Bone cell biology: the regulation of development, structure, and function in the skeleton

Bone cells compose a population of cells of heterogeneous origin but restricted function with respect to matrix formation, mineralization, and resorption. The local, mesenchymal origin of the cells which form the skeleton contrasts with their extraskeletal, hemopoietic relatives under which bone resorption takes place. However, the functions of these two diverse populations are remarkably related and interdependent. Bone cell regulation, presently in its infancy, is a complicated cascade involving a plethora of local and systemic factors, including some components of the skeletal matrices and other organ systems. Thus, any understanding of bone cell regulation is a key ingredient in understanding not only the development, maintenance, and repair of the skeleton but also the prevention and treatment of skeletal disorders.

Defective pro alpha 2(I) collagen synthesis in a recessive mutation in mice: a model of human osteogenesis imperfecta

Osteogenesis imperfecta (OI) is a heritable disorder of connective tissue associated with fractures, osteopenia, and short stature. OI results from mutations affecting the pro alpha 1 or pro alpha 2 gene of type I collagen. We describe a strain of mice with a nonlethal recessively inherited mutation (oim) that results in phenotypic and biochemical features that simulate moderate to severe human OI. The phenotype of homozygous oim mice includes skeletal fractures, limb deformities, generalized osteopenia, and small body size. Their femurs are smaller and demonstrate marked cortical thinning and fewer medullary trabeculae than those of wild-type mice. Breeding studies show the mutation is inherited in most crosses as a single recessive gene on chromosome 6, near the murine Cola-2 gene. Biochemical analysis of skin and bone, as well as isolated dermal fibroblast cultures, demonstrate that alpha 1(I) homotrimeric collagen accumulates in these tissues and is secreted by fibroblasts. Short labeling studies in fibroblasts demonstrate an absence of pro alpha 2(I) collagen chains. Nucleotide sequencing of the cDNA encoding the COOH-propeptide reveals a G deletion at pro alpha 2(I) nucleotide 3983; this results in an alteration of the sequence of the last 48 amino acids. The oim mouse will facilitate the study of type I collagen-related skeletal disease.

Diverse roles of the tumor necrosis factor family member TRANCE in skeletal physiology revealed by TRANCE deficiency and partial rescue by a lymphocyte-expressed TRANCE transgene

Tumor necrosis factor-related, activation-induced cytokine (TRANCE), a tumor necrosis factor family member, mediates survival of dendritic cells in the immune system and is required for osteoclast differentiation and activation in the skeleton. We report the skeletal phenotype of TRANCE-deficient mice and its rescue by the TRANCE transgene specifically expressed in lymphocytes. TRANCE-deficient mice showed severe osteopetrosis, with no osteoclasts, marrow spaces, or tooth eruption, and exhibited profound growth retardation at several skeletal sites, including the limbs, skull, and vertebrae. These mice had marked chondrodysplasia, with thick, irregular growth plates and a relative increase in hypertrophic chondrocytes. Transgenic overexpression of TRANCE in lymphocytes of TRANCE-deficient mice rescued osteoclast development in two locations in growing long bones: excavation of marrow cavities permitting hematopoiesis in the marrow spaces, and remodeling of osteopetrotic woven bone in the shafts of long bones into histologically normal lamellar bone. However, osteoclasts in these mice failed to appear at the chondroosseous junction and the metaphyseal periosteum of long bones, nor were they present in tooth eruption pathways. These defects resulted in sclerotic metaphyses with persistence of club-shaped long bones and unerupted teeth, and the growth plate defects were largely unimproved by the TRANCE transgene. Thus, TRANCE-mediated regulation of the skeleton is complex, and impacts chondrocyte differentiation and osteoclast formation in a manner that likely requires local delivery of TRANCE.

Tooth eruption: evidence for the central role of the dental follicle

The roles of the gubernaculum dentis, root formation, tooth crown and dental follicle in prefunctional eruption of a mandibular premolar have been studied in nine beagle dogs by radiographic and histologic evaluations of the effects of surgical ablation or removal of these structures on tooth eruption. The dental follicle was the only one of these structures required for the coordinated enlargement of the eruption pathway and formation of bone in the base of the bony crypt, the radiographic and histologic hallmarks of tooth eruption. These data, together with the topographic relationships of the dental follicle to areas of localized bone resorption and formation, are interpreted to mean that the dental follicle may influence, if not coordinate, these processes in tooth eruption.

