Our study found that within the last 3 years, there has been a surge in interest regarding the clinical applications of 3D printing in orthopedics with 62.7% of the current literature being published in these 3 years. This trend could be attributed to recent advancements in material design, lower cost of production and greater evidence of clinical benefits of 3D printing[16, 25, 28, 32]. With the expected rise in the number of publications relating to 3D printing, this scoping review seeks to summarize the current literature on 3D printing in orthopedics and identify areas that are lacking in literature to guide future research.
There is a growing interest surrounding the clinical utility of 3D printed prosthesis and orthosis in orthopedics. Recent studies reported that 3D printed orthosis provided more optimal fracture stabilization, immobilization and increased convenience compared to traditional plaster cast[29, 34, 39]. This technology could be particularly useful in low-income countries where despite them having a high demand for prosthesis and orthosis accounting for 0.5% − 1% of their population, only 17% − 37% of people who require them readily have access to these services[31]. Recent studies have found ways to make 3D printed prosthesis more affordable and comparable to conventional prosthesis[31, 38]. For instance, a recent study by van der Stelt et al reported that it was feasible to construct 3D printed lower limb prosthesis at a more affordable cost of 87 USD compared to the average cost of conventional lower limb prosthesis ranging from 100–200 USD[38]. However, potential barriers of entry for low-income countries to adopt 3D printing technology is the high start-up costs incurred during the initial investment such as purchase of a 3D printer, employment of skilled technicians and CT scans required to construct patient specific prosthesis. There have been promising evidence that 3D printers are becoming more affordable (USD400) and similar short term studies reporting adoption of 3D printing in low-income countries is economically feasible[33]. Hence, future studies should focus on both the clinical and economical benefits of 3D printed prosthesis and orthosis for the long term use. This information could greatly improve the quality of life of people staying in low income countries.
When stratified according to subspecialty, we found that trauma (n = 366) and spine (n = 264) had the most literature relating to 3D printing. Moreover, we found that 3D printing was commonly used in regions with complex anatomies such as the pelvis (n = 318), spine (n = 289) and knee (n = 138). One reason is trauma and spine surgery often deal with complex anatomies and require intraoperative fluoroscopy to aid during surgery. Using 3D printed models for pre-operative planning, surgical simulation, and intraoperative referencing will be particularly beneficial. A recent meta-analysis of level 1 RCTs found that 3D printing significantly reduces operating time, blood loss, use of fluoroscopy time and bone union time while increasing accuracy of the operation compared to conventional surgery which greatly benefit both surgeons and patients[41]. Another possible reason could be due to the ease of integration of 3D printing in trauma and spine. In trauma and spine, CT scans of patients are normally required as part of the routine preoperative planning process. Since the process of 3D printing requires pre-operative CT scans, it is convenient for these specialties to incorporate 3D printing into their daily practice. Whereas specialties where CT scans are less required such as pediatrics and sports surgery tend to have less use of 3D printing in their procedures, making their adoption into daily practice more challenging. This shows the importance of optimizing workflows to enable clinicians to easily utilize technologies such as 3D printing in healthcare. This trend has been seen in institutions where on-site 3D printing labs have been set up in the hospital to enable smooth and seamless workflows from clinical needs to 3D printing.
In addition to trauma and spine, adult reconstruction had the 3rd most amount of literature published in 3D printing. Majority of the studies involving adult reconstruction studied the use of 3D printing for 3D printed implants and surgical planning, in particular managing bone defects in revision arthroplasties[24]. Revision arthroplasties with bone defects due to periprosthetic infection or periprosthetic fracture is a challenging procedure[30]. In a tibial reconstruction arthroplasty, cones and sleeves are increasingly being used for reconstruction of massive tibial bone defects[43]. Precise anatomic reconstruction and biomechanical restoration are challenging to accomplish in using preset cones and sleeves that do not have perfect fit[10]. Currently, studies published have found favorable outcomes with utilizing 3D printed cones in the repair of bone defects[19, 24, 45]. However, there has not been any studies published comparing the outcomes between 3D printed cones and conventional cones in the long term outcome. Besides its use in complex revision arthroplasties, 3D printing is also increasingly popular in creating 3D printed patient specific implant guides for total knee and total hip arthroplasties. The benefit of 3D printed implant guides is its high accuracy in predicting the appropriate sizes for implant components. One study reported, achieving 93% and 89% correctness in predicting the sizes for acetabular and femoral components respectively leading to a 61% reduction in implant inventory size which may help to reduce cost [17]. Additionally, better implant fitting may reduce implant loosening, tissue impingement and improve kinematic alignment which may improve implant longevity and patient’s function and satisfaction outcome. However, the current reports have reported no clinical difference in outcomes between convention implants and patient specific implant guides[9].
