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Mechanical evaluation of various external skeletal fixator-intramedullary pin tie-in configurations using a tubular plastic bone model.

Abstract: Use of external skeletal fixator-intramedullary pin tie-in (ESF-IM pin tie-in) fixators is an adjustable and effective method of fracture fixation in birds. The objective of this study was to evaluate the elements of the ESF-IM tie-in configuration used in birds. Ten variations of constructs were applied to a plastic bone model with a standard gap. Variants included non-tied and tie-in configurations, use of a 6- or 10-min acrylic bar or a thermoplastic connecting bar, variation in the placement of the proximal fixation pin, use of 1. l-ram (0.045-in) or 1.6-min (0.062-in) fixation pins, and configurations of 2, 3, or 4 fixation pins. The various constructs were loaded in bending, torque, and compression, and response variables were determined from resulting load-displacement curves (stiffness, load at 1-min displacement). Results showed that, by using the tie-in configuration, increasing the diameter of the acrylic connecting bar, and increasing the diameter or number of fixation pins, each significantly increased the stiffness in all assessments. Placing the fixation pin distally in the proximal bone model segment increased the stiffness in bending, and adding a fixation pin to the distal bone model segment increased the stiffness in torque and bending. These results quantified the relative importance of specific parameters that effect stiffness and safe load of the ESF-IM tie-in construct as applied to a plastic bone fracture model.

Key words: orthopedic surgery, external skeletal fixator, intramedullary pin tie-in, plastic bone model, biomechanics, avian

Introduction

Over the past 25 years, a variety of fracture-repair techniques have been used in birds. External coaptation, intramedullary (IM) pins, polypropylene rods, IM polymethylmethacrylate with or without polymer rod reinforcement, bone plates, and external skeletal fixator (ESF) devices have been used to repair long bones with varying rates of success. (1-5) In 1995, the ESF-IM pin tie-in fixator was introduced for the treatment of long bone fractures in birds at The Raptor Center (University of Minnesota, St Paul, MN, USA). (4) This technique was used to repair diaphyseal or peri-articular fractures of the humerus, ulna, femur, and tibiotarsus. Clinically, this configuration was found to be superior to any fixation method used previously in terms of adaptability, convenience, cost, and efficacy. (3,4,6-9)

When an ESF is used in combination with an IM pin, the 2 devices complement each other. (3,10-13) The IM pin is easy to place and resists bending forces equally well in all directions because of its proximity to the neutral axis of the bone; however, alone it poorly stabilizes the disruptive forces of shear, torsion, and compression. (3,10-13) In contrast, the ESF is best able to resist the forces of shear, torsion, and compression but poorly resists bending forces. (3,10-13) An ESF requires the placement of 4-8 fixation pins to provide adequate stabilization, and the insertion of these pins may propagate fracture lines.(10,12,13) Furthermore, it is more difficult to achieve proper bone alignment by using an ESF alone. (11) Combining an IM pin with a 2- or 4-pin ESF configured to create a link between the 2 apparatuses (ie, a "tie-in") creates an "I-beam" structure that resists all forces acting on a fracture and prevents migration of the IM pin. (3,9-13) In addition, the ESE-IM pin tie-in reduces the risk of formation of proliferative fibrous tissue and a serum pocket, which is frequently noted when an IM pin is cut short and covered by skin. (10) Lastly, the application of an ESF-IM tie-in is consistent with the biological approach to internal fixation, which consists of a closed or limited surgical approach to the bone for the IM pin placement combined with a closed application of the adjunctive external fixator. (3,4,7,9-13)

Conventional connecting bars and clamps used in small animal surgery can be used in avian fracture repair. They have the advantages of being commercially available, available in various sizes, and easy to apply and remove. However, although these devices are well designed for use in dogs and cats, their weight precludes them from being of optimal use in avian fracture management, especially for birds that weigh <300 g.

Materials other than stainless steel bars and clamps that have been used for the connecting system are methacrylate, thermoplastic orthopedic tape, thermoplastic sheets, and epoxy putties. (1.3,4.9,13-18) Surgeons at The Raptor Center have favored the use of methacrylate over thermoplastic sheets or tape because it is considered less bulky, more rigid, and easier to apply when fixation pins are not well aligned linearly. The use of an acrylic connecting system decreases the weight of the fixator, increases flexibility in pin placement, yields less interference with radiographic evaluation of the fractured bone, and reduces cost relative to steel connecting systems. (l,4,9,17-19)

Several studies about the biomechanical properties of the various devices used for fracture repair were conducted in human and small animal surgery. (10,15-21) Extensive experimental testing of devices has had a positive influence on the overall success of fracture repair by leading to improvement of materials and application techniques. ESFs with increased stiffness were shown to enhance the speed and quality of fracture healing. (20,21) A study that attempted to modify fracture healing by controlled destabilization of the external skeletal fixator demonstrated the importance of being familiar with the mechanical strengths of fixation devices to better understand their effects on fracture healing. (22)

Several studies evaluated the biomechanical properties of ESFs or ESFs combined with an IM pin or an acrylic connecting bar. (19,23,24) However, only 1 study evaluated the biomechanical properties of the ESF-IM pin tie-in configuration. (10) This evaluation was limited to the measurement of the bending strength of an ESF-IM pin tie-in versus a ESF-IM pin non tie-in configuration when using canine femurs. Most reports of the use of the ESF-IM pin tie-in configuration in birds describe the investigators' clinical experience (6-9); however, one unpublished controlled study of pigeons (Columba livia) with ulnar fractures, which were stabilized with an ESF-IM pin tie-in configuration, evaluated healing by using serial radiographs, histologic examination, and biomechanical testing. (7) Methods of application for our study were based on this previously recorded information, clinical experience, and biomechanical principles extrapolated from human and small animal studies. (3,4,6-9) To date, no study has evaluated the biomechanical properties of various configurations of the ESF-IM pin tie-in with a plastic bone model.

The objectives of this study were to determine the effect of the following parameters on fixator stiffness: 1) linking the lM pin to the ESE connecting system; 2) altering the diameter of the fixation pin and the connecting system; 3) increasing the number of fixation pins; 4) altering the fixation pin position in the proximal bone model segment; and 5) altering the material that composes the connecting system. In addition, cadaver humeri extracted from euthanatized redtailed hawks (Buteo jamaicensis) were collected, weighed, and compared with the weight of the ESF-IM pin tie-in apparatuses.

