Intramembranous bone formation after callus distraction is augmented by increasing axial compressive strain.

The mechanical environment is a primary factor in the success of distraction osteogenesis. It is known that the interfragmentary movement during the distraction and maturation phase effects the callus formation. In addition to cyclic compression, other movements like shear and bending influence the bone formation process as shown in previous callus distraction studies. Reports of cartilage presence and endochondral ossification in the regenerative zone have been associated with a lack of fixation stability and delayed healing. So far the effects of the direction of interfragmentary movements could not be studied separately. By means of a unique lateral callus distraction model, we investigated the effects of small (0.1 mm) and moderate (0.6 mm), purely axial compression on ossification during callus maturation in sheep. A distraction device incorporating a mobile titanium plate was mounted on the tibia. Following lateral callus distraction, electromechanically controlled movements allowed purely axial cyclic compression of the tissue regenerate. Seven weeks post-operatively, the tissue regenerates were investigated using µCT, histology and immunohistochemistry. The larger amplitude significantly increased bone formation (Fractional bone volume: 19.4% vs. 5.2%, p = 0.03; trabecular thickness: 0.1 mm vs. 0.06 mm, p = 0.006; mean spicule height: 2.6 mm vs. 1.1 mm, p = 0.02) however, no endochondral ossification occurred. The elimination of shear movement, unimpaired neovascularization as well as the tensile strain stimuli during the distraction phase suppressing chondrogenic differentiation may all contribute to the absence of cartilage. In clinical application of distraction osteogenesis, moderate axial interfragmentary movement augments intramembranous ossification provided shear strain is minimized.

Novel systems for the application of isolated tensile, compressive, and shearing stimulation of distraction callus tissue.

BACKGROUND: Distraction osteogenesis is a procedure widely used for the correction of large bone defects. However, a high complication rate persists, likely due to insufficient stability during maturation. Numerical fracture healing models predict bone regeneration under different mechanical conditions allowing fixation stiffness optimization. However, most models apply a linear elastic material law inappropriate for the transient stresses/strains present during limb lengthening or segment transport. They are also often validated using in vivo osteotomy models lacking precise mechanical regulation due to the unavoidable stimulation of secondary interfragmentary motion during ambulation under finitely stiff fixation. Therefore, in order to create a robust numerical model of distraction osteogenesis, it is necessary to both characterize the new tissue's viscoelasticity during distraction and determine the influence of strictly isolated stimulation in each loading mode (tension, compression, and shear) to account for potential differences in mechanical and histological response. AIM: Two electromechanical fixators with integrated load cells were designed to precisely perform and monitor in vivo lateral distraction and isolated stimulation in sheep tibiae using a mobile, hydroxyapatite-coated titanium plate. The novel surgical procedure circumvents osteotomy, eliminating the undesirable and unquantifiable mechanical stimulation during ambulation. METHODS: After a 10-day post-surgery latency period, two 0.275 mm distraction steps were performed daily for 10 days. The load cell collected data before, during, and after each distraction step and was terminated after no less than one minute from the time of distraction. A 7-day consolidation period separated the distraction phase and 18-day stimulation phase. Stimulation was carried out in isolated tension, compression, or shear while recording force/time data. Each stimulation session consisted of 120 cycles with a magnitude of either 0.1 mm or 0.6 mm in the tension and compression groups and 1.0 mm in the shear group. The animals were euthanized after a 3-day holding period following stimulation. RESULTS: Our initial results show that the tissue progressively stiffens and maintains an increasingly large residual traction. The force curves during compressive stimulation show a progressive drift from compression toward tension. We hypothesize that this behavior may be due to the preferential flow of fluid outward from the tissue and a greater resistance to reabsorption during the plate's return to the starting position.

The effect of a combined thoracic and soft-tissue trauma on blood flow and tissue formation in fracture healing in rats.

