Hydroxyapatite dip coated and uncoated titanium poly-axial pedicle screws: an in vivo bovine model

Alternatives to Traditional Pedicle Screws for Posterior Fixation of the Degenerative Lumbar Spine

BACKGROUND

Traditional pedicle screws are currently the gold standard to achieve stable 3-column fixation of the degenerative lumbar spine. However, there are cases in which pedicle screw fixation may not be ideal. Due to their starting point lateral to the pars interarticularis, pedicle screws require a relatively wide dissection along with a medialized trajectory directed toward the centrally located neural elements and prevertebral vasculature. In addition, low bone mineral density remains a major risk factor for pedicle screw loosening, pullout, and pseudarthrosis. The purpose of this article is to review the indications, advantages, disadvantages, and complications associated with posterior fixation techniques of the degenerative lumbar spine beyond the traditional pedicle screws.

METHODS

Comprehensive literature searches of the PubMed, Scopus, and Web of Science databases were performed for 5 methods of posterior spinal fixation, including (1) cortical bone trajectory (CBT) screws, (2) transfacet screws, (3) translaminar screws, (4) spinous process plates, and (5) fusion mass screws and hooks. Articles that had been published between January 1, 1990, and January 1, 2020, were considered. Non-English-language articles and studies involving fixation of the cervical or thoracic spine were excluded from our review.

RESULTS

After reviewing over 1,700 articles pertaining to CBT and non-pedicular fixation techniques, a total of 284 articles met our inclusion criteria. CBT and transfacet screws require less-extensive exposure and paraspinal muscle dissection compared with traditional pedicle screws and may therefore reduce blood loss, postoperative pain, and length of hospital stay. In addition, several methods of non-pedicular fixation such as translaminar and fusion mass screws have trajectories that are directed away from or posterior to the spinal canal, potentially decreasing the risk of neurologic injury. CBT, transfacet, and fusion mass screws can also be used as salvage techniques when traditional pedicle screw constructs fail.

CONCLUSIONS

CBT and non-pedicular fixation may be preferred in certain lumbar degenerative cases, particularly among patients with osteoporosis. Limitations of non-pedicular techniques include their reliance on intact posterior elements and the lack of 3-column fixation of the spine. As a result, transfacet and translaminar screws are infrequently used as the primary method of fixation. CBT, transfacet, and translaminar screws are effective in augmenting interbody fixation and have been shown to significantly improve fusion rates and clinical outcomes compared with stand-alone anterior lumbar interbody fusion.

LEVEL OF EVIDENCE

Therapeutic Level IV. See Instructions for Authors for a complete description of levels of evidence.

The minipig as a potential model for pedicle screw fixation: morphometry and mechanics

Background

While there are several different animal models for use in the characterization of spinal fixation, none have emerged as a definitive model for comparative studies in spinal fixation methods. The purpose of this study is to establish morphometric data of porcine vertebrae and to characterize the feasibility of pedicle screw fixation in porcine spines for potential comparative human study.

Methods

Four spines from 45 to 50 kg Hanford minipigs were cleaned of soft tissue and analyzed by computed tomography and dual-energy x-ray absorptiometry. Two 5 × 30-mm pedicle screws were placed in each vertebra and tested to failure using a combined moment-load protocol.

Results

Pedicle widths were measured from L6-T5. Widths ranged from 7.15 mm (T6) to 9.24 mm (T14). Posterior cortex to anterior cortex depth ranged from 25.9 to 32.6 mm. Mean bone mineral density was 1.0665 g/cm² (range 1.139–1.016). Force-to-failure demonstrated mean 1171.40 N (+ 115.34).

Conclusion

Our baseline morphometric and compositional data demonstrate that porcine vertebrae can serve as a useful model for comparative studies due to their similar pedicle widths and bone mineral density to the human vertebra. This biomechanical data could provide a baseline comparison for future studies. This study also suggests that the minipig could be a suitable model for comparative studies due to similarities in pedicle width and bone mineral density to the human vertebrae.

Radiographic outcomes of transosseous intradiscal screw fixation in lumbar reconstruction-Imaging results of an experience with an alternative in fixation of the unexpectedly osteopenic spine

OBJECTIVE

To present the results of a new alternative in the technique lumbar pedicle screw reconstruction in osteopenic bone. Pedicle screw fixation is compromised in osteopenic bone and adjunct fixation commonly requires incremental technology that can increase cost and risk, and which may not commonly be available. Readily available low cost techniques are desirable.

