A Comparative Mechanical Study of 3 External Fixator Clamps

OBJECTIVE: To compare external fixator clamps from Kirschner-Ehmer (K-E), Synthes, and Meynard with respect to 6 mechanical parameters. Study Design-A bench test of mechanical properties. METHODS: Specially designed fixtures were used to mechanically test 6 clamps of each type at 2.5, 5.0, and 7.5 Newton-Meters of clamp bolt-tightening torque. RESULTS: Components slipped axially and torsionally in the K-E clamp at higher forces for all parameters except for clamp bolt axis pivot. No bolt axis pivot occurred with the Synthes clamp. Instead, the clamp plastically deformed at the fixator-pin interface. This failure occurred at a higher applied torque than the pivot torque for other clamps. The Meynard clamp withstood significantly greater force than the K-E clamp when torsion was applied to the clamp bolt axis in the clockwise direction. Pivot forces for the K-E clamp were significantly higher than the Meynard clamp in the counterclockwise direction. CONCLUSIONS: Overall, the K-E clamp was able to resist higher axial and torsional forces before slipping than the Meynard clamp or the Synthes clamp. The Synthes clamp was best able to resist torsion around the clamp bolt axis. Torsional resistance at the clamp-fixator pin and clamp-connecting bar interface was the weakest parameter of clamp mechanics. CLINICAL RELEVANCE: The ability to resist motion within a clamp is related to fracture-reduction stability. Knowledge of the mechanical properties of fixator clamps will improve a clinician's ability to apply rigid fixation.     

Mechanical Comparison of Two External Fixator Clamp Designs

Objective —To compare two external fixation clamp designs for their ability to resist movement of a fixation pin in relation to the connecting rod. Study Design —Two designs of external fixator clamps were attached to connecting rods mounted on a jig for mechanical testing. Fixator pins were placed perpendicular to the connecting rod. A mechanical testing machine was used to deflect each 3.2-mm pin at a distance that was 25 mm from the center of the clamp bolt. Both clamp designs were tightened to 4.4, 6.1, and 7.8 newton-meters (N m) torque, and loads were applied in a position ramp through 4 mm and resisting loads were measured. Two clamp orientations were used during load application, such that the deflection of the pin tended to tighten the clamp bolt or tended to loosen the clamp bolt. The tests were videotaped to determine mode of failure. Comparisons of the load/displacement curves for the two external fixator clamp designs were made using nonlinear equational curve fitting methods. The resultant plateau and rise coefficients were compared using analysis of variance. Results —Slippage of the pin in relation to the clamp occurred with the Kirschner-Ehmer clamp tightened to 4.4, 6.1, and 7.8 N-m, and slipping of the pin in relation to the clamp occurred with the experimental clamp design tightened to 4.4 and 6.1 N-m but not to 7.8 N-m. At 7.8 N-m, the 3.2-mm pin deformed plastically with the experimental clamp design. Increasing the torque of the clamp bolt resulted in superior plateau coefficients for both clamp designs. At each level of tightness and in each clamp orientation to applied pin load, the experimental clamp design provided greater plateau coefficients than did the Kirschner-Ehmer clamp design. At 7.8 N m of tightness, the Kirschner-Ehmer clamp and bolt bent, whereas only slight plastic deformation of the experimental clamp design occurred. Conclusions —The experimental external fixator clamp was more secure in resisting fixator pin movement at all levels of tightening compared with the Kirschner-Ehmer-type external fixator clamp. At 7.8 N m of tightening, the new clamp design did not allow slippage of the pin within the clamp. Clinical Significance—The experimental external fixator clamp should result in greater rigidity of fixator configurations, in addition to providing design features that allow addition of a clamp between two installed clamps, sleeved predrilling of pilot holes for all pins, measurement of pin depth, and placement of positive profile pins at all sites.     

Mechanical testing of a steel-reinforced epoxy resin bar and clamp for external skeletal fixation of long-bone fractures in cats

AIMS: To provide veterinarians with confidence when using a commercially available epoxy resin in external skeletal fixators (ESF), testing was conducted to determine exothermia during curing of the epoxy resin compared to polymethylmethacrylate (PMMA), the hardness of the epoxy resin as a bar over 16 weeks, and the strength of the epoxy resin bar compared with metal clamps in similarly constructed Type 1a ESF constructs simulating the repair of feline long bone fractures.

