A critical crosslink region in human-bone-derived collagen type I. Specific cleavage site at residue Leu95

Collagen was extracted from human adult bone by limited pepsin digestion and collagen types were purified by consecutive salt precipitation first under neutral and then under acid conditions. In SDS/PAGE, all collagen type I preparations showed a protein band [alpha 1s(I)] migrating between alpha 1(I) and alpha 2(I) as well as a band [alpha 2s(I)] migrating in front of alpha 2(I). The collagenous nature of the pepsin-stable alpha 1s(I) protein was clearly demonstrated by digestion with human-leucocyte-derived collagenase, immunoblotting with antibodies against collagen type I and amino acid analysis. Partial amino acid sequencing of alpha 1(I) and alpha 1s(I) identified alpha 1s(I) as a shortened alpha 1(I) chain due to a specific cleavage site between residues Leu95 and Asp96 which is in close vicinity to the hydroxylysine-derived crosslink at position 87. In circular dichroism, the proportion of thermally labile collagen molecules was proportional to the amount of shortened alpha 1(I) and alpha 2(I) chains, respectively. The melting temperature was found to be 36 +/- 0.5 degrees C as judged from circular dichroism and susceptibility to proteolysis. Our data provide clear evidence that a shortened alpha 1-derived collagen chain can be extracted from human adult bone whereas it is hardly found in human skin. The unique cleavage site might provide important information about the collagen I molecule embedded in the calcified matrix of human bone.

Isolation and analysis of ribonucleic acids from skeletal tissues

We report a method for the isolation of total cellular RNA from mineralized or cartilaginous tissues. The procedure accommodates the large amount of hydroxyapatite and high buoyant density proteoglycans present in skeletal tissue samples, as well as the low cell density characteristic of these tissues. The procedure can be reliably used for processing a large number of small (100-800 mg) tissue samples. Tissues are homogenized in guanidine hydrochloride solution, then centrifuged at low speed, and filtered to remove the nonsolubilized extracellular matrix proteins. Subsequent high speed density gradient centrifugation produces a high yield of RNA (0.2-0.6 micrograms RNA/mg tissue) which is precipitated in a low pH sodium acetate solution. RNA extracted by this method has been analyzed for the expression of various genes by Northern blotting. In addition to mRNAs of bone- and cartilage-specific proteins, messenger RNA for growth factors, proto-oncogenes, and heat shock proteins can be detected.

[Experimental studies of the effect of direct electric micro-current on the healing processes of bone defects]

As a procedure to accelerate healing of the bone defect, the electric micro-current has attracted much attention in the field of clinical application and experimental research. However, investigation of the calcifying process and comparison of the elements of the newly formed bony callus have not yet been carried out. In order to get deeper understanding of the callus formation and calcifying process in bone defects, this experiment aimed at comparing the healing processes of bone defects in the rabbit humerus between stimulated and non-stimulated control group. The bone specimens were surgically removed, on the 4th postoperative day and 1st, 2nd, 3rd, and 4th postoperative weeks. The specimens were examined by the use of the computer aided microanalyser, the energy dispersive spectrometer, and the scanning electron-microscope (JCMA-733). Histological examination was also made. In the stimulated group, on the 4th day, Ca and P of low concentration were observed around the inner periosteum indicating initiation of the calcification with callus formation. On the 1st week Ca and P were distributed diffusely in the bone defect. On the 2nd and 3rd weeks, the bone defect was almost filled with new bony callus and the calcification became more intense. After the 4th week, the distribution and concentration of Ca and P in the bony callus were similar to those of the surrounding cortical bone. Furthermore, the molar ratio of Ca/P of the new bony callus was much greater in the stimulated group than in the control group, and was rather similar to the molar ratio of Ca/P in the surrounding cortical bone. The results showed that the electric micro-current facilitated not only callus formation but also calcification, thus shortening the healing period of the bone defect.

Monoclonal antibodies against osteonectin show conservation of epitopes across species

Several monoclonal antibodies were produced to bovine osteonectin, a major noncollagenous protein in the extracellular matrix of bone, and four were characterized. These antibodies showed different reactivities in Western immunoblots, immunoprecipitation, and indirect immunofluorescence, indicating that they recognize different epitopes on the protein. The data indicate that an epitope recognized by one of the antibodies is masked in interactions of osteonectin within cells and in the extracellular matrix. The high degree of cross-species immunoreactivity observed against bone osteonectin with these monoclonal antibodies indicates that these common epitopes have been conserved during evolution.

