1 Introduction

The talonavicular joint is a crucial component of the hindfoot complex. Arthrodesis of this joint offers immobilization of the subtalar joint and reduces hindfoot motion.^1-3 As such, talonavicular arthrodesis (TNA) is used for the treatment of foot pathology that requires immobilization of the hindfoot and/or Chopart complex, including deficiency of the posterior tibial tendon, pes valgus deformity, and isolated talonavicular arthritis. Previous studies have demonstrated variable nonunion rates after TNA, ranging from 3% to nearly 40%.^4–12 Despite favorable fusion rates in more recent studies, the combination of torsional and shear forces on the joint have potential to make successful arthrodesis challenging. Current techniques for talonavicular arthrodesis vary and include a variety of constructs using staples, screws, and/or plates.

Nitinol, a nickel and titanium alloy, is widely used in the medical field for cardiac stents, orthodontic wiring, and orthopedic implants.^19-22 Nitinol, a shape memory alloy, is unique due to its ability to restore to its original shape after deformation. Nitinol contains multiple additional desirable properties, including its biocompatibility and super elasticity as well as its fatigue/kink resistance.¹⁹ It undergoes martensitic transformation which involves a change from an ordered cubic crystal structure to a monoclinic crystal.²⁴ Traditional materials such as stainless steel undergo different elastic deformation when compared to natural biologic materials in the body. Nitinol's stress-strain characteristics have been demonstrated to be similar to bone in the human body.²⁴ One advantage of staples is its “memory” which allows for continuous compression. Theoretically, staples should continue to compress across the arthrodesis sites even after cyclic loading, as their mechanism of compression is different than screws. Screws lose compression over time, secondary to both loss of bony contact at the arthrodesis site during the resorptive phase of healing, and from toggling during cyclic loading, which is one reason patients are kept non-weightbearing for extended periods of time. However, staples should maintain compression, even after cycling, secondary to their unique metallurgy. This advantage may allow for earlier weightbearing for patients, which is considered beneficial after talonavicular arthrodesis.⁵

Despite favorable clinical results, there is a lack of data regarding the biomechanical performance of second-generation nitinol compression staples compared to screw fixation for talonavicular arthrodesis. To our knowledge, staples manufactured based on advances in nitinol fabrication techniques, such as selective laser melting to optimize shape structure for shape memory capabilities, have not been effectively tested.¹⁸

Therefore, the objective of this study was to compare fixation using two nitinol compression staples to the reference standard of two fully threaded lag screws for talonavicular arthrodesis with respect to their clinically relevant biomechanical properties. A unique component of the experimental design was the use of a robotic testing system to allow for functional cyclic loading while maintaining a constant load. The study was designed to test the null hypothesis that nitinol compression staples would not be significantly different from fully threaded lag screws based on clinically relevant biomechanical properties measured during functional robotic testing of the hindfoot.

2 Materials/methods

All procedures were performed with approval from the University of Missouri Institutional Review Board. Cadaveric feet (n = 12; 6 matched pairs) were acquired from 6 donors (2F, 4 M, mean age 79.5, mean BMI 23.4)(ScienceCare, Phoenix, AZ). A board-certified orthopaedic surgeon performed a standard dorsal approach to the talonavicular joint. A lamina spreader was used to distract the joint and osteotomes were used to prepare the joint for arthrodesis, ensuring complete denudation of the articular cartilage. The surgeon then applied either nitinol compression staples (DynaNite Nitinol Staple, Arthrex) or fully threaded lag screws (4.0 mm, DePuy-Synthes) to perform surgical arthrodesis of the talonavicular joint; each pair undergoing each procedure in random order and alternating between left and right feet, as follows.•Staple – The joint was manually reduced and compressed, then temporarily stabilized using a transarticular Kirschner wire. The guide for the 20 mm staple was then placed over the lateral aspect of the TN joint, with the center of the drill guide overlying the joint space. The talar side was drilled first, with a pin placed through the drill hole to hold the drill guide in place. The navicular side was then drilled. The drill guide and pin were then removed, and the staple inserted. The insertion device was then removed, and the staple impacted. A second staple was then placed in the same fashion over the medial aspect of the TN joint (Fig. 1

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Fig. 1 Illustration of Nitinol Compression Staple fusion over the Talonavicular Joint.

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Fig. 2 Illustration of Fully Threaded Lag Screw fusion over the Talonavicular Joint.