METHODS: Exothermia of the epoxy resin during curing was tested against PMMA with surface temperatures recorded over the first 15 minutes of curing, using four samples of each product. The hardness of 90 identical epoxy resin bars was tested by subjecting them to cyclic loads (1,000 cycles of 20.5?N, every 7 days) over a 16-week period and impact testing 10 bars every 2 weeks. Ten bars that were not subjected to cyclic loads were impact tested at 0 weeks and another 10 at 16 weeks. Strength of the epoxy resin product, as a bar and clamp composite, was tested against metal SK and Kirschner-Ehmer (KE) clamps and bars in Type 1a, tied-in intramedullary pin, ESF constructs with either 90° or 75° pin placement, subjected to compressive and bending loads to 75?N.

RESULTS: The maximum temperature during curing of the epoxy resin (min 39.8, max 43.0)°C was less than the PMMA (min 85.2, max 98.5)°C (p<0.001). There was no change in hardness of the epoxy resin bars over the 16 weeks of cyclic loading (p=0.58). There were no differences between the median strength of the epoxy resin, SK or KE ESF constructs in compression or bending when tested to 75?N (p>0.05). Stiffness of constructs with 75° pin placement was greater for SK than epoxy resin constructs in compression (p=0.046), and was greater for KE than epoxy resin constructs in bending (p=0.033).

CONCLUSIONS: The epoxy resin tested was found to be less exothermic than PMMA; bars made from the epoxy resin showed durability over an expected fracture healing timeframe and had mechanical strength characteristics comparable to metal bar and clamp ESF constructs.

CLINICAL RELEVANCE: The epoxy resin ESF construct tested in this study can be considered a suitable replacement for SK or KE ESF constructs in the treatment of feline long-bone fractures, in terms of mechanical strength.     

Ex Vivo Biomechanics of Kirschner-Ehmer External Skeletal Fixation Applied to Canine Tibiae

The purpose of this study was to determine the respective contribution of each of the following parameters to the compressive, bending, and torsional rigidity of the Kirschner-Ehmer (KE) external fixation splint as applied to canine tibiae with an osteotomy gap: bilateral versus unilateral splints; increasing the number of fixation pins; altering the diameter of fixation pins and side bars; decreasing side bar distances from the bone; increasing pin separation distances in each pin group; decreasing distances between pin groups; altering pin clamp orientation; and altering side bar conformation. Bilateral splints were 100% (mean) stiffer than unilateral splints, with stiffness enhanced to the greatest extent in mediolateral bending and torsion. Increasing pin numbers stiffened both bilateral (mean, 41%; 8 versus 4) and unilateral splints (mean, 14%; 8 versus 4). Medium KE splints were 85% (mean) stiffer than small KE splints. Decreasing side bar distances to the bone from 1.5 cm to 1.0 cm to 0.5 cm increased stiffness of both bilateral and unilateral splints by a mean of 13% to 35%. Widening pin spacing from 1.67 cm to 2.5 cm increased stiffness in craniocaudal bending only (56% increase, bilateral splints; 73% increase, unilateral splints). Decreasing the distance between pin groups from 5.84 cm to 2.5 cm increased stiffness in torsion between 23% (unilateral splints) and 45% (bilateral splints) and decreased stiffness of unilateral splints by 29% in craniocaudal bending. Altering pin clamp configuration so that the bolts of the clamp were inside the side bar rather than outside the side bar increased stiffness in axial compression only (73% increase, bilateral splints; 54% increase, unilateral splints). Conforming the lateral side bar to the tibiae increased only axial compressive stiffness by 77% but was no different than placing the clamps inside the side bars of an unconformed bilateral splint. These results quantify the relative importance of specific parameters affecting KE splint rigidity as applied to unstable fractures in the dog.     

