External skeletal fixation of canine tibial osteotomies. Compression compared with no compression

Effects of Angled Dynamic Compression Holes in a Tibial Plateau Levelling Osteotomy Plate on Cranially Directed Fragment Displacement

OBJECTIVE

 To compare angled dynamic compression holes in a tibial plateau levelling osteotomy (TPLO) plate to a commercially available TPLO plate in an ovine cadaveric model.

STUDY DESIGN

 Ovine tibias (40 bones) were mounted on a custom-made securement device and radiopaque markers were placed to aid radiographic measurements. A standard TPLO procedure was performed on each tibia with either a custom-made six-hole 3.5 mm angled compression hole plate (APlate) or a six-hole 3.5 mm standard commercial plate (SPlate). Radiographs were obtained before and after tightening of the cortical screws and evaluated by an observer blinded to the plate. Measurements of cranio-caudal displacement (CDisplacement), proximo-distal displacement (PDisplacement) in relation to the long axis of the tibia, and change in tibial plateau angle (TPA) were determined.

RESULTS

 CDisplacement was significantly greater in APlate (median 0.85 mm, Q1-Q3: 0.575-1.325 mm) compared to SPlate (median 0.00 mm, Q1-Q3: -0.35-0.50 mm, p < 0.0001). There were no significant differences in the PDisplacement (median 0.55 mm, Q1-Q3: 0.075-1.00 mm, p = 0.5066) or TPA change (median -0.50°, Q1-Q3: -1.225-0.25°, p = 0.1846) between the two plate types.

CONCLUSION

 APlate increases cranially directed displacement of the osteotomy in a TPLO procedure without causing TPA change. The reduced interfragmentary distance across the whole osteotomy could improve osteotomy healing compared to standard commercial TPLO plates.

Optimization of Connecting Rod Design Parameters for External Fixation System: A Biomechanical Study

The role of connecting rod in healing process of a fractured bone has always been of significant importance for surgeons. Adding a connecting rod to the fixator would be a secure option for increasing stability without increasing infection rate. The roles of 4 design parameters of the connecting rod (ie, connecting rod diameter, elevation, material, and configuration) were assessed by using finite element models to calculate axial stiffness and interfragmentary strain at the fracture gap. Taguchi method was used to achieve an optimal design set for maximizing stability with regard to connecting rod variables. Also, analysis of variance (ANOVA) approach was employed to determine contribution percentage of each design parameter on outputs. For optimizing connecting rod design parameters, an optimal set of variables consisting of 11 mm, 40 mm, 200 GPa, and Type 3 external fixator were determined by Taguchi for connecting rod diameter, elevation, modulus of elasticity, and configuration, respectively. However, as Type 3 external fixator stability is a little more than Type 2, it would be better if Type 3 external fixator in Taguchi suggestion be replaced by Type 2 external fixator to be as minimally invasive as possible. Furthermore, ANOVA results revealed that the connecting rod configuration is the most important parameter with 95% and 96% effectiveness on the interfragmentary strain and axial stiffness.

A Mechanical Comparison of the Compressive Force Generated by Various Headless Compression Screws and the Impact of Fracture Gap Size

Background: There is evidence that interfragmentary fracture gap size may affect the compression achievable with a modern headless compression screw (HCS). This mechanical study compared the compression achieved by 3 commercial HCS systems through various fracture gaps: CAPTIVATE Headless (Globus Medical, Inc, Audubon, Pennsylvania), Synthes (DePuy Synthes, Westchester, Pennsylvania), and Acumed Acutrak 2 (Acumed LLC, Hillsboro, Oregon). Methods: Screws were inserted into a custom test fixture composed of polyurethane synthetic bone foam fragments, separated by a layer of easily compressible polyurethane foam simulating a fracture gap. Compression was measured after final insertion and countersinking. The effect of the interfragmentary fracture gap size on the compression generated was also investigated. Results: The CAPTIVATE Headless 3.0 mm screw (70.1 ± 5.7 N) and the Synthes 3.0 mm screw (64.9 ± 7.3 N) achieved similar compressive forces after final countersink. Similar comparisons were found for the CAPTIVATE Headless 2.5 mm and Synthes 2.4 mm screws, and the CAPTIVATE Headless 4.0 mm and Acutrak 2 Standard screws. The final compression of the CAPTIVATE Headless 2.5 mm and Synthes 2.4 mm screws was not significantly affected when the fracture gap was doubled from 2 to 4 mm, but was reduced significantly by 95.9% with the Acutrak 2 Micro screw. Conclusion: When comparing like-sized screws, the CAPTIVATE, Synthes, and Acutrak 2 HCS systems demonstrated similar potential compressive forces. However, compared with the CAPTIVATE Headless and Synthes HCS systems, which are inserted with a compression sleeve that is not gap distance-dependent, the Acutrak 2 HCS system demonstrated less compression when the simulated fracture gap size was increased to 4 mm.

