Computational comparison of three posterior lumbar interbody fusion techniques by using porous titanium interbody cages with 50% porosity

Biomechanical evaluation of various rigid internal fixation modalities for condylar-base-associated multiple mandibular fractures: A finite element analysis

Condylar-base-associated multiple mandibular fractures are more prevalent than single ones. Direct trauma to mandibular symphysis, body or angle are prone to induce indirect condylar fracture. However, little is known about the effects of various rigid internal fixation modalities in condylar base for relevant multiple mandibular fractures, especially when we are confused in the selection of operative approach. Within the finite element analysis, straight-titanium-plate implanting positions in condylar base contained posterolateral zone (I), anterolateral zone (II), and intermediate zone (III). Von Mises stress (SS) in devices and bone and mandibular displacement (DT) were solved, while maximum values (SSmax and DTmax) were documented. For rigid internal fixation in condylar-base-and-symphysis fractures, I + II modality exhibited least SSmax in screws and cortical bone and least DTmax, I + III modality exhibited least SSmax in plates. For rigid internal fixation in condylar-base-and-contralateral-body fractures, I + III modality exhibited least SSmax in screws and cortical bone, I + II modality exhibited least SSmax in plates and least DTmax. For rigid internal fixation in condylar-base-and-contralateral-angle fractures, I + III modality exhibited least DTmax. The findings suggest that either I + II or I + III modality is a valid guaranty for rigid internal fixation of condylar base fractures concomitant with symphysis, contralateral body or angle fractures.

Effect of Insertional Direction of Pedicle Screw on Screw Loosening: A Biomechanical Study on Synthetic Bone Vertebra under a Physiology-like Load

OBJECTIVES

It is now understood that pedicle screw loosening at the implant-bone interface can lead to poor screw-bone interface purchase and decreased fixation stability. Previous biomechanical tests used cadaveric vertebrae and pull-out or torque loads to assess the effect of the insertional direction of pedicle screws on screw loosening. However, these tests faced challenges in matching biomechanical differences among specimens and simulating in vivo loads applied on pedicle screws. This study aimed to evaluate the effect of the insertional direction of pedicle screws on screw loosening using tension-compression-bending loads and synthetic bone vertebrae.

METHODS

Polyaxial pedicle screws were inserted into nine synthetic bone vertebrae in three directions (three samples per group): cranial, parallel, and caudad (-10°, 0°, +10° of the pedicle screw rod to the upper plane of the vertebra, respectively). Pedicle screws in the vertebrae were loaded using a polyethylene block connected to a material testing machine. Tension-compression-bending loads (100N-250N) with 30,000 cycles were applied to the pedicle screws, and displacements were recorded and then cycle-displacement curve was drawn based on cycle number. Micro-CT scans were performed on the vertebrae after removing the pedicle screws to obtain images of the screw hole, and the screw hole volume was measured using imaging analysis software. Direct comparison of displacements was conducted via cycle-displacement curve. Screw hole volume was analyzed using analysis of variance. The correlation between the displacement, screw hole volume and the direction of pedicle screw was assessed by Spearman correlation analysis.

RESULTS

The smallest displacements were observed in the caudad group, followed by the parallel and cranial groups. The caudad group had the smallest screw hole volume (p < 0.001 and p = 0.009 compared to the cranial and parallel groups, respectively), while the volume in the parallel group was greater than that in the cranial group (p = 0.003). Correlation analysis revealed that the insertional direction of the pedicle screw was associated with the displacement (p = -0.949, p < 0.001) and screw hole volume (p = -0.944, p < 0.001).

CONCLUSION

Strong correlations were found between the insertional direction of the pedicle screw and relevant parameters, including displacement and screw hole volume. Pedicle screw insertion in the caudad direction resulted in the least pedicle screw loosening.

Lumbar instability remodels cartilage endplate to induce intervertebral disc degeneration by recruiting osteoclasts via Hippo-CCL3 signaling

Degenerated endplate appears with cheese-like morphology and sensory innervation, contributing to low back pain and subsequently inducing intervertebral disc degeneration in the aged population.1 However, the origin and development mechanism of the cheese-like morphology remain unclear. Here in this study, we report lumbar instability induced cartilage endplate remodeling is responsible for this pathological change. Transcriptome sequencing of the endplate chondrocytes under abnormal stress revealed that the Hippo signaling was key for this process. Activation of Hippo signaling or knockout of the key gene Yap1 in the cartilage endplate severed the cheese-like morphological change and disc degeneration after lumbar spine instability (LSI) surgery, while blocking the Hippo signaling reversed this process. Meanwhile, transcriptome sequencing data also showed osteoclast differentiation related gene set expression was up regulated in the endplate chondrocytes under abnormal mechanical stress, which was activated after the Hippo signaling. Among the discovered osteoclast differentiation gene set, CCL3 was found to be largely released from the chondrocytes under abnormal stress, which functioned to recruit and promote osteoclasts formation for cartilage endplate remodeling. Over-expression of Yap1 inhibited CCL3 transcription by blocking its promoter, which then reversed the endplate from remodeling to the cheese-like morphology. Finally, LSI-induced cartilage endplate remodeling was successfully rescued by local injection of an AAV5 wrapped Yap1 over-expression plasmid at the site. These findings suggest that the Hippo signaling induced osteoclast gene set activation in the cartilage endplate is a potential new target for the management of instability induced low back pain and lumbar degeneration.

Biomechanical Comparison of Different Surgical Approaches for the Treatment of Adjacent Segment Diseases after Primary Transforaminal Lumbar Interbody Fusion: A Finite Element Analysis

BACKGROUND AND OBJECTIVE

Adjacent segment disease (ASD) is a well-known complication after interbody fusion. Revision surgery is necessary for symptomatic ASD to further decompress and fix the affected segment. However, no optimal construct is accepted as a standard in treating ASD. The purpose of this study was to compare the biomechanical effects of different surgical approaches for the treatment of ASD after primary transforaminal lumbar interbody fusion (TLIF).

METHODS

A finite element model of the L1-S1 was conducted based on computed tomography scan images. The primary surgery model was developed with a single-level TLIF at L4-L5 segment. The revision surgical models were developed with anterior lumbar interbody fusion (ALIF), lateral lumbar interbody fusion (LLIF), or TLIF at L3-L4 segment. The range of motion (ROM), intradiscal pressure (IDP), and the stress in cages were compared to investigate the biomechanical influences of different surgical approaches.

RESULTS

The results indicated that all the three surgical approaches can stabilize the spinal segment by reducing the ROM at revision level. The ROM and IDP at adjacent segments of revision model of TLIF was greater than those of other revision models. While revision surgery with ALIF and LLIF had similar effects on the ROM and IDP of adjacent segments. Compared among all the surgical models, cage stress in revision model of TLIF was the maximum in extension and axial rotation.

CONCLUSION

The IDP at adjacent segments and stress in cages of revision model of TLIF was greater than those of ALIF and LLIF. This may be that direct extension of the surgical segment in the same direction results in stress concentration.

