|Year : 2021 | Volume
| Issue : 1 | Page : 37-44
Effect of axial dynamization in circular external fixation on bone segment vertical and lateral displacements
Erin M Honcharuk1, Alexander M Cherkashin2, William A Pierce3, Chanhee Jo3, David A Podeszwa2, Mikhail L Samchukov2
1 Department of Orthopedic Surgery, Scottish Rite Hospital for Children, Dallas, TX, USA
2 The Center for Excellence in Limb Lengthening and Reconstruction, Scottish Rite Hospital for Children; Department of Orthopedic Surgery, University of Texas Southwestern Medical Center, Dallas, TX, USA
3 Seay Center for Musculoskeletal Research, Scottish Rite Hospital for Children, Dallas, TX, USA
|Date of Submission||26-Apr-2021|
|Date of Decision||21-May-2021|
|Date of Acceptance||26-May-2021|
|Date of Web Publication||30-Jun-2021|
Dr. Mikhail L Samchukov
Texas Scottish Rite Hospital for Children, 2222 Welborn Street, Dallas, TX 75219
Source of Support: None, Conflict of Interest: None
Context: The field of deformity correction with external fixation has continued to advance since with the addition of half-pins, even though they can act to stiffen the construct and undermine healing. Dynamization increases axial motion at the fracture site and improves fracture and osteotomy healing in the experimental and clinical studies. No study compares the lateral and vertical displacements of bone segment in dynamized versus nondynamized frames. Aims: The purpose of this study was to compare the segmentary motion in the axial, coronal, and sagittal planes using nondynamized and dynamized circular external fixation frames. Subjects and Methods: Seven frame models were tested including classic Ilizarov three-wire construct and two frame configurations representing the most common modern half-pin and wire combinations. These models were either nondynamized or dynamized with two types of dynamization modules. Each model underwent axial loading up to 50 kg for 11 cycles. Statistical Analysis Used: One-way analysis of variance testing followed by post hoc Tukey's test. Results: The addition of each half pin sequentially decreased axial motion while increased sagittal motion. Dynamization had a limited effect on the sagittal motion but significantly improved axial motion. The sagittal to axial motion ratio increased with half pins in nondynamized frames but decreased equal to or beyond the Ilizarov three-wire fixation frame with dynamization. At the limit of the dynamizers' motion, there was change in the rate of displacement, suggesting that subsequent motion was strictly from the wires and half-pins. Overall, there was minimal coronal motion. Conclusions: While half pins decrease axial micromotion and increase detrimental sagittal motion, dynamization restores.
Keywords: Axial micromotion, circular external fixation, dynamization, dynamization module
|How to cite this article:|
Honcharuk EM, Cherkashin AM, Pierce WA, Jo C, Podeszwa DA, Samchukov ML. Effect of axial dynamization in circular external fixation on bone segment vertical and lateral displacements. J Limb Lengthen Reconstr 2021;7:37-44
|How to cite this URL:|
Honcharuk EM, Cherkashin AM, Pierce WA, Jo C, Podeszwa DA, Samchukov ML. Effect of axial dynamization in circular external fixation on bone segment vertical and lateral displacements. J Limb Lengthen Reconstr [serial online] 2021 [cited 2021 Jul 28];7:37-44. Available from: https://www.jlimblengthrecon.org/text.asp?2021/7/1/37/320048
| Introduction|| |
Optimization of external skeletal fixation for fracture reduction, deformity correction, and limb lengthening remains an important goal of modern orthopedics. In 1905, Codivilla described femoral osteotomies followed by aggressive, sporadic application of traction through a large diameter pin in the calcaneus attached to a massive, static external fixation device to achieve femoral lengthening. To improve patient mobility, the external devices developed later by Putti, Abbott, and Wagner, consisted of uniplanar monolateral or semi-circular frames utilizing large diameter full pins. Those stiff constructs were often combined with the acute application of traction forces that resulted in delayed fracture healing and decreased new bone formation. A high percentage of these cases with delayed union or nonunion required additional surgical procedures with bone grafting and internal fixation. This suggested that overly rigid external fixation combined with acute traction was undesirable for bone regeneration and healing, and instead, lead to a focus on how to allow micromotion at the fracture or osteotomy site.