Genetic variation in mice affects closed femoral fracture pattern outcomes

The purpose of this study was to determine whether differences in structural and material properties of bone between different mouse strains influence the fracture patterns produced under experimental fracture conditions. Femurs of C57BL/6 (B6), C3H/HeJ (C3H), and DBA/2 (DBA) strains were evaluated using micro-computed tomography (μCT), measurements derived from radiographic images and mechanical testing to determine differences in the geometry and mechanical properties. A fracture device was used to create femoral fractures on freshly sacrificed animals using a range of kinetic energies (∼20-80mJ) which were classified as transverse, oblique, or comminuted. B6 femurs had the lowest bone volume/total volume (BV/TV) and bone mineral density (BMD), thinnest cortex, and had the most variable fracture patterns, with 77.5% transverse, 15% oblique, and 7.5% comminuted fractures. In contrast, C3H had the highest BV/TV, BMD, and thickest cortices, resulting in 97.5% transverse, 2.5% oblique, and 0% comminuted fractures. DBA had an intermediate BV/TV and thickness of cortices, with BMD similar to C3H, resulting in 92.9% transverse, 7.1% oblique, and 0% comminuted fractures. A binomial logistic regression confirmed that bone morphometry was the single strongest predictor of the resulting fracture pattern. This study demonstrated that the reproducibility of closed transverse femoral fractures was most influenced by the structural and material properties of the bone characteristics in each strain, rather than the kinetic energy or body weight of the mice. This was evidenced through geometric analysis of X-ray and μCT data, and further supported by the bone mineral density measurements from each strain, derived from μCT. Furthermore, this study also demonstrated that the use of lower kinetic energies was more than sufficient to reproducibly create transverse fractures, and to avoid severe tissue trauma. The creation of reproducible fracture patterns is important as this often dictates the outcomes of fracture healing, and those studies that do not control this potential variability could lead to a false interpretation of the results.

Effects of Removal and Reinsertion of Headless Compression Screws

PURPOSE

This study investigates the loss of compression when 3 commonly used headless compression screws are backed out (reversed), and assesses the ability to re-establish compression with screws of greater diameter.

METHODS

Two investigators tested 3 screw designs (Acutrak 2, Synthes HCS, Medartis SpeedTip CCS) in 2 diameters and lengths. Each design had 10 test cycles in a polyurethane foam bone model with compression recorded using a washer load cell. A 28-mm screw of the narrower diameter was inserted until 2 mm recessed and then reversed 30°, 60°, 90°, 180°, 270°, 360°, and 720°. After this the screw was removed completely and a 24-mm screw of greater diameter inserted until recessed 2 mm with the compressive force again recorded.

RESULTS

All screws showed an immediate, statistically significant loss of compression at 30° of reversing. The Acutrak 2 Micro screw demonstrated not only the greatest mean compressive force, but also the fastest compressive loss. Insertion of the shorter screw of greater diameter was associated with re-establishment of compression to levels comparable with the original screw.

CONCLUSIONS

This study reaffirms the importance of establishing the correct screw length before insertion due to the immediate loss of compression with reversal of these devices.

CLINICAL RELEVANCE

If a headless compression screw penetrates the far joint surface, the screw should be completely removed and replaced with a shorter screw of greater diameter.

Effect of headless compression screw on construct stability for centre of rotation and angulation-based levelling osteotomy. Biomechanical testing in flexion and torsion

OBJECTIVE

To compare the biomechanical properties of bone and implant constructs when used for the centre of rotation and angulation (CORA) based levelling osteotomy, with and without implantation of a trans-osteotomy headless compression screw tested under three-point flexural and torsional forces; thereby determining the contribution of a trans-osteotomy headless compression screw with regards to stability of the construct.

METHODS

Experimental biomechanical study utilizing 12 pairs of cadaveric canine tibias. Using the CORA based levelling osteotomy (CBLO) procedure, the osteotomy was stabilized with either a standard non-locking CBLO bone plate augmented with a headless compression screw (HCS) or a CBLO bone plate alone. Tibial constructs were mechanically tested in three-point craniocaudal flexural testing or in torsion.

RESULTS

In three-point flexural testing, the difference between the two constructs was not significant. In torsion, the difference in the angle of failure between constructs with a HCS (48.46°) and constructs without a HCS (81.65°) was significant (p = 0.036). Maximum torque achieved by constructs with a HCS (21.7 Nm) was greater than those without (18.7 Nm) (p = 0.056). Stiffness differences between both groups in torsion and bending were not significant. Use of a HCS did increase the stability of the CBLO construct in torsional testing, but not in flexural testing.

Interfragmentary compression profile of 4 headless bone screws: an analysis of the compression lost on reinsertion

PURPOSE

To evaluate the interfragmentary compression force generated by 4 different types of headless compression screws and to examine the effects of removal and reinsertion of the screw.

METHODS

We chose foot bones rather than scaphoids for the model because they were larger and would enable comparison of 2 screw designs in the same bone, thereby controlling for the effect of interspecimen variability. A transverse osteotomy was made in 10 fresh-frozen cadaveric navicular bones and 10 medial cuneiforms. A load cell was used to measure compression between the 2 fragments as a screw was inserted across the fracture. Each bone was tested twice, with an Acutrak Mini (Acumed, Hillsboro, OR; n = 10) and an SBi AutoFIX screw (SBi, Morrisville, PA; n = 10) or an Extremifix (Osteomed, Addison, TX; n = 10) and a Barouk screw (Depuy, Warsaw, IN; n = 10). Compression was recorded at initial insertion and on removal and reinsertion of the screw twice to the same position. Compression was also measured after one additional full turn further than the initial position.