Materials and Methods

Plastic bone model

Testing was completed by using a plastic bone model with dimensions that represented bones typical of a bird weighing 1 kg. Tube sections were taken from the plastic casing of a ballpoint pen (Round Stic medium blue, BIC Corporation, Milford, CT, USA). The ink cartridge was removed, and a 50-mm section of the plastic casing, including the plastic plug located opposite the ballpoint end, was cut off. Two sections of plastic casing were used to represent the 2 segments of a bone with a simple, complete, transverse, mid diaphyseal fracture. The tube sections had an 8-mm external diameter, a 1-mmthick cortex, and a 6-mm internal diameter. One end of each tube section was closed with the plastic plug. The plug fit tightly within the tube lumen and was composed of a 9-mm-long, 6-mmdiameter tubular section, with a 1-mm-thick cortex sealed at one end by a dome-shaped, 8-mm-diameter, 1.5-mm-thick top. Two tube sections, each 50 mm long and closed at 1 end with the plug, were positioned with a 5-mm defect between them to create a standard fracture bone model. The ESF-IM pin tie-in fixator was applied in a prescribed manner by using a jig specifically designed for this experiment. A 2.8-mm (7/64-in) Steinmann pin (Steinmann intramedullary pin, IMEX Veterinary Inc, Longview, TX, USA) was inserted through the end of 1 plastic tube, through the plastic plug, and driven to the middle of the second plastic tube section. Partial end-threaded positive-profile fixation pins (Miniature Interface Half pins; IMEX) of 2 sizes (1.57 mm [0.062 in] or 1.14 mm [0.045 in]) were inserted 10 mm from each end of the bone model in the construct with 2 fixation pins. (Note: For the purposes of this article, hereafter 1.57 mm will be rounded to 1.6 mm and 1.14 mm to 1.1 mm.) In the construct with 4 fixation pins, 2 additional partial end-threaded positive-profile fixation pins were inserted, 1 on each side, 10 mm from the fracture gap. Once the fixation pins were in place, the trocar of the Steinmann pin was driven to the outer end of the second plastic tube section between the fixation pin and the cortex of the plastic tube and adjacent plastic plug. The Steinmann pin was bent at a 90[degrees] angle, 15 mm from its exit through the plug. The acrylic connecting bar was placed 10 mm away and parallel to the bone model (Fig 1). The acrylic (Technovit, Jorgensen Laboratories Inc, Loveland, CO, USA) used to make the connecting system was prepared according to manufacturer's directions. The liquid monomer was added to the powder in a ratio of 2:1 by volume at room temperature, and injected into a thin-walled plastic tube (heat shrinkable tubing, 6 or 10 mm in diameter; Guangzhou Kaiheng K&S Co Ltd, Guangzhou, People's Republic of China) that had been impaled on the fixation and Steinmann pins. All models (plastic bone model with fixators) were weighed in grams with a precision scale (Triple beam balance 720-00, Ohaus Corp, New York, NY, USA).

Ten variations of the model were constructed (Fig 2). The abbreviation for each configuration was organized as follows: the number of fixation pins present in each tube section (ie, 2+2) followed by the size of the fixation pins in parentheses (1.1 mm or 1.6 mm), and the size of the connecting bar in millimeters. Each ESF-IM construct was weighed, along with the plastic bone model. The constructs evaluated were the following:

* 1+1 (1.6) 10-mm: ESF-IM pin (not tied-in) with 2 fixation pins (1.6-ram [0.062-in] diameter)

and a 10-mm (0.375-in) diameter acrylic connecting bar. The Steinmann pin protruded 1 cm from its exit point at the plastic bone extremity, n = 10.

* 1+1 (1.6) 10-mm tie-in: ESF-IM pin tie-in with 2 fixation pins (1.6-mm [0.062-in] diameter)

and a 10-mm (0.375-in) diameter acrylic connecting bar. n = 10.

* 2+2 (1.6) 10-mm tie-in: ESF-IM pin tie-in with 4 fixation pins (l.6-mm [0.062-in] diameter) and a 10-mm (0.375-in) diameter acrylic connecting bar. n = 10.

* 1+1 (1.6) 6-mm tie-in: ESF-IM pin tie-in with 2 fixation pins (1.6-mm [0.062-in] diameter) and a 6-mm (0.25-in) diameter acrylic connecting bar.

n=10.

* 2+2 (1.6) 6-mm tie-in: ESF-IM pin tie-in with 4 fixation pins (1.6-mm [0.062-in] diameter) and a 6-mm (0.25-in) diameter acrylic connecting bar. n = 10.

* 1+1 var. (variation) (1.6) 6-mm tie-in: ESF-IM pin tie-in with a 6-mm (0.25-in) diameter acrylic connecting bar and a variation in the placement of the proximal fixation pins (1.6mm [0.062-in] diameter). This construct was similar to the 2 fixation pins configuration described above, except that the fixation pin usually placed at one end of the plastic bone model near the entrance of the Steinmann pin was placed 10 mm from the fracture gap in the same plastic bone section, n = 6.

* 1+2 (1.6) 6-mm tie-in: ESF-IM pin tie-in with a 6-mm (0.25-in) diameter acrylic connecting bar, 1 fixation pin (1.6-mm [0.062-in] diameter) in the plastic bone segment where the Steinmann pin exits, and 2 fixation pins in the other plastic bone section, n = 6.

* 1+1 (1.1) 6-mm tie-in: ESF-IM pin tie-in with 2 fixation pins (1.1-mm [0.045-in] diameter) and a 6mm (0.25-in) diameter acrylic connecting bar. n = 10.

* 2+2 (1.1) 6-mm tie-in: ESF-IM pin tie-in with 4 fixation pins (1.1-mm [0.045-in] diameter) and a 6-mm (0.25-in) diameter acrylic connecting bar. n = 10.

* 1+1 (1.6) thermoplastic tie-in: ESF-IM pin tie-in with 2 fixation pins (1.6-mm [0.062-in] diameter) and a thermoplastic connecting bar. n = 6. The construct was similar to the 2 fixation pins configuration 1+1 (1.6) 10-min tie-in, except that the fixation pins were bent 90[degrees] on a line parallel to the plastic bone and 20 mm away from it. A sheet of thermoplastic material (Veterinary Thermoplastic; IMEX) 280-mm long (twice the distance from the Steinmann pin to the distal fixation pin) by 40-mm wide and 2.4-min thick was immersed in water close to boiling for 30-40 seconds. When the material was translucent and flaccid, the thermoplastic sheet was applied as shown in the study by Redig.3 The long rectangular piece of soft thermoplastic was placed along one side of the pins, leaving 30 mm of thermoplastic above the bent fixation pins and away from the plastic bone model and an equal distance on each side of the fixation pins. The lateral sides of the sheet that extended past the fixation pins were folded toward each other to "sandwich" the pins. The thermoplastic material was then folded along its long axis to form a rectangular connecting bar positioned 10 mm away from the plastic bone model.