Biomechanical Analysis of Unilateral External Skeletal Fixators Combined with IM-Pin and Without IM-Pin Using Finite-Element Method

OBJECTIVES: To determine if a unilateral external skeletal fixator (ESF) with a carbon fiber connecting rod (IMEX SK) without an intramedullary (IM)-pin is mechanically comparable with a unilateral ESF with a stainless-steel connecting rod (IMEX KE) with an IM-pin. STUDY DESIGN: Finite-element method (FEM)-computer simulation. METHODS: FEM models were validated by comparison against data from mechanical testing. Three-dimensional FEM models of a femur with a mid-diaphyseal fracture with a 20 mm gap were developed with 4 unilateral external skeletal fixator devices (6-pin KE, 6-pin KE IM-pin, 6-pin SK, and 6-pin SK IM-pin). A 300 N load was applied to the femur at the proximal end in a direction of theta = 10 degrees distally and phi = 10 degrees laterally cranially. Relative displacements in x-, y- and z-directions at the gap were obtained and the overall stiffness was calculated as 300 N/total displacement. Load transfer at the pin-bone interface (PBI) was assessed by determining the von Mises stress maxima at the PBI-related nodes. RESULTS: The 6-pin SK had superior mechanical performance compared with the 6-pin KE by exhibiting smaller displacements in all directions and higher stiffness. Compared with the 6-pin KE IM-pin, the 6-pin SK (without IM-pin) was superior in craniocaudal and lateromedial displacements, but inferior in axial displacements, overall stiffness and von Mises stress maxima. The 6-pin SK IM-pin was superior to the 6-pin KE IM-pin based on smaller displacements and higher stiffness. CONCLUSIONS: Although the SK device had superior mechanical performance compared with a KE device in a unilateral configuration, the addition of an IM-pin continues to be a powerful method of enhancing mechanical performance of either IMEX SK or IMEX KE unilateral constructs in clinical cases. CLINICAL RELEVANCE: Based on the results of this FEM study we recommend the use of the "tied-in" IM-pin with the ESF clinically when striving for high rigidity. In less challenging situations, a unilateral SK ESF without IM-pin might provide sufficient rigidity for a successful fracture repair.     

A mechanical comparison of veterinary linear external fixation systems

OBJECTIVE: To compare the fixation rigidity of recently developed external fixation systems (EFSs) to that of the traditional Kirschner-Ehmer (KE) system. STUDY DESIGN: In vitro biomechanical study. SAMPLE POPULATION: Five different EFSs (KE, Secur-U, small SK carbon fiber, small SK titanium, large SK carbon fiber) were assembled into 7 frame geometries to stabilize Delrin plastic rods with a 1-cm gap. METHODS: External skeletal fixation (ESF) constructs were tested in axial compression, torsion, medial-lateral bending, and cranial-caudal bending. Testing was conducted within the elastic range of each fixator. Mean stiffness in each mode was determined from the slope of the linear portion of the load-deformation curve. Comparison of stiffness values of each EFS within each loading mode and frame type was performed with 1-way analysis of variance (P <.05). RESULTS: Mean stiffness values were significantly higher for the large SK EFS in all frame types compared with KE but were equal in torsional stiffness in the double-bar type 1a frame. The small SK EFS with titanium connecting bar had greater stiffness than the KE in all modes for frame types Ia, Ia-accessory bar, and II-modified. No overall difference was detected between the KE EFS and the small SK with carbon fiber rod. The stiffness of the Secur-U type Ia frame with augmentation plate was significantly greater than the KE type Ia with accessory bar. CONCLUSIONS: The newer external fixation systems evaluated in this study provided fixation rigidity equal to or greater than that of the KE system. CLINICAL RELEVANCE: EFSs with increased frame rigidity should permit the use of less complex frame designs while providing fracture stability.     

Mechanical evaluation of two crimp clamp systems for extracapsular stabilization of the cranial cruciate ligament-deficient canine stifle