In 1951, Ilizarov introduced his circular external fixation device utilizing a different biomechanical environment for segmental bone fixation. In contrast to rigid fixation with large diameter pins, bone segments were stabilized by pairs of crossed, tensioned thin wires. This inherently less rigid device provided more interfragmentary axial micromotion while preventing undesirable movement of bone segments inside the frame. Preserved axial micromotion has been found to induce a larger callus formation,,,,,, which increases torsional stiffness of bone callus and improves overall fracture healing.,, As result, Ilizarov-type devices became the dominate circular external fixation frames used worldwide. Over time, the original Ilizarov fixator underwent several important modifications including improvement of existing connecting elements (e.g., hinges and angular distractors) and utilization of computer-assisted six-strut hexapod modules (e.g., Taylor Spatial Frame). There was also an enhancement of segmental bone fixation through the addition of large diameter half pins to tensioned wires (e.g., distal femoral and proximal tibial constructs) or even complete replacement of wires with half pins (e.g., proximal femoral constructs). Although the use of half pins allows a safer trajectory of fixation elements by avoiding neurovascular structures and significantly reduces complications related to muscle penetration by the wires, the rigidity of bone segment fixation is also significantly increased. These changes resulted in decreased interfragmentary axial micromotion and as seen in the previous monolateral frames, impair the quality of newly formed bony tissues in fracture callus and distraction regenerate.,,,
Further research elucidated how motion changes with different frame configurations. Decreased bending motion occurs only with additional fixation placed in the plane of motion while axial motion is limited with the addition of any fixation element., Half pins have been found to have the most effect on reducing axial motion. Thus, the same conclusion may be reached, that increasing stiffness of external fixation frames with pin fixation may delay timely and adequate healing.
In order to decrease rigidity of fixation, new special modules are often added to the modern external fixators. These modules provide guided axial movement (dynamization) to enhance healing while preserving torsional and bending stiffness.,,,,, The first dynamization device was described by De Bastiani et al. in 1984. A telescoping bar in his monolateral external fixator allowed for axial micromotion resulting in improved fracture healing and nonunion treatment. For circular external fixation, surgeons classically have implemented dynamization by gradual untightening of the nuts on connecting rods and partially removing wires and/or half pins over time. More recently, specially designed dynamization modules allowing controlled axial motion have been introduced to circular frames (e.g., Hoffmann LRF, Stryker, Celzach, Switzerland and TrueLok, Orthofix, Verona, Italy).
The positive influence of dynamization on fracture healing and new bone formation during distraction osteogenesis was demonstrated in numerous previous publications.,,,, There is no biomechanical study; however, analyzing the effect of axial micromotion on the stability of bone segment fixation in the coronal and sagittal planes. Therefore, the purpose of this study was to evaluate the motion of bone segments in medial-lateral and anterior-posterior directions during the axial motion provided by dynamization modules in a circular external fixation model. We hypothesized that the addition of dynamization modules to circular frames will allow for increased axial motion during longitudinal loading with minimal effect on both coronal and sagittal plane motions.
| Subjects and Methods|| |
Solid 32-mm diameter and 152-mm long Acetyl (Delrin) cylinders were used to simulate a long bone segment. They were placed eccentrically inside a 150-mm diameter aluminum ring (TrueLok, Orthofix, Verona, Italy) with 25 mm of anterior translation relative to the ring center and attached to the ring using 1.8 mm diameter wires tensioned to 130 kg and/or 5.0-mm diameter half pins (TrueLok, Orthofix, Verona, Italy). One model [Figure 1]a represented the classic Ilizarov three-wire (3WR) fixation pattern with two cross wires inserted at 60° relative to each other and secured directly to the ring and the third wire offset 25 mm distally through connecting posts and bisecting the trajectory of the proximal two wires. Two other models represented the most common modern configurations of wires and half pins used for distal femur, proximal tibia, and distal tibia. In one model [Figure 1]b, the Delrin cylinder was also fixed to the ring by two cross wires inserted at 60° but combined with an anterior half pin (Two wires and one half pin [2WR1HP]) bisecting the angle between wires. The other model [Figure 1]c used a single horizontal wire and two anterior half pins (1WR2HP) each inserted at 45° relative to the wire and 90° relative to each other.
