An In Vitro Comparison of Hollow Ground and Trocar Points on Threaded Positive-Profile External Skeletal Fixation Pins in Canine Cadaveric Bone

OBJECTIVE: To compare the microstructural damage created in bone by pins with lathe-cut and rolled-on threads, and to determine the peak tip temperature and damage created by positive-profile external fixator pins with either hollow ground (HG) or trocar (T) tips during insertion. STUDY DESIGN: An acute, in vitro biomechanical evaluation. SAMPLE POPULATION: Twenty-seven canine tibiae. METHODS: Lathe-cut thread design with T point (LT-T), rolled-on thread design with T point (RT-T), and rolled-on thread design with HG point (RT-HG) pins were evaluated. Twenty pins of each type were inserted under constant drilling pressure into 12 canine tibiae (12 diaphyseal and 8 metaphyseal sites per pin type). Peak pin tip temperature, drilling energy, end-insertional pin torque, and pullout force were measured for each pin. For the histologic study, five pins of each type were inserted into cortical and cancellous sites in 15 additional tibiae. Entry and exit damage, and thread quality were assessed from 100 micron histologic sections by using computer-interfaced videomicroscopy. RESULTS: T-tipped pins reached higher tip temperature in both diaphyseal and metaphyseal bone compared with HG-tipped pins. RT-T pins had higher pullout strength (diaphyseal) and end-insertional torque compared with other combinations. No differences in drilling energy or insertional bone damage was found between the three pin types (P < .05). CONCLUSIONS: T-tipped pins mechanically outperformed HG-tipped pins. Pin tip and thread design did not significantly influence the degree of insertional bone damage. CLINICAL RELEVANCE: T-tipped pins may provide the best compromise between thermal damage and interface friction for maximizing performance of threaded external fixator pins.     

Biomechanical characteristics of allogeneic cortical bone pins designed for fracture fixation.

The biomechanical characteristics of 1.2 mm diameter allogeneic cortical bone pins harvested from the canine tibia were evaluated and compared to 1.1 mm diameter stainless steel pins and 1.3 mm diameter polydioxanone (PDS) pins using impact testing and four-point bending. The biomechanical performance of allogeneic cortical bone pins using impact testing was uniform with no significant differences between sites, side, and gender. In four-point bending, cortical bone pins harvested from the left tibia (204.8 +/- 77.4 N/mm) were significantly stiffer than the right tibia (123.7 +/- 54.4 N/mm, P = 0.0001). The site of bone pin harvest also had a significant effect on stiffness, but this was dependent on interactions with gender and side. Site C in male dogs had the highest mean stiffness in the left tibia (224.4 +/- 40.4 N/mm), but lowest stiffness in the right tibia (84.9 +/- 24.2 N/mm). Site A in female dogs had the highest mean stiffness in the left tibia (344.9 +/- 117.4 N/mm), but lowest stiffness in the right tibia (60.8 +/- 3.7 N/mm). The raw and adjusted bending properties of 1.2 mm cortical bone pins were significantly better than 1.3 mm PDS pins, but significantly worse than 1.1 mm stainless steel pins (P < 0.0001). In conclusion, cortical bone pins may be suitable as an implant for fracture fixation based on initial biomechanical comparison to stainless steel and PDS pins used in clinical practice.     

A Comparison of Acute Pull-Out Strength Between Two-Way and One-Way Transfixation Pin Insertion for External Skeletal Fixation in Canine Bone

This study tested the hypothesis that two-way insertion of an external skeletal fixator trans-fixation pin would weaken the pin-bone interface. Smooth and partially threaded (end) trans-fixation pins were placed in tibiae of 32 cadavers by slow speed drilling or hand placement through a predrilled pilot hole. In one bone of each tibial pair, pins were inserted 2 cm beyond the distal cortex and retracted to a predetermined position (two-way). In the contralateral limb, the pins were inserted in one forward motion to the predetermined position (one-way). The peak force (Newtons) required to extract the pins (pull-out strength) axially at a rate of 1 mm/sec was determined by using a universal testing machine. A significant (p < .05) decrease in pull-out strength was found in pins placed by two-way insertion (674 +/- 410) as opposed to one-way insertion (766 +/- 432). The results of this in vitro study suggest that one-way insertion should be used clinically to decrease weakening of the pin-bone interface and prevent possible failure of external fixators. A significantly greater pull-out strength was found for threaded pins placed in the proximal diaphysis (1459 +/- 330 Newtons) compared to the distal metaphysis (873 +/- 297 Newtons).     

