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

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

Shining dead bone—cause for cautious interpretation of [¹⁸F]NaF PET scans

<p><b>Background and purpose — [¹⁸F]Fluoride ([¹⁸F]NaF) PET scan is frequently used for estimation of bone healing rate and extent in cases of bone allografting and fracture healing. Some authors claim that [¹⁸F]NaF uptake is a measure of osteoblastic activity, calcium metabolism, or bone turnover. Based on the known affinity of fluoride to hydroxyapatite, we challenged this view.</b></p> <p><b>Methods — 10 male rats received crushed, frozen allogeneic cortical bone fragments in a pouch in the abdominal wall on the right side, and hydroxyapatite granules on left side. [¹⁸F]NaF was injected intravenously after 7 days. 60 minutes later, the rats were killed and [¹⁸F]NaF uptake was visualized in a PET/CT scanner. Specimens were retrieved for micro CT and histology.</b></p> <p><b>Results — MicroCT and histology showed no signs of new bone at the implant sites. Still, the implants showed a very high [¹⁸F]NaF uptake, on a par with the most actively growing and remodeling sites around the knee joint.</b></p> <p><b>Interpretation — [¹⁸F]NaF binds with high affinity to dead bone and calcium phosphate materials. Hence, an [¹⁸F]NaF PET/CT scan does not allow for sound conclusions about new bone ingrowth into bone allograft, healing activity in long bone shaft fractures with necrotic fragments, or remodeling around calcium phosphate coated prostheses</b></p