Is callus calcium content an indicator of the mechanical strength of healing fractures? An experimental study in rat metatarsals

Changes in the mechanical properties and the calcium content of healing fracture callus were followed, using rat metatarsals. By 24 weeks post-fracture the mean ultimate tensile stress and elastic modulus were still less than half that of the contralateral unfractured bone, whereas the mean torsional modulus had almost reached that of the unfractured bone. The calcium content of the callus formed immediately between the fractured ends of the bone showed changes which coincided with the increases in mechanical strength and the moduli, thus measurement of callus calcium content would enable the prediction of the strength of a healing fracture.

Fracture healing and osteocalcin metabolism in vitamin K deficiency

Osteocalcin (a vitamin K-dependent, bone-specific protein) is widely accepted as a marker of osteoblastic activity. The present study was conducted to determine if a vitamin K deficiency would affect fracture healing by virtue of an alteration in osteocalcin metabolism. Thirty male Sprague-Dawley rats were divided into two groups. The control group was fed a diet that was lacking in, but offered water replete with vitamin K. The experimental group was fed a vitamin K-deficient diet and was offered water that was lacking in vitamin K. After two weeks, vitamin K deficiency was established in the experimental group as shown by decreased urinary excretion of gamma-carboxyglutamic acid and an elevation of serum prothrombin times to between two to two and one-half times the control values. At this time, a standard, closed femoral fracture was produced. Six weeks later, the animals were killed. The bones were biomechanically tested in torsion. Subsequent to mechanical testing, the calluses were retrieved, and the osteocalcin content and the degree of gamma carboxylation of the osteocalcin in the calluses were measured. The results show that despite significant alterations in the gamma carboxylation of osteocalcin and elevation of prothrombin times to two to two and one-half times the control values, there were no differences in the mechanical properties of the calluses. Furthermore, there were no differences in the content or gamma carboxylation of osteocalcin in these calluses. Apparently, in vitamin K deficiency, fracture callus achieves normal mechanical properties and may have a mechanism for the gamma carboxylation of glutamic acids in osteocalcin despite a substantial depression of this activity in the rest of the body.

Biochemical characterization of fracture callus proteoglycans

The changes in proteoglycan molecules during the initial stages of fracture healing in rats were characterized. Following extraction of callus proteoglycan components with dissociative solvents, the components were purified in a cesium chloride density gradient. The recovered proteoglycans were characterized with respect to their molecular size distribution using gel filtration chromatography and a centrifugal transport methodology. During this early healing period, a decrease was observed in the relative proportion of the aggregate and in the hydrodynamic size and sedimentation coefficients of these molecules. While some molecular degradation could have occurred during the early stages of fracture healing, the dominant change of the proteoglycan molecules seemed to be disaggregation. No significant difference was observed in the proportion of aggregates reformed when exogenous hyaluronate and link glycoproteins were allowed to interact with the two corresponding monomer preparations. The molecular changes of the proteoglycan molecules seem to parallel those occurring during endochondral calcification of rat epiphyseal cartilage.

Levels of active metabolites of vitamin D3 in the callus of fracture repair in chicks

The levels of the active metabolites of vitamin D were measured in the callus and in the epiphyseal growth plate of chicks given radioactive cholecalciferol during fracture healing. Those levels were correlated with the histological findings. Three groups of chicks were studied: a control group with no fracture, chicks with fractures fixed by Kirschner wire, and chicks with unfixed fractures. A significant increase in the levels of the active metabolites was found in the callus during the first few days after fracture. The levels of 25-hydroxycholecalciferol [25(OH)D3] and of 24,25-dihydroxycholecalciferol [24,25(OH)2D3] were higher when there was no fixation, while those of 1,25-dihydroxycholecalciferol [1,25(OH)2D3] were higher after fixation. The concentrations of these metabolites in the proximal epiphysis of the tibia were similar to those found in the callus. Based on these findings it is suggested that the active metabolites of vitamin D are directly involved in the process of fracture repair.

Determination of callus quantity in 4-week-old fractures of the rat tibia

In clinical practice the quantity of fracture callus is usually estimated on the basis of radiographs. In experimental studies on fracture healing, more exact chemical measurements (total protein, DNA, and mineral contents) have been used to quantitate the amount of callus. It is not known, however, how these two parameters correlate with each other. In this study we used different (radiographic, histomorphometric, and chemical) methods to evaluate the quantity of fracture callus in 4-week-old tibial fractures of rats stabilized by intramedullary nailing. The wet and dry weights and the nitrogen, collagen, calcium, phosphorus, and DNA contents of the calluses showed no correlation with the radiographic size of the calluses. There was a strong correlation (p less than 0.001), however, between radiographic and histomorphometric callus sizes. The mass of callus and the chemically defined parameters of callus production thus indicate different properties of the healing bone.

