In vitro mechanical evaluation of torsional loading in simulated canine tibiae for a novel hourglass-shaped interlocking nail with a self-tapping tapered locking design

Three-dimensionally printed osteotomy and reaming guides for correction of a multiplanar femoral deformity stabilized with an interlocking nail in a dog

OBJECTIVE

To describe the use of virtual surgical planning (VSP) and three-dimensionally (3D) printed surgical guides for corrective osteotomies stabilized with an interlocking nail in a dog with a multiplanar femoral deformity.

STUDY DESIGN

Case report.

ANIMALS

An 8-year-old male neutered mixed breed dog weighing 44 kg.

METHODS

A dog was presented for a right grade 3 lateral patellar luxation secondary to a multiplanar femoral deformity due to a suspected femoral malunion. A computed tomography (CT) scan was obtained to create virtual femoral models. Corrective osteotomies were simulated with VSP. Custom osteotomy guides and reaming guides were designed to facilitate the correction and the placement of an interlocking nail. The preoperative femoral model, virtually aligned femoral model, custom osteotomy guides, and reaming guides were 3D printed, sterilized, and utilized intraoperatively. A CT scan was performed postoperatively to assess femoral length and alignment.

RESULTS

Custom osteotomy and reaming guides were used as intended by the VSP. Postoperative femoral length as well as frontal, sagittal, and axial plane alignment were within 0.7 mm, 2.2°, 0.5°, and 1.6°, respectively, of the virtually planned femoral model. Two months postoperatively, the dog was sound on visual gait examination, and the patella tracked in the trochlear groove throughout stifle range of motion and was unable to be manually luxated. Radiographs obtained 2 months postoperatively revealed static femoral alignment and implants. Both osteotomies were discernable with callus bridging.

CONCLUSION

Virtual surgical planning and custom osteotomy and reaming guides facilitated complex femoral corrective osteotomies and interlocking nail placement.

Angle-stable interlocking nailing in a canine critical-sized femoral defect model for bone regeneration studies: In pursuit of the principle of the 3R’s

Introduction: Critical-sized long bone defects represent a major therapeutic challenge and current treatment strategies are not without complication. Tissue engineering holds much promise for these debilitating injuries; however, these strategies often fail to successfully translate from rodent studies to the clinical setting. The dog represents a strong model for translational orthopedic studies, however such studies should be optimized in pursuit of the Principle of the 3R’s of animal research (replace, reduce, refine). The objective of this study was to refine a canine critical-sized femoral defect model using an angle-stable interlocking nail (AS-ILN) and reduce total animal numbers by performing imaging, biomechanics, and histology on the same cohort of dogs.

Methods: Six skeletally mature hounds underwent a 4 cm mid-diaphyseal femoral ostectomy followed by stabilization with an AS-ILN. Dogs were assigned to autograft (n = 3) or negative control (n = 3) treatment groups. At 6, 12, and 18 weeks, healing was quantified by ordinal radiographic scoring and quantified CT. After euthanasia, femurs from the autograft group were mechanically evaluated using an established torsional loading protocol. Femurs were subsequently assessed histologically.

Results: Surgery was performed without complication and the AS-ILN provided appropriate fixation for the duration of the study. Dogs assigned to the autograft group achieved radiographic union by 12 weeks, whereas the negative control group experienced non-union. At 18 weeks, median bone and soft tissue callus volume were 9,001 mm³ (range: 4,939–10,061) for the autograft group and 3,469 mm³ (range: 3,085–3,854) for the negative control group. Median torsional stiffness for the operated, autograft treatment group was 0.19 Nm/° (range: 0.19–1.67) and torque at failure was 12.0 Nm (range: 1.7–14.0). Histologically, callus formation and associated endochondral ossification were identified in the autograft treatment group, whereas fibrovascular tissue occupied the critical-sized defect in negative controls.

Conclusion: In a canine critical-sized defect model, the AS-ILN and described outcome measures allowed refinement and reduction consistent with the Principle of the 3R’s of ethical animal research. This model is well-suited for future canine translational bone tissue engineering studies.

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.

