Angle stable locking reduces interfragmentary movements and promotes healing after unreamed nailing. Study of a displaced osteotomy model in sheep tibiae

Interfragmentary strain measurement post-fixation to guide intraoperative decision making: a narrative review

PURPOSE

Interfragmentary strain influences whether a fracture will undergo direct and indirect fracture healing. Orthopedic trauma surgeons modulate strain and create optimal biomechanical environments for specific fracture patterns using fixation constructs. However, objective intraoperative interfragmentary strain measurement does not currently inform fixation strategy in common practice. This review identifies potential methods and technologies to enable intraoperative strain measurement for guiding optimal fracture fixation strategies.

METHODS

PubMed, Scopus, and Web of Science were methodologically queried for manuscripts containing terms related to "bone fracture," "strain," "measurement," and "intraoperative." Manuscripts were systematically screened for relevance and adjudicated by three reviewers. Relevant articles describing methods to measure interfragmentary strain intraoperatively were summarized.

RESULTS

After removing duplicates, 1404 records were screened initially. There were 49 manuscripts meeting criteria for in-depth review. Of these, four reports were included in this study that described methods applicable to measuring interfragmentary strain intraoperatively. Two of these reports described a method using instrumented staples, one described optical tracking of Kirschner wires, and one described using a digital linear variable displacement transducer with a custom external fixator.

CONCLUSION

The four reports identified by this review describe potential methods to quantify interfragmentary strain after fixation. However, further studies are needed to confirm the precision and accuracy of these measurements across a range of fractures and fixation methods. Additionally, described methods require the insertion and likely removal of additional implants into the bone. Ideally, innovations that measure interfragmentary strain intraoperatively would provide dynamic biomechanical feedback for the surgeon to proactively modulate construct stability.

The Use of Optical Tracking to Characterize Fracture Gap Motions and Estimate Healing Potential in Comminuted Biomechanical Models of Surgical Repair

Fracture healing is stimulated by micromotion at the fracture site, whereby there exists an optimal amount of strain to promote secondary bone formation. Surgical plates used for fracture fixation are often evaluated for their biomechanical performance using benchtop studies, where success is based on overall construct stiffness and strength measures. Integration of fracture gap tracking to this assessment would provide crucial information about how plates support the various fragments present in comminuted fractures, to ensure there are appropriate levels of micromotion during early healing. The goal of this study was to configure an optical tracking system to quantify 3D interfragmentary motion to assess the stability (and corresponding healing potential) of comminuted fractures. An optical tracking system (OptiTrack, Natural Point Inc, Corvallis, OR) was mounted to a material testing machine (Instron 1567, Norwood, MA, USA), with an overall marker tracking accuracy of 0.05 mm. Marker clusters were constructed that could be affixed to individual bone fragments, and segment-fixed coordinate systems were developed. The interfragmentary motion was calculated by tracking the segments while under load and was resolved into compression-extraction and shear components. This technique was evaluated using two cadaveric distal tibia-fibula complexes with simulated intra-articular pilon fractures. Normal and shear strains were tracked during cyclic loading (for stiffness tests), and a wedge gap was also tracked to assess failure in an alternate clinically relevant mode. This technique will augment the utility of benchtop fracture studies by moving beyond total construct response and providing anatomically relevant data on interfragmentary motion, a valuable proxy for healing potential.

Angular stable locking in a novel intramedullary nail improves construct stability in a distal tibia fracture model

INTRODUCTION

Intramedullary nails are frequently used for treatment of unstable distal tibia fractures. However, insufficient fixation of the distal fragment could result in delayed healing, malunion or nonunion. Recently, a novel concept for angular stable nailing was developed that maintains the principle of relative stability and introduces improvements expected to reduce nail toggling, screw migration and secondary loss of reduction. The aim of this study was to investigate the biomechanical competence of the novel angular stable intramedullary nail concept for treatment of unstable distal tibia fractures, compared to a conventional nail locking in a human cadaveric model under dynamic loading.

MATERIALS AND METHODS

Ten pairs of fresh-frozen human cadaveric tibiae with a simulated AO/OTA 42-A3.1 fracture were assigned to 2 groups for reamed intramedullary nailing using either a conventional (non-angular stable) Expert Tibia Nail (ETN) with 3 distal screws or the novel Tibia Nail Advanced (TNA) system with 2 distal angular stable locking low-profile retaining screws. The specimens were biomechanically tested under conditions including initial quasi-static loading, followed by progressively increasing combined cyclic axial and torsional loading in internal rotation until failure of the bone-implant construct. Both tests were monitored by means of motion tracking.

RESULTS

Initial nail toggling of the distal tibia fragment in varus and flexion under axial loading was lower for TNA compared to ETN, being significant in flexion, P = 0.91 and P = 0.03. After 5000 cycles, interfragmentary movements in terms of varus, flexion, internal rotation, axial displacement, and shear displacement at the fracture site were all lower for TNA compared to ETN, with flexion and shear displacement being significant, P = 0.14, P = 0.04, P = 0.25, P = 0.11 and P = 0.04, respectively. Cycles to failure until both interfragmentary 5° varus and 5° flexion were significantly higher for TNA compared to ETN, P = 0.04.

CONCLUSION

From a biomechanical perspective, the novel angular stable intramedullary nail concept provides increased construct stability and maintains it over time while reducing the number of required locking screws without impeding the flexibility of the nail itself and resists better towards loss of reduction under dynamic loading, compared to conventional locking in intramedullary nailed unstable distal tibia fractures.

Biomechanical Performance of BoneHelix® Compared with Elastic Stable Intramedullary Nailing (ESIN) in a Pediatric Tibia Fracture Model

Tibial shaft fractures are common injuries in the pediatric and adolescent populations. Elastic stable intramedullary nailing (ESIN) is the treatment of choice for cases that require surgical stabilization. A new intramedullary device, BoneHelix^® (BH), may be an alternative for use with fractures that cannot be satisfactorily stabilized with ESIN. This study aimed to assess the biomechanical performance of BH compared with ESIN in a porcine tibia fracture model, observing cyclic fatigue and load to failure. Computed tomography was used to monitor the implant position and to rule out unintended damage. No implant or bone failure occurred during the fatigue testing. An increase in the cumulative plastic displacement was observed in both test groups over the loading cycles applied. Both implant–bone constructs displayed a trend toward closure of the osteotomy gap. During the load-to-failure test, the average loads at failure in specimens instrumented with ESIN and BH were 5364 N (±723) and 4350 N (±893), respectively, which were not statistically significant (p = 0.11). The values of both groups were two to three times higher than the estimated maximal load (2000 N) during physiological weight bearing. The biomechanical results thus indicate equivalent performance and stability by the implants tested.

