Despite its clinical significance, metaphyseal fracture healing has received little attention in research and experimental models have been limited. In particular it is not known to what extent the mechanical environment plays a role in metaphyseal fracture healing. Recently, a new murine internal fixation plate has been developed to stabilise fractures in the distal femur under highly standardised conditions. Goal of the current study was to modify this design, in order to be able to evaluate the influence of the fixator bending stiffness on metaphyseal fracture healing in mice.

Adapting the existing single body design, resulting in low flexibility fixation, two new plates were developed with a decreased bending stiffness of approximately 65% and 45% of the original implant (100%). Pilot experiments were performed on 54 animals, whereas the mice were sacrificed and fracture healing assessed radiologically and biomechanically after 14 and 28 days.

MicroCT evaluation confirmed that the osteotomy was created in the trabecular, metaphyseal bone of the distal mouse femora. All bones showed progressive fracture healing over time, with decreased implant stiffness leading to increased periosteal callus formation.

These implants represent an important new research tool to study molecular and genetic aspects of metaphyseal fracture healing in mice under standardized mechanical conditions, in order to improve clinical treatment in challenging situations, such as in osteoporotic bone.

To elucidate the molecular biology of fracture healing, murine models are preferred. We performed a study with the first internal fixation system that allows studying murine fracture healing in a controlled mechanical environment, to characterise the timing of the fracture healing cascade with this model, based on a histological evaluation.

Femoral osteotomies were performed in 68 male C57BL/six mice and stabilised with locking internal fixation plates in either stiff, or defined, flexible configurations. Healing progression was studied at 10 time points between 3 and 42 days post- surgery. After surgery, mice were radiographed to confirm the correct implant positioning. After sacrifice, the extracted femora were processed for decalcified histology. Thin sections were taken as serial transverse sections and stained for subsequent histomorphometric analysis and three-dimensional reconstruction of the different fracture callus tissues.

The surgery was successful in 62 animals. Only six6 (8.8%) animals had to be sacrificed due to complications during surgery. The post-operative radiographs demonstrated a high reproducibility of implant positioning and no implant failure or screw loosening occurred during the experimental period. The improved consistency in surgical technique leading to more uniform results represents a key advantage of this system over other mouse fracture healing models. As such, it may allow a reduction in the sample size needed in future murine fracture healing studies. The histological evaluation confirmed the lack of a periosteal callus, and exclusively endosteal, intramembraneous bone formation in the bones stabilised with the stiff implants. The bones that were stabilised with the more flexible internal fixation plates showed additional endochondral ossification with extensive, highly asymmetrical, periosteal callus formation.

Our results demonstrate that this murine fracture model leads to different healing patterns depending on the flexibility of the chosen plate system. This allows researchers to investigate the molecular biology of fracture healing in different ossification modes by selection of the appropriate fixation.

Successful treatment of bone fractures requires a balance between stability, to restore functional anatomy and allow early mobilisation (and thus avoid dystrophy). The healing occurs through complex interactions of inducing, enabling and inhibitory factors. The mechanical environment (e.g. stress and strain) in/around the fracture site regulates tissue changes throughout the healing process, including the formation of a fibro-cartilaginous callus and its progressive replacement by bone. The mechanical and biological environment is controlled substantially by the selection of the fracture stabilisation method achieving either absolute stability (mostly achieved with compression plating technique) or relative stability allowing a limited amount of dynamic fracture displacement across the fracture gap. A number of treatments may be used to accomplish these conditions, ranging from splinting with a plaster cast, external fixator or an intramedullary nail to rigid internal fixation using plates affixed to the bone fragments. Fixation methods are presently selected on the basis of general guidelines, but nevertheless the optimal stability/instability remains unclear and relies heavily on the surgeon’s experience. With the recently more and more widely used locking plates the question of the optimal fixation technique and applied stability to the fracture zone especially in simple fractures have raised again.

To fill this knowledge gap, an interdisciplinary approach with in vitro and in vivo experiments seems to be essential. Analysing clinical situations and the healing course with mathematical modelling and computational simulations can further aid to understand the healing conditions in respect to stability.