RESULTS

The mean interfragmentary compression generated by the Acutrak Mini screw was greater than that of the SBi AutoFIX screw (96 N vs 22 N). There was a trend toward a greater mean compression generated by the Extremifix screw compared to the Barouk screw (85 N vs 22 N). There was a significant loss of compression upon removal and reinsertion of the screws. An additional full turn of the screw was able to re-establish a large proportion of the original compression.

CONCLUSIONS

The compression forces achieved by headless screw systems appeared to vary according to the screw design, depth of insertion, and the quality of the bone. Substantial compression was lost if the screw was removed and replaced. Some screw designs appeared to require a greater depth of insertion to achieve effective compression, and the number of additional turns required to re-establish compression might vary according to the thread design.

CLINICAL RELEVANCE

Surgeons should be aware of the compression profile of each screw design and the effect of screw removal and reinsertion in the clinical setting of small bone fixation.

Biomechanical analysis of second-generation headless compression screws

INTRODUCTION

Headless Compression Screws (HCS) are commonly utilized for the fixation of small bone and articular fractures. Recently several new second generation HCS (SG-HCS) have been introduced with the purported benefits of improved biomechanical characteristics. We sought to determine and compare the biomechanical efficiencies of these screws.

MATERIAL AND METHODS

Five HCS including four second generation (Mini-Acutrak 2 (Acumed), Twinfix (Stryker), Kompressor Mini (Integra), HCS 3.0 (Synthes)) and one first generation (Herbert-Whipple) were studied. Polyurethane foam blocks that represented osteoporotic cancellous bone (0.16 g/cc) with a simulated transverse fracture at the waist were utilized and five screws of each brand were tested for the generated compression force and fastening torque during insertion with and without pre-drilling.

RESULTS

The generated compression force was highest for Mini-Acutrak 2 (45.41 ± 0.88 N) and lowest for Herbert-Whipple (13.44 ± 2.35 N) and forces of Twinfix, Kompressor Mini, HCS 3.0 were in between in descending order. The compression force of SG-HCS increased slightly without pre-drilling but it was not statistically significant while the fastening torque increased significantly. Slight over-fastening beyond the recommended stage significantly reduced the compression force in Twinfix and Kompressor and had no or moderate effect in other screws.

CONCLUSION

All SG-HCS demonstrated greater biomechanical characteristics than the first generation Herbert-Whipple screw. The Mini-Acutrak 2 with a variable pitch design generated the maximum compression force and showed the most reliability and sustainability. Screws with independently rotating trailing heads (Twinfix and Kompressor Mini) demonstrated loss of compression with extra turns. The increase of fastening torque due to over-fastening and loss of compression at the same time in some screw designs, demonstrated how the fastening torque (applied by the surgeon) can be a misleading measure of the compression force. Application of SG-HCS in osteoporotic bone without pre-drilling can slightly increase the compression force.

In vivo models of bone repair

This review is aimed at clinicians appraising preclinical trauma studies and researchers investigating compromised bone healing or novel treatments for fractures. It categorises the clinical scenarios of poor healing of fractures and attempts to match them with the appropriate animal models in the literature.

We performed an extensive literature search of animal models of long bone fracture repair/nonunion and grouped the resulting studies according to the clinical scenario they were attempting to reflect; we then scrutinised them for their reliability and accuracy in reproducing that clinical scenario.

Models for normal fracture repair (primary and secondary), delayed union, nonunion (atrophic and hypertrophic), segmental defects and fractures at risk of impaired healing were identified. Their accuracy in reflecting the clinical scenario ranged greatly and the reliability of reproducing the scenario ranged from 100% to 40%.

It is vital to know the limitations and success of each model when considering its application.

Optimal mechanical environment of the healing bone fracture/osteotomy

The aim of this paper is to review recent experimental and clinical publications on bone biology with respect to the optimal mechanical environment in the healing process of fractures and osteotomies. The basic postulates of bone fracture healing include static bone compression and immobilisation/ fixation for three weeks and intermittent dynamic loading treatment afterwards. The optimal mechanical strain should be in the range of 100-2,000 microstrain, depending on the frequency of the strain application, type of bone and location in the bone, age and hormonal status. Higher frequency of mechanical strain application or larger number of repetition cycles result in increased bone mass at the healing fracture site, but only up to a certain limit, values beyond which no additional benefit is observed. Strain application and transition period from non-load-bearing to full load-bearing can be modified by implants allowing dynamisation of compression and generating strains at the fracture healing site in a controlled manner.

The fixation strength of scaphoid bone screws: an in vitro investigation using polyurethane foam

PURPOSE

To compare the compression strength and pull-apart resistance of 5 single-piece scaphoid bone compression screws (Acutrak, Asnis, Herbert, Herbert-Whipple, and Little Grafter), with those of 2 dual-component screws (Kompressor and TwinFix).

METHODS

Two blocks of polyurethane foam were compressed with a screw while held in a tension test machine, with the force measured at full insertion of the screw. The 2 blocks were then pulled apart, and the maximum resistive force was measured.