PATIENTS AND METHODS

This is a retrospective review of a prospectively accumulated case series of all patients presenting to the senior author's (DAB) practice for elective lumbar reconstruction at a tertiary spine referral center. All consecutive patients treated by the senior author 2002-2012 who were unexpectedly found to be severely osteopenic at surgery are reported.

RESULTS

In seventy-four cases with imaging and clinical information available at an average of five years after surgery there was no screw lucency or accelerated disc degeneration observed despite these screws purposefully projecting into the suprajacent disc space within the limits of the construct. No patient had presented for instrumentation-related revision surgery of any sort.

CONCLUSION

Transosseous intradiscal screw fixation is a potentially viable alternative in surgical stabilization of the unexpectedly osteopenic lumbar spine.

Osseodensification for enhancement of spinal surgical hardware fixation

Integration between implant and bone is an essential concept for osseous healing requiring hardware placement. A novel approach to hardware implantation, termed osseodensification, is described here as an effective alternative. 12 sheep averaging 65kg had fixation devices installed in their C2, C3, and C4 vertebral bodies; each device measured 4mm diameter×10mm length. The left-sided vertebral body devices were implanted using regular surgical drilling (R) while the right-sided devices were implanted using osseodensification drilling (OD). The C2 and C4 vertebra provided the t=0 in vivo time point, while the C3 vertebra provided the t=3 and t=6 week time points, in vivo. Structural competence of hardware was measured using biomechanical testing of pullout strength, while the quality and degree of new bone formation and remodeling was assessed via histomorphometry. Pullout strength demonstrated osseodensification drilling to provide superior anchoring when compared to the control group collapsed over time with statistical significance (p<0.01). On Wilcoxon rank signed test, C2 and C4 specimens demonstrated significance when comparing device pullout (p=0.031) for both, and C3 pullout tests at 3 and 6 weeks collapsed over time had significance as well (p=0.027). Percent bone-to-implant contact (%BIC) analysis as a function of drilling technique demonstrated an OD group with significantly higher values relative to the R group (p<0.01). Similarly, percent bone-area-fraction-occupancy (BAFO) analysis presented with significantly higher values for the OD group compared to the R group (p=0.024). As a function of time, between 0 and 3 weeks, a decrease in BAFO was observed, a trend that reversed between 3 and 6 weeks, resulting in a BAFO value roughly equivalent to the t=0 percentage, which was attributed to an initial loss of bone fraction due to remodeling, followed by regaining of bone fraction via production of woven bone. Histomorphological data demonstrated autologous bone chips in the OD group with greater frequency relative to the control, which acted as nucleating surfaces promoting new bone formation around the implants, providing superior stability and greater bone density. This alternative approach to a critical component of hardware implantation encourages assessment of current surgical approaches to hardware implantation.

In Vivo Study of Hydroxyapatite-coated Hat Type Cervical Intervertebral Fusion Cage Combined With IGF-I and TGF-β1 in the Goat Model

STUDY DESIGN

An in vivo animal experimental study.

OBJECTIVE

To investigate the hydroxyapatite (HA) coating in a modified fusion cage in improving the results of cervical intervertebral fusion and the role of combination of IGF-I and TGF-β1 treatment in a goat cervical spine interbody fusion model.

MATERIALS AND METHODS

Thirty-two goats were divided into 4 groups (n=8 for each) and underwent C3-4 discectomy and intervertebral fusion by the following methods: group 1, autologous tricortical iliac crest bone graft; group 2, cage only; group 3, cage coated with HA; group 4, cage coated with HA+IGF-I and TGF-β1. Radiography was performed preoperatively, postoperatively, and after 1, 2, 4, 8, and 12 weeks. At the same time points, disk space height, intervertebral angle, and lordosis angle were measured. At 12 weeks postoperatively, the goats were killed and fused segments were harvested. Biomechanical study was performed in flexion, extension, axial rotation, and lateral bending with a nondestructive stiffness method to determine the range of motion and stiffness. All cervical fusion specimens underwent histomorphologic studies.

RESULTS

All 3 cage-treated groups showed significantly higher values for disk space height, intervertebral angle, and lordosis angle compared with the autologous tricortical iliac group at 1, 2, 4, 8, and 12 weeks after surgery (P<0.05). The stiffness of hat-shaped cervical intervertebral fusion cage coated with HA+IGF-I and TGF-β1 in flexion, extension, and lateral bending was significantly greater than that of the other groups (P<0.05), and the stiffness of hat-shaped cervical intervertebral fusion cage coated with HA in extension, axial rotation, and lateral bending was significantly greater than that of fusion with the bone graft and cage-only groups (P<0.05). Histomorphologic evaluation showed better fusion in 3 cage groups than in the bone graft group. In group 4 of the cage coated with HA+IGF-I and TGF-β1, a slightly more advanced bone matrix formation was shown than in groups without coating.

