Application of 3D Printing Technology in the Treatment of Hoffa’s Fracture Nonunion^*

Abstract

Objective

To evaluate a proposed three-dimensional (3D) printing process of a biomodel developed with the aid of fused deposition modeling (FDM) technology based on computed tomography (CT) scans of an individual with nonunion of a coronal femoral condyle fracture (Hoffa’s fracture).

Materials and Methods

Thus, we used CT scans, which enable the evaluation of the 3D volumetric reconstruction of the anatomical model, as well as of the architecture and bone geometry of sites with complex anatomy, such as the joints. In addition, it enables the development of the virtual surgical planning (VSP) in a computer-aided design (CAD) software. This technology makes it possible to print full-scale anatomical models that can be used in surgical simulations for training and in the choice of the best placement of the implant according to the VSP. In the radiographic evaluation of the osteosynthesis of the Hoffa’s fracture nonunion, we assessed the position of the implant in the 3D-printed anatomical model and in the patient’s knee.

Results

The 3D-printed anatomical model showed geometric and morphological characteristics similar to those of the actual bone. The position of the implants in relation to the nonunion line and anatomical landmarks showed great accuracy in the comparison of the patient’s knee with the 3D-printed anatomical model.

Conclusion

The use of the virtual anatomical model and the 3D-printed anatomical model with the additive manufacturing (AM) technology proved to be effective and useful in planning and performing the surgical treatment of Hoffa’s fracture nonunion. Thus, it showed great accuracy in the reproducibility of the virtual surgical planning and the 3D-printed anatomical model.

Keywords
Hoffa’s fracture; pseudarthrosis; printing, three-dimensional; orthopedic surgery

Resumo

Objetivo

Avaliar uma proposta de processo de impressão tridimensional (3D) de um biomodelo preparado com o auxílio da tecnologia de modelagem por deposição de material fundido (fused deposition modeling, FDM, em inglês) a partir de imagens de tomografia computadorizada (TC) de um indivíduo com pseudartrose de fratura coronal do côndilo femoral (fratura de Hoffa).

Materiais e Métodos

Para tanto, utilizamos imagens de TC, que permitem estudar a reconstrução volumétrica 3D do modelo anatômico, além da arquitetura e geometria óssea de sítios de anatomia complexa, como as articulações. Também permite o planejamento cirúrgico virtual (PCV) em um programa de desenho assistido por computador (computer-aided design, CAD, em inglês). Essa tecnologia possibilita a impressão de modelos anatômicos em escala real que podem ser utilizados em simulações cirúrgicas para o treinamento e a escolha do melhor posicionamento do implante de acordo com o PCV. Na avaliação radiográfica da osteossíntese da pseudartrose de Hoffa, verificou-se a posição do implante no modelo anatômico impresso em 3D e no joelho do paciente.

Resultados

O modelo anatômico impresso em 3D apresentou características geométricas e morfológicas semelhantes às do osso real. O posicionamento dos implantes em relação à linha de pseudartrose e pontos anatômicos foram bastante precisos na comparação do joelho do paciente com o modelo anatômico impresso em 3D.

Conclusão

A utilização do modelo anatômico virtual e do modelo anatômico impresso em 3D com a tecnologia de manufatura aditiva (MA) foi eficaz e auxiliou o planejamento e a realização do tratamento cirúrgico da pseudartrose da fratura de Hoffa. Desta forma, foi bastante preciso na reprodutibilidade do planejamento cirúrgico tanto virtual quanto no modelo anatômico impresso em 3D.

Palavras-chave
fratura de Hoffa; pseudoartrose; impressão tridimensional; procedimentos ortopédicos

Introduction

In several fields of medicine, such as orthopedics, anatomical models and personalized implants are manufactured for an accurate preoperative planning, the simulation of surgeries, the training of staff, and better communication with the patient.¹,²,³,⁴

Three-dimensional (3D) printing, also called rapid prototyping (RP) or additive manufacturing (AM), enables the printing of models reproducing the actual anatomy. These can improve the surgeon’s anatomical understanding and interpretation. This technology enables the use of personalized guides to optimize the surgical outcomes. Implants and arthroplasties can be developed according to an individual’s anatomy and can also help in the anatomical assessment of the surgical treatment.⁵,⁶

Virtual 3D planning enables the surgeon to visualize and understand the complex anatomy and to digitally plan a corrective osteotomy to restore the anatomy and normal function or the best implant placement, for example.

