Evaluation of instrumentation and pedicle screw design for posterior lumbar fixation: A pre‐clinical in vivo/ex vivo ovine model

Abstract

Background

Stabilization procedures of the lumbar spine are routinely performed for various conditions, such as spondylolisthesis and scoliosis. Spine surgery has become even more common, with the incidence rates increasing ~30% between 2004 and 2015. Various solutions to increase the success of lumbar stabilization procedures have been proposed, ranging from the device's geometrical configuration to bone quality enhancement via grafting and, recently, through modified drilling instrumentation. Conventional (manual) instrumentation renders the excavated bony fragments ineffective, whereas the “additive” osseodensification rotary drilling compacts the bone fragments into the osteotomy walls, creating nucleating sites for regeneration.

Methods

This study aimed to compare both manual versus rotary Osseodensification (OD) instrumentation as well as two different pedicle screw thread designs in a controlled split animal model in posterior lumbar stabilization to determine the feasibility and potential advantages of each variable with respect to mechanical stability and histomorphology. A total of 164 single thread (82 per thread configuration), pedicle screws (4.5 × 35 mm) were used for the study. Each animal received eight pedicles (four per thread design) screws, which were placed in the lumbar spine of 21 adult sheep. One side of the lumbar spine underwent rotary osseodensification instrumentation, while the contralateral underwent conventional, hand, instrumentation. The animals were euthanized after 6‐ and 24‐weeks of healing, and the vertebrae were removed for biomechanical and histomorphometric analyses. Pullout strength and histologic analysis were performed on all harvested samples.

Results

The rotary instrumentation yielded statistically (p = 0.026) greater pullout strength (1060.6 N ± 181) relative to hand instrumentation (769.3 N ± 181) at the 24‐week healing time point. Histomorphometric analysis exhibited significantly higher degrees of bone to implant contact for the rotary instrumentation only at the early healing time point (6 weeks), whereas bone area fraction occupancy was statistically higher for rotary instrumentation at both healing times. The levels of soft tissue infiltration were lower for pedicle screws placed in osteotomies prepared using OD instrumentation relative to hand instrumentation, independent of healing time.

Conclusion

The rotary instrumentation yielded enhanced mechanical and histologic results relative to the conventional hand instrumentation in this lumbar spine stabilization model.

This study compared manual/conventional vs rotary, osseodensification (OD) instrumentation of the osteotomy while simultaneously testing the effect of macrogeometry modifications (traditional versus compacting thread design) of pedicle screws in a large, translational preclinical animal model. Lumbar spinal fusion procedures have experienced a growth toward instability and deformity conditions, primarily among the elderly (65+ years old) and patients diagnosed with co‐morbidities such as osteoporosis. Literature has indicated that biomechanical success is influenced by a variety of variables (i.e., device's geometry, osteotomy preparation, etc.). The presented work, data and analyses, demonstrated that additive, rotary osseodensification instrumentation yielded improved mechanical and histologic results in lumbar spine stabilization relative to the conventional instrumentation technique.

1. INTRODUCTION

Lumbar fusion and stabilization are common treatment modalities for various conditions (i.e., spondylolisthesis and scoliosis). Spine surgery has become more common, with overall incidences increasing ~30% over an 11‐year interval (2004 through 2015). The reported incidence rate increases to ~75% for demographic of patients over the age of 65. ¹ , ² These increasing trends may be attributed to improvements in diagnostics, procedural and peri‐operative techniques allowing elective procedures in older patients, as well as those affected by comorbidities. Furthermore, the increased frequency of lumbar spine procedures have been correlated with a proportional increase of cost, resulting in an astonishing ~45% increase during the same time period (2004–2015). ¹ , ² To mitigate the rising costs, it has become essential to identify suitable alternative modalities with minimal complications in an effort to avoid secondary procedures that are associated with increased financial burden. ¹ , ²

