Introduction

Modification in joint loading, and specifically shear stress, is found to be an important mechanical factor in the development of osteoarthritis (OA). Cartilage shear stresses can be investigated using finite element (FE) modelling, where typically in vivo joint loading as measured by an instrumented hip prosthesis is used as boundary condition. However, subject-specific gait characteristics substantially affect joint loading. The goal of this study is to investigate the effect of subject-specific joint loading as calculated using a subject-specific musculoskeletal model and integrated motion capture data on acetabular shear stress.

Several studies have shown that gait kinematics[1–3] and hip contact forces (HCFs)[4, 5] of patients following total hip arthroplasty (THA) do not return to normal, although improvements in kinematics are found compared to the pre-surgery. However, the evolution of HCFs after surgery has not been investigated. The goal of this study is to evaluate HCFs during gait in OA patients before and at 2 evaluation moments post-THA.

Fourteen unilateral hip OA patients before and 3- and 12-months post-THA surgery walked at self-selected speed, as well as 18 healthy control subjects. 3D marker trajectories were captured using Vicon (Oxford Metrics, UK) and force data was measured using two AMTI force platforms (Watertown, MA). A musculoskeletal model consisting of 14 segments, 19 degrees of freedom and 88 musculotendon actuators and including wrapping surfaces around the hip joint was used[6]. All analyses were performed in OpenSim 3.1[7]. The model was scaled to the dimensions of each subject using the marker positions of a static pose. A kalman smoother procedure was used to calculate joint angles[8]. Muscle forces were calculated using static optimization, minimizing the sum of squared muscle activations. HCFs were calculated and normalized to body weight (BW). First and second peak HCFs were determined and used for statistical analysis. To determine differences between HCFs of OA patients at the different evaluation moments, a Friedman test was used. In case of a significant difference, post-hoc rank-based multiple comparison tests with a Bonferonni adjustment was used. To compare controls and patients at each evaluation moment separate Man-Whitney U tests were used. Differences in HCFs between the affected and non-affected legs were expressed by a symmetry index (SI), i.e. the ratio between the HCFs of the affected leg over the non-affected leg, averaged over the stance phase of the gait cycle.

At the first and second HCF peaks, no significant differences were found between pre-, 3- and 12-months post-surgery (first peak average HCF: 2.68, 2.72 and 2.78BW respectively; second peak average HCF: 3.21, 3.83 and 3.77BW respectively). Compared to controls, significant differences are found for all evaluation moments at the first and second HCF peaks (average HCF controls: 3.43 and 5.15BW respectively). The SI was below 1 pre- and 3-months post-surgery (0.88 and 0.85 respectively), indicating decreased loading of the affected compared to the non-affected leg. At 12-months post-surgery SI was close to 1 (0.98).

As reported before[4, 5], first or second peak HCFs do not return to normal after THA. Although HCFs increase after THA compared to pre-surgery, significant differences with controls remain. Surprisingly, no significant differences are found between the different evaluation moments of the patients, indicating no clear improvements are found after THA. Further, average HCF peaks at 3- and 12-months post-surgery are similar, indicating no further improvements are found 3-months post-surgery. However, the SI was above or close to 1 at 12-months post-surgery, indicating hip loading evolved to a more symmetrical loading 12-months post-surgery.

In the area of 3D printing, more and more maxillofacial surgery departments are equipped with 3D printers to build their own anatomical models or surgical guides. Prior to be printable, the patients' DICOM imaging data has to be converted to a 3D virtual model, a 3D mesh. The file format most commonly used is the STL (Standard Tesselation Language) file format. Many programs exist that are able to convert DICOM data to STL files. Commercial software, such as Surgicase CMF© are FDA- and CE-approved whereas free programs, available online do not have the approval. However, the latter are often used anyway because of financial reasons. In this article, we investigate whether 6 of these software solutions are equivalent or not.

Thin slice CT imaging data of a patient's mandible (in DICOM file format) was converted to STL meshes with 6 different software solutions. One commercial program, Surgicase CMF©, was used to build the reference model. Then 5 free programs were used to create 5 models of the same mandible, specifying the same thresholding parameters: InVesalius 3.0, 3DimViewer 2.2.4, 3D Slicer, itk-Snap and Seg3D. All of these models were loaded in Netfabb Basic 6.4 to retrieve dimensional data, geometric information and the number of holes in each mesh. Finally, the models were then compared to the reference model using CloudCompare 2.6.2.

All models created with free software differed from the reference model in the 3 dimensions. Mean length difference was −0.74 mm [−2.06; −0.32] (SD: 0.74), mean width difference −0.45 mm [−0.76; −0.25] (SD: 0.19) and mean height difference was 0.41 mm [0.14; 0.62] (SD: 0.18). Although the height was increased in all models, both the length and width were systematically decreased, resulting in an average decrease of volume of −7.1 cm³ [−7.45; −6.77] (SD: 0.32). The number of triangles used to create each mesh ranged from 20944 to 368244, resulting in a variation of the file size from 1023 Ko to 80462 Ko (0.16 to 12.70 times the file size of the reference model). Two of the free programs created meshes with errors, such as the presence of holes (non-watertight meshes) that could be repaired with Netfabb.

