Effect of Window and Hole Pattern Cut-Outs on Design Optimization of 3D Printed Braces

At a Glance

Metadata	Details
Publication Date	2022-06-24
Journal	Frontiers in Rehabilitation Sciences
Authors	Robert A. Rizza, Xuecheng Liu, Vince Anewenter
Institutions	Milwaukee School of Engineering, Children’s Hospital of Wisconsin
Citations	2
Analysis	Full AI Review Included

Executive Summary

This study utilized Finite Element Analysis (FEA) to systematically optimize the geometry, size, and spacing of cut-outs (holes and windows) in 3D printed Thoracic Lumbar Sacral Orthosis (TLSO) braces, balancing structural integrity with weight reduction and patient comfort.

Structural Criteria: Optimization was governed by two primary criteria: a minimum Factor of Safety (FOS) of 2.32 (Pugsley method) and a maximum lateral deformation of 4.9 mm.
Optimal Hole Pattern: The hexagonal geometry was identified as the optimal hole pattern, achieving the target FOS (2.32) while removing the highest volume of material (10.09%).
Optimal Hole Parameters: The ideal hexagonal pattern requires an equivalent diameter of 10 mm and a center-to-center spacing of 12 mm (L/D ratio of 1.2). This results in a surface area of 78.54 mm² per hole.
Unacceptable Patterns: Linear hole patterns (aligned holes) were found to be structurally unacceptable, consistently failing to meet the minimum FOS requirement of 2.32.
Abdominal Window Optimization: The “bib” shape was the only abdominal window geometry that satisfied both the FOS and deformation criteria; circular shapes failed the FOS requirement.
Window Sizing: The size and location of both abdominal and de-rotation windows should be determined by clinical/functional needs (e.g., breathing, spinal derotation) rather than structural constraints, as variations in size had a negligible effect on the brace’s stress and deformation.

Technical Specifications

Parameter	Value	Unit	Context
Target Factor of Safety (FOS)	2.32	Dimensionless	Minimum required structural integrity
Maximum Allowable Deformation	4.9	mm	Structural limit derived from 22° Cobb Angle
Brace Thickness (Baseline)	2	mm	Thickness used for initial FEA model
Material Used (Simulation)	Armadillo (NinjaTek 3D)	Polymer	Plastic material properties used in linear FEA
Secant Modulus (Material)	307.64	MPa	Mechanical property input for FEA
Plastic Polymer Yield Stress	12.89	MPa	Mechanical property input for FEA
Optimal Hole Shape	Hexagon	Geometry	Best balance of FOS and volume removal
Optimal Hole Spacing (L)	12	mm	Center-to-center distance for optimal pattern
Optimal Equivalent Diameter (D)	10	mm	Diameter defining the optimal hole size
Optimal Hole Surface Area	78.54	mm²	Surface area per hole for optimal pattern
Volume Removed (Optimal Hexagon)	10.09	%	Weight reduction achieved by optimal pattern
Applied Traction Load (T7-T9)	3	N/mm	Equivalent load applied along the C curve
Applied Force (T1 level)	34.6	N	Equivalent load applied to the brace
Applied Force (T12 level)	62.5	N	Equivalent load applied to the brace