RESULTS

The dual-component screws (Kompressor and TwinFix) gave greater compression force than the single-component screws, with the Kompressor screw giving statistically significantly greater compression than the TwinFix. The pull-apart resistance forces did not show such clear differences.

CONCLUSIONS

The Kompressor screw achieves the greatest compressive forces and has one of the highest pull-apart forces.

CLINICAL RELEVANCE

When compression and pull-apart resistance are considered, the Kompressor screw has advantages over other methods of scaphoid fixation.

The influence of compression on the healing of experimental tibial fractures

PURPOSE

Experimental studies of the effects of various mechanical conditions and stimuli on bone healing have disclosed an improvement potential in bone fracture mineralization and biomechanical properties. We therefore evaluated the effect of a clinically practicable application of a mechanical compressive interfragmentary stimulus on the healing of experimental tibial diaphyseal fractures.

METHODS

Sixty Male rats received a standardized tibial shaft osteotomy stabilized with a unilateral external fixator with a zero interfragmentary distance, and then randomly assigned to the compression (N=20), control (N=20) or distraction (N=20) group. From days 4 to day 14, the external fixator was either tightened (compression group) or loosened (distraction group) once daily to gradually induce a total axial displacement of the external fixator pin clamps of 1.25 mm. Evaluation at 30 and 60 days post-osteotomy included radiography, dual-energy X-ray absorptiometry (DXA), quantitative CT and mechanical testing.

RESULTS

All fractures healed radiographically with sparse callus. At 60 days, the compression and control groups exhibited significantly less amount of mineralized callus in terms of DXA measured callus area and bone mineral content (BMC) compared to the distraction group. These groups also demonstrated a smaller volume of low-mineralized bone tissue (callus) and a larger volume of highly mineralized bone tissue (cortical bone) measured by QCT than in the distraction group. Both mechanical strength and stiffness was significantly higher in the compression and control groups than in the distraction group at 60 days.

DISCUSSION

Compression did not enhance fracture healing in terms of mineralization, bending strength, or stiffness at the time of union, compared with the control condition. The compression and control groups exhibited improved healing in terms of mechanical strength and stiffness and a more mature callus mineralization compared with the distraction group.

Mechanical evaluation of various external skeletal fixator-intramedullary pin tie-in configurations using a tubular plastic bone model

Use of external skeletal fixator-intramedullary pin tie-in (ESF-IM pin tie-in) fixators is an adjustable and effective method of fracture fixation in birds. The objective of this study was to evaluate the elements of the ESF-IM tie-in configuration used in birds. Ten variations of constructs were applied to a plastic bone model with a standard gap. Variants included non-tied and tie-in configurations, use of a 6- or 10-mm acrylic bar or a thermoplastic connecting bar, variation in the placement of the proximal fixation pin, use of 1.1-mm (0.045-in) or 1.6-mm (0.062-in) fixation pins, and configurations of 2, 3, or 4 fixation pins. The various constructs were loaded in bending, torque, and compression, and response variables were determined from resulting load-displacement curves (stiffness, load at 1-mm displacement). Results showed that, by using the tie-in configuration, increasing the diameter of the acrylic connecting bar, and increasing the diameter or number of fixation pins, each significantly increased the stiffness in all assessments. Placing the fixation pin distally in the proximal bone model segment increased the stiffness in bending, and adding a fixation pin to the distal bone model segment increased the stiffness in torque and bending. These results quantified the relative importance of specific parameters that effect stiffness and safe load of the ESF-IM tie-in construct as applied to a plastic bone fracture model.

Interfragmentary compression forces of scaphoid screws in a sawbone cylinder model

Various screws have been developed to stabilise fractures of the scaphoid. Commonly used are the Herbert, the HBS, the 3-mm AO and the Acutrak screws. Not long ago a new screw, the Twin Fix, was introduced. This is cannulated and similar in shape and appearance to the classical Herbert screw. In our test series we compared the maximum achievable compression forces of the Twin Fix screw with that of three other screws (AO, HBS and Acutrak screws). To avoid the variations of density, stiffness and rigidity in natural bone, a polyurethane sawbone-based test setup was used. The test series included 10 screws of each type. The compression force was measured using a special strain gauge. The mean compression force was significantly higher for the Twin Fix screw (8+/-1N) and the Acutrak screw (7.6+/-0.4/0.6N) in relation to the AO screw (6.8+/-1.0/1.4N) and HBS screw (2+/-1N). We found the Twin Fix and Acutrak screws to be promising in the treatment of scaphoid fractures.

Biomechanics and biology of external fixation of distal radius fractures

External fixation is a versatile and useful tool for management of complex fractures. There is little to choose between the various types of commercially available fixators, and it is important to use one that allows the surgeon adequate versatility and follows sound biomechanical principles. Ligamentotaxis can be used effectively to reduce the most difficult fractures; however, over-distraction and prolonged traction are harmful and should be avoided. Certain types of fractures do not respond to treatment with ligamentotaxis alone and require adjunctive treatment, such as limited internal fixation. A single K-wire significantly adds to the stability of fixation and should be considered in all cases. Understanding the basic mechanical principles and respect for pin-bone biology allow for successful use of external fixation with minimal complications.