Oenology: red wine procyanidins and vascular health

Regular, moderate consumption of red wine is linked to a reduced risk of coronary heart disease and to lower overall mortality, but the relative contribution of wine's alcohol and polyphenol components to these effects is unclear. Here we identify procyanidins as the principal vasoactive polyphenols in red wine and show that they are present at higher concentrations in wines from areas of southwestern France and Sardinia, where traditional production methods ensure that these compounds are efficiently extracted during vinification. These regions also happen to be associated with increased longevity in the population.

Osteopetrosis, a new recessive skeletal mutation on chromosome 12 of the mouse

Osteopetrosis (op/op) is a new mutation in the mouse that is transmitted as an autosomal recessive linked with variant waddler (Va) on chromosome 12. Compared with normal littermates, young op/op mice have excessive accumulations of bone without marrow cavities, increases in bone matrix formation and concentrations of parafollicular cells of the thyroid, and are hypophosphatemic. Osteoclasts from op/op mice are small, few in number and have an abnormal cytoplasmic distribution of the lysosomal enzyme acid phosphatase. In contrast to the three other mutations that transmit osteopetrosis in mice, the skeletal signs of the disease slowly disappear in op/op animals after bone matrix formation declines about 6 weeks after birth from 145 percent to 20 percent of that in normal siblings. The main skeletal defect in op/op mice appears to be a severe restriction in bone remodeling that is capable of slowly removing the excessive skeletal mass characteristic of the disease only after bone formation has declined to one-fifth that of normal littermates.

Tooth eruption: theories and facts

The mechanisms of tooth eruption (i.e., the answer to the question of how and why teeth erupt) has been a matter of long historical debate. This review focuses on human and other mammalian teeth with a time- and spacewise limited period of eruption and analyzes recent observations and experimental data on dogs, rats, primates, and humans in a framework of basic biological parameters to formulate a guiding theory of tooth eruption. Acknowledging basic parameters (i.e., that teeth move in three-dimensional space, erupt with varying speed, and arrive at a functional position that in inheritable) eliminates a number of previously held theories and favors those that accommodate basic parameters, such as alveolar bone remodeling in association with root elongation, with possible correction factors in the form of cementum apposition and periodontal ligament formation. We have critically analyzed, summarized, and integrated recent findings associated with preeruptive movements of developing teeth, the intraosseous stage of premolar eruption in dogs, molar eruption in rodents, and premolar and molar eruption in primates. The variable speeds of eruption are particularly important. We conclude with basic principles of tooth eruption--that is, the type of signals generated by the dental follicle proper, the conditions under which teeth are moved and the clinical understanding to be derived from this knowledge.

Inflicted skeletal injury: a postmortem radiologic-histopathologic study in 31 infants

OBJECTIVE

The objective of this postmortem study was to use high-detail skeletal surveys, specimen radiography, and histopathologic analysis to determine the number, distribution, and age of inflicted skeletal injuries in infants studied at the University of Massachusetts Medical Center from 1984 to 1994.

MATERIALS AND METHODS

Thirty-one infants (average age, 3 months) who died with inflicted skeletal injuries were studied with high-detail skeletal surveys and specimen radiography and histopathologic analysis. The distribution and number of fractures was determined for each technique, and dating was performed on the basis of radiologic and histologic criteria. The skull fractures noted in 13 cases were excluded from the numerical analysis.

RESULTS

The radiologic-histopathologic correlation revealed 165 fractures involving the ribs in 84 (51%), long bones in 72 (44%), bones of the hands and feet in 6 (4%), clavicles in 2 (1%), and spine in 1 (< 1%). Of the 72 long bone fractures, the metaphyses were involved in 64 (89%, or 39% of the total), and the shaft was involved in 8 (11%, or 5% of the total). One hundred sixteen fractures were healing, 36 were acute, and 13 were of indeterminate age. In all but two infants, at least one healing fracture was present. Of fractures diagnosed histopathologically, specimen radiography increased the yield of fractures noted on skeletal survey from 58% to 92%.

CONCLUSION

Most infants who die with inflicted injury have fractures at multiple sites. Metaphyseal and rib fractures are much more common than long bone shaft injuries, the opposite of the pattern found in older children. Because most abused infants who die have evidence of healing fractures at the time of autopsy, aggressive radiologic efforts to identify these injuries in living as well as in decreased infants appear justified.

