Comparison of torsional properties between a Fixateur Externe du Service de Santé des Armées and an acrylic tie-in external skeletal fixator in a red-tailed hawk (Buteo jamaicensis) synthetic tibiotarsal bone model

Rebecca A. Hersh-Boyle 1Veterinary Medical Teaching Hospital, University of California-Davis, Davis, CA 95616.

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Amy S. Kapatkin 2Department of Surgical and Radiological Sciences, University of California-Davis, Davis, CA 95616.

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Tanya C. Garcia 2Department of Surgical and Radiological Sciences, University of California-Davis, Davis, CA 95616.

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Duane A. Robinson 2Department of Surgical and Radiological Sciences, University of California-Davis, Davis, CA 95616.

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David Sanchez-Migallon Guzman 3Department of Medicine and Epidemiology, University of California-Davis, Davis, CA 95616.

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Shannon M. Kerrigan 1Veterinary Medical Teaching Hospital, University of California-Davis, Davis, CA 95616.

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Po-Yen Chou 2Department of Surgical and Radiological Sciences, University of California-Davis, Davis, CA 95616.

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Susan M. Stover 2Department of Surgical and Radiological Sciences, University of California-Davis, Davis, CA 95616.

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Abstract

OBJECTIVE

To compare the torsional mechanical properties of 2 external skeletal fixators (ESFs) placed with 2 intramedullary pin (IP) and transfixation pin (TP) size combinations in a model of raptor tibiotarsal bone fracture.

SAMPLE

24 ESF-synthetic tibiotarsal bone model (polyoxymethylene) constructs.

PROCEDURES

Synthetic bone models were fabricated with an 8-mm (simulated fracture) gap. Four types of ESF-synthetic bone model constructs (6/group) were tested: a FESSA with a 1.6-mm IP and 1.6-mm TPs, a FESSA with a 2.0-mm IP and 1.1-mm TPs, an acrylic connecting bar with a 1.6-mm IP and 1.6-mm TPs, and an acrylic connecting bar with a 2.0-mm IP and 1.1-mm TPs. Models were rotated in torsion (5°/s) to failure or the machine angle limit (80°). Mechanical variables at yield and at failure were determined from load deformation curves. Effects of overall construct type, connecting bar type, and IP and TP size combination on mechanical properties were assessed with mixed-model ANOVAs.

RESULTS

Both FESSA constructs had significantly greater median stiffness and median torque at yield than both acrylic bar constructs; FESSA constructs with a 1.6-mm IP and 1.6-mm TPs had greatest stiffness of all tested constructs and lowest gap strain at yield. No FESSA constructs failed during testing; 7 of 12 acrylic bar constructs failed by fracture of the connecting bar at the interface with a TP.

CONCLUSIONS AND CLINICAL RELEVANCE

Although acrylic bar ESFs have been successfully used in avian patients, the FESSA constructs in this study were mechanically superior to acrylic bar constructs, with greatest benefit resulting from use with the larger TP configuration.

Abstract

OBJECTIVE

To compare the torsional mechanical properties of 2 external skeletal fixators (ESFs) placed with 2 intramedullary pin (IP) and transfixation pin (TP) size combinations in a model of raptor tibiotarsal bone fracture.

SAMPLE

24 ESF-synthetic tibiotarsal bone model (polyoxymethylene) constructs.

PROCEDURES

Synthetic bone models were fabricated with an 8-mm (simulated fracture) gap. Four types of ESF-synthetic bone model constructs (6/group) were tested: a FESSA with a 1.6-mm IP and 1.6-mm TPs, a FESSA with a 2.0-mm IP and 1.1-mm TPs, an acrylic connecting bar with a 1.6-mm IP and 1.6-mm TPs, and an acrylic connecting bar with a 2.0-mm IP and 1.1-mm TPs. Models were rotated in torsion (5°/s) to failure or the machine angle limit (80°). Mechanical variables at yield and at failure were determined from load deformation curves. Effects of overall construct type, connecting bar type, and IP and TP size combination on mechanical properties were assessed with mixed-model ANOVAs.