INTRODUCTION: Previously, it was found that fracture healing is impaired by blunt chest trauma and an additional soft-tissue trauma. The mechanisms leading to this disturbance are largely unknown. Here, we investigated the effect of thoracic and soft-tissue trauma on blood flow of the injured lower leg and on tissue differentiation and callus formation during fracture healing. MATERIALS AND METHODS: Male Wistar rats received either a mid-shaft fracture of the tibia alone (group A), an additional chest trauma (group B), or additional chest and soft-tissue traumas (group C). Peripheral blood flow was determined by Laser Doppler Flowmetry before and after the injury, and on observation days 1, 3, 7, 14, and 28. Quantitative histological analysis was performed to assess callus size and composition. RESULTS: All groups displayed an initial decrease in blood flow during the first 3 days post-trauma. A recovery of the blood flow that even exceeded preoperative levels occurred in group A and later and to a lesser degree in group B, but not in group C. The amount of callus formation decreased with increasing trauma load. More cartilage was formed after 7 days in groups B and C than in group A. At later healing time points, callus composition did not differ significantly. CONCLUSIONS: An increasing injury burden causes a decreasing blood supply capacity and revascularization, and leads to impaired callus formation and an increasing delay in bone healing.

Metaphyseal fracture healing in a sheep model of low turnover osteoporosis induced by hypothalamic-pituitary disconnection (HPD).

We recently established a large animal model of osteoporosis in sheep using hypothalamic-pituitary disconnection (HPD). As central regulation is important for bone metabolism, HPD-sheep develop severe osteoporosis because of low bone turnover. In this study we investigated metaphyseal fracture healing in HPD-sheep. To elucidate potential pathomechanisms, we included a treatment group receiving thyroxine T4 and 17ß-estradiol. Because clinically osteoporotic fractures often occur in the bone metaphysis, HPD-sheep and healthy controls received an osteotomy in the distal femoral condyle. Half of the HPD-sheep were systemically treated with thyroxine T4 and 17ß-estradiol during the healing period. Fracture healing was evaluated after 8 weeks using pQCT, µCT, and histomorphometrical analysis. Bone mineral density (BMD) and bone volume/total volume (BV/TV) were considerably reduced by 30% and 36%, respectively, in the osteotomy gap of the HPD-sheep compared to healthy sheep. Histomorphometry also revealed a decreased amount of newly formed bone (-29%) and some remaining cartilage in the HPD-group, suggesting that HPD disturbed fracture healing. Thyroxine T4 and 17ß-estradiol substitution considerably improved bone healing in the HPD-sheep. Our results indicate that fracture healing requires central regulation and that thyroxine T4 and 17ß-estradiol contribute to the complex pathomechanisms of delayed metaphyseal bone healing in HPD-sheep.

Systemic inflammation induced by a thoracic trauma alters the cellular composition of the early fracture callus.

BACKGROUND: We recently demonstrated that a blunt chest trauma, a strong inducer of the posttraumatic systemic inflammatory response and one of the most critical injuries in polytrauma patients, significantly delayed fracture healing in rats, possibly by the interaction of the systemic inflammation with early regeneration processes locally at the fracture site. The underlying cellular mechanisms, however, have as yet remained unknown. Therefore, the aim of this study was to analyze the cellular and morphologic composition of the early fracture callus after a blunt chest trauma. METHODS: Rats received an osteotomy of the right femur stabilized by an external fixator in combination with a blunt chest trauma or not. The animals were killed after 3, 7, and 35 days, and the fracture calli were analyzed histologically for new tissue formation, polymorphonuclear leucocytes, macrophages, osteoclasts, and the presence of the proinflammatory cytokine interleukin 6. RESULTS: The blunt chest trauma considerably increased the number of polymorphonuclear leucocytes in the callus by Day 3 compared with animals with isolated fractures. The number of macrophages was significantly reduced by the thoracic trauma at Days 3 and 7. The number of osteoclasts was not changed at any postoperative time point. After 3 days, the blunt chest trauma led to a significantly stronger interleukin 6 staining within the periosteal callus in zones of intramembranous ossification. During the time of cortical bridging at Day 35, the amount of newly formed bone was significantly decreased after blunt chest trauma. CONCLUSION: Our results suggest that the systemic posttraumatic inflammation induced by a thoracic trauma disturbed the inflammatory balance during the early healing stage by altering the recruitment of inflammatory cells and cytokine expression locally at the fracture site and thus impaired fracture healing. These findings provide new insights in the pathomechanisms of impaired fracture healing in patients experiencing severe trauma.