One of the most common complications of spinal stabilization is pedicle screw loosening, which is linked to the aging patient population and poor bone quality that leads to secondary instability and, ultimately failure of the device. ³ , ⁴ Screw loosening may result from a variety of clinical scenarios such as technical errors during positioning, inappropriate mechanical transmission of stress on the device(s), and/or poor bone quality. ³ , ⁵ , ⁶ Procedures to salvage failures can be a financially prohibitive endeavor, necessitating secondary admission to hospital coupled with potentially difficult technical operation(s) due to the scarred and distorted anatomy and in certain circumstances the need to use specialized instruments. ⁶ , ⁷ , ⁸

A variety of approaches have been deployed to address the issue of screw failure, including but not limited to geometrical modification of individual screws, for example, thread designs, ⁷ , ⁹ , ¹⁰ surface treatment, ¹¹ and the use of “additive” instrumentation in an effort to improve primary stability and osteointegration. ¹² , ¹³ , ¹⁴ , ¹⁵ , ¹⁶ , ¹⁷ Osseodensification is a form of “additive” instrumentation that has been frequently shown by both our group and others to produce bone compaction (autografting into the trabecular space). ¹³ , ¹⁸ , ¹⁹ , ²⁰ , ²¹ , ²² The compaction achieved using the osseodensification instrumentation results in increased insertion torque levels that result in improved primary stability of implanted hardware. ¹² , ¹³ , ¹⁴ , ¹⁵ , ¹⁶ , ¹⁷ The technique's use of counterclockwise rotation and specialized burrs allows for inducing time dependent stress by modulating the rotating burrs compaction of bone fragments into the osteotomy site walls. ¹⁵ , ¹⁶ , ¹⁷ , ²³ This instrumentation has shown to increase primary stability metrics such as bone‐to‐implant contact and the percentage of bone volume surrounding the implant. Furthermore, it has also been hypothesized that this increase in the autologous bone at the osteotomy site not only has the potential to prevent implant failure, but also improve osteointegration using the preserved bone chips (e.g., autografts) which act as nucleating surfaces at the bone‐to‐implant interface, hastening bone formation and osseointegration. ¹² , ¹³ , ¹⁴ , ¹⁵ , ¹⁶ , ¹⁷ , ²³

While rotary “additive” instrumentation is a relatively novel concept, compacting instrumentation techniques to improve orthopedic hardware stability and healing have been previously attempted by other groups. ²³ , ²⁴ , ²⁵ , ²⁶ , ²⁷ Green et al. ²³ demonstrated the positive effect of bone compaction in increasing bone density and bone‐to‐implant contact utilizing manual dilators. Kold et al. ²⁴ , ²⁵ , ²⁶ , ²⁷ established that manually compacting bone, created a spring‐back effect, which may be important for increasing implant fixation by reducing initial gaps between the implant and bone. The time dependent change in applied load during instrumentation or post healing has also been linked to increased levels of bone formation by Duncan and Turner. ²⁸

In our group's previous study ²⁹ the objective focused on the instrumentation, comparing conventional (hand) instrumentation and “additive” osseodensification rotary drilling protocol used to prepare the osteotomy sites for pedicle‐screws. The results revealed increased primary stability in osteotomies prepared using the rotary instrumentation relative to the conventional hand instrumentation. Another study independently demonstrated that the screw's macrogeometry, specifically screw diameter, significantly contributes to primary stability. ³⁰ Additionally, research has corroborated that cylindrical single‐lead thread screws yield superior biomechanical anchorage relative to conical dual‐lead thread screws. ³¹ , ³²

The present study was designed to evaluate both osteotomy instrumentation and pedicle screw design (specifically the individual threads as shown in Figure 1) effect on osseointegration and soft tissue infiltration after 6‐ and 24‐weeks in vivo using a large, pre‐clinical animal (sheep) model.

FIGURE 1.

Computer aided renderings of (A) RELINE‐O Screw (Nuvasive) and (B) customized “compacting thread design” RELINE Tulip, MOD (Versah LLC) pedicle screws (4.5 mm (D) × 35 mm (L))

2. MATERIALS AND METHODS

2.1. Animal model and surgical procedure

After receiving approval from the Institutional Animal Care and Use Committee (IACUC), 21 skeletally mature female sheep (~24 months and an average weight of 65 kg) were acquired and allowed to acclimate for 1 week prior to any surgical intervention. Anesthesia was induced with sodium pentothal (15‐20 mg/kg) in Normasol solution into the jugular vein and maintained with isofluorane (1.5–3%) in O₂/N₂O (50/50). Animal monitoring included ECG, end tidal CO₂, SpO₂ and body temperature, which was regulated by a circulating hot water blanket.