RESULTS

Both FESSA constructs had significantly greater median stiffness and median torque at yield than both acrylic bar constructs; FESSA constructs with a 1.6-mm IP and 1.6-mm TPs had greatest stiffness of all tested constructs and lowest gap strain at yield. No FESSA constructs failed during testing; 7 of 12 acrylic bar constructs failed by fracture of the connecting bar at the interface with a TP.

CONCLUSIONS AND CLINICAL RELEVANCE

Although acrylic bar ESFs have been successfully used in avian patients, the FESSA constructs in this study were mechanically superior to acrylic bar constructs, with greatest benefit resulting from use with the larger TP configuration.

Avian long bone fractures are commonly repaired with an IP tied into an ESF.1 The combination of an IP and ESF has been shown to be mechanically superior to other described ESF techniques.2,3 Methacrylate-based resin (acrylic) connecting bars have been traditionally used, but more recently, FESSAs have been successfully used for long bone fracture repair in small animals4 and birds.5,6

A FESSA is a lightweight, reusable stainless steel connecting bar that was developed by the French Armed Forces for use in repairing fractures of the hands and feet of people.7 Compared with acrylic connecting bars, FESSAs provide additional benefits, including ease of dynamic destabilization and the ability to make intraoperative and postoperative adjustments. Although studies2,3 have been performed to evaluate the biomechanical properties of an ESF with an IP tie-in configuration, to the authors' knowledge, no studies published to date have compared the biomechanical properties of a FESSA system with those of other constructs commonly used for the repair of bone fractures in raptors.

The purpose of the study reported here was to compare the torsional biomechanical properties of 2 ESF constructs created with FESSAs or acrylic connecting bars and placed with 2 IP and TP size combinations in a synthetic red-tailed hawk (Buteo jamaicensis) tibiotarsal bone model as a preliminary step in determining the optimal construct for repair of a common fracture in raptors. The null hypothesis was that the torsional properties would not differ between constructs that included a FESSA and those that included an acrylic connecting bar when used with 2 IP and TP combinations.

Materials and Methods

Preliminary experiments

One construct that comprised an acrylic connecting bara with a 2.0-mm IPb and two 1.1-mm TPsc and 1 construct that comprised a FESSAd with a 2.0-mm IPb and two 1.1-mm TPsc were tested in preliminary experiments with 1 pair of red-tailed hawk tibiotarsal bones. Tibiotarsal bones were from adult red-tailed hawks that were brought in for euthanasia for injuries unrelated to the pelvic limbs. The bones were radiographed to rule out orthopedic disease. An 8-mm gap was created in each tibiotarsal bone at 55% of its length to simulate a comminuted fracture. Each ESF-IP-bone construct was created as described for the synthetic bone model experiments and loaded in torsion to failure with the described mechanical testing methods.

Synthetic bone models

Three pairs of red-tailed hawk tibiotarsal bones were used to determine the size and shape of the synthetic bone models used in remaining experiments. Mean measurements of the 6 bones at the proximal and distal aspects were obtained in the cranial-to-caudal and medial-to-lateral directions to determine the diameter for the model. Bone length was measured from the tibial crest proximally to the most distal aspect of the tibiotarsal-tarsal-metatarsal joint surface. Diaphyseal diameter was measured at the mid-diaphysis in both craniocaudal and mediolateral directions. Medullary cavity diameter was measured as the mean of both radiographic views at mid-diaphyseal length.

A 12.7-mm-diameter solid synthetic (polyoxymethylene) modele of a standard available size was selected for use in the study because it approximated the largest diameter measured in intact bone. The model was divided into a proximal 48-mm segment and a distal 59-mm segment with an 8-mm gap to simulate a common location for a comminuted fracture in raptor tibiotarsal bones (Figure 1)8 A 6.4-mm-diameter hole was drilledf along the length of both segments of the model to replicate a medullary cavity, with 5 mm of the solid model retained at the most proximal and distal aspects of the model to allow placement of an IP through simulated cortical bone and the drilled canal. A 1.32-mmg or 1.73-mm drill bith was used to create a hole in the remaining solid proximal and distal ends of the bone model, depending on the size of the IP selected. The predrilled entry and exit sites for the simulated medullary cavity were created so that use of the model would mimic surgical pin placement, including resistance during IP insertion into a raptor tibiotarsal bone.