|Figure 1: Three types of Delrin cylinder fixation inside the ring: (a) Three-wire pattern with two cross wires attached to the ring and the third wire offset 25 mm distally through connecting posts. (b) Wire half-pin fixation pattern (2WR1HP) with two cross wires combined with an anterior half pin. (c) Wire half-pin fixation pattern (1WR2HP) with one wire combined with two half pins|
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In 3WR model, the ring with the attached Delrin cylinder (proximal dynamic ring) was connected to another 150-mm diameter aluminum empty ring (distal static ring) at 150 mm using four 6-mm diameter threaded rods. In the other two models, proximal and distal rings were interconnected in the similar manner either directly through four threaded rods without dynamization modules (no dynamization [ND]) or using threaded rods with dynamizers [Figure 2]. Two types of dynamization modules were utilized including (1) aluminum Spring-Loaded Dynamization Module (SLDM) or (2) custom-made three dimensional (3D) printed plastic Flex-Disk Dynamization Module (FDDM). Both modules provided for 3.0 mm of maximal axial motion. All described constructs resulted in seven different frame configurations for biomechanical testing including:
- 2WR1HP, spring-loaded dynamization module (2WR1HP-SLDM)
- One wire and two half pins, ND (1WR2HP-ND)
- 1WR2HP, spring-loaded dynamization module (1WR2HP-SLDM)
|Figure 2: Three types of dynamization options between the rings: (a) Four threaded rods provided no dynamization. (b) Four aluminum spring-loaded TrueLok dynamization modules with 3 mm of available axial motion. (c) Four custom-made three-dimensional printed plastic flex-disk dynamization modules with 3 mm of available axial motion|
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In each frame construct, the distal static ring was securely fastened to the loading platform of a servo-electric material testing machine (Bose Electro Force AT3330, TA Instruments, Eden Prairie, MN, USA). To measure the axial, coronal plane, and sagittal plane displacements, the load was applied centrally to the top of the Delrin cylinder through a spherical Delrin ball in a sinusoidal waveform from 0.5 kg to 50 kg for 11 cycles at a rate of 0.1 Hz [Figure 3]. The motion of the distal end of the Delrin cylinder was tracked using a 3D motion capture system (Optotrak, Northern Digital Inc., Ontario, Canada) with a six degrees of freedom marker attached to the bottom of the cylinder recording axial, coronal plane, and sagittal plane displacements along with the applied load. Control of ring deformation was achieved by a second marker directly attached to the proximal dynamic ring.
|Figure 3: Positioning of the construct in the servo-electric material testing machine for the measurement of axial, coronal plane, and sagittal plane displacement under longitudinal loading|
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The trajectories (displacement vs. load) of Delrin cylinder motion were plotted using the Microsoft Excel software. All curves within the groups were consistent without variation. Maximum displacement at 50 kg and slopes for each curve, which describes the stiffness of the construct, were calculated to allow better comparison between different models. Within each plane of motion, the amount of displacement for each model was compared to the other six models using the one-way analysis of variance testing followed by post hoc Tukey's test. Differences were considered statistically significant when P < 0.05.
| Results|| |
The correlation between the axial, coronal plane, and sagittal plane displacements and amount of axial loading as well as associated trajectories of motion is displayed in [Figure 4],[Figure 5], [Figure 6], [Figure 7] and [Table 1], [Table 2], [Table 3]. Multiple comparisons revealed the P value for each pair were <0.05.
