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.     

Stiffness of modified Type 1a linear external skeletal fixators.

Modifications of a Type 1a external skeletal fixator (ESF) frame were evaluated by alternately placing transfixation pins on opposite sides of the connecting rod (Type 1a-MOD) or by placing additional connecting rods on either of the two inside (Type 1a-INSIDE) or two outside (Type 1a-OUTSIDE) transfixation pins. The objective of this study was to evaluate the stiffness of these modifications in terms of axial compression (AC), cranial-caudal bending (CCB), and medial-lateral bending (MLB). We hypothesized that these designs would allow significant increase in unilateral frame stiffness, over Type 1a, without proportional increase in frame complexity or technical difficulty of application. All of the ESF frames were constructed using large IMEX SKtrade mark clamps, 3.2 mm threaded fixation pins, 9.5 mm carbon fibre connecting rods and Delrin rods as bone models. Nine, eight pin frames of each design were constructed, and subjected to repetitive non-destructive loading forces (AC, CCB, MLB) using a materials testing machine. Frame construct stiffness for each force (AC, CCB, MLB) was derived from load-deformation curve analysis and displayed in N/mm. Data revealed the 1a-MOD and 1a-OUTSIDE constructs had significantly increased stiffness in CCB and AC as compared to the Type 1a constructs while all of the modified constructs were significantly stiffer in MLB than the Type 1a constructs.     

In vitro biomechanical properties of linear, circular, and hybrid external skeletal fixation devices for use in large ruminants

OBJECTIVE: To report the biomechanical properties of 3 external skeletal fixation (ESF) devices for use in large ruminants. STUDY DESIGN: In vitro biomechanical testing of ESF constructs. SAMPLE POPULATION: Adult buffalo (weighing, 250-350 kg) tibiae (n=27). METHODS: ESF constructs (bilateral linear fixator [BLF], 4-ring circular external fixator [CEF], and hybrid fixator [HF]) were made using mild (low carbon) steel implants plated with nickel and cadaveric buffalo tibiae. After ESF application, a 1 cm mid-diaphyseal gap was created. Constructs were loaded to failure, on a materials testing machine, in axial compression (n=5/ESF type) and craniocaudal bending (n=3/ESF type). In addition, 3 CEF constructs were tested in intact tibiae under craniocaudal bending. RESULT: In compression, HF was the strongest and most rigid construct; yield load was significantly higher for HF than for BLF or CEF. Under bending, both CEF and HF had similar strength and modulus of elasticity. Strength for BLF was higher than CEF and HF, whereas the reverse was true for modulus of elasticity. CONCLUSIONS: ESF made from mild steel for use in large ruminants could withstand相似文献   

An in vitro biomechanical comparison of interlocking nail constructs and double plating for fixation of diaphyseal femur fractures in immature horses

OBJECTIVE: To compare the biomechanical properties of intact immature horse femurs and 3 stabilization methods in ostectomized femurs. Animal or SAMPLE POPULATION: Eighteen pairs of femurs from immature horses aged 1 to 15 months, and weighing 68 to 236 kg. METHODS: Thirty-four immature horse femurs were randomly assigned to 1 of 5 test groups: 1) interlocking intramedullary nail (IIN) (n = 6); 2) IIN with a cranial dynamic compression plate (I/DCP) (n = 6); 3) 2 dynamic compression plates (2DCP) (n = 8); 4) intact femurs tested to failure in lateromedial (LM) bending (n = 6); and 5) intact femurs tested to failure in caudocranial (CaCr) bending (n = 8). Mid-diaphyseal ostectomies (1 cm) were performed in all fixation constructs. Biomechanical testing consisted of 4 nondestructive tests: CaCr bending, LM bending, compression, and torsion, followed by bending to failure. All groups were tested to failure in LM bending with the exception of 1 group of intact femurs tested to failure in CaCr bending. Stiffness and failure properties were compared among groups. RESULTS: The 2DCP-femur construct had greater structural stiffness in nondestructive bending than the IIN-femur construct in either LM or CaCr bending, and the I/DCP-femur construct in LM bending. Only the I/DCP and 2DCP fixations were similar to intact bone in nondestructive-bending tests. In addition, the 2DCP-femur construct had greater structural and gap torsional stiffness than the I/DCP-femur construct, and greater gap torsional stiffness than the IIN-femur construct. However, all of the fixation methods tested, including the 2DCP-femur construct, had lower structural stiffness in torsional loading compared with intact bone. No significant differences in structural stiffness were found between intact bones and femur constructs tested nondestructively in compression. In resistance to LM bending to failure, the 2DCP-femur construct was superior to the IIN-femur construct, yet similar to the I/DCP-femur construct. Also, evaluation of yield and failure loads revealed no significant differences between intact bone and any of the femur constructs tested to failure in LM bending. CONCLUSIONS: In general, the 2DCP-femur construct provided superior strength and stiffness compared with the IIN and I/DCP-femur constructs under bending and torsion. CLINICAL RELEVANCE: Double plating of diaphyseal comminuted femoral fractures in immature horses may be the best method of repair, because in general, it provides the greatest strength and stiffness in bending and torsion.     