Figure 1—
Figure 1—

Photographs depicting stepwise assembly of an acrylic bar ESF-synthetic bone model construct in a study to compare the torsional mechanical properties of 2 ESFs created with FESSAs or acrylic connecting bars and placed with 2 IP and TP size combinations in a model of raptor tibiotarsal bone fracture. A—A synthetic tibiotarsal bone model was used with an 8-mm gap that simulated a fracture and created proximal (48-mm) and distal (59-mm) segments. B—An IP was placed with a Jacobs chuck and pneumatic drill into and through the predrilled simulated medullary cavity from the proximal to the distal end. C—The IP was bent to a 90° angle 15 mm from the proximal end of the proximal fragment. D—A custom drill guide was applied and attached to the bent end of the IP to enable appropriate hole placement for TPs. E—A custom TP insertion guide was applied, and 2 TPs (1/fragment) were inserted. F—To create the acrylic bar, a manufacturer-supplied corrugated tube was taped to a FESSA bar to maintain straight configuration; the pins were placed through the tube, and the tube was filled with a prepared acrylic mixture by use of an applicator gun and accessories.

Citation: American Journal of Veterinary Research 81, 7; 10.2460/ajvr.81.7.557

Construct design

A FESSAd and an acrylic connecting bara were selected for use with synthetic bone models in the study constructs. The IPs used were smooth, trocartipped pinsb of 2 diameters: 1.6 and 2.0 mm. The TPs used were partially threaded positive profile pinsc of 2 diameters: 1.1 and 1.6 mm.

Step-by-step creation of an acrylic bar ESF construct is shown (Figure 1). A custom aluminum jig was applied to the model to maintain an 8-mm gap between the proximal and distal bone model fragments and to retain alignment while the IP and 2 TPs were placed. The IP was inserted from the proximal to the distal end of the model by use of a pneumatic drilli with a Jacobs chuck attachment.j The IP was bent to a 90° angle with a Kirschner wire benderk 15 mm from the proximal end of the proximal fragment. A custom aluminum drill guide was positioned onto the IP to standardize hole placement for the TPs at the center of the bone model segment that corresponded to the location that would be chosen in a clinical case with a 1.0-mm drill bit.l The drill guide was removed, and a custom aluminum pin guide was applied to the model to standardize insertion of the TPs. The TPs were inserted into the predrilled holes by use of a pneumatic drilli with a Jacobs chuckj attachment. The tube used to create the acrylic connecting bar was then placed parallel to and 10 mm away from the model.

The acrylic bar was assembled by taping a corrugated tubea with an inner diameter of 6 mm to a FESSA to maintain a straight configuration (Figure 1). The acrylic was prepared according to the manufacturer's instructions.9 The liquid monomer was added to the powder at a 2:1 (vol:vol) ratio at room temperature. The bent end of the IP and the TPs were positioned through the tube, and a rubber stopper was applied to the proximal aspect of the tube. The tube was filled from the distal-to-proximal direction with the acrylic mixture by use of an applicator gun and accessories.a The FESSA used to support the acrylic bar was removed when the acrylic hardened, approximately 12 minutes after mixing. The final outer diameter of the acrylic-filled tube was 7.97 mm. Completed acrylic bar constructs were radiographedm,n in a craniocaudal direction (ie, with the acrylic support tubes oriented lateral to the bone model) prior to mechanical testing to assess completeness of the acrylic filling (Figure 2)

Figure 2—
Figure 2—

Composite image of 3 representative radiographs of acrylic bar ESF-synthetic bone model constructs. Notice filling defects within the acrylic columns (arrowheads).