The objective of the present study is to evaluate a proposed 3D printing process of a biomodel developed with the aid of fused deposition modeling (FDM) based on the computed tomography (CT) scans of an individual with nonunion of the coronal femoral condyle fracture (Hoffa fracture).

Materials and Methods

To demonstrate the use of the AM technology in orthopedic surgery, we chose a clinical case of an individual with a Hoffa fracture nonunion. We performed the study of bone geometry, virtual preoperative planning, and preoperative planning with the printing of a 3D anatomical model in full scale (1:1) of the distal region of the femur. In this model, the fixation of the nonunion was simulated with osteosynthesis by plate and screws. After the surgical treatment, the implant position was analyzed (in the anatomical model and in the patient) through the measurement of the angles of the implants oin relation to bone structure landmarks. These measurements enabled us to analyze the reproducibility of the surgical planning and simulation. The present study was performed after approval by the institutional Ethics Committee (CAAE: 94788418.2.0000.5547).

The patient is a 44-year-old male who, in november 2010, suffered a motorcycle accident that resulted in severe polytrauma with an open segmental fracture of the left femur (fracture of the proximal and distal regions). He presented sequelae of the trauma to the left lower limb with nonunion and angular deformity.

After six years of the accident, he started presenting with progressive pain. The X-ray and CT scans were performed, and a neglected Hoffa’s fracture nonunion was diagnosed (Letteneur type III) (►Fig. 1).⁷

Fig. 1
Sagittal, axial, and coronal CT scans.

For the acquisition of bone images to print the anatomical model, we used the GE lightspeed VCT (GE Healthcare, Chicago, IL, United States) CT scanner, manufactured in 2008, with 64 channels. The scans were performed following a specific image acquisition protocol for bone tissue established by the local radiology team, as shown in ►Table 1.

Table 1
Parameters used for the acquisition of computed tomography scans

Parameters	Description
Field of view	17 × 17 cm
Scout	Standard protocol of the scanner
Region of interest	Knee
Voltage (KV)	120
Amperage (mA)	298
Pitch	512 × 512
Colimation	Large body
Slice thickness	0,6 mm
Slice increment	0,969 mm

To create the virtual anatomical model to be used, segmentation was performed (the separation of bone tissue from the CTscans)using the Invesalius (Centro de Tecnologia da Informação [CTI] Renato Acher, Campinas, SP, Brazil) software, version 3.1.1, as shown in ►Fig. 2. The bone segmentation was performed by an automatic algorithm of the software that identified bone tissue in the radiodensity threshold window between 226 and 2014 Hounsfield units (HU). The program created a mask to identify the segmented tissue, which, in this case, wasgreen,asshown in ►Fig. 2. The software enabled the visualization of the reconstructed object in various positions in space, helping to better understand the bone geometry. Volumetric reconstruction of the object was performed, and a file was created in the standard triangle language (STL) format, which can be exported to a computer-aided design (CAD) environment for subsequent virtual modeling of the object or even its 3D printing.

Fig. 2
Screen image of the Invesalius software showing the bone segmentation. Note: The figure shows the images of the axial, sagittal, coronal sections, as well as the image of the reconstructed volume of the segmented bone tissue.

The object was modeled and rendered through the Mesh-mixer (Autodesk, San Rafael, CA, United States) software, version 3.5, with the levelling of the internal and external surface of the bone (►Fig. 3).

Fig. 3
Virtual bone model before rendering with the Meshmixer software. Note: A mesh was developed to better understand the irregularities and flaws on the surface of the object to facilitate their subsequent correction.

