Dynamic interfragmentary motion in fractures during routine patient activity

In silico modelling of long bone healing involving osteoconduction and mechanical stimulation

A lot of evidence has shown the importance of stimulating cell mechanically during bone repair. In this study, we modeled the challenging fracture healing of a large bone defect in tibial diaphysis. To fill the fracture gap, we considered the implantation of a porous osteoconductive biomaterial made of poly-lactic acid wrapped by a hydrogel membrane mimicking osteogenic properties of the periosteum. We identified the optimal loading case that best promotes the formation and differentiation into bone tissue. Our results support the idea that a patient's rehabilitation program should be adapted to reproduce optimal mechanical stimulations.

Concepts and clinical aspects of active implants for the treatment of bone fractures

Nonunion is a complication of long bone fractures that leads to disability, morbidity and high costs. Early detection is difficult and treatment through external stimulation and revision surgery is often a lengthy process. Therefore, alternative diagnostic and therapeutic options are currently being explored, including the use of external and internal sensors. Apart from monitoring fracture stiffness and displacement directly at the fracture site, it would be desirable if an implant could also vary its stiffness and apply an intervention to promote healing, if needed. This could be achieved either by a predetermined protocol, by remote control, or even by processing data and triggering the intervention itself (self-regulated 'intelligent' or 'smart' implant). So-called active or smart materials like shape memory alloys (SMA) have opened up opportunities to build active implants. For example, implants could stimulate fracture healing by active shortening and lengthening via SMA actuator wires; by emitting pulses, waves, or electromagnetic fields. However, it remains undefined which modes of application, forces, frequencies, force directions, time durations and periods, or other stimuli such implants should ideally deliver for the best result. The present paper reviews the literature on active implants and interventions for nonunion, discusses possible mechanisms of active implants and points out where further research and development are needed to build an active implant that applies the most ideal intervention. STATEMENT OF SIGNIFICANCE: Early detection of delays during fracture healing and timely intervention are difficult due to limitations of the current diagnostic strategies. New diagnostic options are under evaluation, including the use of external and internal sensors. In addition, it would be desirable if an implant could actively facilitate healing ('Intelligent' or 'smart' implant). Implants could stimulate fracture healing via active shortening and lengthening; by emitting pulses, waves, or electromagnetic fields. No such implants exist to date, but new composite materials and alloys have opened up opportunities to build such active implants, and several groups across the globe are currently working on their development. The present paper is the first review on this topic to date.

Tissue deformation controlling fracture healing

To achieve optimal flexibility in biological internal fracture fixation two questions require clarification: which biomechanical parameter controls healing and what are the boundary conditions thereof? Fracture movement interacts with callus and local stress and strain are influencing the reaction of the tissue cells. A linear gradient of strain was created inside a sheep tibia osteotomy using an active external fixator. The effect of different amounts of strain applied at 10 stimulation cycles/day on the occurrence of callus and on enabling osseous connection of the fragments was evaluated using micro-radiology to determine the amount of calcified new bone formation and its quality of gap bridging. A strong relation between level of strain and amount of callus was observed. Depending on the strain level different pattern of connections were seen. At the lowest investigated gap strain level of about 7% direct connection of the fragments within the gap occurred. Beyond 13% the callus only connected indirectly outside the gap. At over 36% callus did not connect the fragments anymore comparable to a situation in hypertrophic non-unions. The observed strong relation between interfragmentary strain and reduced osseous bridging may support the hypothesis that the elongation at rupture of connecting tissue plays an important role defining the upper limit for solid bridging. In planning fracture treatment, the amount of fracture mobility resulting in interfragmentary strain may play a crucial role to achieve solid healing.

The role of mechanical stimulation in the enhancement of bone healing

The biomechanical environment plays a dominant role in the process of fracture repair. Mechanical signals control biological activities at the fracture site, regulate the formation and proliferation of different cell types, and are responsible for the formation of connective tissues and the consolidation of the fractured bone. The mechanobiology at the fracture site can be easily manipulated by the design and configuration of the fracture fixation construct and by the loading of the extremity (weight-bearing prescription). Depending on the choice of fracture fixation, the healing response can be directed towards direct healing or towards indirect healing through callus formation. This manuscript summarizes the evidence from experimental studies and clinical observations on the effect of mechanical manipulation on the healing response. Parameters like fracture gap size, interfragmentary movement, interfragmentary strain, and axial and shear deformation will be explored with respect to their respective effects on fracture repair. Also, the role of externally applied movement on the potential enhancement on the fracture repair process will be explored. Factors like fracture gap size, type and amplitude of the mechanical deformation as well as the loading history and its timing will be discussed.

