Results

Manufacturing the socket and the support

According to the aim of our study a 3D-printed transtibial prosthesis socket with the window for real-time ultrasound imaging was designed and it has been validated on healthy individuals for future use in amputees.

 

We designed a measuring tool (Fig. 1) that is made of various parts and can be assembled and disassembled. It consisted of:

• The socket for the leg (2 parts to be connected)
• The support for the probe (2 parts to be connected)
• Screws and nuts (M6)

Assembling procedure is easy due to the sliding mechanism implemented in a design which is clearly shown in an animation below and in Fig. 2.

So the final view when the equipment is on is shown in Fig. 3.

Soft tissue displacement
After testing the socket and performing several experiments to obtain the real-time data we got ultrasound video. The data was post processed in Matlab and the video below shows how the bone part, the interface between fat and muscle and skin were separated to quantify the displacement of the soft tissue relative to the bone.

Fig. 4 (a, b) shows the measured soft tissue displacement over time while walking. The displacement varied cyclically, with peaks corresponding to the push-off phase and troughs occurring during midstance.

On average, maximum peak displacement for the measurement from the anterior side reached approximately 3 mm. As for the lateral side, maximum peak displacement was about 3.5 mm and it is clear that there was more bone displacement compared to the anterior side. Variability between cycles was also observed, particularly during the heel strike and push-off phases. In general, when walking there are 4 main phases:

1. Heel Strike (Initial Contact):
   • When the heel first touches the ground, muscle activation begins.
   • A small displacement occurs as the leg starts to bear weight.
2. Loading Response & Midstance:
   • As the foot fully touches the ground and weight is transferred, the calf muscles engage to stabilize.
   • Displacement increases as tissues move slightly in response to impact forces.
3. Push-Off (Terminal Stance & Pre-Swing):
   • During this phase, the Achilles tendon stores and releases energy, causing rapid motion.
   • The probe detects a significant shift in soft tissue due to muscle contraction and force transmission.
4. Swing Phase:
   • After push-off, the foot lifts off the ground, reducing force on the leg.
   • Displacement decreases as the leg moves forward in the air.

Normalized Gait Cycle Analysis
To compare displacement patterns across multiple steps, individual gait cycles were normalized and averaged (Fig. 6).

 

Table 1. Comparison of anterior and lateral side results

Feature	Anterior Side Displacement	Lateral Side Displacement
Peak Displacement	Lower (~1.5–2 mm) at peak	Higher (~2.5–3 mm) at the start and end
Trough (Minimum)	~0.7 mm around midstance	~1 mm around midstance (40-60%)
Shape of Curve	More gradual curve, smoother changes	More variation in peak locations
Standard Deviation	Larger at the beginning and end	Larger near push-off (70–100%)
Mean and StDev	0.9887 ± 0.4466 mm	1.5588 ± 1.0037 mm

 

Quantitative results

The lateral side has larger peaks and overall mean value compared to the antrerior side, which suggest that soft tissue on the lateral side moves more dynamically, likely due to the fibularis (peroneal) muscles stabilizing the ankle and counteracting inversion/eversion.

Shape of curve

The anterior-side displacement is smoother and follows a gradual U-shaped pattern, while the lateral side shows more abrupt changes in displacement, reflecting the stronger engagement of lateral stabilizing muscles at certain phases.

Standard Deviation Differences

Lateral side variability is highest during push-off (60–100%). This could happen as some individuals may use their fibularis muscles more aggressively during this phase.

Front side variability is highest at the beginning (0–10%) and end (90–100%), which could be due to differences in foot strike patterns or variations in how the tibialis anterior muscle controls dorsiflexion.