Finite element analysis of lattice designed lumbar interbody cage based on the additive manufacturing

Additive manufacturing (AM) methods, which facilitate the production of complex structures with different geometries, have been used in producing interbody cages in recent years. In this study, the effects of Ti6Al4V alloy interbody lattice designed fusion cages between the third and fourth lumbar vertebrae where degenerative disc diseases occur were investigated using the finite element method. Face centered cubic (FCC), body centered cubic (BCC), and diamond structures were selected as the lattice structure suitable for the interbody cage. A kidney shaped interbody lumbar cage was designed. The designated lattice structures were selected by adjusting the cell sizes suitable for the designed geometry, and the mesh configuration was made by the lumbar lattice structure. 400 N Axial force and 7.5 N.m moments were applied to the spine according to lateral bending, flexion, and torsion. 400 N axial force and 7.5 N.m flexion moment is shown high strain and total deformation then lateral bending and torsion on BCC FCC and diamond lattice structured interbody cages. In addition, the effects of lattice structures under high compression forces were investigated by applying 1000 N force to the lattice structures. When von Mises stresses were examined, lower von Mises stress and strains were observed in the BCC structure. However, a lower total deformation was observed in the FCC. Due to the design of the BCC and the diamond structure, it is assumed that bone implant adhesion will increase. In the finite element analysis (FEA), the best results were shown in BCC structures.

Three-Dimensional-Printed Titanium Versus Polyetheretherketone Cages for Lumbar Interbody Fusion: A Systematic Review of Comparative In Vitro, Animal, and Human Studies

Interbody fusion is a workhorse technique in lumbar spine surgery that facilities indirect decompression, sagittal plane realignment, and successful bony fusion. The 2 most commonly employed cage materials are titanium (Ti) alloy and polyetheretherketone (PEEK). While Ti alloy implants have superior osteoinductive properties they more poorly match the biomechanical properties of cancellous bones. Newly developed 3-dimensional (3D)-printed porous titanium (3D-pTi) address this disadvantage and are proposed as a new standard for lumbar interbody fusion (LIF) devices. In the present study, the literature directly comparing 3D-pTi and PEEK interbody devices is systematically reviewed with a focus on fusion outcomes and subsidence rates reported in the in vitro, animal, and human literature. A systematic review directly comparing outcomes of PEEK and 3D-pTi interbody spinal cages was performed. PubMed, Embase, and Cochrane Library databases were searched according to PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analysis) guidelines. Mean Newcastle-Ottawa Scale score for cohort studies was 6.4. A total of 7 eligible studies were included, comprising a combination of clinical series, ovine animal data, and in vitro biomechanical studies. There was a total population of 299 human and 59 ovine subjects, with 134 human (44.8%) and 38 (64.4%) ovine models implanted with 3D-pTi cages. Of the 7 studies, 6 reported overall outcomes in favor of 3D-pTi compared to PEEK, including subsidence and osseointegration, while 1 study reported neutral outcomes for device related revision and reoperation rate. Though limited data are available, the current literature supports 3D-pTi interbodies as offering superior fusion outcomes relative to PEEK interbodies for LIF without increasing subsidence or reoperation risk. Histologic evidence suggests 3D-Ti to have superior osteoinductive properties that may underlie these superior outcomes, but additional clinical investigation is merited.

Biomechanical Evaluation of Rigid Interspinous Process Fixation Combined With Lumbar Interbody Fusion Using Hybrid Testing Protocol

Rigid interspinous process fixation (RIPF) has been recently discussed as an alternative to pedicle screw fixation (PSF) for reducing trauma in lumbar interbody fusion (LIF) surgery. This study aimed to investigate biomechanics of the lumbar spine with RIPF, and also to compare biomechanical differences between two postoperative stages (before and after bony fusion). Based on an intact finite-element model of lumbosacral spine, the models of single-level LIF with RIPF or conventional PSF were developed and were computed for biomechanical responses to the moments of four physiological motions using hybrid testing protocol. It was found that compared with PSF, range of motion (ROM), intradiscal pressure (IDP), and facet joint forces (FJF) at adjacent segments of the surgical level for RIPF were decreased by up to 8.4%, 2.3%, and 16.8%, respectively, but ROM and endplate stress at the surgical segment were increased by up to 285.3% and 174.3%, respectively. The results of comparison between lumbar spine with RIPF before and after bony fusion showed that ROM and endplate stress at the surgical segment were decreased by up to 62.6% and 40.4%, respectively, when achieved to bony fusion. These findings suggest that lumbar spine with RIPF as compared to PSF has potential to decrease the risk of adjacent segment degeneration but might have lower stability of surgical segment and an increased risk of cage subsidence; When achieved bony fusion, it might be helpful for the lumbar spine with RIPF in increasing stability of surgical segment and reducing failure of bone contact with cage.

Effect of Interbody Implants on the Biomechanical Behavior of Lateral Lumbar Interbody Fusion: A Finite Element Study

Porous titanium interbody scaffolds are growing in popularity due to their appealing advantages for bone ingrowth. This study aimed to investigate the biomechanical effects of scaffold materials in both normal and osteoporotic lumbar spines using a finite element (FE) model. Four scaffold materials were compared: Ti6Al4V (Ti), PEEK, porous titanium of 65% porosity (P65), and porous titanium of 80% porosity (P80). In addition, the range of motion (ROM), endplate stress, scaffold stress, and pedicle screw stress were calculated and compared. The results showed that the ROM decreased by more than 96% after surgery, and the solid Ti scaffold provided the lowest ROM (1.2-3.4% of the intact case) at the surgical segment among all models. Compared to solid Ti, PEEK decreased the scaffold stress by 53-66 and the endplate stress by 0-33%, while porous Ti decreased the scaffold stress by 20-32% and the endplate stress by 0-32%. Further, compared with P65, P80 slightly increased the ROM (<0.03°) and pedicle screw stress (<4%) and decreased the endplate stress by 0-13% and scaffold stress by approximately 18%. Moreover, the osteoporotic lumbar spine provided higher ROMs, endplate stresses, scaffold stresses, and pedicle screw stresses in all motion modes. The porous Ti scaffolds may offer an alternative for lateral lumbar interbody fusion.

Computational comparison of anterior lumbar interbody fusion and oblique lumbar interbody fusion with various supplementary fixation systems: a finite element analysis

BACKGROUND AND OBJECTIVE

Anterior lumbar interbody fusion (ALIF) and oblique lumbar interbody fusion (OLIF) have shown a great surgical potential, while it has always been controversial which surgical approach and which type of fixation system should be selected. This study investigated the biomechanical response of ALIF and OLIF with various supplementary fixation systems using the finite element method.

MATERIALS AND METHODS

Lumbar L4-L5 ALIF and OLIF models stabilized by different supplementary fixation systems (stand-alone cage, integrated stand-alone cage, anterior plate, and bilateral pedicle screw) were developed to assess the segmental range of motion (ROM), endplate stress (EPS), and screw-bone interface stress (SBIS).