Prior to surgery, the surgical site was shaved, and an iodine solution applied. Using a scalpel (#10 blade), a ~ 20 cm vertical incision was performed on the lumbar region of the animal at the midline and deepened through the subcutaneous tissue up to the lumbar fascia. Attention was then directed to the interspinous and supraspinous ligaments between L2 and L5, which were sectioned prior to paraspinal muscles sharp and blunt dissection to expose the subperiosteal lamina and roots of the transverse processes of L2‐L3 and L4‐L5. To further destabilize the lumbar spine, the articular capsule between each facet of L2‐L5 was removed to mimic a complete joint derangement. The root of the transverse process was used as reference to place the pedicle screws. On one side, the insertion protocol involved preparation of the osteotomy with a 2 mm pilot drill bit on a contra‐angled handpiece at high speed (700 RPM) under abundant irrigation, followed by manual instrumentation with a 4.0 mm self‐tapping drill bit (Nuvasive, San Diego, CA) to enlarge the osteotomy (Hand). On the contralateral side, the osteotomies were prepared through the additive, rotary, osseodensification instrumentation (Versah LLC, Jackson, MI) beginning with a pilot drill followed by a sequence of twist drills 1828 (2.8 mm), and 2838 (3.8 mm) in counterclockwise fashion under irrigation as per manufacturer instructions.

A total of 164 pedicle screws (D_outer = 4.5 mm × L = 35 mm), divided equally (82 per design) between two distinctive thread designs: (1) Traditional, single lead thread design (RELINE‐O, Nuvasive, San Diego, CA) and (2) A custom modified (MOD), “compacting, single lead, thread design”, (Versah LLC, Jackson, MI) were utilized in a split model (Figure 1). Following preparation of the osteotomy sites, eight pedicle screws (four of each group) were placed to a depth of ~25 mm in the prepared osteotomies (i.e., conventional instrumentation (hand) or additive, osseodensification rotary instrumentation). Each set of screws on L2‐L3 and L4‐L5 were connected on both sides using titanium rods. The surgical distribution and fixation in the split model were designed to allow direct comparisons between instrumentation methods and pedicle screw geometry, where paired level screws were equally destabilized (surgical instrumentation and fixation were randomized to avoid site bias). The surgical site was sutured using an absorbable suture for the muscles and fascia and 2.0 nylon in an interrupted fashion for the skin. The animals were separated into two groups, 12 animals set for 6 weeks, and nine animals set to 24 weeks post‐operatively. Cefazolin (500 mg) was administered intravenously pre‐operatively and post‐operatively. Post‐operatively, food and water ad libitum were offered. The animals were examined daily for wound healing, and general status, from the first day post‐op until time of euthanasia.

Following the necropsy, the vertebrae with devices were removed en bloc. Four pedicle screws, one from each combination: osteotomy preparation and geometric pedicle screw configuration (hand‐prepared osteotomy and traditional screw), were subjected to biomechanical testing and the others for histologic processing, respectively (the rostral and caudal pair of each level to biomechanical and histologic testing, correspondingly).

2.2. Mechanical testing–screw pull out

Pullout strength of selected implants was performed on half of the sample (82 pedicle screws) using a universal testing machine (Instron Series 5560 Norwood, MA) with a cross‐head speed of 1 mm/min. ⁶ , ³³ The force data was analyzed and reported as mean values with the corresponding 95% confidence interval values (mean ± 95% CI). The pull‐out strength was compared using factors including time (6‐ and 24‐weeks), pedicle screw thread‐geometry, as well as surgical instrumentation. It is important to note that after testing these samples were discarded and not utilized for histomorphometric analysis.

