Ex vivo comparison of one versus two distal screws in 8 mm model 11 interlocking nails used to stabilize canine distal femoral fractures

Mechanical evaluation of a novel angle-stable interlocking nail in a gap fracture model

OBJECTIVE

To describe the mechanical characteristics of a novel angle-stable interlocking nail (NAS-ILN) and compare them to those of a locking compression plate (LCP) by using a gap-fracture model.

STUDY DESIGN

Experimental study.

SAMPLE POPULATION

Synthetic bone models.

METHODS

Synthetic bone models simulating a 50 mm diaphyseal comminuted canine tibial fracture were treated with either a novel angle-stable interlocking nail (NAS-ILN) or a locking compression plate (LCP). Maximal axial deformation and load to failure in compression and 4-point bending, as well as maximal angular deformation, slack, and torque to failure in torsion, were statistically compared (P < .05).

RESULTS

In compression, the maximal axial deformation was lower for NAS-ILN (0.11 mm ± 0.03) than for LCP (1.10 mm ± 0.22) (P < .0001). The ultimate load to failure was higher for NAS-ILN (803.58 N ± 29.52) than for LCP (328.40 N ± 11.01) (P < .0001). In torsion, the maximal angular deformation did not differ between NAS-ILN (22.79° ± 1.48) and LCP (24.36° ± 1.45) (P = .09). The ultimate torque to failure was higher for NAS-ILN (22.45 Nm ± 0.24) than for LCP (19.10 Nm ± 1.36) (P = .001). No slack was observed with NAS-ILN. In 4-point bending, the maximal axial deformation was lower for NAS-ILN (3.19 mm ± 0.49) than for LCP (4.17 mm ± 0.34) (P = .003). The ultimate bending moment was higher for NAS-ILN (25.73 Nm, IQR [23.54-26.86] Nm) than for LCP (16.29 Nm, IQR [15.66-16.47] Nm) (P = .002).

CONCLUSION

The NAS-ILN showed greater stiffness in compression and 4-point bending, and a greater resistance to failure in compression, torsion, and 4-point bending, than LCP.

CLINICAL IMPACT

Based on these results, NAS-ILNs could be considered as alternative implants for the stabilization of comminuted fractures.

Evaluation of a Feline Bone Surrogate and In Vitro Mechanical Comparison of Small Interlocking Nail Systems in Mediolateral Bending

OBJECTIVE

 The aim of this study was to (1) evaluate bending structural properties of a machined short fibre epoxy (SFE) feline bone surrogate (FBS), (2) compare the bending behaviour of small angle-stable interlocking nails (I-Loc; Targon) and locking compression plates (LCP) and (3) evaluate the effect of implant removal on FBS bending strength.

METHODS

 Part 1: Feline cadaveric femurs (n = 10) and FBS (n = 4) underwent cyclic four-point bending and load to failure. Part 2: Fracture gap FBS constructs (n = 4/group) were stabilized in a bridging fashion with either I-Loc 3 and 4, Targon 2.5 and 3.0, LCP 2.0 and 2.4, then cyclically bent. Part 3: Intact FBS with pilot holes, simulating explantation, (n = 4/group) underwent destructive bending tests. Bending compliance, angular deformation and failure moment (F_M) were statistically compared (p < 0.05).

RESULTS

 Native bone and FBS were similar for all outcome measures (p > 0.05). The smallest and largest bending compliance and angular deformation were seen in the I-Loc 4 and LCP 2.0 respectively (p < 0.05). While explanted Targon FBS had the lowest F_M (p < 0.05), I-Loc and LCP constructs F_M were not different (p > 0.05).

CONCLUSION

 The similar bending properties of short fibre epoxy made FBS and native feline femurs suggest that this model could be used for mechanical testing of implants designed for feline long bone osteosynthesis. The I-Loc constructs smaller angular deformation which also suggests that these implants represent a valid alternative to size-matched Targon and LCP for feline fracture osteosynthesis. The significantly lower F_M of explanted Targon may increase the risk of secondary fracture following implant removal.

