Anterior Lumbar Interbody Fusion Using a Personalized Approach: Is Custom the Future of Implants for Anterior Lumbar Interbody Fusion Surgery?

Anatomically and mechanically conforming patient-specific spinal fusion cages designed by full-scale topology optimization

Cage subsidence after instrumented lumbar spinal fusion surgery remains a significant cause of treatment failure, specifically for posterior or transforaminal lumbar interbody fusion. Recent advancements in computational techniques and additive manufacturing, have enabled the development of patient-specific implants and implant optimization to specific functional targets. This study aimed to introduce a novel full-scale topology optimization formulation that takes the structural response of the adjacent bone structures into account in the optimization process. The formulation includes maximum and minimum principal strain constraints that lower strain concentrations in the adjacent vertebrae. This optimization approach resulted in anatomically and mechanically conforming spinal fusion cages. Subsidence risk was quantified in a commercial finite element solver for off-the-shelf, anatomically conforming and the optimized cages, in two representative patients. We demonstrated that the anatomically and mechanically conforming cages reduced subsidence risk by 91% compared to an off-the-shelf implant with the same footprint for a patient with normal bone quality and 54% for a patient with osteopenia. Prototypes of the optimized cage were additively manufactured and mechanically tested to evaluate the manufacturability and integrity of the design and to validate the finite element model.

Advancements in Custom 3D-Printed Titanium Interbody Spinal Fusion Cages and Their Relevance in Personalized Spine Care

3D-printing technology has revolutionized spinal implant manufacturing, particularly in developing personalized and custom-fit titanium interbody fusion cages. These cages are pivotal in supporting inter-vertebral stability, promoting bone growth, and restoring spinal alignment. This article reviews the latest advancements in 3D-printed titanium interbody fusion cages, emphasizing their relevance in modern personalized surgical spine care protocols applied to common clinical scenarios. Furthermore, the authors review the various printing and post-printing processing technologies and discuss how engineering and design are deployed to tailor each type of implant to its patient-specific clinical application, highlighting how anatomical and biomechanical considerations impact their development and manufacturing processes to achieve optimum osteoinductive and osteoconductive properties. The article further examines the benefits of 3D printing, such as customizable geometry and porosity, that enhance osteointegration and mechanical compatibility, offering a leap forward in patient-specific solutions. The comparative analysis provided by the authors underscores the unique challenges and solutions in designing cervical, and lumbar spine implants, including load-bearing requirements and bioactivity with surrounding bony tissue to promote cell attachment. Additionally, the authors discuss the clinical outcomes associated with these implants, including the implications of improvements in surgical precision on patient outcomes. Lastly, they address strategies to overcome implementation challenges in healthcare facilities, which often resist new technology acquisitions due to perceived cost overruns and preconceived notions that hinder potential savings by providing customized surgical implants with the potential for lower complication and revision rates. This comprehensive review aims to provide insights into how modern 3D-printed titanium interbody fusion cages are made, explain quality standards, and how they may impact personalized surgical spine care.

Use of Patient-Specific Interbody Cages Through a Minimally Invasive Lateral Approach for Unstable Lumbar Spondylodiskitis

BACKGROUND AND OBJECTIVES

Patients with diskitis/osteomyelitis who do not respond to medical treatment or develop spinal instability/deformity may warrant surgical intervention. Irregular bony destruction due to the infection can pose a challenge for spinal reconstruction. The authors report a lateral approach using patient-specific interbody cages combined with posterior or lateral instrumentation to achieve spinal reconstruction for spinal instability/deformity from spondylodiskitis.

METHODS

This is a retrospective review of 4 cases undergoing debridement, lateral lumbar interbody fusion using patient-specific interbody cages, and supplemental lateral or posterior instrumentation for spinal instability/deformity after spondylodiskitis. The surgical technique is reported, as are the clinical and imaging outcomes.

RESULTS

Four male patients with a mean age of 69 years comprised this study. One had lateral lumbar interbody fusion at L2/3 and 3 at L4/5. The mean hospital stay was 5.8 days. The mean follow-up was 8.5 months (range 6-12 months). There were no approach-related neurological injuries or complications. The mean visual analog scale back pain scores improved from 9.5 to 1.5, and the mean Oswestry disability index improved from 68.5 to 23 at the end of the follow-up. The mean lumbar lordosis increased from 18° to 51°. The segmental angle increased from 6.5° to 18°. The coronal shift was 2.8 cm preoperatively and 0.9 cm postoperatively. The coronal Cobb angle reduced from 8.8° preoperatively to 2.8° postoperatively. On postoperative computed tomography, all patients had interval development of bridging bone across the surgical level through or around the cage. None of them developed cage migration or subsidence.

CONCLUSION

Patients with irregular bony destruction due to diskitis/osteomyelitis may benefit from patient-specific cages for spinal reconstruction to address spinal instability and deformity.

Myth or fact: 3D-printed off-the-shelf prosthesis is superior to titanium mesh cage in anterior cervical corpectomy and fusion?

BACKGROUND

To find out if three-dimensional printing (3DP) off-the-shelf (OTS) prosthesis is superior to titanium mesh cages in anterior cervical corpectomy and fusion (ACCF) when treating single-segment degenerative cervical spondylotic myelopathy (DCSM).

METHODS

DCSM patients underwent ACCF from January 2016 to January 2019 in a single center were included. Patients were divided into the 3DP group (28) and the TMC group (23). The hospital stays, operation time, intraoperative blood loss, and the cost of hospitalization were compared. The Japanese Orthopedic Association (JOA) scores and Neck Disability Index (NDI) were recorded pre-operatively, 1 day, 3, 6, 12, and 24 months post-operatively. Radiological data was measured to evaluate fusion, subsidence, and cervical lordosis. Patients were sent with SF-36 to assess their health-related quality of life (HRQoL).