Is initial interfragmentary compression made to last? An ovine bone in vitro study

Interfragmentary compression, a major principle of fracture treatment, is clinically not quantified and might be lost quickly even without functional loads. We designed an experimental study hypothesizing that (1) compression can be controlled using either lag screw or compression plate, and expecting similar initial compression, (2) loss of interfragmentary compression through relaxation within one hour is reduced with neutralization locking plate next to lag screw compared to compression plate. Twelve ovine femora (N=6) and humeri (N=6) were assigned into groups: Group 1 received a 45° oblique osteotomy at mid-diaphysis and was fixated using a 3.5 mm interfragmentary lag screw and locking compression plate (3.5 mm LCP, DePuy Synthes) as neutralization plate. Group 2 received a transverse osteotomy and was fixated with dynamic compression using compression plate (LCP). Interfragmentary pressure and relative bone fragment displacements were recorded over one hour. Median loss of compression over one hour time (relaxation) were 0.52% in Group 1, and 0.17% in Group 2 (p>0.05). Median rotational displacements amounted to 0.46° for Group 1, and 0.31° for Group 2, and axial displacement to a median of -20 μm in Group 1 and 25 μm in Group 2. Ovine bone interfragmentary stress relaxation maintains compression over the first hour for lag screw with neutralization plate for an oblique fracture line or compression plate for a transverse fracture line. Measured compression forces around 100 N could be overcome by physiological tension loading in bending or torsion, necessitating for instance tension band plating, additional lag screws or absolutive stability.

Bone morphogenetic protein 2-induced cellular chemotaxis drives tissue patterning during critical-sized bone defect healing: an in silico study

Critical-sized bone defects are critical healing conditions that, if left untreated, often lead to non-unions. To reduce the risk, critical-sized bone defects are often treated with recombinant human BMP-2. Although enhanced bone tissue formation is observed when BMP-2 is administered locally to the defect, spatial and temporal distribution of callus tissue often differs from that found during regular bone healing or in defects treated differently. How this altered tissue patterning due to BMP-2 treatment is linked to mechano-biological principles at the cellular scale remains largely unknown. In this study, the mechano-biological regulation of BMP-2-treated critical-sized bone defect healing was investigated using a multiphysics multiscale in silico approach. Finite element and agent-based modeling techniques were combined to simulate healing within a critical-sized bone defect (5 mm) in a rat femur. Computer model predictions were compared to in vivo microCT data outcome of bone tissue patterning at 2, 4, and 6 weeks postoperation. In vivo, BMP-2 treatment led to complete healing through periosteal bone bridging already after 2 weeks postoperation. Computer model simulations showed that the BMP-2 specific tissue patterning can be explained by the migration of mesenchymal stromal cells to regions with a specific concentration of BMP-2 (chemotaxis). This study shows how computational modeling can help us to further understand the mechanisms behind treatment effects on compromised healing conditions as well as to optimize future treatment strategies.

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.

A biomechanical comparison of two plating techniques in lateral clavicle fractures

BACKGROUND

Neer Type IIb lateral clavicle fractures typically lead to dislocation of the medial fragment. Therefore, most surgeons recommend surgical treatment for such a fracture pattern. The use of a locking compression plate with a lateral extension has produced satisfactory results in various studies over recent years. Double-plate fixation is a common technique in the treatment of complex distal radius fractures. The authors use this technique as a routine procedure in the treatment of Neer type IIb fractures. In this biomechanical testing study, the mechanical properties of the two techniques were compared.

METHODS

On 20 clavicles from fresh frozen cadavers a Neer Type IIb fracture-like osteotomy was performed. A cyclic loading test followed by a load-to-failure test was carried out. Parameters for statistical evaluation were the stiffness at cycles 1, 100 and 17,500 as well as the ultimate tensile load and the deformation at the point of failure.

FINDINGS

All specimens withstood the cyclic loading test without any noticeable damage. At cycles 100 and 17,500, the double-plate technique was less stiff. Failure loads were not significantly different from each other, but deformation at the point of failure was significantly greater for the double-plate technique.

INTERPRETATION

Both techniques provided sufficient fixation to the fracture site to endure the cyclic loading test, which is supposed to simulate an incident-free week postoperatively. In summary, the double-plate technique offers biomechanically a feasible alternative to the single-plate technique in lateral clavicle fractures of Neer Type IIb.

Intramedullary nailing biomechanics: Evolution and challenges

This article aims to review the biomechanical evolution of intramedullary nailing and describe the breakthrough concepts which allowed for nail improvement and its current success. The understanding of this field establishes an adequate background for forthcoming research and allows to infer on the path for future developments on intramedullary nailing. It was not until the 1940s, with the revolutionary Küntscher intramedullary nailing design, that this method was recognized as a widespread medical procedure. Such achievement was established based on the foundations created from intuition-based experiments and the first biomechanical ideologies. The nail evolved from allowing alignment and stability through press-fit fixation with nail-cortical wall friction to the nowadays nail stability achieved through interlocking screws mechanical linkage between nail and bone. Important landmarks during nail evolution comprise the introduction of flexible reaming, the progress from slotted to non-slotted nails design, the introduction of nail ‘dynamization’ and the use of titanium alloys as a new nail material. Current biomechanical improvement efforts aim to enhance the bone–intramedullary nail system stability. We suggested that benefit would be attained from a better understanding of the ideal mechano-biological environment at the fracture site, and future improvements will emerge from combining mechanics and biological tools.

Clinical and Research Approaches to Treat Non-union Fracture

PURPOSE OF REVIEW

Impaired healing outcomes or even non-unions after bone injury are still a highly relevant problem in the daily clinical life. Especially within an aging population, the occurrence of bone fractures increases and thus novel treatment approaches to overcome compromised bone regeneration are needed.