The use of a software with CAD technology enables the development of the virtual surgical planning (VSP) and the reduction of fragments (►Fig. 4). The non-consolidated fragments were separated virtually to better study the lesionn, followed by their reduction in the anatomical position, and the positioning of the implants in the desired location. The entire virtual surgery process was performed in a CAD environment using the Meshmixer software (►Fig. 4).

Fig. 4
Images of the reduction of the nonunion fragments.

After the creation of the object in STL format, the file was exported to a G-code generator; G-code is a language used by computer-aided manufacturing (CAM) software to manufacture 3D objects. The main function of the G-code is to instruct the machine to move geometrically in the three dimensions: X, Y, and Z. It is an extremely simple and rudimentary language, consisting of a sequence pf lines of instructions in which each line is responsible for a specific task, and the software is run line by line until the end of the code. The G-code of the object was generated using the Slic3r (developed by Alessandro Ranellucci) free software, version 1.3.0.

After the generation of the G-code, the information was exported to a CAM environment. The Repetier-Host (Hot-World GmbH & Co. KG, Willich, Germany) free software, version 2.0.5, was used to set the printing features, such as the type of thermoplastic, polymer density, filament diameter, extrusion temperature, printing speed, height (width) of the layer, resolution, filling of the structure, and support of the parts.

The manufacturing of the printed model reproduced the internal and external profiles of the cortical layer of the bone, which is completely filled with polymer, representing the actual geometry of the bone (►Fig. 5).

Fig. 5
Images of the 3D-printed bone model with the FDM technology in white ABS, showing the distal femur and fragment of the lateral femoral condyle.

The surgical simulation was performed according to recent studies⁷,⁸ that demonstrate that plate and screws generate more biomechanical stability in the treatment of coronal fracture of the femoral condyle. A 5-hole locking compression plate (LCP) with four 3.5 mm locked screws, a 4.0 mm cancelous screw, a 3.5 mm cortical screw were used.

Fixation of the nonunion of the 3D anatomical model was performed according to the principles of fixation of joint fractures developed by the Arbeitsgemeinschaft für Osteo-synthesefragen (AO) Foundation (►Fig. 6).⁹

Fig. 6
Images of the 3D printed bone fixated with the LCP plate on the lateral portion of the distal femur.

The plate was bent following the geometry of the lateral femoral condyle so that it would be at 90° in relation to the nonunion line. The proper positioning of the implants and the reduction of bone fragments were assessed through perioperative fluoroscopy (►Fig. 7).

Fig. 7
Perioperative fluoroscopy images to assess the positioning of the implants.

After fixating the 3D anatomical model with the implants, stability of the fragment and/or the plate and screws were assessed through manual displacement tests reproducing the flexion and torsion forces.

The anatomical model was sterilized in ethylene oxide to be used during the surgical procedure as a navigation guide (►Fig. 8).

Fig. 8
Use of the 3D-printed anatomical model as a navigation guide for the anatomical landmarks to place the implant according to the virtual planning.

Results

The 3D-printed anatomical model showed geometric and morphological characteristics similar to those of the actual bone. The material used to print the model, the thermoplastic polymer acrylonitrile butadiene styrene (ABS), provided the model with structural strength that enabled the surgical simulation with the placement of the implants, without causing breakage or loosening of the layers of the model. After inserting the implants, the stability of the reduction and fixation of the fragments were assessed.

Using the described method, we were able to print a 3D anatomical model with the characteristics of the actual object, which enabled the development of the VSP, a surgical simulation to plan the best position of the implants and train the surgery team.

The assessment of the reproducibility of the technique, that is, determining if the osteosynthesis performed on the patient’s distal femur followed the virtual planning and the simulation using the 3D-printed anatomical model, was based on the measurements of the angles in comparison to the anatomical landmarks of the distal femur, the nonunion line, and the position of the implant. Angle measurement was performed using the AnimatiPACS (Animati netPACS, Santa Maria, RS, Brazil) software.