RESULTS

The differences in operative time, intraoperative blood loss, and hospital stay were not statistically significant between groups (p > 0.05). Postoperative dysphagia occurred in 2 cases in the 3DP group and 3 cases in the TMC group, which all relieved one week later. The difference in improvement of JOA and NDI between the two groups was not statistically significant (p > 0.05). No hardware failure was found and bony fusion was achieved in all cases except one in the 3DP group. The difference in cervical lordosis (CL), fused segmental angle (FSA), mean vertebral height (MVH), and subsidence rates between groups at each follow-up time point was not statistically significant and the results of the SF-36 were similar (p > 0.05). The total cost was higher in the 3DP group with its higher graft cost (p < 0.05).

CONCLUSION

In treating single-segment DCSM with ACCF, both 3DP OTS prosthesis and TMC achieved satisfactory outcomes. However, the more costly 3DP OTS prosthesis was not able to reduce subsidence as it claimed.

3D printing for spine pathologies: a state-of-the-art review

Three-Dimensional Printing has advanced throughout the years in the field of biomedical science with applications, especially in spine surgeries. 3D printing has the ability of fabricating highly complex structures with ease and high dimensional accuracy. The complexity of the spine's architecture and the inherent dangers of spinal surgery bring the evaluation of 3D printed models into consideration. This article summarizes the benefits of 3D printing based models for application in spine pathology. 3D printing technique is extensively used for fabrication of anatomical models, surgical guides and patient specific implants (PSI). The 3D printing based anatomical models assist in preoperative planning and training of students. Furthermore, 3D printed models can be used for improved communication and understanding of patients about the spinal disorders. The use of 3D printed surgical guides help in the stabilization of the spine during surgery, improving post procedural outcomes. Improved surgical results can be achieved by using PSIs that are tailored for patient specific needs. Finally, this review discusses the limitations and potential future scope of 3D printing in spine pathologies. 3D printing is still in its infancy, and further research would provide better understanding of the technology's true potential in spinal procedures.

Nanoscale Morphologies on the Surface of 3D-Printed Titanium Implants for Improved Osseointegration: A Systematic Review of the Literature

Three-dimensional (3D) printing is serving as the most promising approach to fabricate personalized titanium (Ti) implants for the precise treatment of complex bone defects. However, the bio-inert nature of Ti material limits its capability for rapid osseointegration and thus influences the implant lifetime in vivo. Despite the macroscale porosity for promoting osseointegration, 3D-printed Ti implant surface morphologies at the nanoscale have gained considerable attention for their potential to improve specific outcomes. To evaluate the influence of nanoscale surface morphologies on osseointegration outcomes of 3D-printed Ti implants and discuss the available strategies, we systematically searched evidence according to the PRISMA on PubMed, Embase, Web of Science, and Cochrane (until June 2022). The inclusion criteria were in vivo (animal) studies reporting the osseointegration outcomes of nanoscale morphologies on the surface of 3D-printed Ti implants. The risk of bias (RoB) was assessed using the Systematic Review Centre for Laboratory Animal Experimentation (SYRCLE's) tool. The quality of the studies was evaluated using the Animal Research: Reporting of In Vivo Experiments (ARRIVE) guidelines. (PROSPERO: CRD42022334222). Out of 119 retrieved articles, 9 studies met the inclusion criteria. The evidence suggests that irregular nano-texture, nanodots and nanotubes with a diameter of 40-105nm on the surface of porous/solid 3D-printed Ti implants result in better osseointegration and vertical bone ingrowth compared to the untreated/polished ones by significantly promoting cell adhesion, matrix mineralization, and osteogenic differentiation through increasing integrin expression. The RoB was low in 41.1% of items, unclear in 53.3%, and high in 5.6%. The quality of the studies achieved a mean score of 17.67. Our study demonstrates that nanostructures with specific controlled properties on the surface of 3D-printed Ti implants improve their osseointegration. However, given the small number of studies, the variability in experimental designs, and lack of reporting across studies, the results should be interpreted with caution.

Combining Virtual Surgical Planning and Patient-Specific 3D-Printing as a Solution to Complex Spinal Revision Surgery

With the advent of three-dimensional printing, rapid growth in the field and application in spinal and orthopedic surgery has been seen. This technology is now being applied in creating patient-specific implants, as it offers benefits over the generic alternative, with growing literature supporting this. This report details a unique application of virtual surgical planning and manufacture of a personalized implant in a case of cervical disc replacement failure with severe osteolysis and resultant hypermobility. Where this degree of degenerative bone loss would often necessitate a vertebrectomy to be performed, this case highlights the considerable customizability of 3D-printed patient-specific implants to contour to the bony defects, allowing for a smaller and safer operation, with the achievement of stability as early as 3 months after the procedure, by the presence of osseointegration. With increasing developments in virtual planning technology and 3D printing ability, the future of complex spinal revision surgery may adopt these technologies as it affords the patient a faster, safer, and less invasive and destructive procedure.

Optimization of cervical cage and analysis of its base material: A finite element study

Nowadays, cervical disorders are common due to human lifestyles. Accordingly, the cage design should be optimized as an essential issue. For an optimal design, an objective function is utilized to calculate the proper geometrical parameters. Additionally, the base material of the cage plays a key role in its functionality and final cost. Novel materials are currently introduced with more compatibility with the bone in terms of mechanical and chemical properties. In this study, a cervical cage was modeled based on PEEK material with three types of tooth designs on its surface. The cervical cage is assumed to be implanted between C6 and C7 vertebrae. The geometric parameters of the cage were optimized to minimize the mass by determining allowable stress and subsidence. The effect of complete cortical removal was investigated as a surgical mistake. Finally, a new composition of PEEK/titanium was introduced as the base material of the cage. Ansys 18.2 was used for FEA. The cage with a straight tooth was chosen due to its lower stress and subsidence compared with other designs. Furthermore, the optimized structures of all three tooth designs were determined. The mass and volume of the optimal cages were reduced by 41.47% and 41.52% respectively. Besides, complete cortical resection should not be carried out during fusion surgery, since it may lead to higher subsidence. The composition of PEEK/titanium was chosen as an appropriate base material due to its better performance compared with PEEK or titanium alone.