Complications encountered while using thin-wire-hybrid-external fixation modular frames for fracture fixation. A retrospective clinical analysis and possible support for "Damage Control Orthopaedic Surgery"

One hundred ninety eight adult patients who had sustained long bone fractures were treated by external fixation from admission to bone healing and consolidation. Of these, 135 had sustained high-energy injuries, 39 of them had suffered multi-system injuries. Superficial pin track infection was the most common complication, occurring predominantly in pins located in the femur, upper tibia and upper humerus. There were no cases of deep infection or osteomyelitis. One patient with a femoral shaft fracture developed a DVT although he was on preventive low molecular weight heparin, i.e. sc Clexane 40 mg daily. There were no cases of PE or ARDS. External fixation systems are a minimal invasive surgical modality, which allow three-dimensional fracture fixation after closed or minimal open reduction. They require a good command of surgical anatomy, but provide an optimal preservation of the fracture's soft tissue envelope, the critical biological factor for new bone formation and fracture healing. Recent publications have suggested that in the critically ill patient, minimally invasive fracture fixation surgery may prevent the perpetuation of a reactive, life threatening inflammatory reaction (the "second hit") which may induce the development of multiple organ dysfunction (MODS).

Effect of early axial dynamization on tibial bone healing: a study in dogs

Early axial dynamization and its effect on experimental tibial bone healing was compared with healing under rigid fixation in a time-sequenced manner using dogs. An external fixator that could be rigidly locked or set to allow free axial movement while preventing bending and shear was used. Both tibias were osteotomized and externally fixed, leaving a gap between bone ends of 2 mm. At 1 week, one side was dynamized, whereas the other side was kept rigidly locked as a control. Dogs were euthanized at 1 day and 1, 3, 5, 8, and 11 weeks after dynamization. The outcome measures were static and dynamic load-bearing, periosteal callus development, new bone formation, callus tissue composition, and mechanical strength. Load bearing was higher on the dynamized limbs during standing for the first 5 weeks and during gait for the first 3 weeks after dynamization compared with the controls. Maximum periosteal callus size was reached faster and was distributed more symmetrically on the dynamized side. The periosteal callus area decreased at 12 weeks on the dynamized sides, but there was no significant change in the area on the control sides. Endosteal new bone formation and bone density decreased between 9 and 12 weeks only on the dynamized sides. The dynamized side showed a significantly higher torsional stiffness at 6 weeks than did the controls. There were no significant differences between dynamized and control tibias at other times. Maximum torque also tended to be higher on the dynamized sides at the same time. Early axial dynamization appeared to accelerate callus formation and remodeling and to provide higher mechanical stiffness during early stages of bone healing.

External skeletal fixation. Linear fixators

Linear external skeletal fixators offer some unique advantages over other fixation systems; they are biomechanically versatile, minimize surgical trauma to the fracture environment, and allow for simple staged disassembly to help promote bone healing. Historically, the use of linear external skeletal fixators has been reported to result in numerous complications, primarily related to the pin-bone interface. External skeletal fixators have experienced a resurgence in the past two decades. Increased knowledge of proper pin insertion techniques and advancements in pin design have greatly enhanced the longevity of the pin-bone interface, resulting in fewer complications. This article reviews some of the potential advantages of linear external skeletal fixators and principles of application to help strengthen the pin-bone interface and minimize complications.

Local tissue properties in bone healing: influence of size and stability of the osteotomy gap

To characterize the site-specific mechanical and histological properties in fracture repair and to relate these properties to the initial mechanical situation, an experimental fracture model was used in the metatarsus of 42 sheep. The mechanical situation of a transverse osteotomy was described by three gap sizes (1, 2, or 6 mm) and two amounts of strain (7 or 31%). An external fixator that allowed a defined axial movement provided control of these settings. Nine weeks following surgery, the healing area was dissected and tensile and compressive properties were measured in subregions of the fracture gap and the periosteal callus. The central, sagittal section was used for quantitative histology. We found the quality of the tissue along the osteotomy line to be most important for regaining mechanical stability. Increasing the size of osteotomy gaps resulted in poorer mechanical and histological qualities, and the repair process was less complete. Interfragmentary strain did not significantly influence the repair process. The smaller strain levels had already stimulated the secondary repair process, and this stimulatory effect could not be further enhanced by increasing the amount of strain. Our finding that large gaps between bone segments were not as well healed as were smaller gaps suggests that it is advantageous to avoid large gaps in fracture treatment.

Fracture site motion with Ilizarov and "hybrid" external fixation

OBJECTIVES

To evaluate differences in fracture site motion by using different external fixators.

DESIGN

A wooden dowel was used to simulate a long bone with a transverse diaphyseal fracture. Ilizarov, "hybrid," and strutaugmented "hybrid" external fixation was used to stabilize the "fracture." The wooden dowel was subjected to separate axial, four-point bending, and torsional loads. Fracture site motion in the axial plane, off-axis motion (shear and bending), and rotation were measured.

SETTING

All mechanical testing was performed with a sevohydraulic test frame (MTS Systems, Minneapolis, MN, U.S.A.). Fracture site motion was measured with an interfragment motion device developed in this laboratory.