Biomechanical comparison of a new expandable intramedullary nail and conventional intramedullary nails for femoral osteosynthesis in dogs

Intramedullary nailing of diaphyseal femoral fractures is a commonly used treatment method in dogs because of its biological and biomechanical advantages compared to bone plating. To achieve adequate resistance of the intramedullary nail against torsional and axial compressive forces, additional application of transcortical screws is needed. As these interlocking screws represent a frequent cause of post-operative complications, a new expandable intramedullary nail (EXPN) was developed, which was designed to provide adequate fracture stabilisation without the need for transcortical fixation. The evaluation of the biomechanical properties of the new EXPN with regard to torsional, compressive and bending stability as well as direct comparison to the biomechanical properties of conventional Steinmann (STMN)- and interlocking (ILN) nails was carried out with different biomechanical test arrangements. No significant statistical differences regarding the torsional and bending resistance between the EXPN and ILN group were seen, which indicates that rotatory as well as bending stability of the innovative EXPN is similar to the conventional ILN. Nevertheless, the percentage deviation between the attempted and successfully reached physiological compressive forces was significantly higher (p = 0.045) in the EXPN group compared to the ILN group, which indicates that the compressive stability of the innovative EXPN might be weaker compared to the ILN. In summary, the new EXPN represents an interesting alternative to conventional intramedullary nails. However, in direct comparison to conventional interlocking nails, the EXPN has shown weaknesses in the neutralization of axial compressive forces, which indicates that at least biomechanically the interlocking nail seems advantageous. Further in-vitro and in-vivo investigations are required before clinical use can be recommended.

Mechanical comparison of two small interlocking nails in torsion using a feline bone surrogate

OBJECTIVE

To compare the torsional behavior of two small angle-stable interlocking nails (I-Loc and Targon) with that of locking compression plates (LCP). To evaluate the effect of implant removal on the torsional strength of feline bone surrogates.

STUDY DESIGN

Experimental.

SAMPLE POPULATION

Fracture gap constructs and intact explanted bone surrogates.

METHODS

Fracture gap constructs were stabilized with one of six implants (I-Loc 3 and 4, Targon 2.5 and 3.0, LCP 2.0 and 2.4) and then cyclically tested in torsion (n = 4/group). To simulate implant removal, intact surrogates with implant-specific pilot holes were then twisted to failure (n = 4/group). Torsional compliance (TC; °/Nm), angular deformation (AD; °), and failure torque (F_T ; Nm) were statistically compared (P < .05).

RESULTS

The I-Loc 4 had the smallest TC and AD of all constructs (P < .05). The largest TC (P < .05) was seen with the LCP 2.0. The Targon 2.5 had the largest AD (P < .05) secondary to locking interface slippage. Targon surrogates F_T were the lowest of all groups (P < .05). Conversely, there was no difference between the F_T of the I-Loc, LCP, and intact surrogates (P > .05).

CONCLUSION

We showed that I-Loc nails provided greater torsional stability than size-matched Targon nails and LCPs. Conversely, Targon 2.5 locking interface slippage may jeopardize that construct's stability. Furthermore, the significantly reduced bone surrogate torsional strength provided evidence that the large Targon bolt holes increased the risk of postexplantation iatrogenic fracture.

CLINICAL SIGNIFICANCE

Our results provide evidence to conclude that the small I-Loc nails may be valid alternatives to other osteosynthesis options for feline fracture repair.

An in vitro biomechanical investigation of an interlocking nail system developed for buffalo tibia

Objectives: The objectives of the study were to determine the mechanical properties of a customized buffalo interlocking nail (BIN), intact buffalo tibia, and ostectomized tibia stabilized with BIN in different configurations, as well as to assess the convenience of interlocking nailing in buffalo tibia.

Methods: The BIN (316L stainless steel, 12 mm diameter, 250 mm long, nine-hole solid nails with 10° proximal bend) alone was loaded in compression and three-point bending (n = 4 each); intact tibiae and ostectomized tibiae (of buffaloes aged 5–8 years, weighing 300–350 kg) stabilized with BIN using 4.9 mm standard or modified locking bolts (4 or 8) in different configurations were subjected to axial compression, cranio-caudal three-point bending and torsion (n = 4 each) using a universal testing machine. Mechanical parameters were determined from load-displacement curves and compared using Kruskal-Wallis test (p <0.05).

Results: Intact tibiae were significantly stronger than BIN and bone-BIN constructs in all testing modes. The strength of fixation constructs with eight locking bolts was significantly more than with four bolts. Overall strength of fixation with modified locking bolts was better than standard bolts. Based on technical ease and biomechanical properties, cranio-caudal insertion of bolts into the bone was found better than medio-lateral insertion.

Clinical significance: The eight bolt BINbone constructs could be useful to treat tibial fractures in large ruminants, especially buffaloes.

