Abstract
Background
Nitinol compression staple use in foot and ankle arthrodesis procedures, including for the talonavicular joint, has gained acceptance. A previous study provided evidence for using nitinol compression staples in talonavicular arthrodesis (TNA) based on functional biomechanical testing comparisons to “gold standard” lag screw fixation. This study aimed to further compare the functional biomechanical properties of nitinol compression staple fixation to lag screw fixation for arthrodesis of the talonavicular joint. Body-temperature incubation and ankle inversion and eversion loading sequences were added to previously reported biomechanical testing.
Methods
Robotic testing was performed on cadaveric feet (n = 10; 5 matched pairs) after TNA using either two nitinol compression staples or two fully threaded lag screws. TNA method was randomized, alternating between matched-pairs of left and right feet. After surgical stabilization, specimens were incubated at 38 °C for 24 h to simulate the initial postoperative period in a patient. After plantarflexion and dorsiflexion testing, the specimens underwent inversion and eversion testing, cycling from 20° inversion to 10° eversion for 10 cycles. Displacements were tracked using optical tracking markers. Significant (p < 0.05) differences between staple versus screw fixation cohorts were determined using paired t-Tests.
Results
All specimens completed testing with none experiencing failure at the TNF. No statistically significant differences in functional biomechanical testing properties were noted between nitinol compression staple fixation and lag screw fixation for TNA.
Conclusion
The study findings provide additional support for nitinol compression staple fixation as an option for talonavicular arthrodesis fixation. Taken together, the results of functional biomechanical testing studies have provided sufficient evidence for initiation of a prospective clinical outcomes study using nitinol compression staples for talonavicular arthrodesis fixation at our institution.
Keywords: Nitinol, Talonavicular arthrodesis fixation, Compression staples
1. Introduction
Nitinol is a nickel and titanium combination alloy that has shape memory properties.1 Previous medical uses of nitinol have included cardiac stents, orthodontic brace wiring, and orthopedic implants of various types.2, 3, 4 Recently, nitinol compression staple use in foot and ankle arthrodesis procedures, including for the talonavicular joint, has gained acceptance.5, 6, 7, 8 A previous study by Reddy et al. provided initial evidence for nitinol compression staple fixation in talonavicular arthrodesis based on functional biomechanical testing comparisons to “gold standard” lag screw fixation.9 Based on the dynamic properties of nitinol, which include shape memory and super elasticity that allow for continuous compression over time, additional ex vivo testing is warranted in order to validate nitinol compression staples for clinical use in this indication.1,10 Specifically, Reddy et al. did not consider the effects of equilibration to body temperature on the nitinol staples, nor did they assess inversion and eversion rotation of the foot. As the material properties of nitinol are temperature dependent, the effects of elevation to body temperature after implantation should be simulated. In addition, inversion and eversion rotations are critical functions of the talonavicular joint that are important to assess for clinically relevant biomechanical testing.11 Therefore, this study was designed to further compare the functional biomechanical properties of nitinol compression staple fixation to lag screw fixation for talonavicular arthrodesis after body-temperature incubation with ankle inversion and eversion loading sequences included in the biomechanical testing protocol.
2. Methods
Under our institutional review board's policies for use of cadaveric specimens, cadaver feet (n = 10; 5 matched pairs) from 5 donors (1F, 4 M, mean age 78, mean BMI 24.22), were acquired (ScienceCare, Phoenix, AZ), prepared, instrumented, and biomechanically tested, as previously described.9 Briefly, a single board-certified orthopedic surgeon performed talonavicular stabilization for arthrodesis using a dorsal approach. The joint was prepared for arthrodesis removal of articular cartilage. Nitinol compression staples (DynaNite Nitinol Staple, Arthrex) or fully threaded lag screws (4.0 mm) were used for stabilization of the talonavicular joint as previously described.9 Each matched pair of cadaveric specimens was assigned to treatment in random order; treatment assignment was alternated between left and right feet for each procedure. After surgical stabilization, specimens were incubated at 38 °C for 24 h to simulate the initial postoperative period in a patient. The robotic testing system (Omega 160 IP65 torque-force sensor on the KUKA Kr300 R2500) with attached LED markers (Optotrak Certus optical tracking system) was used to test specimens, with anatomical landmarks digitized (SimVitro software) as previously described (Fig. 1)9.
Fig. 1.
Illustration of biomechanical testing set-Up.
All specimens underwent static compression testing from 30 N to 445 N at 89 N/sec for 1 min. A continuous compressive load of 445 N was applied during cycling from 30° plantarflexion to 15° dorsiflexion for 10 cycles. After plantarflexion and dorsiflexion testing, the specimens underwent inversion and eversion testing. Similarly, 445 N was then applied while cycling from 20° inversion to 10° eversion for 10 cycles. All forces, torques, translations and rotations were monitored at a rate of 100 samples/sec throughout testing by the optical tracking system as well as the robotic coordinate system and the force/torque sensors. Translation was measured in the X-, Y-, and Z-planes.
