Modeling Complex Orthopedic Trauma in Rodents: Bone, Muscle and Nerve Injury and Healing

Orthopedic injury can occur from a variety of causes including motor vehicle collision, battlefield injuries or even falls from standing. Persistent limb pain is common after orthopedic injury or surgery and presents a unique challenge, as the initiating event may result in polytrauma to bone, muscle, and peripheral nerves. It is imperative that we understand the tissue-specific and multicellular response to this unique type of injury in order to best develop targeted treatments that improve healing and regeneration. In this Mini Review we will first discuss current rodent models of orthopedic trauma/complex orthotrauma. In the second section, we will focus on bone-specific outcomes including imaging modalities, biomechanical testing and immunostaining for markers of bone healing/turnover. In the third section, we will discuss muscle-related pathology including outcome measures of fibrosis, muscle regeneration and tensile strength measurements. In the fourth section, we will discuss nervous system-related pathology including outcome measures of pain-like responses, both reflexive and non-reflexive. In all sections we will consider parallels between preclinical outcome measures and the functional and mechanistic findings of the human condition.

A new sequential animal model for infection-related non-unions with segmental bone defect

BACKGROUND

The treatment of fracture-related infections (FRI) is still a challenge for orthopedic surgeons. The prevalence of FRI is particularly high in open fractures with extensive soft-tissue damage. This study aimed to develop a new two-step animal model for non-unions with segmental bone defects, which could be used to evaluate new innovative bone substitutes to improve the therapeutic options in humans with FRI and bone defects.

METHODS

After randomization to infected or non-infected groups, 30 Sprague-Dawley rats underwent a transverse osteotomy of the mid-shaft femur with a 5 mm defect. Additionally, the periosteum at the fracture zone was cauterized at both sides. After intramedullary inoculation with 10³ CFU Staphylococcus aureus (infected group) or PBS (non-infected group), a fracture stabilization was done by intramedullary K-wires. After 5 weeks, the bone healing process was evaluated, and revision surgery was performed in order to obtain increased bone healing. The initial K-wires were removed, and debridement of the osteotomy-gap was done followed by a more stable re-osteosynthesis with an angle-stable plate. After further 8 weeks all rats were euthanized and the bone consolidation was tested biomechanically and the callus formation quantitatively by micro-CT analysis.

RESULTS

We developed and presented a new two-stage non-union animal model through a targeted S. aureus infection. After 5 weeks, all animals showed a non-union irrespective of assignment to the infected and non-infected group. Lane and Sandhu score showed a higher callus formation in the infected group. In all infected animals, the inoculated S. aureus strain was detected in the revision surgery. The second surgery did not improve bone healing, as shown by the Lane Sandhu score and in the μ-CT analysis. Similarly, biomechanical testing showed in both groups a significantly lower maximum torque as compared to the contralateral side (p < 0.0001).

CONCLUSIONS

We were able to successfully develop a new two-stage non-union animal model, which reflects a genuine clinical situation of an infection-related non-union model with segmental bone defects. This model could be used to evaluate various therapeutic anti-infectious and osteoinductive strategies in FRIs.

A Reliable and Reproducible Critical-Sized Segmental Femoral Defect Model in Rats Stabilized with a Custom External Fixator

Orthopedic research relies heavily on animal models to study mechanisms of bone healing in vivo as well as investigate the new treatment techniques. Critical-sized segmental defects are challenging to treat clinically, and research efforts could benefit from a reliable, ambulatory small animal model of a segmental femoral defect. In this study, we present an optimized surgical protocol for the consistent and reproducible creation of a 5 mm critical diaphyseal defect in a rat femur stabilized with an external fixator. The diaphyseal ostectomy was performed using a custom jig to place 4 Kirschner wires bicortically, which were stabilized with an adapted external fixator device. An oscillating bone saw was used to create the defect. Either a collagen sponge alone or a collagen sponge soaked in rhBMP-2 was implanted into the defect, and the bone healing was monitored over 12 weeks using radiographs. After 12 weeks, rats were sacrificed, and histological analysis was performed on the excised control and treated femurs. Bone defects containing only collagen sponge resulted in non-union, while rhBMP-2 treatment yielded the formation of a periosteal callous and new bone remodeling. Animals recovered well after implantation, and external fixation proved successful in stabilizing the femoral defects over 12 weeks. This streamlined surgical model could be readily applied to study bone healing and test new orthopedic biomaterials and regenerative therapies in vivo.