Experimental study in the dog of the non-active role of the tooth in the eruptive process

The role of the tooth in eruption was studied radiographically and histologically after experimental manipulations of the crowns of permanent mandibular premolars in dogs. Crowns were removed and dead crown shells or metal or silicone replicas were substituted into dental follicles just prior to scheduled eruption. These replacements erupted on schedule after formation of the usual eruption pathways and formation of trabecular bone from the base of the bony crypt. Removal of crowns, but without adding replacements, also exhibited these same hallmarks of eruption. We conclude that tooth eruption is a series of metabolic events in alveolar bone characterized by bone resorption and formation on opposite sides of the dental follicle and the tooth does not contribute to this process.

The metaphyseal lesion in abused infants: a radiologic-histopathologic study

The metaphyseal lesions in abused infants have highly distinctive radiologic characteristics. The so-called "bucket-handle" and "corner" fractures often form the basis for the diagnosis of abuse. However, despite the great familiarity with the radiologic appearances, no systematic histopathologic study of the metaphyseal lesions in abused infants has been carried out. An in-depth study of pre- and postmortem radiologic features combined with histologic analyses of the metaphyses from a group of four abused infants provides new insights into the nature of these peculiar lesions. The basic histologic alteration is a subepiphyseal planar series of microfractures through the most immature portion of metaphyseal bone. This fracture results in the isolation of a mineralized disc or portion of a disc that is identifiable radiographically. Depending upon the size of the injury, the degree of involvement of the periphery of the bone, and the radiographic projection, a bucket-handle lesion, corner fracture, or metaphyseal lucency will result. In some cases, the radiographs may be normal even though there is significant histologic alteration. Although cartilaginous injuries may play a role in infant abuse, none of the specimens examined in this study evidenced injury through the germinal layers of cartilage. On the basis of the findings described here, it is recommended that postmortem analysis of all radiographically suspicious metaphyses be carried out in cases of suspected infant homicide. If there are other clinical or pathologic indications to support abuse, a strong argument can be made for removal of radiologically normal but high-risk metaphyses. Instituting these procedures will require closer cooperation between the radiologist and the medical examiner, but the potential for reducing the number of infant homicides demands this enlightened approach.

The origin of osteoclasts: evidence, clinical implications and investigative challenges of an extra-skeletal source

Recent evidence for an extraskeletal origin of osteoclasts and the historical record of the genesis of osteoclasts are examined critically. Reviews of the structure, function and development of osteoclasts from mononuclear precursors, the local regulation of bone resorption and the coupling of bone formation to preceding resorption are presented as a background for discussing the clinical implications for management of osteolytic bone diseases. The roles of osteoclasts and macrophages as phagocytes are compared and contrasted, and recent evidence for macrophage heterogeneity resulting from site-specific monoblastic precursors is reviewed. The implications of these recent developments in macrophage biology are extrapolated to osteoclasts and the existence of site-specific, extraskeletal osteoclast precursors is proposed. Finally, the investigative challenges inherent in these perspectives are discussed.

The cytology of the dental follicle and adjacent alveolar bone during tooth eruption in the dog

Previous studies from our laboratories have shown that premolar eruption in dogs depends upon the presence of the dental follicle and is independent of root or crown growth or attachment to the oral epithelium. The present study is an analysis of the cellular composition of the dental follicle and the cellular investment of the adjacent walls of the bony crypt before and during eruption of the third and fourth mandibular permanent premolars in young beagle dogs. Four premolar follicles and their adjacent bony crypts were examined at 2-week intervals over 12 weeks before and during eruption of these teeth. Tissues were removed, fixed, processed, and oriented so that each follicle and the adjacent crypt wall could be reproducibly examined in vertical and horizontal planes. Mononuclear cells with abundant cytoplasm, euchromatic nuclei, and prominent nucleoli were present in juxtavascular location in the coronal part of the dental follicle; and these cells increased in number immediately preceding and during tooth eruption in parallel with an increase in osteoclasts on the adjacent crypt wall. These data are interpreted to mean that the coronal part of the dental follicle may coordinate the alveolar bone resorption required for tooth eruption by attracting and directing to the crypt wall a population of mononuclear cells, which either become osteoclasts and/or direct osteoclastic activity during tooth eruption.

