Osteoblast precursors and inflammatory cells arrive simultaneously to sites of a trabecular-bone injury

<p>Background and purpose — Fracture healing in the shaft is usually described as a sequence of events, starting with inflammation, which triggers mesenchymal tissue formation in successive steps. Most clinical fractures engage cancellous bone. We here describe fracture healing in cancellous bone, focusing on the timing of inflammatory and mesenchymal cell type appearance at the site of injury</p> <p>Material and methods — Rats received a proximal tibial drill hole. A subgroup received clodronate-containing liposomes before or after surgery. The tibiae were analyzed with micro-CT and immunohistochemistry 1 to 7 days after injury.</p> <p>Results — Granulocytes (myeloperoxidase) appeared in moderate numbers within the hole at day 1 and then gradually disappeared. Macrophage expression (CD68) was seen on day 1, increased until day 3, and then decreased. Mesenchymal cells (vimentin) had already accumulated in the periphery of the hole on day 1. Mesenchymal cells dominated in the entire lesion on day 3, now producing extracellular matrix. A modest number of preosteoblasts (RUNX2) were seen on day 1 and peaked on day 4. Osteoid was seen on day 4 in the traumatized region, with a distinct border to the uninjured surrounding marrow. Clodronate liposomes given before the injury reduced the volume of bone formation at day 7, but no reduction in macrophage numbers could be detected.</p> <p>Interpretation — The typical sequence of events in shaft fractures was not seen. Mesenchymal cells appeared simultaneously with granulocyte and macrophage arrival. Clodronate liposomes, known to reduce macrophage numbers, seemed to be associated with the delineation of the volume of tissue to be replaced by bone. Most fracture healing studies in animal models concern cortical bone in shafts. However, most fractures in patients occur in cancellous bone in the metaphysis, such as the distal radius or in the vertebrae. A growing body of evidence suggests that there are important differences between the healing processes in cortical and cancellous bone.</p

Experimental models for cancellous bone healing in the rat

<div><p><b>Background and purpose —</b> Cancellous bone appears to heal by mechanisms different from shaft fracture healing. There is a paucity of animal models for fractures in cancellous bone, especially with mechanical evaluation. One proposed model consists of a screw in the proximal tibia of rodents, evaluated by pull-out testing. We evaluated this model in rats by comparing it to the healing of empty drill holes, in order to explain its relevance for fracture healing in cancellous bone. To determine the sensitivity to external influences, we also compared the response to drugs that influence bone healing.</p><p><b>Methods —</b> Mechanical fixation of the screws was measured by pull-out test and related to the density of the new bone formed around similar, but radiolucent, PMMA screws. The pull-out force was also related to the bone density in drill holes at various time points, as measured by microCT.</p><p><b>Results —</b> The initial bone formation was similar in drill holes and around the screw, and appeared to be reflected by the pull-out force. Both models responded similarly to alendronate or teriparatide (PTH). Later, the models became different as the bone that initially filled the drill hole was resorbed to restore the bone marrow cavity, whereas on the implant surface a thin layer of bone remained, making it change gradually from a trauma-related model to an implant fixation model.</p><p><b>Interpretation —</b> The similar initial bone formation in the different models suggests that pull-out testing in the screw model is relevant for assessment of metaphyseal bone healing. The subsequent remodeling would not be of clinical relevance in either model.</p></div

Mechanical properties during healing of Achilles tendon ruptures to predict final outcome: A pilot Roentgen stereophotogrammetric analysis in 10 patients-0

<p><b>Copyright information:</b></p><p>Taken from "Mechanical properties during healing of Achilles tendon ruptures to predict final outcome: A pilot Roentgen stereophotogrammetric analysis in 10 patients"</p><p>http://www.biomedcentral.com/1471-2474/8/116</p><p>BMC Musculoskeletal Disorders 2007;8():116-116.</p><p>Published online 26 Nov 2007</p><p>PMCID:PMC2244624.</p><p></p

Mechanical properties during healing of Achilles tendon ruptures to predict final outcome: A pilot Roentgen stereophotogrammetric analysis in 10 patients-1

<p><b>Copyright information:</b></p><p>Taken from "Mechanical properties during healing of Achilles tendon ruptures to predict final outcome: A pilot Roentgen stereophotogrammetric analysis in 10 patients"</p><p>http://www.biomedcentral.com/1471-2474/8/116</p><p>BMC Musculoskeletal Disorders 2007;8():116-116.</p><p>Published online 26 Nov 2007</p><p>PMCID:PMC2244624.</p><p></p

Mechanical properties during healing of Achilles tendon ruptures to predict final outcome: A pilot Roentgen stereophotogrammetric analysis in 10 patients-2

<p><b>Copyright information:</b></p><p>Taken from "Mechanical properties during healing of Achilles tendon ruptures to predict final outcome: A pilot Roentgen stereophotogrammetric analysis in 10 patients"</p><p>http://www.biomedcentral.com/1471-2474/8/116</p><p>BMC Musculoskeletal Disorders 2007;8():116-116.</p><p>Published online 26 Nov 2007</p><p>PMCID:PMC2244624.</p><p></p

Mechanical properties during healing of Achilles tendon ruptures to predict final outcome: A pilot Roentgen stereophotogrammetric analysis in 10 patients-3