Fixation technique influences the monotonic properties of equine mandibular fracture constructs

OBJECTIVE: To determine the optimal fixation technique for equine interdental space fractures by evaluating the biomechanical characteristics of 4 fixation techniques. STUDY DESIGN: In vitro randomized block design. SAMPLE POPULATION: Twenty-seven adult equine mandibles. METHODS: Mandibles with interdental osteotomies were randomly divided into 4 fixation groups (n = 6/group). Fixation techniques were the following: (1) dynamic compression plates (DCP), (2) external fixator (EF), (3) external fixator with interdental wires (EFW), and (4) intraoral splint with interdental wires (ISW). Three intact (nonosteotomized) mandibles were tested as controls. Mandibles were subjected to monotonic cantilever bending until failure. Angular displacement data (radians) were derived from continuously recorded gap width measurements provided by extensometers placed across the osteotomy site. Osteotomy gap width data (mm) at 50 and 100 Nm were selected for standardized comparison of gap width before the yield point and failure point, respectively of all constructs tested. Stiffness (Nm/radian), yield strength (Nm), and failure strength (Nm) were determined from bending moment-angular displacement curves and were compared using ANOVA with appropriate post hoc testing when indicated. Radiographs were obtained prefixation, postfixation, and posttesting. RESULTS: Bending stiffness, yield, and ultimate failure loads were greatest for intact mandibles. Among osteotomized mandibles, stiffness was greatest for DCP constructs (P <.05) and was not significantly different among EF, EFW, and ISW constructs. Yield load was greatest for ISW constructs (P <.05) and was not significantly different among DCP and EFW constructs. Yield and ultimate failure loads were lowest (P <.05) and osteotomy gap width at 50 and 100 Nm were greatest for EF constructs (P =.09 and P <.05, respectively). There was no significant difference in failure loads and osteotomy gap widths among DCP, EFW, and ISW constructs (P <.05). Failure occurred through the screw-bone interface (DCP), acrylic splint (ISW), acrylic connecting bar and/or pin-bone interface (EF, EFW), and wire loosening (EFW). All 3 intact mandibles fractured through the vertical ramus at its attachment to the testing apparatus. CONCLUSIONS: Among osteotomized mandibles, DCP fixation had the greatest stiffness under monotonic bending to failure; however, the relatively low yield value may predispose it to earlier failure in fatigue testing without supplemental fixation. Techniques using tension-band wiring (EFW and ISW) were similar to DCP constructs in yield, failure, and osteotomy displacement, whereas EF constructs were biomechanically inferior to all other constructs. CLINICAL RELEVANCE: DCP fixation is most likely the most stable form of fixation for comminuted interdental space fractures. However, for simple interdental space fractures, ISW fixation may provide adequate stability with minimal invasiveness and decreased expense. Tension-band wiring significantly enhances the strength of type II external skeletal fixators and should be used to augment mandibular fracture repairs.     

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.     

