1.5 (7.2)	0.459	0.301	0.230
Ankle Plantarflex (°)	0.3 (2.2)	−17.6 (7.7)	1.9 (2.9)	−15.1 (7.0)	0.435	0.011 *	<0.001 **
Hip Flexion (°)	26.9 (4.8)	21.2 (4.9)	26.03 (4.5)	25.1 (6.1)	0.013	0.039 *	0.008
Knee Flexion (°)	68.4 (9.3)	62.0(5.8)	63.2 (9.3)	61.6 (6.3)	0.025	0.016 *	0.067
Ankle Dorsiflex (°)	9.3 (4.1)	12.8 (4.4)	10.1 (6.2)	13.7 (4.4)	0.929	0.281	0.004 *
Hip ROM (°)	45.5 (5.3)	38.2 (4.1)	42.1 (6.1)	39.8 (4.3)	0.014 *	0.124	<0.001 **
Knee ROM (°)	68.8 (10.6)	63.1 (5.6)	66.3 (8.6)	63.0 (6.5)	0.033 *	0.024 *	0.168
Ankle ROM (°)	8.9 (3.5)	30.3 (6.4)	8.2 (7.7)	28.8 (5.7)	0.675	0.305	<0.001 **

Note: * = p < 0.05 and ** = p < 0.001.

). Similarly, significant differences in temporal outcomes were observed between the devices, such as stance and swing duration (p < 0.001) (see Table 2). Post hoc analysis indicated that participants using the MUP devices presented a significant increased knee flexion (p = 0.006) and knee ROM (p = 0.011) in the prosthetic limb when compared with the CVP group. On another pairwise comparison, the sound limbs in participants using MUPs showed significant differences in hip flexion (p = 0.005) when compared with the sound limbs of participants with CVPs. Relative to the prosthetic limbs of participants using CVPs, the MUP group showed a significant increase of 5.7° for knee flexion and 4.5° for knee ROM. Hip flexion was significantly reduced in the sound limbs of participants using MUPs compared with the sound limbs of participants with CVPs (about 3.8°, p = 0.005). In addition, temporal analysis showed that the prosthetic limbs of participants with MUPs resulted in a significantly longer stance time (by approximately 47 ms) and a shorter swing time (by approximately 23 ms) compared to the prosthetic limbs of participants with CVPs (p < 0.001). The sound limbs of participants using MUPs were found to be significantly reduced in terms of swing time (about 19 ms) compared with the sound limbs of participants using CVPs (p < 0.001).

Regarding the effects of the limb factor across devices, significant differences were observed in kinematic outcomes such as ankle plantarflexion/dorsiflexion, hip/knee flexion, and hip/ankle ROM between the intact and prosthetic limbs (p < 0.05) (see Table 3). Similarly, temporal outcomes, including stance and swing time, also exhibited significant differences between the prosthetic and intact limbs (p < 0.001). Post hoc analysis indicated that the prosthetic limb significantly increased in hip flexion (approximately 5.7°, p = 0.002) and hip range of motion (about 7.3°, p < 0.001) compared to the intact limb when participants walked with the MUP devices. Ankle kinematics, including ankle plantarflexion/dorsiflexion and range of motion, showed significant differences between the intact and prosthetic limbs in both the MUP and CVP groups. These differences were primarily due to the natural ankle motion in the intact limb versus the minimal motion in the prosthetic limb, which used a solid foot. The difference in ankle range of motion between the intact and prosthetic limb was 21.4° (p < 0.001) for the MUP device, while the CVP device showed a difference of 20.8° (p < 0.001). In the temporal gait measurements, stance time was significantly shorter in the prosthetic limb compared to the intact limb for both devices. Specifically, the prosthetic limb in participants using MUPs showed a reduction of 50 ms (p < 0.001), while the prosthetic limb in participants using CVPs exhibited a 67 ms decrease (p < 0.001). Conversely, swing time was significantly longer in the prosthetic limb, with the prosthetic limb of participants using MUPs showing an increase of 59 ms (p < 0.001), and the prosthetic limb of participants using CVPs showing a 63 ms increase (p < 0.001) compared to the intact limb.