Biomechanics of two external fixator devices used in rat femoral fractures

The use of external fixators allows for the direct investigation of newly formed interfragmentary bone, and the radiographic evaluation of the fracture. We validated the results of a finite element (FE) model with the in vitro stiffness' of two widely used external fixator devices used for in vivo analysis of fracture healing in rat femoral fractures with differing construction (Ti alloy ExFix1 and PEEK ExFix2). Rat femoral fracture fixation was modeled using two external fixators. For both constructs an osteotomy of 2.75 mm was used, and offset maintained at 5 mm. Tufnol, served as standardized substitutes for rat femora. Constructs were loaded under axial compression and torsion. Overall axial and torsional stiffness were compared between the in vitro models and FE results. FE models were also used to compare the fracture movement and overall pattern of von Mises stress across the external fixators. In vitro axial stiffness of ExFix1 was 29.26 N/mm ± 3.83 compared to ExFix2 6.31 N/mm ± 0.67 (p* < 0.05). Torsional stiffness of ExFix1 was 47.5 Nmm/° ± 2.71 compared to ExFix2 at 19.1 Nmm/° ±1.18 (p* < 0.05). FE results predicted similar comparative ratios between the ExFix1 and 2 as the in vitro studies. FE results predicted considerably larger interfragmentary motion in the ExFix2 comparing to ExFix1. We demonstrated significant differences in the stiffness' of the two external fixators as one would expect from such variable designs; yet, importantly we validated the utility of an FE model for the analysis and prediction of changes in fracture mechanics dependent on fixator choice. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:293-298, 2019.

Improving results in rat fracture models: enhancing the efficacy of biomechanical testing by a modification of the experimental setup

Background

Animal fracture models, primarily performed in rats, are crucial to investigate normal and pathological bone healing. However, results of biomechanical testing representing a major outcome measure show high standard deviations often precluding statistical significance. Therefore, the aim of our study was a systematical examination of biomechanical characteristics of rat femurs during three-point bending. Furthermore, we tried to reduce variation of results by individually adapting the span of bearing and loading areas to the bone’s length.

Methods

We examined 40 paired femurs of male Wistar-rats by DXA (BMD and BMC of the whole femur) and pQCT-scans at the levels of bearing and loading areas of the subsequent biomechanical three-point bending test. Individual adjustment of bearing and loading bars was done respecting the length of each specimen. Subgroups of light (< 400 g, n = 22) and heavy (> 400 g, n = 18) animals were formed and analysed separately. We furthermore compared the results of the individualised bending-setting to 20 femurs tested with a fix span of 15 mm.

Results

Femurs showed a length range of 34 to 46 mm. The failure loads ranged from 116 to 251 N (mean 175.4 ± 45.2 N; heavy animals mean 221 ± 18.9 N; light animals mean 138.1 ± 16.4 N) and stiffness ranged from 185 N/mm to 426 N/mm (mean 315.6 ± 63 N/mm; heavy animals mean 358.1 ± 34.64 N/mm; light animals mean 280.8 ± 59.85 N/mm). The correlation of densitometric techniques and failure loads was high (DXA R² = 0.89 and pQCT R² = 0.88). In comparison to femurs tested with a fix span, individual adaptation of biomechanical testing homogenized our data significantly. Most notably, the standard deviation of failure loads (221 ± 18.95 N individualized setting vs. 205.5 ± 30.36 N fixed) and stiffness (358.1 ± 34.64 N/mm individualized setting vs. 498.5 ± 104.8 N/mm fixed) was reduced by at least one third.

