Strength Testing of Definitive Transtibial Prosthetic Sockets Made Using 3D-Printing Technology

Abstract

Introduction Unlike endoskeletal connectors and prosthetic feet, prosthetic sockets are not subjected to structural strength testing using internationally recognized test standards, such as International Organization for Standardization (ISO) 10328. Definitive prosthetic sockets fabricated in the traditional manner have been shown to be inconsistent in the ability to withstand the loads applied by these standards. Persistent concerns regarding the strength and durability of 3D-printed prosthetic sockets are a barrier to clinical adoption of 3D-printing technologies in prosthetic socket fabrication. To develop a robust prosthetic socket design based on 3D-printing technology, an iterative development process was used with integral validation testing using the ISO 10328 loading conditions performed independently by two testing centers. Materials and Methods Twenty-four 3D-printed transtibial prosthetic sockets were tested using ISO 10328 loading conditions designed to represent the greatest atypical elevated loads a 125-kg user would be expected to place on their prosthesis without failure (ultimate strength test at the P6 load level). Several design iterations and variations were tested. One socket of a design that withstood the ultimate strength loading was reprinted and subjected to cyclic testing, up to 3 million cycles. All tests were conducted under the forefoot loading condition (Condition II of the ISO 10328 standard), which generates greater moments at the distal end of the socket than the heel loading condition. Results Early socket designs were unable to withstand the ISO 10328 ultimate strength test to the P6 load level. Successive design improvements increased the strength until a robust final design was achieved. Variations of that final design for different suspension types and a different limb model demonstrated consistent performance. The final design was significantly stronger than the initial design (P = 1.35 × 10−7). The socket that was tested for durability completed the 3 million loading cycles at the P6 load level without damage and withstood the subsequent static proof loads. Conclusions An iterative design process with integral structural testing can result in strong, durable prosthetic sockets made using 3D-printing technology that may be robust to variations in limb size/shape and suspension type.

3D printed transtibial prosthetic sockets: A systematic review

“…Five studies [ 21 , 28 – 31 ] used only 3D printing techniques for socket fabrication; five studies [ 6 , 24 – 27 ] used only traditional manufacturing techniques; and two studies [ 8 , 23 ] used both additives and traditional manufacturing techniques. Among the studies using 3D printing techniques, one study [ 28 ] used multiple manufacturing techniques, i.e., selective laser melting (SLM), selective laser sintering (SLS), and fused deposition modelling (FDM), and the other six studies [ 8 , 21 , 29 – 31 ] used only FDM. Among the traditional manufacturing techniques, seven studies [ 6 , 8 , 23 – 27 ] used lamination for definitive sockets.…”

“…For CF, the particles of carbon fiber in the final socket are not interwoven as they are in LCS reinforced with carbon fiber, hence they are not as strong. Three studies [ 8 , 23 , 31 ] used PLA, two studies [ 29 , 30 ] used PP, and two studies [ 21 , 28 ] used CF, while most LCS were made of carbon fiber and laminated with either nyglass or fiber glass [ 6 , 8 , 23 – 25 ].…”

“…For example, when performing testing at the P5-load level (used for assessing safety for a 100 kg patient), settling and proof tests occur at 920 N and 2013 N, respectively, and the socket is determined to pass the P5-level, i.e., is safe for use by a 100 kg patient, if the ultimate force surpasses 4025 N (twice the proof force) [ 20 ]. Condition I alignment was employed by 25% of studies [ 8 , 23 , 31 ] while 92% of studies performed testing under ISO Condition II [ 8 , 21 , 23 – 30 ]. Among all the studies, only one study used testing below the P5-level and tested one socket at each of the P3- and P4-levels.…”

“…Each relative orientation generated a different load line due to the different fixations of the ankle and knee joints as well as the length of the pylon. In this review, eight studies used the current alignment [ 8 , 21 , 23 – 26 , 28 , 31 ], two studies used the Neo alignment [ 27 , 30 ], and two studies used the Gerchutz alignment [ 6 , 29 ].…”

“…ISO 103280 standards describe socket orientation as a function of loading condition and load level (P-level). Importantly, socket orientation is identical for P5—P8 levels, with the differences only in settling and proof forces and the thresholds for safety [ 21 ]. Therefore, when choosing the orientation for quantitative analysis, tests performed at the P5 –P8 level were grouped together.…”