Fracture healing in weight-bearing and nonweight-bearing bones

An overview of de novo bone generation in animal models

Some of the earliest success in de novo tissue generation was in bone tissue, and advances, facilitated by the use of endogenous and exogenous progenitor cells, continue unabated. The concept of one health promotes shared discoveries among medical disciplines to overcome health challenges that afflict numerous species. Carefully selected animal models are vital to development and translation of targeted therapies that improve the health and well-being of humans and animals alike. While inherent differences among species limit direct translation of scientific knowledge between them, rapid progress in ex vivo and in vivo de novo tissue generation is propelling revolutionary innovation to reality among all musculoskeletal specialties. This review contains a comparison of bone deposition among species and descriptions of animal models of bone restoration designed to replicate a multitude of bone injuries and pathology, including impaired osteogenic capacity.

Loss of mechanosensitive sclerostin may accelerate cranial bone growth and regeneration

OBJECTIVE

Cranial defects can result from trauma, infection, congenital malformations, and iatrogenic causes and represent a surgical challenge. The current standard of care is cranioplasty, with either autologous or allogeneic material. In either case, the intrinsic vascularity of the surrounding tissues allows for bone healing. The objective of this study was to determine if mechanotransductive gene manipulation would yield non-weight-bearing bone regeneration in a critical size calvarial defect in mice.

METHODS

A mouse model of Sost deletion in Sost knockout (KO) mice was created in which the osteocytes do not express sclerostin. A critical size calvarial defect (4 mm in diameter) was surgically created in the parietal bone in 8-week-old wild-type (n = 8) and Sost KO (n = 8) male mice. The defects were left undisturbed (no implant or scaffold) to simulate a traumatic calvariectomy model. Eight weeks later, the animals were examined at necropsy by planimetry, histological analysis of new bone growth, and micro-CT scanning of bone thickness.

RESULTS

Defects created in wild-type mice did not fill with bone over the study period of 2 months. Genetic downregulation of sclerostin yielded animals that were able to regenerate 40% of the initial critical size defect area 8 weeks after surgery. A thin layer of bone covered a significant portion of the original defect in all Sost KO animals. A statistically significant increase in bone volume (p < 0.05) was measured in Sost KO mice using radiodensitometric analysis. Immunohistochemical analysis also confirmed that this bone regeneration occurred through the Wnt pathway and originated from the edge of the defect; BMP signaling did not appear to be affected by sclerostin.

CONCLUSIONS

Mechanical loading is an important mechanism of bone formation in the cranial skeleton and is poorly understood. This is partially due to the fact that it is difficult to load bone in the craniomaxillofacial skeleton. This study suggests that modulation of the Wnt pathway, as is able to be done with monoclonal antibodies, is a potentially efficacious method for bone regeneration that requires further study.

Measurement of mechanical properties on gap healing in a rabbit osteotomy model until the remodeling stage

BACKGROUND

The most important issue in the assessment of fracture healing is to acquire information about the restoration of the mechanical integrity of bone. Many researchers have attempted to monitor stiffness either directly or indirectly for the purpose of assessing strength, as strength has been impossible to assess directly in clinical practice. The purpose of this study was thus to determine the relationship between bending stiffness and strength using mechanical testing at different times during the healing process.

METHODS

Unilateral, transverse, mid-tibial osteotomies with a 2-mm gap were performed in 28 rabbits. The osteotomy site was stabilized using a double-bar external fixator. The animals were divided into four groups (n=7/group/time point; 4, 6, 8 and 12 weeks). A series of images from micro-computed tomography of the gap was evaluated to detect the stage of fracture healing and a 4-point bending test was performed to measure stiffness and strength. Relative stiffness and strength values were also acquired from calculation of ratios relative to those of the non-osteotomized contralateral bones.

FINDINGS

Formation of cortex and medullary canal at the gap was seen in the 12-week group and would represent the remodeling stage. In addition, the relationship between stiffness and strength remained almost linear until at least 12 weeks. However, stiffness recovered much more rapidly than strength.

INTERPRETATION

Strength was not fully restored until the later stages of fracture healing. However, the current study demonstrated that stiffness could be monitored as a surrogate marker of strength until at least the remodeling stage.

Bone repair in mandibular body osteotomy after using 2.0 miniplate system--histological and histometric analysis in dogs

The objective of this study was to evaluate the bone repair along a mandibular body osteotomy after using a 2.0 miniplate system. Nine adult mongrel dogs were subjected to unilateral continuous defect through an osteotomy between the mandibular 3rd and 4th premolars. Two four-hole miniplates were placed in accordance with the Arbeitgeimeinschaft für Osteosynthesefragen Manual. Miniplates adapted to the alveolar processes were fixed monocortically with 6.0-mm-length titanium alloy self-tapping screws, whereas miniplates placed near the mandible bases were fixed bicortically. At 2, 6 and 12 weeks, three dogs were sacrificed per period, and the osteotomy sites were removed, divided into three thirds (Tension Third, TT; Intermediary Third, IT; Compression Third, CT) and prepared for conventional and polarized light microscopy. At 6 weeks, while the CT repaired faster and showed bone union by woven bone formation, the TT and IT exhibited a ligament-like fibrous connective tissue inserted in, and connecting, newly formed woven bone overlying the parent lamellar bone edges. At 12 weeks, bone repair took place at all thirds. Histometrically, proportions of newly formed bone did not alter at TT, IT and CT, whereas significantly enhanced bone formation was observed for the 12-week group, irrespective of the third. The results demonstrated that although the method used to stabilize the mandibular osteotomy allowed bone repair to occur, differences in the dynamics of bone healing may take place along the osteotomy site, depending on the action of tension and compression forces generated by masticatory muscles.