Our study also found that the use of 3D printing has also spread towards usage in medical teaching (n = 56). 3D-printed models allow for improved identification and understanding of complex anatomy, which is often difficult to appreciate with traditional textbooks[21]. 3D-printed model also has advantages over the traditional cadaver specimens where there are concerns of health and safety issues with formalin fluids[27]. Additionally, these 3D models can also be used to teach and inform patients regarding their condition and the surgical procedure that they will be undergoing[37]. Some studies have reported that personalized 3D printed models enhances patients' subjective satisfaction post operatively[46].
We also observed that most of the literature surrounding the use of 3D printing is focused on semi-urgent or elective cases. To our knowledge, there are few papers reporting the use of 3D printing in emergency settings. A likely explanation is that the production of 3D printed pre-operative guides and implants may require a substantial amount of time due to printing. In view that the use of 3D printed pre-operative surgical guides and implants have shown better clinical outcomes compared to conventional management, there is a potential to adopt 3D printing in the management of emergency cases. Thus, more studies focusing on optimizing their clinical workflow with 3D printing would be beneficial towards the implementation and standardization of 3D printing as a standard of care for both emergency and elective settings.
Interestingly, when the papers were stratified according to materials used, papers relating to surgical planning, prosthesis, orthosis, patient education and surgical training mostly used polylactic acid (PLA) and resin. This is contradictory to the materials commonly used at our institution and a previous scoping review - acrylonitrile butadiene styrene (ABS) and PLA[23]. PLA and ABS are commonly used during fused deposition modeling. Fused deposition modeling (FDM) is a 3D printing method where the thermoplastic filament is heated to its melting point and printed layer upon layer to form a 3D object[11]. Meanwhile, resin is commonly used during stereolithography, an optical manufacturing method where UV rays are applied to liquid monomers (photopolymer resin), to tie them together to form polymers. These polymers are then solidified layer by layer[14]. The advantages of FDM over stereolithography are lower initial investment cost and faster processing. However, the disadvantages are that FDM is able to provide less accuracy compared to stereolithography[22]. We hypothesize that as 3D technology is becoming widely adopted and accessible, more institutions are developing the capacity to produce 3D printed objects via stereolithography more efficiently. The added accuracy that stereolithography provides could be beneficial when developing 3D printed surgical planning tools. Hence, explaining the rise in popularity of resin as a material. Another possible reason could be due to a significant number of papers not mentioning the materials used, thus affecting the results observed.
In papers relating to 3D implants, the most used material was titanium (n = 249, 65.01%) while a small percentage of papers used PEEK implants (n = 9, 2.35%). The difference in properties of these materials were mostly studied in spinal surgery. 3D printed titanium cages were found to have significantly lower subsidence rates, revision surgery and higher pain reduction compared to PEEK cages in stand-alone lateral lumbar interbody fusion[2–4]. 3D printed titanium cages were found to have more porous architecture compared to conventional titanium cages. This decreased the Young’s modulus from an E value of 100,000 MPa to E 2,500 MPa making it closer to that of human bone, reducing the stress-shielding effect. In addition, biomechanical studies found that cages with a porous architecture had added benefits such as less stress at the bone-hardware interface[12], increased bone to implant contact surface making the cages more osteoconductive and increased the compressive shear strength under physical force[6, 26, 36]. Hence, these differences in material properties could explain why 3D printed titanium is more widely utilized in literature compared to PEEK.
The strengths of this review are its rigorous methodology, extensive inclusion criteria, and current relevance. We deliberately employed a broad inclusion criterion to capture as many articles on 3D printing in orthopedics to provide a comprehensive and updated understanding on the evolving field of 3D printing. Moreover, the strict adherence to the PRISMA guidelines establishes the methodological integrity of this review.
However, this review was limited by the overall low level of evidence available with most studies being level IV evidence. Furthermore, there was a significant proportion of studies which did not report the type of materials used. Hence, this limited the conclusions we could draw from analyzing the types of materials used. Lastly, the heterogeneity of the included studies precluded a meta-analysis.