Conversion from external fixator to intramedullary nail causes a second hit and impairs fracture healing in a severe trauma model.

In poly-traumatic patients, second hits are known to potentiate the posttraumatic systemic inflammatory response, thus increasing the risk of multi-organ dysfunction. In accordance with "damage control orthopaedic surgery" principles, fractures are initially treated with external fixators, which are replaced by internal osteosynthesis once the immunological status of the patient is considered stable. Recently, we demonstrated that a severe trauma impaired the healing of fractures stabilized by external fixation during the entire healing period. The question arose, whether switching to intramedullary nailing increases the inflammatory response in terms of a second hit, leading to a further impairment of bone healing. Wistar rats received a femoral osteotomy stabilized by an external fixator. Simultaneously half of the rats underwent an additional thoracic trauma. After 4 days, the external fixator was replaced by an intramedullary nail in half of the rats of the two groups. The inflammatory response was evaluated by measuring serum C5a levels. Fracture healing was determined by three-point-bending, µCT, and histomorphometry. The thoracic trauma significantly increased C5a concentrations 6, 24, and 72 h after the second surgical intervention. After 40 days, conversion to intramedullary nailing considerably decreased the flexural rigidity of the callus, with no significant differences between rats with or without thoracic trauma. After 47 days, flexural rigidity in rats subjected to conversion remained decreased compared to animals solely treated by external fixation, particularly in combination with blunt chest trauma. The results indicate that accumulation of second hits after multiple injuries could lead to aggravation of the fracture healing outcome.

Excess dietary methionine does not affect fracture healing in mice.

BACKGROUND: An elevated serum concentration of homocysteine (hyperhomocysteinemia) has been shown to disturb fracture healing. As the essential amino acid, methionine, is a precursor of homocysteine, we aimed to investigate whether excess methionine intake affects bone repair. MATERIAL/METHODS: We analyzed bone repair in 2 groups of mice. One group was fed a methionine-rich diet (n=13), and the second group received an equicaloric control diet without methionine supplementation (n=12). Using a closed femoral fracture model, bone repair was analyzed by histomorphometry and biomechanical testing at 4 weeks after fracture. Blood was sampled to measure serum concentrations of homocysteine, the bone formation marker osteocalcin, and the bone resorption marker collagen I C-terminal crosslaps RESULTS: Serum concentrations of homocysteine were significantly higher in the methionine group than in the control group, while serum markers of bone turnover did not differ significantly between the 2 groups. Histomorphometry revealed no significant differences in size and tissue composition of the callus between animals fed the methionine-enriched diet and those receiving the control diet. Accordingly, animals of the 2 groups showed a comparable bending stiffness of the healing bones. CONCLUSIONS: We conclude that excess methionine intake causes hyperhomocysteinemia, but does not affect fracture healing in mice.

Optimization of intramedullary nailing by numerical simulation of fracture healing.

Due to the annular gap between intramedullary (IM) nails and the endosteal surface, high interfragmentary movement can occur under loading. This could prolong the healing time, particularly for thin IM nails that are often used for unreamed IM nailing. The aims of our study were to determine the influence of the nail diameter on the healing time of human tibial shaft fractures and to investigate whether the healing time could be shortened by increasing the stiffness of the implant material. Therefore, a corroborated numerical model for simulating the fracture healing process in humans was used to simulate the healing process of human tibial fractures treated with IM nails. The calculated healing time (up to 71 weeks) was longest for transverse fractures treated with thin IM nails made of titanium. That the healing time was disproportionately long depended on the nail diameter, and could be greatly reduced by using a thicker nail or using steel instead of titanium. To avoid a prolonged healing time, the nail should be thick, and the annular gap should be as narrow as possible. Alternatively, using steel instead of titanium may also help to avoid a prolonged healing time.

Forces acting on the anterior meniscotibial ligaments.