Citation: American Journal of Veterinary Research 81, 7; 10.2460/ajvr.81.7.557

For the FESSA constructs, assembly was as described for acrylic constructs except that the corrugated tube and injection of acrylic were not used. The FESSA bar was placed as the connecting bar at the same distance from the model as for the acrylic bar with the TPs placed and secured in the holes that matched with the pin placements (Figure 3)

Figure 3—
Figure 3—

Composite photographic image depicting examples from the 4 ESF-synthetic bone model construct groups that underwent torsional mechanical testing. Groups were defined by the connecting bar type and pin size combinations used. A—A FESSA connecting bar with a 1.6-mm IP and 1.6-mm TPs. B—A FESSA connecting bar with a 2.0-mm IP and 1.1-mm TPs. C—An acrylic connecting bar with a 1.6-mm IP and 1.6-mm TPs. D—An acrylic connecting bar with a 2.0-mm IP and 1.1-mm TPs.

Citation: American Journal of Veterinary Research 81, 7; 10.2460/ajvr.81.7.557

Construct groups

Four groups of 6 ESF constructs were prepared for testing (Figure 3). The groups were defined by the type of connecting bar, the IP diameter, and the diameter of the 2 TPs. The construct types included a FESSA with a 1.6-mm IP and 1.6-mm TPs, a FESSA with a 2.0-mm IP and 1.1-mm TPs, an acrylic connecting bar with a 1.6 mm-IP and 1.6-mm TPs, and an acrylic connecting bar with a 2.0-mm IP and 1.1-mm TPs.

Mechanical testing

The synthetic bone model and ESFs were lightly sprayed with black spray paint, and video-tracking markerso were gluedp to the constructs. The video-tracking markers were glued on the distal tip of the IP, on the acrylic rod or FESSA at the level of the IP and each TP, on the bone model at the level of each TP, and on the IP at the midpoint of the gap between the proximal and distal bone model fragments (Figure 4)

Figure 4—
Figure 4—

Photograph of a FESSA-synthetic bone model construct attached to a servohydraulic mechanical testing system for torsional testing. The construct was sprayed with black spray paint, and white video tracking balls were applied to predetermined areas of the construct.

Citation: American Journal of Veterinary Research 81, 7; 10.2460/ajvr.81.7.557

The proximal and distal segments of the synthetic bone model in each construct were fixed into a custom jig attached to a servohydraulic mechanical testing systemq (Figure 4) at a distance of 1.0 cm from each end. This aligned the longitudinal axis of the model with the center of rotation of the mechanical testing system.

Each construct was axially loaded to 5 N, approximately 50% of the mean body weight of an adult (skeletally mature) red-tailed hawk.8 Axial displacement was held constant while the bone was rotated in torsion at 5°/s to failure or to a maximum of 80° (the limit of the machine's rotational capability). Torque and angular actuator displacement data were acquired at 128 Hz. Video recordings were obtained at 60 frames/s with 2 high-resolution cameras.r A single frame of video captured a 3-D calibration cube (10 × 10 × 10 cm) of 8 markers. Measurement of marker movement in the laboratory was assessed by comparing video-captured motion to the mechanical testing system's displacement transducer with each use and was found to be accurate within a 0.01-mm margin of error.

Data reduction

The motion of the tracking markers on the pins and on the bone model was digitized, tracked, and converted to 3-D space in kinematic software.s The torsion angle of the bone model and pins was calculated from the 3-D angle projected in the cross-sectional plane. The gap rotational angle was defined as the angle between the proximal piece of the bone model and the distal piece of the bone model at the gap as assessed by use of the video-tracking markers. The construct angle was defined as the angle between the most proximal part of the construct (at the machine attachment) and the most distal part of the construct (at the machine attachment). The gap shear strain was calculated as bone model torsion angle × bone model radius/gap length.

