Biomechanical Comparison of Sustained Dynamic Compression Screws vs Static Compression Screws in Subtalar Arthrodesis: A Cadaveric Study

Abstract

Background:

Subtalar arthrodesis is a reliable procedure to address subtalar arthritis, though nonunion remains a challenge. Adequate compression is critical to optimizing fusion, but bone resorption and settling impair the ability of standard static compression (SC) screws to maintain compression. Sustained dynamic compression (SDC) screw constructs that use the pseudoelastic property of nitinol may enable more sustained compression than static compression screws.

Methods:

Ten fresh talo-calcaneal cadaveric specimens were instrumented with either 2 SC screws or 2 SDC nitinol screws. Using a custom mechanical fixture, the compression force generated during and after screw insertion was measured in each group. The fixture was allowed to relax over a controlled distance and time to simulate bone resorption. The force at each relaxation point was measured, and the resorption capacity was calculated for each construct.

Results:

Maximum compression at implantation was 430.2 ± 237.2 N for SC and 406.8 ± 117.7 N for SDC. After a 1-minute dwell period, compression was 299.6 ± 194.4 N for SC and 367.9 ± 108.0 N for SDC, corresponding to retention of 69.6% and 90.4% of initial compression, respectively. At 2 mm of simulated resorption, the SDC construct retained 111.1 ± 45.4 N compared with 8.3 ± 18.6 N for SC. The resorption capacity was 4.2 mm compared with 1.3 mm for the sustained dynamic compression and static constructs, respectively.

Conclusion:

We did not detect a difference in initial compressive force between the SC and SDC screws. However, under identical bench conditions, the SDC construct retained more of its initial compression during the first minute and maintained substantially greater compression during simulated bone resorption than the SC construct. These findings support further study of whether sustained compression may help preserve fixation-generated preload during subtalar arthrodesis.

Clinical Relevance:

In a cadaveric subtalar arthrodesis model, novel sustained dynamic compression screws retained more interfragmentary compression than standard static compression screws during early relaxation and simulated bone resorption. This technology may aid surgeons in maintaining compression despite postoperative settling or resorption during subtalar arthrodesis, particularly in higher-risk patients.

Graphical Abstract

Keywords

subtalar arthrodesis nitinol biomechanics compression

Introduction

Subtalar arthrodesis is an orthopaedic procedure performed to eliminate motion at the articulation between the talus and calcaneus.^1,2 This may be performed in isolation or in conjunction with other bony or soft tissue procedures in order to address hindfoot deformity and/or arthritis.^1
-3 This procedure works well to address pain from hindfoot arthritis, but fusion rates can be limited by patient factors, including smoking and BMI.³ Obtaining adequate compression across the fusion site is critical to maximize patients’ likelihood of successful union.^4
-6 Several techniques are employed to obtain stability and compression across the fusion site, typically using one or two screws.^7
-9

Previous biomechanical work has demonstrated that up to 30% of a hindfoot fusion construct’s initial compression is lost just 3 minutes after screw placement because of viscoelastic relaxation of the surrounding bone.^2,3,10 These processes can lead to loss of compressive forces supplied by orthopaedic implants because of joint settling and bone resorption.^4,6,11 To address these changes, sustained dynamic compression devices using the unique properties of nitinol have emerged.¹²

Nitinol is a unique alloy first discovered in 1960, composed of nickel-titanium.^12,13 The alloy demonstrates a unique ability to undergo a solid-state phase transformation of its crystal structure, altering its pliability, stiffness, and elasticity depending on changes in its crystalline structure; these changes are reversible. Medical devices began to take advantage of its pseudoelasticity, and orthopaedic implants are no exception. Early versions used the shape-memory effect to spontaneously restore cold-stored nitinol products to a predetermined shape when heat was applied. Newer devices have been developed to leverage the pseudoelasticity property with an internal nitinol “piston” that is tensioned during fixation.^12,14 Once tensioning is released, the nitinol will attempt to return to its native shortened shape, exerting a sustained compressive force over time. Multiple biomechanical and cadaveric studies have demonstrated superior initial and sustained compression in these devices,^8,15
-17 with some clinical data supporting increased union rates without increased complications from these devices.^14,18