EXPERIMENTAL RESULTS

ALIF showed lower ROM and EPS than OLIF in all motion planes and less SBIS in the most of motion planes compared with OLIF when the anterior plate or pedicle screw was used. ALIF induced higher ROM, while lower EPS and SBIS than OLIF in the majority of motion planes when integrated stand-alone cage was utilized. Using a stand-alone cage in ALIF and OLIF led to cage migration. Integrated stand-alone cage prevented the cage migration, whereas caused significantly larger ROM, EPS, and SBIS than other fixation systems except for the rotation plane. In the most of motion planes, the pedicle screw had the lowest ROM, EPS, and SBIS. The anterior plate induced a slightly larger ROM, EPS, and SBIS than the pedicle screw, while the differences were not significant.

CONCLUSION

ALIF exhibited a better performance in postoperative segmental stability, endplate stress, and screw-bone interface stress than OLIF when the anterior plate or the pedicle screw was used. The pedicle screw could provide the greatest postoperative segmental stability, less cage subsidence incidence, and lower risk of fixation system loosening in ALIF and OLIF. The anterior plate could also contribute to the stability required and fewer complications, while not as effectively as the pedicle screw. Extreme caution should be regarded when the stand-alone cage is used due to the risk of cage migration. The integrated stand-alone cage may be an alternative method; however, further optimization is needed to reduce complications and improve postoperative segmental stability.

Recent advancement in finite element analysis of spinal interbody cages: A review

Finite element analysis (FEA) is a widely used tool in a variety of industries and research endeavors. With its application to spine biomechanics, FEA has contributed to a better understanding of the spine, its components, and its behavior in physiological and pathological conditions, as well as assisting in the design and application of spinal instrumentation, particularly spinal interbody cages (ICs). IC is a highly effective instrumentation for achieving spinal fusion that has been used to treat a variety of spinal disorders, including degenerative disc disease, trauma, tumor reconstruction, and scoliosis. The application of FEA lets new designs be thoroughly "tested" before a cage is even manufactured, allowing bio-mechanical responses and spinal fusion processes that cannot easily be experimented upon in vivo to be examined and "diagnosis" to be performed, which is an important addition to clinical and in vitro experimental studies. This paper reviews the recent progress of FEA in spinal ICs over the last six years. It demonstrates how modeling can aid in evaluating the biomechanical response of cage materials, cage design, and fixation devices, understanding bone formation mechanisms, comparing the benefits of various fusion techniques, and investigating the impact of pathological structures. It also summarizes the various limitations brought about by modeling simplification and looks forward to the significant advancement of spine FEA research as computing efficiency and software capabilities increase. In conclusion, in such a fast-paced field, the FEA is critical for spinal IC studies. It helps in quantitatively and visually demonstrating the cage characteristics after implanting, lowering surgeons' learning costs for new cage products, and probably assisting them in determining the best IC for patients.

The best position of bone grafts in the medial open-wedge high tibial osteotomy: A finite element analysis

BACKGROUND AND OBJECTIVE

The application of wedge-shaped bone grafts can increase the biomechanical stability of knee during the medial open-wedge high tibial osteotomy (MOWHTO) by reducing the von Mises stress of the medial plate and lateral cortical hinge area. However, the optimal position of bone grafts it remains unclear, so we aimed to determine search for the optimal position of the bone grafts in MOWHTO by using finite element analysis.

METHODS

In the finite element analysis, MOWHTO models were established with three different osteotomy distraction heights and assembled into four groups according to different conditions, including the no bone grafts (NBG) group, the anterior bone grafts (ABG) group, the middle bone grafts (MBG) group, and the posterior bone grafts (PBG) group. Based on previous studies, 600 N and 1800 N loads were applied to the knee joint to simulate the static forces during a double and single leg stance to measure the von Mises stress of the medial implant area and lateral hinge area, the maximum displacement of different models, the relative displacement of the osteotomy area and the stress distribution in the bone grafts.

RESULTS

Compared to the NBG and ABG groups, the stress of the lateral cortical hinge area and the medial implate area was significantly lower in the PBG group. For example, under the 600N force load, when the height of the osteotomy area was 10 mm, 15 mm, and 20 mm, the maximum von Mises stress of the medial implate area and lateral cortical hinge area in the NBG group were 140, 141, 172, and 53, 57, 60 MPa, respectively. Compared with the NBG group, the maximum von Mises stress of the medial implate area and lateral cortical hinge area in the PBG group were reduced by 45%, 56%, 63% and 14%, 39%, 68% at distraction height of 10 mm, 15 mm, and 20 mm, respectively. The bone grafts in the posterior parts provide the best stability,with the stress of the middle and posterior bone grafts are mainly concentrated in the edge.

CONCLUSIONS

The posterior part of the osteotomy area is the best position for bone graft placement since it provides optimal stability and reduces von Mises stress in the medial plate and lateral cortex hinge area, with the stress of the posterior bone grafts mainly concentrated in the edge. These findings guide bone graft placement sites in clinical surgery and are a basis for future research on bone graft materials and structures in MOWHTO.

Lumbar interbody fusion with bilateral cages using a biportal endoscopic technique with a third portal

BACKGROUND

Unilateral biportal endoscopic lumbar interbody fusion (ULIF) with one cage results in fewer definitive fusions (Park et al. in Neurosurg Rev 42(3):753-761, 2019). We succeeded in inserting bilateral cages during ULIF.

METHOD

We attempted posterior ULIF for degenerative lumbar spondylolisthesis with bilateral recess stenosis. With the help of a third portal, ULIF with bilateral cage insertion was performed under general anaesthesia.

CONCLUSIONS

We successfully performed ULIF with bilateral cages with the help of a third portal. This procedure may be an alternative for treating lumbar stenosis with instability.

3D-printed titanium cages without bone graft outperform PEEK cages with autograft in an animal model

BACKGROUND CONTEXT

Modernization of 3D printing has allowed for the production of porous titanium interbody cages (3D-pTi) which purportedly optimize implant characteristics and increase osseointegration; however, this remains largely unstudied in vivo.

PURPOSE

To compare osseointegration of three-dimensional (3D) titanium cages without bone graft and Polyether-ether-ketone (PEEK) interbody cages with autologous iliac crest bone graft (AICBG).

STUDY DESIGN

Animal study utilizing an ovine in vivo model of lumbar fusion.

METHODS

Interbody cages of PEEK or 3D-pTi supplied by Spineart SA (Geneva, Switzerland) were implanted in seven living sheep at L2-L3 and L4-L5, leaving the intervening disc space untreated. Both implant materials were used in each sheep and randomized to the aforementioned disc spaces. Computed tomography (CT) was obtained at 4 weeks and 8 weeks. MicroCT and histological sections were obtained to evaluate osseointegration.