2.3. Histological analysis

For the remaining 82 samples, bone‐device blocks were gradually dehydrated in a series of solutions ranging from 70% to 100% ethanol and then embedded in a methyl methacrylate‐based resin. Embedded blocks were then longitudinally cut into sections using a diamond saw (Isomet, 2000, Buehler Ltd., Lake Bluff, IL, USA). The sections were ground on a grinding machine (Metaserv 3000, Buehler, Lake Bluff, IL, USA) under water irrigation with a series of SiC abrasive paper until they were approximately 100 (±10) μm thick. Subsequently the samples were stained with Stevenel's blue and Van Gieson to differentiate the hard, soft and connective tissues. Histology slides were evaluated histomorphometrically (bone to implant contact (%BIC), bone area fraction occupancy (%BAFO), and soft tissue infiltration (STI)) using image analysis software (ImageJ, NIH, Bethesda, MD). Bone to implant contact (BIC) quantifies the degree of osseointegration associated with primary stability by measuring bone in contact with the screw surface and standardized over the screw's surface perimeter. BAFO quantifies the degree of osseointegration derived from secondary stability and is evaluated by measuring the percentage of bone (newly formed and non‐vital autografted/native bone due to instrumentation) within the screw threads compared to the area of the threads, also reported as a percentage. Additionally, soft tissue infiltration (STI) from the root of the transverse process was measured to the first point of contact between bone and screw thread.

2.4. Statistical analysis

All biomechanical and histomorphometric data are presented as mean values with the corresponding 95% confidence interval values (mean ± 95% CI). Removal force (N), %BIC, %BAFO, and STI (mm) value data were analyzed using a linear mixed model with a fixed factor of surgical instrumentation (i.e., Hand vs. Rotary), macro‐geometry (Traditional vs. MOD), and time in vivo (6‐ and 24‐weeks). After administering a significant omnibus test, post‐hoc comparison of the experimental groups means was gathered using Tukey test. The analysis was accomplished using SPSS (IBM SPSS 23, IBM Corp., Armonk, NY). All analyses were conducted at α = 0.05.

3. RESULTS

3.1. Macroscopic assessment

No surgical sites showed any signs of inflammation, infection, or failure of the implant throughout the period of healing. Sharp dissection and clinical inspection demonstrated that all devices were integrated with the bone and clinically stable.

3.2. Biomechanical test

The interfacial integration was evaluated using the mechanical pullout protocol where the pedicle screws were fully displaced from the vertebral bodies. Independent of pedicle screw thread configuration, surgical instrumentation did not yield significant differences in pullout forces at 6 weeks (p = 0.876). At 24 weeks in vivo, rotary instrumentation yielded significantly higher force values (1060 N ± 181) relative to osteotomies prepared using hand instrumentation (769 N ± 181) (p = 0.026) (Figure 2A). Evaluation as a function of pedicle screw and instrumentation yielded no significant difference in outcomes at 6 weeks with an average pullout force of ~750 N for both pedicle screw designs independent of instrumentation (Figure 2B). Analysis of mechanical stability at 24 weeks yielded significantly higher values (p = 0.018) for the MOD pedicle screws placed in osteotomies prepared with rotary instrumentation (1104.6 N ± 256) in comparison to those prepared by manual (hand) instrumentation (663.1 N ± 256) (Figure 2B), whereas no significant differences were detected between rotary and hand instrumentation for the traditional pedicle screw (p = 0.44).

FIGURE 2.

Statistical summary presenting the mean removal load (N) ± 95%CI of pullout force (A) as function of time and instrumentation, where rotary group show significantly greater load bearing capability compared to the hand group at 24‐weeks; and (B) mean peak load with corresponding 95%CI as function of pedicle screw, time, and instrumentation

3.3. Histomorphometry

Analysis of bone‐to‐implant (BIC) as a function of time and pedicle screw type, independent of instrumentation, yielded significantly higher (p = 0.003) values at 6 weeks for rotary (63.9% ± 10.0) compared to osteotomies prepared using hand (41.9% ± 10.0) instrumentation. At 24‐weeks there was no statistical difference (p = 0.176) between hand (55.5% ± 8.7) and rotary (63.9% ± 9.1) (Figure 3A) instrumentation. Similarly, evaluation of bone area fraction occupancy (BAFO) detected significantly higher degrees of bone occupancy for sites prepared with rotary instrumentation at both 6‐weeks (p = 0.041) and 24‐weeks (p = 0.042) (Figure 3B).