INTERVENTION

Comparison was made between a traditional fourring Ilizarov fixator, a "hybrid" fixator using rings and threaded pins attached by a unilateral aluminum bar, and a "hybrid" fixator augmented with a V-shaped strut.

MAIN OUTCOME MEASUREMENT

Load-deformation behavior in axial displacement, shear displacement, and bending displacement were compared between the different configurations under identical conditions of axial loading, torsional loading, and four-point bending. In torsional loading, rotational displacement was also measured.

RESULTS

The Ilizarov configuration allowed significantly less off-axis fracture site motion in all loading modes than either "hybrid" configuration while still allowing axial compression of the fracture ends.

CONCLUSIONS

In a completely unstable fracture with poor bone apposition, the mechanical behavior of a four-ring Ilizarov external fixator is superior to the mechanical behavior of a unilateral "hybrid" frame.

Comparative biomechanical evaluation of different external fixation sidebars: stainless-steel tubes versus carbon fiber rods

Carbon fiber rods were developed to provide radiolucent sidebars for external fixation. In the present study, a single-plane, half-pin, double-bar external fixator construct with either stainless-steel tubes or carbon fiber rods was applied on the anteromedial surface of an osteotomized synthetic human tibia and evaluated for fixation rigidity. Testing was performed with the bone fragments in cortical contact and with a 5-mm midshaft gap between the fragments. The sidebars then were loaded to failure in bending. The results of this study show (a) that the carbon fiber rods were 15% stiffer than the stainless-steel tubes (p = 0.009) and (b) that the external fixator with carbon fiber rods achieved approximately 85% of the fixation stiffness of the external fixator with stainless-steel tubes. The loss of stiffness of the external fixator with carbon fiber rods is most likely due to the clamps being less effective in connecting the carbon fiber rods rigidly to the Schanz screws.

The effects of micromotion and particulate materials on tissue differentiation. Bone chamber studies in rabbits

Motion at the interface between bone and implants for joint replacement may interfere with osseointegration and prosthesis stabilization. Particulate materials may cause foreign body and chronic inflammatory reactions resulting in bone resorption (osteolysis). The micromotion chamber (MC) and the bone harvest chamber (BHC) were implanted in the rabbit tibia, and the effects of micromotion and phagocytosable particulate materials on tissue formation within the chamber were assessed by studying bone ingrowth into a 1-mm pore. Using the MC, one short daily episode of motion (20 cycles/day, 0.5 mm amplitude) for three weeks decreased the amount of bone ingrowth. Using a different pore configuration, the same parameters of motion increased bone ingrowth. Increasing the amplitude of motion (from 0.5 to 0.75 mm), or the number of daily motion periods (from one to two per day) then decreased bone ingrowth. These studies suggest the existence of a window of externally applied strain: a small stimulus may facilitate and a large stimulus may discourage bone formation within the chamber. Cessation of a given set of motion parameters (producing primarily fibrous tissue) for an additional three weeks was accompanied by tissue differentiation into bone. Using the BHC, small, phagocytosable particles of bone cement, high density polyethylene and cobalt chrome alloy, at a concentration of 1.0 x 10(8) particles/mL, caused a foreign body reaction and inhibited the ingrowth of bone. Particles of titanium alloy had no effect on net bone formation. In studies using normal and immunodeficient rats, T lymphocytes were not a prerequisite for macrophages to phagocytose polyethylene particles. In the clinical situation, micromotion and particulate debris may be synergistic in producing prosthetic loosening. If an implant does not undergo osseointegration due to excessive micromotion, the fibrous tissue interface may provide a conduit for the subsequent migration of particles around the implant.

Effects of mechanical stimulation on the differentiation of hard tissues

In 1892, J.L. Wolff believed that bone was a dynamic organ that responded to the biomechanical environment. Research has shown that mechanical stimulation can have a profound effect on the differentiation and development of mesenchymal tissues. It would appear that a 'window' of mechanical strain exists which may facilitate or discourage the accretion of bone. With respect to processes such as fracture healing and ingrowth of bone into porous coated prostheses, it may be possible to modulate the mechanical environment with the application of well-defined, exogenous loads in order to promote a more favourable outcome.

Ex vivo biomechanics of Kirschner-Ehmer external skeletal fixation applied to canine tibiae

