Evolution of a Surgical Technique: A Brief Communication Regarding 5 Cases of Three-Dimensional Printed Sternums.

METHODS
The Central Adelaide Local Health Network Human Research and Ethics Committee (CALHN HREC) was responsible for ethics application approval (Approval Number: 21022). All operations were performed at the Royal Adelaide Hospital, which is the major public hospital within CALHN. For all operations, written informed patient consent was obtained. Each patient was advised of the risks of general anaesthesia and sternal replacement using a combination of titanium and StarPore or simply StarPore alone. No authors are affiliated with an implant manufacturer, nor did they receive any funding. Implants were custom-made under compassionate grounds.

Case study 1
This patient was a 39-year-old woman who previously was treated for left breast carcinoma with a partial mastectomy and axillary clearance, chemotherapy and then oestrogen blockade. Five years later, a computed tomography scan demonstrated the destruction of the sternum. A biopsy proved metastasis to the sternum with no evidence of any other spread. She was approved by a multidisciplinary committee for resection. Anatomics Pty Ltd (Bentleigh East, Victoria, Australia) facilitated the design and creation of the prosthesis. A high-resolution computed tomography scan created Digital Imaging and Communications in Medicine (DICOM) data of the patient and in turn was uploaded via proprietary software.1 This procedure allowed a 3-dimensional (3D) surface rendering of the chest wall and tumour.
The prosthesis was then designed in silico via remote collaboration with the surgeon using their AnatomicsC3D platform.1 An Arcam A1 electron-beam melting machine (CSIRO, Clayton, Australia) was used for the titanium printing.1 The StarPore coating was then added to the 3D-printed titanium prosthesis as a post-processing procedure.1 Lastly, a resection template was 3D printed from a biocompatible material to allow precise placement of resection margins intraoperatively.1
The surgical technique used a resection template, during which the sternal tumour was removed en bloc (Figure). To ensure that the margins where clear, frozen sections of the edges of the wound were sent before proceeding with the next step of the operation. A Vicryl-Prolene mesh was sutured to the edges of the pleura to prevent lung herniation.1
The prosthesis featured rib-attachment clamps with a serrated inner surface, as demonstrated in the Figure.1 These clamps facilitated a secure fit onto each of the costal stumps. Each clamp was then fastened to the costal stumps with titanium screws at 90 degrees. The superior and inferior anchor points on the prosthesis were secured to the corresponding sternal edges with sternal wires. The prosthesis was covered with bilateral pectoral muscle flaps in a double-breasted fashion, which was sutured directly to the StarPore. Chest drains were inserted into each pleura, and bilateral closed-suction drains were inserted under the muscle flaps. Postoperatively the patient was happy with the result and reported a return to normal physical and respiratory function at the 3-month assessment. There were no symptoms of infection or instability of the prosthesis.

Case study 2
This patient was a 47-year-old woman who was diagnosed with breast cancer in 2009. Her cancer was treated with neoadjuvant radiotherapy followed by a left mastectomy and axillary clearance. A recurrence in 2012 was treated with a wide local excision, and in 2015 she was diagnosed with a subsequent isolated metastases involving the sternum.
The prosthesis used in this case was similar to that used in case study 1. It featured a titanium component, and a StarPore component followed the contour of the clamps and anterior flanges, while filling the bulk of the sternal region.
However, there have been several advances in surgical technique that have occurred during the same period. A Gore-Tex mesh was used instead of a Prolene mesh to prevent lung herniation. In addition, superior and inferior anchor points were secured with titanium screws rather than sternal wires through predrilled holes in the titanium portion. Unfortunately, in this case, the implant became infected and ultimately was removed.