Application and postoperative management of external skeletal fixators.

Application of external skeletal fixation involves preoperative assessment of the fracture with regards to healing potential of the bone and stabilizing requirements of the fixator. The fixator can be used alone or with supplemental (IM pin, cerclage, hemicerclage, Kirschner wires, bone screws) fixation to counteract shear, bending, and torsional forces at the fracture site. In addition, cancellous bone grafting can be used to enhance fracture healing. Rigid frames should be based on predrilling pilot holes followed by slow speed or hand insertion of smooth and threaded pins. Precise knowledge of regional anatomy precludes iatrogenic neurovascular or muscular tissue damage, which, subsequently, improves patient morbidity. Postoperative care of the fixator consists of bulky wraps to control pin-skin motion and cleaning of pin tract drainage sites. "Dynamization" or bone loading can be performed during fracture healing to stimulate osteosynthesis. This involves staged disassembly and reduction of frames by removing pins and connecting rods.     

The biomechanical properties of the feline femur

The purpose of this study was to determine the biomechanical properties of feline long bone by testing cadaver bone from mature cats in compression, three-point bending, notch sensitivity and screw pull-out strength. The determination of these properties is of clinical relevance with regard to the forces resulting in long bone fractures in cats as well as the behaviour and failure mode of surgical implants utilized for fracture stabilization and repair in the cat. Cadaveric cat femurs were tested in compression, three-point bending and in three-point bending after the addition of a 2.0 mm screw hole. Cortical screws, 2.7 mm in diameter, were inserted in cadaveric cat femur samples for screw pull-out testing. The mean maximum load to failure of mid diaphyseal feline femurs tested in compression was 4201+/-1218 N. Statistical analysis of the parameter of maximum load tested in compression revealed a statistical difference between sides (p=0.02), but not location (p=0.07), or location by side (p=0.12). The maximum strength of mid diaphyseal feline femurs tested in compression was 110.6+/-26.6 MPa. The modulus of elasticity of mid-diaphyseal cat femurs tested in compression was determined to be 5.004+/-0.970 GPa. The mean maximum load to failure of feline femurs tested in three-point bending was 443+/-98 N. The mean maximum load to failure of feline femurs tested in three-point bending after a 2.0 mm diameter hole was drilled in the mid-diaphyseal region of each sample through both cortices was 471+/-52 N. The mean maximum load required for screw pull-out of 2.7 mm cortical screws placed in feline femurs tested in tension was 886+/-221 N. This data should be suitable for investigating fracture biomechanics and the testing of orthopaedic constructs commonly used for fracture stabilization in the feline patient.     

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.     

Evaluation of compression generated by self compressing Orthofix bone pins and lag screws in simulated lateral humeral condylar fractures.

A simulated lateral humeral condylar fracture was created in each of the 52 humeri collected from 26 dogs. One humerus from each pair was stabilized with a 2.0 mm cortical bone screw which was inserted in lag fashion. The other humerus from each pair was stabilized with a 2.2 mm threaded diameter Orthofix pin inserted across the condyle. Prior to each repair, an antirotational K-wire was placed and then the Pressurex Sensitive film was inserted in the osteotomy site in order to determine the compressive pressure (MPa), compressive force (KN), and area of compression (cm(2)) achieved during fixation. The maximum insertional torque achieved before stripping was measured for each implant. The mean compression generated by insertion of a 2.0 mm lag screw was 20.36 +/- 1.51 MPa compared to 18.88 +/- 1.76 MPa generated by a 2.2 mm Orthofix pin (p < 0.003). The mean area of compression generated by insertion of a 2.0 mm lag screw was 2.39 +/- 1.29 cm(2), compared to 1.16 +/- 0.84 cm(2) generated by insertion of a 2.2 mm Orthofix pin (p < 0.0001). The mean compressive force (compression x area compressed) generated by insertion of a 2.0 mm lag screw was 4.96 +/- 2.90 Kn, compared to 2.20 +/- 1.65 Kn generated by insertion of a 2.2 mm Orthofix pin (p < 0.0001). The mean insertion torque to failure for the lag screws was 0.49 +/- 0.07 NM, compared to 0.91 NM +/- 0.18 NM generated by the Orthofix pins (P < 0.0001). Both repair methods are likely to be acceptable for the repair of similar fractures in small breed dogs.     

