Electron Microprobe Analysis and Tissue Reaction around Titanium Alloy Spinal Implants

Advances in implants and bone graft types for lumbar spinal fusion surgery

The increasing prevalence of spinal disorders worldwide necessitates advanced treatments, particularly interbody fusion for severe cases that are unresponsive to non-surgical interventions. This procedure, especially 360° lumbar interbody fusion, employs an interbody cage, pedicle screw-and-rod instrumentation, and autologous bone graft (ABG) to enhance spinal stability and promote fusion. Despite significant advancements, a persistent 10% incidence of non-union continues to result in compromised patient outcomes and escalated healthcare costs. Innovations in lumbar stabilisation seek to mimic the properties of natural bone, with evolving implant materials like titanium (Ti) and polyetheretherketone (PEEK) and their composites offering new prospects. Additionally, biomimetic cages featuring precisely engineered porosities and interconnectivity have gained traction, as they enhance osteogenic differentiation, support osteogenesis, and alleviate stress-shielding. However, the limitations of ABG, such as harvesting morbidities and limited fusion capacity, have spurred the exploration of sophisticated solutions involving advanced bone graft substitutes. Currently, demineralised bone matrix and ceramics are in clinical use, forming the basis for future investigations into novel bone graft substitutes. Bioglass, a promising newcomer, is under investigation despite its observed rapid absorption and the potential for foreign body reactions in preclinical studies. Its clinical applicability remains under scrutiny, with ongoing research addressing challenges related to burst release and appropriate dosing. Conversely, the well-documented favourable osteogenic potential of growth factors remains encouraging, with current efforts focused on modulating their release dynamics to minimise complications. In this evidence-based narrative review, we provide a comprehensive overview of the evolving landscape of non-degradable spinal implants and bone graft substitutes, emphasising their applications in lumbar spinal fusion surgery. We highlight the necessity for continued research to improve clinical outcomes and enhance patient well-being.

A comparison of the inflammatory host response to particulate debris adjacent to unlocked and locked screws of a growth guidance system for early onset scoliosis

PURPOSE

The SHILLA™ Growth Guidance system is a stainless-steel rod and screw system used for Early Onset Scoliosis which incorporates a unique flanged set screw designed to capture the rod, while allowing it to slide as the patient grows. Concomitant with this design is the potential for generation of wear debris and for an inflammatory host response. We hypothesized that the magnitude of the host response adjacent to the unlocked screws and rods would be greater than the host response to the locked rod/screws.

METHODS

Seven tissue samples adjacent to locked (3) and unlocked screws (4) from three SHILLA patients (mean implantation time of 19 post-operative months) with infantile idiopathic scoliosis were obtained as part of an explant analysis protocol during a PMDA-approved clinical trial in Japan. Gross appearance, high-resolution radiographs, and histology were assessed. ISO Standard 10993 Part 6 was used to assess the host response.

RESULTS

All three locked screw had no metallosis. In contrast, metallosis for unlocked screw tissue samples were rated as "ubiquitous" (2/4), "focal" (1/4), or "absent" (1/4). Microscopic metallic debris was found intracellularly and within interstices of fibrous connective tissues more frequently adjacent to unlocked screws compared to locked screws. Cell type and population scoring consistently showed a modestly larger inflammatory response (macrophages) in the unlocked tissue samples.

CONCLUSIONS

The peri-prosthetic tissue response to the unlocked rods/screws had a higher reactivity grade (slight reaction, Δ = 4.0) per ISO 10993 Part 6 compared to the locked screws in three patients with the SHILLA™ Growth Guidance scoliosis system.

Extensive Periprosthetic Metallosis Associated to Osteolysis and Spinal Instrumentation Failure: Case Report and Literature Review

STUDY DESIGN

Case-report and literature review.

OBJECTIVE

To depict main features of a potentially deleterious postoperative spinal fixation complication.

SUMMARY OF BACKGROUND DATA

Tisular deposit of metal particles from prosthetic systems-metallosis-is an uncommon complication of spinal fixation surgery. Manifestations as chronic postoperative pain, instrumentation failure, infection, or neurological impairment can be developed, but metallosis often appears as an unexpected intraoperative finding.

METHODS

A 70-year-old female underwent several spinal fixation procedures due to progressive degenerative adult scoliosis, who developed instrumentation failure. Unexpected metallosis was evidenced extensively surrounding the dislodged construct due to vertebral osteolysis. Instrumentation replacement and debridement of metallotic tissue was performed. We also conduct a literature review for the terms "spinal metallosis" and "spinal corrosion" on the PubMed/MEDLINE database. Previous publications depicting black/dark staining, discoloration and/or fibrotic tissue, as well as histopathological metal particle deposits, or merely metallosis, were reviewed. Articles reporting individual cases or case-series/cohorts with patient-discriminated findings were included.