|Figure 4: Axial load-displacement curves for tested constructs during 50 kg of gradual axial loading|
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|Figure 5: Coronal plane load-displacement curves for tested constructs during 50 kg of gradual axial loading|
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|Figure 6: Sagittal plane load-displacement curves for tested constructs during 50 kg of gradual axial loading|
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|Figure 7: Axial versus sagittal plane displacement ration curves for tested constructs|
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|Table 1: Axial, coronal-plane, and sagittal plane displacements (mm) at 50 kg of axial loading|
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|Table 2: Axial displacement slope (mm/kg) during gradually increasing axial loading|
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|Table 3: Axial vs. Sagittal displacement ratio slope during axial loading|
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The reference 3WR-ND construct demonstrated proportional increase in the axial displacement due to elastic deformation of wires reaching 3.9 mm at 50 kg of loading with a slope of 0.076 mm/kg [Figure 4] and [Table 1]. Both nondynamized half-pin constructs (2WR1HP-ND and 1WR2HP-ND) demonstrated a similar trajectory of movement as that in the 3WR model and showed a noticeable decrease in motion with a slope of 0.069 mm/kg and 0.037 mm/kg, respectively, and less axial displacement throughout all levels of loading reaching 3.5 mm and 1.9 mm of maximal displacement, respectively.
The addition of dynamization modules increased axial motion as compared to nondynamized 3WR and both half-pin models. Furthermore, while the curves for those nondynamized models appear as a straight line, the constructs with dynamization modules showed a transition point with two distinct curves. During the initial loading, the dynamized constructs show a curve with an increased slope (0.15–0.17 mm/kg and 0.27–0.29 mm/kg for spring-loaded and flex-disk dynamizers, respectively) as compared to the 0.04–0.08 mm/kg slopes of nondynamized constructs [Table 2] indicating more axial displacement achieved with dynamization modules at the same amount of applied force. With continuous loading after the inflection point, however, the dynamized models have linear curves that are almost parallel to those of their nondynamized counterpart. This is further confirmed by the similar slopes in dynamized and nondynamized constructs after the inflection point within the same wire/half pin configuration groups ranging from 0.06 mm/kg to 0.07 mm/kg and from 0.04 mm/kg to 0.06 mm/kg for 1WR2HP and 2WR1HP models, respectively, and reaching from 5.4 mm to 6.8 mm of maximal displacement at 50 kg of loading.
When comparing the displacement trajectory in two different dynamization modules, the flex-disk plastic dynamizers initially provided more axial displacement than the spring-loaded aluminum dynamizers at the same level of axial loading. Spring-loaded dynamizers, for example, required 20 kg of force to produce 3.0 mm of axial displacement while the flex-disk dynamizers allowed for the same amount of displacement at 10 kg of force.
Coronal plane motion
Six of the seven tested constructs demonstrated minimal displacement of the Delrin cylinder in coronal plane throughout the 50 kg of applied axial loading [Figure 5] and [Table 1]. There was no difference in displacement trajectories between the reference 3WR construct as well as nondynamized and dynamized half-pin models with maximal displacement <1 mm ranging between 0.11 mm and 0.60 mm at 50 kg of loading. The sole exception was the 2WR1HP-FDDM model, which showed an initial displacement of 1 mm at 15 kg and maximum displacement of 2 mm at 50 kgs of loading.
Sagittal plane motion
In contrast to the coronal plane, axial loading resulted in noticeable anterior translation of Delrin cylinder in the sagittal plane in all tested constructs [Figure 6] and [Table 1]. Minimal displacement was observed in reference 3WR model reaching 2.3 mm at 50 kg of loading. The use of half-pins when compared to the 3WR construct increased anterior displacement in the sagittal plane. Moreover, loading of nondynamized construct with 2 half pins (1WR2HP-ND) showed a higher maximum displacement of 5.64 mm than the 2WR1HP-ND construct (4.71 mm).
The addition of dynamization modules produced minimal overall changes to sagittal-plane displacement. Those changes depended on the wire/half-pin configuration in the construct as well as the type of dynamization module. For 2WR1HP construct, for example, there were no changes in the amount of anterior displacement with spring-loaded aluminum dynamizers while flex-disk plastic dynamizers produced a 0.3-mm decrease in maximal displacement at 50 kg of loading. The addition of dynamizers for the 1WR2HP construct produced an opposite effect. Spring-loaded dynamizers caused a mild 0.48 mm decrease in final anterior displacement while flex-disk dynamizers produced a 0.8-mm increase at maximal loading.
Axial versus sagittal displacement
The correlation between the amount of anterior displacement and axial displacement at the same levels of loading is demonstrated in [Figure 7] and [Table 3]. Loading of the reference 3WR construct resulted in 0.60 mm/kg slope with gradual increase in sagittal-plane displacement proportional to the increase in the amount of axial displacement reaching 2.3 mm of maximal anterior displacement at 3.9 mm of axial motion.