Construct torque was plotted against construct angular deformation (grip-to-grip). A yield point Y was defined as a change in stiffness following a linear elastic region at low torque (< 500 N·mm). Failure was defined as a ≥ 60% decrease in torque. The angle limit was defined as 80° of rotation. Preyield stiffness was derived as the slope of the least squares linear regression fit through the middle third of the data between the start of the test and the yield point. Torque at yield, torque at failure, and angle values were determined from measurements at the respective yield and failure points. Yield and failure cumulative energies were calculated from the integral under the curve to the yield and failure points, respectively.

Statistical analysis

Differences in mechanical properties between connecting bar types (FESSA or acrylic) and pin size combinations (1.6-mm IP and 1.6-mm TPs or 2.0-mm IP and 1.1-mm TPs) were assessed with mixed-model ANOVA.t Group construct comparisons were assessed with type 3 fixed effects from the ANOVA. The interaction between connecting bar type and pin sizes was included as a fixed effect. Normality of variable distributions and of the residuals for ANOVA was assessed with the Shapiro-Wilk test. When the data were not normally distributed, a rank transformation was performed, and the same mixed-model ANOVA was performed. Values of P < 0.05 were considered significant for all comparisons.

Results

Preliminary experiments

In experiments with 1 pair of cadaveric red-tailed hawk bones, testing of both the FESSA and acrylic bar ESFs with a 2.0-mm IP and 1.1-mm TPs resulted in bending of the TPs at the 80° torsion angle. None of the models showed signs of bone failure.

Mean ± SD measurements for the proximal diameter of the 6 cadaveric red-tailed hawk bones used for preliminary experiments were 6 ± 0.9 mm in the cranial-to-caudal direction and 10 ± 1.9 mm in the medial-to-lateral direction. Mean ± SD measurements for the distal diameter were 7 ± 0.8 mm in the cranial-to-caudal direction and 5 ± 0.2 mm in the lateral-to-medial direction. Mean ± SD medullary cavity diameter at mid-diaphysis was 4.7 ± 0.5 mm.

Radiographic evaluation of acrylic bar constructs

Air bubble defects were present in all acrylic connecting bars. The number, locations, and sizes of defects varied. The defect sizes were ≤ 2 mm in all acrylic bars, and the number of defects detected ranged from 2 to 20/bar.

Mechanical testing

On comparison of the 4 construct groups, both FESSA constructs had significantly greater stiffness and median torque at yield than both acrylic bar constructs (Figure 5; Table 1). The FESSA connecting bar constructs with a 1.6-mm IP and 1.6-mm TPs had significantly greater median stiffness and significantly lower median gap shear strain except when compared with the acrylic bar constructs that included a 2.0-mm IP and 1.1-mm TPs at yield.

Figure 5—
Figure 5—

Representative torque-actuator angle deformation curves (mean values) for the 4 ESF-synthetic bone model construct groups (a FESSA connecting bar with a 1.6-mm IP and 1.6-mm TPs [thin black line], a FESSA connecting bar with a 2.0-mm IP and 1.1-mm TPs [thick black line], an acrylic connecting bar with a 1.6-mm IP and 1.6-mm TPs [thin gray line], and an acrylic connecting bar with a 2.0-mm IP and 1.1-mm TPs [thick gray line]) during torsional mechanical testing to failure (n = 6/group). The yield point (arrowheads) was defined as a sharp, short decrease in torque when low torque (ie, 19% to 46% of maximum torque) was present. The failure point was defined as a ≥ 60% decrease in torque. Six of 6 constructs composed of an acrylic connecting bar with a 1.6-mm IP and 1.6-mm TPs failed. One of 6 constructs composed of an acrylic connecting bar with a 2.0-mm IP and 1.1-mm TPs failed.

Citation: American Journal of Veterinary Research 81, 7; 10.2460/ajvr.81.7.557

Table 1—

Comparison of mechanical properties obtained at yield during torsional mechanical testing for 4 ESF-synthetic bone model construct groups (n = 6/group).