RESULTS

The histopathological analysis of our patient revealed dense fibroconnective tissue with black metallic pigment associated. She evolved with great pain relief in the immediately postoperative period. The patient achieved pain-free standing with significant pharmacotherapy reduction and independent ambulation. The literature search retrieved 26 articles for "spinal metallosis" and 116 for "spinal corrosion"; 16 articles met selection criteria. Approximately 60% of the reported cases accounted for patients younger than 30 years old, mainly related to expandable fixation system (65%) for idiopathic scoliosis. Usually, the symptoms were correlated with abnormal radiological findings: instrumentation breakage, dislodgement, loosening, expandable systems fracture. All the reviewed patients evolved free of pain and neurologically recovered.

CONCLUSION

Instrumentation removal and metallosis debridement seems to be useful for symptomatic patients, but remains controversial on fixed asymptomatic patients. If solid fusion has not been achieved, extension, and reinforcement of the failed fixation could be required.Level of Evidence: 4.

Resistance of coated polyetheretherketone lumbar interbody fusion cages against abrasion under simulated impaction into the disc space

In order to improve osseointegration, polyetheretherketone (PEEK) interbody fusion cages are frequently surface coated. The bonding strength of the coatings is mostly investigated under unrealistic loading conditions. To close this gap, in this study, uncoated and coated cages were loaded in a clinical setup in order to investigate their real resistance against abrasion. Six uncoated, six calcium phosphate (CaP) nanocoated, and six titanium (Ti) nanocoated PEEK cages were tested in this study. The experimental setup was designed to mimic cage impaction into the intervertebral disc space using polyurethane (PU) foam blocks as vertebral body substitutes. The cage surface was inspected before and after impaction, and their weight was measured. Impaction resulted in abrasion at the tip of the ridges on the implant surface. The mean weight loss was 0.39 mg for the uncoated cages, 0.57 mg for the CaP nanocoated cages, and 0.75 mg for the Ti nanocoated cages. These differences were statistically significant. In conclusion, differences between the three cage types were found concerning the amount of abrasion. However, all three cages lost less weight than a comparative Ti plasma spray coated cage, which showed a mean weight loss of 2.02 mg. This may be because the plasma spray coating is significantly thicker than the two nanocoatings. If compared with the permitted amount of weight loss derived from an FDA guidance document, which is approximately 1.7 mg, the wear of the Ti plasma spray coated cage is above this limit, whereas the wear for all other cage types is below.

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.

Does impaction of titanium-coated interbody fusion cages into the disc space cause wear debris or delamination?

BACKGROUND CONTEXT

A large number of interbody fusion cages are made of polyetheretherketone (PEEK). To improve bone on-growth, some are coated with a thin layer of titanium. This coating may fail when subjected to shear loading.

PURPOSE

The purpose of this testing was to investigate whether impaction of titanium-coated PEEK cages into the disc space can result in wear or delamination of the coating, and whether titanium cages with subtractive surface etching (no coating) are less susceptible to such failure.

STUDY DESIGN/SETTING

A biomechanical study was carried out to simulate the impaction process in clinical practice and to evaluate if wear or delamination may result from impaction.

MATERIALS AND METHODS

Two groups of posterior lumbar interbody fusion cages with a similar geometry were tested: n=6 titanium-coated PEEK and n=6 surface-etched titanium cages. The cages were impacted into the space in between two vertebral body substitutes (polyurethane foam blocks). The two vertebral body substitutes were fixed in a device, through which a standardized axial preload of 390 N was applied. The anterior tip of the cage was positioned at the posterior border of the space between the two vertebral body substitutes. The cages were then inserted using a drop weight with a mass representative of a surgical hammer. The drop weight impacted the insertion instrument at a maximum speed of about 2.6 m/s, which is in the range of the impaction speed in vivo. This was repeated until the cages were fully inserted. The wear particles were captured and analyzed according to the pertinent standards.

RESULTS

The surface-etched titanium cages did not show any signs of wear debris or surface damage. In contrast, the titanium-coated PEEK cages resulted in detached wear particles of different sizes (1-191 µm). Over 50% of these particles had a size <10 µm. In median, on 26% of the implants' teeth, the coating was abraded. Full delamination was not observed.

CONCLUSIONS

In contrast to the surface-etched implants, the titanium-coated PEEK implants lost some coating material. This was visible to the naked eye. More than half of all particles were of a size range that allows phagocytosis. This study shows that titanium-coated implants are susceptible to impaction-related wear debris.