This presentation will give an overview on the role of the mechanical environment in fracture healing, and demonstrating clinical examples that highlight the relevance of this research.

Introduction: Over the last 10 years minimally invasive plate osteosynthesis (MIPO) has gained more and more popularity over the conventional open surgical approach (ORIF). Numerous clinical case collection studies reported the MIPO technique as a good, alternative method. While MIPO offers some advantages over ORIF, it also has significant drawbacks, such as more demanding surgical technique and increased radiation exposure. In an attempt to compare these approaches, a previous animal study 1 did not show any significant differences in fracture healing outcome. Using a new developed, standardised severe trauma model on the sheep femur 2, this study examines the effects of the surgical approaches on fracture healing.

Methods: 24 sheep (Merino wethers, mean age 5.6years, mean weight 39.1kg) underwent the trauma model 2 with a severe soft tissue damage and a multifragmentary, distal femur fracture as well as initial stabilisation with an external fixator. After five days of soft tissue recovery, the animals were definitively operated with an internal fixator (LCP) randomised either by a minimally invasive or open approach. The sheep were sacrificed after 4 and 8 weeks (two groups), mechanical testing performed and statistically analysed with ANOVA test.

Results: After 4 weeks, torsional rigidity is significantly higher in the MIPO group (30.1r10.6(SD)%) of fractured to intact bones, p< 0.05) compared to ORIF group (9.8r12.4(SD)%), while ultimate torque also shows increased values for MIPO technique (p=0.11). After 8 weeks, the differences in mechanical properties levelled out, but still higher values for the MIPO group (p=0.36/p=0.26).

Conclusion: In the early stage of fracture healing, minimally invasive plate osteosynthesis shows advanced healing pattern compared to open fixation technique. This advantage seems to level out over time.

Volume and density of fracture callus are important outcome parameters in fracture healing studies. These values provide an indication for the recovery of the mechanical function of the bone. Traditionally, fracture callus’ have been evaluated from radiographs, which represent 2D projections of the three-dimensional structures, therefore such an analysis can be affected by many artefacts. With the availability of Computer Tomography (CT) scanners for the evaluation of healing bones, it is now possible to perform precise, three-dimensional reconstructions of the fracture callus and therefore to evaluate true three-dimensional callus volumes and bone mineral densities. We wanted to make use of this technology in the evaluation of a study looking at the healing of a multifragmentary fracture in sheep after 4 and 8 weeks of healing time (Wullschleger et al, ANZORS, 2006). Our goal was to develop a protocol that would allow for the standardised calculation of cortical bone and callus tissue volumes with minimal user influence. Here, we report on the development of this evaluation protocol and some early results.

A clinical CT scanner was used to scan the experimental limbs, immediately after the sheep had been euthanized. Further analysis of the CT dataset was accomplished with the commercial computer software Amira. The region of interest was cropped to a 9 cm section of the bone shaft, guaranteed to comprise the entire fracture callus. Next, the cortical bone and the callus tissue were segmented by choosing appropriate threshold values for the measured grey levels. The volume of the segmented regions was then calculated by the software.

The application of this protocol to six CT scans from our experimental study resulted in average callus volumes of 12.21 ± 1.96 (standard deviation) cm2 after 4 weeks healing time and 14.28 ± 1.58 cm2 after 8 weeks healing time.

In conclusion, we demonstrated the feasibility of using CT data for a quantitative 3D analysis of callus volumes. While this technique is undoubtedly superior to the estimation of callus volumes from two-dimensional radiographs, the absolute accuracy of the results will need to be determined by comparison with histological data.

Bilateral mandibular lengthening is widely accepted during mandibular distraction osteogenesis. However, distraction osteogenesis are sometimes associated with clinical complications such as open bite deformity, lateral displacement of temporo-mandibular joint, premature consolidation and pin loosening. Although distraction osteogenesis aims to develop pure tensile strain on the regenerate tissue however, in real life situation due to differences in device orientation, materials and misalignment it is often subjected to complex stress and strain regimes.

The objective of this study was to characterise the mechanical environment (stress and strain) in the Finite Element Models (FEM) of regenerate tissue during two different device orientations:

(a) device placed parallel to the mandibular body

Furthermore, the influence of misalignment from above two idealised orientations was also investigated.