PURPOSE: The purpose of this study was to investigate the forces occurring in human anterior meniscotibial attachment structures under various loading conditions. METHODS: Twelve human knee joints were exposed to eight loading conditions (tibial rotations and varus/valgus stress) using a previously described knee joint simulator. Subsequently, the joints were axially compressed (1,000 N at 0° 30° and 60° knee flexion) using a materials testing machine. Then, we performed a tensile test to failure of the ligaments. Finally, we used the strains that occurred during the loading tests and the force-elongation diagrams obtained from the tensile test to recursively assess the resulting forces. RESULTS: In the anterior meniscotibial ligaments, we found maximum mean strains of 3.8 ± 2.3% under external moments and 1.5 ± 0.9% for axial compression. With an ultimate load of 454 ± 220 N for the anterolateral meniscotibial ligament and 397 ± 275 N for the anteromedial meniscotibial ligament, we estimated maximum forces of up to 50.2 N for the knee simulator tests and 22.6 N for the axial compression tests. CONCLUSIONS: The low forces found in the meniscal ligaments suggest that for normal daily activities, meniscal replacement implants and allografts do not require a very rigid fixation at their bony insertions. However, it remains unknown, what level of force occurs in the meniscotibial ligaments under traumatic situations or impact knee loads. Furthermore, the results of the present study could help to optimize meniscal re-fixation and to improve the properties of meniscal replacement materials, such as tissue-engineered artificial menisci. Moreover, the results could be used for the validation of finite element models of the knee joint with the main focus on the meniscus and its biomechanical relevance for tibiofemoral contact pressure.

The anaphylatoxin receptor C5aR is present during fracture healing in rats and mediates osteoblast migration in vitro.

BACKGROUND: There is evidence that complement components regulate cytokine production in osteoblastic cells, induce cell migration in mesenchymal stem cells, and play a regulatory role in normal enchondral bone formation. We proved the hypothesis that complement might be involved in bone healing after fracture. METHODS: We investigated the expression of the key anaphylatoxin receptor C5aR during fracture healing in rats by immunostaining after 1, 3, 7, 14, and 28 days. C5aR expression was additionally analyzed in human mesenchymal stem cells (hMSC) during osteogenic differentiation, in human primary osteoblasts, and osteoclasts by reverse transcriptase polymerase chain reaction and immunostaining. Receptor functionality was proven by the migratory response of cells to C5a in a Boyden chamber. RESULTS: C5aR was expressed in a distinct spatial and temporal pattern in the fracture callus by differentiated osteoblast, chondroblast-like cells in cartilaginous regions, and osteoclasts. In vitro C5aR was expressed by osteoblasts, osteoclasts, and hMSC undergoing osteogenic differentiation. C5aR was barely expressed by undifferentiated hMSC but was significantly induced after osteogenic differentiation. C5aR activation by C5a induced strong chemotactic activity in osteoblasts, and in hMSC, which had undergone osteogenic differentiation, being abolished by a specific C5aR antagonist. In hMSC, C5a induced less migration reflecting their low level of C5aR expression. CONCLUSIONS: Our in vitro and in vivo results demonstrated the presence of C5aR in bone forming osteoblasts and bone resorbing osteoclasts. It is suggested that C5aR might play a regulatory role in fracture healing in intramembranous and in enchondral ossification, one possible function being the regulation of cell recruitment.

The effect of both a thoracic trauma and a soft-tissue trauma on fracture healing in a rat model.

BACKGROUND AND PURPOSE: There is some clinical evidence that fracture healing is impaired in multiply injured patients. Nothing is known, however, about the effects of various types of injuries and their contribution to a possible disturbance of the fracture-healing process. We investigated the effect of a thoracic trauma and an additional soft-tissue trauma on fracture healing in a rat tibia model. METHODS: 3 groups of rats were operated: group A with a simple fracture of the tibia and fibula, group B with a fracture and an additional thoracic trauma, and group C with a fracture, thoracic trauma, and an additional soft-tissue trauma. The fracture and the soft-tissue injury were produced by a special guillotine-like device and the thoracic trauma by a blast wave generator. After one day, the serum level of IL-6 was quantified, and at the end of the study (28 days) the mechanical properties and the callus volume of the healed tibia were determined. RESULTS: Increasing the severity of the injury caused IL-6 levels to more than double 1 day after injury. It halved the load to failure in mechanical tests and led to reduced callus volume after 28 days of healing. INTERPRETATION: Fracture healing is impaired when additional thoracic trauma and soft tissue trauma occurs.