Biodegradable interbody cages for lumbar spine fusion: Current concepts and future directions

Lumbar fusion often remains the last treatment option for various acute and chronic spinal conditions, including infectious and degenerative diseases. Placement of a cage in the intervertebral space has become a routine clinical treatment for spinal fusion surgery to provide sufficient biomechanical stability, which is required to achieve bony ingrowth of the implant. Routinely used cages for clinical application are made of titanium (Ti) or polyetheretherketone (PEEK). Ti has been used since the 1980s; however, its shortcomings, such as impaired radiographical opacity and higher elastic modulus compared to bone, have led to the development of PEEK cages, which are associated with reduced stress shielding as well as no radiographical artefacts. Since PEEK is bioinert, its osteointegration capacity is limited, which in turn enhances fibrotic tissue formation and peri-implant infections. To address shortcomings of both of these biomaterials, interdisciplinary teams have developed biodegradable cages. Rooted in promising preclinical large animal studies, a hollow cylindrical cage (Hydrosorb™) made of 70:30 poly-l-lactide-co-d, l-lactide acid (PLDLLA) was clinically studied. However, reduced bony integration and unfavourable long-term clinical outcomes prohibited its routine clinical application. More recently, scaffold-guided bone regeneration (SGBR) with application of highly porous biodegradable constructs is emerging. Advancements in additive manufacturing technology now allow the cage designs that match requirements, such as stiffness of surrounding tissues, while providing long-term biomechanical stability. A favourable clinical outcome has been observed in the treatment of various bone defects, particularly for 3D-printed composite scaffolds made of medical-grade polycaprolactone (mPCL) in combination with a ceramic filler material. Therefore, advanced cage design made of mPCL and ceramic may also carry initial high spinal forces up to the time of bony fusion and subsequently resorb without clinical side effects. Furthermore, surface modification of implants is an effective approach to simultaneously reduce microbial infection and improve tissue integration. We present a design concept for a scaffold surface which result in osteoconductive and antimicrobial properties that have the potential to achieve higher rates of fusion and less clinical complications. In this review, we explore the preclinical and clinical studies which used bioresorbable cages. Furthermore, we critically discuss the need for a cutting-edge research program that includes comprehensive preclinical in vitro and in vivo studies to enable successful translation from bench to bedside. We develop such a conceptual framework by examining the state-of-the-art literature and posing the questions that will guide this field in the coming years.

Influence of coronal-morphology of endplate and intervertebral space to cage subsidence and fusion following oblique lumbar interbody fusion

Background

Endplate morphology is considered to be one of the influencing factors of cage subsidence after lumbar interbody fusion (LIF). Previous radiographic evaluations on the endplate mostly used sagittal X-ray or MRI. However, there are few studies on the CT evaluation of the endplate and intervertebral space (IVS), especially the evaluation of coronal morphology and its influence on subsidence and fusion after LIF. We aimed to measure and classify the shapes of the endplate and IVS using coronal CT imaging and evaluate the radiographic and clinical outcomes of different shapes of the endplate/IVS following oblique lateral lumbar interbody fusion (OLIF).

Methods

A total of 137 patients (average age 59.1 years, including 75 males and 62 females) who underwent L4-5 OLIF combined with anterolateral fixation from June 2018 to June 2020 were included. The endplate concavity depth (ECD) was measured on the preoperative coronal CT image. According to ECD, the endplate was classified as flat (< 2 mm), shallow (2–4 mm), or deep (> 4 mm). The L4-5 IVS was further classified according to endplate type. The disc height (DH), DH changes, subsidence rate, fusion rate, and Oswestry Disability Index (ODI) in different endplate/IVS shapes were evaluated during 1-year follow up.

Results

The ECD of L4 inferior endplate (IEP) was significantly deeper than that of L5 superior endplate (SEP) (4.2 ± 1.1 vs 1.6 ± 0.8, P < 0.01). Four types of L4-5 IVS were identified: shallow-shallow (16, 11.7%), shallow-flat (45, 32.9%), deep-shallow (32, 23.4%), and deep-flat (44, 32.1%). A total of 45 (32.9%) cases of cage subsidence were observed. Only one (6.3%) subsidence event occurred in the shallow-shallow group, which was significantly lower than in the other three groups (19 shallow-flat, 6 deep-shallow, and 19 deep-flat) (P < 0.05). Meanwhile, the shallow-shallow group had the highest fusion rate (15, 93.8%) and the highest rate of reach minimal clinically important difference (MCID) ODI among the four types. For a single endplate, the shape of L4 IEP is the main influencing factor of the final interbody fusion rate, and the shallow shape L4 IEP facilitates fusion ( OR = 2.85, p = 0.03). On the other hand, the flat shape L5 SEP was the main risk factor to cage subsidence (OR = 4.36, p < 0.01).

Conclusion

The L4-5 IVS is asymmetrical on coronal CT view and tends to be fornix-above and flat-down. The shallow-shallow IVS has the lowest subsidence rate and best fusion result, which is possibly because it has a relatively good degree in matching either the upper or lower interface of the cage and endplates. These findings provide a basis for the further improvements in the design of OLIF cages.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12891-022-05584-3.

Rapid Personalised Virtual Planning and On-Demand Surgery for Acute Spinal Trauma Using 3D-Printing, Biomodelling and Patient-Specific Implant Manufacture

Three-dimensional printing is a rapidly growing field, with extensive application to orthopaedics and spinal surgery. Three-dimensional-printed (3DP) patient-specific implants (PSIs) offer multiple potential benefits over generic alternatives, with their use increasingly being described in the spinal literature. This report details a unique, emergency case of a traumatic spinal injury in a 31-year-old male, acquired rurally and treated with a 3DP PSI in a tertiary unit. With increasing design automation and process improvements, rapid, on-demand virtual surgical planning (VSP) and 3DP PSIs may present the future of orthopaedics and trauma care, enabling faster, safer, and more cost-effective patient-specific procedures.