OBJECTIVE: To compare the mechanical properties and interoperator variabilities of 2 crimp clamp systems for extracapsular, fabello-tibial, nylon loop stabilization of the cranial cruciate ligament-deficient stifle in dogs. STUDY DESIGN: In vitro mechanical testing. METHODS: Three operators with different grip strengths each secured 20 standardized nylon loops using stainless-steel crimp clamps: 10 using a Veterinary Instrumentation system (45 kg [100 lb] test nylon leader line, 12 mm crimp clamps) and 10 using a Securos system (36 kg [80 lb] test nylon leader line, 36 kg [80 lb] crimp clamps). Loops were tensile loaded to failure in a materials testing machine. RESULTS: Mean ultimate load and mean stiffness were significantly higher for the Securos (336.9 N, 60.6 N/mm) than for the Veterinary Instrumentation system (113.8 N, 37.0 N/mm). For both systems, ultimate load was subject to interoperator variability. CONCLUSIONS: The Securos loops were significantly stronger and stiffer than the Veterinary Instrumentation loops for all operators, but significant differences between operators for ultimate load existed for both systems. CLINICAL RELEVANCE: Securos fabello-tibial sutures will withstand greater loads than Veterinary Instrumentation sutures and this is particularly true for sutures created by surgeons with reduced grip strength. It may be necessary to use more than 1 Veterinary Instrumentation suture to match the ultimate load and stiffness of a Securos suture.     

Use of an aiming device for application of a type-II (bilateral) external fixator to a fractured tibia of a dog

An aiming device was used to guide insertion of fixation pins in a type-II (bilateral) external fixator stabilizing an open canine tibial fracture. This device, designed by the Swiss AO group, has multiple applications in orthopedic surgery, because it accurately locates the exit point of a pin or drill hole on the far side of a bone or fractured bone fragment. When used with the type-II external fixator, it greatly facilitates pin placement by ensuring that, as a pin emerges from the bone, it is in line with the second clamp on the opposite connecting bar.     

Relative stiffness and stress of type I and type II external fixators: acrylic versus stainless-steel connecting bars--a theoretical approach

OBJECTIVE: To compare the stiffness and pin stresses of three sizes of external fixator systems with stainless-steel and acrylic connecting bars. STUDY DESIGN: Finite element analysis. METHODS: Small, medium, and large external fixator systems of type I and type II configurations were modeled for finite element analysis. Each model was evaluated with a standard stainless-steel and three different diameters of acrylic connecting bar. Displacements and stresses were calculated for the loading modes of axial compression, medio-lateral bending, cranio-caudal bending, and torsion. The location of the pin experiencing maximum stress was determined for all configurations and loading modes. RESULTS: Acrylic column diameters of 9.53 mm for the small external fixator system and 15.9 mm for the medium external fixator system provide equivalent stiffness and maximum pin stresses to those provided by the standard stainless-steel connecting bars (3.2- and 4.8-mm diameter, respectively). The largest diameter acrylic column tested (31.75-mm) produced lower stiffness and higher maximum pin stresses than the standard stainless-steel connecting bar (11.1-mm diameter). CONCLUSIONS: When applying a small or medium external fixator, an acrylic column of 9.53-mm or 15.9-mm diameter, respectively, can be used. For a large external fixator system, an acrylic column of diameter >31.75 mm is required. CLINICAL RELEVANCE: The sizes of acrylic connecting bars for use in small and medium external fixator systems have been determined. Large systems should incorporate the standard stainless-steel connecting bar.     

Comminuted supracondylar humeral fractures repaired with a modified type I external skeletal fixator construct

Highly comminuted supracondylar humeral fractures were stabilised in six large-breed dogs with a modified type I external fixator using a craniomedial acrylic connecting column and an Intramedullary pin which was incorporated into the connecting frame. This construct provided sufficient stability to allow satisfactory bone healing in five of the six dogs, while premature removal of the intramedullary pin and external fixator resulted in subsequent refracture of the humerus in the remaining dog. Limb function at the time of final assessment was considered excellent in two dogs, good in three dogs and poor in one dog. The craniomedial acrylic connecting column simplified application of this modified type 1(a) configuration to the humerus by reducing the number of clamps required; the acrylic column also facilitated contouring of the cranlomedial connecting column to the brachium and was readily extended proxlmally to engage the intramedullary pin. In addition, the acrylic connecting column allowed placement of intramedullary pins of varying diameter.     

