To examine in detail the events that lead to healing under different mechanical conditions, experimental fractures of the rabbit tibia were immobilized with either a metal compression plate or a plastic plate, to produce stable or unstable mechanical conditions respectively. To fracture the bone, a small saw cut is made in the medial cortex and three-point pressure is then applied to fracture the remaining cortical bone. A small amount of bone is removed by the saw cut. This is of no consequence when a plastic plate is used because the plate is designed so that a small gap remains between the fractured surfaces. When a compression plate is used, a gap approximately 100 $\mu$ m wide remains under the plate. In addition, despite the use of compression, only part of the fractured surfaces are normally in contact, so other narrow fracture gaps may remain. The fractures were examined at experimental times from 4 days to 1 year by both light and electron microscopy. In stable mechanical conditions, a thin layer of periosteal callus starts to form within 4 days and healthy mesenchymal cells and capillaries are adjacent to the fracture. By 7 days the callus is fully developed and is covered by a new periosteum. Over the following weeks it becomes more compact as the cavities are filled by bone. The endosteal callus, if formed, is small. From 9 days onwards the fracture gaps of between 30 and 100 $\mu$ m are invaded by mesenchymal cells; macrophages remove the debris and fibroblasts lay down a collagenous matrix. Capillaries are numerous. By 2 weeks, these gaps are lined by osteoblasts producing transverse layers of new bone on the fractured surfaces. The gap is rapidly filled by transversely orientated bone to achieve union by 3-4 weeks. Resorption is limited to very small, isolated regions of the fractured surfaces. The gaps between 10 and 30 $\mu$ m wide appear to be too narrow for cells and capillaries to enter, and they are widened by osteoclastic action. Mesenchymal cells and capillaries then enter and bone is laid down on the surfaces of the newly excavated cavities. After union, the region is recognized as a wide band of irregularly organized bone. In compressed regions, areas of actual contact are very few, and narrow gaps can be seen. Union is achieved by direct Haversian remodelling across gaps up to 9 $\mu$ m wide. Remodelling starts at about 3 weeks and it is part of the normal process that occurs after union of a fractured cortex. The events in the fractures healing under unstable mechanical conditions diverge immediately. The initial periosteal reaction is similar, but cancellous bone forms only away from the fracture gap. Mesenchymal cells in a fibrous matrix, but with very few, if any, associated capillaries, persist as a thick layer over the fracture gap. At 6 days islands of cartilage begin to develop, and by 12 days the whole central area is filled by a cartilaginous matrix containing thick and thin collagen fibrils and elastic fibres. The cartilage is replaced by bone by 3-4 weeks. At this point, healthy cells and capillaries are found near the fracture gap. A small endosteal callus may form in a similar way. The fracture gaps are now invaded. The debris is removed by macrophages, and fibroblasts produce a collagenous matrix. Osteoclasts cover the fractured surfaces, but the amount of resorption is minimal. Capillaries also enter the gap. Within 1-2 weeks the gap is filled by transverse lamellae of bone. Once the fractured cortex is united, the events are similar in both series of fractures. Resorption of any callus starts. In the periosteal callus, large cavities form particularly along the callus-cortex junction. Over the next 3 months the callus is resorbed and the cortex is extensively remodelled. By 18 weeks no callus remains. The cortical bone is compact, but large cavities filled with fat cells may occur. The overall dimensions of the bone may be larger than that of the contralateral normal tibia, but the layer of cortical bone is thinner. Thus the plate, whether of metal or plastic, affects the structure of the bone. This study serves to emphasize that the mechanical stability of a healing fracture determines the composition and size of the periosteal callus and the time at which union across a fracture gap is achieved. The fracture fragments must be mechanically stable, so that there is no movement at the fracture gap before cells can enter and lay down new bone. This may be achieved immediately, though artificially, by a metal compression plate, or slowly, though naturally, by the formation of a large callus of cancellous bone. The movements and differentiation of the cells involved in healing are discussed. This experimental fracture model will form a basis for future experimental studies of healing fractures.