Biomechanical Comparison of the Trocar Tip Point and the Hollow Ground Tip Point for Smooth External Skeletal Fixation Pins

Objective —To compare the insertional characteristics of external fixator pins with hollow ground (HG), modified HG, and trocar (T) points.
Study Design —An acute, in vitro biomechanical evaluation.
Sample Population—Thirteen radii from canine cadavers.
Methods —A total of 16 T-tipped and 16 HG-tipped pins were inserted into 8 canine radii. Ten pins of each modification of the HG tip (length of the cutting edge reduced by 0.127 mm and 0.254 mm, respectively) were inserted into another five radii. All pins were inserted with low-speed power drilling and 80 N drilling load. Differences between peak tip temperature, drilling energy, and pullout force were determined for each pin type at both diaphyseal and metaphyseal locations.
Results —HG-tipped pins showed a 40% lower tip temperature in diaphyseal bone, a 25% reduction in drilling energy in diaphyseal bone, and a reduction of pullout force in both diaphyseal (65%) and metaphyseal (50%) bone compared with T-tipped pins. HG 0.254-mm pins generated higher tip temperatures and had greater pullout than HG pins in diaphyseal bone.
Conclusions —The HG tip was a more efficient design; however, the reduction in pullout force suggests that, because a better hole was drilled, radial preload is reduced. Reduction of the cutting edge by 0.254 mm increased the pullout force but also increased the temperatures.
Clinical Relevance —Thermal and microstructural damage are reduced by the HG tip, but pin-bone interface stability is also compromised. The use of a tip with 0.254 mm reduction in the cutting edge may optimize the biological and mechanical factors at the pin-bone interface.     

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.     

In vitro biomechanical comparison of three methods for internal fixation of femoral neck fractures in dogs

The in vitro biomechanical properties of three methods for internal fixation of femoral neck fractures were evaluated. Fifty cadaveric femura from Beagle dogs were used. Ten intact femora served as controls. In 40 femura, an osteotomy of the femoral neck was performed to simulate a transverse fracture. With the remaining 30 femura, three repair methods (two medium Orthofix pins, a 2.7 mm cortical bone screw placed in lag fashion and an anti- rotational Kirschner wire, or three divergent 1.1 mm Kirschner wires) were used to stabilize the osteotomies, and 10 osteotomies were stabilised per repair method. These 30 femura where then subject to monotonic loading to failure. Construct stiffness and load to failure were measured. In the remaining 10 femura, pressure sensitive film was placed at the osteotomy site prior to stabilization with either two Orthofix pins (n = 5) or a screw placed in lag fashion (n = 5) to determine the compressive pressure (MPa), compressive force (KN) and area of compression (cm2). There was no significant difference in the stiffness or load to failure for the three repair methods evaluated. There was no significant difference in the compressive pressure, compressive force or area of compression in osteotomies stabilized with Orthofix pins and 2.7 mm bone screws.     

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.     

Holding power of three different pin designs in the femur and ulna of the common buzzard (Buteo buteo)

External skeletal fixation is generally considered the best stabilization technique for immobilizing avian long bone fractures, but one of its major complications is the failure of bone-fixation pin interface or the loss of holding power. Consequently, this study is aimed at elucidating which pin design offers more pull-out strength in certain bones of the common buzzard (Buteo buteo). To achieve this objective, three pin designs (a smooth design and two negative profile threaded designs, with different thread pitch) were placed in five positions along the femur and ulna of the common buzzard. The pin pull-out strength was measured with the purpose of comparing medullary and pneumatic bones, insertion sites, and pin designs. Threaded pins with negative profile showed greater holding power than smooth pins (P < 0.05). When comparing holding power between the ulna and femur, no differences were found for smooth pins, whereas threaded pins showed more pull-out strength in the ulna than in the femur (P < 0.05). There were no differences observed related to pin location along the same bone when considering the same pin type. These results suggest that negative profile threaded pins have more holding power than smooth pins and that pneumatic bones provide less pull-out strength to negative profile threaded pins than medullary bones.     