Programable Active Fixator System for Systematic In Vivo Investigation of Bone Healing Processes

This manuscript introduces a programable active bone fixator system that enables systematic investigation of bone healing processes in a sheep animal model. In contrast to previous systems, this solution combines the ability to precisely control the mechanical conditions acting within a fracture with continuous monitoring of the healing progression and autonomous operation of the system throughout the experiment. The active fixator system was implemented on a double osteotomy model that shields the experimental fracture from the influence of the animal's functional loading. A force sensor was integrated into the fixator to continuously measure stiffness of the repair tissue as an indicator for healing progression. A dedicated control unit was developed that allows programing of different loading protocols which are later executed autonomously by the active fixator. To verify the feasibility of the system, it was implanted in two sheep with different loading protocols, mimicking immediate and delayed weight-bearing, respectively. The implanted devices operated according to the programmed protocols and delivered seamless data over the whole course of the experiment. The in vivo trial confirmed the feasibility of the system. Hence, it can be applied in further preclinical studies to better understand the influence of mechanical conditions on fracture healing.

Improvement of clinical fracture healing - What can be learned from mechano-biological research?

The most significant predictors of reoperation following operative management of fractures are the presence of a third degree open fracture, remaining fracture gaps and a transverse fracture. However clinical studies provide no information regarding the involvement of various soft tissues or how the mechanical environment affects revascularisation and bone healing. Here the results of experimental and numerical mechano-biological studies on fracture healing are summarized to provide guidance toward clinical treatment of fractures. In experimental studies, isolated muscle crush appeared to only temporarily impair fracture healing, with no significant effect to the final bone healing, whereas a more severe muscle trauma significantly reduced callus formation and biomechanical properties of the healed bones. An intraoperative trauma can furthermore impede vascularization. Surgical removal of the haematoma or periosteum disturbs fracture healing. While reaming for intramedullary nailing reduced blood flow in the bone during the early phase of bone healing, it did not affect the stiffness or strength of the final bone healing. The optimal conditions for rapid vascularization and bone healing result from fracture fixation that minimizes shearing movements in the healing zone while allowing moderate compressive movements. Bone healing is increasingly delayed with increasing fracture gap size and critical-size defects do not heal sufficiently independent of the mechanical environment. The stiffness of fracture fixation systems like nails and external fixators applied in clinical treatments frequently display a too low stiffness, whereas plate systems often cause a too stiff fixation that suppresses bone healing.

Optimal satellite rod constructs to mitigate rod failure following pedicle subtraction osteotomy (PSO): a finite element study

BACKGROUND CONTEXT

Pedicle subtraction osteotomy (PSO) is a challenging restoration technique for sagittal imbalance and is associated with significant complications. One of the major complications is rod fracture and there exists a need for a biomechanical assessment of this complication for various instrumentation configurations.

PURPOSE

To evaluate and compare the global range of motion (ROM), rod stress distribution, and the forces on the pedicle subtraction site in various instrumentation configurations using finite element analysis.

STUDY DESIGN/SETTING

A computational biomechanical analysis.

METHODS

A previously validated osseoligamentous three-dimensional spinopelvic finite element model (T10-pelvis) was used to develop a 30° PSO at the L3 level. In addition to the standard bilateral cobalt chromium primary rod instrumentation of the PSO model, various multirod configurations including constructs with medially, laterally, and posteriorly affixed satellite rods and the short-rod technique were assessed in spinal physiological motions. T10-S1 global ROM, maximum von Mises stress on the rods and at the PSO level, factor of safety (yield stress of the rod material/maximum actual stress in the rod) and the load acting across the PSO site were compared between various instrumentation configurations. The higher the factor of safety the lesser the chances of rod failure.

RESULTS

Among all multirod constructs, posteriorly affixed satellite rod construct showed the greatest motion reduction compared to the standard bilateral rod configuration followed by medially and laterally affixed satellite rod constructs. Compared to the standard bilateral rod configuration, recessed short-rod technique resulted in 4% to 49% reduction in T10-S1 ROM recorded in extension and lateral bending motions, respectively, while the axial rotation motion increased by approximately 31%. Considering the maximum stress values on the rods, the recessed short-rod technique showed the greatest factor of safety (FOS = 4.1) followed by posteriorly (FOS = 3.9), medially (FOS = 3), laterally affixed satellite rod constructs (FOS = 2.8), and finally the standard bilateral rod construct (FOS = 2.7). By adding satellite rods, the maximum von Mises stress at the PSO level of the rods also reduced significantly and at this level resulted in the greatest FOS in the posteriorly affixed satellite rod construct. Compared to the standard bilateral rod construct, the load magnitude acting on the osteotomy site decreased by 11%, 16%, and 37% in the laterally, medially, and posteriorly affixed satellite rod constructs, respectively, and did not change with the short-rod technique.

CONCLUSIONS

Adding satellite rods increases the rigidity of the construct, which results in an increase in the stability and the reduction of the global ROM. Additionally, having satellite rods reduces the stress on the primary rods at the PSO level and shifts the stresses from this PSO region to areas adjacent to the side-by-side connectors. The data suggest a significant benefit in supplementing medial over lateral satellite rods at the PSO by reducing stress on the primary rods. Except the recessed short-rod technique, all other multirod constructs decrease the magnitude of the load acting across the osteotomy region, which could cause a delayed or non-union at the PSO site.