RECENT FINDINGS

The gold standard to treat delayed or non-healing bone injuries is still the use of autologous bone grafts to foster regeneration. Besides its successful treatment outcome, it also has disadvantages: a second surgery is needed in order to harvest the bone material and the material is highly limited. Looking into the recent literature, a multitude of different research approaches were already conducted to identify new possible strategies to treat impaired bone regeneration: application of mesenchymal stromal cells, platelet lysates, growth factors, interference in the immune system, or bone formation stimulation by ultrasound. This review gives an overview of the treatment approaches actually performed in the clinic as well as at the bench in the context of compromised bone healing. It clearly highlights the complexity of the nature of non-healing bone fractures as well as patient-dependent factors influencing the healing process.

The antegrade angle-stable locking intramedullary nail for type-C distal femoral fractures: a thirty four case experience

INTRODUCTION

This is a retrospective study that provides initial experience and verifies the effectiveness of the newly-designed antegrade interlocking angle-stable intramedullary nail (IAIN) combined with half-threaded cancellous screws in the management of type-C (AO/OTA classification) distal femoral fractures.

METHODS

During a period of 30 months, 34 patients (mean age 43.1 years) with type-C (AO/OTA classification) fractures of the distal femur were treated with IAIN and half-threaded cancellous screws were reviewed. Peri-operative and post-operative parameters were analyzed.

RESULTS

All of the fractures healed in a mean time of 12.6 weeks with no incidences of malunion, nonunion or infection. No secondary failure of fixation occurred. Partial weight bearing was initiated in an average of 7.4 weeks post-operatively, with full weight bearing initiated in 13.8 weeks. All of the patients, except for one, gained full extension. The mean flexion of the knee joint was 110.1°, while the mean Hospital for Special Surgery (HSS) knee score was 85.2.

CONCLUSION

The IAIN and half-threaded cancellous screws provided a reliable fixation that facilitated uncomplicated outcomes and uneventful early mobilization in treating type-C fractures of the distal femur.

Role of the fibula in the stability of diaphyseal tibial fractures fixed by intramedullary nailing

BACKGROUND

For tibial fractures, the decision to fix a concomitant fibular fracture is undertaken on a case-by-case basis. To aid in this clinical decision-making process, we investigated whether loss of integrity of the fibula significantly destabilises midshaft tibial fractures, whether fixation of the fibula restores stability to the tibia, and whether removal of the fibula and interosseous membrane for expediency in biomechanical testing significantly influences tibial interfragmentary mechanics.

METHODS

Tibia/fibula pairs were harvested from six cadaveric donors with the interosseous membrane intact. A tibial osteotomy fracture was fixed by reamed intramedullary (IM) nailing. Axial, torsion, bending, and shear tests were completed for four models of fibular involvement: intact fibula, osteotomy fracture, fibular plating, and resected fibula and interosseous membrane.

FINDINGS

Overall construct stiffness decreased slightly with fibular osteotomy compared to intact bone, but this change was not statistically significant. Under low loads, the influence of the fibula on construct stability was only statistically significant in torsion (large effect size). Fibular plating stiffened the construct slightly, but this change was not statistically significant compared to the fibular osteotomy case. Complete resection of the fibula and interosseous membrane significantly decreased construct torsional stiffness only (large effect size).

INTERPRETATION

These results suggest that fixation of the fibula may not contribute significantly to the stability of diaphyseal tibial fractures and should not be undertaken unless otherwise clinically indicated. For testing purposes, load-sharing through the interosseous membrane contributes significantly to overall construct mechanics, especially in torsion, and we recommend preservation of these structures when possible.

Biomechanical effects of angular stable locking in intramedullary nails for the fixation of distal tibia fractures

Treatment of distal tibia shaft fractures using intramedullary nailing requires stable fixation of the distal fragment to prevent malunion. Angular stable locking for intramedullary nails pledge to provide increased mechanical stability. This study tested the hypothesis that intramedullary nails with angular stable interlocking screws would have increased construct stiffness, reduced fracture gap movement and enhanced fatigue failure compared to nails with conventional locking having the same diameter. Biomechanical experiments were performed on 24 human cadaveric tibiae which obtained a distal fracture and were fixed by three different techniques: conventional locking with 8- and 10-mm-diameter nails and angular stable locking with 8-mm nails. Stiffness of the implant–bone construct and movement of the fragments were tested under axial loading and torsion. The constructs were tested to failure under cyclic fatigue loading. Analysis of variance and Kaplan–Meier survival analysis were used for statistical assessment. Axial stiffness of the 10-mm nail was about 50% larger compared to both 8-mm nail constructs independent of the type of locking mode (p < 0.01). No differences were found in axial performance between angular stable and conventional locking neither under static nor under cyclic testing conditions (p > 0.5). Angular stability significantly decreased the clearance under torsional load by more than 50% compared to both conventionally locked constructs (p = 0.03). However, due to the larger nail diameter, the total interfragmentary motion was still smallest for the 10-mm nail construct (p < 0.01). Although the 10-mm nail constructs survived slightly longer, differences between groups were minor and not statistically significant (p = 0.4). Our hypothesis that angular stable interlocking of intramedullary nails would improve mechanical performance of distal tibia fracture fixation was not confirmed in a physiologically realistic loading scenario. Whether minor mechanical advantages provided by angular stability of the locking screws would improve biological tissue response cannot be concluded from this biomechanical study.

Biomechanical evaluation of shape-memory alloy staples for internal fixation-an in vitro study

Background

The field of orthopaedics is a constantly evolving discipline. Despite the historical success of plates, pins and screws in fracture reduction and stabilisation, there is a continuing search for more efficient and improved methods of fracture fixation. The aim of this study was to evaluate shape-memory staples and to compare them to a currently used implant for internal fracture fixation. Multi-plane bending stability and interfragmentary compression were assessed across a simulated osteotomy using single and double-staple fixation and compared to a bridging plate.

Methods

Transverse osteotomies were made in polyurethane blocks (20 × 20 × 120 mm) and repairs were performed with one (n = 6), or two (n = 6) 20 mm nitinol staples, or an eight-hole 2.7 mm quarter-tubular plate (n = 6). A pressure film was placed between fragments to determine contact area and compressive forces before and after loading. Loading consisted of multi-planar four-point bending with an actuator displacement of 3 mm. Gapping between segments was recorded to determine loads corresponding to a 2 mm gap and residual post-load gap.

