Static structural testing of trans-tibial composite sockets

Abstract

The purpose of this investigation was to quantify the structural strength of various trans-tibial composite sockets. To conduct the study, loading parameters and methods were developed that emulate the International Standards Organisation (ISO) standards for structural testing of lower limb prostheses since specific guidelines for the testing of the trans-tibial socket portion of a prosthesis have not yet been established. The experimental set-up simulated the instant of maximum loading during the late stance phase of gait. Ten trans-tibial sockets were evaluated. Five different reinforcement materials and two resin types were used to construct the sockets. A standard four hole distal attachment plate was used to connect the socket and pylon. Each sample was loaded to failure in a servo-hydraulic materials test machine at 100 N/s. None of the composites in the study met the ISO 10328 standards for level A100, loading condition II (4025 N), as required for other prosthetic componentry. All failures occurred at the site of the pyramid attachment plate. Ultimate strength and failure type were material dependent. Load point deflection was significantly different for the resin variable (p < 0.05). Statistical differences according to reinforcement material were noted in composite weight and strength-to-weight ratio (p < 0.05). The fibre volume fraction was also estimated and recorded. Reinforcement material type was the primary determinant of performance for the tested samples. Carbon reinforcements performed better than fibreglass reinforcements of similar weave type. The greatest ultimate strength and strength-to-weight ratio was observed with the unidirectional carbon reinforcement.

Using mechanical testing to assess texturing of prosthetic sockets to improve suspension in the transverse plane and reduce rotation

“…Standardization (ISO) 10328 standard [25] testing methods to include assessment of prosthetic sockets [26][27][28][29], yet these tests focus on socket strength and are not helpful in the assessment of suspension forces. While it may be relatively straightforward to assess longitudinal displacement of a socket on a mock residual limb using testing conditions that resemble those experienced during ambulation or activities of daily living [20,30,31], assessing rotational displacement is more challenging given the cost of multi-axial material testing systems [32].…”

“…With respect to the test set-up and protocol we developed, it was limited to bi-axial loading, where compression and rotation were applied simultaneously. Even though bi-axial testing is more complex than the uniaxial testing typically used when evaluating sockets [20,[26][27][28][29][30][31] it does not fully simulate the range of loading directions and moments a prosthesis user would experience during every day activities. Given these limitations, our results represent a best case scenario with respect to maintaining suspension, and likely overestimates the torques that might occur at 5˚and 7.5r otation angles when adding texture to 3D printed sockets worn by persons with transtibial amputation.…”

Recommendations on Composite Socket Fabrication Based Upon Experimental Results

“…In order to compare them against each other, a baseline series of panels was chosen. For each resin investigated, three panels were made using the two most common reinforcements, the baseline carbon fabric (1) and fiberglass (2). The three panels were 1) five layers of baseline carbon fibers, represented by the blue line marked CCCCC, 2) the baseline carbon with the middle (of five) layer replaced with fiberglass, represented by the maroon line marked CC-FG-CC, and 3) the baseline carbon with the second and forth layers replaced with fiberglass, represented by the green line marked C-FG-C-FG-C.…”

An Investigation of the Structural Strength of Transtibial Sockets Fabricated Using Conventional Methods and Rapid Prototyping Techniques

“…7 The second study found that for socket reinforced with carbon weighing between 616 -795g, failure occurred between 4247-5663 N. 5 Different material lay-ups and socket attachment methods were found to increase the strength of laminated composite socket. 5,7,9 This study also concluded that modular components began to fail above 5400 N of force. 5 The sockets tested at Condition II in the current study, weighed between 410-450 g and broke between 4384 and 6505 N. This is approximately double the load reported by the first study and similar to results in the second study, despite sockets in this study weighing much less.…”