Evaluating modified socket casting and molding vs. total surface bearing techniques in unilateral transtibial amputation: Effects on rotational control, socket fit satisfaction, and interface pressure

Abstract

Objective(s):

To achieve functional and comfortable prostheses, great attention must be paid to socket design, as it represents a critical interface between the residual limb and the prosthetic limb. This study aimed to evaluate the outcomes of unilateral transtibial amputees utilizing modified socket casting/molding technique in comparison to total surface bearing (TSB) socket.

Methods:

Eight individuals with unilateral transtibial amputation participated in this cross-over design trial. Each participant used both the TSB and modified socket casting/molding designs. Four weeks after the utilization of each socket, the assessment was conducted on socket fit satisfaction (measured through the Comprehensive Lower Limb Amputee Socket Survey), socket rotation (evaluated using a standard meter), socket/liner interface pressure (monitored with 10 force-sensitive resistors), and the amputees' decision regarding the retention of one of the sockets after the trials (gauged via a numerical rating scale).

Results:

The amputees experienced greater stability (P = 0.02) and improved lateral rotational control (P = 0.01) during prosthetic gait when using the modified socket, compared with the TSB socket. Local pressure on the anterodistal tibia and the head of the fibula was not statistically significant (P > 0.05) between 2 socket types. The decision of participants to retain the modified socket was significantly higher than that for the TSB socket after the trial period (P = 0.03).

Conclusion:

The modified socket design may offer advantages by providing improved rotational control during walking and reducing local pressure on the anterodistal tibia and fibula head. In addition, it maintains the total surface contact concept.

Introduction

According to the World Health Organization, the frequency of limb amputations is increasing globally.¹ Trauma, vascular diseases, and diabetes are the most common causes of major limb amputations. In the United States, among 1.6 million individuals with amputation, 86% have undergone lower-limb amputations, with transtibial amputations accounting for 28% of this population.¹

After a lower-limb amputation, amputees should initiate a rehabilitation program to achieve the highest possible level of independence. The use of a prosthetic leg can help minimize the adverse effects of disabilities, restore functional activity, and enhance autonomy. A key factor in the success or failure of prosthesis use is the quality of the socket fit, which significantly influences load transmission, prosthesis stability, and control.² Therefore, to attain a functional and comfortable prosthesis, great attention must be paid to socket design.²

The patellar tendon-bearing (PTB) socket was the first proposed load-bearing socket designed to deliberately load the limb in its pressure-tolerant areas. However, applying localized pressures may lead to callus formation, skin abrasions, and breakdown.^3,4 In contrast, the total surface bearing (TSB) socket is designed to reduce localized pressure. This design evolved with the advent of gel liner interfaces, which maximize the load-bearing surface area of the limb by applying uniform pressure.⁴ Amputees using the TSB socket may experience greater comfort during activities, enhance speed and balance, and improved walking symmetry compared with those using the PTB socket.³ Although the TSB socket reduces localized pressures, gel liners are limited in their effectiveness at alleviating localized loads on certain areas of residual limb, such as the anterodistal region of the tibia and the fibular head. It appears that the PTB design is more successful in providing rotational control compared with the TSB design.⁴

The new concept of transtibial amputation socket design, known as the modified socket, aims to combine the advantages of both designs, including localized pressure on the residual limb and socket rotation.⁴ Dickinson et al scanned the residual limbs and their prosthetic sockets of 163 transtibial amputees, which included PTB and TSB sockets. They found that the prosthetists combined the concepts from both socket types, resulting in participants not having purely TSB or PTB design sockets.⁵

To our knowledge, no published research compares the rotational characteristics of prostheses, socket satisfaction, and socket-liner interface pressure distribution between hands-on modified socket designs and PTB or TSB socket designs.^5,6 Existing studies have primarily focused on the rotation of prostheses using various socket suspension types.^7-9 No prior studies have quantitatively assessed direction-specific socket rotation (e.g., medial vs. lateral displacement) using objective tools. Existing literature has largely relied on subjective questionnaires or overall rotational control comparisons between suspension systems, without reporting the magnitude or direction of rotation.^9-11

The aim of this study is to examine the rate of change in prosthetic socket-liner interface pressure, rotational control, and socket fit satisfaction in unilateral transtibial amputees using prosthetic limbs. This will be accomplished by comparing modified socket casting/molding designs with TSB casting/molding design.