FIGURE 3.

Statistical summary presenting the mean (A) BIC and (B) BAFO (%) ± 95%CI as function of time and instrumentation. (Hand and Rotary instrumentation). (C) BIC and (D) BAFO as function of time, instrumentation (Hand and Rotary instrumentation) and pedicle screw design

Evaluating %BIC as a function of instrumentation and pedicle screw design at 6 weeks, significantly higher values were recorded at the earlier healing time (6‐weeks) for both traditional and MOD pedicle screws placed in osteotomies prepared with rotary instrumentation (p = 0.026 and p = 0.039, respectively) (Figure 3C). At 24‐weeks, no significant differences in %BIC were detected between pedicle screw design or instrumentation (Figure 3C). Analysis of %BAFO as function of pedicle screw and instrumentation detected a significant (p = 0.043) increase for the rotary relative to hand instrumentation for traditional pedicle screws at 6 weeks (Figure 3D). No significant differences were detected for %BAFO irrespective of screw design and instrumentation at 24 weeks (Figure 3D).

Analysis of STI as a function of both instrumentation and pedicle screw design, yielded significantly higher levels for the hand instrumentation for both traditional (p = 0.041) and MOD (p = 0.023) pedicle screws at 6 weeks (Figure 4). At 24‐weeks, significantly lower STI was observed for MOD pedicle screws placed in rotary prepared osteotomies relative to hand instrumentation (p = 0.019), while no difference (p = 0.219) was observed for traditional pedicle screws independent of instrumentation (Figure 4).

FIGURE 4.

Bar graphs presenting the mean length of soft tissue infiltration (STI) (%) ± 95%CI as function of time, instrumentation (Hand and Rotary instrumentation) and pedicle screw design

All pedicle screws considered for qualitative and quantitative histomorphometric analysis demonstrated osseointegration upon initial evaluation (Figures 5 and 6). Independent of pedicle screw design, distinctive osseointegration patterns were observed when evaluating hand versus rotary instrumentation (Figure 5). Qualitative analysis of histomicrographs revealed newly regenerated bone in contact with the pedicle screws, particularly screws placed into osteotomies prepared using hand instrumentation (Figure 5A–d). For implants placed under rotary instrumentation, bone fragments from osseodensification rotary instrumentation were observed to be surrounded by newly formed bone in proximity of the implant independent of screw design (Figure 5E–H). At 24 weeks (Figure 6), both hand (Figure 6A–D) and rotary (Figure 6E–H) instrumentation groups presented interfacial bone remodeling with increased degrees of lamellar bone presence relative to 6 weeks. The implants placed in osteotomies prepared with rotary instrumentation (Figure 6E–H) resulted in the presence of bone fragments.

FIGURE 5.

Six‐week in vivo histologic section of traditional pedicle screw placed using (A,C) hand and (E, G) rotary instrumentation, and MOD pedicle screw inserted in osteotomies prepared by (B, D) hand and (F, H) rotary instrumentation. The red arrow on (A) depicts soft tissue infiltration. Higher magnification of (C) traditional and (D) MOD pedicle screws placed by hand instrumentation revealed new bone in contact with the implant, whereas bone fragments (blue arrows) embedded into newly formed bone was observed in close proximity and within the threads of both (G) traditional and (G) MOD pedicle screws

FIGURE 6.