Case studies 3 to 5
The first of these was a 52-year-old woman with oligometastasis from breast cancer. The second was a 62-year-old woman with primary sternal chondrosarcoma, and the third was a 65-year-old woman with thyroid metastasis to the manubrium.
These 3 cases were similar to each other in several aspects. However, they also differed in several respects from the first 2 cases. First, there were no titanium components in the prosthesis that use only complete StarPore, as demonstrated in the Figure. Second, there were no predrilled anchor points, which allowed the possibility of intraoperative modification. Finally, the prosthesis was secured with titanium screws and a combination of sternal and fibre wires for bone and cartilage, respectively.
Case 3 was similar to the first 2 case studies because Gore-Tex mesh was used to prevent lung herniation. However, in cases 4 and 5, we switched back to Prolene mesh because it was thought to be more porous and therefore should reduce the likelihood of seroma formation. Unfortunately, case 3 did develop a seroma that was subsequently drained. See the Table, which compares the different surgical techniques and materials used in all 5 cases.

DISCUSSION
Anatomics Pty Ltd (Bentleigh East, Australia) was the responsible for the manufacturing and engineering of each of the prostheses. The surgeon designed each prosthesis using the company platform. Four key steps are involved in this process. First, a computed tomography scan was performed with 1-mm original fine slices. Second, an online planning session (1-2 days) took place with the engineering team. Third, designs and a prototype were developed and sent to the surgeon for initial review. Subsequently online planning sessions took place between the surgeon and the engineers. Fourth, the product took approximately 10-20 days to manufacture, depending on the materials used in the design process.
StarPore is a porous high-density polyethylene implant material that can be customized and used effectively in facial and cranial reconstruction.2 The implants are porous, creating the potential for integration of the implants with surrounding tissue, which would be a desirable outcome to stabilize the implants over long periods of time.2 There are limited case reports in the literature of a 3D printed prosthesis used for sternal reconstruction that is part titanium and part StarPore.3–6 Furthermore, there are currently no published case reports of 3D printed sternums composed completely of StarPore.
StarPore reconstructions demonstrate several significant advantages. First, StarPore allows easy intraoperative modification. The surgeon can easily suture soft tissue anywhere in the implant and can screw anywhere through material without predrilling holes.7 Furthermore, one can soak the material in antiseptic liquid and then in an antibiotic wash. In addition it is lighter in weight than polyetheretherketone (PEEK) or titanium, which allows more manipulation and provides some flexibility, making it easier to implant.7 From a postoperative perspective, the material does not cause imaging artefact (computed tomography/x-ray/magnetic resonance imaging), which is important for oncology cases. Furthermore, initial results suggest that soft tissue grows into the implant within 4 weeks of the operation, which should help with stabilization. Moreover, this theoretical tissue integration and fibrovascular ingrowth should help to reduce infection and tissue erosion. Finally, in terms of longevity, StarPore material has 510 K FDA approval for craniofacial surgery (CMF) and 10 years of clinical safety in CMF. It is important to note that this technology is innovative and that the long-term implications of this prosthesis are yet to be fully understood. However, there is no suggestion at this point that the prosthesis should be removed after 10 years.
Interestingly, despite lighter weight and intraoperative malleability, there is no inferiority in terms of resistance and strength against the usual loads placed on the sternum.8 Girotti et al. conducted a study to examine different prosthetic materials used for chest wall prostheses that are placed under critical fracture conditions.8 These critical conditions included 3 forms of loads–sternal, vertical and lateral. There was no inferiority found with StarPore.
A significant limitation of all cases was the cost, with all 3D printed prostheses exceeding $20,000 AUD for the sternum alone. Furthermore, another issue is the potential delay of the operation, given the required planning and manufacturing processes. One may argue this is an acceptable outcome because the operation can be planned within 30 days with adequate planning and communication.

CONCLUSION
The 3D printed sternums are an innovative step forward that can provide relief to individuals with a customized and precise fit. The evolution of available materials has meant that the surgeon involved in these cases has shifted from using a pure titanium prosthesis to StarPore. StarPore has significant advantages over its counterparts in its light weight, intraoperative malleability, anti-infective properties and radiologic compatibility while also boasting a strength similar to that of other materials when testing sternal, lateral and vertical load forces.