The intent of this study is to compare the compression provided by a sustained dynamic compression (SDC) screw to traditional static compression (SC) screws in isolated subtalar arthrodesis using a cadaveric model. Prior subtalar biomechanical studies have focused primarily on construct configuration and initial compression rather than sustained maintenance of compression during early relaxation and simulated resorption.^8,9,19,20 A matched-pair cadaveric model permits clinically representative screw placement while partially controlling for anatomic variability and bone quality between constructs. Based on existing literature on nitinol devices, we hypothesize that SDC screws will provide similar initial compression with superior sustained compression compared to SC screws.

Methods

Preparation of Specimens

This study was approved by the Institutional Review Board at our institution (C.2025.103n). Ten hindfoot specimens from 5 cadaveric donors were obtained by the Center for Emergency Health Services (Spring Branch, TX). The mean age at death of cadaveric specimens was 79.5 years (range: 56-94 years). Three specimens were female and 2 were male. No specimens had underlying foot deformities that would preclude subtalar arthrodesis. Resorption capacity was defined a priori as the cumulative interfragmentary displacement at which measured compression was fully lost. An a priori power analysis based on resorption capacity data obtained from previous Sawbones testing indicated that 4 paired specimens would be sufficient to detect between-group differences with α = 0.05 and power = 0.80; therefore, 5 paired specimens were tested to provide redundancy for specimen preparation and variation in cadaver anatomy. Specimens were disarticulated at the tibiotalar, talonavicular, and calcaneocuboid joints, leaving an isolated subtalar articulation. The capsular and ligamentous attachments between the calcaneus and talus were left largely intact to aid with instrumentation.

Specimens were then instrumented with either 2 standard, 7.0- mm SC screws (Tiger Headless; Enovis) or two 7.0-mm SDC screws (DynaNail Helix; Enovis) (Figure 1). The SC implant is a headless, partially threaded and cannulated screw, and the SDC implant is a headless, partially threaded screw with a nitinol center. All screws were placed in a retrograde fashion from the calcaneus into the talus. Both screws were placed in a lag-by-design technique. We ensured all threads for each screw were in the distal fusion segment in the talus and that both screws were placed in clinically representative, nonintersecting, approximately parallel trajectories. Each pair received 2 static and 2 dynamic implants (eg, the left specimen was instrumented with the SC implants and the right instrumented with the SDC implants) to account for differences in bone quality and anatomy. A random number generator was used to determine which side (right or left) of each specimen received the static or dynamic device.

Figure 1.

Photographic depiction of the static compression (SC) and sustained dynamic compression (SDC) screws.

Construct Assembly

Each talus-calcaneus pair was first mounted in a custom-designed compression measurement fixture that maintained a controlled interfacial gap across the subtalar joint. This testing construct has been previously used in prior biomechanical studies.^19,20 The fixture allowed precise, incremental adjustment of the joint gap to simulate progressive bone resorption while isolating compression generated by the implant-bone interface. The talus and calcaneus were rigidly secured to the fixture using four 5.0-mm smooth pins (2 per bone), positioned to avoid interference with planned screw trajectories (Figure 2).

Figure 2.

Demonstration of mounting of specimens and biomechanical testing. (A) The calcaneus and talus are pinned in place and mounted to the measuring jig; a force sensor at the base of the jig measures the compression force between the 2 mounting panels. (B) Turning a screw that connects the 2 mounting panels allows for initial distraction, followed by gradual relaxation over fixed distances to simulate resorption. (C) The subtalar joint is distracted, and then instrumented. (D) Screws have been inserted into the specimen, and the compression force is measured.