CONCLUSIONS

HA coating can improve the fusion effect of the cervical intervertebral cage, and IGF-I and TGF-β1 can enhance bone fusion.

Biomechanical evaluation and surface characterization of a nano-modified surface on PEEK implants: a study in the rabbit tibia

Polyether ether ketone (PEEK) is today frequently used as a biomaterial in different medical operations due to its excellent mechanical and chemical properties. However, the untreated surface of PEEK is bioinert and hydrophobic, and it does not osseointegrate in its pure form. The aim of this study was to evaluate a unique nano-modified surface of PEEK with respect to osseointegration. Forty-eight threaded, non-cutting PEEK implants were inserted bilaterally in the tibia of 24 rabbits. Half of the implants (n=24) were coated with nanocrystalline hydroxyapatite (test) and the remaining implants (n=24) were left uncoated (control). Half of the animals (n=12) were euthanized after 3 weeks of healing and the remaining (n=12) after 12 weeks. The implant retention was measured with a removal torque apparatus. Surface analysis was performed with interferometry, scanning electron microscopy, and X-ray photon spectroscopy to relate the removal torque to the applied surface. The test implants revealed a significantly higher retention after 3 weeks (P=0.05) and 12 weeks (P=0.028) compared to controls. The result of the present study proves that the addition of nanocrystalline hydroxyapatite coating to PEEK surfaces significantly increases its removal torque and biocompatibility.

Influence of micro- and nano-hydroxyapatite coatings on the osteointegration of metallic (Ti6Al4 V) and bioabsorbable interference screws: an in vivo study

The purpose of this study is to show and compare the fixation and osteointegration capability of metallic and bioabsorbable interference screws. For this, 8×20-mm interference screws were implanted into the bone tunnel in the proximal tibial metaphysis of sheep. The nano- (25 nm±0.8) and microscale (25 μm±0.5) hydroxyapatite were both dip-coated on Ti6Al4 V interference screws via an in vivo study. After the initial 12 weeks of postoperative, the pullout test, histopathology, X-ray diffraction and scanning electron microscopy examinations were performed. This multidisiplined work showed that the coated screws particularly those with nano-sized-HA coating and the bioabsorbable screws enhanced fixation and provided better stabilization, bone ingrowth and osteointegration than that of uncoated and microscale HA-coated screws. The bioabsorbable screws showed better histopathologic results.

Preparation and Characterization of Biphasic Calcium Phosphate Coatings on 316L Stainless Steel Fabricated by Electrophoretic Deposition

Biphasic calcium phosphate (BCP) ceramic is a synthetic biomaterial exhibiting a chemical composition similar to that of tooth mineral. Therefore, it is viably used in coating metallic implants manufactured from metals and alloys, such as titanium and stainless steel. In the present study, electrophoretic deposition (EPD) has been attempted for depositing BCP coatings on 316L Stainless Steel substrate followed by vacuum sintering at 800 °C for 1 h. The surface morphology, thickness, compositions and microstructure of the BCP coated 316L SS was investigated by scanning electron microscope (SEM), energy dispersive spectrometer (EDS), X-ray diffraction (XRD) and the bond strength of the coating was measured.

[Orthopedic treatment of vertebral compression fractures in osteoporosis]

Vertebral compression fracture is the most common type of fractures in osteoporosis increasing the mortality and morbidity of the systemic disease. Adequate treatment of the vertebral compression fractures is always in the focus of the national and international spine meetings and one of the most innovative fields in the spine care is the surgical therapy of the osteoporotic spine. Here, the authors summarize the orthopedic treatment options for vertebral compression fractures based on a literature review and their own institutional experience.

In vitro dissolution and corrosion study of calcium phosphate coatings elaborated by pulsed electrodeposition current on Ti6Al4V substrate

Calcium-deficient hydroxyapatite (Ca-def-HAP) coatings on titanium alloy (Ti6Al4V) substrates are elaborated by pulsed electrodeposition. In vitro dissolution/precipitation process is investigated by immersion of the coated substrate into Dulbecco's Modified Eagle Medium (DMEM) from 1 h to 28 days. Calcium and phosphorus concentrations evolution in the biological liquid are determined by Induced Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES) for each immersion time. Physical and chemical characterizations of the coating are performed by scanning electron microscopy (SEM) associated to Energy Dispersive X-ray Spectroscopy (EDXS) for X-ray microanalysis. Surface modifications are investigated by an original method based on the three-dimensional reconstruction of SEM images (3D-SEM). Moreover, corrosion measurements are carried out by potentiodynamic polarization experiments. The results show that the precipitation rate of the Ca-def HAP coating is more pronounced in comparison with that of stoichiometric hydroxyapatite (HAP) used as reference. The precipitated bone-like apatite coating is thick, homogenous and exhibits an improved link to the substrate. Consequently, the corrosion behaviour of the elaborated prosthetic material is improved.