Conclusions

Total variation observed in any trait reflects biological and methodological variation. Precision of the method hence affects the statistical power of the study. By simply adapting the setting of the biomechanical testing, interindividual variation could be reduced, which improves the precision of the method significantly.

Electronic supplementary material

The online version of this article (10.1186/s12891-018-2155-y) contains supplementary material, which is available to authorized users.

Osteotomy models - the current status on pain scoring and management in small rodents

Fracture healing is a complex regeneration process which produces new bone tissue without scar formation. However, fracture healing disorders occur in approximately 10% of human patients and cause severe pain and reduced quality of life. Recently, the development of more standardized, sophisticated and commercially available osteosynthesis techniques reflecting clinical approaches has increased the use of small rodents such as rats and mice in bone healing research dramatically. Nevertheless, there is no standard for pain assessment, especially in these species, and consequently limited information regarding the welfare aspects of osteotomy models. Moreover, the selection of analgesics is restricted for osteotomy models since non-steroidal anti-inflammatory drugs (NSAIDs) are known to affect the initial, inflammatory phase of bone healing. Therefore, opioids such as buprenorphine and tramadol are often used. However, dosage data in the literature are varied. Within this review, we clarify the background of osteotomy models, explain the current status and challenges of animal welfare assessment, and provide an example score sheet including model specific parameters. Furthermore, we summarize current refinement options and present a brief outlook on further 3R research.

Does Anticoagulant Medication Alter Fracture-Healing? A Morphological and Biomechanical Evaluation of the Possible Effects of Rivaroxaban and Enoxaparin Using a Rat Closed Fracture Model

Low molecular weight heparin (LMWH) is routinely used to prevent thromboembolism in orthopaedic surgery, especially in the treatment of fractures or after joint-replacement. Impairment of fracture-healing due to increased bone-desorption, delayed remodelling and lower calcification caused by direct osteoclast stimulation is a well-known side effect of unfractioned heparin. However, the effect of LMWH is unclear and controversial. Recent studies strongly suggest impairment of bone-healing in-vitro and in animal models, characterized by a significant decrease in volume and quality of new-formed callus. Since October 2008, Rivaroxaban (Xarelto) is available for prophylactic use in elective knee- and hip-arthroplasty. Recently, some evidence has been found indicating an in vitro dose independent reduction of osteoblast function after Rivaroxaban treatment. In this study, the possible influence of Rivaroxaban and Enoxaparin on bone-healing in vivo was studied using a standardized, closed rodent fracture-model. 70 male Wistar-rats were randomized to Rivaroxaban, Enoxaparin or control groups. After pinning the right femur, a closed, transverse fracture was produced. 21 days later, the animals were sacrificed and both femora harvested. Analysis was done by biomechanical testing (three-point bending) and micro CT. Both investigated substances showed histomorphometric alterations of the newly formed callus assessed by micro CT analysis. In detail the bone (callus) volume was enhanced (sign. for Rivaroxaban) and the density reduced. The bone mineral content was enhanced accordingly (sign. for Rivaroxaban). Trabecular thickness was reduced (sign. for Rivaroxaban). Furthermore, both drugs showed significant enlarged bone (callus) surface and degree of anisotropy. In contrast, the biomechanical properties of the treated bones were equal to controls. To summarize, the morphological alterations of the fracture-callus did not result in functionally relevant deficits.