To measure the position of the implant, the anatomical landmarks were defined on anteroposterior (AP) and Lateral view (L) radiographs, with the tangential tracing of the lines in the anatomical landmarks to form angles with the implants or the nonunion line. On the AP view, a line was defined as a tangential landmark to the distal end of the femoral condyles. On the Lateral view, a line was defined as a tangential landmark to the posterior cortex of the distal femoral metaphysis. (►Fig. 9).

Fig. 9
Anatomical references on the AP and Lateral view radiographs with the tangential tracing of lines on the landmarks and the formation of angles to measure the position of the implants.

The radiographs (on AP and Lateral views) of the patient’s knee and the 3D-printed anatomical model show that the position of the plate and screws in relation to the defined anatomical landmarks (articular surface of the femoral condyles and the posterior cortical layer of the distal metaphysis of the femur) are different (►Table 2).

Table 2
Measurements of the positioning angles of the nonunion line, the plate, and screws in relation to the anatomical landmarks of the distal femur

	X-ray view	Knee	Anatomical model
Nonunion line	Anteroposterior	41.2°	38,1°
	Lateral	43.8°	61,5°
Plate	Anteroposterior	49,6°	71,8°
	Lateral	36,9°	26,0°
Proximal screw	Anteroposterior	57,2°	74,6°
	Lateral	57,0°	32,1°
Distal screw	Anteroposterior	52,5°	62,8°
	Lateral	50,9°	37,3°

The patient’s knee radiographs on Lateral view and the printed anatomical model show a proximity of the values of the angles between the plate and screws in relation to the nonunion line (►Table 3). The differences in percentage regarding the positioning of the implants in the patient’s knee in relation to the anatomical model were of 3.5% on the plate, 18% on the proximal screw, and 4% on the distal screw.

Table 3
Measurements of the positioning angles of the plate and screws in relation to the nonunion line

	X-ray view	Nonunion line
		Knee	Anatomical model
Plate	Lateral	93.0°	89.8°
Proximal screw	Lateral	70.2°	85.7°
Distal screw	Lateral	76.1°	79.4°

Discussion

Before the advent of computer systems to visualize digitized images, such as the picture archiving and communication system (PACS), CT and magnetic resonance imaging (MRI) scans were printed on film, with the loss of valuable information in the process. The PACS changed the way of analyzing the images, providing another dimension to their interpretation, since it enables the dynamica visualization of the 3D object from various angles. The volumetric reconstruction of the studied structure can be exported to CAD software for modeling and rendering, which make it possible to print the object.¹⁰

According to several authors,⁶,¹¹,¹²,¹³,¹⁴ currently, the 3D printing process in the medical field (used for teaching, planning, and surgical training, and the development of surgical guides, implants, and prostheses) consists of the steps showed in ►Fig. 10.

Fig. 10
3D printing process in medicine.

Kim et al.¹⁵ reported their clinical experience with the use of 3D printing techniques in orthopedic trauma, which enables a better understanding of the fracture and its anatomical relationships, and may be applied in the preoperative planning, in medical education, and in surgical training and simulation.

In the present study, the process used for the acquisition of CT scans and rendering and modeling in a CAD environment to create a virtual bone model for 3D printing through FDM technology was successful. According to the literature, the parameters in the acquisition of CT scans, such as the width and number of slices, are determining factors for the accuracy of the volumetric reconstruction. The width of the slices was smaller than 1mm, which generated enough data for a volumetric reconstruction that was very close to that of the actual bone. Segmentation was performed automatically by a computer algorithm with the identification of bone tissue in the threshold window between 226 HU and 2014 HU, which enabled an accurate segmentation, with little need for the removal of artifacts and smoothing the surface during rendering and modeling.

The use of CAD software, such as Invesalius (used for bone segmentation) and Meshmixer (used for modeling and rendering), enabled us to perform the anatomical reduction of bone fragments during the VSP.