The distraction protocol in this study was similar to previous study of Loboa et al (2005). FE models were developed at four stages: end of latency, distraction day two, distraction day five and distraction day eight. At each time period a distraction of 1mm was applied to the model as it is most widely used distraction rate. In these models two primary distraction vectors were simulated; first when the device is parallel to the body of the mandible and second when the device is parallel to the axis of distraction.

Results indicate that when the device is placed parallel to the mandible the effect of distraction vector variation due to misalignment in transverse plane (±150 at an interval of 50 ; + indicate lateral and indicates medial) is symmetric and variation in the stress and strain regimes on regenerative tissue are less than 3%. However, when the device is placed parallel to axis of distraction the corresponding change is asymmetric and almost double in magnitude. The greatest differences were seen when misalignment is towards lateral side (+150). Similarly in the sagittal plane variations up to 17% were developed due to 0- 400 change in the distraction vector orientation. Thus the orientation of device which determines the distraction vector plays an important role in determining the mechanical environment around regenerative tissue. The results suggest that implications of misalignment of the device and its sensitivity from the ideal situation should be well understood during clinical planning.

With the development and popularisation of minimally invasive surgical methods and implants for fracture fixation, it is increasingly important that available implants are pre-contoured to the specific anatomical location for which they are designed. Due to differences in the bone morphology it is impossible to design single implants that are universally applicable for the entire human population. A recent study on the fit of a distal periarticular medial tibia plate to Japanese bones supported the need for shape optimisation [1]. The present study aimed to quantify and optimise the fit of the same plate for an extended dataset of Japanese tibiae.

Forty-five 3D models of the outer bone contour of Japanese tibiae were used. The average age of the specimens was 67 years with an average height of 156 cm. All bone models were considered to be within a normal range without any bony pathology. An anatomical fit of the plate was defined with four criteria [1]. The current plate shape was optimised based on the quantitative results of the plate fitting. Two different optimised plate shapes were generated.

The current plate shape achieved an anatomical fit on 13% of tibias from the dataset. Plate 1 achieved an anatomical fit for 42% and Plate 2 a fit for 67% of the bone models. If either Plate 1 or Plate 2 is used, then the anatomical fit can be increased from 13% to 82% for the same dataset. For 12 (27%) of the 45 bones both modified plate shapes were fitting.

The results for the fit of the current plate shape are comparable to findings of a similar study on the anatomical fitting of proximal tibia plates [2]. The obtained results indicate that for the available dataset no further modification is warranted for the shaft region of the modified plates. Further optimization of the distal regions of Plate 1 and Plate 2 will be possible. This study shows that in order to achieve an anatomical fit of the plate to the medial Malleolus at least two different plate shapes will be required.

Volume and density of fracture callus are important outcome parameters in fracture healing studies. These values provide an indication for the recovery of the mechanical function of the bone. Traditionally, fracture callus’ have been evaluated from radiographs, which represent 2D projections of the three-dimensional structures; therefore such an analysis can be affected by many artefacts. With the availability of Computer Tomography (CT) scanners for the evaluation of healing bones, it is now possible to perform precise, three-dimensional reconstructions of the fracture callus and therefore to evaluate true three-dimensional callus volumes and bone mineral densities. We wanted to make use of this technology in the evaluation of a study looking at the healing of a multifragmentary fracture in sheep after 4 and 8 weeks of healing time (Wullschleger et al, ANZORS, 2006). Our goal was to develop a protocol that would allow for the standardised calculation of cortical bone and callus tissue volumes with minimal user influence. Here, we report on the development of this evaluation protocol and some early results.

A clinical CT scanner was used to scan the experimental limbs, immediately after the sheep had been euthanized. Further analysis of the CT dataset was accomplished with the commercial computer software Amira. The region of interest was cropped to a 9 cm section of the bone shaft, guaranteed to comprise the entire fracture callus. Next, the cortical bone and the callus tissue were segmented by choosing appropriate threshold values for the measured grey levels. The volume of the segmented regions was then calculated by the software.