In vitro evaluation of the 18 and 36 kg Securos Cranial Cruciate Ligament Repair System

OBJECTIVE: To evaluate the mechanical properties of the 18 and 36 kg Securos Cranial Cruciate Ligament Repair System. STUDY DESIGN: In vitro mechanical evaluation. SAMPLE POPULATION: Loop constructs of 18, 27, and 36 kilogram test (kgt) nylon leader line (NLL) secured with Securos crimp-clamps (SCC, n=40 per NLL test weight) or by a clamped square knot (CSK; n=40/NLL test weight). METHODS: The 36 kg SCC were used for the 27 and 36 kgt NLL, and 18 kg SCC were used for the 18 kgt NLL. Loop constructs were mounted on a material testing machine, and distracted at 500 mm/min for static tests, and for cyclic tests at 500 mm/min to a distraction limit of 6 mm (18 kgt) or 7.5 mm (27 and 36 kgt) for 49 cycles, until failure. Constructs were tested at 20 degrees C except for 1 group of 27 kgt CSK loops tested at 40 degrees C. Load at failure, elongation, and stiffness was recorded and compared between groups under static or cyclic testing conditions. RESULTS: All 27 and 36 kgt loops failed by disruption of NLL contained within the knot or crimp-clamp, whereas 18 kgt SCC loops failed by the NLL pulling through the crimp-clamp. The 18 kg SCC loops had considerable variability in ultimate load and elongation (coefficient of variation 29.6% and 18.3%, respectively). There was no significant difference in elongation between 27.3 kgt CSK loops tested at 20 degrees C and 40 degrees C. Generally, in both static and cyclic testing, SCC constructs formed with 27.3 or 36.4 kgt NLL performed as well or better than CSK constructs, resulting in loops that were strong, underwent minimal elongation, and had high stiffness. CONCLUSION: The results support use of the 36 kg Securos system but not the 18 kg Securos system (with the clamp and crimping device used). The significantly lower load required for failure, slippage through the clamp, and substantial variability suggested that the crimp tube diameter or the crimping device tested may be inappropriate for use with 18 kgt NLL. CLINICAL RELEVANCE: Surgeons should be aware that crimp-clamp design is important in controlling suture slippage or breakage within the clamp, and that novel systems should undergo mechanical testing with the size suture material they are intended to secure before clinical use.     

Evaluation of strength at the acrylic-pin interface for variably treated external skeletal fixator pins

Objective: To report pullout force to failure at the acrylic–pin interface for variably treated 3.2 mm external skeletal fixator pins. Study Design: In vitro biomechanical evaluation. Sample Population: 3.2 mm external skeletal fixator pins in polymethylmethacrylate bars. Methods: 3.2 mm external skeletal fixator pins were used for each of 5 treatment groups: polished, unpolished, 3 notched, 5 notched, and machine knurled. Each pin was seated into a 2‐cm‐diameter acrylic connecting bar and tested in pullout force to failure. Each group consisted of 6 pins. The force required to remove the pins from the acrylic bar was measured and compared between groups. Results: Significant differences between treatment groups were determined (P<.05). Within a construct group failure mode was consistent. Fracture of the acrylic bar was only seen with knurled pin ends. Conclusions: When using 2 cm acrylic bars in external skeletal fixation (ESF), a knurled pin shaft or a pin surface with 5 notches should be considered to improve the overall stability of the ESF construct.     

Evaluation of a nontoxic rigid polymer as connecting bar in external skeletal fixators

OBJECTIVE: To investigate the mechanical characteristics of a nontoxic, low-cost, rigid polymer (RP) and to compare the structural and mechanical properties of a full-frame external skeletal fixator (ESF) with either RP connecting bars, polymethylmethacrylate (PMMA) connecting bars, or stainless-steel (SS) clamps and connecting bars. STUDY DESIGN: In vitro mechanical evaluation. METHODS: Mechanical properties were assessed using an in vitro bone fracture model with a bilateral uniplanar ESF (type II). Identical ESF were built with connecting bars using RP (n = 8), PMMA (n = 8), and SS connecting bars and clamps (System Meynard; n = 3). Nondestructive mechanical tests were performed in uniaxial compression (AC) and craniocaudal (CC) 4-point bending, as well as fatigue AC. Composite stiffness for each specimen and for each loading mode was calculated from 6 replicate measures using the slope of the load displacement curve at small displacements. RESULTS: RP, PMMA, and SS ESF constructs yielded mean +/- SD composite stiffness values of 227 +/- 15, 381 +/- 30, and 394 +/- 9 N/mm in AC and of 35 +/- 2, 24 +/- 2, and 15 +/- 0 N/mm in CC, respectively. CONCLUSIONS: Structural and mechanical properties of RP are satisfactorily rigid and fatigue resistant for its use as a connecting bar in ESF. CLINICAL RELEVANCE: RP connecting bars in an ESF are a reliable, versatile, nontoxic and inexpensive option for the veterinary surgeon.     