CLINICAL SIGNIFICANCE

The study evaluates the mechanical performance of various satellite rod instrumentation configurations following PSO to predict the risk factors for rod fracture and thereby mitigate the rate of clinically relevant failures.

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.

Control of the micromovements of a composite-material nail design: A finite element analysis

BACKGROUND

Intramedullary nail fixation is the most accepted modality for stabilizing long bone midshaft fractures. The commercially used nails are fabricated from Stainless Steel or Titanium. Composite-materials (CM) mainly carbon-fiber reinforced polymers (CFRP) have been gaining more interest and popularity due to their properties, such as modulus of elasticity close to that of bone, increased fatigue strength, and radio-opacity to irradiation that permits a better visualization of the healing process. The use of CFRP instead of metals allows better control of different directional movements along a fracture site. The purpose of this analysis was to design a CM intramedullary nail to enable micromovements as depicted on a finite element analysis method.

METHODS

We designed a three-dimentional femoral nail model. Three CFRP with different laminates arrangements, were included in the analysis. The finite element analysis involved applying vertical and horizontal loads on each of the designed and tested nails.

RESULTS

The nails permitted a transverse micromovement of 0.75mm for the 45° lay-up and 1.5mm for the 90° lay-up for the CM, 1.38mm for the Titanium and 0.74mm for the Stainless Steel nails. The recorded axial movements were 0.53mm for the 45° lay-up, 0.87mm for the 90° lay-up, 0.46mm for the unsymmetrical lay-up CM, 0.046 for the Titanium and 0.02 for the Stainless Steel nails. Overall, the simulations showed that nail transverse micromovements can be reduced by using 45° carbon fiber orientations. Similar results were observed with each metal nails.

INTERPRETATION

We found that nail micromovements can be controlled by changing the directional stiffness using different lay-up orientations. These results can be useful for predicting nail micromovements under specified loading conditions which are crucial for stimulating callus formation in the early stages of healing.

The healing stages of an intramedullary implanted tibia: A stress strain comparative analysis of the calcification process

AIMS

The extended usage of unreamed tibial nailing resulted in reports of an increased rate of complications, especially for the distal portion of the tibia. Unreamed nailing favours biology at the expense of the achievable mechanical stability, it is therefore of interest to define the limits of the clinical indications for this method. Extra-articular fractures of the distal tibial metaphysis, meta-diaphyseal junction, and adjacent diaphysis are distinct in their management from impaction derived ''pilon'' type fractures and mid-diaphyseal fractures. The goals of this work were to gain a thorough understanding of the load-sharing mechanism between unreamed nail and bones in a fractured tibia. With this purpose a complete model of the human leg was realised, simulating a mid-diaphyseal fracture, classified as A2 type 1, according to the AO classification. The analysis of the entire chain allows to have a complete picture of the stress distribution and of the most stressed bones and soft tissues, but, more importantly can overcome problems connected with boundary conditions imposed at single bony components.

METHODS

Model consists of six bony structures: pelvis, femur, patella, fibula, tibia, and a simplified lump of the feet, configured in a standing up position. Their articular cartilage layers, were simulated by 3D membranes of opportune stiffness connecting the different segments. Moreover an unreamed intra-medullary nail Expert Tibial Nail (DePuy Synthes(®)) stabilized the fractured tibia. A load of 700 N has been applied at the top of pelvis and a part the feet, at the tip, was rigidly fixed. Five different contact interfaces have been imposed at the different bony surfaces in contact.

RESULTS

Three different conditions were analysed: the initially healthy tibia, the A2 type 1 fractured tibia with the Expert tibial nail implanted, and the follow up stage after complete healing of tibia. Non-linear finite element analysis of the models were performed with Abaqus version 5.4 (Hibbitt, Karlsson and Sorensen, Inc., Pawtucket, RI) using the geometric non linearity and automatic time stepping options.

CONCLUSION

The obtained results reveal interesting consequences deriving by taking into account how the stress shielding can influence the integrity and resistance of bones, in order to identify the mechanical reasons for the unfavourable clinical results, and to identify borderline indications due to biomechanical factors. The evolution of treatment options for these fractures has been closely linked to developments in implant technology and surgical technique. Further developments in this area, particularly with respect to minimally invasive plating techniques and nail design are ongoing.