Clinical application of the small I-Loc interlocking nail in 30 feline fractures: A prospective study

OBJECTIVE

To describe medium-term functional outcome after nail osteosynthesis in feline traumatology and report clinically relevant recommendations for I-Loc angle-stable interlocking nail use in cats.

STUDY DESIGN

Prospective clinical study.

SAMPLE POPULATION

Client-owned cats (n = 29).

METHODS

Consecutive cases with femoral, tibial, or humeral fractures were included. Outcome measures included fracture and surgical procedure description, limb alignment, nail size vs body weight (BW), percentage of nail medullary canal (MC) fill, time to limb function at clinical union (CU), and complications. Descriptive statistics were reported and compared with historical data.

RESULTS

Bone distribution was 53.3% femora, 30% tibiae, and 16.7% humeri. There were six epimetaphyseal and 24 diaphyseal fractures. Overall, 67% of fractures were comminuted. Open reduction and minimally invasive techniques were used in 73% and 27% of cases, respectively. Seventeen I-Loc 3 (cat mean BW 4.4 ± 2.2 kg) and 13 I-Loc 4 (cat mean BW 5.2 ± 1.2 kg) nails were placed with mean MC fill of ≤50%. Average time to CU was 7.2 weeks. At CU, lameness had resolved or was mild in every cat, and all cats ultimately regained full limb function. No major complications were encountered.

CONCLUSION

Because of improved CU times, excellent functional outcomes, and low complication rate, our results provide evidence that I-Loc nails are safe and effective for feline traumatology.

CLINICAL SIGNIFICANCE

The I-Loc may be advantageous for fixation of epimetaphyseal fractures. Because of feline bone specific dimensional constraints, I-Loc 3 is likely appropriate for all feline humeri and most tibiae, while I-Loc 4 is well sized for feline femora.

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.

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.

Interlocking Nails and Minimally Invasive Osteosynthesis

Reviews of clinical outcomes led to the foundation of a new approach in fracture management known as biological osteosynthesis. As intramedullary rods featuring cannulations and locking devices at both extremities, interlocking nails are well suited for bridging osteosynthesis. Unique biological and mechanical benefits make them ideal for minimally invasive nail osteosynthesis and an attractive, effective alternative to plating, particularly in revisions of failed plate osteosynthesis. Thanks to a new angle-stable locking design, interlocking nailing indications have been expanded to osteosynthesis of epi-metaphyseal fractures, including those with articular involvement and angular deformities such as distal femoral varus and associated patellar luxations.

Ex vivo torsional properties of a 2.5 mm veterinary interlocking nail system in canine femurs

Objective: To evaluate the torsional properties of the Targon® Vet Nail System (TVS) in small canine femurs and to compare these properties to those of the 2.4 mm LC-DCP® plates.

Methods: Thirty-six cadaveric femurs were allocated to three groups (n = 12). In all bones, points just distal to the lesser trochanter and just proximal to the fabellae were marked and a midshaft transverse osteotomy was performed. Group 1: bones were fixed with the 2.5 mm TVS with the bolts applied at the pre-identified marks. Group 2: A TVS system with 25% shorter inter-bolt distance was used. Group 3: A 7-hole 2.4 mm LCDCP® plates were applied. All constructs were tested non-destructively for 10 cycles, followed by an acute torsion to failure.

Results: Torque at yield was 0.806 ± 0.183 and 0.805 ± 0.093 Nm for groups 1 and 2 and 1.737 ± 0.461 Nm for group 3. Stiffness was 0.05 ± 0.01, 0.05 ± 0.007, and 0.14 ± 0.015 Nm/° for groups 1 to 3 respectively. Maximal angular displacement under cyclic loading was 16.6° ± 2.5°, 15.6° ± 2.1°, and 7.8° ± 1.06° respectively. There was no significant difference for any of the parameters between groups 1 and 2. Both torque at yield and stiffness were significantly greater between group 3 and groups 1 and 2.

Clinical significance: The TVS had approximately half the torsional strength and approximately 1/3 of the stiffness of the 2.4 mm bone plate. Slippage of the locking mechanism was probably the cause of the early failure. The system should be considered as a low-strength and low-stiffness system when compared to bone plates.