 Construct groupP value (intergroup comparison)
VariableFESSA, 1.6-mm IP, 1.6-mm TPs (1)FESSA, 2.0-mm IP, 1.1-mm TPs (2)Acrylic bar, 1.6-mm IP, 1.6-mm TPs (3)Acrylic bar, 2.0-mm IP, 1.1-mm TPs (4)1 vs 23 vs 41 vs 32 vs 41 vs 42 vs 3
Construct angle (°)6.02 (3.26–9.61)10.31 (6.8–15.2)5.45 (5.11–8.61)6.05 (4.46–7.09)0.0120.7390.0060.9620.0140.775
Torque (N·m)0.32 (0.18–0.51)0.39 (0.25–0.50)0.18 (0.13–0.26)0.16 (0.12–0.21)0.3080.308< 0.0010.0100.0010.001
Stiffness (N·m/°)0.05 (0.05–0.06)0.04 (0.03–0.04)0.03 (0.03–0.04)0.03 (0.03–0.03)0.0040.4290.003< 0.0010.018< 0.001
Gap shear strain (°·mm/mm)3.75 (1.67–6.98)13.22 (6.08–25.46)9.74 (7.14–15.02)5.2 (3.71–7.94)< 0.0010.0030.004< 0.0010.9020.184
Gap rotation (°)4.73 (2.1–8.79)16.65 (7.66–32.07)11.86 (9.00–18.92)6.55 (4.67–10)< 0.0010.0030.004< 0.0010.9020.184

Data are shown as median (range). Synthetic tibiotarsal bone models (12.7-mm diameter with a 6.4-mm-diameter simulated medullary cavity) were divided into a proximal 48-mm segment and distal 59-mm segment separated by an 8-mm gap to simulate a fracture. Each construct was loaded in torsion at 5°/s in rotation under actuator displacement control and axially loaded to 5 N. Axial actuator displacement was held constant while the construct rotated to failure or to a maximum angle of 80°, and torque and angular actuator displacement data were acquired at 128 Hz throughout the test. All constructs reached a yield point prior to failure or the end of the test.

Values of P < 0.05 were considered significant.

None of the 6 FESSA constructs failed prior to reaching the maximum rotational angle of 80°, whereas 6 of 6 acrylic bar constructs with a 1.6-mm IP and 1.6-mm TPs failed before this point (Figure 5). Five of the 6 latter constructs had failure of the acrylic bar at the interface with the proximal TP (Figure 6) The remaining acrylic bar construct in this group had failure of the acrylic bar at the interface with the distal TP. At failure, acrylic bar constructs with a 1.6-mm IP and 1.6-mm TPs had a median construct angle of 31.07° (range, 23.52° to 65.85°), median torque of 0.63 N·m (range, 0.58 to 1.03 N·m), median gap shear strain of 48.82°·mm/mm (range, 1.59° to 61.18°·mm/mm), and median stiffness of 0.02 N·m/° (range, 0.01 to 0.02 N·m/°). One acrylic bar construct with a 2.0-mm IP and 1.1-mm TPs had failure of the acrylic bar at the interface with the proximal TP prior to reaching the maximum angle of 80°; this construct had a construct angle of 61.4°, torque of 0.46 N·m, gap shear strain of 71.30°·mm/mm, and stiffness of 0.004 N·m/°.

Figure 6—
Figure 6—

Representative photographs (orthogonal views) showing deformation after testing of an acrylic bar construct with a 1.6-mm IP and 1.6-mm TPs at failure. Failure of the acrylic bar occurred at the interface with the proximal TP. A—Cranial-caudal view of the construct. B—Lateral-medial view of the construct.

Citation: American Journal of Veterinary Research 81, 7; 10.2460/ajvr.81.7.557

The statistical model adjusted for pin size combination revealed that the median torque and median stiffness at yield were significantly greater for FESSA constructs than for acrylic bar constructs (Table 2). Comparison of the performance of the 2 IP and TP combinations in the statistical model adjusted for connecting bar type revealed that 1.6-mm IPs with 1.6-mm TPs had significantly greater stiffness at yield than 2.0-mm IPs with 1.1-mm TPs. There was no significant difference in median gap shear strain between connecting bar types or between the 2 IP and TP size combinations.