Results

Staple fixations showed statistically significant higher mean compressive loads and contact areas across the osteotomy compared to plate fixations. Double-staple constructs were superior to single-staple constructs for both parameters (p < 0.001). Double-staple constructs were significantly stiffer and endured significantly larger loads before 2 mm gap formation compared to other constructs in the dorsoventral plane (p < 0.001). However, both staple constructs were significantly less stiff and tolerated considerably lower loads before 2 mm gap formation when compared to plate constructs in the ventrodorsal and right-to-left lateral loading planes. Loading of staple constructs showed significantly reduced permanent gap formation in all planes except ventrodorsally when compared to plate constructs.

Conclusions

Although staple fixations were not as stable as plate fixations in particular loading planes, double-staple constructs demonstrated the most consistent bending stiffness in all planes. Placing two perpendicular staples is suggested instead of single-staples whenever possible, with at least one staple applied on the compression side of the anticipated loading to improve construct stability.

Monitoring Healing Progression and Characterizing the Mechanical Environment in Preclinical Models for Bone Tissue Engineering

The treatment of large segmental bone defects remains a significant clinical challenge. Due to limitations surrounding the use of bone grafts, tissue-engineered constructs for the repair of large bone defects could offer an alternative. Before translation of any newly developed tissue engineering (TE) approach to the clinic, efficacy of the treatment must be shown in a validated preclinical large animal model. Currently, biomechanical testing, histology, and microcomputed tomography are performed to assess the quality and quantity of the regenerated bone. However, in vivo monitoring of the progression of healing is seldom performed, which could reveal important information regarding time to restoration of mechanical function and acceleration of regeneration. Furthermore, since the mechanical environment is known to influence bone regeneration, and limb loading of the animals can poorly be controlled, characterizing activity and load history could provide the ability to explain variability in the acquired data sets and potentially outliers based on abnormal loading. Many approaches have been devised to monitor the progression of healing and characterize the mechanical environment in fracture healing studies. In this article, we review previous methods and share results of recent work of our group toward developing and implementing a comprehensive biomechanical monitoring system to study bone regeneration in preclinical TE studies.

Initiation and early control of tissue regeneration - bone healing as a model system for tissue regeneration

INTRODUCTION

Tissue regeneration in itself is a fascinating process that promises repeated renewal of tissue and organs.

AREAS COVERED

This article aims to illustrate the different strategies available to control tissue regeneration at a very early stage, using bone as an exemplary tissue. The aspects of a controlled inflammatory cascade to achieve a balanced immune response, cell therapeutic approaches for improved tissue formation and angiogenesis, guiding the organization of newly formed extracellular matrix by biomaterials, the relevance of mechanical signals for tissue regeneration processes, and the chances and limitations of growth factor treatments are discussed.

EXPERT OPINION

The currently available knowledge is reviewed and perspectives for potential new targets are given. This is done under the assumption that early identification of risk patients as well as the application of early intervention strategies is possible.

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 new angle stable nailing concept for the treatment of distal tibia fractures

PURPOSE

Surgical treatment of distal tibial fractures demands a stable fracture fixation while minimizing the irritation to the soft tissues by approach and implant. Biomechanical studies have demonstrated superior performance for angular-stable locked nails over standard locked nails in distal tibial fractures. The experimental Retrograde Tibial Nail (RTN) is a minimally invasive local intramedullary osteosynthesis, which has been under design by our group. We conducted a biomechanical comparison in composite tibiae of the Retrograde Tibial Nail against the Expert Tibial Nail (Synthes®). Our hypothesis was that the RTN would provide equivalent biomechanical stability with respect to extra-axial compression, torsion and load to failure testing, in an extra-articular distal tibia fracture model.

METHODS

Biomechanical composite bone testing was conducted in 14 biomechanical composite tibiae in an AO 43 A3 fracture model. In both groups, triple angle stable interlocking was performed in the distal fragment.

RESULTS

Results show a statistically non-significant higher stability of the ETN during the axial loading tests. Torsional stability testing resulted in a statistically superior performance for the RTN (p = 0.018). Destructive extra-axial compression resulted in failure of six ETN constructs, while all RTN specimens survived the maximal load.

CONCLUSIONS

The experimental Retrograde Tibial Nail provides the key features for the treatment of distal tibial fractures. It combines a minimally invasive local intramedullary osteosynthesis with the ability to securely fix the fracture by multiple angle stable locking options.

Fracture near press-on interlocking enhances callus mineralisation in a sheep midshaft tibia osteotomy model

INTRODUCTION

Factors which impair fracture healing after intramedullary (IM) nailing of long bone fractures range from surgical and biological factors to mechanical parameters. Mechanical parameters known to prolong bony consolidation are share forces at the site of the fracture. Fracture near press-on interlocking reduces share forces directly at the fracture site and is hypothesised to enhance callus mineralisation. A sheep model of midshaft tibia osteotomies evaluates the technique.

MATERIALS AND METHODS

Fracture near interlocking was achieved by surfacing a custom made nail with special hutches that enable firm screw seating on top of the nail ("golf ball" structure). Virtual (fine element analysis (FEA)) and biomechanical pilot tests were completed before in vivo application in 12 adult female German black sheep. Midshaft tibia osteotomy was performed creating a subcritical 7 mm gap for delay in union. One group (n=6) was treated with reamed IM nailing employing the custom made nail and in addition to proximal and distal standard interlocking a fracture near press on interlocking was employed. A second group of six sheep without additional press on interlocking served as control. 10 weeks after operation the quality of fracture healing was determined by micro-CT.

RESULTS

The FEA showed that axial loading up to 4000N did not lead to implant fatigue. Fracture near press on interlocking led to significantly more callus mineralisation compared to the conventional interlocking procedure (0.567 g/cm(3) ± 0.106 g/cm(3) versus 0.434 g/cm(3) ± 0.0836 g/cm(3), p=0.043).

CONCLUSIONS

Fracture near press on interlocking increases callus mineralisation in a subcritical osteotomy model in sheep. The results indicate that the reduction of share forces at the fracture site after nailing procedures may be effective in reducing the time until bony consolidation.