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All model configurations were tested in bending (by using a 4-point bending test), torsion, and axial compression. The same models were used for the 3 tests. Compression tests, followed by torsion tests, were performed first because they were conducted within the elastic deformation limits of the models. Then the models were subjected to 4-point bending tests until permanent deformation was created.

All tests were performed by using a servohydraulic materials testing machine (Endura TEC, Minnetonka, MN, USA) under displacement control. For the torsion and axial compression tests, each end of the test specimen was clamped to the actuator of the testing machine by custom-made clamps (Fig 3). Torsion forces were applied to the test specimen at the rate of 10[degrees]/s until a maximum displacement of 50[degrees] was reached. Axial compression forces were applied to the test specimen at the rate of 1 mm/s until a maximum displacement of 4 mm was reached.

The test jig used for the 4-point bending test had the outer supports separated by a distance of 50.7 mm and the inner supports separated by a distance of 19 mm for applying bending force to the fracture model on each side of the fracture gap (Fig 4). The plane was defined as being perpendicular to the axis of the fixation pins. The load was applied at the rate of 1 mm/s until a displacement of 8 mm was achieved on each side. This displacement permanently deformed the model.

For all tests, load displacement and torque rotation were measured. For all load-displacement curves, initial stiffness (rigidity), and safe load were determined. The stiffness was calculated from the initial slope of the linear portion of the load-displacement curve. Safe load was defined as the load that was required to produce a maximum movement of 1 mm between opposing points at a fracture gap. (18,19,25,26)

Bone specimen collection and preparation

Sixteen humeri were collected from cadavers of red-tailed hawks euthanatized for clinical or humane reasons at The Raptor Center. The bones were collected from hawks in good body condition and that weighed 900-1300 g. The wing was disarticulated at the shoulder, and the weight of the entire wing was recorded by using a precision scale. All soft tissues were dissected away from each specimen, and the humerus was isolated and weighed on a precision scale.

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Statistical analysis

An all pairwise multiple comparison of the means procedure (Student-Newman-Keuls test) was used to compare the stiffness (torsion, compression, and bending) and safe load (torsion, compression, and bending) of the different configurations. Values of P < .05 were considered significant. A 2-sample t test was used to compare the mean weights of the configurations evaluated. Values of P < .05 were considered significant.

[FIGURE 4 OMITTED]

Results

The results for stiffness and safe load for all configurations and loading modes are presented in Table 1 and Figures 5-7. Means and SDs of the weights of the plastic bone model and ESF-IM pin tie-in composites are presented in Table 2. Three constructs failed during the testing in axial compression, and 1 construct failed during the 4point bending test. When under axial compression, 2 of the 2+2 (1.6) 6-mm constructs failed at a load of 199 N and 3.88-mm displacement and 218.4 N and 3.7-mm displacement, respectively. One 1+1 (1.6) 6-mm construct failed at a load of 148.8 N and 3.7-mm displacement. During the bending test, one 1+2 (1.6) 6-mm construct failed at a load of 832.4 N and 5.58-mm displacement. All 4 constructs failed by breakage of the acrylic connecting bar at the junction of the bar and the fixation pin closest to the bent Steinmann pin. The data from these 4 constructs were not included in the statistical analysis.

Effect of the tie-in

A significant difference was seen in stiffness in torque (P < .001), axial compression (P < .001), and bending (P < .001) between the 1+1 (1.6) 10mm and 1+1 (1.6) 10-mm tie-in constructs. The mean stiffness was 57% higher in torque, 34% higher in axial compression, and 17% higher in bending with the tie-in configuration. There was also a significant difference in safe load in torque (P < .001) and axial compression (P < .001) between the 1+1 (1.6) 10-mm and 1+1 (1.6) 10min tie-in constructs. The mean safe load was 50% higher in axial compression and 64% higher in torque for the tie-in configuration. Mean safe loads did not differ significantly in bending. The mean weight of the 1+1 (1.6) 10-mm tie-in configuration was 16% higher than the 1+1 (1.6) 10-mm configuration (P < .01).

Effect of the connecting bar

6-rnm versus 10-mm acrylic connecting bar." There was a significant difference in stiffness in torque (P < .001), axial compression (P < .001), and bending (P = .016) between the 2+2 (1.6) 10mm tie-in and 2+2 (1.6) 6-mm tie-in constructs. The mean stiffness was 16% higher in torque, 23% higher in axial compression, and 19% higher in bending for the 2+2 configuration with a 10-mm versus a 6-mm acrylic connecting bar. A significant difference was also seen in stiffness in torque (P < .001) and bending (P < .001) between the 1+1 (1.6) 10-mm tie-in and 1+1 (1.6) 6-mm tie-in configurations. The mean stiffness was 16% higher in torque and 10% higher in bending for the 1+1 (1.6) 10-mm tie-in configuration. Axial compression between these constructs was not significantly different. There was a significant difference in the safe load only in torque between the 1+1 (1.6) 10-mm tie-in and 1+1 (1.6) 6-mm tie-in configurations as well as between the 2+2 (1.6) 10-mm tie-in and 2+2 (1.6) 6-mm tie-in configurations (P = .007 and P < .001, respectively). Axial compression and bending were not significantly different between these configurations. The mean safe load in torque was 17% higher for the 1+1 configuration with a 10min versus the 6-mm acrylic connecting bar and 20% higher for the 2+2 configuration with a 10ram versus the 6-mm acrylic connecting bar. The mean safe load was not different in axial compression and bending between these configurations. The mean weights of the 1+1 (1.6) 10-mm tie-in and 2+2 (1.6) 10-mm tie-in configurations were 47% and 44% higher, respectively, than those of the 1+1 (1.6) 6-mm tie-in and 2+2 (1.6) 6mm tie-in configurations (P < .01).

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[FIGURE 6 OMITTED]

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6-mm and 10-mm acrylic connecting bar versus the thermoplastic connecting bar: A significant difference in stiffness in torque (P < .001) and bending (P = .003) was seen between the 1+1 (1.6) 10-mm tie-in and 1+1 (1.6) thermoplastic tie-in configurations. The mean stiffness was 19% higher in torque and 8% higher in bending for the 10-mm acrylic connecting bar compared with the thermoplastic connecting bar when using the same configuration. There was no significant difference between these constructs in axial compression. There was a significant difference in the safe load in torque (P < .001) between these 2 configurations. The mean safe load was 11% higher in torque for the configuration with the 10mm acrylic connecting bar compared with the one with the thermoplastic connecting bar. There was no significant difference in stiffness in torque, axial compression, or bending between the 1+1 (1.6) 6-mm tie-in and the 1+1 (1.6) thermoplastic tie-in constructs. There was also no significant difference in safe load in torque and axial compression between these configurations. The safe load in bending for the 1+1 (1.6) thermoplastic tie-in could not be evaluated because malpositioning of the model in the test frame yielded no data. The mean weight of the 1+1 (1.6) thermoplastic tie-in configuration was higher than both the 1+1 (1.6) 10-mm tie-in and the 1+1 (1.6) 6-mm tie-in configurations (48% and 117%, respectively) (P < .05).