Outcomes of Surgical Reconstruction Using Custom 3D-Printed Porous Titanium Implants for Critical-Sized Bone Defects of the Foot and Ankle

BACKGROUND

Treating critically sized defects (CSDs) of bone remains a significant challenge in foot and ankle surgery. Custom 3D-printed implants are being offered to a small but growing subset of patients as a salvage procedure in lieu of traditional alternates such as structural allografts after the patient has failed prior procedures. The long-term outcomes of 3D-printed implants are still unknown and understudied because of the limited number of cases and short follow-up durations. The purpose of this study was to evaluate the outcomes of patients who received custom 3D-printed implants to treat CSDs of the foot and ankle in an attempt to aid surgeons in selecting appropriate surgical candidates.

METHODS

This was a retrospective study to assess surgical outcomes of patients who underwent implantation of a custom 3D-printed implant made with medical-grade titanium alloy powder (Ti-6Al-4V) to treat CSDs of the foot and ankle between June 1, 2014, and September 30, 2019. All patients had failed previous nonoperative or operative management before proceeding with treatment with a custom 3D-printed implant. Univariate and multivariate odds ratios (ORs) of a secondary surgery and implant removal were calculated for perioperative variables.

RESULTS

There were 39 cases of patients who received a custom 3D-printed implant with at least 1 year of follow-up. The mean follow-up time was 27.0 (12-74) months. Thirteen of 39 cases (33.3%) required a secondary surgery and 10 of 39 (25.6%) required removal of the implant because of septic nonunion (6/10) or aseptic nonunion (4/10). The mean time to secondary surgery was 10 months (1-22). Multivariate logistic regression revealed that patients with neuropathy were more likely to require a secondary surgery with an OR of 5.76 (P = .03).

CONCLUSION

This study demonstrated that 74% of patients who received a custom 3D-printed implant for CSDs did not require as subsequent surgery (minimum of 1-year follow-up). Neuropathy was significantly associated with the need for a secondary surgery. This is the largest series to date demonstrating the efficacy of 3D-printed custom titanium implants. As the number of cases using patient-specific 3D-printed titanium implant increases, larger cohorts of patients should be studied to identify other high-risk groups and possible interventions to improve surgical outcomes.

LEVEL OF EVIDENCE

Level IV, case series.

3D printing applications in spine surgery: an evidence-based assessment toward personalized patient care

PURPOSE

Spine surgery entails a wide spectrum of complicated pathologies. Over the years, numerous assistive tools have been introduced to the modern neurosurgeon's armamentarium including neuronavigation and visualization technologies. In this review, we aimed to summarize the available data on 3D printing applications in spine surgery as well as an assessment of the future implications of 3D printing.

METHODS

We performed a comprehensive review of the literature on 3D printing applications in spine surgery.

RESULTS

Over the past decade, 3D printing and additive manufacturing applications, which allow for increased precision and customizability, have gained significant traction, particularly spine surgery. 3D printing applications in spine surgery were initially limited to preoperative visualization, as 3D printing had been primarily used to produce preoperative models of patient-specific deformities or spinal tumors. More recently, 3D printing has been used intraoperatively in the form of 3D customizable implants and personalized screw guides.

CONCLUSIONS

Despite promising preliminary results, the applications of 3D printing are so recent that the available data regarding these new technologies in spine surgery remains scarce, especially data related to long-term outcomes.

Biomaterials for Interbody Fusion in Bone Tissue Engineering

In recent years, interbody fusion cages have played an important role in interbody fusion surgery for treating diseases like disc protrusion and spondylolisthesis. However, traditional cages cannot achieve satisfactory results due to their unreasonable design, poor material biocompatibility, and induced osteogenesis ability, limiting their application. There are currently 3 ways to improve the fusion effect, as follows. First, the interbody fusion cage is designed to facilitate bone ingrowth through the preliminary design. Second, choose interbody fusion cages made of different materials to meet the variable needs of interbody fusion. Finally, complete post-processing steps, such as coating the designed cage, to achieve a suitable osseointegration microstructure, and add other bioactive materials to achieve the most suitable biological microenvironment of bone tissue and improve the fusion effect. The focus of this review is on the design methods of interbody fusion cages, a comparison of the advantages and disadvantages of various materials, the influence of post-processing techniques and additional materials on interbody fusion, and the prospects for the future development of interbody fusion cages.

Spinal Implant Osseointegration and the Role of 3D Printing: An Analysis and Review of the Literature

The use of interbody implants for spinal fusion has been steadily increasing to avoid the risks of complications and donor site morbidity when using autologous bone. Understanding the pros and cons of various implant designs can assist the surgeon in choosing the ideal interbody for each individual patient. The goal of these interbody cages is to promote a surface area for bony ingrowth while having the biomechanical properties to support the axial skeleton. Currently, the majority of interbody implants consists of metal or polyether ether ketone (PEEK) cages with bone graft incorporated inside. Titanium alloy implants have been commonly used, however, the large difference in modulus of elasticity from bone has inherent issues. PEEK implants have a desirable surface area with the benefit of a modulus of elasticity closer to that of bone. Unfortunately, clinically, these devices have had increased risk of subsidence. More recently, 3D printed implants have come into the market, providing mechanical stability with increased surface design for bony ingrowth. While clinical outcomes studies are limited, early results have demonstrated more reliable and quicker fusion rates using 3D custom interbody devices. In this review, we discuss the biology of osseointegration, the use of surface coated implants, as well as the potential benefits of using 3D printed interbodies.