The Flexible Axial Stimulation (FAST) intramedullary nail provides interfragmentary micromotion and enhanced torsional stability

BACKGROUND

Recent advances in intramedullary (IM) nailing have focused on removing free play at the nail-screw interface to provide enhanced construct torsional stiffness. These changes also increase axial construct stiffness and reduce axial interfragmentary movement, which is required for optimal secondary fracture healing. This study tested whether a novel intramedullary nail, the Flexible Axial Stimulation (FAST) nail, can simultaneously provide controlled axial interfragmentary motion with enhanced torsional stiffness.

METHODS

Novel tibial nails and matched controls (N=6 per group) were tested in a cadaveric osteotomy fracture model and in explanted bench testing. In cadaver and bench tests, nails were tested in axial tension/compression, torsion, bending, and shear. Overall construct stiffness values were calculated in each loading mode and axial and torsional low-load micromotion plateaus were quantified.

FINDINGS

The novel nails produced 1 mm of controlled axial interfragmentary motion, which was associated with a 22% reduction in axial stiffness compared to standard controls (P=0.026, effect size 2.5). The novel constructs also allowed less low-load torsional movement compared to the controls (3.8 deg vs. 7.1 deg, P=0.010, effect size 1.9), which was associated with a 14% increase in overall construct torsional stiffness (P=0.003, effect size 1.3). There were no observable differences in performance between the novel and control nails in anteroposterior/mediolateral bending or shear.

INTERPRETATION

These results suggest that an IM nailing construct can provide axial interfragmentary motion while retaining high torsional stiffness, a combination which may potentially enhance healing.

Long-term stability of angle-stable versus conventional locked intramedullary nails in distal tibia fractures

Background

In the last years intramedullary nailing has become the treatment of choice for most displaced diaphyseal tibia fractures. In contrast intramedullary nailing of distal tibia fractures is accompanied by problems like decreased biomechanical stability. Nevertheless the indications for intramedullary nailing have been extended to include even more distal fractures. The purpose of this study was to compare long-term mechanical characteristics of angle-stable versus conventional locked intramedullary nails in the treatment of unstable distal tibia fractures. Therefore, the effect of time on the mechanical properties of biodegradable sleeves was assessed.

Methods

8 pairs of fresh, frozen porcine tibiae were used. The expert tibial nail (Synthes) was equipped with either three conventional locking screws (CL) or the angle-stable locking system (AS), consisting of a special ASLS screw and a biodegradable sleeve. Biomechanical testing included torsional and axial loading at different time-points over 12 weeks.

Results

The AS group showed a significantly higher torsional stiffness at all time-points (at least 60%) compared to the CL group (p < 0.001). The neutral zone was at least 5 times higher in the CL group (p < 0.001). The mean axial stiffness was maximum 10% higher (week 6) in the angle-stable locked group compared to the conventional group. There was no significant change of the torsional mechanical characteristics over the 12 weeks in both groups (p > 0.05). For axial stiffness and range of motion significant differences were found in the AS group.

Conclusions

The angle-stable locking system (ASLS) with the biodegradable sleeve provides significantly higher long-term stability. Especially the differences determined under torsional loading in this study may have clinical relevance. The ASLS permits the potential to decrease complications like secondary loss of reduction and mal-/non-union.

Intramedullary nailing and angulation prevention in distal metaphyseal tibial fractures

Intramedullary nailing, which is preferred in tibial diaphyseal fractures, is also frequently used in distal third tibial fractures. Various angular deformities, including varus/valgus deformity, may be observed during postintramedullary nailing. Orthopedic surgeons use several methods to prevent this problem.In this study, at least 2 static locking screws were placed proximal and distal to the nail during intramedullary nailing of distal third tibial fractures. No additional supportive methods were used. The efficacy of this technique in the prevention of postoperative angular deformities was retrospectively investigated. Thirty-four patients with distal third tibial fractures who were treated with intramedullary nailing were included in the study. Angulations were measured in the anteroposterior and lateral planes on plain radiographs obtained preoperatively, on postoperative day 1, and after fracture union. Angulations measured on postoperative day 1 were compared with those measured after fracture union, and an increase was observed. Based on statistical analyses, the increase in the angulations was not significant.In distal third tibial fractures, when fixation was performed by placing 2 static screws distal and proximal to the intramedullary nail following adequate reduction, the angulations that developed during the period until union were not significant in terms of causing deformity, although additional fixation methods are not used.

Auxiliary locking plate improves fracture stability and healing in intertrochanteric fractures fixated by intramedullary nail

BACKGROUND

Intertrochanteric fractures present a significant management challenge due to their low inherent stability. The objective of this study was to determine whether an auxiliary locking plate decreases interfragmentary motions and improves fracture healing in intertrochanteric fractures treated by intramedullary nail.

METHODS

Biomechanical tests and a clinical retrospective study in intertrochanteric to subtrochanteric nonunions were performed. Six synthetic femurs were osteotomized intertrochanterically and fixated with a long gamma nail and an additional locking compression plate. Mechanical tests were conducted that simulated the hip joint force during gait cycle. Following the initial test, the locking compression plate (LCP) was removed from each specimen and the test was repeated. Interfragmentary motions, strains on implants and osteosynthesis stiffness were determined. For the clinical part of the study, 13 intertrochanteric to subtrochanteric nonunions were treated with revisional long gamma nail and additional locking compression plate. Complications and time to union were determined.

FINDINGS

Biomechanically, interfragmentary rotation was 48% smaller (P=0.047) and interfragmentary shear movement was 42% smaller (P=0.007) with locking compression plate. Strains on the nail decreased by 20-27% (P<0.027) and the osteosynthesis stiffness increased by 23% (P=0.005) with locking compression plate. Clinically, fracture healing was achieved in eleven out of 13 patients after 9.0months (range 4 to 22months).

INTERPRETATION

The findings of our study indicate that auxiliary locked plating considerably improves biomechanical performance and results in successful healing of unstable intertrochanteric to subtrochanteric femur fractures.

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.

A novel intramedullary nail for micromotion stimulation of tibial fractures

BACKGROUND

Animal studies and clinical trials have suggested that early application of controlled axial micromotion can accelerate healing of long bone fractures compared to rigid fixation. However, experimental investigations of micromotion constructs have been limited to external fixators, which have a higher incidence of complications than intramedullary nails. The purpose of this study was to assess whether a novel intramedullary nail design can generate stimulatory micromotion under minimal weight-bearing loads typical of the early healing period.