Effect of the fixation pin size, 1.6 mm (0.062 in) versus 1.1 mm (0.045 in)

There was a significant difference in stiffness in torque (P < .001), axial compression (P < .001), and bending (P < .001) between the 1+1 (1.6) 6-mm tie-in and 1+1 (1.1) 6-mm tie-in constructs. The mean stiffness was 136% higher in torque, 120% higher in axial compression, and 10% higher in bending for the 1+1 configuration with 1.6-mm (0.062-in) fixation pins compared with the same configuration when using 1.1-mm (0.045-in) fixation pins. There was also a significant difference in the safe load in torque (P < .001) and axial compression (P < .001) between these 2 constructs. The mean safe load was 130% higher in torque and 52% higher in axial compression for the 1+1 configuration with the 1.6-mm (0.062-in) versus the 1.1-mm (0.045-in) fixation pins. The difference in safe load between these 2 configurations could not be evaluated in bending, because malpositioning of the 1+1 (1.1) 6-mm tie-in construct yielded no data.

There was a significant difference in stiffness in torque (P < .001) and axial compression (P .001) between the 2+2 (1.6) 6-mm tie-in and 2+2 (1.1) 6-mm tie-in constructs. The mean stiffness was 94% higher in torque and 96% higher in axial compression for the 2+2 configuration when using 1.6-mm (0.062-in) fixation pins compared with the same configuration when using 1.1-mm (0.045-in) fixation pins. There was no significant difference in stiffness in bending between these 2 constructs. There was a significant difference in safe load in torque (P < .001), axial compression (P < .001), and bending (P = .002) between the 2+2 (1.6) 6-mm tie-in and 2+2 (1.1) 6-mm tie-in constructs. The mean stiffness was 77% higher in torque, 90% higher in axial compression, and 9% higher in bending for the 2+2 configuration when using the 1.6-mm (0.062-in) versus 1.1-mm (0.045in) fixation pins. The mean weight of the 1+1 (1.6) 6-mm tie-in and 2+2 (1.6) 6-mm tie-in configurations were 5% and 7% higher than the 1+1 (1.1) 6mm tie-in and 2+2 (1.1) 6-mm tie-in configurations, respectively (P < .01).

Effect of the fixation pin position in a 2-pin configuration

There was a significant difference in stiffness in bending (P < .001) between the 1+1 (1.6) 6-mm tie-in and 1+1 var. (1.6) 6-mm tie-in configurations. The mean stiffness was 7% higher in bending for the 1+1 var. (1.6) 6-mm tie-in configuration. Differences in stiffness in torque and axial compression were not significant between these constructs. There were also no significant differences in the safe load in torque, axial compression, and 4-point bending between these 2 configurations.

Effect of the number of fixation pins

Addition of a third fixation pin to form a 3 fixation pin configuration." There was a significant difference in stiffness in torque (P < .001) and bending (P < .001) between the 1+1 (1.6) 6-mm tie-in and 1+2 (1.6) 6-mm tie-in configurations. The mean stiffness was 33% higher in torque and 14% higher in bending for the 1+2 configuration. Stiffness in axial compression was not significantly different between these 2 constructs.

There was a significant difference in safe load in torque (P = .005) between the 1+1 (1.6) 6mm tie-in and 1+2 (1.6) 6-mm tie-in configurations; the mean safe load was 29% higher in torque for the 1+2 configuration. There was no significant difference in the safe load in axial compression or bending between these 2 constructs. There was a significant difference in stiffness in torque (P < .001), axial compression (P < .001), and bending (P < .001) between the 1+2 (1.6) 6mm tie-in and 2+2 (1.6) 6-mm tie-in configurations. The mean stiffness was 17% higher in torque, 66% higher in axial compression, and 8% higher in bending for the 2+2 configuration.

There was a significant difference in safe load in torque (P < .001), axial compression (P < .001), and bending (P = .003) between these 2 constructs. The mean safe load was 13% higher in torque, 64% higher in axial compression, and 12% higher in bending for the 2+2 configuration. The mean weight of the 1+2 (1.6) 6-mm tie-in configurations was 3% higher than the 1+1 (1.6) 6-mm tie-in and 3% lower than the 2+2 (1.6) 6mm tie-in configurations (P < .01).

Addition of 2 fixation pins to form a 2+2 fixation pin configuration." [Note: In the 2+2 (1.6) 10-mm tie-in configuration group, 1 test result for axial compression was treated as an outlier and was removed from the data set.] There was a significant difference in stiffness in torque (P < .001), axial compression (P < .001), and bending (P < .001 and P = .002, respectively) between the group 1+1 (1.6) 6-mm tie-in and 2+2 (1.6) 6-mm tie-in configurations as well as between the group 1+1 (1.6) 10-mm tie-in and 2+2 (1.6) 10-mm tie-in configurations. The mean stiffness was 56% higher in torque, 53% higher in axial compression, and 23% higher in bending for the 2+2 (1.6) 6-mm versus the 1+1 (1.6) 6-mm configuration, and it was 56% higher in torque, 76% higher in axial compression, and 34% higher in bending for the 2+2 (1.6) 10-mm versus the 1+1 (1.6) 10-mm configuration. There was a significant difference in the safe load in torque (P < .001), axial compression (P < .001), and bending (P < .001) between the 1+1 (1.6) 6-mm tie-in and 2+2 (1.6) 6mm tie-in configurations as well as between the 1+1 (1.6) 10-mm and 2+2 (1.6) 10-mm configurations. The mean safe load was 46% higher in torque, 51% higher in axial compression, and 19% higher in bending for the 2+2 (1.6) 6-mm versus the 1+1 (1.6) 6-mm configuration, and it was 50% higher in torque, 53% higher in axial compression, and 21% higher in bending for the 2+2 (1.6) 10-mm versus the 1+1 (1.6) 10-mm configuration. The mean weight of the 2+2 (1.6) 6- or 10-mm tie-in configuration was 5% higher than the 1+1 (1.6) 6- or 10-mm tie-in configuration, respectively (P < .01).

Wing and cadaver humeri

The weight of the red-tailed hawk wing disarticulated at the shoulder was 98.2 g -13.6 g (mean [+ or -] SD). The weight of the humeri was 6.97 [+ or -] 1.08 g (mean [+ or -] SD).