Prediction of the time course of callus stiffness as a function of mechanical parameters in experimental rat fracture healing studies--a numerical study

Numerous experimental fracture healing studies are performed on rats, in which different experimental, mechanical parameters are applied, thereby prohibiting direct comparison between each other. Numerical fracture healing simulation models are able to predict courses of fracture healing and offer support for pre-planning animal experiments and for post-hoc comparison between outcomes of different in vivo studies. The aims of this study are to adapt a pre-existing fracture healing simulation algorithm for sheep and humans to the rat, to corroborate it using the data of numerous different rat experiments, and to provide healing predictions for future rat experiments. First, material properties of different tissue types involved were adjusted by comparing experimentally measured callus stiffness to respective simulated values obtained in three finite element (FE) models. This yielded values for Young's moduli of cortical bone, woven bone, cartilage, and connective tissue of 15,750 MPa, 1,000 MPa, 5 MPa, and 1 MPa, respectively. Next, thresholds in the underlying mechanoregulatory tissue differentiation rules were calibrated by modifying model parameters so that predicted fracture callus stiffness matched experimental data from a study that used rigid and flexible fixators. This resulted in strain thresholds at higher magnitudes than in models for sheep and humans. The resulting numerical model was then used to simulate numerous fracture healing scenarios from literature, showing a considerable mismatch in only 6 of 21 cases. Based on this corroborated model, a fit curve function was derived which predicts the increase of callus stiffness dependent on bodyweight, fixation stiffness, and fracture gap size. By mathematically predicting the time course of the healing process prior to the animal studies, the data presented in this work provides support for planning new fracture healing experiments in rats. Furthermore, it allows one to transfer and compare new in vivo findings to previously performed studies with differing mechanical parameters.

Adjustable stiffness, external fixator for the rat femur osteotomy and segmental bone defect models

The mechanical environment around the healing of broken bone is very important as it determines the way the fracture will heal. Over the past decade there has been great clinical interest in improving bone healing by altering the mechanical environment through the fixation stability around the lesion. One constraint of preclinical animal research in this area is the lack of experimental control over the local mechanical environment within a large segmental defect as well as osteotomies as they heal. In this paper we report on the design and use of an external fixator to study the healing of large segmental bone defects or osteotomies. This device not only allows for controlled axial stiffness on the bone lesion as it heals, but it also enables the change of stiffness during the healing process in vivo. The conducted experiments have shown that the fixators were able to maintain a 5 mm femoral defect gap in rats in vivo during unrestricted cage activity for at least 8 weeks. Likewise, we observed no distortion or infections, including pin infections during the entire healing period. These results demonstrate that our newly developed external fixator was able to achieve reproducible and standardized stabilization, and the alteration of the mechanical environment of in vivo rat large bone defects and various size osteotomies. This confirms that the external fixation device is well suited for preclinical research investigations using a rat model in the field of bone regeneration and repair.

Interaction of age and mechanical stability on bone defect healing: an early transcriptional analysis of fracture hematoma in rat

Among other stressors, age and mechanical constraints significantly influence regeneration cascades in bone healing. Here, our aim was to identify genes and, through their functional annotation, related biological processes that are influenced by an interaction between the effects of mechanical fixation stability and age. Therefore, at day three post-osteotomy, chip-based whole-genome gene expression analyses of fracture hematoma tissue were performed for four groups of Sprague-Dawley rats with a 1.5-mm osteotomy gap in the femora with varying age (12 vs. 52 weeks - biologically challenging) and external fixator stiffness (mechanically challenging). From 31099 analysed genes, 1103 genes were differentially expressed between the six possible combinations of the four groups and from those 144 genes were identified as statistically significantly influenced by the interaction between age and fixation stability. Functional annotation of these differentially expressed genes revealed an association with extracellular space, cell migration or vasculature development. The chip-based whole-genome gene expression data was validated by q-RT-PCR at days three and seven post-osteotomy for MMP-9 and MMP-13, members of the mechanosensitive matrix metalloproteinase family and key players in cell migration and angiogenesis. Furthermore, we observed an interaction of age and mechanical stimuli in vitro on cell migration of mesenchymal stromal cells. These cells are a subpopulation of the fracture hematoma and are known to be key players in bone regeneration. In summary, these data correspond to and might explain our previously described biomechanical healing outcome after six weeks in response to fixation stiffness variation. In conclusion, our data highlight the importance of analysing the influence of risk factors of fracture healing (e.g. advanced age, suboptimal fixator stability) in combination rather than alone.