METHODS

Eight cadaver tibiae were reamed, osteotomised, and implanted with commercially-available IM nails fitted with a custom insert that allowed 1mm of axial micromotion after proximal/distal interlocking. Specimens were mounted in a materials testing machine and subjected to cyclic axial loading while interfragmentary motion was measured using an extensometer. Implants were also tested in standard statically-locked mode.

FINDINGS

The average force required to cause distraction of the fracture gap in micromotion mode was 37.0 (SD 21.7) N. The mean construct stiffness was 1046.8 (SD 193.6) N/mm in static locking mode and 512.4 (SD 99.6) N/mm in micromotion mode (significantly different, P<0.001).

INTERPRETATION

These results support the development of a micromotion-enabled IM nail because the forces required to cause interfragmentary movements are very low, less than the weight of the hanging shank and foot. In contrast to rigid-fixation nails, which require significant weight-bearing to induce interfragmentary motion, the micromotion-enabled nail may allow movement in non-weight-bearing patients during the early healing period when the benefits of mechanical stimulation are most critical.

Fracture healing under healthy and inflammatory conditions

Optimal fracture treatment requires knowledge of the complex physiological process of bone healing. The course of bone healing is mainly influenced by fracture fixation stability (biomechanics) and the blood supply to the healing site (revascularization after trauma). The repair process proceeds via a characteristic sequence of events, described as the inflammatory, repair and remodeling phases. An inflammatory reaction involving immune cells and molecular factors is activated immediately in response to tissue damage and is thought to initiate the repair cascade. Immune cells also have a major role in the repair phase, exhibiting important crosstalk with bone cells. After bony bridging of the fragments, a slow remodeling process eventually leads to the reconstitution of the original bone structure. Systemic inflammation, as observed in patients with rheumatoid arthritis, diabetes mellitus, multiple trauma or sepsis, can increase fracture healing time and the rate of complications, including non-unions. In addition, evidence suggests that insufficient biomechanical conditions within the fracture zone can influence early local inflammation and impair bone healing. In this Review, we discuss the main factors that influence fracture healing, with particular emphasis on the role of inflammation.

The primary stability of angle-stable versus conventional locked intramedullary nails

PURPOSE

The aim of this study was to compare the initial biomechanical characteristics of the angle-stable locking system for intramedullary nails using the new biodegradable sleeve with conventional locking in the treatment of unstable distal tibial fractures.

METHODS

Eight pairs of fresh, frozen porcine tibiae were used for this study. The expert tibial nail (Synthes) was equipped with either conventional locking screws (CL) or the angle-stable locking system (AS). This system consists of a special ASLS screw with a biodegradable sleeve. For this investigation distal tibias (5.5 cm) were used and the nails were locked with three screws in both groups. Biomechanical testing included non-destructive torsional and axial loading.

RESULTS

The AS group showed a significantly higher torsional stiffness (70%) compared to the CL group. The range of motion was 0.5 times smaller for the AS constructs. The neutral zone was eight times higher in the CL group (p < 0.001). In axial loading the AS group also showed a 10% higher axial stiffness and a 12% lower range of motion (p < 0.001).

CONCLUSION

The angle-stable locking system (ASLS) using a special screw and sleeve locking for intramedullary nails provides a significantly higher primary stability. The differences determined in this study may have clinical relevance particularly for torsional loads. For the new biodegradable angle-stable sleeve we found a comparable stability to the PEEK-based sleeve system. This system has the potential to decrease complications such as secondary loss of reduction and mal-/non-union.

Intramedullary nailing in fracture treatment: history, science and Küntscher's revolutionary influence in Vienna, Austria

Although the first intermedullary fixation technique was already reported in 1886, successfully inter-medullary nailing did not start until November 1939 when Küntscher's revolutionary technique was applied for the first time. Whereas Küntscher initially stated that his "marrow nail" was suitable for almost every fracture type as well as for other procedures including fixation of osteotomies, joint arthrodesis and pseudarthro-sis treatment he tried to develop an own nail for every possible fracture type through the years. Undoubtedly, Küntscher has to be considered one of the most influential surgeons. Nevertheless, he was never offered a university position and a lot of people did not acknowledge his brilliancy until his death in 1972. Only in Vienna the willingness to seize Küntscher's ideas was high. Therefore, in addition to a historic overview and to fundamental knowledge referring to reamed and unreamed respectively to static and dynamic nailing Küntscher's influence on Viennese researchers is presented in our paper.

Advances in intramedullary nail fixation in foot and ankle surgery

Tibiotalocalcaneal arthrodesis for the treatment of complex foot and ankle deformities are extremely challenging cases. Technological advances in intramedullary nail fixation have improved the biomechanical properties of available fixation constructs in recent years. Nails designed specifically to accommodate hindfoot anatomy, advancement in the understanding of optimal screw orientation, fixed angle technology, the availability of spiral blade screws, and features designed to achieve compression across the arthrodesis site have provided the foot and ankle surgeon with a greater armamentarium for performing tibiotalocalneal arthrodesis. Although advances may help to improve clinical results, small sample sizes and the low-level evidence of study designs limit the evaluation of how these advances affect clinical outcomes.

Mechanobiology of bone healing and regeneration: in vivo models

Mechanical boundary conditions are well known to influence the regeneration of bone and mechanobiology is the study of how mechanical or physical stimuli regulate biological processes. In vivo models have been applied over many years to investigate the effects of mechanics on bone healing. Early models have focused on the influence of mechanical stability on healing outcome, with an interest in parameters such as the magnitude of interfragmentary movement, the rate and timing of application of micromotion and the number of loading cycles. As measurement techniques have been refined, there has been a shift in orders of magnitude from investigations targeted at the organ level to those targeted at the tissue level and beyond. An understanding of how mechanics influences tissue differentiation during repair and regeneration crucially requires spatial and temporal knowledge of both the local mechanical environment in the healing tissue and a characterization of the tissues formed over the course of regeneration. Owing to limitations in the techniques available to measure the local mechanical conditions during repair directly, simulation approaches, such as the finite element method, are an integral part of the mechanobiologist's toolkit, while histology remains the gold standard in the characterization of the tissue formed. However, with rapid advances occurring in imaging modalities and methods to characterize tissue properties, new opportunities exist to better understand the role of mechanics in the biology of bone regeneration. Combined with developments in molecular biology, mechanobiology has the potential to offer exciting, new regenerative treatments for bone healing.

