In vitro evaluation of the effect of fracture configuration on the mechanical properties of standard and novel interlocking nail systems in bending.

OBJECTIVE: To investigate the effect of fracture configuration on the mechanical properties of standard interlocking nails (ILNs) and a novel angle-stable ILN (ILNn) in bending. STUDY DESIGN: In vitro experimental study. SAMPLE POPULATION: Synthetic tibial gap fracture bone models. METHODS: Bone models, featuring a 5 or 120 mm central defect, respectively, mimicking a simple diaphyseal and a comminuted fracture involving both metaphyses, were implanted with 6 or 8 mm screwed or bolted standard ILNs (ILN6s, ILN6b, ILN8s, ILN8b, respectively) or an ILNn. Specimens were tested in 4-point bending. Construct angular deformation (AD) and slack were statistically compared (P<.05). RESULTS: With increasing gap size, standard ILN construct AD increased significantly by approximately 27% in ILN8b and by up to 105% in ILN6s. Similarly, standard ILN construct slack significantly increased by approximately 33% in ILN8b (from approximately 4.2 degrees to approximately 5.6 degrees) and by up to approximately 130% in ILN6s (from approximately 7 degrees to approximately 16 degrees). Conversely, there was no difference in the ILNn construct AD (approximately 4 degrees) regardless of gap size. ILNn AD was the lowest of all groups and occurred without slack. CONCLUSIONS: This study demonstrated that the angle-stable ILNn provided construct stability regardless of fracture configuration, whereas the intrinsic slack of standard ILNs could jeopardize construct stability in a fracture configuration involving the metaphyses. CLINICAL RELEVANCE: Use of standard ILNs may be optimal in diaphyseal fractures where circumferential nail/cortical contact could augment repair stability. Conversely, the angle-stable ILNn may represent a reliable fracture stabilization method for diaphyseal fractures as well as fractures involving the metaphyseal regions.

In vitro mechanical evaluation of medial plating for pantarsal arthrodesis in dogs.

OBJECTIVE: To compare the bending properties of pantarsal arthrodesis constructs involving either a commercially available medial arthrodesis plate (MAP1) or a specially designed second-generation plate (MAP2) implanted in cadaveric canine limbs and evaluate the effect of calcaneotibial screw (CTS) augmentation on the structural properties of both constructs. SAMPLE POPULATION: 5 pairs of canine hind limbs. PROCEDURES: Within pairs, specimens were stabilized with an MAP1 or MAP2 and loaded to 80% of body weight, with and without CTS augmentation. Compliance, angular deformation (AD), and plate strains were compared. RESULTS: Construct compliance and AD did not differ between plates. Maximum plate strain was lower in the MAP2 than in the MAP1 (difference of approx 30%). Augmentation with a CTS reduced compliance, AD, and strains in MAP1 constructs but had no effect on those variables in MAP2 constructs. CONCLUSIONS AND CLINICAL RELEVANCE: Because of lower peak strains, the MAP2 may be less susceptible to failure than the MAP1. Furthermore, CTS augmentation was unnecessary with MAP2s, which could minimize intra- and postoperative morbidity. Compared with what is known for dorsal plates, MAP2 constructs were associated with approximately 35% less AD. As a result of improved local stability, one might anticipate earlier fusion of the talocrural joint with an MAP2. In addition, plate peak strain was approximately 3.5 times lower in MAP2s than in dorsal plate constructs, which should result in greater fatigue resistance. The use of MAP2s may be a better alternative to both MAP1s and dorsal plates and could contribute to lower patient morbidity.

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.

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.