Twenty‐four‐week in vivo histologic section of Traditional pedicle screw placed using (A, C) hand and (E, G) rotary, and MOD pedicle screw inserted in osteotomies prepared by (B, D) hand and (F, H) rotary instrumentation. Relative to their counterparts at 6 weeks in vivo (Figure 5) higher magnification of (C) traditional and (D) MOD pedicle screws placed by hand instrumentation showed increased degrees of lamellar structures. For the (G, H) rotary instrumentation groups, relative to their 6 weeks counterparts, qualitatively less bone fragments (blue arrows) were observed in close proximity and within the threads of both (G) traditional and (H) MOD pedicle screws. For both (G) traditional and (H) MOD pedicle screws, lamellar bone configuration was observed in close proximity of the implants

4. DISCUSSION

This study compared hand (conventional) versus rotary (“additive”) instrumentation, to confirm and validate the observations of our previous study ²⁹ while simultaneously testing the effect of macrogeometry modifications (traditional vs. compacting thread design) of the pedicle screws on biomechanical and histological parameters. The study was designed as a randomized split model allowed for a paired comparison of the two variables while minimizing potential bias from anatomical variation (e.g., bone density, volume, and shape).

Indications for lumbar spinal fusion surgery have experienced a growth toward instability and deformity conditions, mainly among the elderly population (65+ years old) and patients affected by co‐morbidities, particularly osteoporosis. ¹ , ² Taking into consideration the projection of the current demographic trend, which indicates that by 2050 the population of elderly patients will outnumber adolescents and youth (15–24 years old), ³⁴ suitable protocols and materials/devices that will deliver positive outcomes in a reproducible manner along with limited complications are highly desirable. ¹ , ² , ³⁵ Dating back to one of the first instrumented spinal fusion procedures completed by bounding with silver wire, the field has seen an incredible evolution over time. ³⁶ This evolution has seen a surge of new and/or refined consumables, instrumentation protocols, as well as tools (equipment). ³⁵ , ³⁷ , ³⁸ , ³⁹ , ⁴⁰ Despite substantial research and development, pedicle screw loosening and instability of the constructs has remained an issue particularly in the elderly population and osteoporotic patients. ⁴¹ , ⁴² Despite the plethora of osteosynthesis implants currently available, until recently minimal consideration has been given to the preparation of the osteotomy required for placement of the devices, a paramount factor for biomechanical stability and osseointegration. ¹² , ¹³ , ¹⁴ , ¹⁸ , ²⁴ , ²⁵ , ²⁶ , ²⁷ , ²⁹ , ⁴³ This work aimed to build on our group's earlier studies ¹⁴ , ²⁹ , ⁴⁴ , ⁴⁵ where we explored the novel rotary instrumentation sequence for endosteal implants used in oral and maxillofacial and orthopedics, for placing spinal lumbar fusion multiaxial pedicle screws. Current work and hypothesis support that rotary instrumentation relies on the unconventional design of the drills, which rotate in counterclockwise direction, compacting the bone particulate into the trabecular space wall. Concurrently, this compaction into the wall increases the bone density at the bone‐implant interface while inducing residual strain manifested in spring‐back effect. This increased bone density results in increased primary stability while simultaneously facilitating secondary stability and subsequently, enhanced osteointegration ¹² , ¹³ , ¹⁴ , ¹⁵ , ¹⁶ , ¹⁷ , ¹⁸ due to the preservation of autologous bone particulate present between the threads and osteotomy wall.

Biomechanical success is influenced by a variety of variables (i.e., implant macrogeometry, microgeometry, and osteotomy preparation technique). ³⁷ , ³⁸ , ³⁹ , ⁴⁰ , ⁴⁶ , ⁴⁷ In terms of mechanical pull‐out testing, our results showed increased values in the rotary group compared with the hand instrumentation at 24 weeks (~1100 N) but not at the earlier healing time of 6 weeks. The superiority of mechanical stability for the rotary group concurs with observations from our previous study. ²⁹ However, it must be noted that our previous investigation presented lower values relative to the present study due to mechanical testing method variation, where pull‐out strength to interfacial failure was used ²⁹ unlike full pullout as in the present study. This observation indicates that the effect of compacting the bone particulate against the osteotomy wall yields a substantial effect on osseointegration and biomechanical stability over time by providing autograft bone particles able to enhance osteogenesis rather than primary stability due to increased bone density at implant interface.