<p><b>Copyright information:</b></p><p>Taken from "Mechanical properties during healing of Achilles tendon ruptures to predict final outcome: A pilot Roentgen stereophotogrammetric analysis in 10 patients"</p><p>http://www.biomedcentral.com/1471-2474/8/116</p><p>BMC Musculoskeletal Disorders 2007;8():116-116.</p><p>Published online 26 Nov 2007</p><p>PMCID:PMC2244624.</p><p></p>ad placement). Increase in stiffness is mostly caused by increasing modulus, but there is a negative correlation between increase in modulus and transverse area (symbolized by drawn line on bottom plane)

Marrow compartment contribution to cortical defect healing

<p><b>Background and purpose — Healing of shaft fractures is commonly described as regards external callus. We wanted to clarify the role of the bone marrow compartment in the healing of stable shaft fractures.</b></p> <p><b>Patients and methods — A longitudinal furrow was milled along the longitudinal axis of the femoral shaft in mice. The exposed bone marrow under the furrow was scooped out. The mice were then randomized to no further treatment, or to receiving 2 silicone plugs in the medullary canal distal and proximal to the defect. The plugs isolated the remaining marrow from contact with the defect. Results were studied with histology and flow cytometry.</b></p> <p><b>Results — Without silicone plugs, the marrow defect was filled with new bone marrow-like tissue by day 5, and new bone was seen already on day 10. The new bone was seen only at the level of the cortical injury, where it seemed to form simultaneously in the entire region of the removed cortex. The new bone seemed not to invade the marrow compartment, and there was a sharp edge between new bone and marrow. The regenerated marrow was similar to uninjured marrow, but contained considerably more cells. In the specimens with plugs, the marrow compartment was either filled with loose scar tissue, or empty, and there was only minimal bone formation, mainly located around the edges of the cortical injury.</b></p> <p><b>Interpretation — Marrow regeneration in the defect seemed to be a prerequisite for normal cortical healing. Shaft fracture treatment should perhaps pay more attention to the local bone marrow.</b></p

Different mechanisms activated by mild versus strong loading in rat Achilles tendon healing

<div><p>Background</p><p>Mechanical loading stimulates Achilles tendon healing. However, various degrees of loading appear to have different effects on the mechanical properties of the healing tendon, and strong loading might create microdamage in the tissue. This suggests that different mechanisms might be activated depending on the magnitude of loading. The aim of this study was to investigate these mechanisms further.</p><p>Methods</p><p>Female rats had their right Achilles tendon cut transversely and divided into three groups: 1) unloading (calf muscle paralysis by Botox injections, combined with joint fixation by a steel-orthosis), 2) mild loading (Botox only), 3) strong loading (free cage activity). Gene expression was analyzed by PCR, 5 days post-injury, and mechanical testing 8 days post-injury. The occurrence of microdamage was analyzed 3, 5, or 14 days post-injury, by measuring leakage of injected fluorescence-labelled albumin in the healing tendon tissue.</p><p>Results</p><p>Peak force, peak stress, and elastic modulus of the healing tendons gradually improved with increased loading as well as the expression of extracellular matrix genes. In contrast, only strong loading increased transverse area and affected inflammation genes. Strong loading led to higher fluorescence (as a sign of microdamage) compared to mild loading at 3 and 5 days post-injury, but not at 14 days.</p><p>Discussion</p><p>Our results show that strong loading improves both the quality and quantity of the healing tendon, while mild loading only improves the quality. Strong loading also induces microdamage and alters the inflammatory response. This suggests that mild loading exert its effect via mechanotransduction mechanisms, while strong loading exert its effect both via mechanotransduction and the creation of microdamage.</p><p>Conclusion</p><p>In conclusion, mild loading is enough to increase the quality of the healing tendon without inducing microdamage and alter the inflammation in the tissue. This supports the general conception that early mobilization of a ruptured tendon in patients is advantageous.</p></div

Results from gene expression and microdamage analyses.

<p><b>A)</b> Gene expression for extracellular matrix genes, 5 days after tendon injury. The fold change of genes for <i>Collagen I</i> (<i>COL1A1</i>), <i>collagen III</i> (<i>COL3A1</i>), <i>collagen V</i> (<i>COL5A1</i>), and <i>lysyl oxidase</i> (<i>LOX</i>) are shown. Three different loading conditions were tested; unloading (Botox and steel-orthosis), mild loading (Botox), and strong loading (free cage activity), N = 11–12. <b>B)</b> Leakage of fluorescent protein (BSA-FITC) as a sign of bleeding and microdamage: 3, 5, or 14 days after tendon injury. The result describes the fraction of the fluorescence in the tendon tissue compared to the fluorescence in the blood plasma (ratio of (counts per second / mg specimen) / (counts per second / mg blood plasma)). Two different loading conditions were tested: mild (Botox) and strong loading (N = 12). The rats were intravenously injected with BSA-FITC 1 hour before euthanasia. The fluorescence detected in the tendon tissue was normalized to the tissue weight and the fluorescence detected in the blood plasma. * p < 0.001.</p

Mechanical properties of rat Achilles tendons, 8 days post-injury.

<p>Mechanical properties of rat Achilles tendons, 8 days post-injury.</p