Osteoclast biology: lessons from mammalian mutations

Major contributions to and confirmations of osteoclast biology have been made by experimental investigations of the osteopetrotic mutations in mammals. Congenital osteopetrosis is a bone disease characterized by a generalized increase in skeletal mass due to decreased osteoclast function. Abnormalities of skeletal growth and the failures of marrow cavity development and tooth eruption are secondary to reduced bone resorption of heterogeneous cause. Elucidation of pathogenetic pathways and unraveling of the cell biology of the osteoclast have proceeded hand-in-hand. This is illustrated by the variable differentiation and activation of osteoclasts among mutations and by demonstrations that the disease in certain animals and children can be cured by providing competent stem cells for osteoclasts via bone marrow transplantation. Congenital absence of carbonic anhydrase II (CA II) in children results in a syndrome that included osteopetrosis because osteoclasts are unable to function in the absence of CA II. The resistance of all mutations to the hypercalcemic effects of parathyroid hormone and recent reports of elevated blood levels of 1,25 dihydroxyvitamin D have broadened the scope of pathogenetic possibilities for osteopetrosis and regulatory possibilities for osteoclasts. Immunological effects including reductions in natural killer cell activity, superoxide and interleukin-2 production make osteopetrotic mutants potential models for studying the role of the immune system in osteoclast biology. Furthermore, coexistence of osteopetrosis with rickets and osteoblast abnormalities and the failure of cell transplants to cure the disease in some mutations illustrate the utility of the osteopetroses for exploring the role of matrix as mentor in osteoclast biology. Thus, understanding congenital osteopetrosis and osteoclast biology are likely to continue together.

Morphological evidence of reduced bone resorption in osteopetrotic (op) mice

Osteopetrosis, a metabolic bone disease in which a generalized accumulation of bone mass reduces or obliterates marrow cavities, is inherited as an autosomal recessive in several mammalian species. A recently discovered mutation in mice, the osteopetrosic (op) mutation, exhibits an elevation in bone matrix synthesis and a resistance to the hypercalcemic effects of exogenous parathyroid extract when young mutants are compared with normal littermates. This investigation examined the number, cytology, and ultrastructure of osteoclasts and the structure of bone surfaces in op mice in a morphologic assessment of bone resorption. Compared with normal littermates, op mice have a severe deficiency of osteoclasts, which also contain unusual toluidine blue-positive and electron-dense cytoplasmic inclusions and hypertrophy of clear zones and ruffled borders. Marrow spaces in op mice contained large numbers of megakaryocytes and large lipoid masses. Bone surfaces exhibiting evidence of resorption by scanning electron microscopy in normal littermates showed no evidence of resorption in op mice. Instead, these areas were characterized by morphologic features of bone formation. These data offer morphologic evidence of a reduction of bone resorption in this mutation. They are interpreted to mean 1) that op mice have a severe reduction in numbers or proliferative capacity of osteoclast precursors, which may be related to the cellular inclusions in the osteoclast population, and 2) that hypertrophy of the ruffled borders and clear zones of op osteoclasts is a compensatory attempt to increase bone resorption.

The hematogenous origin of osteoclasts: experimental evidence from osteopetrotic (microphthalmic) mice treated with spleen cells from beige mouse donors

The excessive skeletal mass and reduced bone resorption characteristic of osteopetrosis in microphthalmic (mi) mice can be corrected by irradiation and transfer of spleen cells from a normal littermate. Osteoclasts in beige (bg) mice, a mutation without osteopetrosis, have giant lysosomal granules. These two facts were exploited to trace osteoclast lineage. Microphthalmic mice treated with whole-body irradiation and spleen cells from a beige donor resorbed the excessive skeletal mass and recovered from osteopetrosis. Furthermore, osteoclasts in treated mi mice had giant lysosomal granules and resembled those found in bg donors when examined by light and transmission electron microscopy. These data provide direct evidence for a hematogenous origin of osteoclasts in mammals.

Rib fractures in 31 abused infants: postmortem radiologic-histopathologic study

PURPOSE

To examine the morphologic alterations of fractures of the lateral and anterior rib arcs and costochondral junction (CCJ) to better understand the factors that influence radiographic visualization and to gain insight into the mechanism of injury in rib fractures of abused infants.

MATERIALS AND METHODS

Thirty-one infants (average age, 3 months) who died with inflicted skeletal injuries were studied with high-detail, pre- or postmortem skeletal surveys, or both, and radiography of specimens, with histologic analysis. The distribution and number of fractures were determined for each technique, and dating was performed on the basis of radiographic and histologic criteria. The radiologic features were correlated with the pathologic findings in comparable histologic sections.