A Review on Development of Bio-Inspired Implants Using 3D Printing

Biomimetics is an emerging field of science that adapts the working principles from nature to fine-tune the engineering design aspects to mimic biological structure and functions. The application mainly focuses on the development of medical implants for hard and soft tissue replacements. Additive manufacturing or 3D printing is an established processing norm with a superior resolution and control over process parameters than conventional methods and has allowed the incessant amalgamation of biomimetics into material manufacturing, thereby improving the adaptation of biomaterials and implants into the human body. The conventional manufacturing practices had design restrictions that prevented mimicking the natural architecture of human tissues into material manufacturing. However, with additive manufacturing, the material construction happens layer-by-layer over multiple axes simultaneously, thus enabling finer control over material placement, thereby overcoming the design challenge that prevented developing complex human architectures. This review substantiates the dexterity of additive manufacturing in utilizing biomimetics to 3D print ceramic, polymer, and metal implants with excellent resemblance to natural tissue. It also cites some clinical references of experimental and commercial approaches employing biomimetic 3D printing of implants.

Is there a variance in complication types associated with ALIF approaches? A systematic review

Purpose

Anterior lumbar interbody fusion (ALIF) is a well-established alternative to posterior-based interbody fusion techniques, with approach variations, such as retroperitoneal, transperitoneal, open, and laparoscopic well described. Variable rates of complications for each approach have been enumerated in the literature. The purpose of this study was to elucidate the comparative rates of complications across approach type.

Methods

A systematic review of search databases PubMed, Google Scholar, and OVID Medline was made to identify studies related to complication-associated ALIF. PRISMA guidelines were utilised for this review. Meta-analysis was used to compare intraoperative and postoperative complications with ALIF for each approach.

Results

A total of 4575 studies were identified, with 5728 patients across 31 studies included for review following application of inclusion and exclusion criteria. Meta-analysis demonstrated the transperitoneal approach resulted in higher rates of retrograde ejaculation (RE) (p < 0.001; CI = 0.05–0.21) and overall rates of complications (p = 0.05; CI = 0.00–0.23). Rates of RE were higher at the L5/S1 intervertebral level. Rates of vessel injury were not significantly higher in either approach method (p = 0.89; CI =  − 0.04–0.07). Rates of visceral injury did not appear to be related to approach method. Laparoscopic approaches resulted in shorter inpatient stays (p = 0.01).

Conclusion

Despite the transperitoneal approach being comparatively underpowered, its use appears to result in a significantly higher rate of intraoperative and postoperative complications, although confounders including use of bone morphogenetic protein (BMP) and spinal level should be considered. Laparoscopic approaches resulted in shorter hospital stays; however, its steep learning curve and longer operative time have deterred surgeons from its widespread adaptation.

Vulnerability of the L5 nerve root during anterior lumbar interbody fusion at L5-S1: case series and review of the literature

OBJECTIVE

Nerve root injuries associated with anterior lumbar interbody fusion (ALIF) are uncommonly reported in the literature. This case series and review aims to describe the etiology of L5 nerve root injury following ALIF at L5-S1.

METHODS

The authors performed a single-center retrospective review of prospectively collected data of patients who underwent surgery between 2017 and 2019 who had postoperative L5 nerve root injuries after stand-alone L5-S1 ALIF. They also reviewed the literature with regard to nerve root injuries after ALIF procedures.

RESULTS

The authors report on 3 patients with postoperative L5 radiculopathy. All 3 patients had pain that improved. Two of the 3 patients had a neurological deficit, one of which improved.

CONCLUSIONS

Stretch neuropraxia from overdistraction is an important cause of postoperative L5 radiculopathy after L5-S1 ALIF. Judicious use of implants and careful preoperative planning to determine optimal implant sizes are paramount.

Three-Dimensional-Printed Drill Guides for Occipitothoracic Fusion in a Pediatric Patient With Occipitocervical Instability

BACKGROUND

Pediatric occipitothoracic fusion can be challenging because of small size pedicles and thin occipital bone. Three-dimensional (3D) printing technology can help with accurate screw insertion but has not been described for occipital keel plate positioning so far.

OBJECTIVE

To describe the novel use of 3D technology to position occipital keel plates during pediatric occipitothoracic fixation.

METHODS

A young boy with segmental spinal dysgenesis presented with asymmetrical pyramidal paresis in all limbs. Developmental abnormities of the cervical spine caused a thinned spinal cord, and because of progressive spinal cord compression, surgical intervention by means of occipitothoracic fixation was indicated at the age of 3 yr.Because of the small-size pedicles and thin occipital bone, the pedicle screws and occipital plates were planned meticulously using 3D virtual surgical planning technology. The rods were virtually bent in order to properly align with the planned screws. By means of 3D-printed guides, the surgical plan was transferred to the operating theater. For the occipital bone, a novel guide concept was developed, aiming for screw positions at maximal bone thickness.

RESULTS

The postoperative course was uneventful, and radiographs showed good cervical alignment. After superimposing the virtual plan with the intraoperative acquired computed tomography, it was confirmed that the occipital plate positions matched the virtual plan and that pedicle screws were accurately inserted without signs of breach.

CONCLUSION

The use of 3D technology has greatly facilitated the performance of the occipitothoracic fixation and could, in the future, contribute to safer pediatric spinal fixation procedures.

Implications of 3-Dimensional Printed Spinal Implants on the Outcomes in Spine Surgery

Three-dimensional printing (3DP) applications possess substantial versatility within surgical applications, such as complex reconstructive surgeries and for the use of surgical resection guides. The capability of constructing an implant from a series of radiographic images to provide personalized anatomical fit is what makes 3D printed implants most appealing to surgeons. Our objective is to describe the process of integration of 3DP implants into the operating room for spinal surgery, summarize the outcomes of using 3DP implants in spinal surgery, and discuss the limitations and safety concerns during pre-operative consideration. 3DP allows for customized, light weight, and geometrically complex functional implants in spinal surgery in cases of decompression, tumor, and fusion. However, there are limitations such as the cost of the technology which is prohibitive to many hospitals. The novelty of this approach implies that the quantity of longitudinal studies is limited and our understanding of how the human body responds long term to these implants is still unclear. Although it has given surgeons the ability to improve outcomes, surgical strategies, and patient recovery, there is a need for prospective studies to follow the safety and efficacy of the usage of 3D printed implants in spine surgery.