Biomechanical comparison of a circular external skeletal fixator construct to pin and tension band wire fixation for the stabilization of olecranon osteotomies in dogs: a cadaveric study

OBJECTIVE: To compare anatomic reduction and the biomechanical properties of a circular external skeletal fixator (CESF) construct to pin and tension band wire (PTBW) fixation for the stabilization of olecranon osteotomies in dogs. STUDY DESIGN: Cadaveric study. ANIMALS: Forelimbs from 12 skeletally mature mixed-breed dogs, weighing 23 to 28 kg. METHODS: An olecranon osteotomy was stabilized with either a CESF construct or PTBW fixation. A single distractive load to failure was applied to each specimen through the triceps tendon. Osteotomy reduction and biomechanical properties were compared between fixation groups. RESULTS: Reduction was not significantly different (gap: P =.171; malalignment: P =.558) between fixation groups. Osteotomies stabilized with the CESF had greater stiffness (P <.0001) and maximum load sustained (P <.0001) compared to PTBW fixation. There was no significant difference for yield load (P =.318) or for load at 1 mm of axial displacement (P =.997) between fixation groups. Failure of fixation occurred by bending of the intramedullary Steinmann pin and the fixation wires in the CESF specimens and by untwisting of the tension band wire knot with pullout and bending of the Kirschner wires in the PTBW specimens. CONCLUSIONS: Specimens stabilized with the CESF construct had similar reduction and yield load, greater stiffness and maximum load sustained, and less elastic deformation than specimens stabilized with PTBW fixation. CLINICAL RELEVANCE: The CESF construct may provide a biomechanically favorable alternative to PTBW fixation for stabilization of olecranon osteotomies in dogs, and its application warrants clinical investigation.     

Biomechanical analysis of 3 fixation techniques in rabbit radius and humerus bones

The objective of this study was to compare the strength and stiffness of various fixation methods applied to the long bones of the rabbit forelimb. Twenty rabbit radius/ulna and 20 rabbit humeri were randomly assigned to 1 of 4 groups. Control bones remained intact, whereas all others were osteotomized to create fracture models that were fixated with locking plate and locking screws (LP), veterinary cuttable plate (VCP) with cortical screws, or external skeletal fixator constructs (ESF), and tested in 4-point bending until failure. Load/deformation curves were generated for each sample and used to calculate stiffness (slope of the curve) and strength (load to failure). Intact controls had greater strength and stiffness than any fixation techniques in the rabbit radius/ulna and humeri samples. Locking plate and VCP constructs had greater stiffness than ESF when applied to the radius, whereas locking plate constructs were stronger than VCP or ESF when applied to the humerus. Overall, the LP construct had characteristics most closely resembling those of the intact control regarding strength in the humerus. Therefore, fracture fixation with a LP would provide the greatest strength in humeral fracture repairs in the rabbit.     

An in vitro biomechanical investigation of an MP35N intramedullary interlocking nail system for repair of third metacarpal fractures in adult horses

OBJECTIVE: To compare monotonic mechanical properties of gap-ostectomized third metacarpal bones (MC3) stabilized with an MP35N interlocking nail system with contralateral intact bones. ANIMALS OR SAMPLE POPULATION: Twenty-four pairs of cadaveric equine MC3s. METHODS: Third metacarpal bones were divided into 4 mechanical testing groups (6 pairs per group): compression, palmarodorsal (PD) and mediolateral (ML) 4-point bending, and torsion. One MC3 from each pair was randomly selected as an intact specimen, and the contralateral gap ostectomized bone was stabilized with a 4-hole, 14-mm-diameter, 250-mm-long, MP35N intramedullary nail, and four, 7-mm-diameter, 60-mm-long MP35N interlocking screws (constructs). Mechanical testing properties were compared between intact specimens and constructs with a paired t test (significance set at P <.05). RESULTS: Intact specimens were significantly stronger and stiffer than constructs in all testing modes except PD bending. Constructs achieved mean yield strengths that were 57% (compression), 81% (PD bending), 68% (ML bending), and 78% (torque) of intact specimens. Constructs achieved mean stiffnesses that were 53% (compression), 58% (PD bending), 41% (ML bending), and 47% (torque) of intact specimens. CONCLUSION: Monotonic yield mechanical properties of MP35N intramedullary interlocking nail-stabilized, gap-ostectomized MC3 were lower than those of paired intact bones but exceeded reported in vivo loads for dorsopalmar bending and compression and estimated in vivo torsional loads. CLINICAL RELEVANCE: Considering the benefits associated with intramedullary interlocking nail fixation of fractures, this system should be considered for use for repair of MC3 fractures with applicable fracture configurations.     

Effects of ring diameter and wire tension on the axial biomechanics of four-ring circular external skeletal fixator constructs.