Biomechanical design of less invasive stabilization system femoral plates: computational evaluation of the fracture environment

Less Invasive Stabilization System femoral plates are currently accepted as a suitable fixation technique for supra-intercondylar femoral fractures. However, general agreement does not exist regarding the optimum design of this fixator type. Therefore, the aim of this article is to reduce the intrinsic Less Invasive Stabilization System complications by clarifying, from a biomechanical point of view, how the number of screws, the screw connection type (unicortical or bicortical), or the structured position of the screws can influence the outcome of the fracture site. These studies include a specific finite element analysis that determines how several biomechanical variables, such as the movement at the fracture site, are influenced by the preconditions of bone healing. The results of this study show that the screw type affects the mechanical stabilization of the femur to a greater extent than the material type of the Less Invasive Stabilization System femoral plates. The most significant differences among all the analyzed configurations are observed in the shear interfragmentary strain between screw types. Values are approximately 50% higher with unicortical screws than with bicortical ones.

Numerical simulation of callus healing for optimization of fracture fixation stiffness

The stiffness of fracture fixation devices together with musculoskeletal loading defines the mechanical environment within a long bone fracture, and can be quantified by the interfragmentary movement. In vivo results suggested that this can have acceleratory or inhibitory influences, depending on direction and magnitude of motion, indicating that some complications in fracture treatment could be avoided by optimizing the fixation stiffness. However, general statements are difficult to make due to the limited number of experimental findings. The aim of this study was therefore to numerically investigate healing outcomes under various combinations of shear and axial fixation stiffness, and to detect the optimal configuration. A calibrated and established numerical model was used to predict fracture healing for numerous combinations of axial and shear fixation stiffness under physiological, superimposed, axial compressive and translational shear loading in sheep. Characteristic maps of healing outcome versus fixation stiffness (axial and shear) were created. The results suggest that delayed healing of 3 mm transversal fracture gaps will occur for highly flexible or very rigid axial fixation, which was corroborated by in vivo findings. The optimal fixation stiffness for ovine long bone fractures was predicted to be 1000–2500 N/mm in the axial and >300 N/mm in the shear direction. In summary, an optimized, moderate axial stiffness together with certain shear stiffness enhances fracture healing processes. The negative influence of one improper stiffness can be compensated by adjustment of the stiffness in the other direction.

Disadvantages of interfragmentary shear on fracture healing--mechanical insights through numerical simulation

The outcome of secondary fracture healing processes is strongly influenced by interfragmentary motion. Shear movement is assumed to be more disadvantageous than axial movement, however, experimental results are contradictory. Numerical fracture healing models allow simulation of the fracture healing process with variation of single input parameters and under comparable, normalized mechanical conditions. Thus, a comparison of the influence of different loading directions on the healing process is possible. In this study we simulated fracture healing under several axial compressive, and translational and torsional shear movement scenarios, and compared their respective healing times. Therefore, we used a calibrated numerical model for fracture healing in sheep. Numerous variations of movement amplitudes and musculoskeletal loads were simulated for the three loading directions. Our results show that isolated axial compression was more beneficial for the fracture healing success than both isolated shearing conditions for load and displacement magnitudes which were identical as well as physiological different, and even for strain-based normalized comparable conditions. Additionally, torsional shear movements had less impeding effects than translational shear movements. Therefore, our findings suggest that osteosynthesis implants can be optimized, in particular, to limit translational interfragmentary shear under musculoskeletal loading.

Analysis of the Effect of Ring Stiffness on the Mechanical Performance of a Two-Ring Ilizarov Fixator

The two-ring Ilizarov fixator is superior, in terms of space and weight savings, to the traditional four-ring Ilizarov fixator. But the stiffness of the two-ring Ilizarov fixator is low. This weakness causes the two-ring Ilizarov fixator to be hardly used in corrective surgery. It has been shown that the configurations of the fixator, such as ring diameter and cross angle of the wires, can affect the stiffness of the fixator. In this study, the focus was on the effects of the properties of the ring, such as ring diameter, ring deformation, and ring material, on the stiffness of the two-ring Ilizarov fixator. The finite element analysis (FEA) technique was employed to model all the two-ring Ilizarov fixators using the ABAQUS FEA software. The following findings were achieved: (1) the radial deformation of the ring has an almost linear relationship with the vertical displacement of the bone especially when the radial deformation is larger, (2) the change in the stiffness of the two-ring Ilizarov fixator caused by the variation of the wire angle is due to the deformation of the ring, (3) the pretension on the wire is greatly reduced after it is attached to the ring, and (4) the influence of ring material on the stiffness of the two-ring Ilizarov fixator is less when the fixator wire angles are 90 deg-90 deg rather than 0 deg-0 deg. Based on these findings, in a real clinical application, the stiffness acting in a fixator-bone system during the course of a treatment and the stiffness of the growing bone can be deduced in a nonintrusive way.

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.

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.

Assessment of inducible fracture micromotion in distal radial fractures using radiostereometry

OBJECTIVES

This study examined the potential for measuring dynamic inducible micromotion (DIMM) between fragments in healing distal radial fractures using radiostereometry (RSA).

DESIGN

Prospective imaging study.

SETTING

University teaching hospital.