Healing of dentate or edentulous mandibular fractures treated with rigid or semirigid plate fixation--an experimental study in dogs

The right mandibles of 40 dogs were fractured with a bending clamp, at a site being either dentate or edentulous, and treated with compression-plates or miniplates. After 1, 2, 4, 6, 8, 12 and 24 weeks the mandibles were examined by microradiography and histology. Some variation in the healing mode within the groups was observed, interindividual as well as intra-individual in different layers of the same mandible. Generally, healing displayed slightly more time-related progression with compression plates compared with miniplates and similarly in dentate fracture sites when compared with edentulous ones. The so-called indirect healing mode through cartilaginous callus was found to take place in the group with miniplates and edentulous fracture sites, whereas in the three other groups a type of direct bone healing through woven bone in the presence of more or less callus could be observed. The conclusions drawn from these findings are: that compression plates as well as miniplates are suitable for treatment of mandibular fractures in dogs, and the presence of teeth appears to exert a stabilizing effect on the fracture.

Fracture stiffness measurement in the assessment and management of tibial fractures

Fracture stiffness results obtained from patients with tibial fractures treated with external skeletal fixation are reviewed. A threshold value of bending fracture stiffness at which removal of the external fixator and functional loading of the fracture can be prescribed is proposed to be 15 N m degree(-1). The logarithm of the bending stiffness was found to give a linear relationship with time post-fracture, thus implying that stiffness increases exponentially with time. From three previous studies of mechanical measurements of fracture healing, the fracture stiffnesses at which independent weight-bearing was permitted were determined and were found to be comparable with the threshold value of stiffness proposed in this paper. A review of previously published data for the in-vivo stiffness of intact tibiae showed that the stiffness required for functional healing of a tibial fracture is between 17 and 25% of that of the intact tibia.

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.

Role of mechanical loading in the progressive ossification of a fracture callus

The progressive ossification pattern in a fracture callus was predicted based on a theory that relates the local stimulus for ossification to the tissue mechanical loading history. Two-dimensional finite element analyses of a fracture callus were considered at three different stages of ossification. The sites of callus ossification represented in the initial model were predicted by previous analyses relating mechanical stress and vascularity to the differentiation of mesenchymal tissue in the early callus. The zones of further ossification, bone bridging, and bone consolidation predicted in the present study were found to be similar to the ossification patterns that have been documented by other researchers. The approach used to predict fracture healing is identical to that of previous studies predicting joint morphogenesis, with the exception that fracture healing requires continuous, attached skeletal elements, whereas joint morphogenesis requires discontinuous, articulating skeletal elements.

Correlations between mechanical stress history and tissue differentiation in initial fracture healing

A general theory for the role of intermittently imposed stresses in the differentiation of mesenchymal tissue is presented and then applied to the process of fracture healing. Two-dimensional finite element models of a healing osteotomy in a long bone were generated and the stress distributions were calculated throughout the early callus tissue under various loading conditions. These calculations were used in formulating theoretical predictions of tissue differentiation that were consistent with the biochemical and morphological observations of previous investigators. The results suggest that intermittent hydrostatic (dilatational) stresses may play an important role in influencing revascularization and tissue differentiation and determining the morphological patterns of initial fracture healing.

Vascular reorganization and return of rigidity in fracture healing

The present study addresses the relationship between vascular reorganization and the biomechanical changes occurring during canine long bone fracture healing. A middiaphyseal osteotomy of the radius was performed, and angiographic studies were carried out using India ink injections from 2 to 12 weeks postosteotomy. The radii were harvested, subjected to nondestructive four-point bend testing on an Instron, and then sectioned and cleared according to the Spaltholtz technique. The observed microangiographic patterns were correlated with the calculated rigidity values. Quantitative bone blood flow measurements around the osteotomy were performed on a different group of animals, using the radiotracer microsphere technique, at 3, 6, and 9 weeks postosteotomy to correlate with our qualitative microangiographic results. We demonstrated a direct and predictable relationship between the phase of vascular reorganization and the rigidity. This correlation was present even in our delayed union specimens.

Weight bearing: its lack of effect on fracture healing

The load transmitted through fresh fractures of the tarsometatarsus of adult chickens was varied by 40% more, or less, than the body weight to determine the effect of load-bearing on fracture healing. The healing fracture was examined for size and strength at 14 days, but no significant differences were detected in any group. The technique was sensitive enough to detect the differences between healing fractures at 7, 10 and 14 days. In this experiment load transmission was not found to be an important factor in fracture healing.

The biomechanical behavior of healing canine radii and ribs

An experimental study and accompanying theoretical study of the biomechanics of healing canine ribs and radii have been conducted. Biomechanical behavior of the healing bones was determined experimentally by whole bone structural tests in four point bending. The experimental results indicated that the non-weight-bearing ribs regained bending stiffness more rapidly than bending stiffness, while the weight-bearing radii regained bending stiffness more rapidly than bending strength. These differences were not found to correlate with differences in overall callus biochemistry, but physical examinations of the bone suggested that differences in callus morphological details between the two bone types might offer an explanation. Utilizing a theoretical model, a parameter study of the influence of callus geometry and morphology on bending behavior was conducted. The results of the parameter study demonstrate that the experimentally observed variations in the amount and distribution of callus materials and in bone geometry can produce the qualitative biomechanical differences measured experimentally. The results of the parameter study also demonstrate that bending stiffness must be used with caution as a healing criterion since it does not necessarily reflect structural strength.