Overall, the only difference observed with respect to %BIC and %BAFO was found as a function of instrumentation, where the rotary group demonstrated significantly higher values relative to the hand instrumentation especially at the earlier time in vivo. This is likely due to the nature of the instrumentation, since the rotary technique utilizes an “additive” method whereas the hand instrumentation uses a subtractive method, lacking the autografting effect. With respect to the geometry (e.g., thread design) of the devices, there were no detectable changes at either healing time and instrumentation. As seen with BIC, the BAFO presented significantly higher values in the rotary instrumentation group, suggesting that rotary instrumentation promotes stronger initial bone formation, possibly due to the autografted bone particles acting as nucleating surfaces for new bone formation as we have previously described. ¹³ , ¹⁴ , ²⁹ , ⁴⁸

The rotary instrumentation effect on osseointegration is further observed on analysis of histology. Densification around the implants is visualized in the samples placed into osteotomies prepared with rotary instrumentation, as this group presented greater volume of woven bone as well as bone‐implant contact relative to samples instrumented with the hand instrumentation, especially at the 6‐week time point. Non‐vital bone fragments were observed in greater frequency in the rotary group relative to the manual instrumentation group. These autologous bone particles are residual fragments that act as nucleating surfaces, acting as autologous grafts promoting new bone regeneration around the pedicle screws, yielding higher degrees of initial osseointegration at 6 weeks that subsequently likely translated in higher mechanical stability at 24 weeks as well as lower degrees of soft tissue infiltration on the mechanically loaded pedicle screws. While the macrogeometry of the pedicle screws did not yield any significant influence on the mechanical (e.g., pullout force) or histological parameters (i.e., %BIC, %BAFO, STI), it may be speculated that the vertebral bone may not provide adequate morphology to promote a distinguished mechanical advantage to the compacting design of pedicle screw.

The current work provided further insight on the effect of instrumentation (i.e., rotary vs. hand) on pedicle screw biomechanical and histological behavior although associated with some limitations. First, the health and young age of the animals utilized do not adequately mimic pathologic conditions in terms of both biomechanical stability and bone quality. Second, the relative short follow‐up does not allow for the evaluation of screw integration and implant stability under long‐standing stress conditions. Finally, the potential intrinsic bias of the split model in the spine setting where the sharing of load in a paired system inevitably creates a mutual interference on the mechanical response potentially affecting the outcome.

In conclusion, based on the presented data and analyses, rotary osseodensification instrumentation yielded significantly improved mechanical and histologic results in lumbar spine stabilization relative to the conventional hand instrumentation technique. Further studies are warranted to determine whether pedicle screw design plays a substantial role on lumbar spinal fixation over extended periods of time and under different mechanical conditions.

AUTHOR CONTRIBUTIONS

Lukasz Witek MSci, PhD —Dr. Witek contributed his time for the study design, assisted with all surgical procedures, supervised all data collection, lead data analysis, writing and revisions of this paper. Paulo Eduardo Lima Parente BSci—Mr. Parente was part of the data organization, statistical analysis, writing and image compilation. Andrea Torroni MD, PhD—Dr. Torroni assisted with all the surgeries, and was a crucial part in the revising and editing of this manuscript along with data analysis. Michael Greenberg BSci—Mr. Greenberg was part of the data organization, statistical analysis, writing and image compilation. Vasudev Vivekanand Nayak BE, MSci—Mr. Nayak was part of the data organization, statistical analysis, writing and image compilation. Jacques Henri Hacquebord MD—Dr. Hacquebord an orthopedic surgeon also lead the surgeries on all procedures, and was a crucial part in the revising and editing of this manuscript along with data analysis. Paulo G. Coelho MD, DDS, PhD, MBA—Dr. Coelho is one of the lead PIs, who along with Dr. Witek came up with the idea of using these devices in this large animal model. Dr. Coelho was the lead surgeon on all procedures and was a crucial part in the writing of this manuscript along with the review.

CONFLICT OF INTEREST

The authors have no conflict of interest to declare.

Witek, L. , Parente, P. E. L. , Torroni, A. , Greenberg, M. , Nayak, V. V. , Hacquebord, J. H. , & Coelho, P. G. (2023). Evaluation of instrumentation and pedicle screw design for posterior lumbar fixation: A pre‐clinical in vivo/ex vivo ovine model. JOR Spine, 6(2), e1245. 10.1002/jsp2.1245