External fixation of the lumbar spine in a canine model

OBJECTIVE: To compare the mechanical properties of two types of external skeletal fixation of the lumbar spine with polymethylmethacrylate (PMMA)/Steinmann pin fixation in a canine unstable spine model. STUDY DESIGN: Cadaver study. SAMPLE POPULATION: Lumbar spines of 17 mature large-breed dogs. METHODS: Spine stiffness (N-m/deg) in flexion, extension, and rotation under physiological loading conditions and spine strength (N-m) in flexion were determined. Spines were destabilized at L3-L4, instrumented and retested. Fixation techniques included four-pin PMMA (PMMA4), eight-pin PMMA (PMMA8), eight-pin biplanar type I external skeletal fixator (ESF) (SK), and eight-pin spinal arch ESF (ARCHES). RESULTS: All fixation groups were as stiff as intact spines in extension and rotation and were significantly stiffer in flexion. In flexion, both PMMA8 and ARCHES were significantly stiffer than SK, and PMMA8 was significantly stiffer than PMMA4. In rotation, PMMA8 and ARCHES were significantly stiffer than SK, and in flexion to failure, PMMA8 and ARCHES were significantly stiffer than PMMA4. CONCLUSIONS: External skeletal spinal fixation (ESSF) has mechanical properties comparable to more commonly used PMMA/pin internal fixation techniques. CLINICAL RELEVANCE: External fixation of the canine spine has several potential advantages over internal fixation including minimal dissection for pin placement, the ability to span affected vertebrae with placement of implants distant from the site of injury, postoperative adjustability, and complete removal of implants after healing. This study supports the biomechanical stability ESSF of the canine lumbar spine. Further studies are indicated to evaluate zones of consistently safe and secure placement of pins and clinical efficacy.     

Static Strength Evaluation of Six External Skeletal Fixation Configurations

The relative strength of six different configurations of external skeletal fixation was determined by subjecting them to axial compression, and shear and torsion forces while measuring the load they would sustain before failing. The double clamp, single connecting bar, double connecting bar, quadrilateral frame, full pin splintage and three- dimensional tent configurations were found to be successively stronger in all three of the tests with the exception of the quadrilateral frame being more resistant to shear force than the full pin splintage configuration.     

Nonlinear increasing axial gap stiffness in type II external skeletal fixation: a mechanical study

OBJECTIVE: To quantify the effect on gap stiffness and cranial to caudal bending stiffness of conversion of the 6 distal clamps of planar bilateral fixator models to sliding clamps and the effect of attachment of composite beams to the sliding clamp models. STUDY DESIGN: Mdash;Mechanical testing performed on models. SAMPLE POPULATION: Five models using birch dowels and a commercially available external skeletal fixator system. METHODS: A segmentally comminuted, middiaphyseal fracture was simulated with the use of wooden dowels, and a bilateral 6-pin fixator was applied to create each of 5 models. The models were mechanically tested with all fixed clamps, with the 6 distal clamps converted to sliding clamps and with composite beams attached to the sliding clamp models. Testing was carried out in axial loading with physiologically relevant loads for a canine model, and in bending in the cranial to caudal plane. RESULTS: Sliding clamp fixators with composite beams attached exhibited a nonlinear increase in axial loading gap stiffness as load increased. The composite beam group also exhibited an increase in cranial to caudal bending stiffness as compared with fixed clamp and sliding clamp models. CONCLUSIONS: Using composite beam elements, planar bilateral external fixators can be constructed such that the fracture site would undergo controlled amounts of displacement at low loads and lessening displacement at higher loads. CLINICAL RELEVANCE: The nonlinear stiffness profile attained by the addition of composite beam elements to a planar external fixator allows controlled axial micromotion at the fracture site. Because controlled axial micromotion appears to stimulate fracture healing, a nonlinear stiffness profile of this type should enhance fracture healing.     