Discussion

A standardized bone model that used a plastic tube made from ink pens was chosen to simulate the avian bone composed of a large medullary cavity and thin cortex. A 5-mm fracture gap was selected to avoid potential contact between the bone fragments, thus preventing load sharing between the fixator and the plastic bone model and ultimately allowing for the evaluation of the apparatus alone. The length and size of the plastic bone, the sizes and positions of the fixation pins, and the distance between the plastic bone and the connecting system were chosen to simulate clinical application of the ESF-IM pin fixator to the humerus of a medium-sized bird such as a redtailed hawk. (3-4) Partial end-threaded positive profile pins with roughened shafts were used because they increase the bond between the acrylic or thermoplastic connecting rod and the fixation pin as well as improve the bone purchase. (16,27) The displacement of 50[degrees] (torsion test) and 4 mm (axial compression test) were chosen after two 1+1 (1.6) 10-mm tie-in and two 1+1 (1.1) 6-mm tie-in configurations were tested in a preliminary study (A.J.V.W., unpublished data, 2002). These displacements did not create appreciable permanent damage to the constructs and provided similar data after 5 consecutive repetitions of axial compression or torsion.

Ten test models were used for each of 7 configurations (1+1 [1.6] 10-min, 1+1 [1.6] 10-mm tie-in, 2+2 [1.6] 10-mm tie-in, 1+1 [1.6] 6-mm tie-in, 2+2 [1.6] 6-mm tie-in, 1 + 1 [ 1.1 ] 6-mm tie-in, and 2+2 [1.1] 6-mm tie-in); however, after considering the results obtained with these configurations (low variability within the group and large variability between the groups tested), the number of test models was reduced to 6 for each of the 3 remaining groups (1+1 var. [1.6] 6-mm tie-in, 1+2 [1.6] 6-min tie-in, and 1+1 [1.6] thermoplastic tie-in) to reduce the cost and length of the study.

Three test models failed during testing, and their data were not included in the statistical analysis. In the group with the 2+2 (1.6) 10-min tie-in configuration, 1 test result for axial compression was treated as an outlier and was removed from the data set. This outlier performed as expected in torque and 4-point bending tests, but the measurement taken in axial compression was approximately 50% inferior to the other constructs in the group. The SD in this group was approximately 10% of the mean.

The 4 models that failed did so at the same point, that is, the junction between the acrylic connecting bar and the fixation pin closest to the bent Steinmann pin. The reason for this is not known, but it can be speculated that, because the Steinmann pin and the proximal fixation pin are located very close to each other, a very rigid section is created in the connecting bar, followed by a more flexible segment. When the plastic bone was loaded, the forces were concentrated on the proximal fixation pin-connecting bar junction.

Torsion, compression, and bending results for both stiffness and safe load were calculated for each model to allow comparisons among the various configurations. Stiffness and safe loads of the constructs are important characteristics to evaluate because of their potential clinical relevance. Stiffness is important in determining how well the apparatus can resist permanent deformation, and safe load is important in determining how well the apparatus resists displacement at the fracture gap when subjected to a load. Because of the technical complexity involved in measuring the displacement of the bone segment at the level of the fracture gap, the load or torque at 1.0-min or a 15.9[degrees] displacement of the actuator of the test machine was used to determine the safe load. This 1-mm displacement cut point selected followed the discussion of the allowable movement at the fracture gap by Finlay et al (25) and Goodship and Kenwright. (26) This simplification was deemed appropriate because it produced repeatable thresholds. The trade-off between the complexity of measurement and reproducibility of the results was believed to be warranted.

It was once believed that fixation systems needed to be as stiff as possible to encourage primary bone healing over callus healing; however, a certain amount of interfragmentary motion was shown to be advantageous for secondary (callus) bone healing. (26,28,29) Although the ideal stiffness of an ESF is not known, interfragmentary movement within the range of 0.2-1 mm has been reported to be optimal for bone healing in animal experiments. (26,28) This factor gives the safe load major clinical importance when considering physiologic loading conditions. If the safe load is too low, then excessive movement will occur at the fracture gap, and an adequate environment for healing will not be present. (26,28)

During the torque test, 60% of the load-displacement curves demonstrated a temporary short and small deviation from the regular curve line at 20[degrees]-40[degrees] of rotation. This deviation was attributed to the motion of the fixation pin in contact with the Steinmann pin trocar. The tip of the Steinmann pin was wedged between the fixation pin and the cortex of the plastic bone. This is done in clinical application of these fixators because avian bones do not have a metaphysis with dense trabecular bone in which to seat the pin. The end of the Steinmann pin has a sharp tip with 3 flat surfaces (trocar). When torsion forces were applied to the plastic bone, the 2 plastic-tube segments rotated on their axis in opposite directions. At the extremity, where the Steinmann pin trocar is located, the fixation pin moved alongside the tip of the Steinmann pin and from one flat surface of the trocar to another one, which created a small and very short deviation on the load-displacement curve in both places. This short "bump" on the load-displacement curve was inconsequential because the slope of the curve was not modified.

The results of this study indicate that the tie-in configuration increased the stiffness compared with a non-tied-in configuration by at least 57% in torque and at least 34% in axial compression, with only a 16% increase in weight. The increase in torque and compression was even more marked in the safe load. The stiffness was also increased in bending (17%). The lack of a significant difference in bending in the safe load was probably because of the relatively large Steinmann pin opposing the majority of the bending forces and minimizing the effect of the other components of the fixator.

In testing analogous configurations with different sizes of acrylic connecting bars (10- and 6-mm diameters), a significant increase in stiffness was observed with the 10-mm bar. The increase in stiffness was greater for the 2+2 (1.6) 10-mm tie-in configuration than for the 1+1 (1.6) 10-mm tie-in configuration. The differences between the 2 connecting bar sizes were less pronounced in the safe load results where the only significant difference was in torque for the apparatus with a 10-mm connecting bar. A 17% and 20% increase in the safe load for the 1+1 (1.6) 10-mm tie-in and 2+2 (1.6) 10-mm tie-in configurations, respectively, were observed compared with the same configurations when using a 6-mm connecting bar. To explain the lack of difference in safe load in axial compression in this study, it can be theorized that the Steinmann pin linked to the connecting bar takes some of the load applied to the plastic bone model and minimizes the effect of the connecting bar strength in small displacement. To account for the lack of difference in the safe load in bending, one can hypothesize that, because the Steinmann pin is taking the majority of the load, the strength of the connecting bar has only a limited importance in opposing the first part of the plastic deformation.