OBJECTIVE: To determine relative effects of ring diameter and wire tension on axial biomechanical properties of 4-ring circular external skeletal fixator constructs. SAMPLE POPULATION: 4-ring circular external skeletal fixator constructs and artificial bone models. PROCEDURE: 4-ring constructs were assembled, using 50-, 66-, 84-, or 118-mm-diameter rings. Two 1.6-mm-diameter fixation wires were attached to opposing surfaces of each ring at intersection angles of 90 degrees and placed through a gap-fracture bone model. Three examples of each construct were loaded in axial compression at 7 N/s to a maximum load of 400 N at each of 4 wire tensions (0, 30, 60, and 90 kg). Response variables were determined from resulting load-displacement curves (construct stiffness, load at 1 mm of displacement, displacement at 400 N). RESULTS: Ring diameter and wire tension had a significant effect on all response variables and had a significant interaction for construct stiffness and displacement at 400 N. Significant differences within all response variables were seen among all 4 ring diameters and all 4 wire tensions. As ring diameter increased, effect of increasing wire tension on gap stiffness and gap displacement at 400 N decreased. Ring diameter had a greater effect than wire tension on all response variables. CONCLUSIONS AND CLINICAL RELEVANCE: Although effects of wire tension decrease as ring diameter increases, placing tension on wires in larger ring constructs is important because these constructs are inherently less stiff. The differential contribution of ring diameter, wire tension, and their interactions must be considered when using circular external skeletal fixators.     

Effect of various distal ring-block configurations on the biomechanical properties of circular external skeletal fixators for use in dogs and cats

OBJECTIVE: To evaluate mediolateral, axial, torsional, and craniocaudal bending behavior of 6 distal ring-block configurations commonly used to stabilize short juxta-articular bone segments in small animals. SAMPLE POPULATION: 8 circular external skeletal fixator constructs of each of 6 distal ring-block configurations. The distal ring-block configurations were composed of combinations of complete rings, incomplete rings, and drop wires. PROCEDURE: Constructs were nondestructively loaded in axial compression, craniocaudal bending, mediolateral bending, and torsional loading by use of a materials testing machine. Gap stiffness was determined by use of the resultant load displacement curve. RESULTS: Circular external skeletal fixator configurations and constructs significantly affected gap stiffness in all testing modes. Within each loading mode, gap stiffness was significantly different among most configurations. In general, complete ring configurations were significantly stiffer than similar incomplete ring configurations, and addition of a drop wire to a configuration significantly increased stiffness of that configuration. CONCLUSIONS AND CLINICAL RELEVANCE: When regional anatomic structures permit, the use of complete ring configurations is preferred over incomplete ring configurations. When incomplete ring configurations are used, the addition of a drop wire is recommended.     

In vitro evaluation of five canine tibial plateau leveling methods

OBJECTIVE: To compare application time, accuracy of tibial plateau slope (TPS) correction, presence and magnitude of rotational and angular deformities, and mechanical properties of 5 canine tibial plateau leveling methods. SAMPLE POPULATION: 27 canine tibial replicas created by rapid prototyping methods. PROCEDURE: The application time, accuracy of TPS correction, presence and magnitude of rotational and angular deformation, and construct axial stiffness of 3 internal fixation methods (tibial plateau leveling osteotomy, tibial wedge osteotomy, and chevron wedge osteotomy [CWO]) and 2 external skeletal fixation (ESF) methods (hinged hybrid circular external fixation and wedge osteotomy linear fixation [WOLF]) were assessed. RESULTS: Mean bone model axial stiffness did not differ among methods. Mean application time was more rapid for WOLF than for other methods. Mean TPSs did not differ from our 5 degrees target and were lower for ESF methods, compared with internal fixation methods. Mean postoperative rotational malalignment did not differ from our target or among groups. Mean postoperative medio-lateral angulation did not differ from our target, except for CWO. Internal fixation methods lead to axially stiffer constructs than ESF methods. Reuse of ESF frames did not lead to a decrease in axial stiffness. CONCLUSIONS AND CLINICAL RELEVANCE: The 5 tibial plateau leveling methods had acceptable geometric and mechanical properties. External skeletal fixation methods were more accurate as a result of precise data available for determining the exact magnitude of correction required to achieve a 5 degrees TPS.     