[1]
T. K. Hunt,et al.
Studies on inflammation and wound healing: angiogenesis and collagen synthesis stimulated in vivo by resident and activated wound macrophages.
,
1984,
Surgery.
[2]
G. Bagby,et al.
The effect of compression on the rate of fracture healing using a special plate.
,
1958,
American journal of surgery.
[3]
J. Greiff.
Bone healing in rabbits after compression osteosynthesis: a comparative study between the radiological and histological findings.
,
1979,
Injury.
[4]
E. Canalis,et al.
Effect of bone-derived growth factor on DNA, RNA, and proteoglycan synthesis in cultures of rabbit costal chondrocytes.
,
1982,
Metabolism: clinical and experimental.
[5]
J. Wray,et al.
Cellular proliferation in the formation of fracture callus in the rat tibia.
,
1973,
Clinical orthopaedics and related research.
[6]
S M Perren,et al.
A method for making reproducible experimental fractures of the rabbit tibia.
,
1982,
Injury.
[7]
C. ANDREW L. BASSETT,et al.
Influence of Oxygen Concentration and Mechanical Factors on Differentiation of Connective Tissues in vitro
,
1961,
Nature.
[8]
B. Pauli,et al.
10 – Vascularity of Cartilage*
,
1983
.
[9]
C A Bassett,et al.
Biologic significance of piezoelectricity
,
1967,
Calcified tissue research.
[10]
A. Caplan,et al.
The establishment of vascular-derived microenvironments in the developing chick wing.
,
1983,
Developmental biology.
[11]
G. W. Bernard,et al.
Osteoclast formation in vitro from bone marrow mononuclear cells in osteoclast-free bone.
,
1981,
The American journal of anatomy.
[12]
D. Beebe,et al.
Hyaluronate in vasculogenesis.
,
1983,
Science.
[13]
B. Mckibbin,et al.
The biology of fracture healing in long bones.
,
1978,
The Journal of bone and joint surgery. British volume.
[14]
R. Ross,et al.
A macrophage-dependent factor that stimulates the proliferation of fibroblasts in vitro.
,
1976,
The American journal of pathology.
[15]
C. R. Howlett,et al.
Formation of bone and cartilage by marrow stromal cells in diffusion chambers in vivo.
,
1980,
Clinical orthopaedics and related research.
[16]
E. J. Hargadon,et al.
Is conservative treatment of displaced tibial shaft fractures justified?
,
1984,
The Journal of bone and joint surgery. British volume.
[17]
A. Friedenstein.
Stromal mechanisms of bone marrow: cloning in vitro and retransplantation in vivo.
,
1980,
Haematology and blood transfusion.
[18]
R. Duthie,et al.
The histochemistry of the preosseous stage of bone repair studied by autoradiography; the effect of cortisone.
,
1955,
The Journal of bone and joint surgery. British volume.
[19]
J. Vacanti,et al.
Control of tumor growth in animals by infusion of an angiogenesis inhibitor.
,
1980,
Proceedings of the National Academy of Sciences of the United States of America.
[20]
F. Jotereau,et al.
The developmental relationship between osteocytes and osteoclasts: A study using the quail-chick nuclear marker in endochondral ossification☆
,
1978
.
[21]
B. Hall,et al.
The repair of fractured membrane bones in the newly hatched chick
,
1975,
The Anatomical record.
[22]
A. Ham,et al.
CHAPTER 10 – Repair and Transplantation of Bone
,
1971
.
[23]
A. Ham.
A HISTOLOGICAL STUDY OF THE EARLY PHASES OF BONE REPAIR
,
1930
.
[24]
G E Smith,et al.
THE MOST ANCIENT SPLINTS
,
1908,
British medical journal.
[25]
S. Olerud,et al.
Fractures of the tibial shaft; a critical evaluation of treatment alternatives.
,
1974,
Clinical orthopaedics and related research.
[26]
R. Ross,et al.
The role of the macrophage in wound repair. A study with hydrocortisone and antimacrophage serum.
,
1975,
The American journal of pathology.