Table 2—

Comparison of mechanical properties obtained at yield during torsional mechanical testing for the 24 constructs in Table 1 on the basis of connecting bar type and pin size combination.

 Connecting bar type Pin size combination 
VariableFESSAAcrylicP value2.0-mm IP with 1.1-mm TPs1.6-mm IP with 1.6-mm TPsP value
Construct angle (°)7.95 (3.26–15.2)5.87 (4.46–8.61)0.0196.94 (4.46–15.2)5.45 (3.26–9.61)0.033
Torque (N·m)0.36 (0.18–0.51)0.17 (0.12–0.26)0.0010.23 (0.12–0.5)0.21 (0.13–0.51)0.672
Stiffness (N·m/°)0.05 (0.03–0.06)0.03 (0.03–0.04)< 0.0010.03 (0.03–0.04)0.04 (0.03–0.06)< 0.001
Gap shear strain (°·mm/mm)6.36 (1.67–25.46)7.7 (3.71–15.02)0.6357.04 (3.71–25.46)7.06 (1.67–15.02)0.164
Gap rotation (°)8.01 (2.1–32.07)9.7 (4.67–18.92)0.6358.87 (4.67–32.07)8.9 (2.1–18.92)0.164

Properties of constructs with different types of connecting bars were compared in a statistical model with adjustment for pin size, and properties of constructs with different pin size combinations were compared in a statistical model with adjustment for connecting bar type.

See Table 1 for remainder of key.

Discussion

The null hypothesis was rejected. The FESSA constructs were stiffer and more resistant to torsional forces than acrylic bar constructs overall and when the statistical model was adjusted for pin size combination, and constructs with a 1.6-mm IP and 1.6-mm TPs were stiffer than those with a 2.0-mm IP and 1.1-mm TPs when the statistical model was adjusted for connecting bar type. Furthermore, none of the FESSA constructs failed during torsional mechanical testing, whereas acrylic constructs with either pin size combination had failures (6/6 with a 1.6-mm TP and 1.6-mm IPs and 1/6 with a 2.0-mm IP and 1.1-mm TPs) prior to reaching the maximum angle of 80°, indicating that FESSA constructs had superior mechanical qualities. The FESSA constructs with a 1.6-mm IP and 1.6-mm TP had the lowest median gap strain and the greatest median stiffness at yield of the constructs tested.

The acrylic constructs had failure of the connecting bar at the TP interfaces. Radiographs obtained at the time of acrylic construct assembly revealed air bubbles in various parts of the acrylic connecting bars despite adherence to the manufacturer instructions when filling the tubes. Air bubbles in the regions of TPs may have contributed to failure of the constructs. Results of previous studies10,11 have indicated that porosity in methacrylate-based resin is detrimental to its strength. Injecting under pressure with a nozzle has been shown to be the best method to decrease porosity in acrylic connecting bars. In the present study, failure of the acrylic constructs primarily at the interface with the proximal TP was similar to a finding reported by Van Wettere et al2 on evaluation of acrylic tie-in ESFs constructs with plastic bone models. A possible reason for failure at the proximal pin location was the rigid portion of the connecting bar between the ESF tie-in pin and the proximal TP creating a bending moment arm at the proximal pin. This would concentrate the forces at the pin and connecting bar interface.

The combination of the pin sizes selected for the present study was clinically relevant for tibiotarsal bone repair in red-tailed hawks. The pin diameters and the pin size combinations used were within the established guidelines for avian fracture repair with consideration for the diameter of the bone and medullary canal.12,13 The FESSA constructs with a 1.6-mm IP and 1.6-mm TPs had the best mechanical testing results of the constructs used in the study; although larger TPs add stiffness, they must not be large enough to cause a critical defect resulting in bone failure when used in patients. We selected maximal IP and TP size combinations that fit the synthetic bone model used. It is unclear whether maximizing stiffness is needed for clinical success in the treatment of fractures with an ESF in raptors. Bueno et al1 showed that TPs with diameters as small as 20% of the medullary canal diameter were adequate for tibiotarsal bone healing in raptors. However, the median stiffness at yield for the stiffest construct used in the present study (50 N·mm/°) was still 30% of that found for intact tibiotarsal bones from red-tailed hawk cadavers (170.8 N·mm/°).8