Methods

Participants

This trial utilized a cross-over design. All participants were recruited from 3 prosthetic centers in Tehran, Iran. Data collection was performed from July 2021 to August 2022. Participants were invited to take part in this study if they met the following inclusion criteria: age >18 years, a history of unilateral transtibial amputation with a minimum length of 13 cm from the inferior edge of the patella to the distal end of the residual limb, prior successful experience using a silicone liner with a pin/lock system allowing us to isolate the effects of socket design, use of a prosthetic leg for more than 1 year, a K3 activity level with/without a walking aid, stable limb volume without issues in the residual limb, ability to bear pressure on the whole surface of the residual limb, sufficient upper limb strength to don/off the prosthesis, and absence of pain or wound in the residual limb. Participants were excluded if they exhibited any significant, progressive, or nonprogressive neurological, rheumatological, or orthopedic deficits that impaired mobility. In addition, those unable to complete the questionnaire because of cognitive disorders, language barriers were also excluded.

The consent form, approved by the Ethical Committee of the University of Social Welfare and Rehabilitation Sciences (code IR.USWR.REC.1400.075), was read and signed by all participants before their involvement in the study. The sample size was estimated based on parametric assumptions from previous studies reporting interface pressure values, using a paired t test model to detect within-subject differences.¹¹ The power and alpha errors were set at 0.80 and 0.05, respectively. The calculation yielded a total sample size of 8 participants, to which 20% (2 cases) was added to account for potential dropouts, resulting in a final sample size of 10.

The TSB socket

The positive cast mold for the prosthetic socket was fabricated based on the Össur's transtibial hand casting/modification technique. Before casting, participants rolled on the gel liner over the residual limb, which was isolated with a single layer of plastic film to ensure complete coverage of the liner. Bony landmarks, including the patella, fibular head, crest of the tibia, and anterodistal aspect of the tibia, were marked. The circumference of the residual limb was recorded at 2-cm intervals from the patellar tendon to the distal end. Participants were instructed to maintain their knee in a relaxed extension. A 4-ply Plaster of Paris slab was then prepared to encompass the distal sensitive area of the residual limb. The plaster impression was applied in a circumferential figure-of-eight manner. The pressure-tolerant regions included the patellar tendon, pretibial muscles, medial shaft of the tibia, lateral shaft of the fibula, and the posterior compartment. A positive mold was created by filling the negative cast with liquid plaster, which was allowed to dry. After removing the negative cast from the positive model, the volume of the proximal end at the level of the posterior compartment was reduced by 3% to 5%, focusing on the pressure-tolerant areas.¹²

The modified socket

The casting process for the modified socket was similar to that of the TSB socket, with some key differences: (1) participants did not use a liner during casting, (2) the casting involved 2 steps. In the first step, the prosthetist used the 4-ply plaster of Paris slab to pressure on the tibialis anterior muscle and the distal medial flare of the tibia (Figure 1a). Although the cast was remained on to the residual limb, the second step was conducted. During this step, the prosthetist applied pressure to the residual limb similar to the TSB method, but with increased pressure on the patellar tendon compared with the TSB method (Figure 1b). (3) After preparing and molding the positive cast, emphasis was placed on adding flares to bony prominences and reducing plaster on the pressure-tolerant areas (Figure 2). The liner was then rolled onto the positive cast, and a temporary socket was created using 5 inches of fiberglass casting tape under suction. (4) The fiberglass socket was filled with medical liquid plaster for complete the socket preparation.

Figure 1.

(a) First step of casting. The prosthetist used the 4-ply plaster of Paris slab to pressure on the tibialis anterior muscle and the distal medial flare of the tibia. (b) Second step of casting. The prosthetist applied pressure to the residual limb similar to the TSB method, but with increased pressure on the patellar tendon compared with the TSB method.

Figure 2.

Adding flares to bony prominences and reducing plaster on the pressure-tolerant areas. From the left to the right, the pink colors showed the adding flare on the anterior, posterior, medial, and lateral sides, respectively.