RESULTS

Of 165 fractures, 84 (51%) involved the ribs. Only 30 rib fractures (36%) were visible with skeletal survey examination. Lateral and anterior arc fractures tended to impact along the inner cortex of the rib. CCJ fractures tended to involve the inner aspect of the osteochondral interface with an associated osseous fragment.

CONCLUSION

Acute and healing rib fractures are common in infants who died with inflicted injury; detection is technique-dependent. Use of high-detail skeletal radiography to identify these injuries in live and deceased infants appears justified.

Radiologic contributions to the investigation and prosecution of cases of fatal infant abuse

In 1984 we started a two-year program in Worcester (Mass.) and Boston to provide additional radiologic data for the medical investigation of suspected fatal infant abuse. During that period the investigation of 12 cases of unexplained infant death included the review of complete radiographic skeletal surveys by a pediatric radiologist. Autopsies were supplemented with resection, high-detail radiography, and histologic study of all non-cranial sites of suspected osseous injury. Thirty-four bony injuries were noted, including 12 acute and 16 healing fractures of the long-bone metaphyses and posterior-rib arcs in patterns indicative of infant abuse. The investigations determined that there were eight cases of abuse, two accidental deaths, and two natural deaths (sudden infant death syndrome). At this writing, the radiologic and osseous histologic studies appear to have influenced the determination of the manner of death in six of the eight cases of abuse and the criminal prosecution in four of the five convictions. These findings suggest that a thorough postmortem radiologic evaluation followed by selected histologic studies can have an impact on the investigation and prosecution of cases of fatal infant abuse.

The role of three-dimensional information in health care and medical education: the implications for anatomy and dissection

The purposes of medical education can be summarized as learning how to take an effective history, perform a physical examination, and perform diagnostic and therapeutic procedures with minimal risk and maximal benefit to patients. Because patients are three-dimensional (3-D) objects, health care and medical education involve learning and applying 3-D information. The foundation begins in anatomy where students form and confirm or reform their own 3-D ideas and images of the development and structure of the human body at all levels of organization. Students go on to understand the interdependence of structure and function in health and disease. The basic questions for those teaching anatomy are "How do we learn and use 3-D information?" and "How is it taught most effectively?" These are not easy questions for teachers and are rarely asked by those who currently defend or reframe curricula. Unfortunately, there is little information on how we learn 3-D information and no evidence-based literature on the relative long-term vocational effectiveness of methods for teaching it. It is clear that we learn in several distinct modalities and that our students represent a spectrum of learning styles. To support the 3-D learning essential to both medical education and health care, anatomical societies need to provide answers to the following questions: Do the opportunities of dissection (visual, tactile, time, discovery, group process, mentoring) contribute to short- and long-term learning of 3-D information? If so, how? Does dissection offer significant advantages over other methods for learning, confirming, and using 3-D information in anatomy? Answers to these questions will provide a rational basis for decisions about curricular changes in anatomy courses (if, where, and when dissection should occur). This, in turn, will link these changes to society's ultimate purposes for medical education and health care rather than to the fiscal concerns of the businesses of health care and medical education, which is the current practice.

Human anatomy: a foundation for education about death and dying in medicine

In most medical schools, little curricular time is devoted to the art of medicine, and this is particularly evident with respect to death education. We make a case for including education on death and dying in medical schools, specifically its early introduction in the anatomy course. Studies indicate that whereas dissection of cadavers is an exciting discovery for most students, for many it is traumatic and if not addressed, students may use depersonalization and denial as their approach to suffering. The dissecting experiences in two different medical schools are described. The University of Massachusetts program developed in a traditional curriculum and explores humanistic issues with lectures and group discussions. Parallels are drawn between dissection and patient care, and coping styles are discussed openly. In the problem-based curriculum at Dalhousie Medical School, death and grief are discussed in the first week of medical school, and students are given information about the body donor program and support systems for students. This program is part of a longitudinal curriculum on death and dying. In both schools, students tour the dissecting rooms before the course begins and organize memorial events for body donors at the end of the academic year. These examples illustrate how death education can begin early in the medical curriculum and contribute to the development of practitioners who are sensitive to broader issues of human mortality.