Opinion Piece: Patient-Specific Implants May Be the Next Big Thing in Spinal Surgery

The emergence of 3D-Printing technologies and subsequent medical applications have allowed for the development of Patient-specific implants (PSIs). There have been increasing reports of PSI application to spinal surgery over the last 5 years, including throughout the spine and to a range of pathologies, though largely for complex cases. Through a number of potential benefits, including improvements to the implant–bone interface and surgical workflow, PSIs aim to improve patient and surgical outcomes, as well as potentially provide new avenues for combating challenges routinely faced by spinal surgeons. However, obstacles to widespread acceptance and routine application include the lack of quality long-term data, research challenges and the practicalities of production and navigating the regulatory environment. While recognition of the significant potential of Spinal PSIs is evident in the literature, it is clear a number of key questions must be answered to inform future clinical and research practices. The spinal surgical community must selectively and ethically continue to offer PSIs to patients, simultaneously allowing for the necessary larger, comparative studies to be conducted, as well as continuing to provide optimal patient care, thereby ultimately determining the exact role of this technology and potentially improving outcomes.

Effect of Bicortical Interfragmentary Screw Size on the Fixation of Metacarpal Shaft Fractures: A 3-Dimensional-Printed Biomechanical Study

Purpose

Spiral metacarpal fractures fixed with 2 non-lagged, interfragmentary cortical screws were tested to failure. The effect of screw size (1.2 mm, 1.5 mm, 2.0 mm, and 2.3 mm) on construct strength was tested in 3-point bending.

Methods

Three-dimensional-printed metacarpal test models were reproduced from computed tomography scans to reduce the confounding variables of bone density and anatomy, often encountered when using cadavers.

Results

No significant difference was found between the screw sizes, and the peak failure force was similar. Drill bit fracture and deformation during the insertion of the smallest screw (1.2 mm) as well as model failure during the insertion of the largest screw (2.3 mm) were found in some cases.

Conclusions

Screws of 1.5 mm and 2.0 mm in diameter were of sufficient strength and did not have the issues encountered with smaller or larger screws. Concerns from previous authors regarding intraoperative fracture were consistent with the pre-testing failure of some 2.3-mm models.

Clinical Relevance

Screws of 1.5 mm or 2 mm appear adequate for the fixation of spiral fracture patterns in metacarpal shafts using bicortical non-lagged technique with a low risk of fixation complications.

Comparison in clinical performance of surgical guides for mandibular surgery and temporomandibular joint implants fabricated by additive manufacturing techniques

Additive manufacturing (AM) offers great design freedom that enables objects with desired unique and complex geometry and topology to be readily and cost-effectively fabricated. The overall benefits of AM are well known, such as increased material and resource efficiency, enhanced design and production flexibility, the ability to create porous structures and on-demand manufacturing. When AM is applied to medical devices, these benefits are naturally assumed. However, hard clinical evidence collected from clinical trials and studies seems to be lacking and, as a result, systematic assessment is yet difficult. In the present work, we have reviewed 23 studies on the clinical use of AM patient-specific surgical guides (PSGs) for the mandible surgeries (n = 17) and temporomandibular joint (TMJ) patient-specific implants (PSIs) (n = 6) with respect to expected clinical outcomes. It is concluded that the data published on these AM medical devices are often lacking in comprehensive evaluation of clinical outcomes. A complete set of clinical data, including those on time management, costs, clinical outcomes, range of motion, accuracy of the placement with respect to the pre-operative planning, and extra complications, as well as manufacturing data are needed to demonstrate the real benefits gained from applying AM to these medical devices and to satisfy regulatory requirements.

Mechanical properties and fluid permeability of gyroid and diamond lattice structures for intervertebral devices: functional requirements and comparative analysis

Current intervertebral fusion devices present multiple complication risks such as a lack of fixation, device migration and subsidence. An emerging solution to these problems is the use of additively manufactured lattice structures that are mechanically compliant and permeable to fluids, thus promoting osseointegration and reducing complication risks. Strut-based diamond and sheet-based gyroid lattice configurations having a pore diameter of 750 µm and levels of porosity of 60, 70 and 80% are designed and manufactured from Ti-6Al-4V alloy using laser powder bed fusion. The resulting structures are CT-scanned, compression tested and subjected to fluid permeability evaluation. The stiffness of both structures (1.9-4.8 GPa) is comparable to that of bone, while their mechanical resistance (52-160 MPa) is greater than that of vertebrae (3-6 MPa), thus decreasing the risks of wither bone or implant failure. The fluid permeability (5-57 × 10^-9 m²) and surface-to-volume ratios (~3) of both lattice structures are close to those of vertebrae. This study shows that both types of lattice structures can be produced to suit the application specifications within certain limits imposed by physical and equipment-related constraints, providing potential solutions for reducing the complication rate of spinal devices by offering a better fixation through osseointegration.

Revision of a Failed C5-7 Corpectomy Complicated by Esophageal Fistula Using a 3-Dimensional-Printed Zero-Profile Patient-Specific Implant: A Technical Case Report

BACKGROUND

Esophageal fistulae are rare, though serious, complications of anterior cervical surgery. Hardware-related issues are important etiologic factors. Patient-specific implants (PSIs) have increasingly been adapted to spinal surgery and offer a range of benefits. Zero-profile implants are a recent development primarily aimed at combating postoperative dysphagia. We report the first use of a 3-dimensional (3D)-printed zero-profile PSI in managing implant failure with migration and a secondary esophageal fistula.

METHODS

A 68-year-old female had a prior C5-7 corpectomy with cage and plate fixation, as well as posterior C3-T1 lateral mass fixation, complicated by anterior plate displacement, resulting in pseudoarthrosis and an esophageal fistula. A 3D-printed zero-profile PSI was designed and implanted as part of a revision procedure to assist in recovery, prevent recurrence, and facilitate bony fusion.