Improvement of the shear fixation stability of intramedullary nailing

BACKGROUND

The healing outcome of long bone fractures is strongly influenced by the mechanical environment. High interfragmentary movement at the fracture site is detrimental to the fracture healing process. Long bone fractures stabilized with thin intramedullary nails commonly used for unreamed intramedullary nailing might be very flexible in shear direction and therefore critical for the fracture healing outcome. The aims of this study were to simulate the shear interfragmentary movement during gait for a human tibia treated with intramedullary nailing and to investigate if this movement could be lowered by implant design modifications.

METHODS

The shear movement was calculated with a 3D finite element model based on computer tomograph images of a cadaver bone-implant complex of a transverse tibia fracture treated with a Stryker T2 Standard Tibial Nail. This model was validated through in vitro test results under pure shear, axial, bending and torsional loading.

FINDINGS

High shear movements of approximately 4mm were calculated during gait. These shear movements could be reduced by approximately 30% either by implant modifications or the use of a 1mm thicker nail. Combining the implant modifications with a 1mm thicker nail, the shear movements could be reduced by 54%.

INTERPRETATION

The increase of the fixation stiffness by using an implant material with a high Young's modulus in combination with an angle-stable nail-screw fixation helps to reduce the shear movement during gait and possibly to lower the risk of a prolonged healing time with unreamed intramedullary nailing.

Intramedullary nailing vs. palmar locked plating for unstable dorsally comminuted distal radius fractures: a biomechanical study

BACKGROUND

The purpose of this study was to compare the stability of a 2.4mm palmar locking compression plate and a new intramedullary nail-plate-hybrid Targon DR for dorsally comminuted distal radius fractures.

METHODS

An extraarticular 10mm dorsally open wedge osteotomy was created in 8 pairs of fresh frozen human radii to simulate an AO-A3-fracture. The fractures were stabilized using one of the fixation methods. The specimens were loaded axially with 200 N and dorsal-excentrically with 80 N. 2000cycles of dynamic loading and axial loading-to-failure were performed.

FINDINGS

Axial loading revealed that intramedullary osteosynthesis (Targon DR: 369 N/mm) was significantly (p=0.017) stiffer than plate osteosynthesis (Locking compression plate: 131 N/mm). With 214 N/mm the intramedullary nail also showed higher stability during dorsal excentric loading than the Locking compression plate with 51 N/mm (p=0.012). After 2000 cycles of axial loading with 80 N the Targon DR-group was significantly stiffer than the Locking compression plate-group under both loading patterns. Neither group showed significant changes in stiffness after 2000 cycles. Under dorsal excentric loading the Targon DR-group was still significantly stiffer with 212 N/mm than the Locking compression plate-group with 45 N/mm (p=0.012). The load to failure tests demonstrated higher stability of intramedullary nailing (625 N) when compared to plate osteosynthesis (403 N) (p<0.025).

INTERPRETATION

The study shows that intramedullary fixation of a distal AO-A3 radial fracture is biomechanically more stable than volar fixed-angle plating under axial and dorsal-excentric loading in an experimental setup.

Influence of the fixation stability on the healing time--a numerical study of a patient-specific fracture healing process

BACKGROUND

The healing outcome of long bone fractures is strongly influenced by the interfragmentary movement of the bone fragments. This depends on the fixation stability, the optimum value of which is still not known. The aim of this study was to simulate a patient-specific human healing process using a numerical algorithm and to retrospectively analyse the influence of the fixation stability on the healing time.

METHODS

The healing simulation was processed as an initial value problem. This was iteratively solved based on two mechanical (invariants of the strain tensor, calculated through a finite element analysis) and five biological state variables (local tissue composition and blood perfusion) using a previously published fuzzy logic algorithm. For validation purposes, the calculated interfragmentary movement was compared to in vivo measurements of this patient. By changing clinically adjustable parameters of the fixation device, the influence of the fixation stability on the healing time was analysed.

FINDING

The time course showed good agreement of the interfragmentary movement compared with the in vivo measurements. The predicted healing time was strongly influenced by the fixation stability, i.e. by changing the parameters of the fixation device, it was possible to significantly reduce the healing time.

INTERPRETATION

The time to heal could be greatly reduced by modification of the fixator design, i.e. increasing the fixation stiffness. When using external fixation devices, this could be achieved by decreasing the free bending length of the pins, using a stiff fixation body and a stiff connection between the pins and the body.

Angle stable interlocking screws improve construct stability of intramedullary nailing of distal tibia fractures: a biomechanical study

INTRODUCTION

Intramedullary nailing is the treatment of choice for most displaced tibial shaft fractures. The ability to maintain a mechanically stable fixation becomes more difficult the further the fracture extends distally or proximally or when unreamed tibial nails are used. We assumed that a new angular stable locking option would provide improved stability and reduced interfragmentary movements in a distal tibia in vitro fracture model.

MATERIALS AND METHODS

Left and right bones of 8 pairs of human cadaveric tibiae were randomly assigned to either a group with conventional locked or a group with angular stable locked intramedullary nails. Nails of 10-mm-diameter were used after reaming up to 11 mm. A transverse distal osteotomy was performed and the specimens were tested mechanically under eccentric axial load. A video optical measurement system was used to determine the angular displacement of the osteotomy gap during loading.

RESULTS

Construct stiffness, maximum load of the bone-nail construct and gap angle at 0.5 kN load were measured. The group with the angular stable locking option showed significantly higher stiffness values and reduced fracture gap motion compared to the group with conventional locked nails.

DISCUSSION

A new angular stable locking option of intramedullary nails provides higher stability in terms of construct stiffness and reduced interfragmentary movements in a distal tibia in vitro fracture model.