A two-factor repeated measures ANOVA revealed significant interaction effects between the device and limb for hip and knee kinematics (Table 3). Hip and knee kinematic measurements differed between the MUP and CVP groups and within intact and prosthetic limbs (see Figure 5). Post hoc analysis indicated no significant differences in hip and knee flexion angles between the intact and prosthetic limbs in the CVP group. However, the prosthetic limbs in the MUP group exhibited significant peak hip flexion that was approximately 5.7° higher than the intact limb group (p = 0.002); moreover, knee flexion was found to be approximately 6.6° higher than for the intact limbs in the MUP group. As a result, the prosthetic limbs in the MUP group demonstrated significantly higher hip ROM (approximately 7.3°, p < 0.001), and knee ROM was about 5° higher than for the intact limbs. In contrast, no significant interactions between limbs and devices were found for the temporal and spatial gait parameters (see Table 2).

3.3. Gait Symmetry Index (GSI)

In amputees, the GSI score is used to evaluate how well a prosthetic limb mimics the natural movement of the intact limb. Temporal and spatial measurements did not show significant differences, indicating a similar stride time, stride length, and step length between the devices (see

Table 4. Result of gait symmetry index (GSI)—temporal and spatial.

	Mean GSI (SEM)		Paired t-Test p < 0.05	Effect Size
	MUP	CVP		Cohen’s d	SE
Stride Duration (s)	0.5 (0.2)	−0.2 (0.4)	0.120	0.364	0.309
Stance Duration (s)	−7.8 (1.4)	−10.2 (1.8)	0.185	0.308	0.249
Swing Duration (s)	9.4 (1.6)	10.2 (1.8)	0.838	−0.046	0.221
Speed (m/s)	−1.2 (1.3)	−0.7 (0.8)	0.747	−0.073	0.284
Stride Length (m)	−0.3 (1.4)	−1.6 (0.9)	0.473	0.164	0.333
Step Length (m)	−0.8 (8.4)	−14.9 (8.0)	0.135	0.349	0.255

Note: %GSI = 0% indicates perfect symmetry. Negative or positive %GSI indicates that the prosthetic and intact limb are functioning with asymmetry.

). However, significant differences were observed in the symmetry index for discrete kinematic outcomes. Hip flexion with the CVP demonstrated 19% greater symmetry compared to the MUP (p = 0.012), with a Cohen’s d of 0.619, indicating a moderate-to-large effect. Knee flexion with the CVP showed 8% more symmetry than with the MUP device (p = 0.026), with a Cohen’s d of 0.541, reflecting a moderate effect highlighting both hip flexion and knee flexion superior symmetry with the CVP. Conversely, ankle plantarflexion with the MUP exhibited 24.4% greater symmetry than with the CVP device (p = 0.013), with a Cohen’s d of 0.610, a moderate-to-large effect highlighting superior symmetry with the MUP, potentially enhancing propulsion mechanics. Range of motion (ROM) outcomes further underscore these trends. Hip ROM symmetry was significantly greater with the MUP (p = 0.014), with a Cohen’s d of 0.606 (moderate-to-large effect), while knee ROM showed a moderate effect (Cohen’s d = 0.482, p = 0.044) favoring the MUP (see

Table 5. Result of gait symmetry index (GSI)–kinematic (degrees).

	Mean GSI (SEM)		Paired t-Test p < 0.05	Effect Size
	MUP	CVP		Cohen’s d	SE
Hip Extension (°)	9.1 (4.9)	11.1 (7.9)	0.780	−0.063	0.228
Knee Extension (°)	18.7 (19.4)	25.1 (23.0)	0.806	−0.056	0.275
Ankle Plantarflexion (°)	−163.4 (46.0)	−267.9 (61.3)	0.013 *	0.610	0.161
Hip Flexion (°)	24.2 (5.5)	5.2 (5.)	0.012 *	0.619	0.293
Knee Flexion (°)	9.7 (2.8)	1.7 (4.2)	0.026 *	0.541	0.209
Ankle Dorsiflexion (°)	−32.9 (10.9)	−36.4 (12.2)	0.738	0.076	0.198
Hip ROM (°)	17.1 (2.9)	5.1 (3.5)	0.014 *	0.606	0.333
Knee ROM (°)	7.8 (3.6)	0.6 (4.0)	0.044 *	0.482	0.209
Ankle ROM (°)	−109.2 (6.5)	−120.4 (10.3)	0.323	0.227	0.284

).

4. Discussion

The Mercer Universal Prostheses (MUP) concept involves a standardized socket alignment and a universal pre-made socket design. This study aimed to investigate the immediate effects of MUPs on gait symmetry, focusing on kinematic, temporal, and spatial gait parameters. Sagittal joint angles (hip, knee, and ankle) were the primary focus due to their consistency and minimal error in IMU-based kinematic measurements [28]. As expected, kinematic outcomes showed some degree of difference between the intact and prosthetic limbs in transtibial amputees who were using both CVPs and MUPs immediately post fitting. Both the temporal and spatial outcomes remained consistent between the two devices. Gait symmetry also showed small differences between MUPs and CVPs.