With the specimen secured in the fixture, guidewire placement was performed under fluoroscopy. The near cortex was opened using the implant-specific cortical opening drill, followed by preparation of the screw path with the manufacturer-recommended drill. Prior to screw insertion, the fixture was adjusted to distract the talus and calcaneus such that the opposing articular surfaces did not contact one another throughout the resorption simulation; this ensured that the measured load reflected fixation-generated interfragmentary compressive preload rather than direct articular contact. Since the articular surface of the posterior facets of the talus and calcaneus never touched, the joint surfaces were not prepped as is often done in an arthrodesis procedure. Screw lengths were determined using the implant-specific depth gauge.

All constructs were assembled using a standardized dorsal-then-plantar insertion sequence. In SDC constructs, the dorsal screw was fully seated and the nitinol compression element released. After a 60-second dwell to provide a standardized equilibration interval and allow activation of the compression element, the plantar screw was inserted and its compression element released, followed by an additional 60-second dwell prior to measurement. SC constructs followed the same insertion order and dwell periods to maintain procedural consistency across groups.

Compression Measurement and Simulated Resorption Protocol

Interfragmentary compressive force across the subtalar interface was recorded using the integrated load measurement system of the custom fixture (Transducer Technologies). Maximum compression at implantation was defined as the peak force measured immediately following insertion of the second screw, and compression after 1 minute was defined as the plateau force measured after the subsequent 60-second dwell period.

Simulated bone resorption was then introduced in a stepwise manner by relaxing the fixture to decrease the interfragmentary gap in fixed increments. Sequential gap decreases corresponding to 0.25-mm, 0.5-mm, 1.5-mm, 2.5-mm, and subsequent increments were applied until compressive force was no longer detected. At each resorption increment, the construct was allowed to equilibrate for 60 seconds as a standardized interval for this benchtop model prior to force acquisition. This protocol was continued until the measured compressive force reached zero, indicating complete loss of fixation-generated preload.

Outcomes for this phase included maximum interfragmentary compression measured immediately following insertion of the second screw (zero-time delay), interfragmentary compression after a 1-minute dwell period prior to initiation of simulated resorption, and normalized compression at 1 minute expressed as a percentage of maximum compression. Additional outcomes included interfragmentary compression measured at 1.0 mm and 2.0 mm of simulated resorption, as well as resorption capacity, defined as the cumulative interfragmentary displacement at which measured compression was fully lost.

Statistical Analysis

Data were analyzed using 1-tailed paired t tests assuming a normal (Gaussian) distribution, with statistical significance set at α = 0.05 (GraphPad Prism version 10.0). Normalized compression retained at 1 minute and resorption capacity were considered the primary mechanistic outcomes. Initial compression and compression measured at fixed resorption intervals were considered supportive exploratory comparisons, and exact P values are reported.

Results

We did not detect a difference in maximum interfragmentary compression measured immediately following insertion of the second screw between fixation constructs (430.2 ± 237.2 N for SC vs 406.8 ± 117.7 N for SDC; mean paired difference [SDC – SC], −23.4 N; 95% CI, −225.6 to 178.7; Figure 3A, P = .382). Compression decreased for both constructs after the 1-minute dwell period (299.6 ± 194.4 N for SC vs 367.9 ± 108.0 N for SDC; mean paired difference [SDC – SC], 68.3 N; 95% CI, −53.5 to 190.1; Figure 3B, P = .097). However, the SDC construct retained a substantially greater proportion of its initial compression, maintaining 90.2% ± 6.9% at 1 minute compared with 66.6% ± 11.9% for the SC construct (mean paired difference [SDC – SC], 23.6 percentage points; 95% CI, 7.7-39.5; Figure 3C, P = .007).

Figure 3.