Revision characteristics of cement-augmented, cannulatedfenestrated pedicle screws in the osteoporotic vertebral body: a biomechanical in vitro investigation

In generalized osteoporosis, instrumentation with cement-augmented pedicle screws is an amplification of the therapeutic spectrum. Early clinical results are promising for both solid and cannulated screws; however, there are concerns regarding the revision characteristics of these screws, especially for the cannulated-fenestrated type with its continuous cement interconnection from the core of the screw to surrounding bone tissue. In a human cadaver model, bone mineral density (BMD) was assessed radiographically. Spinal levels T9–L4 were instrumented left unilaterally, transpedicularly by using cannulated-fenestrated pedicle screws with the dimensions 6.5 × 45 mm. Polymethylmethacrylate cement (1.5 ml) was injected through the screws into each vertebra. After polymerization of the cement, the extraction torque was recorded. For both implantation and explantation of the screws, a fluoroscope was used to guarantee correct screw and cement positioning and to observe possible co-movements—that is, any movement of the cement mass within the vertebral body upon removal of the screw. For comparison, the extraction torque of same-dimension pedicle screws was recorded in a nonosteoporotic, non–cement-augmented instrumentation. The BMD was 0.60 g/cm2, a level that corresponds to a severe grade of osteoporosis. For removal of the screws, the median and mean extraction torques were 34 and 49 ± 44 Ncm, respectively. No co-movements of the cement mass occurred within the vertebral body. In the nonosteoporotic control, BMD was 1.38 g/cm2. The median and mean extraction torques were 123 and 124 ± 12 Ncm, respectively. Thus, the revision characteristics of cement-augmented, cannulated-fenestrated pedicle screws are not problematic, even in cases of severe osteoporosis. The winglike cement interconnection between the screw core and surrounding bone tissue is fragile enough to break off in the event of an extraction torque and to release the screw. There is no proof to support the theoretical fear that while trying to remove a screw, the composite of screw and cement would not break but instead would rotate as a whole in the osteoporotic vertebral body.

Inter-observer reliability of detecting Dynesys pedicle screw using plain X-rays: a study on 50 post-operative patients

Dynamic stabilisation system for the spine (Dynesys) relies on titanium screw purchase within the pedicle. Decision on osseointegration is important especially when the patient becomes symptomatic following initial good outcome. Loose screws make the construct non-functional especially in symptomatic patients. Removal of Dynesys may become necessary to control patient's symptoms. In this study, we report interobserver reliability of X-ray for the interpretation of Dynesys pedicle screw osseointegration based on the diagnosis of "halo zone" and "Double halo zones" surrounding loose screw. Lumbar spine X-ray images of 50 patients in two views (AP and lateral) were selected from a random sample of 420 Dynesys patients. The total number of pedicle screws reviewed in this study was 260. The investigators (observers) were asked to state whether or not each pedicle screw is loose using "radiolucent zone sign". Observers were two expert orthopaedic spine consultant surgeons and one expert consultant radiologist and four specialist registrars (SpR) in orthopaedics and radiology. SpR assessments were repeated after 4 months with instructions to use "double-halo sign" for loose screws. The evaluation of interobserver agreement was performed by obtaining a Kappa (K) index. Using "radiolucent zone sign", Kappa Index (KI) among three consultants was 0.2198 at 95% CI (0.0520, 0.4916) while for all of the seven assessors (3 consultants and 4 SpR), KI was 0.1462 at 95% CI (0.0332, 0.2592). The use of "double-halo sign" was associated with KI of 0.666 at 95% CI (SE 0.83) among all of the 4 SpR. Based on plain X-ray "radiolucent zone sign", the inter-observer reliability of detecting loose Dynesys pedicle screw was poor (Kappa index of 0.2). On the other hand, using plain X-ray "double-halo sign" was associated with improved inter-observer reliability and validity.

Pedicle screw surface coatings improve fixation in nonfusion spinal constructs

STUDY DESIGN

Biomechanical and histologic analysis.

OBJECTIVE

To compare the strength of the bone-screw interface of standard uncoated pedicle screws with screws treated with hydroxyapatite (HA), titanium plasma spray (TPS), and a composite HA-TPS coating.