Biomechanical Analysis of Stacked Plating Techniques to Stabilize Distal Radial Fractures in Small Dogs

Objective— To evaluate the fatigue life of partially stacked and fully stacked (1.5/2.0 and 2.0/2.7 mm) veterinary cuttable plates (VCP) in a fracture gap model of the distal aspect of the radius. Study Design— In vitro biomechanical study. Methods— Constructs (n=4/group) were assembled for each of 8 groups using 8‐hole plates (1.5/2.0 and 2.0/2.7 mm VCP) in the following configurations: unstacked; 2‐hole stacked centered over the gap (COG); 4‐hole stacked COG; and fully stacked. Plate(s) were secured to 2 separate polyvinylchloride pipe lengths, mounted to a mechanical testing system with a custom jig, and were loaded in axial compression for 10⁶ cycles at 10 Hz or until failure at 6–60 N for the 1.5/2.0 mm VCP and 10–100 N for the 2.0/2.7 mm VCP. Differences in number of cycles, stiffness, and failure mode were recorded. Results— All construct failures occurred through a screw hole adjacent to the gap. Fully stacked and 4‐hole stacked 1.5/2.0 and 2.0/2.7 mm VCP withstood 10⁶ cycles. Fatigue life and stiffness of the 1.5/2.0 or 2.0/2.7 mm unstacked constructs were significantly less than the other constructs. Differences were identified in stiffness among the 1.5/2.0 mm stacked constructs and in fatigue life among the 2.0/2.7 mm VCP stacked constructs. Conclusion— Four‐hole partially stacked VCP (either 1.5/2.0 or 2.0/2.7 mm) have comparable mechanical properties to fully stacked VCP. Clinical Relevance— Partial stacking of 2 holes of VCP on both sides of the fracture gap may provide sufficient strength for healing, but this premise must be tested in vivo.     

Effect of Fixation Pin Insertion on the Bone-Pin Interface

Smooth and partially threaded 3.12 mm (1/8 inch) trochar-tipped Steinmann pins were inserted transversely through both diaphyseal cortices of eight mature canine tibias using five methods. Angular velocity (revolutions per minute) during insertion and temperature elevation due to friction during penetration of the second cortex were recorded. The force required for extraction of the pins from the bone and the histologic appearance of the bone-pin interface were determined for one-half of the pins 2 days after insertion and for one-half of the pins 56 days after insertion. The increase in temperature was similar for all methods of insertion except high speed power, which was significantly greater (p < 0.05). The force required for axial pin extraction was similar for pins inserted by hand chuck, predrilled, and low speed power methods after both 2 and 56 days. Pins inserted by high speed power and hand drill required force similar to the others for extraction after 2 days but significantly less force (p < 0.05) for extraction after 56 days. The partially threaded pins required significantly greater force (p < 0.01) extraction after both 2 and 56 days. Histologic examination revealed increased mechanical bone damage surrounding hand chuck inserted pins, increased bone necrosis surrounding high speed power inserted pins, and increased inflammatory changes surrounding hand drill inserted pins.     