An In Vitro Biomechanical Comparison of Dynamic Condylar Screw Plate Combined with a Dorsal Plate and Double Plate Fixation of Distal Diaphyseal Radial Osteotomies in Adult Horses

Objective— To compare stiffness and strength of a dynamic condylar screw plate combined with dorsal broad dynamic compression plate (DCS–bDCP) fixation with double broad dynamic compression plate (dbDCP) fixation used to repair oblique distal fractures of adult equine radii. Study Design— Experimental. Sample Population— Adult equine radii (n=10 pair). Methods— An unconstrained three‐dimensional loading–measurement system was used to determine stiffness of a 50 mm long intact, and then DCS–bDCP or dbDCP‐plated osteotomized/ostectomized segment of radii when subjected to a nondestructive sequence of compression, torsion, and lateral‐to‐medial (LM), medial‐to‐lateral (ML), cranial‐to‐caudal (CrCa), and caudal‐to‐cranial (CaCr) bending. Uniform load over the entire length of construct identified its weakest characteristics during torsion and LM and CrCa bending to failure. Results— No difference was observed between osteotomized/ostectomized DCS–bDCP and dbDCP construct stiffness for all 6 loading modes, and strength for all 3 failure loads. Ostectomized DCS–bDCP and dbDCP construct stiffness was significantly lower than osteotomized radii, the latter approaching intact for axial, LM, and CrCa bending. Most frequent failure was bone fracture through exit site of a screw located adjacent to osteotomy/ostectomy. Conclusions— DCS–DCP and dbDCP constructs had comparable strength and stiffness when repairing osteotomies/ostectomies in equine adult radius bone. Fracture reduction increased stiffness that approached intact bone for loads that placed the unplated side in compression. Clinical Relevance— DCS–bDCP and dbDCP constructs are comparable in stiffness and strength when applied to oblique distal diaphyseal osteotomies/ostectomies in equine radius bone. However, the DCS's localized effect on distal epiphyseal structure because of additional bone removal remains to be investigated under in vivo articular loading conditions.     

Biomechanical Comparison of Two Alternative Tibial Plateau Leveling Osteotomy Plates with the Original Standard in an Axially Loaded Gap Model: An In Vitro Study

Objective— To compare the axial compression stiffness of osteotomized canine tibiae stabilized with Slocum, Securos, or Synthes plates after a tibial plateau leveling osteotomy (TPLO) procedure. Study Design— In vitro, paired comparison of cadaveric tibial constructs subjected to mechanical testing under an axial load. Sample Population— Canine tibiae (n=16 pairs) from skeletally mature male and female dogs of various breeds (18–55 kg). Methods— Tibial pairs (n=16) were randomly assigned to 1 of 2 study cohorts (n=8 pairs/cohort): cohort 1, tibial osteotomy stabilization with a Slocum or a Securos plate, or cohort 2, tibial osteotomy stabilization with a Slocum or a Synthes plate. One tibia from each pair was stabilized with 1 of each plate design assigned to the cohort after TPLO. A 3.2 mm osteotomy gap was maintained during plate application in all constructs. Load and axial displacement were recorded while constructs were loaded to 2000 N in axial compression. Failure loads were not reported because no distinct yield point or failure point was evident within the load range for many specimens. Failure modes were recorded for each construct, and photographs of typical failures were obtained. Stiffness (N/mm) was calculated from load–displacement curves. Paired comparisons of mean stiffness were performed within study groups using a paired t‐test. Significance was set at P<.05. Results— The mean construct stiffnesses for the Slocum (383±183 N/mm) and Securos (258±64.1 N/mm) constructs were not significantly different (P=.164; power=0.566). The mean construct stiffness for the Synthes constructs (486±91.0 N/mm) was significantly greater than that of the Slocum constructs (400±117 N/mm); P=.0468. Modes of failure for the Slocum (16/16) and Securos (8/8) constructs included plastic deformation of the implant with valgus deformity combined with fibular luxation (2/16 Slocum; 1/8 Securos) or fibular fracture (2/16 Slocum; 4/8 Securos). Most Synthes constructs underwent elastic deformation (7/8). One Synthes construct fractured in the saggital plane through the tibial plateau depression at the point of load application. Conclusions— The Slocum and Securos plate/tibia construct have similar stiffness, whereas the Synthes/tibia constructs are significantly stiffer than the Slocum/tibia constructs. Modes of fixation failure observed in this model were consistent with TPLO fixation failures observed clinically. Clinical relevance— Construct stiffness in axial load varies with implant type. Implants that confer higher stiffness to the construct may result in greater fixation stability in tibial metaphyseal osteotomies.     