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.

Segmented interlocking nail: An in vivo evaluation of a novel humeral osteotomy fixation device in a caprine model

Objectives: To describe a novel humeral fixation device, the insertion technique, healing of humeral osteotomies, and clinical outcomes in a caprine model over a six month period.

Methods: Fourteen mature female Boer/Nubian cross goats with a mean body weight of 50.7 kg were implanted with a proprietary segmented interlocking nail (SILN) in both humeri. Each goat had one humerus randomly selected for mid-diaphyseal osteotomy.

Results: Immediately after surgery all but one goat was able to stand, although none of the goats were weight bearing on the osteotomy limb. During the six month study, clinical lameness was always associated with the osteotomy limb. One month after surgery, lameness for twelve of the goats was grade 2/5 or better. At three months, 11 of the 14 did not exhibit any signs of lameness. On radio-graphic images, notable malalignment of the osteotomy was observed, although all osteotomies went to bone union.

Clinical significance: The results of this study suggest that despite misalignment, the SILN maintained adequate osteotomy fixation to achieve bone union in the research model studied, with reduced morbidity and early return to function with bilateral implantation. The SILN used in this study allowed intramedullary fixation of humeral diaphyseal osteotomies with a limited and safe surgical approach.

Effect of bending direction on the mechanical behaviour of 3.5 mm String-of-Pearls and Limited Contact Dynamic Compression Plate constructs

OBJECTIVE

To compare the bending properties of String-of-Pearls® (SOP) and Limited Contact Dynamic Compression Plate® (LC-DCP) constructs in orthogonal bending directions.

METHODS

3.5 mm SOP and LC-DCP plates were fixed to a bone model simulating a comminuted tibial fracture. Specimens were non-destructively tested in both mediolateral and craniocaudal bending for 10 cycles. Bending stiffness and total angular deformation were compared using parametric analyses (p <0.05).

RESULTS

For both constructs, stiffness was significantly less when bending moments were applied against the thickness of the plates (mediolateral bending) than against the width (craniocaudal bending). When compared to the mediolateral plane, bending constructs in the craniocaudal plane resulted in a 49% (SOP group) and 370% (LC-DCP group) increase in stiffness (p <0.001). Mediolateral bending stiffness was significantly greater in the SOP than the LC-DCP constructs. Conversely, in craniocaudal bending, SOP constructs stiffness was significantly less than that of the LC-DCP constructs. The differences between the two constructs in total angular deformation had an identical pattern of significance.

CLINICAL SIGNIFICANCE

This study found that SOP showed less variability between the orthogonal bending directions than LC-DCP in a comminuted fracture model, and also described the bi-planar bending behaviour of both constructs. Although not exhibiting identical bending properties in both planes, SOP constructs had a more homogenous bending behaviour in orthogonal loading directions. The difference between the SOP with a circular cross sectional shape compared to the rectangular shape of standard plates is probably responsible for this difference.

Biomechanical comparison of two locking plate constructs under cyclic torsional loading in a fracture gap model. Two screws versus three screws per fragment

OBJECTIVES

The number of locking screws required per fragment during bridging osteosynthesis in the dog has not been determined. The purpose of this study was to assess the survival of two constructs, with either two or three screws per fragment, under cyclic torsion.

METHODS

Ten-hole 3.5 mm stainless steel locking compression plates (LCP) were fixed 1 mm away from bone surrogates with a fracture gap of 47 mm using two bicortical locking screws (10 constructs) or three bicortical locking screws (10 constructs) per fragment, placed at the extremities of each LCP. Constructs were tested in cyclic torsion (range: 0 to +0.218 rad) until failure.

RESULTS

The 3-screws constructs (29.65 ± 1.89 N.m/rad) were stiffer than the 2-screws constructs (23.73 ± 0.87 N.m/rad), and therefore, were subjected to a greater torque during cycling (6.05 ± 1.33 N.m and 4.88 ± 1.14 N.m respectively). The 3-screws constructs sustained a significantly greater number of cycles (20,700 ± 5,735 cycles) than the 2-screws constructs (15,600 ± 5,272 cycles). In most constructs, failure was due to screw damage at the junction of the shaft and head. The remaining constructs failed because of screw head unlocking, sometimes due to incomplete seating of the screw head prior to testing.

