Dual Pitch Titanium-Coated Pedicle Screws Improve Initial and Early Fixation in a Polyetheretherketone Rod Semi-Rigid Fixation System in Sheep
Introduction
Lumbar degenerative diseases significantly impair quality of life, with spinal fusion combined with rigid instrumentation being a conventional treatment. However, concerns about adjacent segment degeneration and implant failure have driven interest in dynamic or semi-rigid stabilization systems. Polyetheretherketone (PEEK) rods, with their modulus of elasticity between cortical and cancellous bone, offer a semi-rigid alternative that reduces stress on implants while preserving physiological motion. Despite advancements, pedicle screw loosening remains a critical challenge in non-fusion systems. Modifications to screw design, such as surface coatings and thread geometry, aim to enhance fixation. This study evaluates the biomechanical and histological performance of dual pitch titanium-coated pedicle screws (DPTCPS) in a PEEK rod semi-rigid system, focusing on initial and early fixation in a sheep model.
Materials and Methods
Implant Design
Four types of Ti-6Al-4V pedicle screws were tested:
- Standard Pitch Pedicle Screw (SPPS): Double-threaded design with 3.0 mm pitch in the pedicle region.
- Dual Pitch Pedicle Screw (DPPS): Single-threaded design with 6.0 mm pitch in the pedicle region.
- Standard Pitch Titanium-Coated Pedicle Screw (SPTCPS): SPPS with plasma-sprayed titanium coating (roughness Ra = 4.54 µm, thickness = 386.6 µm, porosity = 29%).
- Dual Pitch Titanium-Coated Pedicle Screw (DPTCPS): DPPS with identical titanium coating.
All screws had a 4.5 mm diameter, 25.0 mm length, and 3.0 mm pitch in the vertebral body region. PEEK rods (6.35 mm diameter, 55.0 mm length) were used for semi-rigid fixation.
Animal Model
- Initial Fixation (0-week): Fifty-four fresh sheep spine specimens (L2–L5) were instrumented with four screws and two rods.
- Early Fixation (6-month): Sixty-four sheep underwent L3–L4 screw implantation. Bone mineral density (BMD) was measured via dual-energy X-ray absorptiometry to ensure uniformity.
Biomechanical Testing
- Axial Pull-Out Test: Screws were subjected to tension at 5 mm/min using a material testing system (Instron 3367) to determine maximum pull-out force.
- Toggle Testing: A ±200 N craniocaudal load was applied incrementally (25 N every 20 cycles) until 2 mm displacement occurred.
- Cyclic Loading: A 6 Nm bending moment (20–100 N load) followed by 12 Nm (20–200 N) was applied for 1,000 cycles each. Maximum displacement was recorded.
Histological Analysis
After 6 months, bone-screw interfaces were stained with Van-Gieson to quantify bone-to-implant contact (%) and assess osseointegration.
Results
Biomechanical Performance at 0-Week (Initial Fixation)
- Axial Pull-Out Strength: DPTCPS (557.0 ± 25.2 N) and SPTCPS (622.6 ± 25.2 N) outperformed SPPS (459.1 ± 19.1 N) (P < 0.05).
- Toggle Resistance: DPTCPS withstood higher loads (343.4 ± 16.5 N vs. SPPS: 237.5 ± 12.9 N) and more cycles (124.7 ± 13.5 vs. SPPS: 41.9 ± 4.3) (P < 0.05). SPTCPS showed intermediate results (289.9 ± 12.8 N; 79.5 ± 11.8 cycles).
- Cyclic Loading: DPTCPS exhibited minimal displacement (1.80 ± 0.13 mm) compared to SPPS (3.76 ± 0.19 mm) and SPTCPS (2.46 ± 0.20 mm) (P < 0.05).
Biomechanical Performance at 6-Months (Early Fixation)
- Axial Pull-Out Strength: DPTCPS (908.4 ± 33.6 N) and SPTCPS (925.9 ± 53.9 N) exceeded SPPS (646.5 ± 59.4 N) (P < 0.05). No difference existed between DPTCPS and SPTCPS (P > 0.05).
- Toggle Resistance: DPTCPS demonstrated superior load resistance (496.9 ± 17.9 N vs. SPPS: 370.3 ± 16.4 N) and cycle tolerance (249.1 ± 11.0 vs. SPPS: 149.9 ± 11.1) (P < 0.05). SPTCPS performed worse than DPTCPS (414.1 ± 12.8 N; 199.8 ± 7.2 cycles).
- Cyclic Loading: DPTCPS maintained stability (0.96 ± 0.11 mm displacement), significantly lower than SPPS (2.39 ± 0.14 mm) and SPTCPS (1.82 ± 0.12 mm) (P < 0.05).
Histological Findings
After 6 months, DPTCPS exhibited robust osseointegration with 58.3% ± 7.0% bone-to-implant contact, compared to 36.5% ± 4.4% for SPPS (P < 0.05). New bone infiltrated the titanium coating without inflammation or particle degradation. SPPS interfaces showed fibrous tissue and gaps.
Discussion
The DPTCPS design synergizes dual-pitch threading and titanium coating to optimize mechanical stability and biological integration. The 6.0 mm pitch in the pedicle region reduces thread density, minimizing cancellous bone compaction and preserving trabecular integrity, while the titanium coating enhances surface roughness for bone ingrowth.
At initial fixation, titanium coating increased pull-out strength by 21.3% (DPTCPS vs. SPPS), highlighting its role in immediate stability. However, SPTCPS outperformed DPTCPS in axial pull-out (622.6 N vs. 557.0 N), suggesting standard threading may offer superior short-term resistance in unloaded conditions. By 6 months, this difference vanished (DPTCPS: 908.4 N vs. SPTCPS: 925.9 N; P > 0.05), indicating that dual-pitch design and osseointegration compensate over time.
Toggle and cyclic loading data underscore DPTCPS superiority in resisting repetitive shear forces, critical for spinal motion preservation. The 1.80 mm displacement for DPTCPS at 0-week and 0.96 mm at 6 months reflect sustained stability under dynamic loads, reducing micromotion-induced loosening risks.
Histologically, titanium coating facilitated direct bone bonding, eliminating fibrous interfaces common in uncoated screws. The absence of inflammatory responses confirms coating biocompatibility, addressing concerns associated with hydroxyapatite coatings, which may degrade or incite osteolysis.
Conclusion
DPTCPS combines dual-pitch threading and titanium coating to enhance initial and early fixation in semi-rigid PEEK systems. Biomechanical tests confirm superior resistance to pull-out, toggle, and cyclic forces, while histology validates enhanced osseointegration. These findings support DPTCPS as a promising solution for reducing screw loosening in dynamic spinal stabilization.
doi.org/10.1097/CM9.0000000000000335
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