RESULTS

MicroCT demonstrated osseous in-growth of native cancellous bone in the trabecular architecture of the 3D-pTi interbody cages and no interaction between the PEEK cages with the surrounding native bone. Qualitative histology revealed robust osseointegration in 3D-pTi implants and negligible osseointegration with localized fibrosis in PEEK implants. Evidence of intramembranous and endochondral ossification was apparent with the 3D-pTi cages. Quantitative histometric bone implant contact demonstrated significantly more contact in the 3D-pTi implants versus PEEK (p<.001); region of interest calculations also demonstrated significantly greater osseous and cartilaginous interdigitation at the implant-native bone interface with the 3D-pTi cages (p=.008 and p=.015, respectively).

CONCLUSIONS

3D-pTi interbody cages without bone graft outperform PEEK interbody cages with AICBG in terms of osseointegration at 4 and 8 weeks postoperatively in an ovine lumbar fusion model.

CLINICAL SIGNIFICANCE

3D-pTi interbody cages demonstrated early and robust osseointegration without any bone graft or additive osteoinductive agents. This may yield early stability in anterior lumbar arthrodesis and potentially bolster the rate of successful fusion. This could be of particular advantage in patients with spinal neoplasms needing post-ablative arthrodesis, where local autograft use would be ill advised.

Finite Element Analysis of a Novel Fusion Strategy in Minimally Invasive Transforaminal Lumbar Interbody Fusion

Purpose

To evaluate the biomechanics of a novel fusion strategy (hybrid internal fixation+horizontal cage position) in minimally invasive transforaminal lumbar interbody fusion (MIS-TLIF).

Methods

MIS-TLIF finite element models for three fusion strategies were created based on computed tomography images, namely, Model-A, hybrid internal fixation (ipsilateral pedicle screw and contralateral translaminar facet screw fixation)+horizontal cage position; Model-B, bilateral pedicle screw (BPS) fixation+horizontal cage position; and Model-C, BPS fixation+oblique 45° cage position. A preload of 500 N and a moment of 10 Nm were applied to the models to simulate lumbar motion, and the models' range of motion (ROM), peak stress of the internal fixation system, and cage were assessed.

Results

The ROM for Models A, B, and C were not different (P > 0.05) but were significantly lower than the ROM of Model-INT (P < 0.0001). Although there were subtle differences in the ROM ratio for Models A, B, and C, the trend was similar. The peak stress of the internal fixation system was significantly higher in Model-A than that of Models B and C, but only the difference between Models A and B was significant (P < 0.05). The peak stress of the cage in Model-A was significantly lower than that of Models B and C (P < 0.01).

Conclusion

Hybrid internal fixation with horizontal single cage implantation can provide the same biomechanical stability as traditional fixation while reducing peak stress on the cage and vertebral endplate.

Bone Remodelling Around Solid and Porous Interbody Cages in the Lumbar Spine

Spinal fusion is an effective surgical treatment for intervertebral disc degeneration. However, the consequences of implantation with interbody cages on load transfer and bone remodelling in the vertebral bodies has scarcely been investigated. Using detailed 3D models of an intact and implanted lumbar spine and the strain energy density based bone remodelling algorithm, this study investigated the evolutionary changes in bone density distributions around porous and solid interbody cages. Follower load technique and submodelling approach were employed to simulate applied loading conditions on the lumbar spine models. The study determined the relationship between mechanical properties and parametrical characteristics of porous Body-centered-cubic (BCC) models, which corroborated well with Gibson-Ashby and exponential regression models. Variations in porosity affected the peri-prosthetic stress distributions and bone remodelling around the cages. In comparison to the solid cage, stresses and strains in the cancellous bone decreased with an increase in cage porosity; whereas the range of motion increased. For the solid cage, increase in bone density of 20-28% was predicted in the L4 inferior and L5 superior regions; whereas the model with 78% porosity exhibited a small 3-5% change in bone density. An overall increase of 9-14% bone density was predicted in the L4 and L5 vertebrae after remodelling for solid interbody cages, which may influence disc degeneration in the adjacent segment. In comparison to the solid cage, an interbody cage with 65-78% porosity could be a viable and promising alternative, provided sufficient mechanical strength is offered.

Outcomes of Transforaminal Lumbar Interbody Fusion Using Unilateral Versus Bilateral Interbody Cages

Objective

To assess the impact of bilateral versus unilateral interbody cages on outcomes for minimally invasive transforaminal lumbar interbody fusion (MIS TLIF) procedures.

Methods

A retrospective review for primary, elective, single-level MIS TLIF procedures with bilateral posterior instrumentation from 2008–2020 was performed. Patients were grouped according to unilateral or bilateral interbody cage use. Procedures performed without static interbody cages or indicated for trauma, infection, malignancy were excluded. Patient-reported outcomes (PROs) included visual analogue scale (VAS), Oswestry Disability Index, 12-item Short Form health survey physical composite score (SF-12 PCS), PatientReported Outcome Measurement Information System physical function (PROMIS-PF). PROs were collected preoperatively and postoperatively. Change in PROs (Δ) was calculated and compared between groups. Achievement of minimum clinically important difference (MCID) was calculated using established values from the literature. Achievement rates were compared between groups using logistic regression.

Results

The study included 151 patients, with 111 unilateral and 40 bilateral cage placements. Charlson Comorbidity Index, diabetes, and insurance status differed between groups (p < 0.050). Prevalence of degenerative and isthmic spondylolisthesis (both p ≤ 0.002), operative level (p = 0.003), and postoperative length of stay (p = 0.022) significantly differed between groups. The unilateral group had lower 1-year arthrodesis rates (p = 0.035). Preoperative VAS leg (p = 0.017) and SF-12 PCS (p = 0.045) were worse for the unilateral group. ΔPROMIS-PF was greater for the bilateral group at 2 years (p = 0.001). Majority of patients achieved an overall MCID for all PROs, except VAS leg (bilateral group).

Conclusion

While preoperative status and postoperative arthrodesis rates differed, patients achieved an MCID at similar rates regardless of use of unilateral or bilateral cages.

Computational comparison of three different cage porosities in posterior lumbar interbody fusion with porous cage

Porous interbody cages, manufactured using additive laser melting technology, have recently been used in lumbar fusion surgery. The major advantage of a porous cage is the presence of space inside the cage for bone ingrowth. However, the biomechanical effects of different porosities on the lumbar segment with and without bone fusion (ingrowth) are still unclear. Hence, the present study aimed to compare the biomechanical responses, including the stress and range of motion (ROM) of the lumbar L3-L4 segments with three different types of porous cages along with a posterior instrument (PI) with and without bone fusion using computer simulation. A lumbar L3-L4 segment model with a PI and porous cages was used in this study. Three different porosities, namely 12.5, 41.2, and 80.84% were used. The diameter of the pores of the porous cage was uniformly set to 0.5 mm. In addition, a traditional PEEK cage was used in this study. Two different bone statuses, with and without bone fusion (ingrowth into the pores of the porous cage and the inner space of the PEEK cage), were considered. The results indicated that although the contact pressure on the bone surface reduced, the cage stress increased with increasing cage porosity. Furthermore, cage stress and contact pressure also increased in cases with bone fusion compared with those without bone fusion. The contact pressure on the bone surface with a cage porosity of 80.8% decreased by 40% (from 943.1 to 575.5 MPa), 37.7% (from 133 to 82.9 MPa), 40.4% (from 690.8 to 412 MPa), and 34.2% (from 533 to 351.1 MPa), respectively, for flexion, extension, lateral bending, and rotation, respectively, compared with that with a cage porosity of 12.5%. The rotational ROM of the PEEK cage with bone fusion was clearly larger than those of the porous cages. Porous cages have recently become popular owing to improved manufacturing technology. This study provides scientific data on the strength and weakness of porous cages with different porosities for clinical use.