With regard to clinical application, the 10-mm acrylic connecting bar was significantly heavier (45%) than the 6-mm bar. The constructs with a 6-mm connecting bar (1+1 [1.6] and 2+2 [1.6] 6-mm tie-in configurations) weighed 9.62 g and 10.45 g, respectively, without the plastic bone sections, which is 30% to 50% heavier than the mean weight of the red-tailed hawk humeri collected for this study. Compared with the mean weight of the entire red-tailed hawk wing, the mean weight of the 1+1 (1.6) tie-in apparatus represented approximately 10% of the wing weight when using a 6-mm bar, 17% with a 10-mm bar, and 26% with a thermoplastic bar. For clinical application, a 6-mm-diameter acrylic connecting bar should provide sufficient stiffness, because it is only slightly weaker than a similar construct when using a 10-mm acrylic bar. Clinical data from The Raptor Center showed that a 1+1 (1.6) 10-mm tie-in configuration produces the stability required for bone healing in 1-kg birds. (3,4,9)

The testing of a 1+1 (1.6) tie-in configuration with a thermoplastic connecting bar showed no significant difference in stiffness and safe load (torsion, compression, and bending) compared with a similar construct that included a 6-mm acrylic connecting bar. However, the mean weight of the 1+1 (1.6) thermoplastic tie-in fixator itself was 26.7 g, which was 117% heavier than the mean weight of the same configuration with a 6-mm acrylic connecting bar. Relative to the mean weight of a red-tailed hawk humerus, the acrylic connecting bar was preferable for clinical application.

Only the 2 sizes of fixation pins most commonly used in avian fracture repair at The Raptor Center were tested in this study. In comparing similar configurations when using the 2 pin sizes, the 1.6-min pin resulted in significant increases relative to the 1.1-mm pin in torque and axial compression in both stiffness and safe load. These increases were expected, because the stiffness of a pin is directly related to the fourth power of the radius. From the results, it can be concluded that, for the same fixator configuration, the use of 1.6-mm (0.062-in) instead of 1.1-mm (0.045-in) fixation pins will double the stiffness of the apparatus in torque and compression. The difference in bending stiffness was small (10% when comparing the 1+1 [1.1] 6-mm tie-in and the 1+1 [1.6] 6-mm tie-in) or not significant (when comparing 2+2 [1.1] 6-mm tie-in and 2+2 [1.6] 6-mm tie-in). The mean safe load in torque and compression were also significantly increased for the 1.6-mm compared with the 1.1-mm fixation pins (130% and 52%, respectively, for the 1+1 configuration; 77% and 90%, respectively, for the 2+2 configuration).

Unfortunately, the bending safe load between the 1+1 (1.6)6-mm tie-in and 1+1 (1.1)6-mm tie-in configurations could not be compared because of an error in the placement of the model in the load frame. However, there was a 9% increase in the bending safe load for the 2+2 (1.1) 6-mm tie-in compared with the 2+2 (1.6) 6-mm tie-in configuration. These results showed the effect of fixation pin diameter on bending stiffness and the safe load of an ESF-IM pin tie-in configuration when using one size of Steinmann pin used in avian orthopedic surgery. Because the Steinmann pin diameter was much larger than the fixation pin diameter, the Steinmann pin opposed the majority of the bending forces and masked the effect of the fixation pin size. Clinically, maximizing the fixation pin diameter will increase the stiffness of the apparatus.

The evaluation of the configuration 1+1 var. (1.6) 6-mm tie-in revealed a small but significant increase in bending stiffness (7%) and no significant difference in torque and axial compression stiffness when compared with the standard 1+1 (1.6) 6-mm tie-in configuration. There was no difference in the safe load (torsion, compression, and bending) between these 2 configurations. By placing a fixation pin closer to the fracture site, we expected to increase the stiffness as well as reduce the movement at the fracture site. (30) The lack of difference in safe load could be explained by the fact that the displacement measured was the testing machine's actuator displacement rather than the displacement of the plastic bone cortex at the fracture gap. No explanation could be found for the lack of difference in stiffness in torque and compression.

When compared with the 1+1 (1.6) 6-mm tie-in configuration, the stiffness of the 1+2 (1.6) 6-mm tie-in configuration showed an increase in torque (33%) and bending (14%), but no significant difference was observed in axial compression. The safe load results showed a significant increase only in torque (29%). This was unanticipated, because the addition of 1 fixation pin was expected to increase the axial compression stiffness as well as the safe load. The lack of increase in the safe load could be because the displacement measured was the displacement of the actuator and not that occurring at the fracture gap.

The addition of 2 fixation pins to a 1+1 (1.6) 6-mm tie-in configuration increased the stiffness and safe load by 50% in torque and axial compression and by 20% in bending. Adding 1 fixation pin near the fracture gap in 1 bone model segment provided little advantage other than increasing the resistance to torsion, but adding 1 fixation pin in each bone segment increased the strength of the fixator significantly in all assessments. This supports the use of more fixation pins in both segments for better stabilization of unstable fractures. (30) Another reason to use more pins per fragment in clinical situations is disbursement of the weight-bearing load (which is concentrated at the pin-bone interface) over a larger area and a decreased incidence of pin loosening. (19,27,30) Placing additional fixation pins may involve penetrating large muscle masses, especially around the proximal humerus, femur, and tibiotarsus, which may predispose to drainage around the pins because of increased motion between the pins and soft tissues. (19,27,30) Other risks associated with placing more pins are the risk of iatrogenic damage to the bone by inserting a fixation pin close to an undetected fissure line, competition for space in the medullary cavity between the Steinmann pin and the fixation pins, and disrupting the blood supply at the fracture site. From clinical data at The Raptor Center, constructs similar to the 1+1 (1.6) 10-mm tie-in configuration used in this study provided good stabilization and clinical outcomes in birds that weighed approximately 1 kg. (3,4,9) It remains to be proven clinically or experimentally that configurations stiffer than a 1+1 (1.6) 10-mm tie-in would improve healing.

The fixator configurations evaluated were loaded at a constant rate. The effect of varying the loading rate or cyclic loading was not evaluated. Cyclic testing under physiologic loads would have simulated more closely the forces applied to the constructs in clinical conditions, but the various physiologic loads applied to bones in avian species are unknown. The 1-mm displacement used to define the safe load was not measured at the fracture gap; therefore, movement of the bone segment at the fracture gap itself may have exceeded the 1-mm threshold considered the upper limit of movement conducive to fracture healing. In addition, despite the fact that no changes were observed in a short preliminary study, the use of the same model for the 3 different tests may have affected their mechanical properties and influenced the results of the tests performed subsequently.

In summary, this study quantified specific parameters that affect the ESF-IM pin stiffness and safe load. Among other differences observed, linking the intramedullary pin to the connecting bar, increasing the diameter of the acrylic connecting bar, and increasing the diameter or doubling the number of fixation pins significantly increased the stiffness in all assessments. The biomechanical data generated should help clinicians in selecting appropriate ESF-IM pin tie-in configurations for clinical cases.