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Prior to and after surgical instrumentation, specimens were maintained at 37 C in a dedicated incubator to simulate body temperature. After surgical instrumentation was completed for each specimen, soft tissues were removed to expose 3 inches of the tibia and fibula proximal to the ankle joint. The exposed bones were potted in a 2.5-inch diameter tube using low-temperature metal alloy. The specimens were then mounted in the robotic testing system (KUKA Kr300 R2500 Ultra with Omega 160 IP65 force/torque sensor load cells) using high-precision memory-lock clamps. An aluminum plate covered with non-skid tape and mounted to the lower load cell served as an anti-slip simulated floor surface for the foot and the foot was further secured to the plate using nylon plastic cable supports. Specimens were kept hydrated with physiologic saline throughout testing.

Optical tracking markers (Optotrak Certus) were attached to the talus bone and navicular bone and anatomical landmarks were digitized using a probe to register the specimen in the testing system using a foot/ankle-specific software module (SimVitro)(Fig. 3). After mounting and digitization, the specimen was moved to a position of neutral flexion and preloaded with 30 N of compressive force to simulate initial postoperative loading while all other axes were commanded to 0 force and 0 torque in order to achieve neutral static loading. Calibration was performed to verify the accuracy of the robotic coordinate system for optical tracking. After calibration, the specimen was loaded at 89 N/sec from 30 N to 445 N for 1 min. Next, a continuous compressive load of 445 N was applied while cycling from 30° plantarflexion to 15° dorsiflexion for 10 cycles. Specimens that completed cyclic testing without evidence of TNA failure underwent 25° inversion to 20° eversion for 10 cycles while maintaining a 445 N continuous compressive load. Throughout testing, all forces, torques, translations and rotations were continuously monitored at a rate of 100 samples per second. Translation was measured in the X, Y, and Z planes (Fig. 4). Movement in the X direction was parallel to the talonavicular joint, the Y direction was perpendicular to the TN joint, and the Z plane was along the medial-lateral plane of the joint. Roll was defined as rotation around the X axis, pitch as rotation around the Z axis, and yaw as rotation around the Y axis (Fig. 5). Since the start point for translation/rotation was ‘0’, negative values were defined by the respective direction of the translation/rotation. A clinical photo of the biomechanical testing set up is demonstrated in Fig. 6.

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Fig. 3 Illustration of Biomechanical set-up. Specimens received either Nitinol Compression Staples or Fully Threaded Lag Screw Fusion of the Talonavicular Joint.

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Fig. 4 Illustration of the X, Y, Z axis of the talonavicular joint with Fully Threaded Lag Screw fusion.

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Fig. 5 Illustration of roll, pitch, and yaw of the talonavicular joint with Fully Threaded Lag Screw fusion.

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Fig. 6 Image of equipment set-up for testing.

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3 Statistical analysis

Means and standard deviations (SD) were calculated for each measured variable. Staple fixation was compared to screw fixation to assess for statistically significant differences for each variable using paired t-Tests (SigmaStat). Significance was set a priori at p < 0.05.

4 Results

All specimens completed the entire testing protocol without TNA failure.

Nitinol compression staple fixation specimens were biomechanically similar to lag screw fixation specimens while undergoing 10 cycles of simulated ankle motion (15 ° dorsiflexion to 30 ° plantarflexion), with a 445 N continuous compressive load. The only statistical difference occurred during static compression testing (Tables 1–3). Nitinol compression staple fixation was associated with significantly greater roll rotation when compared to lag screw fixation during static testing (p = 0.009). During this phase of testing, the navicular of the nitinol specimens rolled slightly externally, while the navicular of the lag screw fixation specimens rolled slightly internally in relation to the talus.

5 Discussion

Nickel-titanium alloy (e.g. Nitinol) compression staples have previously been used in foot and ankle surgery, and numerous studies have demonstrated high radiographic union rates for forefoot, midfoot, and hindfoot arthrodesis.^13,14,15 Herrera-Perez et al. conducted a series of 33 patients undergoing isolated subtalar arthrodesis. They found that rates of radiographic fusion and complications were similar between cannulated screws and compression staples.¹⁶ Our study expanded on this principal and applied them to the talonavicular joint. We hypothesized that nitinol compression staples would be statistically equivalent in all measured biomechanical properties when compared to fully threaded lag screws. Our biomechanical results demonstrated no statistically significant difference when comparing nitinol compression staples to fully threaded lag screws in all measured properties except roll during static compression testing only, and provides further support for their use in the clinical setting.

The only difference noted on biomechanical testing was during roll on static compression testing, but not during cyclic loading. An explanation for this difference is likely secondary to the difference in orientation of the staples and the screws, as well as the slightly increased pliability of the staples when compared to screws. However, this difference was not noted during cyclic loading, which is when roll would be the most likely to occur during the transition from initial contact to push off during the gait cycle. While there was increased translation during roll, the displacement was minimal and returned to their resting position. This indicates that while staples have a slight increase in their pliability, they are still able to maintain the reduction, even after translation occurs. It is also unclear how clinically relevant this motion is, especially considering it occurred during static testing only.