Early interfragmentary compression behavior following fixation. (A) Interfragmentary compression measured immediately after insertion of the second screw. (B) Interfragmentary compression measured after a 1-minute dwell period. (C) Normalized compression retained at 1 minute, expressed as a percentage of maximum compression within each specimen. Bars represent mean ± SD. Statistical comparisons were performed using 1-tailed paired t tests; exact P values are shown.

Compression retention during simulated resorption diverged between constructs. After 1.0 mm of simulated resorption, the SDC construct maintained 152.3 ± 80.4 N compared with 67.2 ± 63.3 N for the SC construct (mean paired difference [SDC – SC], 85.1 N; 95% CI, −6.7 to 176.8; Figure 4A, P = .031). By 2.0 mm of simulated resorption, the SC construct had lost nearly all compression, whereas the SDC construct maintained 111.1 ± 45.4 N compared with 8.3 ± 18.6 N for SC (mean paired difference [SDC – SC], 102.8 N; 95% CI, 38.8-166.8; Figure 4B, P = .006). Consistent with these findings, resorption capacity was greater for the SDC construct, which tolerated an average of 4.2 mm of simulated resorption prior to complete loss of compression compared with 1.3 mm for the SC construct (Figure 4C; P < .001).

Figure 4.

Compression retention during simulated resorption and resorption capacity. (A) Interfragmentary compression measured after 1.0 mm of simulated resorption. (B) Interfragmentary compression measured after 2.0 mm of simulated resorption. (C) Resorption capacity, defined as the cumulative simulated resorption distance sustained before complete loss of compression. Bars represent mean ± SD. Statistical comparisons were performed using 1-tailed paired t tests; exact P values are shown.

Discussion

This biomechanical study compared 2 fixation constructs for subtalar arthrodesis: a standard SC screw construct and a novel SDC nitinol screw that allows for additional sustained compression after the device has been released. In a cadaveric lower extremity model, both devices achieved similar amounts of initial compression, although the SC screw construct retained only roughly 60% of its initial compressive force after just 1 minute, compared with roughly 90% retained by the SDC screw construct. With simulated bony resorption, the SDC screws maintained compressive forces up to about 4 mm of resorption, whereas the SC screws had nearly no compression remaining by roughly 1.5 mm. These findings suggest that traditional screw fixation may result in a relatively precipitous loss of compression across the arthrodesis site, an effect that may be further compounded by bony resorption over time. SDC technologies, such as the device tested in this study, may therefore more effectively maintain compression over time, potentially improving arthrodesis rates of the subtalar joint.

Although subtalar arthrodesis is a successful surgery for treating subtalar arthritis, nonunion remains a concern. Rates of nonunion after subtalar arthrodesis have ranged from 4% to over 20% in some cohorts.^3,21
-23 Risk factors for nonunion include smoking, prior trauma involving the subtalar joint, diabetes, and fusion at adjacent joint articulations, among others.^3,22,23 A study by Easley et al³ retrospectively reviewed the outcomes of 148 isolated subtalar arthrodesis procedures and found an overall nonunion rate of 16%. When they removed patients with known risk factors for nonunion (smoking history, those with prior ankle fusions, etc), the nonunion rate dropped to 4%. Jennison et al²³ evaluated patients who underwent subtalar arthrodesis and had a preexisting ipsilateral tibiotalar arthrodesis (n = 18) and compared them with patients without an adjacent arthrodesis (n = 53). In patients without an adjacent tibiotalar fusion, the union rate was 86.8% compared with only 44.4% in the patients with an ipsilateral ankle fusion. Surgical techniques to enhance union include two screw fixation constructs, use of autograft and biologics, adequate joint preparation, and stable compression across the fusion site.^21,24,25 The results of the present study suggest that the SDC screw construct may allow for enhanced, sustained compression across the subtalar joint compared with standard SC screws. Whether this mechanical advantage translates into improved fusion rates, particularly in higher-risk individuals, requires clinical study.