Biomechanical Evaluation of a Crimp Clamp System for Loop Fixation of Monofilament Nylon Leader Material Used for Stabilization of the Canine Stifle Joint

Objective —To test a crimp clamp system designed to secure monofilament nylon leader (MNL) material commonly used as lateral fabellotibial sutures (LFS) in extra-articular stabilization of the canine stifle joint.
Study Design — In vitro biomechanical tests of MNL loops secured with either the crimp clamp system or knots were performed. Suture loops (n = 94) were created from 27.3 kg tensile strength MNL and fastened with knots or crimp clamps. Tests were conducted on steam-sterilized, ethylene-oxide-sterilized, and nonsterilized MNL sutures. Loops were evaluated in single load tests and cycled tests. Values for load to failure, initial loop tension, loop elongation, mode of failure, and point of failure were determined.
Results —Crimp-clamped loops were superior to knotted loops in all parameters tested in both cycled and noncycled tests. Loop failure generally occurred by breaking within 3 mm of the fixation in both clamped and knotted tests. Loop elongation after cycling was greater in the knotted loops compared with clamped loops ( P < .001). Load to failure was greater in clamped tests than in knotted tests (P < .001), regardless of sterilization technique used. Significantly higher initial loop tension could be achieved with the clamp system compared with knot fixation ( P < .001).
Conclusions —The crimp clamp system provides superior in vitro loop fixation characteristics compared with knot fixation in 27.3 kg test MNL.
Clinical Relevance —Based on the results of biomechanical testing and the known biocompatibility of the system's implant components, clinical trials using the crimp clamp system are warranted.     

Epoxy Putty for Free-Form External Skeletal Fixators

Objective —To evaluate the suitability of epoxy putty for use as a connecting beam material in a free-form external skeletal fixator.
Design —Mechanical evaluation of beams and the pin-material interface of commonly used methacrylates and the proposed epoxy putty.
Procedure —The apparent modulus, bending strength, and toughness of 10 beams of three methacrylates (Technovit, APEF System, Bone Cement) and three epoxy putties (Oatey Epoxy Putty, All-Metals PowerPoxy, and Plumber's PowerPoxy) were determined in three-point bending. The shear strength of smooth and roughened-shaft pins embedded in the three methacrylates and the Oatey Epoxy Putty was determined by pull-out testing.
Results —The epoxy putties had similar strength, greater apparent modulus, and reduced toughness when compared with the methacrylates. The shear strength of the smooth pin interface with the Oatey Epoxy putty was greater than that with the methacrylates. The interface with roughened pins was much stronger than that with smooth pins for all materials tested.
Clinical Relevance —Epoxy putty is a suitable material for free-form external fixators. It is easy to handle, inexpensive, and has suitable setting times and mechanical properties.     

The Kirschner-Ehmer splint in small animal orthopedics

The Kirschner-Ehmer splint can be used to treat open, infected or highly comminuted fractures, gunshot fractures, nonunions, delayed unions, mandibular fractures and angular deformities in association with osteotomy, as well as to immobilize joints and as an adjunct to other fixation devices. After the fracture is reduced, 2-4 percutaneous pins are inserted with a Jacobs hand chuck through one or both cortices at 45-60 degrees to the longitudinal axis of the bone and attached to a connecting bar with clamps. Complications are minimized by not inserting the pin through large muscle masses, the fracture hematoma, large blood vessels or the incision line, avoiding encroachment of soft tissue with the clamps, and restricting the animal's activity during healing.     

Placement of Multiple Full Pins for External Fixation Technique and Results in Four Dogs

A technique using a third bar and guide clamp was developed to permit reduction and stabilization of distal limb fractures with full pins and two connecting bars, one on each side of the bone. The system uses only single Kirschner clamps. Four distal limb fractures were stabilized using this technique. Closed reduction was used in three dogs to avoid devitalization of fragments and avoid contamination of a closed fracture. The fracture site stability achieved with this configuration allowed early return to weight bearing and rapid bone healing.