PATIENTS

Nine patients with low-impact distal radial fractures.

INTERVENTION

Volar locked plating of the fracture with insertion of tantalum beads into bone fragments. RSA examinations at 1 day and then 2, 6, 26, and 52 weeks. Motion at the fracture site was induced by maximal voluntary hand grip using a Jamar dynamometer. Radiographs were analyzed using locally developed and UMRSA software.

MAIN OUTCOME MEASUREMENTS

DIMM and migration were calculated as translations and rotations of the main distal segment. Clinical precision was assessed under repeatability conditions.

RESULTS

Precision (as 95% error limit) ranged from 0.06 to 0.13 mm and 0.5 to 0.8 degrees for migration, and from 0.10 to 0.14 mm and 0.6 to 1.0 degrees for DIMM. DIMM was characterized by axial and dorsal compression with dorsiflexion. The median DIMM of patients reached a maximum at 2 weeks: mainly as 0.3 mm axial compression, 0.3 mm dorsal compression, and 2.5 degrees dorsiflexion. DIMM ceased by 26 weeks, indicating union of all fractures. Fracture collapse continued until the 26-week measurement, ranging between 0.2 and 2.8 mm axially. Instability of some intraosseous markers was observed.

CONCLUSIONS

The precision of this RSA method was sufficient to observe inducible movements occurring during fracture healing. This has the potential for quantifying rates of fracture union and improving understanding of the available treatments.

The ABJS Nicolas Andry Award: Tissue engineering of bone and ligament: a 15-year perspective

Musculoskeletal repair is a major challenge for orthopaedic surgeons. The burden of repair is compounded by supply constraints and morbidity associated with autograft and allograft tissue. We report 15 years of research regarding tissue engineering and biological substitutes for bone and ligaments. Our approach has focused on biomaterial selection, scaffold development, cell selection, cell/material interaction, and growth factor delivery. We have extensively tested poly(ester), poly(anhydride), poly(phosphazene) derivatives, and composite materials using biocompatibility, degradation, and mechanical analyses for bone and ligament tissue engineering. We have developed novel three-dimensional matrices with a pore structure and mechanical properties similar to native tissue. We also have reported on the attachment, growth, proliferation, and differentiation of cells cultured on several scaffolds. Through extensive molecular analysis, in vitro culture condition analysis, and in vivo evaluation, our findings provide new methods of bone tissue regeneration using three-dimensional tissue engineered scaffolds, bioactive bone cement composite materials, and three-dimensional tissue engineered scaffolds for ligament regeneration.

Optimum loading mode for axial stiffness testing in limb lengthening

The axial stiffness of the regenerate is an indicator of bone healing after fracture or distraction osteogenesis. The axial stiffness may be assessed clinically by measuring the sharing of load between fixator and limb during loading. The aim of this study was to find out how to perform the stiffness test in order to minimize the influence of confounding factors to the test result. We investigated whether the test score was influenced by two factors: 1) the magnitude of the external load applied to the limb during the test; and 2) the patient's position during the test. The problem was approached by both a clinical study and by theoretical calculations. Thirty-three patients undergoing leg lengthening were tested regularly during the consolidation period. The stiffness test was executed with both high and low load, and in a standing and sitting position. There were significant differences in results between both the tests with high and low load, and between the standing and sitting tests. This indicated that both the magnitude of force and patient position during the test influenced the test result. Accordingly, these factors represent sources of error and must be taken into consideration when performing an axial stiffness test. The result of the theoretical calculations confirmed the result. We recommend performing the test while the patient is sitting, and to apply no more than 20% of the individual's body weight. It is also recommended that the same load be used in every test, when testing a patient several times during the treatment period.

Initial vascularization and tissue differentiation are influenced by fixation stability

Fracture healing requires a certain degree of mechanical stability and an adequate blood supply. The hypothesis of the present study was that increased interfragmentary shear leads to a reduced initial vascularization and prolonged healing. The aim of the study was to quantitatively analyze the histological appearance of vascularization and tissue differentiation with regard to fracture stability during the course of healing. A mid-shaft osteotomy of the tibia was performed in two groups of sheep and stabilized with either a rigid or semirigid external fixator, differing in bending stiffness. Interfragmentary movements and ground reaction forces were evaluated in vivo during a 9-week period. The sheep were sacrificed at 2, 3, 6, and 9 weeks postoperatively. The tibiae were tested biomechanically and histological sections from the callus were prepared for analysis of tissue differentiation and vascularization. Larger interfragmentary shear movements in the semirigid fixator group were associated with a reduced initial blood supply. At 6 weeks the semirigid fixator group showed a significantly lower percentage of mineralized bone and a higher amount of fibrous tissue leading to a significantly lower stiffness of the callus than the rigid fixator group. This initial delay in healing was compensated for in the later stages with the production of greater volumes of callus tissue so that both groups showed the same callus stiffness at 9 weeks. However, the rigid fixator group showed signs of the beginning of callus remodeling at the latest time points suggesting a faster bone healing. The results indicate the important role of the initial mechanical stability specifically in the vascularization of an osteosynthesis. Further studies should illustrate the precise role of mechanical conditions on the regulation of angiogenesis during early bone healing.