Free programs able to convert volume imaging data to a printable virtual mesh do not provide equivalent results. Variations were noted in the three plane of space with a systematic difference between free programs and the commercial FDA-approved one. While the length and width were less than a millimeter different to the reference, the dimension that most varied was the length with a difference reaching −2.06 mm with itk-Snap. Geometric data also varied significantly, the number of triangles composing the meshes being much different than the reference, resulting in variable file sizes. This traduces the fact that algorithms used by the programs are not the same. In the era of 3D printing made directly accessible in surgical departments, great attention should be paid to the accuracy of the models created with free software.

Reconstructing mandibular and maxillary bone defects with free vascularized bone flaps requires to take into account the aesthetic and functional requirements to consider subsequent placement of dental implants. It implies a three-dimensional conformation of the bone fragment. This is usually done by making osteotomies on the bone harvested. The aim of our study was to evaluate the interest of virtual planning and 3D printing using free software and a consumer printer in this indication.

Invesalius® software (Technology of Information Renato Archer Center, Campinas, Brazil) was used to build virtual models from the patients' CT scan imaging data. The surgical procedure was planned using Meshmixer® (Autodesk, San Rafael, United States). Meshlab® software (Visual Computing Lab, Pisa, Italy) was used to design cutting guides for the flap harvest and modelling. 3D printing of these guides with a consumer printer (Ultimaker 2® Ultimaker B.V., Geldermalsen, the Netherlands) allowed the transfer of the planning to the operating room.

Three patients requiring mandibular reconstruction underwent an iliac crest free flap, a fibula free flap and a scapula free flap, and could benefit from this technique. In each case, the bone resection was performed virtually and the positioning of the bone available at the donor site was simulated on screen. This allowed to anticipate the position and orientation of the cutting planes on the bone flap. From the anatomy of the donor site and the cutting planes, harvest templates and cutting guides could be designed by computer. Planning the conformation of the bone flap to the recipient site has allowed an anatomical, aesthetic and functional reconstruction of the bone defect.

Surgeon-made virtual planning and “low cost” 3D printing helps harvest the bone flap and position and orient the osteotomies to adapt it to the defect. They provide, both the patient and the surgeon, reduced operative time and better anticipation of the result, particularly in the context of the maxillofacial reconstruction. Compared to commercially available custom-made devices, this technique allows the manufacture of the guides without delay and at a cheap price.

The management of maxillofacial injuries requires restoring the contours of the facial skeleton to achieve an aesthetic outcome. When fractures are simple, open reduction and rigid fixation with stock titanium osteosynthesis plates is usually sufficient. However, when the damage is more substantial (when the fracture is comminuted or in case of a bone defect) anatomical landmarks are lost and the reconstruction requires the use of titanium meshes. These meshes are usually modelled intraoperatively to restore the contours of the bone. This can be a tough and time consuming task in case of minimal invasive approach and intraoperative edema. When the injury is unilateral, printing a 3D anatomical model of the mirrored unaffected side is an easy way to accurately pre-bend the mesh preoperatively. With the emergence of “low cost” consumer 3D printers, the aim of our study was to evaluate the cost of this technique in a department of maxillofacial surgery.

The first part of the study was to evaluate free software solutions available online to determine which of these could be used to create 3D virtual models from the patients' volume imaging data, mirror the model and export an STL file suitable for 3D-printing with a consumer 3D-printer. The second part was to identify the desktop 3D-printers commercially available according to the different technology used, their prices and that of consumables required.

Five free software solutions were identified to create STL meshes of the patient's anatomy from thin slice CT scan DICOM data. Two more were available to repair, segment and mirror them to provide a clean STL file suitable for 3D printing with a desktop 3D printer. The prices of 2 different printers were then listed for each of the 3 additive manufacturing technologies available to date. Prices ranged from 2,299 € for the Ultimaker 2+© (Fuse Deposition Modeling, FDM), to 4,999 € for the Sintratec© printer (Selective Laser Sintering, SLS), the Formlabs 2© (stereolithography) being at an intermediate price of 3,299 €. Finally, the cost of the manufacture of a model was calculated for each of these printers. Considering a model of a supraorbital ridge printed to restore the anterior wall of the frontal sinus, the volume of the mesh is around 20 cm³. This represents a cost of less than 1 € with the FDM technology, 4.70 € with stereolithography and 1.50 € with the SLS printer.

Since patents of additive manufacturing have become part of the public domain, the cost of 3D printing technology has fallen drastically. Desktop printers are now an investment accessible to a surgery department and the cost of the material is low. This allows the surgeons, by the mean of free software, to directly create 3D models of their patients' anatomy, mirror them if needed and manufacture a template to pre-bend titanium meshes that will be subsequently sterilized for the surgery. Having the printer in the department reduces manufacturing lead times and makes this technique possible even for urgent cases.