Indonesian Researchers 4D Print Spacers for Minimally Invasive Transforaminal Lumbar Interbody Fusion (MI-TLIF)

Researchers at Universitas Indonesia are investigating the potential of 4D printed spacers for spinal surgeries, outlining their recent, published findings in ‘Modelling the shape memory properties of 4D printed polylactic acid (PLA) for application of disk spacer in minimally invasive spinal fusion.’ Authors Nindya Aprilia Alief, Sugeng Supriadi, and Yudan Whulanza created a disk spacer able to morph according to its environment.

Using PLA as the material for the disk spacer, the goal was to make a patient-specific implant that can help to stabilize the lumbar segment during a surgical treatment called Minimally Invasive Transforaminal Lumbar Interbody Fusion (MI-TLIF). This procedure is less aggressive and less traumatic to the body and to the patient, but a disk spacer is required for a successful outcome.

Pattern of disk spacer model with hollow spaces. a) Uniformed vertical hollow spaces in 1 mm width. b) Uniformed vertical hollow spaces in 2 mm width. c) Non-uniformed vertical hollow spaces varies from 1 to 3 mm width.

“In order to obtain the right shape transformation, the expansion and thermal distribution of 4D printed polylactic acid model were investigated,” stated the researchers. “It is indicated that specific PLA structure possesses thermal shape memory behavior that can be thermomechanically trained into temporary shape and return to their permanent shape when heated.”

The researchers created several models of the disk spacer, designing a pattern for the temporary shape. The disk spacers were 50 mm long, 18 mm wide, with two identical parts, partitioned. Once the structure is created, hollow areas are added, with the three models varying in detail. PLA was chosen because of its clarity and rigidity, and most importantly, its biocompatibility.

Representative of the PLA model deformation after 3 seconds. a) Solid disk spacer model. b) Uniformed vertical hollow spaces in 1 mm width. c) Uniformed vertical hollow spaces in 2 mm width. d) Non-uniformed vertical hollow spaces varies from 1 to 3 mm width.

After 3D printing, a beaker was filled with 300 ml of hot water, with each model immersed at 60 °C, the glass temperature of PLA.

“… the time needed for a PLA pattern to expand has an average of 0.0513 second. The model (a) needs 0.064 seconds, model (b) needs 0.07 seconds, and model (c) needs 0.02 seconds. Afterwards, the model has reached the final expansion. Models are expanded towards x-axis and y-axis paralleled to the shape of the hollow spaces. Each model has adjacent value of maximum total displacement. Subsequently, the model (a) has maximum total displacement as much as 0.94 mm, the model (b) has 0.94 mm, and the model (c) has 0.95 mm, which all of the values are located on the tip of the pattern.

“Moreover, the time needed for a pattern to completely reach the external temperature has an average of 0.052 seconds. Hence, the model (a) needs 0.058 seconds, model (b) needs 0.058 seconds, and model (c) needs 0.04 seconds.”

Side by side comparison before and after adding stimulus. Left: a PLA model before adding stimulus and right: after adding the stimulus.

The authors noted that non-uniformed hollow spaces were superior in the amount of time required for thermal expansion, explaining that less time required for deformation, the better, as that means there would be less valuable time needed to affix the 4D planted structure during the medical procedure.

“This paper presents a study of thermal shape memory behavior of the PLA pattern model. The time needed for a model to expand is less than one second. The pattern expands toward x-axis and y-axis and the expansion happens gradually until it completely expands,” concluded the researchers. “The expansion value between simulation and experimental result are much the same yet the values from the experiment are on the different direction. However, based on the simulation result, nonuniformed hollow spaces pattern has favorable result that could be a reference for the future research in order to design the suitable pattern for the 4D PLA model.

“The current pattern that has been developed also need to combine with other different pattern in order to achieve desired transformation.”

The impacts that 3D and 4D printing are making in the medical field are stunning in many cases, allowing researchers to bioprint brain tumors to understand them better, 3D print splints to open up breathing airways, and fabricate mandibular implants to improve chewing, facial structure, and much more. Find out more about the potential for 4D technology in creating disk spacers for spinal fusion surgery here. What do you think of this news? Let us know your thoughts! Join the discussion of this and other 3D printing topics at 3DPrintBoard.com.