Introduction

Pectus excavatum is the most common congenital chest wall deformity [1]. There is a wide spectrum of severity, with extreme cases manifesting clinically as reduced exercise tolerance from compromised cardiopulmonary function. This would generally prompt consideration of surgical correction to improve function and may require an intrathoracic procedure (Nuss, Ravitch procedures) which carries substantial risks [2, 3]. Yet, the majority of cases do not have significant cardiorespiratory compromise and can be managed conservatively from a functional perspective [4, 5]. However, an uncorrected deformity may adversely affect the psychological well-being of the patient. Whilst this can be corrected with surgery, the risks associated with an intrathoracic procedure become harder to justify [6, 7]. Extrathoracic camouflage techniques using a variety of non-degradable implant materials have been used to improve pectus excavatum deformity with a safer risk profile than intrathoracic procedures [6].

Silicone implants have a long and well-reported history of use, mostly in the discipline of breast surgery [8], but also in other soft tissue augmentation [9]. They can also be manufactured in a customised way [6]. The complication profile is extensively documented and includes capsular contracture, implant migration, rupture, extrusion and infection [10, 11]. Silicone implants have been implicated in the development of Breast Implant Associated Anaplastic Large Cell Lymphoma (BIA-ALCL) [12, 13]. These are significant drawbacks, particularly for young adults seeking correction of pectus excavatum. Permanent implantable materials other than silicone, such as polyurethane and porous polyethylene, have also been used but not as widely [14, 15]. The use of polyurethane implants has also been implicated in the development of BIA-ALCL [16]. On the other hand, since polyethylene implants are permanent, they have a complication profile that is similar to silicone prostheses [17, 18]. An alternative autologous method to reconstructing pectus excavatum defects has been suggested using fat graft with encouraging results [19]. However, this technique is limited by unpredictable fat resorption [20], so its best use is in minor deformities.

Scaffold-guided tissue engineering (SGTE) is a promising approach to address large volume defects, where an interdisciplinary framework of biology, material science and biomedical engineering principles are employed to modulate the regeneration of vascularized soft tissue. Scaffolds are engineered as highly porous implants serving as substrates to guide cellular interactions and tissue formation. We propose a new camouflage technique using a biodegradable, patient-matched scaffold which is 3D printed from medical grade polycaprolactone (mPCL). The highly porous implant is immediately filled with an autologous fat graft. mPCL has bioresorbable properties, avoiding many of the issues with permanent implants [21,22,23,24,25,26]. This offers a SGTE approach to generate autologous tissue for the purpose of pectus excavatum reconstruction.

Surgical technique

Pre-operative

Patients are selected evaluating their pectus excavatum deformity by taking a focussed history and performing an examination, exercise stress testing and imaging. Computed tomography (CT) scans using thin slice (1 mm) and standard helical acquisition are performed of the patient’s chest wall deformity. Computer-aided design is used to generate a porous scaffold model to sit on the sternum and costal cartilages to fill the pectus excavatum defect. Scaffolds are created by additive manufacturing using mPCL in a GMP-certified cleanroom facility (BellaSeno GmbH, Leipzig). Scaffolds are printed with a 0–90 degree lay down pattern, filament diameter of 300–350 μm and internal pore size of 6 mm (Fig. 1). Scaffolds are quality control tested, plasma-treated, gamma sterilised, packaged and shipped (Appendix 1).

Fig. 1
figure 1

Pectus excavatum scaffold in sterilised packaging prior to implantation

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Operative technique

The surgical procedure is performed under a general anaesthesia in a supine position. Antibiotics (2 g cephazolin) are administered intravenously during induction. An incision is made in the lower parasternal region. Medial fibres of pectorals major are released from their origin to create a pocket superficial to the sternum and costal cartilages (see Fig. 2A). The scaffold is compressed to a width that could be accommodated through the incision and re-expanded to its original dimensions once placed into the pocket. Compression without implant damage is able to be performed due to the known mechanical properties of porous PCL scaffolds [27]. As a result of extensive pre-operative planning, there should be an excellent fit (Fig. 2B). Fat graft harvest sites on the patient’s abdomen and thighs are infiltrated with tumescent fluid (1 L 0.9% sodium chloride mixed with 200 mg of Ropivacaine). Fat graft is harvested and prepared using the Coleman fat transfer method [28], and fat graft is injected into the scaffold with a standard fat injection cannula. The wound is closed in layers with 3-0 monocryl and dressed with a dry dressing.

Fig. 2
figure 2

Intraoperative view. Panel A shows dissection of pocket above the sternum and costal cartilages by releasing medial origin of the pectoralis major. Panel B shows an excellent scaffold fit inside the pocket. The patient has Klippel-Trenaunay syndrome with a large lymphovascular malformation affecting her left chest which is pixelated

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Post-operative

Patients are discharged from the hospital once they are comfortable, and the surgical wound is stable. Patients are reviewed at 1 week, 4 weeks, 12 weeks and 9 months post-operatively to evaluate clinical outcomes. An MRI scan is performed 10 weeks after the surgery to evaluate radiological outcomes.