Angle stable nails provide improved healing for a complex fracture model in the femur

BACKGROUND

Conventional nails are being used for an expanding range of fractures from simple to more complex. Angle stable designs are a relatively new innovation; however, it is unknown if they will improve healing for complex fractures.

QUESTIONS/PURPOSES

When comparing traditional and angle stable nails to treat complex open canine femur fractures, the current study addressed the following questions: do the two constructs differ in (1) radiographic evidence of bone union across the cortices; (2) stability as determined by toggle (torsional motion with little accompanying torque) and angular deformation; (3) biomechanical properties, including stiffness in bending, axial compression, and torsional loading, and construct failure properties in torsion; and (4) degree of bone tissue mineralization?

METHODS

Ten hounds with a 1-cm femoral defect and periosteal stripping were treated with a reamed titanium angle stable or nonangle stable nail after the creation of a long soft tissue wound. Before the study, the animals were randomly assigned to receive one of the nails and to be evaluated with biomechanical testing or histology. After euthanasia at 16 weeks, all operative femora were assessed radiographically. Histological or biomechanical evaluation was conducted of the operative bones with nails left in situ compared with the nonoperative contralateral femora.

RESULTS

Radiographic and gross inspection demonstrated hypertrophic nonunion in all 10 animals treated with the nonangle stable nail, whereas six of 10 animals treated with the angle stable nail bridged at least one cortex (p = 0.023). The angle stable nail construct demonstrated no toggle in nine of 10 animals, whereas all control femora exhibited toggle. The angle stable nail demonstrated less angular deformation and toggle (p ≤ 0.005) and increased compressive stiffness (p = 0.001) compared with the conventional nonangle stable nail. Histology demonstrated more nonmineralized tissue in the limbs treated with the conventional nail (p = 0.005).

CONCLUSIONS

Angle stable nails that eliminate toggle lead to enhanced yet incomplete fracture healing in a complex canine fracture model.

CLINICAL RELEVANCE

Care should be taken in tailoring the nail design features to the characteristics of the fracture and the patient.

A modified intramedullary nail interlocking design yields improved stability for fatigue cycling in a canine femur fracture model

Intramedullary nailing has evolved to become the standard of care for most diaphyseal femoral and tibial fractures, as well as an expanding number of metaphyseal fractures. Owing to the unstable nature of some fractures, the intramedullary device may be subjected to significant stresses owing to a lack of solid cortical contact after nailing. In such cases, excessive interfragmentary motion (due to construct toggle) has been shown to occur. Such motion increases the likelihood of a non- or delayed-union. In the current study, two versions of a modified, angle stable interlocking design were subjected to fatigue testing in a segmental defect fracture model representing a canine femur. As a control, a third group of constructs were stabilized with a traditional nail that allowed a small amount of toggle. All constructs were subjected to 50,000 fatigue cycles representing 12 weeks of cage activity at physiologic levels of combined axial-torsional loading. Torsional testing pre- and post-fatigue revealed 4.6 +/- 1.3 degrees of toggle in the traditional nail and no toggle with the angle stable nail designs. The stable nails were also significantly stiffer in axial compression and torsion before and after cycling. These data indicate that the enhanced stability of the modified interlocking designs can be maintained throughout fatigue cycling in a challenging fracture model.

Development of an interlocked nail for segmental defects in the rabbit tibia

Previous animal models have been developed to study intramedullary nailing for challenging segmental defects in the tibia. In large animals, interlocked nail fixation created a stable environment suitable to study new bone growth technologies placed in the defect. To our knowledge, there are no comparable interlocked tibial defect models for the rabbit in which new technologies could be evaluated. Such a model would be helpful since the rabbit is a popular initial model for orthopedic research studies owing to its wide availability and low cost. While numerous studies have nailed the rabbit tibia, all were non-locked implants that allowed some degree of instability between the fracture fragments. In addition, the non-locked nails were constructed of stainless steel, whereas human nails are increasingly made from titanium alloy. In the current study, an interlocked titanium nail was developed for the rabbit tibia. It was implanted in cadaver tibiae and subjected to fatigue cycling in combined compression and bending at physiologic levels to 21,061 cycles. This duration is estimated to represent 12 weeks of gait by the animal. Before and after fatigue cycling, monotonic testing was performed in compression and bending at physiologic levels. The intact contralateral limbs served as controls. All limbs completed the cycling; the instrumented limbs exhibited interfragmentary cyclic strain amplitudes during fatigue (616 ± 139 microstrain), which was significantly greater than the control limbs (136 ± 35 microstrain). Monotonic strain amplitudes for the test limbs in bending and compression were 4839 ± 1028 and 542 ± 122 microstrain, respectively; corresponding values for the control bones were 407 ± 118 and 95 ± 38 microstrain, respectively. These data are similar to those presented in prior studies in larger bone models. The current study presents one method for interlocked nail fixation for this complex tibial shaft fracture in a small animal.