Experimental blunt chest trauma impairs fracture healing in rats.

In poly-traumatic patients a blunt chest trauma is an important trigger of the posttraumatic systemic inflammatory response. There is clinical evidence that fracture healing is delayed in such patients, however, experimental data are lacking. Therefore, we investigated the influence of a thoracic trauma on fracture healing in a rat model. Male Wistar rats received either a blunt chest trauma combined with a femur osteotomy or an isolated osteotomy. A more rigid or a more flexible external fixator was used for fracture stabilization to analyze whether the thoracic trauma influences regular healing and mechanically induced delayed bone healing differently. The blunt chest trauma induced a significant increase of IL-6 serum levels after 6 and 24 h, suggesting the induction of a systemic inflammation, whereas the isolated fracture had no effect. Under a more rigid fixation the thoracic trauma considerably impaired fracture healing after 35 days, reflected by a significantly reduced flexural rigidity (three-point-bending test), as well as a significantly diminished callus volume, moment of inertia, and relative bone surface (µCT analysis). In confirming the clinical evidence, this study reports for the first time that a blunt chest trauma considerably impaired bone healing, possibly via the interaction of the induced systemic inflammation with local inflammatory processes.

Biomechanical principles and mechanobiologic aspects of flexible and locked plating.

The goal of minimally invasive surgery in extramedullary internal fixation has led to the development of flexible plates, bridging plates, and locked internal fixators. The change from conventional compression plates to these new implants, however, resulted in different biomechanics of fixation and different mechanobiologic processes for fracture healing. The aim of a flexible fixation is the stimulation of fracture healing by callus formation. Fracture healing follows mechanobiologic rules based mainly on interfragmentary strain, which is dependent on the stability of the fixation construct and the type of fracture. Knowledge of the mechanobiologic processes and the factors influencing the stability of fracture fixation are necessary for the surgeon to choose the correct technique for fracture fixation. Problems in the selection of the correct technique and limitations with the available implants as well as possible future developments are discussed.

Accelerated aging phenotype in mice with conditional deficiency for mitochondrial superoxide dismutase in the connective tissue.

The free radical theory of aging postulates that the production of mitochondrial reactive oxygen species is the major determinant of aging and lifespan. Its role in aging of the connective tissue has not yet been established, even though the incidence of aging-related disorders in connective tissue-rich organs is high, causing major disability in the elderly. We have now addressed this question experimentally by creating mice with conditional deficiency of the mitochondrial manganese superoxide dismutase in fibroblasts and other mesenchyme-derived cells of connective tissues in all organs. Here, we have shown for the first time that the connective tissue-specific lack of superoxide anion detoxification in the mitochondria results in reduced lifespan and premature onset of aging-related phenotypes such as weight loss, skin atrophy, kyphosis (curvature of the spine), osteoporosis and muscle degeneration in mutant mice. Increase in p16(INK4a) , a robust in vivo marker for fibroblast aging, may contribute to the observed phenotype. This novel model is particularly suited to decipher the underlying mechanisms and to develop hopefully novel connective tissue-specific anti-aging strategies.

A small scale cell culture system to analyze mechanobiology using reporter gene constructs and polyurethane dishes.

Mechanical forces are translated into biochemical signals and contribute to cell differentiation and phenotype maintenance. Mesenchymal stem cells and their tissue-specific offspring, as osteoblasts and chondrocytes, cells of cardiovascular tissues and lung cells are sensitive to mechanical loading but molecules and mechanisms involved have to be unraveled. It is well established that cellular mechanotransduction is mediated e.g. by activation of the transcription factor SP1 and by kinase signaling cascades resulting in the activation of the AP1 complex. To investigate cellular mechanisms involved in mechanotransduction and to analyze substances, which modulate cellular mechanosensitivity reporter gene constructs, which can be transfected into cells of interest might be helpful. Suitable small-scale bioreactor systems and mechanosensitive reporter gene constructs are lacking. To analyze the molecular mechanisms of mechanotransduction and its crosstalk with biochemically induced signal transduction, AP1 and SP1 luciferase reporter gene constructs were cloned and transfected into various cell lines and primary cells. A newly developed bioreactor and small-scale 24-well polyurethane dishes were used to apply cyclic stretching to the transfected cells. 1 Hz cyclic stretching for 30 min in this system resulted in a significant stimulation of AP1 and SP1 mediated luciferase activity compared to unstimulated cells. In summary we describe a small-scale cell culture/bioreactor system capable of analyzing subcellular crosstalk mechanisms in mechanotransduction, mechanosensitivity of primary cells and of screening the activity of putative mechanosensitizers as new targets, e.g. for the treatment of bone loss caused by both disuse and signal transduction related alterations of mechanotransduction.