RESULTS

Optimal implant placement was achieved on the basis of preoperative virtual surgical planning. By 1 month postoperatively the patient had significantly improved, with evidence of esophageal fistula resolution and radiographic evidence of optimal implant placement.

CONCLUSIONS

Zero-profile 3D-printed PSIs may combat common and serious complications of anterior cervical surgery including postoperative dysphagia and esophageal fistulae. Further research is required to validate their widespread use for either cervical corpectomy or diskectomy and interbody fusion.

Selective Laser Melting and Electron Beam Melting of Ti6Al4V for Orthopedic Applications: A Comparative Study on the Applied Building Direction

The 3D printing process offers several advantages to the medical industry by producing complex and bespoke devices that accurately reproduce customized patient geometries. Despite the recent developments that strongly enhanced the dominance of additive manufacturing (AM) techniques over conventional methods, processes need to be continually optimized and controlled to obtain implants that can fulfill all the requirements of the surgical procedure and the anatomical district of interest. The best outcomes of an implant derive from optimal compromise and balance between a good interaction with the surrounding tissue through cell attachment and reduced inflammatory response mainly caused by a weak interface with the native tissue or bacteria colonization of the implant surface. For these reasons, the chemical, morphological, and mechanical properties of a device need to be designed in order to assure the best performances considering the in vivo environment components. In particular, complex 3D geometries can be produced with high dimensional accuracy but inadequate surface properties due to the layer manufacturing process that always entails the use of post-processing techniques to improve the surface quality, increasing the lead times of the whole process despite the reduction of the supply chain. The goal of this work was to provide a comparison between Ti6Al4V samples fabricated by selective laser melting (SLM) and electron beam melting (EBM) with different building directions in relation to the building plate. The results highlighted the influence of the process technique on osteoblast attachment and mineralization compared with the building orientation that showed a limited effect in promoting a proper osseointegration over a long-term period.

3D-printed Patient-specific Spine Implants: A Systematic Review

STUDY DESIGN

Systematic review.

OBJECTIVE

To review the current clinical use of 3-dimensional printed (3DP) patient-specific implants in the spine.

SUMMARY OF BACKGROUND DATA

Additive manufacturing is a transformative manufacturing method now being applied to spinal implants. Recent innovations in technology have allowed the production of medical-grade implants with unprecedented structure and customization, and the complex anatomy of the spine is ideally suited for patient-specific devices. Improvement in implant design through the process of 3DP may lead to improved osseointegration, lower subsidence rates, and faster operative times.

METHODS

A comprehensive search of the literature was conducted using Ovid MEDLINE, EMBASE, Scopus, and other sources that resulted in 1842 unique articles. All manuscripts describing the use of 3DP spinal implants in humans were included. Two independent reviewers (N.W. and N.E.S.) assessed eligibility for inclusion. The following outcomes were collected: pain score, Japanese Orthopedic Association (JOA) score, subsidence, fusion, Cobb angle, vertebral height, and complications. No conflicts of interest existed. No funding was received for this work.

RESULTS

A total of 17 studies met inclusion criteria with a total of 35 patients. Only case series and case reports were identified. Follow-up times ranged from 3 to 36 months. Implant types included vertebral body replacement cages, interbody cages, sacral reconstruction prostheses, iliolumbar rods, and a posterior cervical plate. All studies reported improvement in both clinical and radiographic outcomes. 11 of 35 cases showed subsidence >3 mm, but only 1 case required a revision procedure. No migration, loosening, or pseudarthrosis occurred in any patient on the basis of computed tomography or flexion-extension radiographs.

CONCLUSIONS

Results of the systematic review indicate that 3DP technology is a viable means to fabricate patient-matched spinal implants. The effects on clinical and radiographic outcome measures are still in question, but these devices may produce favorable subsidence and pseudoarthrosis rates. Currently, the technology is ideally suited for complex tumor pathology and atypical bone defects. Future randomized controlled trials and cost analyses are still needed.

LEVEL OF EVIDENCE

IV-systematic review.

Three-dimensional printing in spine surgery: a review of current applications

In recent years, the use of three-dimensional printing (3DP) technology has gained traction in orthopedic spine surgery. Although research on this topic is still primarily limited to case reports and small cohort studies, it is evident that there are many avenues for 3DP innovation in the field. This review article aims to discuss the current and emerging 3DP applications in spine surgery, as well as the challenges of 3DP production and limitations in its use. 3DP models have been presented as helpful tools for patient education, medical training, and presurgical planning. Intraoperatively, 3DP devices may serve as patient-specific surgical guides and implants that improve surgical outcomes. However, the time, cost, and learning curve associated with constructing a 3DP model are major barriers to widespread use in spine surgery. Considering the costs and benefits of 3DP along with the varying risks associated with different spine procedures, 3DP technology is likely most valuable for complex or atypical spine disorder cases. Further research is warranted to gain a better understanding of how 3DP can and will impact spine surgery.

Bone ongrowth and mechanical fixation of implants in cortical and cancellous bone

BACKGROUND

What is the right surface for an implant to achieve biological fixation? Surface technologies can play important roles in encouraging interactions between the implant surface and the host bone to achieve osseointegration. Preclinical animal models provide important insight into in vivo performance related to bone ongrowth and implant fixation.

METHODS

A large animal model was used to compare the in vivo response of HA and plasma-sprayed titanium coatings in a well-reported adult ovine model to evaluate bone ongrowth in terms of mechanical properties in cortical sites, and histology and histomorphometry in cortical and cancellous sites at 4 and 12 weeks.

RESULTS

Titanium plasma-sprayed surfaces outperformed the HA-coated samples in push-out testing in cortical sites while both surfaces supported new bone ongrowth and remodeling in cortical and cancellous sites.

CONCLUSIONS

While both HA and Ti plasma provided an osteoconductive surface for bone ongrowth, the Ti plasma provided a more robust bone-implant interface that ideally would be required for load transfer and implant stability in the longer term.