The addition of half pins to the nondynamized constructs caused more vertical slopes (2.99 mm/kg and 1.38 mm/kg for 1WR2HP-ND and 2WR1HP-ND models, respectively) with a significant increase in the amount of anterior displacement at the same level of axial motion. For 1 mm of axial motion, for example, the 3WR construct produced 0.5 mm of anterior displacement, while for the 1WR2HP-ND and 2WR1HP-ND half-pin models, it resulted in 1.5 mm and 3 mm of anterior movement, respectively. A similar tendency was observed for 2 mm of axial motion producing 1 mm, 2.5 mm, and 5.5 mm of anterior displacement for the 3WR, 1WR2HP-ND, and 2WR1HP-ND constructs, respectively.
The addition of dynamization modules to the half-pin constructs significantly reduced amount of anterior displacement as compared to nondynamized models. Slopes for spring-loaded dynamizers were similar to each other ranging between 0.52 mm/kg and 0.60 mm/kg and almost completely overlapping with the 0.60 mm/kg slope of the 3WR construct. Flex-disk plastic dynamizers further reduced the amount of anterior displacement, demonstrating more horizontal slopes for both half-pin constructs ranging between 0.16 mm/kg and 0.23 mm/kg. The reduction of anterior displacement in the dynamized constructs was especially noticeable within the 3 mm axial working limit of the dynamization modules. Those 3 mm of initial axial motion produced only 1.5–2.0 mm of anterior displacement for spring-loaded dynamizers while anterior displacement for flex-disk dynamizers was <1 mm. Continuous axial displacement in dynamized constructs resulted in a gradual increase in the amount of anterior displacement similar to that in nondynamized models producing two distinct curves with an inflexion point around-3–4 mm.
| Discussion|| |
To our knowledge, this is the first report on the biomechanics of the 3D displacement of a simulated bone fragment in a dynamizable circular external fixator model during the application of axial loading. We were able to evaluate axial motion of the segment, document displacement in coronal and sagittal planes, and demonstrate how that displacement changes with the use of two different dynamization modules. We have confirmed that additional fixation through the use of half pins does indeed increase stability but only in the axial direction.
Previous literature has similarly found that axial compression decreased with additional fixation., However, in the current study, the utilization of dynamizers allowed for substantially more axial displacement than occurred in either the nondynamized half pin or all wire constructs. Domb et al. proposed that dynamization increases the force seen at the fracture site leading to more rapid bone callus formation and remodeling. Earlier research also appreciated the clinical importance in evaluating frame constructs.,, Richardson et al. reported that unlocking an external fixator used to treat tibia fractures created progressive movement that closed the fracture gap. Goodship and Kenwright forced axial micromotion 17 min a day in goats with a tibial osteotomy and frame. This led to an earlier radiographic appearance of callus and increased torsional stiffness. Claes et al. used external fixators with and without a telescopic system on sheep metatarsal osteotomies. Dynamization increased callus width by 70% and tensile strength by 45% and decreased the occurrence of fibrous cartilage at the osteotomy site. Furthermore, they found that the tibias that maintained the most interfragmentary movement throughout the dynamization process had the most callus. In a clinical study, Kenwright et al. evaluated tibial fractures treated with either dynamized or nondynamized frames. Not only was there a long callus that developed around the fracture in the dynamized group but that group also had a shorter time to unsupported weight bearing and healing.
Given that both types of dynamization modules in this study only allow for maximum of 3 mm of motion, it is not surprising that we found a change in axial displacements between 2.6 and 4.7 mm. This range of inflection points suggests that even while the dynamizers are displacing, there continues to be some amount of motion from the elastic deformation of wires and half pins themselves. However, once the dynamizers have reached their limit, displacement then comes entirely from the fixation elements. This is demonstrated by the slope being more similar to the nondynamized frames and the curves are more parallel to each other. We do not know of any other study that has demonstrated a change in the displacement moment in dynamized frames.