Introduction of a new interlocked intramedullary nailing device for stabilization of critically sized femoral defects in the rat: A combined biomechanical and animal experimental study

The goals of this study were to develop a new intramedullary, rotation-stable locking device and evaluate it biomechanically and in vivo for maintenance of a critical size osteotomy gap in a model of conscious pseudarthrosis. In standardized osteotomized rat femora (5 mm osteotomy gap) two different rotation- and axial-stable locking devices (group pS + cS) were tested in vitro with respect to biomechanics and compared to a control group without an additional locking device (K; n = 6 for each group). For in vivo studies, 27 male Sprague Dawley rats (250-300 g) underwent a femoral defect osteotomy of critical size and were stabilized by one of the three methods (n = 9 for each group). All groups were examined radiologically postoperatively, after 14 days, and after 12 weeks. In vitro testing revealed higher compression and torsional rigidities for the two locking devices (p < 0.05) compared to the control group (compression rigidity: pS = 103.6 +/- 13.2; cS = 91.3 +/- 10.9; K = 52.8 +/- 8.4 N/mm; torsional rigidity: pS = 5.9 +/- 0.9; cS = 4.3 +/- 1.4; K = 0.4 +/- 0.1 Nmm/ degrees ). In vivo, group K and pS exhibited up to two thirds wire dislocation and reduction of the osteotomy gap, while dislocation was less frequent in the cS group. Thus, the locking device with compression of the wire showed advantages in rotational and axial stability for a critically sized defect, though the osteotomy gap could not be maintained in all cases over the 12-week period. Nevertheless, our data corroborate the necessity of an internal fixation device with sufficient axial and rotational stability.

Two Techniques for Supplementing Interlocking Nail Repair of Fractures of the Humerus, Femur, and Tibia: Results in 12 Dogs and Cats

OBJECTIVE

To describe 2 devices for improving stabilization of inadequately stabilized interlocking nail (ILN) repairs of the humerus, tibia, and femur in dogs and cats.

STUDY DESIGN

Prospective study.

ANIMALS

Twelve client-owned dogs and cats.

METHODS

Two devices to further stabilize ILN repair of inadequately stabilized diaphyseal fractures were developed. Device 1 was an axial extension for the ILN that was connected to a conventional type I external skeletal fixator (ESF) with a short connecting bar. Device 2 had hybrid ILN bolt/ESF pins that were used to lock the ILN and serve as the pins for a type I ESF. Devices were used at the initial surgery when the stability of ILN repair was considered inadequate based on palpable fracture segment movement, insufficient medullary canal filling of the ILN at the fracture site, or when the ILN was used in a buttress mode. Outcome was obtained by recheck examinations, radiography, and telephone interview.

RESULTS

Device 1 was applicable to fractures of the humerus and femur, but was not used for fractures of the tibia because the ILN extension would have interfered with the stifle. No gross loosening of the ILN/ESF extension connection to the ILN occurred. Device 2 was easily placed and used in the humerus, femur, and tibia. Device 2 allowed removal of the ILN interlock to one or both main fracture segments non-invasively. Clinically, both devices added stability compared with ILN repair alone. Both devices facilitated controlled destabilization of the fracture repair as healing progressed. Complications of pin tract infection, and premature hybrid bolt/ESF pin loosening resulting in premature ESF removal each occurred in 1 patient. Four of 28 hybrid ILN/ESF pins were grossly loose at 4- or 6-week postoperative recheck examinations. Outcomes were excellent (9), good (1), fair (1), and poor (1).

CONCLUSIONS

Inadequately stabilized ILN repair of fractures can be stabilized by use of either device, both of which also permit controlled destabilization of the repair during healing. Device 2 can be used when non-invasive removal of the ILN interlock is desired during healing.

CLINICAL RELEVANCE

These 2 devices should be considered as alternative methods for stabilization of inadequately stabilized ILN repairs in dogs and cats, or when controlled destabilization of an ILN fracture repair is desired.