Shear movement at the fracture site delays healing in a diaphyseal fracture model

This study tested the hypothesis that interfragmentary axial movement of transverse diaphyseal osteotomies would result in improved fracture healing compared to interfragmentary shear movement. Ten skeletally mature merino sheep underwent a middiaphyseal osteotomy of the right tibia, stabilized by external fixation with an interfragmentary gap of 3 mm. A custom made external fixator allowed either pure axial (n=5) or pure shear movement (n=5) of 1.5 mm amplitude during locomotion by the animals. The movement of the osteotomy gap was monitored weekly in two sheep by an extensometer temporarily attached to the fixator. After 8 weeks the sheep were killed, and healing of the osteotomies was evaluated by radiography, biomechanical testing, and undecalcified histology. Shear movement considerably delayed the healing of diaphyseal osteotomies. Bridging of the osteotomy fragments occurred in all osteotomies in the axial group (100%), while in the shear group only three osteotomies (60%) were partially bridged. Peripheral callus formation in the shear group was reduced by 36% compared to the axial group (p<0.05). In the axial group bone formation was considerably larger at the peripheral callus and in between the osteotomy gaps but not in the intramedullary area. The larger peripheral callus and excess in bone tissue at the level of the gap resulted in a more than three times larger mechanical rigidity for the axial than for the shear group (p<0.05). In summary, fixation that allows excessive shear movement significantly delayed the healing of diaphyseal osteotomies compared to healing under axial movement of the same magnitude.

The initial phase of fracture healing is specifically sensitive to mechanical conditions

Interfragmentary movements affect the quality and quantity of callus formation. The mounting plane of monolateral external fixators may give direction to those movements. Therefore, the aim of this study was to determine the influence of the fixator mounting plane on the process of fracture healing. Identically configured fixators were mounted either medially or anteromedially on the tibiae of sheep. Interfragmentary movements and ground reaction forces were evaluated in vivo during a nine week period. Histomorphological and biomechanical parameters described the bone healing processes. Changing only the mounting plane led to a modification of interfragmentary movements in the initial healing phase. The difference in interfragmentary movements between the groups was only significant during the first post-operative period. However, these initial differences in mechanical conditions influenced callus tissue formation significantly. The group with the anteromedially mounted fixator, initially showing significantly more interfragmentary movements, ended up with a significantly smaller callus diameter and a significantly higher callus stiffness as a result of advanced fracture healing. This demonstrates that the initial phase of healing is sensitive to mechanical conditions and influences the course of healing. Therefore, initial mechanical stability of an osteosynthesis should be considered an important factor in clinical fracture treatment.

Interfragmentary movements in the early phase of healing in distraction and correction osteotomies stabilized with ring fixators

BACKGROUND

Experimental analyses have demonstrated the impact of mechanical conditions on bone healing. In critical clinical cases the mechanical conditions may be even more demanding than those in experimental studies. This study set out to examine the gap movements in distraction and correction osteotomies and to determine the suitability of initial fixation.

PATIENTS AND METHODS

Interfragmentary movements, ground reaction forces, and stability (ground reaction force divided by interfragmentary movement) were measured in 18 patients with tibial osteotomies stabilized by Ilizarov hybrid constructs until either bone union or conversion to internal fixation occurred (9 distraction treatments, 9 correction osteotomies). Consolidation was determined by clinical evaluation and standard radiographic techniques.

RESULTS

In both groups cocontraction led to gap movements comparable to level walking. Although the in vitro stiffness was slightly increased in the correction constructs, the interfragmentary movement in vivo was initially comparable between the groups. Patients undergoing distraction returned later to full weight bearing than patients undergoing correction treatment. In the correction group the stability increased with treatment time, while in the distraction group the stability remained relatively small.

CONCLUSIONS

The in vivo mechanical conditions in challenging clinical cases appear far more demanding than those in experimental studies. In distraction, mechanical conditions at the defect appear to be more critical than during correction osteosynthesis. According to the persistence of shear motion, even after 80 days of treatment, it may from the clinical point of view be important to maintain interfragmentary compression during the whole healing process and thereby reduce shear.