Influence of gender and fixation stability on bone defect healing in middle-aged rats: a pilot study

BACKGROUND

Gender and stability of fixation independently influence bone regeneration but their combined effects are unclear.

QUESTIONS/PURPOSES

In a pilot study we determined the combined influence of gender and fixation stability on the callus of middle-aged rats regarding (1) biomechanical properties; (2) bridging over time; (3) callus formation; and (4) callus size, geometry, mineralization, and microstructure.

METHODS

We osteotomized the left femur of 32 Sprague-Dawley rats (12 months old). Femurs were externally fixed with a gap of 1.5 mm in four groups of eight animals each: female semirigid, male semirigid, female rigid, and male rigid. Qualitative and quantitative in vivo radiographic analyses were performed twice weekly. Six weeks postoperatively, harvested femora were evaluated using micro-CT and biomechanical testing.

RESULTS

Torsional stiffness and maximum torque at failure were higher in male and in semirigidly fixed fractures. Radiographic analysis revealed earlier bridging and callus formation in both male groups. Micro-CT analysis showed a larger callus size, altered geometry, and microstructure in males and semirigidly fixed animals, whereas mineralization was similar in all animals.

CONCLUSION

Our data suggest female gender represents an independent risk factor for bone healing in middle-aged rats. Although healing in females was delayed compared with males, they exhibited a similar response (superior callus properties) to a more semirigid fixation.

CLINICAL RELEVANCE

While female gender appears to reflect a risk for impaired bone healing in middle-aged female rats, clinical studies would be required to confirm the finding in humans.

A 5-mm femoral defect in female but not in male rats leads to a reproducible atrophic non-union

INTRODUCTION

The objectives of this study were to (1) establish a reproducible atrophic non-union model in rats by creation of a segmental femoral bone defect that allows, (2) in-depth characterization of impaired healing, and (3) contrast its healing patterns to the normal course. Hypothesis was that a 5-mm bone defect in male rats would deviate from uneventful healing patterns and result in an atrophic non-union.

MATERIALS AND METHODS

A femoral osteotomy was performed in two groups of 12-week-old male rats (1 vs. 5 mm gap) stabilized with an external fixator. Bone healing in these models was evaluated by radiology, biomechanics, and histology at 6 or 8 weeks. The evaluation of the 5-mm group revealed in some cases a delayed rather than a non-union, and therefore, a group of female counterparts was included.

RESULTS

The creation of a 5-mm defect in female rats resulted in a reproducible atrophic non-union characterized by sealing of the medullary canal, lack of cartilage formation, and negligible mechanical properties of the callus. In both gap size models, the male subjects showed advanced healing compared to females.

DISCUSSION AND CONCLUSION

This study showed that even under uneventful healing conditions in terms of age and bone defect size, there is a sex-specific advanced healing in male compared to female subjects. Contrary to our initial hypothesis, only the creation of a 5-mm segmental femoral defect in female rats led to a reproducible atrophic non-union. It has been shown that an atrophic non-union exhibits different healing patterns compared to uneventful healing. A total lack of endochondral bone formation, soft tissue prolapse into the defect, and bony closure of the medullary cavity have been shown to occur in the non-union model.

Mechanical characterization of external fixator stiffness for a rat femoral fracture model

Clinical and experimental studies have shown that several mechanical factors influence the fracture healing process. One such factor, interfragmentary movement, is affected by loading and the stiffness of the fixation device. This study evaluated the stiffness of different external fixation devices for a rat femoral fracture model, using in vitro and analytical methods. The contribution to the stiffness of the fixation construct was dominated by the flexibility of the pins in relation to their offset, diameter, and material properties. The axial stiffness increased with decreasing offset and increasing pin diameter. Titanium pins resulted in significantly lower axial stiffness compared to stainless steel pins of the same design. The fixator body material and fixator length had a less pronounced influence on fixation stiffness. Mechanically characterized external fixation devices will allow in vivo study of the fracture healing process utilizing pre-calculated fracture fixation stiffness. These characterized fixation devices will allow controlled manipulation of the axial and shear interfragmentary movement to achieve a flexible fixation resulting in callus formation compared to a more rigid fixation limiting callus formation in a rat femoral fracture model.