An interesting finding of this study is the effect that simulated bone resorption had on compression forces across the fusion site. Bone resorption and settling occur after arthrodesis and can have a serious impact on the sustained compression of various implants. A biomechanical study by Yakacki et al²⁰ simulated bony resorption in a TTC model and found a roughly 90% loss of initial compression with only 1 mm of simulated bony resorption. Several studies have demonstrated that following TTC fusion, varying degrees of bone resorption or settling occur.^26,27 Kildow et al²⁶ followed 15 patients radiographically after TTC nailing with a pseudoelastic implant. They noted that within the first 3 months postoperatively, the nitinol element had shortened by roughly 2.38 mm, indicating that bone resorption occurs relatively quickly after arthrodesis. Similarly, Conklin et al²⁷ radiographically followed patients who underwent TTC nailing with bulk femoral allograft ankle arthrodesis after failed total ankle arthroplasty, and similarly noted nitinol migration postoperatively. In the present study, the SC construct lost nearly all compression by 2 mm of simulated resorption, whereas the SDC construct maintained measurable compression at that distance and out to an average resorption capacity of 4.2 mm. Continued compression in this model should not be interpreted as unlimited tolerance to postoperative settling or bone loss, as this study did not evaluate thread-bone interface failure, cyclic loading, or the biologic consequences of sustained compression.

This study is not without limitations. First, this is a cadaveric study, and the viscoelastic properties of cadaver bone may not perfectly replicate live bone characteristics. Additionally, some capsular and ligamentous attachments were left intact to aid instrumentation and may have influenced the measured construct response, although the same preparation and testing conditions were used for both groups. Second, nitinol is a temperature sensitive alloy, and testing was conducted at room temperature. Accordingly, the absolute compressive forces measured here may differ from those generated in vivo, and higher physiologic forces could alter implant behavior and the thread-bone interface response. Third, there may have been slight variations in specimen measurements using the custom fixture. Prior to each measurement the fixture was zeroed to minimize this risk, and we used the same fixture for all measurements to mitigate measurement bias. Finally, the results of this study cannot definitively support that the SDC screw will improve fusion rates; rather the results demonstrate more sustained compression over time and simulated bone resorption. Clinical studies of this novel implant are necessary to evaluate whether this mechanical advantage translates into improved fusion rates in the setting of subtalar arthrodesis.

Conclusion

The results of this biomechanical study demonstrate that in a cadaveric subtalar arthrodesis model, the sustained dynamic compression (SDC) construct maintained a greater proportion of its initial compression over the first minute after implantation and preserved compression over a greater range of simulated bone resorption than the static compression (SC) construct. These findings support further study of whether sustained compression may help preserve fixation-generated preload during subtalar arthrodesis.

Supplemental Material

sj-pdf-1-fao-10.1177_24730114261449242 – Supplemental material for Biomechanical Comparison of Sustained Dynamic Compression Screws vs Static Compression Screws in Subtalar Arthrodesis: A Cadaveric Study

Supplemental material, sj-pdf-1-fao-10.1177_24730114261449242 for Biomechanical Comparison of Sustained Dynamic Compression Screws vs Static Compression Screws in Subtalar Arthrodesis: A Cadaveric Study by James Baker, Mariah Arave, James Johnson, David Safranski and Jeannie Huh in Foot & Ankle Orthopaedics

Footnotes

ORCID iDs

James Baker, MD,

Mariah Arave, MD,

James Johnson, PhD,

Jeannie Huh, MD,

Ethical Considerations

Ethical approval was obtained from the Institutional Review Board at Brooke Army Medical Center (approval ID: C.2025.103n).

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: Enovis provided the implants for this study free of charge. No financial compensation, direct or indirect, was provided by Enovis for this work.

Declaration of Conflicting Interests

The authors declared the following potential conflicts of interest with respect to the research, authorship, and/or publication of this article: James Johnson, PhD, and David Safranski, PhD, are paid employees of Enovis, the manufacturer of one of the devices tested in this work. Disclosure forms for all authors are available online.

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