Mineral parameters in early fracture repair

In an effort to define and characterize the initial mineralization product of fracture-healing, we studied the mineral components within a model of endochondral osseous repair. Fracture calluses from the tibiae of rats and rabbits undergoing endochondral fracture-healing were analyzed, in toto and following density fractionation, by physicochemical and crystallographic techniques. Significant changes in mineral composition, crystal size, and density occurred in the early phases of fracture repair. In the rat, two weeks after fracture, the calcium-to-phosphorus ratio was higher than that of the mineral component, possibly due to calcium-binding to some of the macromolecules known to be present. The earliest mineral was poorly crystallized hydroxyapatite with a high carbonate content. Crystal perfection improved rapidly and approached that of normal diaphyseal bone within eight weeks after endochondral fracture in both the rabbit and the rat.

[Effect of vitamin D2 and 1 alpha-hydroxycholecalciferol on bone tissue in rats with femoral fractures]

Formation of the callus at the site of a femoral fracture in rats without vitamin D deficiency is accompanied by an increase in the specific mass and mineralization of the unaffected pair bone, which indicates general intensification of the processes of mineralization occurring in the skeleton during fractures. Vitamin D deficiency leads to hypocalcemia, retardation of callus mineralization, and drastic demineralization of the intact thighbone thus pointing to pathological enhancement of skeleton resorption as the main source of calcium required for covering bodily requirements. Like vitamin D2 in a dose of 0.300 microgram, administration of 1 alpha-hydroxyvitamin D3 (1 alpha-HCD3) in a dose of 0.025 microgram daily to rats with femoral fractures kept on the vitamin D-deficient diet provides for effective calcium homeostasis maintenance and mineralization of the callus and unaffected bones. This demonstrates high biological activity of 1 alpha-HCD3 and its efficacy in promoting the processes of mineralization during fractures. The increase in the phosphorus content in the diet until the calcium/phosphorus ratio reaches 1:2 (instead of the optimal 1:1), aggravates hypocalcemia and sharply enhances demineralization of the intact bone in vitamin d-deficient rats with femoral fractures and reduces mineralization of the callus in rats given 1 alpha-HCD3. The data obtained indicate the necessity of reliable correction of potential vitamin D deficiency and optimization of the calcium-phosphorus ratio in the diet and preparations, as well as a possibility of applying 1 alpha-HCD3 in the combined treatment of fractures.

Comparative biochemical studies of the callus matrix in immobilized and non-immobilized fractures

Fracture healing in both non-immobilized and immobilized rat fracture callus, though different radiographically and histologically; is characterized by similar biochemical changes in the ground substance. Although the callus formed in the immobilized model is appreciably smaller than that formed when the fracture is not immobilized, the chemical composition of the organic matrix of the existing callus undergoes similar modifications regardless of fixation and histological response. Previous studies of endochondral fracture repair have demonstrated an elevation in fracture callus hexosamine content during the early stages of healing of the non-immobilized fracture. This corresponded histologically to a predominance of cartilaginous tissue in the callus matrix. In the present study, the biochemistry, histology, and roentgenographic features of stabilized and non-stabilized rat fractures are compared. Similar elevations in hexosamine are noted at two weeks in both models. These elevations are attributed to changes in proteoglycan rather than glycoprotein concentration of the tissue.

[Biochemical and immunological characteristics of serum proteins following bone injuries in rats]

The protein, lipo-and glycoprotein components of blood and callus were studied in 145 Wistar rats after fracture of the femur. The injury was associated with the development of hypoproteinemia and disparately oriented shifts in the content of serum fractions: decreasing of the content of albumin and gamma-globulin and elevation of the concentration of chlorine-soluble glucoproteins (seromucoid fraction) and alpha-globulins. The injury did not entail changes in the antigen composition of the serum. However, the content of 5 out of 41 antigens detected fell down significantly. Some of serum proteins were detectable in a 7-day callus.