Although both socket designs aimed to maintain total surface contact, the modified design incorporated distinct biomechanical and fabrication modifications compared with the TSB design. Specifically, during the casting process, the modified technique involved manual pressure application over key anatomical landmarks, including the tibialis anterior, distal medial tibial flare, and patellar tendon—maneuvers that were not part of the TSB protocol. Furthermore, in the positive mold modification stage, the modified design featured anatomical flaring around bony prominences to accommodate pressure-sensitive regions, whereas the TSB socket relied exclusively on uniform volume reduction in pressure-tolerant areas. These targeted modifications in the modified socket approach were intended to improve rotational stability and optimize load distribution across the limb-socket interface.

Socket preparation

A 10-mm polypropylene plastic socket was fitted for the patient to check its comfort and fit during gait. Subsequently, the final socket was prepared utilizing a vacuum frame and suction technique. The prosthetist secured the socket on the bench and provided dynamic alignment during the fitting process.

Study protocol

All participants used both modified and TSB sockets, with half receiving the modified design socket followed by the TSB socket, and the other half receiving the TSB socket followed by the modified design. Participants were randomly assigned to one of the sequences by selecting 1 of 2 sealed envelopes. Each participant used a 6-mm (mm) thick gel liner with a pin/lock suspension system. Each socket was used for 4 weeks, with evaluations conducted immediately after this period. The participants began using the second socket on the same day they discontinued use of the first socket. Outcomes after the first socket usage were measured before fitting the second socket. The washout period between socket changes was no longer than 1 h, as participants needed their prostheses for daily activities. No outcomes were measured immediately after switching sockets. All assessments were conducted by a single assessor who was blinded to group allocation. All steps of prosthetic fitting in this research included inclusive criteria considering, stump casting, positive cast molding, socket fabrication and fitting, bench and dynamic adjustment of prosthetic were done by an expert prosthetist who had more than 15 years of experience in this field. The protocol of this study was registered in the Iranian Registry of Clinical Trials (IRCT20211019052810N1).

Measurements

Socket fit satisfaction

Socket fit satisfaction was investigated using the Persian version of the Comprehensive Lower Limb Amputee Socket Survey (CLASS) questionnaire.¹³ This 15-items self-reported questionnaire consists of 4 subscales: stability, suspension, comfort, and appearance. The stability, suspension, and comfort subscales consist of 4 items each, evaluating the prosthetic socket fit during sitting, standing, walking, and navigating stairs. The appearance subscale includes 3 items that assess the participants’ satisfaction with the socket’s appearance while standing, sitting, and wearing tight pants. For each item, participants rated their perception of the prosthetic socket fit on a 4-point ordinal response scale, where 1 indicated the worst condition and 4 indicated the best. A score of 0 signified that the activity was not applicable to the amputee. The CLASS score for each subscale was expressed on a 0%–100% scale, with higher scores indicating greater satisfaction.¹³

Socket selection

The satisfaction level regarding the selection and retention of a socket after trial was measured using an 11-point Numerical Rating Scale, ranging from 0 to 10, where 0 represented no satisfaction and 10 indicated maximum satisfaction with the socket.

Socket-liner interface pressure

The socket-liner interface pressure was assessed using 10 Force Sensitive Resistors (No. 402, Interlink Electronics, USA) positioned over patellar tendon (sensor 1), middle of the anterior side of the tibia (sensor 2), anterodistal tibia (sensor 3), distal medial tibial flare (sensor 4), head of fibula (sensor 5), distal fibula (sensor 6), soft tissue on the medial side of the tibia (sensor 7), soft tissue on the lateral side of the tibia (sensor 8), middle of the posterior side of the residual limb (sensor 9), and distal posterior side of the residual limb (sensor 10). The sensors were secured in place using thin paper tape. A thin nylon socks was used over the sensors to ensure they did not move while the socket was worn. Interface pressure was measured twice in 3 static standing positions and during walking. We reported the mean peak pressure from the 2 assessments for each sensor. The 3 static positions included bilateral standing (50% of total weight on each leg), bilateral standing with the prosthetic side bearing 25% of total weight, and full weight bearing on the prosthetic side. To assess interface pressure during walking, participants traversed a 3-m direct walkway without barriers.¹⁴ The Force Sensitive Resistor was calibrated by applying a range of force values and measuring the corresponding resistance values.^14,15