Administration of colony stimulating factor-1 corrects some macrophage, dental, and skeletal defects in an osteopetrotic mutation (toothless, tl) in the rat

The toothless (tl/tl) mutation in the rat results in a paucity of osteoclasts and osteopetrosis that cannot be corrected by bone marrow transplantation. In the present study we demonstrate that tl/tl rats also have profound deficiencies of femoral, peritoneal, and pleural cavity macrophages. Furthermore, the macrophage colony stimulating activity of post-endotoxin sera from tl/tl rats is substantially reduced, suggesting that, as in the case of the op mutation in mice, the basis of the tl mutation is a deficiency of the macrophage growth factor, colony stimulating factor-1 (CSF-1). Consistent with this suggestion, treatment of tl/tl rats from birth for up to six weeks with CSF-1 reduced the osteopetrosis, increased body weight, and permitted tooth eruption. In addition, CSF-1 treatment induced large numbers of osteoclasts in tl/tl bones and macrophages in the peritoneal cavity and bone marrow. Persistence of metaphyseal sclerosis, however, indicated that the disease was not totally corrected by this treatment. These studies indicate that the basis of the tl mutation is most likely another CSF-1 deficiency, and further emphasize the role of this growth factor in osteoclast differentiation.

Osteopetrosis in the toothless (t1) rat: presence of osteoclasts but failure to respond to parathyroid extract or to be cured by infusion of spleen or bone marrow cells from normal littermates

Osteoclast have been observe for the first time in toothless (t1) rats, a mutation with inherits osteopetrosis as an autosomal recessive. The ability of t1 rats to raise the serum calcium concentration after injection of parathyroid extract was severely limited when compared with normal littermates. In addition, osteopetrosis in t1 rats is not cured by radiation and infusion of normal spleen or bone marrow cells from normal littermates, a method know to cure osteopetrosis in mutants of this and other species. This indirect evidence for a reduction in bone resorption as a cause of osteopetrosis in this mutation and the failure of transplanted cells to cure the disease are discussed in relation to the development and function of osteoclasts.

Ultrastructural features of the dental follicle associated with formation of the tooth eruption pathway in the dog

The dental follicle is a loose connective tissue layer that surrounds the developing and erupting tooth. The follicle is necessary for tooth eruption in dogs and specific cellular changes occur in the follicle at the onset of tooth eruption. In particular, within the coronal region of the follicle next to areas of subsequent bone resorption there is an increase in mononuclear cells which have the ultrastructure features of monocytes and contain specific granules characteristic of preosteoclasts. The follicle has an extensive microvasculature and monocytes are often seen adjacent to capillaries and venules. Monocytes increase in number in direct proportion to the increase in osteoclasts that form the eruption pathway and decrease in number as soon as this activity is completed. It is postulated that monocytes enter the follicle from the microvasculature and then migrate to the walls of the bony crypt to participate in the formation of the eruption pathway.

Blindness following bilateral radical neck dissection

Blindness after bilateral radical neck dissection is a rare complication. A recent patient, who suffered total blindness after simultaneous bilateral radical neck dissection, is the fifth case reported. It is, however, the first with pathological study of the optic tracts. Detailed microscopic examination revealed bilateral intraorbital hemorrhagic optic nerve infarction without evidence of embolization or ophthalmic artery occlusion. The probable etiology of this event is an episode of prolonged hypotension. An additional etiologic factor may be increased resistance to blood flow caused by venous hypertension, resulting from bilateral internal jugular vein ligation.

Factors affecting visualization of posterior rib fractures in abused infants

Rib fractures frequently are encountered in abused infants and commonly occur in the posterior rib arcs. Fractures occurring near the costovertebral articulations are rarely identified radiographically in the acute phase, and callus formation usually is the first indication of injury. To assess the factors influencing the visibility of fractures near the costovertebral articulations in abused infants, 103 posterior rib fractures occurring in 16 abused infants were studied radiologically. The plain radiologic studies were correlated with the pathologic findings in 15 ribs from four patients. The limited visibility of fractures relates to (1) the frequent superimposition of the transverse process over the rib fracture site, (2) a fracture line that crosses at an obliquity to the radiographic beam, and (3) nondisplacement of rib fragments due to preservation of the posterior periosteum. Fresh fractures invisible on a frontal projection are clearly defined when the rib is viewed axially with postmortem radiography. These findings explain the reported superior sensitivity of radionuclide bone scans vs radiography in the identification of fresh posterior rib fractures. A knowledge of the factors influencing the visibility of these important injuries is useful in planning an appropriate diagnostic evaluation in cases of suspected infant abuse.