Our analysis revealed significant alterations in kinematics, particularly for prosthetic limbs, immediately after fitting with the MUP. Participants exhibited greater knee flexion (about 5.7°) and hip flexion (about 6.6°) in the prosthetic limb compared to the intact limb (refer to Figure 6). This resulted in an increased hip and knee range of motion (3.4° and 2.5°, respectively) in the MUP group, with no significant differences in the CVP group. These findings suggest that the MUP device influences biomechanics, and specifically inter-limb symmetry, differently compared to the CVP device, particularly in the hip and knee joint angles. Immediately after fitting, the MUP device appears to enhance joint mobility, especially in the prosthetic limb, leading to greater hip/knee flexion and hip/knee ROM, which may contribute to a more balanced and efficient gait. This increased mobility could reduce compensatory movements and the risk of overuse injuries in the intact limb. Clinically speaking, these results suggest that the MUP may be more effective in promoting natural gait patterns, which could improve long-term mobility and quality of life for amputees. Additionally, the clinical implication of the MUP as an alternative low-cost prosthetic device could significantly reduce the cost of prosthetic healthcare in low-income countries, making it a viable option for improving accessibility and affordability. Future research should explore the longitudinal effects of these kinematic adaptations on gait function, stability, and comfort.

To contextualize these findings, the hip and knee range of motion (ROM) from the CVP device in this study were compared to those reported in Laing’s 2018 study on Vietnamese amputees using conventional prostheses. Laing et al. reported a hip ROM of 37° in the sound limb and 36° in the prosthetic limb, while this study found hip ROMs of about 40° in the intact limb and 42° in the prosthetic limb. Additionally, Laing’s study also reported a knee ROM of 70.7° in the intact limb and 61.4° in the prosthetic limb, whereas this study reported approximately 63° in the intact limb and 66.3° in the prosthetic limb. The cadence in this study (approximately 75 steps/min) was lower than that in Laing’s study (about 96 steps/min), which may account for some of the discrepancies in kinematic measurements [29,30,31]. This study acknowledges the absence of existing literature directly comparing the gait kinematics of dynamically aligned prostheses to the “neutral alignment” approach utilized in the MUP-fitting context, highlighting a gap that warrants further investigation.

The kinematic measurements in this study were obtained using Noraxon IMU technology, which was validated in healthy control subjects by Berner et al. (2020) and Park et al. (2021), demonstrating that sagittal joint angles can be accurately compared with those obtained using optical motion capture (OMC) systems, with hip angle differences within ±1° [18,19]. However, Park’s study noted that knee and ankle joint angles may be overestimated and underestimated, respectively, during the swing phase, which is an important consideration for clinical practice [17]. In this study, the differences in hip and knee kinematics between the MUP and CVP devices in both the prosthetic and sound limbs were minimal; for example, peak hip flexion differed by approximately 1°, hip ROM differed by 3.5°, and differences in peak knee flexion and ROM were approximately 5.6° and 4.5°, respectively. In the sound limb, the MUP group presented with a slightly reduced both hip flexion and hip ROM (i.e., about 3.8° for hip flexion and 1.5° for hip ROM). The knee kinematic differences in the sound limb between the MUP and CVP groups were found to be negligible (<1°). In addition, the differences between intact and prosthetic limbs were increased, at approximately 5.7° for hip flexion and 7.3° for hip ROM in the MUP group, while the CVP group showed slight differences in hip flexion (<1°), at 2.3° for hip ROM. Similarly, knee kinematics were found to have slight differences between the intact and prosthetic limbs in the CVP group (1.5° for knee flexion and <1° for knee ROM), while these differences were found to be higher in the MUP group (about 6.6° for knee flexion and 5° for knee ROM) (see Figure 6). These differences in hip and knee kinematics between limbs and devices, as measured by the Noraxon Utium IMU system, were considered to be within the clinically acceptable error margin of 5–7° when comparing IMU versus gold-standard OMC measurements [19]. Thus, kinematics were similar between CVP and MUP devices within both prosthetic and intact limbs, relative to measurement error.