Acknowledgments. We thank R. E. Sherman, PhD, for help with the statistical analysis of the results and D. Cooper, BS, for his technical assistance. This project was supported by grants from the Association of Avian Veterinarians; the University of Minnesota, College of Veterinary Medicine, Small Companion Animal Research Grant; and the Midwest Orthopaedic Research Foundation, Orthopaedic Biomechanics Laboratory.

References

(1.) Bennett RA, Kuzma AB. Fracture management in birds. J Zoo Wildl Med. 1992;23:5-38.

(2.) Mathews KG, Wallace LJ, Redig PT, et al. Avian fracture healing following stabilization with intramedullary polyglycolic acid rods and cyanoacrylate adhesive vs. polypropylene rods and polymethylmethacrylate. Vet Comp Orthop Traumatol. 1994;7:158-169.

(3.) Redig PT. The use of an external skeletal fixator-intramedullary pin tie-in (ESF-IM fixator) for treatment of long bone fractures in raptors. In: Lumeij JT, Remple JD, Redig PT, et al. Raptor Biomedicine III. Lake Worth, EL: Zoological Education Network; 2000:239-254.

(4.) Redig PT. Effective methods for management of avian fractures and other orthopedic problems. Proc Eur Assoc Avian Vet. 2001:26-41.

(5.) Wan PY, Adair HS, Patton CS, Faulk DL. Comparison of bone healing using polydioxanone and stainless steel intramedullary pins in transverse, midhumeral osteotomies in pigeons (Columba livia). J Zoo Wildl Med. 1994;25:264-269.

(6.) Garcia-Gramser A, Rodriguez-Quinos J, Benito de la Vibora J, et al. Treatment of fractures in birds by intramedullary pin-external skeletal fixator tie-in technique: a review of 34 cases. Proc Eur Assoc Avian Vet. 2003:353-357.

(7.) Abu J. Evaluation of the Effects of Intramuscular Implantation of Avian Demineralized Bone Matrix (ADBM) and the Use of ADBM in Created Ulna Defects Managed by the Intramedullary Pin External Skeletal Fixator (IM-ESF) Tie-in Technique in Pigeons (Columba livia domestica) [PhD thesis]. St Paul, MN: University of Minnesota; 2002.

(8.) Linn K. Use of an intramedullary pin/external skeletal fixator tie-in technique for repair of long bone fractures in birds. Proc Int Wildl Rehab Council Conf. 1996:107-111.

(9.) Redig PT. Fractures. In: Samour J, ed. Avian Medicine. London: Harcourt; 2000:131-165.

(10.) Aron DN, Foutz TL, Keller WG, et al. Experimental and clinical experience with an IM pin external skeletal fixator tie-in confguration. Vet Comp Orthop Traumatol. 1991;4:86-94.

(11.) Aron DN, Palmer RH, Johnson AL. Biological strategies and a balanced concept for repair of highly comminuted long bone fractures. Compend Contin Educ Pract Vet. 1995;17:35-49.

(12.) Beck JA, Simpson DJ. Type 1-2 hybrid external fixator with tie-in intramedullary pin for treating comminuted distal humeral fractures in a dog and a cat. Aust Vet J. 1999;77:18-20.

(13.) Shani J, Shahar R. The unilateral external fixator and acrylic connecting bar, combined with I.M. pin, for the treatment of tibial fractures. Vet Comp Orthop Traumatol. 2002;15:104-110.

(14.) Martinez SA, Arnoczky SP, Flo GL, Brinker WO. Dissipation of heat during polymerization of acrylics used for external skeletal fixator connecting bars. Vet Surg. 1997;26:290-294.

(15.) Okrasinski EB, Pardo AD, Graehler RA. Biomechanical evaluation of acrylic external skeletal fixation in dogs and cats. J Am Vet Med Assoc. 1991;199:1590-1593.

(16.) Roe S, Keo T. Epoxy putty for free-form external skeletal fixators. Vet Surg. 1997;26:472-477.

(17.) Shahar R. Relative stiffness and stress of type I and type II external fixators: acrylic versus stainless steel connecting bars--a theoretical approach. Vet Surg. 2000;29:59-69.

(18.) Willer RL, Egger EL, Histand MB. Comparison of stainless steel versus acrylic for the connecting bar of external skeletal fixators. J Am Anim Hosp Assoc. 1991;27:541-548.

(19.) McPherron MA, Schwarz PD, Histand MB. Mechanical evaluation of half-pin (type 1) external skeletal fixation in combination with a single intramedullary pin. Vet Surg. 1992;21:178-182.

(20.) Hart MB, Wu JJ, Chao EYS, Kelly PJ. External skeletal fixation of canine tibial osteotomies--compression compared with no compression. J Bone Joint Surg Am. 1985;67:598-605.

(21.) Wu JJ, Shyr HS, Chao EYS, Kelly PJ. Comparison of osteotomy healing under external fixation devices with different stiffness characteristics. J Bone Joint Surg Am. 1984;66:1258-1264.

(22.) De Bastiani G, Aldegheri R, Brivio LR. The treatment of fractures with a dynamic axial fixator. J Bone Joint Surg Br. 1984;66:538-545.

(23.) Brinker WO, Verstraete MC, Soutas-Little RW. Stiffness studies on various configurations of external fixators. J Am Anim Hosp Assoc. 1985;21:801-808.

(24.) Egger EL. Static strength evaluation of six external skeletal fixation configurations. Vet Surg. 1983;12:130-136.

(25.) Finlay JB, Moroz TK, Rorabeck CH, et al. Stability of ten configurations of the Hoffmann external-fixation frame. J Bone Joint Surg Am. 1987;69:734-744.

(26.) Goodship AE, Kenwright J. The influence of induced micromovement upon the healing of experimental tibial fractures. J Bone Joint Surg Br. 1985;67:650-655.

(27.) Bennett RA, Egger EL, Histand MB, Ellis AB. Comparison of the strength and holding power of 4 pin designs for use with half-pin (type I) external skeletal fixation. Vet Surg. 1987; 16:207-211.

(28.) Duda GN, Kirchner H, Wilke HJ, Claes L. A method to determine the 3-D stiffness of fracture fixation devices and its application to predict interfragmentary movement. J Biomech. 1998;31:247-252.

(29.) Kenwright J, Goodship AE. Controlled mechanical stimulation in the treatment of tibial fractures. Clin Orthop Relat Res. 1989;241:36-47.