Biomechanical comparison of orthofix pins and cortical bone screws in a canine humeral condylar fracture model

OBJECTIVE: To compare shear stability of simulated humeral lateral condylar fractures reduced with either a self-compressing pin or cortical bone screw. STUDY DESIGN: In vitro biomechanical tests. SAMPLE POPULATION: Bilateral cadaveric canine humeri (n=18) without evidence of elbow disease. METHODS: Lateral condylar fracture was simulated by standardized osteotomy. Bone fragments were stabilized with a self-compressing pin or a cortical bone screw (2.7 or 3.5 mm) inserted in lag fashion. Specimens were mounted in a materials testing system and the condylar fragment displaced in a proximal direction until failure. Mechanical testing variables derived from load-deformation curves were compared between stabilization methods using a Student's paired t-test. RESULTS: There were no statistically significant differences for mechanical testing variables between pin and screw stabilized specimens at expected walk and trot loads. Three yield points subjectively coincided with yield of the interfragmentary interface (Y1), bone at the implant interface (Y2), and implant deformation (Y3). Displacements at Y1 were 48-156% greater for pin than screw stabilized specimens. Y2 and Y3 loads were higher for screw than pin stabilized specimens, but likely supraphysiologic for dogs convalescing after surgical repair. CONCLUSIONS: A self-compressing pin or a cortical bone screw inserted in lag fashion both provided adequate strength in applied shear to sustain expected physiologic loads through the repaired canine elbow during postoperative convalescence. CLINICAL RELEVANCE: Because self-compressing pins were easy to implant and mechanical properties were not significantly different than cortical screws at expected physiologic loads, pins should be considered for the repair of traumatic humeral condylar fractures.     

An In Vitro Comparison of the Holding Strength of Partially Threaded vs Nonthreaded Intramedullary Pins

The pin holding strengths of partially threaded and nonthreaded intramedullary pins were compared in an in vitro study. The mean percent holding strength of partially threaded pins to nonthreaded pins was 102%. No relationships could be determined when the holding strength of the pins was evaluated as a function of the dog's weight, bone length, and length of pin within the bone. It was concluded that no statistical difference in in vitro pin holding strength exists between the two types of pins.     

Comparison of insertion characteristics of two types of hydroxyapatite-coated and uncoated positive profile transfixation pins in the third metacarpal bone of horses

OBJECTIVE: To determine the effect of 2 hydroxyapatite pin coatings on heat generated at the bone-pin interface and torque required for insertion of transfixation pins into cadaveric equine third metacarpal bone. SAMPLE POPULATION: Third metacarpal bone pairs from 27 cadavers of adult horses. PROCEDURES: Peak temperature of the bone at the cis-cortex and the hardware and pin at the trans-cortex was measured during insertion of a plasma-sprayed hydroxyapatite (PSHA)-coated, biomimetic hydroxyapatite (BMHA)-coated, or uncoated large animal transfixation pin. End-insertional torque was measured for each pin. The bone-pin interface was examined grossly and histologically for damage to the bone and coating. RESULTS: The BMHA-coated transfixation pins had similar insertion characteristics to uncoated pins. The PSHA-coated pins had greater mean peak bone temperature at the cis-cortex and greater peak temperature at the trans-cortex (70.9 +/- 6.4(o)C) than the uncoated pins (38.7 +/- 8.4(o)C). The PSHA-coated pins required more insertional torque (10,380 +/- 5,387.8 Nmm) than the BMHA-coated pins (5,123.3 +/- 2,296.9 Nmm). Four of the PSHA-coated pins became immovable after full insertion, and 1 gross fracture occurred during insertion of this type of pin. CONCLUSIONS AND CLINICAL RELEVANCE: The PSHA coating was not feasible for use without modification of presently available pin hardware. The BMHA-coated pins performed similarly to uncoated pins. Further testing is required in an in vivo model to determine the extent of osteointegration associated with the BMHA-coated pins in equine bone.     

Rotational strength of double-pinning techniques in repair of transverse fractures in femurs of dogs

Transverse midshaft fractures of femurs from freshly euthanatized dogs were stabilized by means of 6 methods: (1) 3.5-mm bone plate and screws, (2) single intramedullary pin, (3) double intramedullary pins retrograded proximally and driven distally to the level of the femoral trochlea, (4) double intramedullary pins retrograded distally and driven proximally into the trochanteric region, (5) double intramedullary pinning in Rush pin fashion, and (6) multiple intramedullary pinning that filled the medullary cavity at the fracture site. All bones were subjected to torsional stress. The measured strain was converted to forces of torque and correlated with bone diameter to normalize the data. The forces of torque from each fixation technique were compared with each other and with the mean torque force necessary to fracture intact femurs. Torsional shear applied to plated femurs resulted in failure at a mean level of 33.8% of the calculated theoretic moment. Torsional forces were concentrated at one end of the plate and catastrophically failed at that point, whereas the fracture site remained rigidly fixed. There was no significant difference in the initial moment of torsional failure between the single intramedullary pin technique (0.05 Nm) and the double-pinning techniques (0.03 to 0.04 Nm). The multiple-pinning technique was 1.8 to 3 times as effective in resisting rotational forces, compared with the other pinning techniques, but not significantly so.     