The purpose of this study was to determine the respective contribution of each of the following parameters to the compressive, bending, and torsional rigidity of the Kirschner-Ehmer (KE) external fixation splint as applied to canine tibiae with an osteotomy gap: bilateral versus unilateral splints; increasing the number of fixation pins; altering the diameter of fixation pins and side bars; decreasing side bar distances from the bone; increasing pin separation distances in each pin group; decreasing distances between pin groups; altering pin clamp orientation; and altering side bar conformation. Bilateral splints were 100% (mean) stiffer than unilateral splints, with stiffness enhanced to the greatest extent in mediolateral bending and torsion. Increasing pin numbers stiffened both bilateral (mean, 41%; 8 versus 4) and unilateral splints (mean, 14%; 8 versus 4). Medium KE splints were 85% (mean) stiffer than small KE splints. Decreasing side bar distances to the bone from 1.5 cm to 1.0 cm to 0.5 cm increased stiffness of both bilateral and unilateral splints by a mean of 13% to 35%. Widening pin spacing from 1.67 cm to 2.5 cm increased stiffness in craniocaudal bending only (56% increase, bilateral splints; 73% increase, unilateral splints). Decreasing the distance between pin groups from 5.84 cm to 2.5 cm increased stiffness in torsion between 23% (unilateral splints) and 45% (bilateral splints) and decreased stiffness of unilateral splints by 29% in craniocaudal bending. Altering pin clamp configuration so that the bolts of the clamp were inside the side bar rather than outside the side bar increased stiffness in axial compression only (73% increase, bilateral splints; 54% increase, unilateral splints). Conforming the lateral side bar to the tibiae increased only axial compressive stiffness by 77% but was no different than placing the clamps inside the side bars of an unconformed bilateral splint. These results quantify the relative importance of specific parameters affecting KE splint rigidity as applied to unstable fractures in the dog.

Fractures of the tibia

ESF is an ideal fixation for most tibial fractures. Many ESF configurations have been used in repair of tibial fractures including ESF supplementation of intramedullary pin or interfragmentary screw fixation. Tibial fractures are often comminuted or open, and the tibia is prone to postoperative infections after open reduction and internal fixation. ESF is ideal for stabilization of open tibial fractures and management of associated soft tissue injury.

Influence of bone staple design on interfragmentary compression

Staple fixation, most commonly thought of as static, has been studied with regard to effecting dynamic compression. Features of staple design that relate to the creation of interfragmentary compression have been analyzed, and a simple manipulative technique to effect compression is illustrated. Compressive forces of bone fixation staples were measured using pressure-sensitive film. Thirteen staple designs were tested in a wood-block model and in cadaveric bone. Staples with legs splayed outward produced higher compressive forces than standard staple designs. Internal beveling of the staple leg tips also produced high compressive forces, while external beveling produced distraction. Staples with legs tapered over their entire length produced compression, but caused splitting in the wood-block model. Our results show that outward splaying of the staple legs is a simple method to increase compression in any staple design.

The determination of bone fracture properties by dual-energy X-ray absorptiometry and single-photon absorptiometry: a comparative study

Dual-energy X-ray absorptiometry (DEXA) and single-photon absorptiometry (SPA) were used to quantitate the structural strength and local material properties of healing tibial osteotomies in 32 dogs. Dogs were divided into four equal groups, euthanatized at either 2, 4, 8, or 12 weeks, and imaged with DEXA and SPA. Invasive techniques were used to determine (1) the torsional properties of the bone, (2) the local stiffness properties and calcium content within the bone, and (3) new bone formation and porosity by histology. There were no differences between SPA and DEXA in their associations with the torsional properties of bone. SPA and DEXA had strong correlations with the ultimate torque (R2 = 0.76, 0.51) and the torsional stiffness (R2 = 0.68, 0.53) of bone. SPA and DEXA of periosteal callus, endosteal callus, and cortical bone had similar associations with indentation stiffness, calcium content, new bone formation, and porosity. SPA of gap tissue had significantly stronger associations with these four parameters than DEXA (P less than 0.05). Correlation coefficients (R2) with these local material properties ranged as high as 0.82 for SPA with new bone formation in the gap tissue and 0.73 for DEXA with indentation stiffness of periosteal callus.

Role of interfragmentary strain in fracture healing: ovine model of a healing osteotomy

It has been hypothesized that the histological pattern of fracture healing is controlled at least in part by the local mechanical strains in the interfragmentary region. To test this "interfragmentary strain hypothesis," we applied cyclic bending deformations to tibial osteotomies in 11 sheep. An instrumented flexible plate spanning a 1-mm osteotomy gap was deformed to create a gradient of tissue elongation from 10% under the plate to 100% at the opposite cortex. The cyclic deformations were applied three times per minute, 24 h per day, for 1-5 weeks. However, as a result of tissue differentiation, the bone-plate complex increased in stiffness with healing time, resulting in a marked reduction of the gap deformation at approximately 4 weeks. Fracture healing was evaluated using vascular injection of India ink and conventional histology. A nonlinear three-dimensional finite element model of the interfragmentary tissue at the initial stage of healing was used to predict the complex tissue strains. The ingrowth of vascularized soft tissue into the interfragmentary gap, as well as the subsequent differentiation of this tissue, occurred earlier and to a greater degree in regions of lower strain. In contrast, the proliferation of callus tissue was greatest at the periosteal and endosteal surfaces of the cortex opposite the plate. Direct comparison of the finite element predictions with the histology demonstrated that the spatial distribution of bone resorption at the fracture fragment ends directly corresponded to the locations of elevated tissue strain and stress. However, there was no consistent numerical relationship between the magnitude of these local peak strains and the corresponding volume of cortical bone resorption over the bone cross section.