Socket rotation

For assessing rotational stability of the socket, the anterior-upper edge of the socket was marked with a 1-mm thick marker. A 100-mm standard tape measure was placed on the front of the gel liner (over the anterior superior socket). The marked section was set at 50 mm using a laser line (Figure 3). We instructed the participants to rotate the residual limb within the socket medially and laterally while maintaining their prosthetic foot in place. Participants tried to have bilateral standing (50% of total weight on each leg) during the test. The rotation was quantified after the participants walked 3 m along a direct walkway without barriers. To determine the inter- and intrarater reliability of the measurements, each socket's rotation was recorded twice by 2 assessors once at the beginning and once at the end of all tests conducted on the same day. The mean of these measurements was reported as the amount of rotation for both static medial and lateral rotations, as well as after walking 3 m.

Figure 3.

Socket rotation was assessed by using a 100-mm standard tape meter was placed on the front of the gel liner (over the anterior superior socket).

Statistical analyses

SPSS version 26.0 was employed for data analysis, with a significance level set at P < 0.05. Participants’ demographic information was collected and reported as means, standard deviations, numbers, and percentages. Because of the small sample and the non-normal distribution of our data, the Wilcoxon signed-rank test was ultimately used to compare the fit satisfaction, rotation, and preference between the TSB and the modified design sockets. Cronbach alpha was calculated to assess inter/intrarater reliability concerning the rotational assessment method.

Results

Among the 10 participants included in the study, 2 participants dropped out because of relocation to another town. Eight individuals with mean age: 44.13 ± 18.52 years and mean body mass index: 23.46 ± 1.73 kg/m² were ultimately included. The mean time since amputation was 9.28 ± 17.32 years. One participant used a cane as a daily assistive device and was permitted to use it during the assessment. The causes of amputation included trauma (n = 3), diabetes (n = 3), burns (n = 1), and vascular disease (n = 1).

As shown in Table 1, the stability of the modified socket was significantly greater than that of the TSB socket (P = 0.02). Furthermore, satisfaction with the selection and retention of the socket after the trial was significantly higher for the modified socket compared with the TSB socket (P = 0.03) (Figure 4).

Table 1.

Result of CLASS subscales in 2 types of sockets.

CLASS subscales	Modified socket	TSB socket	P
Stability (%)	81.25 ± 16.02	68.75 ± 16.02	0.02a
Suspension (%)	78.12 ± 18.89	71.87 ± 20.59	0.34
Comfort (%)	72.65 ± 21.11	73.43 ± 21.32	0.91
Appearance (%)	61.45 ± 30.51	59.37 ± 32.86	0.85

Mean ± SD.

^aDifference is significant at the level α<0.05.

Figure 4.

Participants’ satisfaction with the selected socket after the trial period, measured using an 11-point Numerical Rating Scale (0 = no satisfaction, 10 = maximum satisfaction). Values represent mean ± standard deviation.

The socket-liner interface pressure is shown in Table 2. No significant differences were observed in interface pressure between the modified and TSB sockets (P > 0.05).

Table 2.

Interface pressure between socket and liner.