Mechanical evaluation of pin and tension-band wire factors in an olecranon osteotomy model

OBJECTIVE: To evaluate the effect of altering pin and wire diameter, wire position and configuration, and osteotomy angle on applied load and absorbed strain energy in a pin and tension-band wire (PTBW) fixation model. STUDY DESIGN: In vitro mechanical study. SAMPLE POPULATION: Delrin models (n=96). METHODS: PTBW was applied to Delrin olecranon osteotomy models. A control configuration was defined and then altered, 1 variable (wire diameter, pin diameter, wire-hole position, wire configuration, osteotomy angle) at a time, to create 11 test configurations. Tensile force was applied and displacement at the caudal aspect of the osteotomy was measured. Fixation strength, in terms of tensile load and strain energy, was compared between control and each test configuration at 4 osteotomy displacements. RESULTS: Models with larger wire, pins, or combined figure-of-eight/lateral wires were stronger than control, whereas those with smaller wire, pins, or a solitary lateral wire were weaker. The superior strength of the larger wire was apparent for all assessed osteotomy displacement. CONCLUSIONS: PTBW fixation strength increases as implant diameter is increased, with wire diameter having greatest effect. Lateral wire configuration is weaker than figure-of-eight, but can be added to figure-of-eight configuration to increase strength. Wire-hole position and osteotomy angle have little effect on PTBW strength. CLINICAL RELEVANCE: Wire diameter is the key determinant of PTBW strength, whereas pin diameter is somewhat less critical. Wire passage through an additional hole proximally provides equivalent strength and may avoid soft-tissue entrapment and subsequent loosening.     

Fatigue analysis of plates used for fracture stabilization in small dogs and cats

OBJECTIVE: To evaluate the fatigue life of stacked and single, veterinary cuttable plates (VCP) and small, limited contact, dynamic compression plates (LC-DCP). STUDY DESIGN: In vitro biomechanical study. METHODS: Fracture models (constructs; n = 8) were assembled for each of 6 groups all with 8-hole plates: 2.0 mm LC-DCP; 2.4 mm LC-DCP; single 1.5/2.0 mm VCP; stacked 1.5/2.0 mm VCP; single 2.0/2.7 mm VCP; and stacked 2.0/2.7 mm VCP. Plate(s) were secured to 2 polyvinylchloride pipe lengths, mounted in a testing system with a custom jig, and subjected to axial loading (10-100 N) for 1,000,000 cycles at 10 Hz or until failure. Differences in number of cycles to failure among groups were compared. Failure mode was determined. RESULTS: All LC-DCP and single VCP constructs failed before 1,000,000 cycles. Stacked 2.0/2.7 mm VCP constructs withstood 1,000,000 cycles without failure. ANOVA and Fisher's least significant difference tests demonstrated significantly more cycles to failure for the stacked 1.5/2.0 mm VCP and stacked 2.0/2.7 mm VCP compared with the single 1.5/2.0 mm VCP, single 2.0/2.7 mm VCP, 2.0 mm LC-DCP, or 2.4 mm LC-DCP. Constructs that failed did so through a screw hole adjacent to the gap. CONCLUSION: Stacked VCP constructs have greater fatigue lives than comparably sized LC-DCP or single VCP constructs. Plates with 2.4 mm screws were not significantly different from the comparable construct with 2.0 mm screws. CLINICAL RELEVANCE: Although these data reveal that stacked VCP create a superior construct with respect to cyclic fatigue, surgeons must decide whether this is a clinical advantage on a case-by-case basis.