Key Methodologies

Spine Model and Load Determination: A finite element model of the spine (vertebrae E=10 GPa, disc E=4.2 MPa) was constructed based on literature data to establish equivalent mechanical loads applied to the brace (3 N/mm traction, 34.6 N at T1, 62.5 N at T12).
Brace Geometry Acquisition: A Computer Aided Design (CAD) model of the Thoracic Lumbar Sacral Orthosis (TLSO) was generated from an optical scan of an existing brace and resized to fit the spine model.
Baseline FEA: An initial FEA simulation (Ansys) was performed on the solid 2 mm thick brace model, applying the calculated loads and constraints (fixed bottom, constrained displacement at top) to establish baseline stress and deformation distributions.
Hole Pattern Comparison: Four geometries (Circle, Triangle, Diamond, Hexagon) were tested using constant equivalent diameter (10 mm) and spacing (15 mm, L/D=1.5). The patterns were sketched and “wrapped” onto low-stress areas of the brace surface.
Hexagonal Optimization: Based on initial results, the hexagonal pattern was selected for further optimization. Multiple FEA iterations varied spacing (L) and equivalent diameter (D) to achieve the target FOS (2.32) and deformation (less than 4.9 mm).
Window Geometry Analysis: Abdominal window shapes (Circular, Trapezoidal, “Bib”) and De-rotation window shapes (Oval/Boot) were analyzed. For the abdominal window, the “bib” shape was identified as the only structurally viable option.
Window Size and Location Sensitivity: Simulations varied the size and placement of the windows to determine the sensitivity of the FOS and deformation to these parameters, concluding that structural performance was largely insensitive to these changes.

Commercial Applications

Customized Orthotics Manufacturing: Direct application in the design and 3D printing of patient-specific TLSO braces, ensuring optimal structural performance while maximizing weight reduction and patient comfort (breathability).
Design for Additive Manufacturing (DfAM): Provides validated design rules for incorporating complex cut-out patterns (specifically hexagonal arrays) into load-bearing plastic components to achieve superior strength-to-weight ratios.
Biomechanical Simulation Services: The methodology establishes a standardized, systematic approach for using FEA to validate and optimize medical support devices against defined biomechanical criteria (FOS and deformation limits).
Pediatric Medical Devices: Crucial for designing braces for young children, where abdominal windows must be sized functionally to permit normal abdominal breathing without compromising the corrective moment generated by the brace.

View Original Abstract

Background There are many different Thoracic Lumbar Sacral Orthosis style brace designs available in the market for the correction of scoliosis deformity. Hole cut out patterns, are commonly used in brace designs. These cut-outs may be subdivided into two groups: hole patterns and windows. Hole patterns are an array of holes which are implemented to lighten the weight of a brace and allow for the skin to breathe. Windows provide space for spinal derotation and/or breathing. From an examination of the literature, it appears that a systematic analysis of the effect of these cut-outs on the structural integrity and functionality of the brace has not been undertaken. Furthermore, there is a lack of understanding on the effect of spacing, size and geometry of the cut-outs on the mechanical behavior of the brace. Method of Approach In this study, Finite Element Analysis is employed to examine the mechanical response of the brace to these cut-outs. Geometry for the Thoracic Lumbar Sacral Orthosis was obtained by scanning an existing brace using an optical scan and converted into a Computer Aided Design model. A systematic approach was undertaken where cut-out geometry, spacing and size was varied. The deformation and stress in the thickness of the brace was ascertained from the Finite Element Analysis. An appropriate factor of safety for the structural analysis was determined using a standardized approach and used to quantify the structural integrity of the brace due to the cut-out. Various geometries were analyzed for the hole patterns including circle, triangle, diamond, and hexagon. For the window, the geometries considered were circle, trapezoidal and the “bib” geometry. Results It was found that linear hole patterns where the holes are aligned do not provide a desirable structural factor safety. Furthermore, among all the possible geometries, the hexagonal cut-out was the best structurally while reducing the weight of the brace the most. The optimal spacing was found to be 12 mm, and the optimal hole surface area was found to be 78.54 mm 2 . For the windows in the abdominal area, the “bib” shape provided the best structural integrity and generated the lowest amount of deformation. An increase in the size of this window had a small effect on the stress but an almost negligible effect on the deformation. Conclusions A hexagonal hole pattern should be used with a spacing of 12 mm and each hole should have a surface area of 78.54 mm 2 . Windows in the abdominal area should be of “bib” shape. The size of the window cut-outs does not affect the brace stress and deformation significantly. Thus, the size of these windows should be based on the functional aspects of the brace, i.e., the minimum required size needed to permit the patient to breathe comfortably as in the case of the abdominal window or to allow for proper derotation, as in the case of the derotation window.

Tech Support

Original Source

DOI: https://doi.org/10.3389/fresc.2022.889905

References

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