The application of this protocol to six CT scans from our experimental study resulted in average callus volumes of 12.21 ± 1.96 (standard deviation) cm2 after 4 weeks healing time and 14.28 ± 1.58 cm2 after 8 weeks healing time.

In conclusion, we demonstrated the feasibility of using CT data for a quantitative 3D analysis of callus volumes. While this technique is undoubtedly superior to the estimation of callus volumes from two-dimensional radiographs, the absolute accuracy of the results will need to be determined by comparison with histological data.

Bone fluid flow transports nutrients to, and carries waste from, the bone cells embedded in the bony matrix. In long bones, it is driven by the blood pressure differentials between the medullary cavity and the periosteal surface and it is enhanced by mechanical loading. Loading of bone tissue deforms the bone matrix and changes the volume of the medullary cavity. Both mechanisms alter the interstitial fluid flow in the bone cortex. The former changes the volume of the fluid cavities in the cortex, while the latter modifies the intramedullary pressure (IMP). This study aims to investigate and compare, for the first time, the effects of these two mechanisms combined on the interstitial fluid flow in the bone cortex.

A hydraulic-fluid method is proposed to investigate the enhancement of IMP induced by the external loading. An intact sheep tibia is represented by a hollow cylinder, with the bone marrow being completely constrained in the cavity and assumed to behave as an icompressible liquid. The cortex is supposed to be a purely elastic material, and its permeability is ignored at this stage. The numerical results show that an axial compressive load of 500 N increases the IMP from 4000 Pa to 48900 Pa.

The influence of the enhanced IMP on the interstitial fluid flow is examined in a subsequent poroelastic analysis. At this stage, the cortex is assumed to be a biphasic material that permits fluid perfusion. The poroelastic analyses were conducted for both initial and enhanced IMPs. The results of the simulations demonstrate that the external load induces very high interstitial pressure. The highest pressure could be 25 times higher than the initial marrow pressure, but its magnitude decreases quickly. Furthermore, the influence of the IMP on the interstitial pressure is limited to the inner half of the cortical wall adjacent to the endosteal surface. However, the influence becomes more significant with decreasing load-induced interstitial pressure.

In conclusion, these simulations suggest that the increase in IMP during mechanical loading further enhances interstitial fluid movements in cortical bone, which highlights the importance of mechanical loading for the maintenance of healthy bones.

In recent years, plate osteosynthesis in metaphyseal and diaphyseal long bone fractures has been increasingly applied in a minimally invasive fashion. Several clinical studies describe a beneficial effect of the smaller additional soft tissue damage, resulting in satisfying fracture and soft tissue healing. However, is the surgical soft tissue damage really evidently smaller and the recovery faster?

A trauma model with severe, circumferential soft tissue damage to the distal right thigh and a distal multifragmentary (AO type C) femur fracture was carried out on 24 male sheep. After five days temporary external fixation, an internal fixator was placed either by a conventional open lateral approach or by minimally invasive technique. To assess the soft tissue damage and its recovery within the first 14 days, local compartment pressure monitoring as well as daily measurements of systemic markers (Creatin Kinase, CK and Lactate Dehydrogenase, LDH) in blood were performed. The local monitoring with a special probe (Neurovent PTO, Raumedic AG, Germany) within the quadriceps muscle allowed the measurement of compartment pressure (CP), as well as temperature.

The CK and LDH levels responded to the severe trauma with high peaks within the first 48 hours post trauma. After the internal fixator operations CK levels illustrate a significantly lower increase (p< 0.05) in the minimally invasive group compared to the open approach group in the first two days postoperatively. LDH levels show lower values for the minimally invasive group (p=0.06).

The values of CP present an initial increase after the trauma and then higher values (p=0.08) after the open plating operation. For the intracompartmental temperature no statistical differences were found, too (p=0.17).

These results, with reduced additional soft tissue damage and faster recovery in the minimally invasive approach group, reflect the clinical experience and expectations. However, while minimally invasive plate osteo-synthesis is certainly a desired option for fracture fixation, good surgical skills are required to insure that the reduced surgical trauma is in line with optimal fracture healing. The influence of the two different approaches on the bone healing per se, as well as the influence on soft tissue functionality, has yet to be demonstrated.