The concept of osseointegration and bone matrix expression

Osseointegration has been defined as the direct structural and functional connection between ordered, living bone and the surface of a load-carrying implant. To date, this concept has been described by descriptive histological and ultrastructural criteria but not by biochemical means. This review evaluates the basic science work performed on this concept and then applies the concept to the principle of osseous healing. Specific studies are cited where alterations in the healing response are due to clinical management of implant placement and how studies of surface properties may lead to further insights on implant design and prognosis. In addition, a review of bone expression as a function of in vitro stress applications is given. This is followed by an indepth review of the collagens and noncollagenous proteins, described to date, within isolated bone matrix. It is this collagenous matrix (especially type I) that is described as being close to and oriented with a glycoprotein component next to the implant surface. In turn, the large family of noncollagenous proteins are important in mediating bone proliferation, matrix accumulation, orientation, mineralization, and turnover. This section is followed by a discussion of specific growth factors as they may relate to osseous healing around an implant.

Radial and tibial fracture repair with external skeletal fixation. Effects of fracture type, reduction, and complications on healing

Twenty-eight consecutive fractures of the canine radius and tibia were treated with external skeletal fixation as the primary method of stabilization. The time of fixation removal (T1) and the time to unsupported weight-bearing (T2) were correlated with: (1) bone involved; (2) communication of the fracture with the external environment; (3) severity of the fracture; (4) proximity of the fracture to the nutrient artery; (5) method of reduction; (6) diaphyseal displacement after reduction; and (7) gap between cortical fragments after reduction. The Kruskal-Wallis one-way analysis of variance was used to test the correlation with p less than .05 set as the criterion for significance. The median T1 was 10 weeks and the median T2 was 11 weeks. None of the variables correlated significantly with either of the healing times; however, there was a strong trend toward longer healing times associated with open fractures and shorter healing times associated with closed reduction. Periosteal and endosteal callus uniting the fragments were observed radiographically in comminuted fractures, with primary bone union observed in six fractures in which anatomic reduction was achieved. Complications observed in the treatment of these fractures included: bone lysis around pins (27 fractures), pin track drainage (27 fractures), pin track hemorrhage (1 fracture), periosteal reaction around pins (27 fractures), radiographic signs consistent with osteomyelitis (12 fractures), degenerative joint disease (2 dogs), and nonunion (1 fracture). Valgus or rotational malalignment resulted in 16 malunions of fractures. One external fixation device was replaced and four loose pins were removed before the fractures healed. One dog was treated with antibiotics during the postoperative period because clinical signs of osteomyelitis appeared.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanical performance of the standard Orthofix external fixator

Static and fatigue tests of the standard Orthofix unilateral external fixator (Orthofix SRL, Verona, Italy) were performed. Under similar fixation configurations, the Orthofix device offered higher bending stiffness in both directions, equal torsional stiffness, and lower axial stiffness when compared to the Hoffmann-Vidal quadrilateral frame with full pins. The bending resistance of the Orthofix ball joint was found to be proportional to its locking cam tightening torque. After applying 2 million loading cycles to the bone ends fixed by the device, the overall stiffness characteristics of the frame did not change significantly. Repetitive manual tightening and loosening of the ball joint caused abrasive wear on the cam and bushing surfaces. The locking position of the cam migrated for a mean of 45 degrees. After 50 cycles of tightening and bending to failure, the ball joint locking strength was reduced by 20% to 25%, but the stiffness did not change. Wear and stripping of the seat of the fixator body locking screw and the pin fixation screw threads were also noted. Based on the test results, the standard Orthofix device could be re-used, but certain fixator components must be inspected and replaced. The ball joint locking cam and fixation screws required periodic tightening during clinical application to prevent loss of frame stiffness under repetitive loading. Modifications of the fixator design are recommended to improve its mechanical performance.

Correlations between mechanical stress history and tissue differentiation in initial fracture healing

A general theory for the role of intermittently imposed stresses in the differentiation of mesenchymal tissue is presented and then applied to the process of fracture healing. Two-dimensional finite element models of a healing osteotomy in a long bone were generated and the stress distributions were calculated throughout the early callus tissue under various loading conditions. These calculations were used in formulating theoretical predictions of tissue differentiation that were consistent with the biochemical and morphological observations of previous investigators. The results suggest that intermittent hydrostatic (dilatational) stresses may play an important role in influencing revascularization and tissue differentiation and determining the morphological patterns of initial fracture healing.

Guidelines for external fixation frame rigidity and stresses

Using results from FEM analyses and experiments as references, analytical methods are applied to develop simple approximate formulas to relate frame rigidity, maximal pin stresses, and peak pin-bone stresses in external fracture fixation (EFF) configurations in axial loading to the most important frame, pin, and bone parameters. It is found that, in a realistic range, the parameters can be adapted to vary the frame rigidity from about 13 N/mm to 17,000 N/mm, thereby reducing the maximal stresses in the pins and at the pin-bone interface by a factor of 140. In particular, when compromises have to be established in the frame characteristics in order to ensure a flexible configuration and limit the stress values at the same time, the formulas presented can provide useful guidelines. The side-bar separation and the pin modulus, in particular, can be adapted to decrease the rigidity, while only moderately increasing the stresses, thereby reducing changes for pin failure, pin-bone loosening, and pin-tract infection. A nomogram is presented for a quick reference to estimated relations between frame characteristics, rigidity, and stresses. It is believed that this material may be of use in EFF design and applications in clinical and animal experimental trials.