CLINICAL SIGNIFICANCE

Omitting the third innermost locking screw during bridging osteosynthesis led to a reduction in fatigue life of 25% and construct stiffness by 20%. Fracture of the screws is believed to occur sequentially, starting with the innermost screw that initially shields the other screws.

Effect of Intramedullary Rod Diameter on a String of Pearls Plate-Rod Construct in Mediolateral Bending: An In Vitro Mechanical Study

OBJECTIVE

To evaluate the effect of intramedullary rod (IMR) diameter on the mechanical behavior of string of pearls (SOP) plate-rod constructs.

STUDY DESIGN

In vitro mechanical study.

SAMPLE POPULATION

Synthetic bone models (n = 24).

METHODS

Locking 3.5 mm SOP plates were fixed to a tibial bone model with a 50 mm fracture gap. Four experimental groups (n = 4) were tested: monocortical SOP construct alone and monocortical SOP constructs augmented with a 2.4, 3.2, or 4.0 mm IMR corresponding to 24, 32, or 40% filling of the medullary cavity diameter (SOP-24, SOP-32, SOP-40). Control groups (n = 4) were stabilized with either a bicortical SOP plate (SOP-B) or a 3.5 mm limited contact dynamic compression plate (LC-DCP) with a 4.0 mm IMR filling 40% of the medullary cavity diameter (LC-DCP-40). Specimens were tested in mediolateral bending. Construct compliance (CC) and angular deformation (AD) were compared between construct types (P < .05).

RESULTS

CC and AD incrementally decreased with increasing IMR diameter (P < .001). There were no statistical differences between SOP-24 and SOP-B (P = .806) or between SOP-32 and LC-DCP-40 (P = .773), which was also the least compliant of all constructs (P < .001). AD followed an identical pattern of significance.

CONCLUSIONS

Biological osteosynthesis often relies on more compliant bridging constructs to promote beneficial micromotion at the fracture. Our study suggests use of a smaller diameter IMR (SOP-32) is comparable to a conventional plate-rod construct (LC-DCP-40). Should greater compliance be desired, an even smaller diameter IMR (SOP-24) may prove beneficial while as stable as an accepted bicortical construct (SOP-B).

Effect of screw type and distribution on the torsional stability of 3.5 mm string of pearls locking plate constructs

OBJECTIVE

To evaluate the effects of screw type (mono- [M] versus bicortical [B]), number, and position on torsional stability of String of Pearls (SOP) locking plate constructs.

STUDY DESIGN

In vitro mechanical study.

METHODS

SOP plates (n = 32) were applied to bone models and divided into 8 groups named according to screw type (M or B) and position in each fragment relative to the fracture gap starting at the outermost plate hole. Positive and negative controls were MMM and BBB, respectively. Specimens were non-destructively tested in torsion. Compliance and angular deformation were statistically compared (P < .002).

RESULTS

The MMM construct was most compliant (P < .001). Compliance decreased in groups with a single bicortical screw (P < .001). Compared to the MMM group, torsional compliance decreased in constructs where a single monocortical screw was replaced with a bicortical screw (P < .001). Compared with a centrally positioned bicortical screw, constructs with a bicortical screw in either outer- or innermost position were 15% and 23% less compliant, respectively (P < .001). Addition of a second bicortical screw/fragment further decreased compliance (P < .001). No significant difference was found between groups with 2 bicortical screws. The BBB construct was least compliant (P < .001). Group responses for angular deformation followed the same pattern of significance recorded for compliance.

CONCLUSIONS

A minimum of 1 bicortical screw/fragment should be used to increase torsional stability of 3.5 mm SOP constructs. Positioning this screw at the inner- or outermost positions relative to the fracture is preferred, with the innermost position providing the greatest improvement in stability. Should further torsional stability be desired, increasing the number of bicortical screws is recommended. Clinically, these results may assist with preoperative planning of various fracture patterns.