The use of 3D printing technology has been growing exponentially in several fields, including orthopedic surgery, and it is relevant in creation of biomodels, surgical tools (such as guides and templates), implants and prostheses.

The anatomical model may also facilitate the communication among the medical team and the patient and their family, providing information about the type of surgical treatment, promoting a better understanding of the clinical picture and of the posible complications, as well as better adherence to the treatment, which contributes to improve the doctor-patient relationship.⁴,¹⁶,¹⁷,¹⁸,¹⁹ The participant in the present study was instructed on the severity and prognosis of the joint injury. For this purpose, we used the printed anatomical model submitted to the surgical simulation, with the nonunion fixed with the implants chosen.

Studies⁴,¹²,¹⁷,²⁰ show the use of anatomical models with AM technology in surgical training. A wide variety of domains including simulation accuracy, anatomical similarity, training in the use of surgical instruments, use printed models for training surgeons. Several studies have demonstrated the effective application of 3D printing in medical education in orthopedics,⁴,¹¹,²⁰,²¹,²²,²³,²⁴,²⁵,²⁶ mainly in surgical procedures in sites of complex anatomy.⁶

There are several CAD software that enable the performance of the VSP to better understand the spatial geometry and anatomical relations. It is possible to program less invasive surgical approaches and anticipate the reduction of bone fragments, simulating the definitive osteosynthesis.²⁷,²⁸ The printing of biomodels provides additional information when compared to conventional imaging scans and increases the knowledge about the lesion.⁵,¹¹,²²,²⁹

Some meta-analyses and systematic reviews³,³⁰ on pre-operative planning and 3D printing-assisted surgery in orthopedic trauma suggest that this method reduces the surgical time, the levels of intraoperative blood loss, and the fluoroscopy time.

In the present study, the printing of the 3D anatomical model with the FDM technology enabled us to develop a careful VSP, to plan the position of the implants, and train the surgical team. Moreover, the anatomical model fixated with the implants was used as a navigation guide during surgery. There was some technical difficulty in positioning the anatomical model so it would correspond to the precise position of the pantient’s knee on the AP radiograph. Due to the lack of other anatomical structures adhered or attached to it, the model did not have geometric and anatomical landmarks that would enable the precise reproduction of specifc X-ray technique for the knee. Thus, the angles reveal that the patient’s knee and the anatomical model were not in the same position at the time the X-ray was performed, as shown in ►Table 2.

In order to achieve a stable osteosynthesis, the screws and the plate must be at an angle close to 90° in relation to the nonunion line, according the AO techniques,⁹ which was achieved in the present study, as shown in ►Table 3.

Despite the difference in the angles of the radiograph of the patient’s knee and those of the anatomical model, the positioning of the implants was adequate, and we were able to reproduce the technique of placing the plate and screws as in the simulation in the anatomical model. This resulted on great fixation stability, and enabled a satisfactory reduction and the anatomical reconstruction of the joint.

According to the radiographic analysis, great accuracy was verified in the reproducibility of the VSP and in the surgical simulation compared with the outcome of the osteosyn-thesis of the Hoffa’s fracture nonunion.

Conclusion

The protocol suggested for the acquisition and segmentation of the CT scans of the knee of a patient with Hoffa’s fracture nonunion proved to be effective for the volumetric reconstruction and rendering of the 3D anatomical model. The model enabled the performance of a careful VSP, as well as the simulation of the ostheosyntesis. According to the radiographic analysis, great accuracy was observed in the reproducibility of the surgical simulation. The use of the 3D-printed model proved to be effective and useful in the planning and surgical treatment of Hoffa’s fracture nonunion.

• Financial Support
  The authors declare they have received no financial support from public, commercial, or non-profit sources.
• *
  Work developed at the Federal Technological University of Paraná, Curitiba, Paraná, Brazil.

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Publication Dates

• Publication in this collection
  11 Aug 2023
• Date of issue
  Mar-Apr 2023

History

• Received
  01 Mar 2022
• Accepted
  28 Apr 2022
• Published
  03 Oct 2022