Interfragmentary motion in tibial osteotomies stabilized with ring fixators

Relative movement of bone fragments affects healing processes. In vivo data exist for patients with reduced transverse fractures only. The gap movements that occur under more complex conditions such as in tibial osteotomies, however, are unknown. The goal of this study was to determine the initial gap movements in tibial correction osteotomies, to monitor movements during the early healing period, and to determine the suitability of initial fixation stability in relation to daily activities. The interfragmentary movements were measured in six patients with correction osteotomies stabilized by Ilizarov ring fixator constructs until union or until conversion to internal fixation. Consolidation was determined by clinical evaluation and standard radiographic techniques. Co-contraction led to gap movements comparable with level walking or standing. Shear generally exceeded axial compression. Although ground reactions and fixation stiffness were comparable with those reported for reduced fractures, movement magnitudes generally were larger than 2 mm. The shear movement component reflected the nature of the mechanical conditions at the bone gap. In a direct comparison with data from animal experiments, the local mechanical environment at the bone gap seemed unstable rather than overly stable. The method introduced in this study opens the perspective of adjusting osteosynthesis stability to the specific needs of each patient.

Mechanical boundary conditions of fracture healing: borderline indications in the treatment of unreamed tibial nailing

Unreamed nailing favors biology at the expense of the achievable mechanical stability. It is therefore of interest to define the limits of the clinical indications for this method. The extended usage of unreamed tibial nailing resulted in reports of an increased rate of complications, especially for the distal portion of the tibia. The goals of this work were to gain a thorough understanding of the load-sharing mechanism between unreamed nail and bone in a fractured tibia, to identify the mechanical reasons for the unfavorable clinical results, and to identify borderline indications due to biomechanical factors. In a three-dimensional finite element model of a human tibia, horizontal defects were stabilized by means of unreamed nailing for five different fracture locations, including proximal and distal borderline indications for this treatment method. The loading of the bone, the loading of the implant and the inter-fragmentary strains were computed. The findings of this study show that with all muscle and joint contact forces included, nailing leads to considerable unloading of the interlocked bone segments. Unreamed nailing of the distal defect results in an extremely low axial and high shear strain between the fragments. The results suggest that mechanical conditions are advantageous to unreamed nailing of proximal and mid-diaphyseal defects. Apart from biological reasons, clinical problems reported for distal fractures may be due to the less favorable mechanical conditions in unreamed nailing. From a biomechanical perspective, the treatment of distal tibial shaft fractures by means of unreamed nailing without additional fragment contact or without stabilizing the fibula should be carefully reconsidered.

Deformation across the zone of callotasis during loading. radiostereometric analysis in a patient with achondroplasia

The present study demonstrates that high-resolution radiostereometric analysis (RSA) can be used to assess global longitudinal compressive deformation across the callotasis zone during loading. In an achondroplastic patient operated with bifocal lengthening of the tibia by use of the Ilizarov external fixator, the axial compressive intersegmental strain in the proximal lengthening zone under a load of 71% of body weight was 7.7 mm. The proximal lengthening zone was 51.0 mm, and accordingly the overall linear strain across the callotasis was 15.1%. This large strain value found in distraction osteogenesis 6 weeks after end of distraction is not consistent with classical theory of the magnitude of micromotion needed for adequate stimulation of bone formation in fracture healing. The increased axial displacement did not stimulate bone healing and delayed union was observed. This one single observation does not allow for any conclusions to be drawn about the relationship of strain to fracture healing, but further and refined use of the RSA method will certainly improve our understanding of the role of axial strains in distraction osteogenesis.

External ring fixators: an overview

External fixation is widely used in the fixation of fractures and limb deformities. The mechanical characteristics of a specific external fixator are major factors in determining the biomechanical environment at a fracture/osteotomy site and, hence, affect the healing process. Although the optimal biomechanical environment for healing of a fracture or an osteotomy is unknown, a specific range of interfragmentary motion exists which promotes healing. It is therefore desirable that the mechanics of an external fixator can be manipulated to enable the surgeon to control the range of interfragmentary motion. The characteristics of an external fixator are defined by a large number of variables. Therefore, to gain control over the degree of interfragmentary motion, an understanding of the effect of each variable and how it interacts with the others to determine the overall characteristics of the device is required. For the past two decades, individual components and whole-frame configurations have been studied in depth. This article provides a summary of previous work concerning the mechanics of external ring fixators and how they affect the biomechanical environment at the fracture/osteotomy site.

Rapid application fracture fixators - an evaluation of mechanical performance

OBJECTIVE

This study evaluates the mechanical performance of the Pinless and Centrafix fixators for rapid application to tibial fractures in a disaster or battlefield scenario.

DESIGN

Comparative study based on measurements made in the laboratory.

BACKGROUND

The Pinless and Centrafix fixators may be considered for rapid application to stabilise fractures in emergency conditions without the aid of electrical equipment such as power drills for bone screw insertion or image intensifiers to facilitate bone alignment.

METHODS

Stiffnesses, maximum service loads and fatigue strengths of the fixators were measured in the orientations of loading that correspond to walking and stretcher-bearing. These properties were compared with measurements on three conventional fixators, the AO, Shearer and Triax.