SUMMARY OF BACKGROUND DATA

Transpedicular screw fixation has become the gold standard in the treatment of various thoracolumbar spinal conditions. Pedicle screw loosening, however, has been reported, especially in mechanically demanding constructs or in vertebrae with low bone mineral density.

METHODS

Six mature porcine were instrumented with 4 types of titanium monoaxial pedicle screws (uncoated, HA-only coated, TPS-only coated, and HA-TPS composite coated) in a systematically varied, single-blinded fashion. After a 3-month survival period, the spines were harvested en-bloc and "time zero" control screws were instrumented in adjacent vertebrae. Screw placement and bone mineral density were evaluated with a postharvest computed tomography, and the strength of the tissue-implant interface was evaluated with a torsional screw extraction analysis (60 screws) and a nondecalcified histologic analysis (16 screws).

RESULTS

At 3 months postoperative, peak torque increased for all 3 types of coated screws (increased fixation) and decreased significantly for the uncoated screws (P < 0.001). Although 3-month peak torque was not statistically different between the 3 screw coatings, 4 of 10 TPS-only coated screws had a peak torque that was nearly 0 (<0.1 N m) versus only 1 of 10 HA-only screws and 0 of 10 HA-TPS composite screws. Histologic analysis confirmed the biomechanical findings with improved osseointegration in the HA-only and HA-TPS composite screws.

CONCLUSION

Pedicle screw coatings that promote mechanical interlocking, TPS, or direct osteoblast bonding(HA) increased screw fixation in this nonfusion model. More non-HA coated screws, however, were thought to be "loose" with a nearly zero peak extraction torque and fibrous encapsulation. Increased osseointegration with HA may result in a decreased incidence of screw loosening and improved outcomes of transpedicular spinal instrumentation in nonfusion procedures.

Bioceramic dip-coating on Ti-6Al-4V and 316L SS implant materials

The focus of the present study is based on more economical and rapid bioceramic coating on the most common implant substrates such as Ti-6Al-4V and 316L SS used often in orthopedics. For ceramic dip coating of implant substrates, Hydroxyapatite (HA) powder, Ca10(PO4)6(OH)2, P2O5, Na2CO3 and KH2PO4 are used to provide the gel. Ceramic films on sandblasted substrates have been deposited by using a newly manufactured dip-coating apparatus. Sample characterization is evaluated by SEM and XRD analysis. A smooth and homogeneous coating films have been obtained and average of 20 MPa bonding strength has been achieved for both Ti-6Al-4V and 316L SS alloys after sintering at 750 degrees C under flowing argon. The level of importance of the process parameters on coating was determined by using analysis of variance (ANOVA). The current process appears to be cheap, easy, and flexible to shape variations and high production rates for orthopedic applications.

Characterization andin vitro evaluation of biphasic calcium pyrophosphate–tricalciumphosphate radio frequency magnetron sputter coatings

The objective of this study was to characterize the physicochemical, dissolution, and osteogenic properties of radio frequency magnetron sputtered dicalcium pyrophosphate/tricalciumphosphate (Pyro/TCP) and hydroxylapatite (HA) coatings. Therefore Pyro/TCP and HA coatings were deposited on grit-blasted titanium discs. The results showed that the deposited coatings were amorphous and changed into a crystalline structure after IR heat-treatment of 550 degrees C for HA and 650 degrees C for Pyro/TCP. Heat-treated HA coatings appeared to be stable during immersion in simulated body fluid (SBF), that is no changes in the XRD pattern were observed. Also, no dissolution of the coating was observed by scanning electron microscopy (SEM). Energy dispersive spectroscopy (EDS) revealed that the Ca/P ratio of the HA coatings remained constant during SBF immersion. On the other hand, the heat-treated Pyro/TCP coatings showed a surface reaction of calcium pyrophosphate into a beta-tricalcium phosphate phase during SBF immersion. This was confirmed by EDS analysis. Rat bone marrow-derived osteoblast-like cells cultured on the heat-treated substrates showed that cell proliferation and differentiation occurred on both types of bioceramic coatings. No significant differences for proliferation and early differentiation were observed between cells cultured on heat-treated Pyro/TCP and HA at individual time points. However, osteocalcin expression, a late marker for osteoblast-like cell differentiation, was significantly increased after 12 days of culture on HA-coatings. These results were confirmed by SEM observations and suggest increased osteogenic properties for HA-coatings over Pyro/TCP-coatings. Additional research is necessary to obtain conclusive evidence on the in vivo osteogenic capacity of Pyro/TCP coatings.