Finite element study on the influence of pore size and structure on stress shielding effect of additive manufactured spinal cage

The stress shielding effect occurs when the orthopedic implant reduces the load delivered to the bone, causing inefficient stress transfer to the host bone. The usage of porous additive manufactured (AM) cages reduces the stress shielding effect and promotes bone ingrowth also. The purpose of this work is to study the stress and deformation on porous hybrid spinal cages under different loading conditions using Finite Element Analysis (FEA). The spinal cages consisting of three porous structures with pore sizes ranging from 0.4 to 0.6 mm were investigated for stress shielding and fatigue strength. The results showed a significant reduction in stress shielding for the studied designs and conclude that the pore size has a greater significant effect on stress shielding than the porous structure in spinal cages.

Prediction of the influence of vertical whole-body vibration on biomechanics of spinal segments after lumbar interbody fusion surgery

BACKGROUND

Previous studies have shown that for healthy spine, cyclic loading encountered due to whole-body vibration exposure generated higher responses in spinal tissues than static loading. However, how whole-body vibration affects spine biomechanics after interbody fusion surgery is poorly understood. This study aimed at comparing the effects of vibration loading on spinal segments between postsurgical and healthy lumbar spines.

METHODS

A validated finite element model of healthy human lumbosacral spine was modified to simulate interbody fusion at L4-L5 level considering the statuses immediately after surgery (before bony fusion) and after bony fusion. Biomechanical responses at its adjacent levels for the healthy and fusion models to a sinusoidal axial vibration load of ±40 N and the corresponding static axal loads (-40 N and 40 N) were computed using transient dynamic and static analyses, respectively.

FINDINGS

For both healthy and fusion models, vibration amplitudes of the predicted responses were significantly higher than the corresponding changing amplitudes under static loads. Specifically, the increasing effect of vibration load in disc bulge, disc stress and intradiscal pressure at L3-L4 level reached 255.9%, 215.0% and 224.4% for the healthy model, 157.4%, 177.8% and 171.8% for the fusion model (before bony fusion), 141.9%, 152.6% and 160.1% for the fusion model (after bony fusion).

INTERPRETATION

Although whole-body vibration is still more dangerous for the lumbar spine after interbody fusion surgery than static loading, the sensitivity of adjacent segment in postsurgical spine to vibration loading is decreased compared with healthy spine, especially when reaching to bony fusion.

Numerical analysis of the mechanical behaviour of intact and implanted lumbar functional spinal units: Effects of loading and boundary conditions

The objective of this study was to develop an improved finite element (FE) model of a lumbar functional spinal unit (FSU) and to subsequently analyse the deviations in load transfer owing to implantation. The effects of loading and boundary conditions on load transfer in intact and implanted FSUs and its relationship with the potential risk of vertebral fracture were investigated. The FE models of L1-L5 and L3-L4 FSUs, intact and implanted, were developed using patient-specific CT-scan dataset and segmentation of cortical and cancellous bone regions. The effect of submodelling technique, as compared to artificial boundary conditions, on the elastic behaviour of lumbar spine was examined. Applied forces and moments, corresponding to physiologic movements, were used as loading conditions. Results indicated that the loading and boundary conditions considerably affect stress-strain distributions within a FSU. This study, based on an improved FE model of a vertebra, highlights the importance of using the submodelling technique to adequately evaluate the mechanical behaviour of a FSU. In the intact FSU, strains of 200-400 µε were observed in the cancellous bone of vertebral body and pedicles. High equivalent stresses of 10-25 MPa and 1-5 MPa were generated around the pars interarticularis for cortical and cancellous regions, respectively. Implantation caused reductions of 85%-92% in the range of motion for all movements. Insertion of the intervertebral cage resulted in major deviations in load transfer across a FSU for all movements. The cancellous bone around cage experienced pronounced increase in stresses of 10-15 MPa, which indicated potential risk of failure initiation in the vertebra.

The anterior and traverse cage can provide optimal biomechanical performance for both traditional and percutaneous endoscopic transforaminal lumbar interbody fusion

BACKGROUND

Transforaminal lumbar interbody fusion (TLIF) is a well-established surgical treatment for patients with lumbar degenerative disc disease; however, the optimal position for the interbody fusion cage in TLIF procedures for reducing cage-related complications remains uncertain. The present study aims to compare the biomechanical effects between different cage positions in TLIF and percutaneous endoscopic-TLIF (PE-TLIF).

METHOD

An intact finite element model of L3-L5 from computed tomography images of a 25-year-old healthy male without any lumbar disease was reconstructed and validated. TLIF and PE-TLIF were performed on L4-L5 with bilateral pedicle screws fixation. Two surgical finite element models were subjected to loads with six degrees of freedom. The range of motion (ROM) and von Mises stress of the implantations and endplates were measured for the anterior, middle, and posterior district and the traverse or oblique direction of the cage respectively.

RESULTS

As the cage was implanted forward, the ROMs in the fused L4-L5 segments and the von Mises stress of the cage and endplates decreased while the von Mises stress of the screws increased; this was also shown in the traverse cage when compared with the oblique cage (A-90-compared with A-45- had a 31.3%, 1.7%, 12.6%, and 5.7% decrease in FL, EX, LB and AR). The ROMs (TLIF A-45 increase of 80.8%, 23.8%, and 12.2% in FL, EX, and LB when compared with PE-TLIF), cage stress, and endplate stress of PE-TLIF were lower than those of TLIF.

CONCLUSIONS

Considering the ROM of the fusion segments, implanting the cage in the anterior district in the traverse direction can effectively enhance the fusion segment stiffness, thus contributing to the stability of the lumbar spine after fusion. It can also cause less cage stress and endplate stress, which indicates its beneficial effect in avoiding cage injury or subsidence. However, the higher stress of the pedicle screws and rods indicates higher failure risk. PE-TLIF had better biomechanical performance than TLIF. Therefore, it is recommended that the surgeon implant the cage in the anterior district of the L5 vertebra's upper endplate in the traverse direction using the PE-TLIF technique.