Movement in the X direction was perpendicular to the talonavicular joint (TNJ), Y direction parallel to the TNJ, and the Z direction superior and inferior to the TNJ. Rotation around the X-axis was roll, rotation around the Y-axis was pitch, and rotation around the Z-axis was yaw. Negative values were obtained depending on the direction of translation or rotation under static compression or dynamic (plantarflexion, dorsiflexion, inversion, eversion) loading, with the translation or rotation start point defined as ‘0’. (Fig. 2).
Fig. 2.
Illustration of the X, Y, Z axes and roll, pitch, and yaw of the talonavicular joint with threaded lag screw fixation for talonavicular arthrodesis.
Descriptive statistics were calculated to report means and standard deviations (SD) for each measured variable. Nitinol staple fixation was compared to lag screw fixation for statistically significant differences in translation and rotation using paired t-Tests (SigmaStat). Significance was set at p < 0.05.
3. Results
All specimens underwent full testing protocol. None of the samples experienced failure at the TNF.
No statistically significant differences in functional biomechanical testing properties were noted between nitinol compression staple fixation and lag screw fixation for TNA (Table 1).
Table 1.
Mean translation and rotation for static and dynamic cyclical compression testing in plantarflexion, dorsiflexion, inversion, and eversion of the foot to assess talonavicular arthrodesis fixation.
| Mean (SD) Translation and Rotation for Static Compression | |||
|---|---|---|---|
| Translation | Mean Staple | Mean Screw | p-value |
| X | 0.84 (1.1) | 0.38 (1.3) | 0.39 |
| Y | 0.57 (1.6) | 0.53 (1.9) | 0.93 |
| Z | 0.3 (0.9) | 0.46 (1.5) | 0.53 |
| Rotation | |||
| Roll | 0.38 (1.7) | 0.82 (1.8) | 0.86 |
| Pitch | 0.53 (1) | 0.16 (1.3) | 0.82 |
| Yaw | −4.7 (1.7) | −3.4 (1.9) | 0.6 |
| Mean (SD) Translation and Rotation for Plantarflexion | |||
|---|---|---|---|
| Translation | Mean Staple | Mean Screw | p-value |
| X | −1.3 (1.7) | −0.44 (1) | 0.18 |
| Y | 0.34 (0.4) | 0.26 (0.3) | 0.73 |
| Z | −0.26 (0.9) | −0.4 (0.5) | 0.32 |
| Rotation | |||
| Roll | −0.5 (1.8) | 0.94 (2) | 0.51 |
| Pitch | 1.5 (0.8) | 2.2 (1.6) | 0.54 |
| Yaw | 1.6 (2.9) | 2.9 (2.3) | 0.78 |
| Mean (SD) Translation and Rotation for Dorsiflexion | |||
|---|---|---|---|
| Translation | Mean Staple | Mean Screw | p-value |
| X | 1.4 (2.1) | 1.1 (1.1) | 0.33 |
| Y | 0.98 (0.5) | 0.62 (1.4) | 0.58 |
| Z | −0.18 (0.7) | −0.4 (1) | 0.77 |
| Rotation | |||
| Roll | 3 (1.3) | 3.3 (1.5) | 0.73 |
| Pitch | −0.6 (0.7) | −0.2 (2) | 0.99 |
| Yaw | 5.2 (3.3) | 3.4 (1.5) | 0.68 |
| Mean (SD) Translation and Rotation for Inversion | |||
|---|---|---|---|
| Translation | Mean Staple | Mean Screw | p-value |
| X | 0.88 (1.2) | 0.8 (0.9) | 0.22 |
| Y | 0.74 (1.1) | 0.9 (1.5) | 0.73 |
| Z | 0.28 (1.2) | 0.5 (1.5) | 0.78 |
| Rotation | |||
| Roll | −1.4 (3) | −1.1 (3.7) | 0.92 |
| Pitch | 2.3 (2.3) | 3.3 (2.9) | 0.59 |
| Yaw | 2.8 (1.4) | 2.3 (1.7) | 0.55 |
| Mean (SD) Translation and Rotation for Eversion | |||
|---|---|---|---|
| Translation | Mean Staple | Mean Screw | p-value |
| X | −2.1 (1.4) | −1.3 (1.1) | 0.3 |
| Y | 0.46 (0.5) | 0.71 (0.8) | 0.41 |
| Z | 0.7 (0.6) | 0.66 (0.5) | 0.89 |
| Rotation | |||
| Roll | 1.9 (0.9) | 1.5 (1.4) | 0.54 |
| Pitch | −1.6 (2.4) | −1.8 (2.2) | 0.95 |
| Yaw | −3.6 (1.6) | −3.4 (1.6) | 0.75 |
4. Discussion
The study results provide further preclinical evidence for the use of nitinol compression staples as an option for talonavicular arthrodesis fixation based on their equivalent functional biomechanical properties to “gold standard” lag screws after 24-h incubation at body temperature. Incubation of the specimens at the upper-end of reported average human body temperature (38 °C)12, 13, 14 ensured that the temperature-dependent effects of nitinol staples that would occur in patients in the initial postoperative period were considered.5, 6, 7 Body temperature activates the shape memory and super elastic properties of nitinol, which may account for the improved biomechanical properties of nitinol staples noted after 24-h incubation when compared to those reported in a previous study.9