Parameter	Socket	Sensor 1 (N)	Sensor 2 (N)	Sensor 3 (N)	Sensor 4 (N)	Sensor 5 (N)	Sensor 6 (N)	Sensor 7 (N)	Sensor 8 (N)	Sensor 9 (N)	Sensor 10 (N)
50% of the total weight	Modified	1811.81 ± 808.47	1078.25 ± 922.10	1974.43 ± 841.90	1215.25 ± 561.58	1994.50 ± 512.04	2108.06 ± 524.52	1405.93 ± 535.94	1946.06 ± 209.57	2061.68 ± 414.86	2036.81 ± 406.31
	TSB	1292.0 ± 662.15	1266.93 ± 651.27	2491.25 ± 692.67	1507.37 ± 592.68	1983.12 ± 582.73	1675.12 ± 683.33	1184.37 ± 595.63	1671.12 ± 555.78	2112.31 ± 619.14	1889.43 ± 689.06
	P	0.07	0.20	0.41	0.21	0.09	0.07	0.40	0.16	0.57	0.48
25% of the total weight	Modified	1767.62 ± 551.34	1247.11 ± 533.14	1623.62 ± 1031.03	939.43 ± 509.18	1698.31 ± 738.45	1511.62 ± 602.72	1498.31 ± 815.21	1610.81 ± 715.30	1795.56 ± 671.05	1837.12 ± 643.60
	TSB	1337.87 ± 795.79	1128.87 ± 533.85	1979.68 ± 684.24	1453.12 ± 594.14	1775.31 ± 738.20	1552.06 ± 465.93	1049.81 ± 863.21	1514.62 ± 543.90	1983.93 ± 567.72	1911.43 ± 572.85
	P	0.26	0.40	0.57	0.12	0.57	0.16	0.32	0.78	0.67	0.57
Total weight bearing	Modified	1764.56 ± 333.53	1109.81 ± 779.35	2387.43 ± 739.37	1505.87 ± 1003.75	1914.56 ± 879.10	1930.00 ± 440.38	1187.43 ± 465.83	1905.00 ± 580.07	2122.75 ± 459.81	1971.18 ± 663.16
	TSB	1380.87 ± 804.28	1461.93 ± 774.67	2186.37 ± 959.52	1620.87 ± 667.63	2248.56 ± 451.28	1687.56 ± 865.24	1081.62 ± 572.43	1642.37 ± 766.27	2010.06 ± 526.41	1822.81 ± 712.11
	P	0.12	0.26	0.31	0.16	0.89	0.40	0.57	0.28	0.78	0.40
Walking	Modified	2081.37 ± 658.79	1466.87 ± 1064.68	2543.00 ± 814.72	1426.31 ± 584.77	1983.12 ± 582.73	2295.62 ± 432.90	1451.62 ± 897.19	2184.81 ± 649.97	2390.562 ± 491.59	2414.06 ± 264.57
	TSB	1811.81 ± 808.47	1266.93 ± 651.27	2491.25 ± 692.67	1507.37 ± 592.68	2373.56 ± 527.18	1675.12 ± 683.33	1184.37 ± 595.63	1671.12 ± 555.78	2112.31 ± 619.14	1889.43 ± 689.06
	P	0.57	0.40	0.67	0.09	0.89	0.33	0.07	0.21	0.67	0.40

Abbreviation: N, Newton.

Mean ± SD.

^aDifference is significant at the level α < 0.05.

As shown in Figure 5, the value of lateral rotation was significantly higher in the TSB socket compared with the modified socket after 3 m of walking (P = 0.01). Lateral rotation of the TSB socket in a static position was significantly greater than medial rotation (P = 0.01). Conversely, the difference between static lateral and medial rotation in the modified socket was not statistically significant (P = 0.67). The intrarater and inter-rater reliability of the socket rotation assessment method are detailed in Table 3.

Figure 5.

Mean ± standard deviation of socket rotation (in mm) during static testing and after 3-m walking (*difference is significant at the level α < 0.05).

Table 3.

Reliability of the socket rotation considering method.

Parameter	Socket		Static lateral rotation (mm)	ICC	Static medial rotation (mm)	ICC	Rotation after walking (mm)	ICC
Intrarater reliability	Modified	Rater 1. Test 1	0.75 ± 0.88	0.87	0.25 ± 0.46	0.85	0.50 ± 0.53	0.87
		Rater 1. Test 2	0.75 ± 0.70		0.37 ± 0.51		0.62 ± 0.51
		Rater 2. Test 1	0.75 ± 0.88	0.87	0.25 ± 0.46	1.00	0.75 ± 0.46	0.72
		Rater 2. Test 2	0.75 ± 0.70		0.25 ± 0.46		0.50 ± 0.53
	TSB	Rater 1. Test 1	1.87 ± 1.55	0.97	0.50 ± 0.75	0.94	1.62 ± 0.74	0.78
		Rater 1. Test 2	1.87 ± 1.64		0.62 ± 0.74		1.62 ± 0.51
		Rater 2. Test 1	1.87 ± 1.45	0.98	0.62 ± 1.06	1.00	1.62 ± 0.74	0.78
		Rater 2. Test 2	2.00 ± 1.41		0.62 ± 1.06		1.62 ± 0.51
Inter-rater reliability	Modified	Rater 1. Test 1	0.75 ± 0.88	1.00	0.25 ± 0.46	1.00	0.50 ± 0.53	0.72
		Rater 2. Test 1	0.75 ± 0.88		0.25 ± 0.46		0.75 ± 0.46
		Rater 1. Test 2	0.75 ± 0.70	1.00	0.37 ± 0.51	0.85	0.62 ± 0.51	0.87
		Rater 2. Test 2	0.75 ± 0.70		0.25 ± 0.46		0.50 ± 0.53
	TSB	Rater 1. Test 1	1.87 ± 1.55	0.96	0.50 ± 0.75	0.74	1.62 ± 0.74	1.00
		Rater 2. Test 1	1.87 ± 1.45		0.62 ± 1.06		1.62 ± 0.74
		Rater 1. Test 2	1.87 ± 1.64	0.95	0.62 ± 0.74	0.65	1.62 ± 0.51	1.00
		Rater 2. Test 1	2.00 ± 1.41		0.62 ± 1.06		1.62 ± 0.51