A new device to control mechanical environment in bone defect healing in rats

Mechanical conditions have a significant influence on the biological processes of bone healing. Small animal models that allow controlling the mechanical environment of fracture and bone defect healing are needed. The aim of this study was to develop a new animal model that allows to reliably control the mechanical environment in fracture and bone defect healing in rats using different implant materials. An external fixator was designed and mounted in vitro to rat femurs using four Kirschner-wires (titanium (T) or steel (S)) of 1.2 mm diameter. The specimens were distracted to a gap of 1.5mm. Axial and torsional stiffness of the device was tested increasing the offset (distance between bone and fixator crossbar) from 5 to 15 mm. In vivo performance (well-being, infection, breaking of wires and bone healing) was evaluated in four groups of 24 Sprague-Dawley rats varying in offset (7.5 and 15 mm) and implant material (S/T) over 6 weeks. Torsional and axial stiffness were higher in steel compared to titanium setups. A decrease in all configurations was observed by increasing the offset. The offset 7.5 mm showed a significantly higher torsional (S: p<0.01, T: p<0.001) and axial in vitro stiffness (S: p<0.001, T: p<0.001) compared to 15 mm offset of the fixator. Although in vitro designed to be different in mechanical stiffness, no difference was found between the groups regarding complication rate. The overall-complication rate was 5.2%. In conclusion, we were able to establish a small animal model for bone defect healing which allows modeling the mechanical conditions at the defect site in a defined manner.

Influence of age and mechanical stability on bone defect healing: age reverses mechanical effects

Non-unions and delayed healing are still prevalent complications in fracture and bone defect healing. Both mechanical stability and age are known to influence this process. However, it remains unclear which factor dominates and how they interact. Within this study, we sought a link between both factors. In 36 female Sprague-Dawley rats, the left femur was osteotomized, distracted to an osteotomy gap of 1.5 mm and externally fixated. Variation of age (12 vs. 52 weeks - biologically challenging) and fixator stiffness (mechanically challenging) resulted in 4 groups (each 9 animals): YS: young semi-rigid, OS: old semi-rigid, YR: young rigid and OR: old rigid. Qualitative and quantitative radiographical analyses were performed at weeks 2, 4 and 6 after surgery. Six weeks post-op, rats were sacrificed and femora were harvested for biomechanical testing (torsional stiffness (TS) and maximum torque at failure (MTF)). Six weeks after surgery, TS showed a significant interaction between age and fixation stiffness (p<0.0001). TS in YR was significantly higher than that in the other groups (YS: p<0.001; OR: p<0.001; OS: p<0.001). Additionally, YS showed a significantly higher TS compared to the OS (p=0.006) and OR (p=0.046). Testing of MTF showed a significant interaction of both variables (p=0.0002) and led to significant differences between OR and YS (p<0.001), OS (p=0.046) and YR (p<0.001). The YR showed a higher MTF compared to YS (p=0.012) and OS (p=0.001), whereas OR's MTF was inferior compared to OS. At 2-week follow-up, YR (p=0.006), and at 6-week follow-up, YS and YR (p=0.032) showed significantly higher radiographic scores. At 2-week follow-up, YS's callus was larger than that of the old groups (OS: p=0.025; OR: p=0.003). In YR a significantly smaller callus was observed compared to YS at time points 4 and 6 weeks (p=0.002 for both) and compared to OS at 6-week follow-up (p=0.03). The effect of age seems to invert the effect of mechanical properties of the callus, which was not correlated to callus size. Optimization of mechanics alone seems to be not sufficient. The underlying mechanisms and causes of the age-related influences and their clinical counterparts need to be further investigated.