Comparison of dynamic response of three TLIF techniques on the fused and adjacent segments under vibration

To explore which TLIF techniques are advantageous in reducing the risk of complications and conducive to bone fusion under the vibration. The L1-L5 finite element lumbar model was modified to simulate three different TLIF techniques (a unilateral standard cage, a crescent-shaped cage, and bilateral standard cages). The results showed that the crescent-shaped cage may reduce the risk of subsidence and provide a more stable and suitable environment for vertebral cell growth under the vibration compared to the other TLIF techniques. Unilateral cage may increase the risk of adjacent segment disease and cage failure including fatigue failure under vibration.

Porous fusion cage design via integrated global-local topology optimization and biomechanical analysis of performance

Porous fusion cage is considered as a satisfactory substitute for solid fusion cage in transforaminal lumbar interbody fusion (TLIF) surgery due to its interconnectivity for bone ingrowth and appropriate stiffness reducing the risk of cage subsidence and stress shielding. This study presents an integrated global-local topology optimization approach to obtain porous titanium (Ti) fusion cage with desired biomechanical properties. Local topology optimizations are first conducted to obtain unit cells, and the numerical homogenization method is used to quantified the mechanical properties of unit cells. The preferred porous structure is then fabricated using selective laser melting, and its mechanical property is further verified via compression tests and numerical simulation. Afterward, global topology optimization is used for the global layout. The porous fusion cage obtained by the Boolean intersection between global structural layout and the porous structure decreases the solid volume of the cage by 9% for packing more bone grafts while achieving the same stiffness to conventional porous fusion cage. To eliminate stress concentration in the thin-wall structure, framework structures are constructed on the porous fusion cage. Although the alleviation of cage subsidence and stress shielding is decelerated, peak stress on the cage is significantly decreased, and more even stress distribution is demonstrated in the reinforced porous fusion cage. It promises long-term integrity and functions of the fusion cage. Overall, the reinforced porous fusion cage achieves a favorable mechanical performance and is a promising candidate for fusion surgery. The proposed optimization approach is promising for fusion cage design and can be extended to other orthopedic implant designs.

Development and Validation of a Computationally Efficient Finite Element Model of the Human Lumbar Spine: Application to Disc Degeneration

INTRODUCTION

This study develops and validates an accurate, computationally efficient, 3-dimensional finite element model (FEM) of the human lumbar spine. Advantages of this simplified model are shown by its application to a disc degeneration study that we demonstrate is completed in one-sixth the time required when using more complicated computed tomography (CT) scan-based models.

METHODS

An osseoligamentous FEM of the L1-L5 spine is developed using simple shapes based on average anatomical dimensions of key features of the spine rather than CT scan images. Pure moments of 7.5 Nm and a compressive follower load of 1000 N are individually applied to the L1 vertebra. Validation is achieved by comparing rotations and intradiscal pressures to other widely accepted FEMs and in vitro studies. Then degenerative disc properties are modeled and rotations calculated. Required computation times are compared between the model presented in this paper and other models developed using CT scans.

RESULTS

For the validation study, parameter values for a healthy spine were used with the loading conditions described above. Total L1-L5 rotations for flexion, extension, lateral bending, and axial rotation under pure moment loading were calculated as 20.3°, 10.7°, 19.7°, and 10.3°, respectively, and under a compressive follower load, maximum intradiscal pressures were calculated as 0.68 MPa. These values compare favorably with the data used for validation. When studying the effects of disc degeneration, the affected segment is shown to experience decreases in rotations during flexion, extension, and lateral bending (24%-56%), while rotations are shown to increase during axial rotation (14%-40%). Adjacent levels realize relatively minor changes in rotation (1%-6%). This parametric study required 17.5 hours of computation time compared to more than 4 days required if utilizing typical published CT scan-based models, illustrating one of the primary advantages of the model presented in this article.

CONCLUSIONS

The FEM presented in this article produces a biomechanical response comparable to widely accepted, complex, CT scan-based models and in vitro studies while requiring much shorter computation times. This makes the model ideal for conducting parametric studies of spinal pathologies and spinal correction techniques.

Integration of a Three-Dimensional-Printed Titanium Implant in Human Tissues: Case Study

A titanium alloy implant of appropriate pore size can potentially enhance osseointegration and soft tissue integration. However, the human clinical application of such implants has not been reported. Here, we present a case of limb salvage surgery for a bone tumor using customized three-dimensional (3D)-printed Ti6Al4V radius and ulna implants. The patient presented with local recurrence at the proximal junction of the ulna and underwent a re-wide excision. Single forearm bone surgery was performed using another 3D-printed implant after resection of the recurrent tumor with an ulnar implant. Host osseointegration and soft tissue integration of the retrieved implant were quantified through histological evaluation. The total tissue integration rates of the implant at the proximal and distal bone junctions were 45.96% and 15.03%, respectively. The mesh structure enhanced bone integration by up to 10.81% in the proximal and by up to 8.91% in the distal bone junction. Furthermore, the soft tissue adhesion rates of the implant shaft were 59.50% and 50.26% in the axial and longitudinal cuts, respectively. No area was left unoccupied throughout the shaft of the implant. Overall, these results indicate that the 3D-printed Ti6Al4V titanium alloy implant with a rough surface has considerable tissue integration ability.

Computational comparison of bone cement and poly aryl-ether-ether-ketone spacer in single-segment posterior lumbar interbody fusion: a pilot study

Posterior lumbar interbody fusion (PLIF) with a spacer and posterior instrument (PI) via minimally invasive surgery (MIS) restores intervertebral height in degenerated disks. To align with MIS, the spacer has to be shaped with a slim geometry. However, the thin spacer increases the subsidence and migration after PLIF. This study aimed to propose a new lumbar fusion approach using bone cement to achieve a larger supporting area than that achieved by the currently used poly aryl-ether-ether-ketone (PEEK) spacer and assess the feasibility of this approach using a sawbone model. Furthermore, the mechanical responses, including the range of motion (ROM) and bone stress with the bone cement spacer were compared to those noted with the PEEK spacer by finite element (FE) simulation. An FE lumbar L3-L4 model with PEEK and bone cement spacers and PI was developed. Four fixing conditions were considered: intact lumbar L3-L4 segment, lumbar L3-L4 segment with PI, PEEK spacer plus PI, and bone cement spacer plus PI. Four kinds of 10-NM moments (flexion, extension, lateral bending, and rotation) and two different bone qualities (normal and osteoporotic) were considered. The bone cement spacer yielded smaller ROMs in extension and rotation than the PEEK spacer, while the ROMs of the bone cement spacer in flexion and lateral bending were slightly greater than with the PEEK spacer. Compared with the PEEK spacer, peak contact pressure on the superior surface of L4 with the bone cement spacer in rotation decreased by 74% (from 8.68 to 2.25 MPa) and 69.1% (from 9.1 to 2.82 MPa), respectively, in the normal and osteoporotic bone. Use of bone cement as a spacer with PI is a potential approach to decrease the bone stress in lumbar fusion and warrants further research.