C3-C5 Chordoma Resection and Reconstruction with a Three-Dimensional Printed Titanium Patient-Specific Implant

BACKGROUND

With this case report, we aim to add to the clinical literature on the use of three-dimensional printed patient-specific implants in spinal surgery, show the current state of the art in patient-specific implant device design, present thorough clinical and radiographic outcomes, and discuss the suitability of titanium alloy as an implant material for patients with cancer.

CASE DESCRIPTION

A 45-year-old man presented with neck and left arm pain combined with shoulder weakness. Imaging revealed significant destruction of the C3-C5 vertebrae, and chordoma diagnosis was confirmed by biopsy. Gross total tumor resection including multilevel corpectomy was performed in combination with reconstruction using a three-dimensional printed titanium custom implant. Custom-designed features aimed to reduce reconstruction time and result in good clinical and radiographic outcomes. Clinical scores improved postoperatively and remained improved at 17-month postoperative follow-up: visual analog scale score 10/10 preoperatively improved to 2-6/10 at 17 months; Neck Disability Index 46% preoperatively improved to 32% at 17 months. Neither dysphagia nor dysphonia remained after surgical soft tissue swelling subsided. The patient was successfully treated with proton beam therapy after surgery, with no tumor recurrence at 17-month follow-up. Radiographic assessment showed incomplete fusion at 3 months, with clinically insignificant implant subsidence (2.7 mm) and no implant migration or failure at 14 months.

CONCLUSIONS

Computer-aided preoperative planning with three-dimensional printed biomodels and custom implant resulted in relatively quick and simple reconstruction after tumor resection, with good clinical and radiographic outcomes at 17 and 14 months, respectively. For patients with primary tumors who may require follow-up radiotherapy or postoperative magnetic resonance imaging, metals used in the devices cause significant imaging artifact.

3D printed anatomical (bio)models in spine surgery: clinical benefits and value to health care providers

The applications of three-dimensional printing (3DP) for clinical purposes have grown rapidly over the past decade. Recent advances include the fabrication of patient specific instrumentation, such as drill and cutting guides, patient specific/custom long term implants and 3DP of cellular scaffolds. Spine surgery in particular has seen enthusiastic early adoption of these applications. 3DP as a manufacturing method can be used to mass produce objects of the same design, but can also be used as a cost-effective method for manufacturing unique one-off objects, such as patient specific models and devices. Perhaps the first, and currently most widespread, application of 3DP for producing patient specific devices is the production of patient specific anatomical models, often termed biomodels. The present manuscript focuses on the current state of the art in anatomical (bio)models as used in spinal clinical practice. The biomodels shown and discussed include: translucent and coloured models to aid in identification of extent and margins of pathologies such as bone tumours; dynamic models for implant trial implantation and pre-operative sizing; models that can be disassembled to simulate surgical resection of diseased tissue and subsequent reconstruction. Biomodels can reduce risk to the patient by decreasing surgery time, reducing the probability of the surgical team encountering unexpected anatomy or relative positioning of structures and/or devices, and better pre-operative planning of the surgical workflow including ordered preparation of the necessary instrumentation for multi-step and revision procedures. Conversely, risks can be increased if biomodels are not accurate representations of the anatomy, which can occur if MRI/CT scan data is simply converted into 3DP format without interpretation of what the scan represents in terms of patient anatomy. A review and analysis of the cost-benefits of biomodels shows that biomodels can potentially reduce cost to health care providers if operating room time is reduced by 14 minutes or more.

Does implantation site influence bone ingrowth into 3D-printed porous implants?

BACKGROUND CONTEXT

The potential for osseointegration to provide biological fixation for implants may be related to anatomical site and loading conditions.

PURPOSE

To evaluate the influence of anatomical site on osseointegration of 3D-printed implants.

STUDY DESIGN

A comparative preclinical study was performed evaluating bone ingrowth in cortical and cancellous sites in long bones as well as lumbar interbody fusion with posterior pedicle screw stabilization using the same 3D-printed titanium alloy design.

METHODS

3D-printed dowels were implanted in cortical bone and cancellous bone in adult sheep and evaluated at 4 and 12 weeks for bone ingrowth using radiography, mechanical testing, and histology/histomorphometry. In addition, a single-level lumbar interbody fusion using cages based on the same 3D-printed design was performed. The aperture was filled with autograft or ovine allograft processed with supercritical carbon dioxide. Interbody fusions were assessed at 12 weeks via radiography, mechanical testing, and histology/histomorphometry.

RESULTS

Bone ingrowth in long bone cortical and cancellous sites did not translate directly to interbody fusion cages. While bone ingrowth was robust and improved with time in cortical sites with a line-to-line implantation condition, the same response was not found in cancellous sites even when the implants were placed in a press fit manner. Osseointegration into the porous walls with 3D porous interbody cages was similar to the cancellous implantation sites rather than the cortical sites. The porous domains of the 3D-printed device, in general, were filled with fibrovascular tissue while some bone integration into the porous cages was found at 12 weeks when fusion within the aperture was present.

CONCLUSION

Anatomical site, surgical preparation, biomechanical loading, and graft material play an important role in in vivo response. Bone ingrowth in long bone cortical and cancellous sites does not translate directly to interbody fusions.

3D printing in spine surgery

The applications of three-dimensional (3D) printing, or additive manufacturing, to the field of spine surgery continue to grow in number and scope especially in recent years as improved manufacturing techniques and use of sterilizable materials have allowed for creation of 3D printed implants. While 3D printing in spine surgery was initially limited to use as visual aids in preoperative planning for complex pathology, it has more recently been used to create intraoperative patient-specific screw guides and templates and is increasingly being used in surgical education and training. As patient-specific treatment and personalized medicine gains popularity in medicine, 3D printing provides a similar option for the surgical fields, particularly in the creation of customizable implants. 3D printing is a relatively new field as it pertains to spine surgery, and as such, it lacks long-term data on clinical outcomes and cost effectiveness; however, the apparent benefits and seemingly boundless applications of this growing technology make it an attractive option for the future of spine surgery.