Characteristic molar ratios of magnesium, carbon dioxide, calcium and phosphorus in the mineralizing fracture callus and predentine

From fracture callus in different stages of mineralization the contents of Mg, CO2, Ca and P were determined and compared with those found in predentine. It was found that the Mg- and CO2-contents are high in relation to Ca and P values during the prestages and early stages of mineralization. These relatively high Mg- and CO2-values are connected with high Mg/Ca, Mg/P, CO2/Ca and CO2/P ratios which strongly decrease with the increasing degree of mineralization. These results seem to be part of a general controlling mechanism of the calcification.

The effect of a constant direct current on the repair of an experimental osseous defect

To investigate the effect of a constant direct electrical current on the mineral deposition in an experimental osseous defect, 8 microamperes of current were applied across a circular osteotomy placed in the lateral cortex of an avian metatarsus. Mineral deposition was measured to quantitate bone formation in vivo using iodine-125 absorptiometry. The electrical current had no effect either on the initiation or the rate of mineral deposition. However, the quantity of mineralized callus was significantly greater in the treated than untreated bones.

[Crystallographic investigation of distraction callus (author's transl)]

Physical examination using electron diffraction and thermogravimetry was performed on the distraction callus of six metacarpal bones of sheep. Brushite was observed in this callus beside apatite, and the number of crystals found was less than in physiological nature bone. From the crystallographic point of view the distraction callus was found similar to the callus known in fracture healing.

Fracture healing in weight-bearing and nonweight-bearing bones

Animal models demonstrated the differences in the process of fracture healing in weight-bearing and nonweight-bearing bones. Canine rib fractures regained strength properties rapidly; in canine radial fractures the return to normal stiffness levels was primary. The structural differences in fracture healing are probably functional and due more to biologic reorganization than to any measurable change in the biochemical components of the callus.

[Concentration of lysozyme during mineralization in callous tissue of healing fractures (author's transl)]

Investigations have suggested that lysozyme (E.C. 3.2.1.17) is involved in bone mineralization. High concentrations of lysozyme is found in the growth plate near cartilage bone junction, where it is located at the collagen fibrils and in the ground substance. Quantitative studies of lysozyme levels were made in ossifying tissue of healing fractures, to confirm the existence of this relationship on bone repair. Callous tissue, serum samples and normal bone was collected from 42 rats at 15 intervalls during a 50 day healing period. Agar gel diffusion test was used for quantitation of lysozyme. Electrophoresis of tissue extract and standard henn egg white lysozyme served as control. Lysozyme levels in callous tissue increased significantly (4--5-fold) from 4.--21. day p. trauma and subsequently decreased. The concentration in serum samples did not change significantly. Changes in Ca concentration and histological studies during tests confirm a direct relationship between bone mineralization and lysozyme level changes.

Effect of local application of fluoride on healing of experimental bone fractures in rabbits

The effect of local application of low fluoride concentrations on the healing of experimental bone-radii fractures in 20 rabbits was examined. No external fixation was required after the osteotomy and gelfoam being used as a medium for fluoride application. Histologic mineral and mechanical strength measurements were carried out on the healing callus. There were no significant changes in the ashpercentage and the fluoride concentration of the ash between the experimental and control callus. A significantly higher percentage of ossified tissue was found in the fluoride treated callus. The breaking strength was found to be significantly higher in the bone fractures treated with fluoride, whereas there was no notable difference between the surface areas. It is postulated that the greater amount of ossified tissues, the differences in organization of the bone trabecules, and/or an improved mineral crystallinity may be responsible for this result.

Improved healing of experimental long bone fractures in rabbits by delayed internal fixation

Experiments in rabbits were conducted to test the clinical propositions that a) the risk of nonunion in certain long bone fractures can be sharply reduced by open reduction and internal fixation, and b) that there is an optimum critical period following trauma in which this procedure should be carried out to facilitate healing. Bilateral standard open fractures of rabbit radii were internally fixed by intramedullary K-wires immediately or at 5, 10, and 17 days after trauma. These bones were subjected to stress and histometric analyses at 4-56 days after the initial fractures. The observations suggested that the pace of fracture healing could be optimized by time-delay surgery. The most exuberant and strongest calluses developed when the bones were fixed 10 days after fracture, and the rate of healing was improved. Delay times of 5 or 17 days did not provide better healing than that achieved by an immediate operation. Callus quality, in terms of the proportion of fiber bone, lamellar bone, and cartilage, was similar in all groups. The tensile strengths of the bones during callus formation were highly correlated with the ratio of callus/cortical bone areas (p less than 0.01).