Integrated Design Approaches for 3D Printed Tissue Scaffolds: Review and Outlook

Emerging 3D printing technologies are enabling the fabrication of complex scaffold structures for diverse medical applications. 3D printing allows controlled material placement for configuring porous tissue scaffolds with tailored properties for desired mechanical stiffness, nutrient transport, and biological growth. However, tuning tissue scaffold functionality requires navigation of a complex design space with numerous trade-offs that require multidisciplinary assessment. Integrated design approaches that encourage iteration and consideration of diverse processes including design configuration, material selection, and simulation models provide a basis for improving design performance. In this review, recent advances in design, fabrication, and assessment of 3D printed tissue scaffolds are investigated with a focus on bone tissue engineering. Bone healing and fusion are examples that demonstrate the needs of integrated design approaches in leveraging new materials and 3D printing processes for specified clinical applications. Current challenges for integrated design are outlined and emphasize directions where new research may lead to significant improvements in personalized medicine and emerging areas in healthcare.

Bony ingrowth potential of 3D-printed porous titanium alloy: a direct comparison of interbody cage materials in an in vivo ovine lumbar fusion model

BACKGROUND CONTEXT

There is significant variability in the materials commonly used for interbody cages in spine surgery. It is theorized that three-dimensional (3D)-printed interbody cages using porous titanium material can provide more consistent bone ingrowth and biological fixation.

PURPOSE

The purpose of this study was to provide an evidence-based approach to decision-making regarding interbody materials for spinal fusion.

STUDY DESIGN

A comparative animal study was performed.

METHODS

A skeletally mature ovine lumbar fusion model was used for this study. Interbody fusions were performed at L2-L3 and L4-L5 in 27 mature sheep using three different interbody cages (ie, polyetheretherketone [PEEK], plasma sprayed porous titanium-coated PEEK [PSP], and 3D-printed porous titanium alloy cage [PTA]). Non-destructive kinematic testing was performed in the three primary directions of motion. The specimens were then analyzed using micro-computed tomography (µ-CT); quantitative measures of the bony fusion were performed. Histomorphometric analyses were also performed in the sagittal plane through the interbody device. Outcome parameters were compared between cage designs and time points.

RESULTS

Flexion-extension range of motion (ROM) was statistically reduced for the PTA group compared with the PEEK cages at 16 weeks (p-value=.02). Only the PTA cages demonstrated a statistically significant decrease in ROM and increase in stiffness across all three loading directions between the 8-week and 16-week sacrifice time points (p-value≤.01). Micro-CT data demonstrated significantly greater total bone volume within the graft window for the PTA cages at both 8 weeks and 16 weeks compared with the PEEK cages (p-value<.01).

CONCLUSIONS

A direct comparison of interbody implants demonstrates significant and measurable differences in biomechanical, µ-CT, and histologic performance in an ovine model. The 3D-printed porous titanium interbody cage resulted in statistically significant reductions in ROM, increases in the bone ingrowth profile, as well as average construct stiffness compared with PEEK and PSP.

Biomechanical Analysis of Porous Additive Manufactured Cages for Lateral Lumbar Interbody Fusion: A Finite Element Analysis

BACKGROUND

A porous additive manufactured (AM) cage may provide stability similar to that of traditional solid cages and may be beneficial to bone ingrowth. The biomechanical influence of various porous cages on stability, subsidence, stresses in cage, and facet contact force has not been fully described. The purpose of this study was to verify biomechanical effects of porous AM cages.

METHODS

The surgical finite element models with various cages were constructed. The partially porous titanium (PPT) cages and fully porous titanium (FPT) cages were applied. The mechanical parameters of porous materials were obtained by mechanical test. Then the porous AM cages were compared with solid titanium (TI) cage and solid polyetheretherketone (PEEK) cage. The 4 motion modes were simulated. Range of motion (ROM), cage stress, end plate stress, and facet joint force (FJF) were compared.

RESULTS

For all the surgical models, ROM decreased by >90%. Compared with TI and PPT cages, PEEK and FPT cages substantially reduced the maximum stresses in cage and end plate in all motion modes. Compared with PEEK cages, the stresses in cage and end plate for FPT cages decreased, whereas the ROM increased. Comparing FPT cages, the stresses in cage and end plate decreased with increasing porosity, whereas ROM increased with increasing porosity. After interbody fusion, FJF was substantially reduced in all motion modes except for flexion.

CONCLUSIONS

Fully porous cages may offer an alternative to solid PEEK cages in lateral lumbar interbody fusion. However, it may be prudent to further increase the porosity of the cage.

Evaluation of a polyetheretherketone (PEEK) titanium composite interbody spacer in an ovine lumbar interbody fusion model: biomechanical, microcomputed tomographic, and histologic analyses

BACKGROUND CONTEXT

The most commonly used materials used for interbody cages are titanium metal and polymer polyetheretherketone (PEEK). Both of these materials have demonstrated good biocompatibility. A major disadvantage associated with solid titanium cages is their radiopacity, limiting the postoperative monitoring of spinal fusion via standard imaging modalities. However, PEEK is radiolucent, allowing for a temporal assessment of the fusion mass by clinicians. On the other hand, PEEK is hydrophobic, which can limit bony ingrowth. Although both PEEK and titanium have demonstrated clinical success in obtaining a solid spinal fusion, innovations are being developed to improve fusion rates and to create stronger constructs using hybrid additive manufacturing approaches by incorporating both materials into a single interbody device.

PURPOSE

The purpose of this study was to examine the interbody fusion characteristic of a PEEK Titanium Composite (PTC) cage for use in lumbar fusion.

STUDY DESIGN/SETTING

Thirty-four mature female sheep underwent two-level (L₂-L₃ and L₄-L₅) interbody fusion using either a PEEK or a PTC cage (one of each per animal). Animals were sacrificed at 0, 8, 12, and 18 weeks post surgery.

MATERIALS AND METHODS

Post sacrifice, each surgically treated functional spinal unit underwent non-destructive kinematic testing, microcomputed tomography scanning, and histomorphometric analyses.

RESULTS

Relative to the standard PEEK cages, the PTC constructs demonstrated significant reductions in ranges of motion and a significant increase in stiffness. These biomechanical findings were reinforced by the presence of significantly more bone at the fusion site as well as ingrowth into the porous end plates.

CONCLUSIONS

Overall, the results indicate that PTC interbody devices could potentially lead to a more robust intervertebral fusion relative to a standard PEEK device in a clinical setting.