The motion was minimal in the coronal plane with all constructs. Given that one wire was maintained fully across the frame in this plane, it is expected that any cantilever bending in that place would be controlled.
Although axial motion decreased with enhanced fixation, sagittal-plane motion actually increased with the addition of half pins. In fact, the two half-pin constructs had the least axial displacement but one of the highest levels of anterior displacement. There was not a significant change in the anterior displacement with the utilization of dynamizers, especially at the maximum force applied. Previous research, demonstrated that for bending motion to decrease, additional fixation had to be in the plane of bending. Lewis et al. confirmed that half pins can cause cantilever bending with axial load. The appearance of cantilever motion is concerning when applied clinically. Soft-tissue irritation and pin-site infection may still occur as result of pin motion. The constant angular force on the half pins may result in an angular deformity and resultant malunion. Domb et al. in a study of pediatric femoral fractures treated with external fixators randomized with or without dynamization found that dynamization decreased varus/valgus angulation. In the current study, at the initiation of force, there is an initial slight reduction in the displacement as compared to nondynamized models. We cannot be sure if this would also be enough to limit procurvatum deformity often seen in circular frames during tibial lengthening. Therefore, further research to evaluate a change in the rate and degree of procurvatum after introduction of dynamization would be useful.
What was particularly significant in the current study is the correlation between axial motion and anterior displacement for each construct. The use of half pins in nondynamized models significantly increases the amount of anterior displacement per axial displacement as compared to the traditional three-wire fixation. With the utilization of dynamization modules in half-pin models, amount of anterior translation per same degree of axial displacement reduced significantly as compared to nondynamized constructs. It was especially noticeable in 1WR2HP fixation where 1.5 mm of axial motion resulted in 5.5 mm of anterior displacement in the nondynamized model dropped to 0.5–0.75 mm with the addition of dynamization modules; thus, it stayed well below the 3 mm compression working limit of the dynamizers. A similar tendency was observed in the 2WR1HP fixation with reduction of anterior displacement from 4.5 mm at 3 mm of axial motion to 0.5–2.0 mm. Obviously, spring-loaded aluminum dynamization modules still resulted in the larger anterior displacement similar to the traditional 3WR construct while the reduction of anterior displacement with flex-disk plastic dynamizers was more dramatic and less than 1 mm for both half pin fixation models. However, continuous axial displacement beyond 3 mm in dynamized constructs resulted in gradual increase in the amount of anterior displacement similar to that in nondynamized models.
We do recognize the limitations of this study. First, the Delrin plastic cylinders were used to simulate bone segments. Real bone structure is different, potentially resulting in different response to the same amounts of loading. Moreover, the addition of muscles and other soft tissues surrounding the bone could further deviate our measurements changing overall displacements and the inflection that may occur at a different amount of loading. In addition, fixation of the Delrin cylinder was achieved in a standard 150 mm diameter ring using either traditional three thin, tensioned wires or a combination of wire(s) and half pin(s). The use of smaller or larger ring sizes as well as constructs with more half pins may be more or less resistant to axial loading affecting axial motion, displacements in coronal and sagittal planes, and overall effect of dynamization modules on those displacements. Finally, the tested constructs were exposed to gradually increasing pure axial loading. The combination of axial and lateral loading in more common clinical settings (e.g., tripping over a step or jumping) could potentially result in different reaction compared to our models. However, although it is difficult to extrapolate clinical results from a biomechanical study, we do feel that our findings are applicable to the clinical implementation of dynamization in circular external fixation. We are currently working on a patient cohort study to better evaluate the clinical outcomes of axial micromotion using dynamization modules.
| Conclusions|| |
Circular external fixation constructs provide axial motion for attached bone segment under the application of a longitudinal loading due to elastic deformation of fixation components. The amount of axial motion decreases with replacement of thin tensioned wires by more rigid half pins requiring higher forces to achieve the desired amount of axial displacement. The addition of dynamization modules with a preset amount of axial displacement allows the targeted amount of axial motion at significantly reduced levels of loading. In addition, dynamization modules do not affect coronal-plane displacements but significantly reduce anterior displacements of bone segments present during axial loading in nondynamized constructs.
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Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7]
[Table 1], [Table 2], [Table 3]