ICC; intraclass correlation coefficient (average measures).

Mean ± SD.

Discussion

Our findings indicate that amputees experienced greater stability and lateral rotational control during gait when using the modified socket compared with the TSB socket. Local pressure on the anterior distal tibia and the fibular head was reduced in the modified socket relative to the TSB socket; however, the rate of reduction was not statistically significant. Amputees expressed a preference for the modified socket design for their daily activities.

Several studies have explored the different socket types used in transtibial amputations. Selles et al¹⁰ assessed the PTB and TSB sockets satisfaction among 26 adults with unilateral transtibial amputation. They reported that PTB and TSB sockets performed equally well in satisfaction. The discrepancies between their findings and our results could be related to differences in socket designs and the questionnaires used. Selle et al compared the TSB socket with the PTB socket using a questionnaire focused on total prosthetic satisfaction, while we compared the TSB with the modified socket, which integrates features of both PTB and TSB sockets, utilizing the CLASS questionnaire that focused on the prosthetic socket fit satisfaction. In addition, Laing et al¹⁶ investigated hydrostatic sockets fabricated via pressurized casting, aiming for uniform pressure distribution, and compared them with conventional PTB sockets using functional mobility metrics (Timed Up and Go Test, 6-Minute Walk Test, and Symmetry Index). Their sockets employed Pelite liners and cuff suspension, in contrast to our modified design sockets, which combine targeted pressure zones (e.g., patellar tendon, medial tibial flare) with total contact principles, using silicone liners and pin/lock suspension. Given these fundamental differences in casting technique, suspension type, and outcome measures, a direct comparison is not methodologically appropriate between this study and Laing et al’s study. Our study focused on socket fit satisfaction, interface pressure, and rotational stability parameters not assessed in Laing et al’s work.¹⁶ In their investigation, Kahle et al emphasized the influence of posterior trimline height on user mobility in hydrostatic vs. patellar tendon-bearing sockets. However, in our study, posterior trimlines were intentionally standardized across both socket types (TSB and modified) to isolate the effects of casting and mold modification. Therefore, trimline-dependent functional differences reported by Kahle et al are not applicable to our controlled comparison.¹⁷

We observed no significant differences in pressure distribution between the modified and TSB socket designs across both pressure-tolerant (soft tissue) and intolerant (bony tissue) areas of the residual limb. Despite the observed pressure reduction on the fibular head and anterodistal tibia, the modified design did not produce local pressure comparable to that of the PTB socket design. Our results support the hybrid socket concept, which, by combining elements of both PTB and TSB socket designs, aims to reduce pressure on the fibula’s head and the anterodistal prominences of the tibia. This concept allows for rotation control akin to the PTB design while maintaining favorable interface pressure distributions similar to those of the TSB design.⁴ Doumleton et al compared the dynamic interface pressure distributions of pressure-cast (hand-off) and hand-cast sockets on unilateral transtibial prostheses. Their findings suggested that interface pressure in the hand-off socket design was significantly higher than that in the hand-cast design. In our study, both socket castings were performed by hand; thus, differences in our results may relate to variations in socket designs and the methods used for pressure testing. We reported pressure in static weight-bearing conditions and after walking, whereas Doumleton et al¹⁸ measured interface pressure during the early, mid, and late stance phases of gait.