Schipper et al.²³ concluded that radiographic union was seen in 91.6% of joints using a nitinol staple construct and radiographic union was seen in 88.2% of joints using a combined nitinol staple and screw construct, both fusions occurring at the talonavicular joint. They found no significant difference between the two group when looking at radiographic union or revision surgery rates. Results from our biomechanical analysis offers further nonclinical support to their results by showing similar translation in all planes between the two groups. It should also be noted that Schipper et al. compared a combination nitinol staple to a staple and screw construct, which differed from this study, which compared an isolated dual staple construct to isolated dual screw construct.

O'Neil et al.¹⁹ sought to evaluate whether the addition of a nitinol staple-plate to a single cannulated screw increased the mechanical stability for a talonavicular fixation construct. They cyclically loaded 1000 cycles at 20 N, increasing by 20 N until reaching failure in an ex vivo model of one cannulated screw versus a plate with 2 screws with an 18 mm nitinol staple across the talonavicular joint. Their results showed increased stability of the talonavicular joint when adding a nitinol staple with a cannulated screw. Our study expanded on these concepts to evaluate if nitinol staples alone could provide adequate stability and encourage talonavicular joint arthrodesis when compared to fully threaded lag screws. We were able to show that nitinol staples were similar to screw fixation in an ex vivo model.

In a 1996 study, Meyer et al. found no statistical difference in fixation strength between screws and staples at the talonavicular, subtalar, or calcaneocuboid joints in triple arthrodesis on cadaveric feet.¹⁷ Our study tested initial biomechanical properties between screws and staples, demonstrating staple fixation is similar to screw fixation. Initial biomechanical properties we observed, coupled with the Meyer et al. study further supports the use of Nitinol compression staples in a clinical setting.

While our results support the use of isolated staple fixation for talonavicular arthrodesis, it is not without its limitations. The primary limitation of this study centers on the cadaveric biomechanical model. While several steps were taken to simulate normal foot motion during ambulation, it is difficult to completely replicate anatomic foot motion. This study measured biomechanical properties of cadaveric feet shortly after screws or staples were installed. In a clinical setting, the talonavicular joint would have time to fuse before loading or performing weight-bearing activities, thus negating some of the effects of the memory function of nitinol staples. However, this function of the metallurgy may be more effective during the initial resorptive phase of healing. Specimens were also kept at 37 °C to maximize the effect of the nitinol metallurgic properties, testing occurred at ambient temperature, which could have subtle effects on the staple's properties. However, this would then underestimate the biomechanical strength of the staples, thus providing a conservative estimate of their functional abilities. Another limitation is the difference in bone mineral density from patient to patient. This could potentially cause differing strengths to our fixation strategies. We attempted to minimize this factor through paired matching of feet specimen. In order to prevent excessive motion during testing, the specimens, they were secured to the non-skid aluminum plate with the use of zip ties. This solution could have caused unique stress risers and subtle effects on the motion of the talonavicular joint. However, care was taken to ensure that all specimens were tied down in a similar manner to decrease this confounding effect.

In conclusion, nitinol staples offer several advantages over conventional talonavicular arthrodesis techniques include efficient, repeatable insertion technique using a low-profile implant to minimize soft tissue irritation, and continuous compression across the arthrodesis site during the resorptive phase of healing. We found that nitinol compression staples were similar to fully threaded lag screws when used for talonavicular arthrodesis in a biomechanical model. There was no significant difference in all measured properties with the exception of roll during static compression testing. Further studies should focus on comparing nitinol compression staples to other fixation methods in the clinical setting.

Funding/sponsorship

This research did not receive any specific grant from funding agencies in the public, commercial or not-for-profit sectors.

Authors contribution

Prati Jahnu Reddy, MD—Data curation, investigation, writing—original draft, review/editing.

Blaine Manning, MD—Conceptualization, funding acquisition, methodology.

Will Bezold, MD-- Conceptualization, data curation, investigation, methodology.

Ashwin Garlapaty, MD—Investigation, data curation, writing—original draft, review/editing.

Kyle Schweser, MD—Conceptualization, data curation, funding acquisition, investigation, methodology, project administration, supervision, writing-review and editing.

James Cook, PhD, DVM, OTSC-- Conceptualization, formal analysis, funding acquisition, validation, methodology, project administration, supervision, writing-review and editing.