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Arnaud J. Van Wettere, DVM, MS, Larry J. Wallace, DVM, MS, Dipl ACVS, Patrick T. Redig, DVM, PhD, Craig A. Bourgeault, BS, and Joan E. Bechtold, PhD

From the Raptor Center (Van Wettere, Redig) and the Department of Veterinary Clinical Sciences (Wallace) College of Veterinary Medicine, University of Minnesota, St Paul, MN 55108, USA; and the Orthopaedic Biomechanics Laboratory, Midwest Orthopaedic Research Foundation, Minneapolis, MN 55404, USA (Bourgeault, Bechtold). Present address (Van Wettere): Department of Population Health and Pathobiology, College of Veterinary Medicine, North Carolina State University, 4700 Hillsborough St, Raleigh, NC 27606, USA.
Table 1. Stiffness and safe loads (mean [+ or -] SD) of 10 external
skeletal fixator-intramedullary pin configurations tested
by 3 loading modes in the humeri of red-tailed hawks.

Loading mode            Configuration                Stiffness
                                                 (mean [+ or -] SD)

Axial             1+1 (1.6) 10-mm non-tie-in    24.57 [+ or -] 3.76
compression (a)   1+1 (1.6) 10-mm tie-in        32.91 [+ or -] 6.21
                  2+2(l.6) 10-mm tie-in         59.54 [+ or -] 5.21
                  1+1 (1.6) thermo. tie-in      31.84 [+ or -] 4.34
                  1+1 (1.6) 6-mm tie-in         31.72 [+ or -] 4.13
                  2+2 (1.6) 6-mm tie-in         48.43 [+ or -] 3.97
                  1+1 var. (1.6) 6-mm tie-in    26.09 [+ or -] 2.76
                  1+2 (1.6) 6-mm tie-in         29.21 [+ or -] 2.74
                  1+1 (1.1) 6-mm tie-in         14.42 [+ or -] 1.73
                  2+2 (1.1) 6-mm tie-in         24.70 [+ or -] 1.34
Bending (a)       1+1 (1.6) 10-mm non-tie-in    655.7 [+ or -] 32.3
                  1+1 (1.6) 10-mm tie-in        769.2 [+ or -] 19.7
                  2+2 (1.6) 10-mm tie-in        1028.4 [+ or -] 26.8
                  1+1 (1.6) thermo. tie-in      708.1 [+ or -] 21.4
                  1+1 (1.6) 6-mm tie-in         700.2 [+ or -] 17.3
                  2+2 (1.6) 6-mm tie-in         862.5 [+ or -] 12.4
                  1+1 var. (1.6) 6-mm tie-in    748.3 [+ or -] 16.7
                  1+2 (1.6) 6-mm tie-in         797.6 [+ or -] 42.0
                  1+1 (1.1) 6-mm tie-in         634.9-[+ or -] 17.8
                  2+2 (1.1) 6-mm tie-in         857.9 [+ or -] 32.0
Torque (b)        1+1 (1.6) 10-mm non-tie-in   0.0146 [+ or -] 0.0006
                  1+1 (1.6) 10-mm tie-in       0.0229 [+ or -] 0.0027
                  2+2 (1.6) 10-mm tie-in       0.0357 [+ or -] 0.0020
                  1+1 (1.6) thermo. tie-in     0.0186 [+ or -] 0.0099
                  1+1 (1.6) 6-mm tie-in        0.0198 [+ or -] 0.0017
                  2+2 (1.6) 6-mm tie-in        0.0308 [+ or -] 0.0022
                  1+1 var. (1.6) 6-mm tie-in   0.0179 [+ or -] 0.0022
                  1+2 (1.6) 6-mm tie-in        0.0264 [+ or -] 0.0012
                  1+1 (1.1) 6-mm tie-in        0.0084 [+ or -] 0.0009
                  2+2 (1.1) 6-mm tie-in        0.0159 [+ or -] 0.0013

Loading mode            Safe load
                    (mean [+ or -] SD)

Axial              43.89 [+ or -] 10.05
compression (a)    66.00 [+ or -] 6.81
                  101.15 [+ or -] 14.40
                   65.22 [+ or -] 8.56
                   59.86 [+ or -] 14.30
                   90.23 [+ or -] 13.12
                   47.51 [+ or -] 3.49
                   55.16 [+ or -] 5.97
                   39.28 [+ or -] 9.56
                   47.36 [+ or -] 4.52
Bending (a)         112.3 [+ or -] 4.5
                    115.3 [+ or -] 2.5
                   139.1 [+ or -] 13.9
                          error
                    114.8 [+ or -] 2.5
                    136.3 [+ or -] 5.7
                   104.1 [+ or -] 10.2
                    121.1 [+ or -] 9.6
                          error
                   124.1 [+ or -] 11.8
Torque (b)        0.2703 [+ or -] 0.0233
                  0.4439 [+ or -] 0.0509
                  0.6658 [+ or -] 0.0566
                  0.3992 [+ or -] 0.0379
                  0.3814 [+ or -] 0.0354
                  0.5574 [+ or -] 0.0558
                  0.3341 [+ or -] 0.0538
                  0.4907 [+ or -] 0.0588
                  0.1648 [+ or -] 0.0247
                  0.3141 [+ or -] 0.0370

Abbreviations: thermo. indicates thermoplastic; var.,
variation; N, newton; N m, newton meter.

(a) Stiffness is expressed in N/mm, and the safe load is
expressed in newtons.

(b) Stiffness is expressed in N m/degree, and the safe
load is expressed in N m.

Table 2. Weight (mean [+ or -] SD) of a plastic bone model
and 10 external skeletal fixator-intramedullary pin
configurations tested in the humeri of red-tailed hawks.

Configuration                    Weight (mean [+ or -] SD) (g)

1+1 (1.6) 10-mm                       18.44 [+ or -] 0.43
1+1 (1.6) 6-mm tie-in (a)             14.62 [+ or -] 0.2
1+1 (1.6) 10-mm tie-in                21.39 [+ or -] 0.3
2+2 (1.6) 6-mm tie-in                 15.45 [+ or -] 0.16
2+2(l.6) 10-mm tie-in                 22.20 [+ or -] 0.24
1+1 (1.1) 6-mm tie-in                 13.97 [+ or -] 0.12
2+2 (1.1) 6-mm tie-in (a)             14.53 [+ or -] 0.22
1+1 var. (1.6) 6-mm tie-in            14.24 [+ or -] 0.15
1+2 (1.6) 6-mm tie-in                 14.97 [+ or -] 0.18
1+1 (1.6) thermoplastic tie-in        31.72 [+ or -] 0.33

Abbreviation: var. indicates variation.

(a) With the exception of these 2 weights, the weight of each
configuration was significantly different from all others.
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Title Annotation:Original Studies
Author:Van Wettere, Arnaud J.; Wallace, Larry J.; Redig, Patrick T.; Bourgeault, Craig A.; Bechtold, Joan E
Publication:Journal of Avian Medicine and Surgery
Article Type:Report
Geographic Code:1USA
Date:Dec 1, 2009
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