Computationally designed lattices with tuned properties for tissue engineering using 3D printing

Tissue scaffolds provide structural support while facilitating tissue growth, but are challenging to design due to diverse property trade-offs. Here, a computational approach was developed for modeling scaffolds with lattice structures of eight different topologies and assessing properties relevant to bone tissue engineering applications. Evaluated properties include porosity, pore size, surface-volume ratio, elastic modulus, shear modulus, and permeability. Lattice topologies were generated by patterning beam-based unit cells, with design parameters for beam diameter and unit cell length. Finite element simulations were conducted for each topology and quantified how elastic modulus and shear modulus scale with porosity, and how permeability scales with porosity cubed over surface-volume ratio squared. Lattices were compared with controlled properties related to porosity and pore size. Relative comparisons suggest that lattice topology leads to specializations in achievable properties. For instance, Cube topologies tend to have high elastic and low shear moduli while Octet topologies have high shear moduli and surface-volume ratios but low permeability. The developed method was utilized to analyze property trade-offs as beam diameter was altered for a given topology, and used to prototype a 3D printed lattice embedded in an interbody cage for spinal fusion treatments. Findings provide a basis for modeling and understanding relative differences among beam-based lattices designed to facilitate bone tissue growth.

Use of Pig as a Model for Mesenchymal Stem Cell Therapies for Bone Regeneration

Bone is a plastic tissue with a large healing capability. However, extensive bone loss due to disease or trauma requires extreme therapy such as bone grafting or tissue-engineering applications. Presently, bone grafting is the gold standard for bone repair, but presents serious limitations including donor site morbidity, rejection, and limited tissue regeneration. The use of stem cells appears to be a means to overcome such limitations. Bone marrow mesenchymal stem cells (BMSC) have been the choice thus far for stem cell therapy for bone regeneration. However, adipose-derived stem cells (ASC) have similar immunophenotype, morphology, multilineage potential, and transcriptome compared to BMSC, and both types have demonstrated extensive osteogenic capacity both in vitro and in vivo in several species. The use of scaffolds in combination with stem cells and growth factors provides a valuable tool for guided bone regeneration, especially for complex anatomic defects. Before translation to human medicine, regenerative strategies must be developed in animal models to improve effectiveness and efficiency. The pig presents as a useful model due to similar macro- and microanatomy and favorable logistics of use. This review examines data that provides strong support for the clinical translation of the pig model for bone regeneration.

Digital Anatomical Measurement for Anterolateral Fixation of Middle and Lower Thoracic Vertebrae

Background

The key to its successful application is to determine the best entry point for the vertebral screw(s). This study aimed to provide a reference for clinical anterolateral fixation through digital measurement of computed tomography (CT) data to identify relevant anatomical positions in the middle and lower thoracic vertebrae (T4–T12) of 30 adults.

Material/Methods

We performed digital measurement of anatomical positions in the middle and lower thoracic vertebrae (T4–T12) of 30 adults. Abbreviations: Left height of vertebral body, LHV; Right height of vertebral body, RHV; Anterior height of vertebral body, AHV; Middle height of vertebral body, MHV; Posterior height of vertebral body, PHV; Superior sagittal diameter of vertebral body, SSDV; Superior transverse diameter of vertebral body, STDV; inferior sagittal diameter of vertebral body, ISDV; Inferior transverse diameter of vertebral body, ITDV; (1) Left (right) height of vertebral body, [L(R)HV]; Anterior (middle, posterior) height of vertebral body [A(M,P)HV]; Superior (inferior) sagittal diameter of vertebral body, [S(I)SDV]; Superior (inferior) transverse diameter of vertebral body, [S(I)TDV].

Results

The transverse diameters of vertebral bodies were always larger than the sagittal diameter for 3~4 mm. The distance between 2 vertebrae (interval of 1 vertebra) range were (52–56) mm for T4–T7 and (44–48) mm for T8–T12, and the surgeons could collate these data to choose a suitable stick length.

Conclusions

Bone graft should prune into laterigrade cuboid, it can recover A-P and bilateral physiological functions load, and the height of the vertebral body increased from T4 to T12.

Biomechanical investigation of titanium elastic nail prebending for treating diaphyseal long bone fractures

This study numerically investigated the deformation of titanium elastic nails prebent at various degrees during implantation into the intramedullary canal of fractured bones and the mechanism by which this prebending influenced the stability of the fractured bone. Three degrees of prebending the implanted portions of the nails were used: equal to, two times, and three times the diameter of the intramedullary canal. Furthermore, a simulated diaphyseal fracture with a 5-mm gap was created in the middle shaft portion of the bone fixed with two elastic nails in a double C-type configuration. End caps were simulated using a constraint equation. To confirm that the simulation process is able to present the mechanical response of the nail inside the intramedullary, an experiment was conducted by using sawbone for validation. The results indicated that increasing the degrees of nail prebending facilitated straightening the nails against the inner aspect of canal after implantation, with increase in stability under torsion. Furthermore, reducing nail prebending caused a larger portion of the nails to move closer to the loading site and center of bone after implantation; the use of end caps prevented the nail tips from collapsing and increased axial stability. End cap use was critical for preventing the nail tips from collapsing and for increasing the stability of the nails prebent at a degree equal to the diameter of the canal with insufficient frictional force between the nail and canal. Therefore, titanium elastic nail prebending in a double C-type configuration with a degree three times the diameter of the canal represents a superior solution for treating transverse fractures without a gap, whereas that with a degree equal to the diameter of the intramedullary canal and combined with end cap use represents an advanced solution for treating comminuted fractures in a diaphyseal long bone fracture.

Computational comparison of tibial diaphyseal fractures fixed with various degrees of prebending of titanium elastic nails and with and without end caps

INTRODUCTION

Elastic stable intramedullary nailing (ESIN) is a treatment strategy for the management of diaphyseal long-bone fractures in adolescents and children, but few studies have investigated the mechanical stability of tibial diaphyseal fractures treated with various degrees of prebending of the elastic nails. Therefore, the aim of this study was to compare the mechanical stability, including the gap deformation and nail dropping, of a tibia fracture with various fracture sites and fixed with various degrees of prebending of the elastic nails by the finite element method. Furthermore, the contribution of end caps to stability was taken into consideration in the simulation.

METHODS

A tibia model was developed with a transverse fracture at the proximal, middle and distal parts of the diaphysis, and fixed with three degrees of prebending of elastic nails, including those equal to, two times and three times the diameter of the intramedullary canal. The outer diameter of the nail used in the computation was 3.5mm, and the fractured tibia was fixed with two elastic double C-type nails. Furthermore, the proximal end of each nail was set to free or being tied to the surrounding bone by a constraint equation to simulate with or without using end caps.

RESULTS

The results indicated that using end caps can prevent the fracture gap from collapsing by stopping the ends of the nails from dropping back in all prebending conditions and fracture patterns, and increasing the prebending of the nails to a degree three times the diameter of the canal reduced the gap shortening and the dropping distance of the nail end in those without using end caps under axial compression and bending. Insufficient prebending of the nails and not using end caps caused the gap to collapse and the nail to drop back at the entry point under loading.

CONCLUSIONS

Using end caps or increasing the prebending of the nails to three times the diameter of the canal is suggested to stop the nail from dropping back and thus produce a more stable structure, with less gap deformation, in the management of a simulated tibial diapyhseal fracture by using titanium elastic nails with a double C-shape.