Case presentation

A 22-year-old female presented as an adult to our institution with severe pectus excavatum (Haller index = 9.75, New York Heart Association Class II Heart Failure). The patient was affected by Klippel-Trenaunay syndrome with a large lymphovascular malformation of the left chest and upper limb. The chest wall component had been serially excised as a child incorporating the breast and nipple-areola complex. Previous treatment for the pectus excavatum included three Nuss bar procedures which were abandoned intraoperatively due to the risk of injury to intrathoracic organs. On the first attempt, the patient suffered a cardiac arrest due to right atrial compression, which resolved upon removal of the Nuss bar. A temporising tissue expander had been inserted 4 years prior to this procedure but had migrated inferiorly, causing discomfort and subsequently removed. A customised silicone implant was designed which would have weighed 730 g and may have further compromised cardiorespiratory function. An alternative lightweight solution was considered. The patient had suffered upper limb deep vein thrombosis in recent years and was managed with upper limb compression therapy and anticoagulation (Rivaroxaban 20 mg daily).

The patient was reviewed by our institution’s clinical ethics committee where it was agreed that offering the patient an experimental treatment was acceptable. The process of written consent was as for a clinical trial. Based on CT imaging, a custom-made scaffold was designed and manufactured. The scaffold had dimensions of 169 × 94 × 49 mm and a volume of 328 cc and weighed 35.8 g (Figure 1). We performed the described surgical technique to reconstruct the patient’s pectus excavatum deformity. A 60-mm incision was used through an existing scar on the patient’s right inferior parasternal region. The 94-mm-wide scaffold was able to be compressed through the 60-mm incision and subsequently re-expanded to fill the defect with an excellent fit. One hundred thirty cubic centimetre of the fat graft was harvested and injected into the scaffold. Post-operatively, the patient recommenced her usual anticoagulant regimen and was discharged on post-operative day two with simple oral analgesia. The patient attended post-operative reviews and an MRI scan was performed ten weeks after the surgery.

There were no major post-operative complications. The patient developed a tender fluid collection superficial to the implant after an episode of minor trauma 3 weeks after surgery. Ultrasound-guided aspiration was unsuccessful, in keeping with early presentation haematoma with a volume estimated at 30 ml. This was managed conservatively and settled over the following 2 weeks. There were no other complications.

There was considerable improvement in the appearance of the pectus excavatum deformity with restitution of the position of the right breast after correction of its base position (Fig. 3). Clinically, there has been no migration of the implant, and this has been confirmed on MRI using a standard DIXON fat suppression method to show and quantify the amount of adipose tissue. Segmentation showed retention of 69 ml of adipose tissue, representing 21% filling of the total potential scaffold volume at 10 weeks (see Fig. 4). The patient tolerated the scaffold well and was extremely satisfied with the outcome at a 9-month review. There was no change in exercise tolerance on an exercise stress test conducted before and after the operation.

Fig. 3
figure 3

Pre-operative (A, B, C) and post-operative (C, D, E). See improvement in pectus excavatum appearance and restitution of the right breast base position. The patient has Klippel-Trenaunay syndrome with a large lymphovascular malformation affecting her left chest which is pixelated

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Fig. 4
figure 4

Magnetic resonance imaging DIXON method of an implanted scaffold filled with fat graft to correct a pectus excavatum deformity at 10 weeks. Fat suppressed (A) and water suppressed (B) showing large islands of adipose tissue retained in the scaffold

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Discussion

A patient-matched approach is important to pectus excavatum reconstruction due to the varying presentation of deformities. Chavoin et al. [6] demonstrated the utility of customised silicone implant to improve the cosmetic and psychological impact of pectus excavatum. They also demonstrated that malformations were better corrected with computer-aided design technology, as opposed to the creation of a cast mould. However, almost half of these patients were aware of the prosthesis during increased activity. They did not report any evidence of capsular contracture over a mean of eight years follow-up. The risk of implant failure is a lingering concern.

In clinically relevant volumes, the weight of large silicone implants is significant, which impacts patient tolerability. In our case, the patient was unwilling to accept a 730 g silicone implant. The 35.8-g porous scaffold compares favourably. Grappolini et al. [15] also recognised the benefit of using a porous construct to improve patient comfort. They used a permanent porous polyethylene implant wrapped in a pedicled omental flap. An added benefit of a porous construct is that it allows tissue infiltration reducing the risk of implant migration. However, polyethylene implants are permanent and have a complication profile that is similar to silicone prostheses [17, 18].

Our SGTE approach utilises the advantages of a highly porous implant, as well as using a biodegradable material (mPCL), which avoids issues seen in the permanent prosthesis. The use of mPCL has a safe degradation profile, and devices fabricated in mPCL have been FDA approved and CE mark registered [29]. The use of a scaffold offers grafted fat protection against external mechanical forces, which can inhibit survival, and facilitates the ingrowth of resident adipocytes and vasculature [30]. Large animal studies have also shown the survival of fat graft when using SGTE [31, 32]. In this case, we were able to retain a significant volume of fat graft, despite grafting into an avascular structure. There is also an opportunity for additional fat grafting procedures to further fill the scaffold, although this has not been performed in this case as the patient is currently satisfied with their outcome.

The extent to which SGTE translates in the human will be borne out in serial imaging studies. If successful, this technique has the potential to transform soft tissue augmentation. This case illustrates the first-in-human trial of a bioresorbable custom-made 3D-printed scaffold with autologous fat transfer to camouflage a pectus excavatum defect not previously reconstructable by conventional techniques.