OBJECTIVE: To describe a novel interlocking nail (ILN) and locking system and compare the torsional properties of constructs implanted with the novel ILN or a standard 8-mm ILN (ILN8) by use of a gap-fracture model. SAMPLE POPULATION: 8 synthetic specimens modeled from canine tibiae. PROCEDURES: An hourglass-shaped ILN featuring a tapered locking mechanism was designed. A synthetic bone model was custom-made to represent canine tibiae with a 50-mm comminuted diaphyseal fracture. Specimens were repaired by use of a novel ILN or an ILN8 with screws. Specimens were loaded for torsional measurements. Construct compliance and angular deformation were compared. RESULTS: Compliance of the ILN8 was significantly smaller than that of the novel ILN. Mean +/- SD maximum angular deformation of the ILN8 construct (23.12 +/- 0.65 degrees) was significantly greater, compared with that of the novel ILN construct (9.45 +/- 0.22 degrees). Mean construct slack for the ILN8 group was 15.15 +/- 0.63 degrees, whereas no slack was detected for the novel ILN construct. Mean angular deformation for the ILN8 construct once slack was overcome was significantly less, compared with that of the novel ILN construct. CONCLUSIONS AND CLINICAL RELEVANCE: Analysis of results of this study suggests that engineering of the locking mechanism enabled the novel hourglass-shaped ILN system to eliminate torsional instability associated with the use of current ILNs. Considering the potential deleterious effect of torsional deformation on bone healing, the novel ILN may represent a biomechanically more effective fixation method, compared with current ILNs, for the treatment of comminuted diaphyseal fractures.

The Effect of Axial Load in the Tibia on the Response of the 90 degrees Flexed Knee to Blunt Impacts with a Deformable Interface.

Lower extremity injuries are a frequent outcome of automotive accidents. While the lower extremity injury criterion is based on fracture of bone, most injuries are of less severity. Recent studies suggest microscopic, occult fractures that have been shown to be precursors of gross bone fractures, may occur in the kneecap (patella) for impacts with rigid and deformable interfaces due to excessive levels of patello-femoral contact pressure. One method of reducing this contact pressure for a 90 degrees flexed knee is to provide a parallel pathway for knee impact loads into the tibial tuberosity. Yet, blunt loads onto the tibial tuberosity can cause posterior drawer motion of the tibia, leading to injury or rupture of the posterior cruciate ligament (PCL). Recently studies have shown that axial loads in the tibia, which are measured during blunt loading on the knee in typical automobile crashes, can induce anterior drawer motion of the tibia and possibly help unload the PCL. The purpose of the current study was to explore the effect of combined anterior knee loading (AKL) and axial tibia loading (ATL), on response and injury for the 90 degrees flexed human knee. In repeated impacts with increasing ATL the stiffness of the knee to an AKL impact increased. For a 3 kN AKL, the stiffness of the knee increased approximately 26% when the ATL was increased from 0 kN to 2 kN. For 6 kN and 9 kN AKL, the stiffness was increased approximately 17% and 20%, respectively, when the ATL was increased from 0 kN (uniaxial) to 4 kN (biaxial). The effect, however, was not statistically significant at the 9 kN AKL level. The posterior tibial drawer was shown to increase with increased AKL and decrease with increased levels of ATL at an average of 0.3 mm per kN ATL for both the 3 kN and 6 kN ATL scenarios. For 9 kN AKL this drawer displacement was significantly reduced for biaxial versus uniaxial impacts, from 7.4+/-1.4 mm to 5.8+/-0.6 mm, respectively. Additionally, the percentage of the load carried by the tibial tuberosity increased with an ATL. For AKL impacts of 3, 6, and 9 kN, the percentage of load carried by the tibial tuberosity increased from 2.1% (range 0-19%) to 4.9% (0-36%), 2.1% (0-15%) to 6.9% (0-36%), and 8.7% (0-25%) to 12.7% (0-33%), respectively, between uniaxial and biaxial tests. The biaxial loading scenario also resulted in a reduction of the patello-femoral (PF) contact force as the ATL was increased. Ten knee impacts resulted in PCL tears at an average peak load of 12.7+/-2.4 kN in biaxial impacts (n=5) and 12.0+/-3.1 kN for uniaxial impacts (n=5). These PCL injured specimens had an average age of 62+/-11.3 years. The remaining specimens (n=11, 78+/-12.9 years of age) had bone fractures at approximately 8.9+/-3.1 kN. This study showed that combinations of compressive ATL and AKL reduced the PF contact force and had a stiffening effect on the response of the knee impacting a stiff but deformable interface. Furthermore, ATL reduced the posterior drawer of the tibia, which is the current basis for PCL injury in the knee, although it did not reduce the incidence of PCL injury in this study. While the current injury tolerance criterion reflects the vulnerability of the PCL to injury by limiting tibial drawer to 15 mm, the current dummy design does not incorporate the stiffening effect of an ATL that may occur at the same time as knee contact with an instrument panel during a typical automotive crash.