RESULTS

The Centrafix stiffnesses were 31 N/mm (axial), 1 N/ degrees (torsional shear), 0.4 N/ degrees (coronal plane bending), 4 N/ degrees (sagittal plane bending) and 11 N/mm (transverse shear) and strengths were 95 N (axial) and 1.9 Nm (bending). Corresponding Pinless stiffnesses were 43 N/mm, 0.7 N/ degrees, 0.3 N/ degrees, 8 N/ degrees and 50 N/mm, and strength was 55 N (axial).

CONCLUSIONS

The stiffness and strength of both rapid application fixators in simulated walking was judged to be low, and additionally the stiffness and strength of the Centrafix in simulated stretcher-bearing was judged to be low.

RELEVANCE

The Pinless is not recommended for weight-bearing subjects with unstable fractures. The Centrafix is not recommended for stretcher-bearing or weight-bearing with unstable fractures.

Stiffness, strength and healing assessment in different bone fractures--a simple mathematical model

External fixators can only be removed safely when fractures have healed sufficiently to restore mechanical integrity to the bone. A bending stiffness of 15 N m/ degrees has been suggested as a means of estimating mechanical integrity. To examine whether this end point stiffness value can be applied to all fractures, the present study examined the degree of variability in predicted stiffness and strength that arises from variations in bone dimensions. Results imply that there is no common value for the end-point of bending stiffness in different bones. At an end point value of 15 N m/degrees, the maturity of the fracture repair tissue (represented by its elastic modulus) can vary 500-fold between an adult femur with a 0.5-mm gap to a child's mid diaphyseal tibia with a 1.0-mm gap. Fortunately, the strength does not vary by as large an extent as the modulus. However, even though two fractures each have reached a stiffness of 15 N m/degrees, a fracture in a bone of 50 mm diameter may exhibit only 60% of the strength of repair in a bone of 30 mm diameter. Therefore, caution should be exercised when using the bending stiffness as an end point indicator for different bones.

A mathematical stiffness matrix for characterising mechanical performance of the Orthofix DAF

This study investigates a method for characterising the 3D support to a bone fracture site that is provided by a fracture stabilising device. The method is demonstrated with the Orthofix DAF unilateral external fixator, for which a mathematical stiffness matrix is defined using experimental measurements in six degrees of freedom. Single forces or bending moments are applied to a model fracture stabilised by the fixator, and 3D inter fragmentary displacements are measured by an instrumented spatial linkage. The 6x6 stiffness matrix for the fracture site is calculated from the product of the vector of forces and moments and the inverse of the vector of displacements. A transformation matrix is used to determine the stiffness matrix for a range of anatomical angles (between the plane of the fixator frame and the sagittal plane). Comparison between measured displacements (for an angle of 30 degrees) and the corresponding calculated displacements showed agreement to within 8%. The method enables fracture site stiffness for a fixation device to be characterised comprehensively, and its properties to be identified in comparison with other devices. It also provides the means of analysing inter fragmentary motion that may arise from physiological loading, regardless of complexity.

Do axial dynamic fixators really produce axial dynamization?

The use of dynamic external fixators for the treatment of long bone fracture is widespread and well accepted. It is claimed that dynamization, i.e. small micromovements of compression/distraction at the fracture site, can be produced by allowing sliding of an inner rod within an outer housing. However, as the forces on the fixator are not direct but transferred from the bone via bone pins, there is a bending moment on the fixator. This produces "self-locking" and effectively prevents axial movements. We have studied the effect of this moment on the binding properties of the Orthofix system. The amount of movement at a simulated fracture site allowed before this locking occurs was measured and its implications discussed. It would appear that true axial dynamization does not take place using this system.

Mechanical influences on tibial fracture healing

Selected studies are summarized that measure interfragmentary fracture displacements in 6 degrees of freedom at intervals throughout healing in groups of patients with tibial diaphyseal fractures treated by external skeletal fixation. The results are compared with those obtained from experimental studies in which the ideal mechanical conditions for fracture healing were predicted. A finite element analysis model of the healing tibial fracture also was developed. Measured data were used for the analysis, and stress and strain patterns were defined for different stages of healing. Interfragmentary movement measured in the first 6 weeks after injury usually is a magnitude smaller in patients treated by external fixation than in patients treated with cast immobilization. This movement can be much smaller than that predicted to be optimal by experimental studies. A greater amplitude can be achieved, even in stable fractures, by ensuring patient activity. The interfragmentary movement is elastic during loading activity and is generally sinusoidal during steady walking. At the time of dynamization (the unlocking of the frame), a permanent set occurs at the fracture site in all planes. The cyclical movement range in each plane often decreases immediately after unlocking. The model analysis study of fracture healing predicts that tissue damage may occur in the later (hard callus) phase of healing, even while the fixation device is in place, because of abnormally high stresses and strains. This study indicates that fracture mechanics should be controlled more rigorously to provide amplitudes of movement in the first 4 to 6 weeks after fracture. The rigidity of fixation should be increased in the subsequent weeks until the fracture has healed and the frame is removed.