Mechanical stimulation alters pleiotrophin and aggrecan expression by human intervertebral disc cells and influences their capacity to stimulate endothelial migration.

STUDY DESIGN: The influence of mechanical load on pleiotrophin (PTM) and aggrecan expression by intervertebral disc (IVD) cells, and the effects of disc cell conditioned medium on endothelial cell migration was investigated. OBJECTIVE: To examine possible interactions of mechanical loads and known pro- and antiangiogenic factors, which may regulate disc angiogenesis during degeneration. SUMMARY OF BACKGROUND DATA: Pleiotrophin expression can be influenced by mechanical stimulation and has been associated with disc vascularization. Disc aggrecan inhibits endothelial cell migration, suggesting an antiangiogenic role. A possible interplay between these factors is unknown. METHODS: The influence of the respective predominant load (cyclic strain for anulus fibrosus and hydrostatic pressure for nucleus pulposus cells) on PTN and aggrecan expression by IVD cells was determined by real-time RT-PCR and Western blotting (PTN only). The effects of IVD cell conditioned medium on endothelial cell migration were analyzed in a bioassay using human microvascular endothelial (HMEC-1) cells. RESULTS: Application of both mechanical loads resulted in significant alterations of gene expression of PTN (+67%, P = 0.004 in anulus cells; +29%, P = 0.03 in nucleus cells) and aggrecan (+42%, P = 0.03 in anulus cells, -25%, P = 0.03 in nucleus cells). These effects depended on the cell type, the applied load, and timescale. Conditioned media of nucleus pulposus cells enhanced HMEC-1 migration, but this effect was diminished after 2.5 MPa hydrostatic pressure, when aggrecan expression was diminished, but not 0.25 MPa, when expression levels were unchanged. CONCLUSION: Mechanical loading influences PTN expression by human IVD cells. Conditioned media from nucleus pulposus cell cultures stimulated HMEC-1 endothelial cell migration. This study demonstrates that the influence of mechanical loads on vascularization of the human IVD is likely to be complex and does not correlate simply with altered expression of known pro- and antiangiogenic factors.

A novel model to study metaphyseal bone healing under defined biomechanical conditions.

INTRODUCTION: Experimental studies on metaphyseal fractures are rare and do not control the biomechanical conditions in the healing zone. This study aimed to develop an improved experimental model, which characterizes and controls the biomechanical condition in the fracture gap of a metaphyseal fracture. MATERIALS AND METHODS: A partial osteotomy model in the distal femur of the sheep was developed. The osteotomy was located in the region of the trochlea groove. The osteotomy gap was 3 mm wide. The retro-patellar force acting on the joint in vivo causes a bending of the trochlea resulting in a narrowing of the osteotomy gap. To limit and control this interfragmentary movement, stainless steel plates of various thicknesses were implanted into the osteotomy gap. Forces acting on the trochlea were analyzed and a load-deflection curve of the model was determined in vitro. A pilot study on two sheep was performed using the new model with two different interfragmentary movements of 0.3 or 1 mm. Eight weeks, post-operatively, the sheep were sacrificed and undecalcified histology was performed. RESULTS: The biomechanical analysis of the joint forces and the in vitro load-deflection behavior of the trochlea revealed that the forces acting on the trochlea were high enough to cause an interfragmentary movement of 1 mm in the osteotomy gap. This was confirmed by an X-ray of the sheep, which showed a closing of the proximal osteotomy gap under weight-bearing conditions. The histological section revealed no external callus formation. The sheep with the 0.3 mm interfragmentary movement showed almost complete bridging of the osteotomy gap with woven bone whereas the sheep with the 1 mm interfragmentary movement exhibited new bone formation only at the borderline of the osteotomy but larger areas with connective tissue or even fibrous cartilage in the center of the gap. CONCLUSION: This metaphyseal bone-healing model provides defined and adjustable biomechanical conditions. The histological images demonstrated intramembranous and endochondral bone healing in the osteotomy gap without callus formation. The model therefore seems appropriate to study metaphyseal bone healing under differing mechanical conditions.