Nonlinear stiffness profiles of external fixators constructed with composite rods

OBJECTIVE: To determine if composite connecting rods confer nonlinear stiffness characteristics on unilateral and bilateral external skeletal fixators (ESF) in cranial-caudal bending and axial loading. STUDY DESIGN: Mechanical testing performed on models. SAMPLE POPULATION: Six models of 6-pin ESF constructs composed of birch dowels, a commercial ESF system, and composite connecting rods. METHODS: Unilateral and bilateral ESF configurations were assembled using either specially designed composite titanium and silicone (composite group) or solid titanium (solid group) connecting rods. Mechanical testing was performed in axial loading and 4-point cranial-caudal bending. Stiffness was determined at a low and high-load range, and was considered increasing and nonlinear if the stiffness at high loads was greater than at low loads. RESULTS: The stiffness of the solid group was linear in all testing modes and configurations. Bilateral composite fixators had a nonlinear increasing stiffness in axial loading and cranial-caudal bending. Unilateral composite fixators had a nonlinear increasing stiffness in axial loading, but not cranial-caudal bending. Solid connecting rods conferred a higher stiffness in all testing modes and configurations. CONCLUSIONS: Composite connecting rods resulted in nonlinear increasing axial and bending stiffness in bilateral fixators, and in axial load in unilateral fixators. CLINICAL RELEVANCE: Conventional ESF can be constructed so that the stiffness increases as load increases. This provides the surgeon with additional options to control the local mechanical environment of a healing fracture, which may be used to enhance fracture healing.     

An in vitro biomechanical investigation of an intramedullary nailing technique for repair of third metacarpal and metatarsal fractures in neonates and foals

OBJECTIVE: To evaluate a dorsoproximal extra-articular approach for insertion of 8.25-mm, solid-titanium, intramedullary (IM) interlocking nails into ostectomized foal third metacarpal (MC3) and third metatarsal (MT3) bones; to compare the monotonic mechanical properties of IM nail constructs with paired intact bones; and to determine the effects of age, body weight, fore- or hindlimb, and left or right limb on the mechanical testing variables. ANIMAL OR SAMPLE POPULATION: Twenty bone pairs (10 MC3, 10 MT3) collected from 10 foals of various weights and ages. METHODS: One bone from each pair was randomly selected to be ostectomized and stabilized using an 8.25-mm, solid-titanium IM nail, and four 3.7-mm titanium interlocking screws (construct). Constructs and contralateral intact bone specimens were tested in axial compression and palmaro-/plantarodorsal (PD) 4-point bending. Monotonic mechanical properties were compared between intact specimens and constructs with an ANOVA; significance was set at P <.05. RESULTS: Nail insertion caused bone failure in 6 MC3 and 2 MT3. In general, mean mechanical testing values indicated that intact specimens were significantly stronger and stiffer than constructs for all age and weight ranges when tested in compression and PD 4-point bending (P <.05). Bone strength and stiffness of intact specimens tested in compression and bending tended to increase linearly with age and weight. CONCLUSION: IM interlocking nail fixation of gap-ostectomized MC3 and MT3 with 8.25-mm IM nails and 3.7-mm interlocking screws did not achieve sufficient strength or stiffness to be recommended as the sole means of repair for comminuted MC3 and MT3 fractures in young foals. CLINICAL RELEVANCE: IM interlocking nail fixation of foal cannon bone fractures may be useful to decrease soft-tissue disruption at the fracture site; however, there is a risk of bone failure associated with extra-articular insertion. This method should be combined with other forms of external coaptation for added stability in axial compression and PD bending.     

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.     