The increase in maximum hip and knee ROM angles immediately after fitting the prosthetic limb in participants using MUPs could be attributed to the MUP’s lighter weight compared to the CVP in this study [5]. According to Bateni’s 2004 study, changing prosthetic components from steel to titanium helped to reduce the physiological cost index (PCI), increase the amputee’s relative speed, and emphasize kinematic changes [32]. These findings suggest that the type of prosthesis and walking speed are crucial factors influencing the ROM in transtibial amputees, highlighting the importance of considering these variables in future prosthetic design and fitting processes. Nevertheless, in the present study, gait speed did not differ significantly between the MUP and CVP devices.

No significant interactions were observed in temporal and spatial gait parameters immediately after fitting the MUP device. However, stance and swing times were significantly affected by both the device and limb factors. Within-subject analysis revealed that the prosthetic limb had a significantly shorter stance phase and a prolonged swing phase, while the intact limb exhibited the opposite pattern. This reflects typical amputee gait characteristics, where the intact limb is favored for stability [23,29,30,31,32]. The MUP device led to slight improvements in balancing stance and swing times between the intact and prosthetic limbs. Notably, changes in the prosthetic limbs of participants using MUPs caused a reduction in stance and swing times in the intact limb, suggesting patient acceptance of the MUP device immediately after fitting. However, these changes could also indicate that the patients did not feel fully confident so soon after receiving the MUP device.

Gait symmetry is often associated with reduced energy expenditure, lower risk of overuse injuries, and better overall mobility. The differences in symmetry observed between prosthetic and intact limbs across the devices could therefore have important implications for long-term outcomes in prosthetic users. The lack of significant differences in temporal and spatial measurements (such as speed, stride time, stride length, and step length) suggests that both the MUP and CVP devices allow for similar overall gait mechanics. The prosthetic users in this study maintained consistent walking temporal–spatial gait parameters regardless of which device they used. The CVP device was associated with better symmetry between intact and prosthetic limbs for both hip and knee flexion compared to the MUP. Specifically, hip flexion symmetry was 19% better, and knee flexion symmetry was 8% better when using the CVP device. This suggests that the CVP device may promote more balanced movement between the prosthetic and intact limbs at these joints, which could be important for maintaining a stable and efficient gait. In contrast, the MUP device showed 24.4% greater symmetry in ankle plantarflexion compared to the CVP device. This implies that the MUP device might be better at replicating the natural movement of the ankle, leading to a more balanced motion in this specific joint. The C-shaped design of MUPs (see Figure 2) aims to mimic the natural ankle motion by enhancing dorsiflexion/plantarflexion and improving overall ankle ROM [5,6]; this could allow more natural ankle motion, which would explain the difference in the ankle joint motion ROM compared to the CVP’s SACH foot [7,8,33]. Collectively, these findings suggest that each device has specific strengths. The CVP device appears to promote better symmetry in hip and knee movements since it is the participant’s current, habitual device. On the other hand, the MUP device exhibits less symmetry at hip and knee joints immediately after fitting, but seems to enhance symmetry between intact and prosthetic limbs at the ankle, thus providing more natural ankle movement. This study also acknowledges the immediate effects of MUPs on experienced prosthetic users. Importantly, the GSI has some limitations. Human gait is complex, and the GSI is just a single number that may not capture all aspects of gait symmetry. Moreover, the GSI in amputees during training sessions can be influenced by various factors such as walking speed, fatigue, or the environment in which the gait is assessed [25]; therefore, it should be interpreted with an understanding of its limitations and in the context of the broader assessment of gait.

5. Conclusions and Suggestions

Overall, the results imply that, immediately post fitting, the MUP device was shown to be similar to the CVP device in terms of both kinematic and temporospatial parameters, and these outcomes were found to be within an acceptable margin of error. The MUPs initially alter the prosthetic user’s joint kinematics, particularly in the prosthetic limb, which could potentially lead to better outcomes. However, the impact on overall gait efficiency and user confidence might require more time and adaptation. Zhang et al. (2019) emphasized that testing new prosthetic interventions within a few hours can yield unreliable outcomes, as prosthetic gait compensation requires longer accommodation periods [34]. To achieve accurate gait assessments and minimize deviations in gait variables, evaluations should be conducted within 10 weeks to 3 months after fitting [25,34,35,36,37,38]. Therefore, it is recommended that this study be replicated with a longer training and rehabilitation period (ideally 3 months after fitting) to precisely observe the effect of the MUP design on the kinematic and temporal–spatial gait parameters on the gait symmetry of the transtibial amputees. Quality of life metrics (i.e., SF-36) have also been suggested to investigate long-term training to evaluate both the outcome performance and gait adaptation of MUPs.

6. Patents

Mercer Universal Prostheses—US patent number: US20110320010A1 and US8870968B2.