Early dynamization by reduced fixation stiffness does not improve fracture healing in a rat femoral osteotomy model

Dynamization of fracture fixation is used clinically to improve the bone healing process. However, the effect of early dynamization remains controversial. This study evaluated the effect of early dynamization, by reduced stiffness of fixation on callus stiffness and size after 5 weeks of healing in a rat diaphyseal femoral osteotomy. An external unilateral fixator allowed either a rigid (R-group; n = 8) or a flexible (F-group; n = 8) fixation. The dynamized group (D-group: n = 8) had a rigid fixation for 1 week, and then a flexible fixation for the remaining 4 weeks. The pre- and postoperative activity of the rats was measured. After 5 weeks, the rats were sacrificed, and healing was evaluated by biomechanical and densitometric methods. The R-group had a higher activity more closely approaching preoperative levels, compared to the D-group throughout all time points measured. This difference was significant after 14 days and 21 days. The flexural rigidity of the R-group was 82% (tested in the anterior-posterior direction; p = 0.01) and 93% (tested in the medial-lateral direction; p = 0.002) greater than the flexural rigidity of the D-group. The rigid fixation led to a stiffer callus with a smaller callus volume, but better mineralized tissue in the whole callus and at the level of the osteotomy gap than the flexible or the dynamized fixation. Early dynamization did not improve healing compared to rigid or flexible fixation in a rat femoral osteotomy model.

A new animal model for bone atrophic nonunion: fixation by external fixator

A new small animal model of bone atrophic nonunion was established for investigating the process of bone regeneration by performing cauterization of the periosteum, removal of the local bone marrow, and stabilization with external fixation. The model allows the creation of an atrophic nonunion without the need for a critical size defect. Furthermore, it provides reproducible, well-defined mechanical conditions and minimized physical interference of the implant with the biological processes in the healing zone. Eighty adult Sprague-Dawley rats received an osteotomy of the left femur, stabilized with an external fixator. In half of the animals, the periosteum proximal and distal to the osteotomy was destroyed by cauterization and the adjacent bone marrow was removed (nonunion group). At 2 and 8 weeks after surgery, radiological, biomechanical, histological, and histomorphometrical analyses showed a typical physiological healing in the control group, while the nonunion group was characterized by resorption of the bone ends with some callus formation distant to the osteotomy. At both time points, the callus was composed of significantly less bone and significantly more connective tissue (p < 0.001). In addition, the torsional strength of the osteotomized femur was significantly less in the nonunion group than in the control group, which was comparable to that of the intact femur (p < 0.001). In conclusion, the present model allows the induction of an atrophic nonunion without the need of a critical size defect. It is reproducible, provides standardized biomechanical conditions, and allows minimized interaction of the implant with the healing zone.

Angle-stable and compressed angle-stable locking for tibiotalocalcaneal arthrodesis with retrograde intramedullary nails. Biomechanical evaluation

BACKGROUND

Retrograde intramedullary nailing is an established procedure for tibiotalocalcaneal arthrodesis. The goal of this study was to evaluate the effects of angle-stable locking or compressed angle-stable locking on the initial stability of the nails and on the behavior of the constructs under cyclic loading conditions.

METHODS

Tibiotalocalcaneal arthrodesis was performed in fifteen third-generation synthetic bones and twenty-four fresh-frozen cadaver legs with use of retrograde intramedullary nailing with three different locking modes: a Stryker nail with compressed angle-stable locking, a Stryker nail with angle-stable locking, and a statically locked Biomet nail. Analyses were performed of the initial stability of the specimens (range of motion) and the laxity of the constructs (neutral zone) in dorsiflexion/plantar flexion, varus/valgus, and external rotation/internal rotation. Cyclic testing up to 100,000 cycles was also performed. The range of motion and the neutral zone in dorsiflexion/plantar flexion at specific cycle increments were determined.

RESULTS

In both bone models, the intramedullary nails with compressed angle-stable locking and those with angle-stable locking were significantly superior, in terms of a smaller range of motion and neutral zone, to the statically locked nails. The compressed angle-stable nails were superior to the angle-stable nails only in the synthetic bone model, in external/internal rotation. Cyclic testing showed the nails with angle-stable locking and those with compressed angle-stable locking to have greater stability in both models. In the synthetic bone model, compressed angle-stable locking was significantly better than angle-stable locking; in the cadaver bone model, there was no significant difference between these two locking modes. During cyclic testing, five statically locked nails in the cadaver bone model failed, whereas one nail with angle-stable locking and one with compressed angle-stable locking failed.

CONCLUSIONS

Regardless of the bone model, the nails with angle-stable or compressed angle-stable locking had better initial stability and better stability following cycling than did the nails with static locking.

Development of an in vitro three dimensional loading-measurement system for long bone fixation under multiple loading conditions: a technical description

The purpose of this investigation was to design and verify the capabilities of an in vitro loading-measurement system that mimics in vivo unconstrained three dimensional (3D) relative motion between long bone ends, applies uniform load components over the entire length of a test specimen, and measures 3D relative motion between test segment ends to directly determine test segment construct stiffness free of errors due to potting-fixture-test machine finite stiffness.Intact equine cadaveric radius bones, which were subsequently osteotomized/ostectomized and instrumented with bone plates were subjected to non-destructive axial, torsion, and 4-point bending loads through fixtures designed to allow unconstrained components of non-load associated 3D relative motion between radius ends. 3D relative motion between ends of a 50 mm long test segment was measured by an infrared optical tracking system to directly determine its stiffness. Each specimen was then loaded to ultimate failure in either torsion or bending. Cortical bone cross-section diameters and published bone biomechanical properties were substituted into classical mechanics equations to predict the intact test segment theoretical stiffness for comparison and thus loading-measurement system verification.Intact measured stiffness values were the same order of magnitude as theoretically predicted. The primary component of relative motion between ends of the test segment corresponded to that of the applied load with the other 3D components being evident and consistent in relative magnitude and direction for unconstrained loading of an unsymmetrical double plate oblique fracture configuration. Bone failure configurations were reproducible and consistent with theoretically predicted.The 3D loading-measurement system designed: a) mimics unconstrained relative 3D motion between radius ends that occurs in clinical situations, b) applies uniform compression, torsion, and 4-point bending loads over the entire length of the test specimen, c) measures interfragmentary 3D relative motion between test segment ends to directly determine stiffness thus being void of potting-fixture-test machine stiffness error, and d) has the resolution to detect differences in the 3D motion and stiffness of intact as well osteotomized-instrumented and ostectomized-instrumented equine radii.

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.