Mechanical comparison of an interlocking nail locked with conventional bolts to extended bolts connected with a type-IA external skeletal fixator in a tibial fracture model

OBJECTIVE: To compare the structural properties of interlocking nails (ILNs) locked with bolts (ILNb) to ILN locked with extended bolts connected with a type-IA external skeletal fixator (ILN-ESF) in a fracture gap model. STUDY DESIGN: Experimental study. SAMPLE POPULATION: Synthetic tibial bone substitutes. METHODS: Custom-made synthetic tibial bone substitutes were implanted with standard ILNs locked with either bolts or extended bolts connected to an external skeletal fixation (ESF). Constructs were tested in torsion, bending, and axial compression (n=4/testing mode). Data, consisting of construct compliance and associated deformation, were compared using t-tests. RESULTS: The ILN-ESF construct compliance and deformation were significantly less than those of the ILNb construct in torsion, bending, and compression (P<.001). Slack was present in the ILNb construct under torsion and bending, but not in the ILN-ESF construct, regardless of testing mode. CONCLUSIONS: Substitution of locking bolts with extended bolts connected to an ESF significantly reduced the construct compliance and overall deformation in torsion, bending, and compression. Furthermore, the inherent slack of the ILNb was eliminated by the use of an ESF in torsion and bending. CLINICAL RELEVANCE: The improvement in structural properties of the ILN-ESF constructs could diminish interfragmentary motion at the fracture site and potentially improve bone healing.     

Effect of a supplemental plate on the stiffness of a type I external fixator

OBJECTIVE: To assess the effect of a supplemental plate on the stiffness of a six-pin unilateral external skeletal fixator. STUDY DESIGN: Mechanical testing performed on models. METHODS: Wooden (birch) dowels were used to create five models of a fracture. A commercially available external fixation system was applied to the model with a uniform unilateral six-pin fixator design. The models were mechanically tested with and without a supplemental plate attached to the 2 clamps adjacent to the fracture gap. Testing was conducted in axial loading, medial to lateral bending, and cranial to caudal bending. RESULTS: Results showed a 4.42-fold increase in stiffness in axial load, a 4.23-fold increase in stiffness in medial to lateral bending, and a 1.94-fold increase in stiffness in cranial to caudal bending with the addition of the plate. CONCLUSIONS: The addition of a supplemental plate increases the mechanical stiffness of unilateral fixators. This was especially true in axial load and medial to lateral bending. CLINICAL RELEVANCE: A supplemental plate can be used with unilateral fixators to increase stiffness of the fixator. Conversely, the plate can be removed to decrease stiffness without the removal of fixation pins.     

An In Vitro Biomechanical Study of a Multiplanar Circular External Fixator Applied to Equine Third Metacarpal Bones

The biomechanical characteristics of a 4-ring circular multiplanar fixator applied to equine third metacarpal bones with a 5 mm mid-diaphyseal osteotomy gap were studied. Smooth Steinmann pins, either 1/8 inch, 3/16 inch, or 1/4 inch, were driven through pilot holes in the bone in a crossed configuration and full pin fashion and fastened to the fixator rings using cannulated fixation bolts. The third metacarpal bone fixator constructs were tested in three different modes (cranial-caudal four-point bending, axial compression, and torsion). Loads of 2,000 N were applied in bending and axial compression tests and a load of 50 N ± m was applied during testing in torsion. Fixator stiffness was determined by the slope of the load displacement curves. Three constructs for each pin size were tested in each mode. Comparisons between axial stiffness, bending stiffness, and torsional stiffness for each of the three different pin sizes were made using one-way analysis of variance. There was no visually apparent deformation or permanent damage to the fixator frame, and no third metacarpal bone failure in any of the tests. Plastic deformation occurred in the 1/8 inch pins during bending, compression, and torsion testing. The 3/16 inch and 1/4 inch pins elastically deformed in all testing modes. Mean (±SE) axial compressive stiffness for the 1/8 inch, 3/16 inch, and 1/4 inch pin fixator constructs was: 182 ± 16 N/mm, 397 ± 21 N/mm, and 566 ± 8.7 N/mm; bending stiffness was 106 ± 3.3 N/mm, 410 ± 21 N/mm, and 548 ± 12 N/mm; and torsional stiffness was 6.15 ± 0.82 N.m/degree, 7.14 ± 0.0 N±m/degree, and 11.9 ± 1.0 N.m/degree respectively. For statically applied loads our results would indicate that a 4-ring fixator using two 1/4 inch pins per ring may not be stiff enough for repair of an unstable third metacarpal bone fracture in a 450 kg horse.