Increased cortical remodeling after osteotomy causes posttraumatic osteopenia.

Following a fracture, substantial bone mineral loss can occur at the affected limb. The aim of this study was to analyze the changes in cortical bone around the site of a fracture. We analyzed bone mineral density by quantitative computed tomography and quantified changes in cortical remodeling by histomorphometry adjacent to an experimental osteotomy in sheep metatarsals. In the cortical bone around the osteotomy, we found a statistically significant 16% reduction in app.BMD within 9 weeks following surgery. This reduction was explained (R=-0.71, P<0.01) by a more than 6 fold increase in bone remodeling activity within cortical bone at the affected limb. The remodeling activity significantly increased between surgery and week 6, but remained unchanged between week 6 and week 9. We conclude from these findings that posttraumatic bone mineral loss adjacent to a fracture is related to an elevated number of active osteons, indicating a significant increase in bone remodeling activity. Load shielding by the osteosynthesis material and local recruitment of bone mineral are likely causes for this increased remodeling. This post-traumatic bone loss is likely to contribute significantly to frequently observed healing complications like refracture, failure of implant fixation, implant loosening, or cut out.

Mechanical regulation of osteoclastic genes in human osteoblasts.

Bone adaptation to mechanical load is accompanied by changes in gene expression of bone-forming cells. Less is known about mechanical effects on factors controlling bone resorption by osteoclasts. Therefore, we studied the influence of mechanical loading on several key genes modulating osteoclastogenesis. Human osteoblasts were subjected to various cell stretching protocols. Quantitative RT-PCR was used to evaluate gene expression. Cell stretching resulted in a significant up-regulation of receptor activator of nuclear factor-kappaB ligand (RANKL) immediate after intermittent loading (3x3h, 3x6h, magnitude 1%). Continuous loading, however, had no effect on RANKL expression. The expression of osteoprotegerin (OPG), macrophage-colony stimulating factor (M-CSF), and osteoclast inhibitory lectin (OCIL) was not significantly altered. The data suggested that mechanical loading could influence osteoclasts recruitment by modulating RANKL expression in human osteoblasts and that the effects might be strictly dependent on the quality of loading.

A new laser scanning technique for imaging intervertebral disc displacement and its application to modeling nucleotomy.

BACKGROUND: Nucleotomy is a standard procedure for treating disc prolapse. It can reduce intervertebral disc height, flattening and displacing the disc, which could lead to a painful narrowing of the foramina due to nerve root compression. The purpose of this study was to investigate the disc displacement of a complete spinal segment with and without nucleotomy. We hypothesized that a nucleotomy under a certain load combination might amplify disc displacement. METHODS: A laser scanner was developed for recording three-dimensional disc displacement of six loaded L4-5 specimens for three conditions: intact, disc with vertebral bodies and subsequent nucleotomy. Specimens were exposed to pure moments of 7.5 N m in the three principal anatomical directions. Disc displacement was obtained at maximal deflection. A finite element model was validated and subsequently utilized to determine disc displacement. The task of the finite element model was to provide supplemental data for the posterolateral region, which could not be measured from intact specimens. FINDINGS: Disc displacement measurements of intact specimens were limited to the anterior part of discs, whereas the finite element model was able to provide the missing data of the dorsal disc region. The simulation of load combinations showed that the highest disc displacement was 1.9 mm at the lateral or posterolateral region. The nucleotomy increased the disc displacement up to 2.1mm, whereas the displacement zenith migrated posterolaterally. INTERPRETATION: These results could be a possible explanation for disadvantages of nucleotomy as a treatment. With the methodology presented here, we would be able to assess the performance of nucleus implants by determining the disc displacement map. This could also give us appropriate information of the annular deformation, which is needed for the development of motion preserving implants.