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.

Quantifying and comparing torsional strains after olecranon plating

PURPOSE

Any torsion experienced at a fracture site will directly translate into shearing forces and has been regarded as detrimental to healing. The purpose of this study was to determine which plating system currently on the market controls torsional forces about comminuted olecranon fractures most effectively.

METHODS

Five olecranon plates (Acumed, Synthes-SS, Synthes-Ti, ITS/US Implants and Zimmer) were implanted to stabilise a simulated comminuted fracture pattern in 50 fresh-frozen, cadaveric elbows. All specimens were evaluated by dual energy X-ray absorptiometry (DXA) scan to determine bone density. Three-dimensional displacement analysis was conducted to assess fragment motion through physiologic cyclic arcs of motion. The specimens were cycled through progressive physiologic loads (0.18-5.6 kg). Movements of the fragments were statistically compared amongst the different implants using one-way analysis of variance (ANOVA) and Tukey Honestly Significant Difference (HSD) post hoc comparisons with a critical significance level of α=0.05.

RESULTS

DXA bone mineral densities (BMDs) ranged from 0.465 to 0.927, with an average of 0.714. The Acumed, Synthes-SS, Synthes-Ti and Zimmer plates allowed <1° of torsion up to 1.6 kg of load. The differences between these plates at this load were not statistically significant. The ITS/US Implants plate, however, allowed significantly more torsion above loads of 2.6 kg (p=0.045) compared with all other plates. The ITS/US Implants plate allowed over 2° of torsion at 2.6 kg (p=0.012), and nearly 3° at 3.6 kg (p=0.045). The Zimmer plate consistently allowed more torsion than the Acumed plate or either of the Synthes plates, but the differences were not statistically significant.

CONCLUSION

Regardless of which olecranon plate is used, the authors recommend limiting postoperative rehabilitation loads to below 1.6 kg in an effort to minimise the detrimental effects of torsion on healing. If loads over 1.6 kg are anticipated, the authors recommend the use of the Acumed plate or either of the Synthes plates.

In Vitro Mechanical Comparison of Screwed, Bolted, and Novel Interlocking Nail Systems to Buttress Plate Fixation in Torsion and Mediolateral Bending

OBJECTIVE

To compare standard interlocking nails (ILN) with a newly designed ILN featuring an angle-stable locking mechanism (ILNn).

STUDY DESIGN

Six experimental groups.

SAMPLE POPULATION

Bone models (n=48) treated with 6 and 8 mm nails locked with screws or bolts (ILN6s, ILN8s, ILN6b, ILN8b, respectively), ILNn, and a 3.5 mm broad-DCP (br-DCP); n=4/testing mode.

METHODS

Specimens were tested in torsion or 4-point bending. Construct compliance, deformation, and slack were statistically compared (P<.05).

RESULTS

Regardless of testing mode, construct compliance was greater with smaller ILN. Screwed constructs were more compliant than bolted ones, with a significant difference between ILN6s and ILN6b in torsion. Plated constructs were significantly more compliant than the ILNn. Angular deformation was consistently greater with smaller ILN. Screwed ILN constructs sustained approximately 2 x the torsional deformation of the bolted ones (approximately 36 degrees [ILN6s] versus approximately 18 degrees [ILN6b]). Comparatively, ILNn constructs had significantly less torsional (approximately 8 degrees) and bending (approximately 4 degrees) deformation than other constructs. Whereas standard ILN constructs had slack in both modes, ILNn and br-DCP construct deformations consistently occurred without slack.

CONCLUSIONS

Use of bolts rather than screws improved ILN mechanical behavior, but neither locking mechanism completely counteracted torsion and bending forces. Conversely, the ILNn angle-stable locking system eliminated torsional and bending slack, resulting in comparable mechanical performances between ILNn and plated constructs.

CLINICAL RELEVANCE

The angle-stable locking mechanism of the new ILN eliminates all slack in the system; thus, interfragmentary motion will likely be reduced compared with standard ILN, which may improve the local environment for fracture healing.

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.