The present study included inversion and eversion in the functional biomechanical testing protocol based on the clinical relevance to function of the talonavicular joint.11 Despite the importance for comprehensive assessment of TNA, inversion and eversion are rarely included in biomechanical testing protocols due to the difficulty in maintain adequate cadaveric foot contact during functional testing. The specimen mounting and compression-loaded (weightbearing) protocols developed and implemented for 6-degree-of-freedom robotic system allowed for valid functional plantarflexion, dorsiflexion, inversion, and eversion assessments following TNA. Importantly, the results of this comprehensive testing suggested that nitinol staple fixation maintained functional biomechanical equivalence to lag screw fixation, including dynamic cyclic inversion and eversion loading.
Limitations to the present study should be considered when interpreting and applying the results. As a preclinical cadaveric biomechanical model study, normal foot motion during ambulation could only be simulated and not fully replicated. In addition, the biomechanical testing was performed immediately after the talonavicular joint was instrumented such that the influences of tissue healing, bone fusion and remodeling, and postoperative restriction and rehabilitation protocols were not assessed. As such, this model provides a “worst case scenario” for evaluating the biomechanical properties of the fixation methods such that the results describe a conservative estimate of their functional abilities. Patient-related variables including age, sex, BMI, and bone mineral density were also not directly assessed, but were mitigated as confounding variables through paired matching of specimens.
The results of this study further support using nitinol compression staples as an option for talonavicular arthrodesis fixation. Taken together, the results of functional biomechanical testing studies have provided sufficient evidence for initiation of a prospective clinical outcomes study using nitinol compression staples for talonavicular arthrodesis fixation at our institution.
Ethical Statement
Not applicable.
Funding Statement
The Barry J. Gainor Resident Research & Scholarship Endowment Fund and University of Missouri Orthopaedic Association Research & Education Fund provided financial support for this research.
Arthrex provided nitinol compression staples for the study design. No other external funding was used in the completion of this study.
Guardian/Patient's Consent
Not applicable.
CRediT authorship contribution statement
Ashwin Garlapaty: Investigation, Data curation, Writing – original draft, Writing – review & editing. James L. Cook: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Supervision, Validation, Writing – original draft, Writing – review & editing. Will Bezold: Investigation, Project administration, Data curation, Writing – original draft, Writing – review & editing. Kyle Schweser: Conceptualization, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Supervision, Validation, Writing – original draft, Writing – review & editing.
Declaration of Competing interest
• James L Cook receives research support from AO Trauma; receives IP royalties, is a paid consultant, and receives research support from Arthrex, Inc; receives research support from Collagen Matrix Inc; receives research support from DePuy, A Johnson & Johnson Company; is on the editorial or governing board of Journal of Knee Surgery; is a board or committee member for Midwest Transplant Network; is a board or committee member, receives IP royalties and research support from Musculoskeletal Transplant Foundation; receives research support from National Institutes of Health (NIAMS & NICHD); receives research support from Orthopaedic Trauma Association; receives research support from Purina; receives research support from Regenosine; receives research support from SITES Medical; receives publishing royalties, financial or material support from Thieme; is a paid consultant for Trupanion; and receives research support from U.S. Department of Defense.
• Kyle Schweser is a board or committee member for AAOS; is a board or committee member for AO North America; receives research support from Arthrex, Inc; receives IP royalties from ODi; and is a board or committee member for Orthopaedic Trauma Association.
• Ashwin Garlapaty and Will Bezold have no conflict of interest to disclose.
Acknowledgements
The author team would like to acknowledge Stacy Cheavens for her contribution of the illustrated artwork used in this manuscript, and Adelina Colbert for preparing this manuscript for submission.
Arthrex provided nitinol compression staples for the study design.
The Barry J. Gainor Resident Research & Scholarship Endowment Fund and University of Missouri Orthopaedic Association Research & Education Fund provided financial support for this research.
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