The study reported here was limited by its reliance on a synthetic tibiotarsal bone model. Use of a bone model eliminates the variability expected to be found among individual raptor bones and thus allows for more direct comparison among ESF constructs. However, the properties of the ESFs used in this study may differ when used with cadaver bones or when applied to patients. All constructs were tested by torsion in a single cycle to failure. The application of torsional loads was a simplification of the loads experienced in vivo. However, the results of direct comparison among constructs do offer important information that can be expected to have clinical relevance in raptors with tibiotarsal fractures. Little is known about avian bone healing.14 There is no established interfragmentary gap shear strain limit for bone healing in raptors. It has been shown that a lower gap shear strain is more advantageous for bone healing in mammalian species,15,16 suggesting that of the constructs assessed in the present study, a FESSA construct with a 1.6-mm IP and 1.6-mm TP would be the preferred choice for fracture repair in tibiotarsal bones of red-tailed hawks. Although ESFs with acrylic connecting bars have been successfully used in avian patients, the results of the present study indicated that FESSA constructs with similar IP and TP configurations have superior mechanical properties against torsional forces, and this consideration may warrant the greater cost for these products, compared with that of acrylic constructs.

Acknowledgments

Funded by a grant from the Association of Avian Veterinarians Research Committee and by the Center of Companion Animal Health at the University of California-Davis.

The authors have no conflict of interest or financial disclosures to report.

The authors thank Chrisoula Toupadakis Skourtakis for assistance with figures and images.

Presented in abstract form abstract at the 46th Annual Meeting of the Veterinary Orthopedic Society, Breckenridge, Colo, February 2019.

ABBREVIATIONS

ESF

External skeletal fixator

FESSA

Fixateur Externe du Service de Santé des Armées

IP

Intramedullary pin

TP

Transfixation pin

Footnotes

a.

Acrylx, IMEX Veterinary Inc, Longview, Tex.

b.

IP, IMEX Veterinary Inc, Longview, Tex.

c.

Miniature interface positive profile pin, IMEX Veterinary Inc, Longview, Tex.

d.

FESSA, 8 × 150 mm, Jorgensen Laboratories, Loveland, Colo.

e.

Delrin white acetal rod, 0.5-inch diameter, McMaster-Carr, Santa Fe Springs, Calif.

f.

Black-oxide high speed drill bit, 0.25-inch diameter, McMaster-Carr, Santa Fe Springs, Calif.

g.

Black-oxide high speed drill bit No. 55, McMaster-Carr, Santa Fe Springs, Calif.

h.

Black-oxide high speed drill bit No. 51, McMaster-Carr, Santa Fe Springs, Calif.

i.

Small air drive No. 511.10, DePuy Synthes, West Chester, Pa.

j.

No. 511.73, DePuy Synthes, West Chester, Pa.

k.

No. 30016, IMEX Veterinary Inc, Longview, Tex.

l.

Black-oxide high speed steel 1.0-mm drill bit, McMaster-Carr, Santa Fe Springs, Calif.

m.

NEXT digital radiography system, SOUND, Carlsbad, Calif.

n.

HF100/30+ x-ray generator, MinXray Inc, Northbrook, Ill.

o.

White 0.125-inch diameter polypropylene balls, McMaster-Carr, Santa Fe Springs, Calif.

p.

Loctite Super Glue Gel Control, Henkel Adhesives, Düsseldorf, Germany.

q.

Model 809, MTS Systems Corp, Minneapolis, Minn.

r.

S-PRI (1,280 × 1,280 pixels), AOS Technologies AG, Baden, Switzerland.

s.

Motus 10, Contemplas, Kempten, Germany.

t.

PROC MIXED, SAS, version 9.4, SAS Institute Inc, Cary, NC.

References

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