Several trials have reported vertical displacements of the residual limb within the prosthetic socket ranging from 5 to 40 mm, which may be attributed to the suspension system and measurement techniques used. Angular movements in the sagittal plane have been documented at approximately 5 to 20 degrees.⁴ In this trial, we aimed to assess the rotational movement of the socket over the residual limb during a static test and after 3 m of walking. In static position, the rotation of the TSB socket was greater than that of the modified socket in both medial and lateral directions; however, these differences were not statistically significant. In both socket designs, the mean lateral rotation exceeded the mean medial rotation. After 3 m of walking, lateral rotation was observed in both socket designs, with the TSB socket exhibiting significantly greater lateral rotation compared with the modified socket. This suggests that the modified socket provides more rotational stability on the residual limb during gait than the TSB socket. We could not identify any trials that reported the value of socket rotation for various socket designs. Previous studies primarily focused on the rotational movements of sockets using different suspension systems. These studies employed Likert-based self-reported questionnaires to assess prosthetic socket rotation but did not quantify the rate or direction of rotation.^7,8,11,19 In contrast to our findings, Gholizadeh et al⁹ reported enhanced rotational control in the TSB socket compared with the PTB socket. This discrepancy may be related to differences in the suspension systems utilized in their study. The amputee in their study used a TSB socket with a mediolateral suspension strap, which may have facilitated rotation, whereas a pin/lock suspension system was used with the PTB socket. In our trial, we ensured that the suspension system and socket/residual limb interface materials were consistent across both socket designs for all participants. The influence of suspension systems and socket/residual limb interface materials on socket rotation control was demonstrated by Ali et al,⁷ which reported significant differences among silicone liners with shuttle lock systems, seal-in liners, and polyethylene foam liners. In this study, the modified socket design functions as a biomechanical intermediary—deliberately integrating rotational control and anatomical contouring strategies from PTB with the even pressure distribution and comfort features of TSB. This integrative design appears to preserve the functional stability of PTB while avoiding its key disadvantages (i.e., high pressure peaks over specific load-bearing regions), and simultaneously upholds the pressure-equalizing philosophy of TSB while compensating for its typical lack of rotational control.⁴

Limitations

The limited sample size and the recruitment of amputees exclusively at the K3 activity level may restrict the generalizability of our findings. Moreover, we were unable to assess interface pressure during various phases of gait or on different surfaces, such as uneven terrain or while descending and ascending ramps and stairs. Lack of validity consideration of our rotation method measurement, inability to control for some confounding variables (like muscle tonus, perspiration during static testing), and long-term follow-up to evaluate effects after limb volume changes or other dynamic variables were other limitations in our research. Future research should aim to validate this method using gold-standard motion analysis tools. This highlights the need for future long-term follow-up research to evaluate interface pressure across diverse conditions and gait phases, utilizing a larger sample size and a broader range of activity levels. Although we used a simple measurement tool in the clinical setting for rotational consideration, it is recommended to use other types of measurement devices with focusing on their validity, not only reliability.

Conclusion

The modified socket may provide advantages in rotational control during ambulation while reducing local pressure on the anterodistal tibia and the fibular head. It can also maintain the total surface contact concept. Given these benefits, we recommend its implementation in routine clinical practice.

Author contributions

All authors have substantially contributed to conception of study design, analysis, and drafting the manuscript. All authors have read and approved the final manuscript.

Ethics review and approval

The author(s) disclosed ethics review and approval for the research described in this article: The protocol of this cross-over trial was approved by the Ethical Committee of the University of Social Welfare and Rehabilitation Sciences in code IR.USWR.REC.1400.075. All subjects signed written consent forms before filling out the questionnaires.

Clinical trial registration

The protocol of this study was coded (IRCT20211019052810N1) in the Iranian Registry of Clinical Trials.

Availability of data and material

The raw data are available from the corresponding author on reasonable request.

Funding

The authors disclosed that they received no financial support for the research, authorship, and/or publication of this article.

Supplemental material

No supplemental digital content is available in this article.