X-ray anatomy of the skeleton

The skeleton goes through a complex development path. It begins with the formation of the connective tissue skeleton. From the second month of intrauterine life, the latter is gradually transformed into a cartilaginous skeleton (only the cranial vault, facial bones and clavicle bodies do not go through the cartilaginous stage). Then a long transition from the cartilaginous to the bone skeleton occurs, which is completed on average by the age of 25. The process of ossification of the skeleton is well documented with the help of X-rays.

In a newborn, most bones do not yet have ossification centers at their ends and are made of cartilage, so the epiphyses are not visible on radiographs and the radiographic joint spaces appear unusually wide. In subsequent years, ossification centers appear in all epiphyses and apophyses. Fusion of the epiphyses with metaphyses and apophyses with diaphyses (so-called synostosis) occurs in a certain chronological order and, as a rule, is relatively symmetrical on both sides.

Analysis of the formation of ossification centers and the timing of synostosis is of great importance in radiation diagnostics. The process of osteogenesis can be disrupted for one reason or another, and then congenital or acquired anomalies in the development of the entire skeleton, individual anatomical areas, or an individual bone occur.

Using radiological methods, various forms of skeletal ossification disorders can be identified: asymmetry in the appearance of ossification points.

Among the wide variety of bones (humans have more than 200 of them), it is customary to distinguish tubular (long: humerus, forearm bones, femur, shin bones; short: clavicles, phalanges, metacarpal and metatarsal bones), spongy (long: ribs, sternum; short: vertebrae, carpal bones, metatarsus and sesamoid bones), flat (bones of the skull, pelvis, scapula) and mixed (bones of the base of the skull) bones.

The position, shape and size of all bones are clearly reflected in radiographs. Since X-rays are absorbed mainly by mineral salts, the images show mainly dense parts of the bone, i.e. bone beams and trabeculae. Soft tissues - periosteum, endosteum, bone marrow, vessels and nerves, cartilage, synovial fluid - do not give a structural X-ray image under physiological conditions, as well as the fascia and muscles surrounding the bone. All these formations are partially distinguished on sonograms, computer and especially magnetic resonance tomograms.

The bone trabeculae of the spongy substance consist of a large number of closely adjacent bone plates that form a dense network resembling a sponge, which is the basis for the name of this type of bone structure - spongy. In the cortex, the bone plates are located very densely. The metaphyses and epiphyses consist mainly of spongy substance. It gives a special bone pattern on the radiograph, composed of intertwined bone trabeculae. These bone trabeculae and trabeculae are located in the form of curved plates connected by transverse crossbars, or have the form of tubes that form a cellular structure. The ratio of bone trabeculae and trabeculae to bone marrow spaces determines the bone structure. On the one hand, it is determined by genetic factors, and on the other hand, throughout a person's life it depends on the nature of the functional load and is largely determined by living conditions, work, and sports activities. On radiographs of tubular bones, diaphyses, metaphyses, epiphyses and apophyses are distinguished. The diaphysis is the body of the bone. The medullary canal is distinguished throughout its entire length. It is surrounded by compact bone substance, which causes an intense uniform shadow along the edges of the bone - its cortical layer, which gradually becomes thinner towards the metaphyses. The outer contour of the cortical layer is sharp and distinct, in places where ligaments and muscle tendons are attached it is uneven.

An apophysis is a protrusion of bone near the epiphysis that has an independent ossification nucleus; it serves as the site of origin or attachment of muscles. Articular cartilage does not cast a shadow on radiographs. As a result, a light band called the X-ray joint space is determined between the epiphyses, i.e. between the articular head of one bone and the glenoid cavity of the other bone.

The X-ray image of flat bones differs significantly from the picture of long and short tubular bones. In the cranial vault, the spongy substance (diploic layer) is well differentiated, bordered by thin and dense outer and inner plates. In the pelvic bones, the structure of the spongy substance is distinguished, covered at the edges by a fairly pronounced cortical layer. Mixed bones in the X-ray image have different shapes, which can be correctly assessed by taking pictures in different projections.

A special feature of CT is the image of bones and joints in axial projection. In addition, computer tomograms reflect not only bones, but also soft tissues; it is possible to judge the position, volume and density of muscles, tendons, ligaments, the presence of pus accumulations, tumor growths, etc. in soft tissues.

An extremely effective method of examining the muscles and ligamentous apparatus of the extremities is sonography. Tendon ruptures, lesions of their cuffs, effusion in the joint, proliferative changes in the synovial membrane and synovial cysts, abscesses and hematomas in soft tissues - this is far from a complete list of pathological conditions detected by ultrasound examination.

Radionuclide visualization of the skeleton deserves special attention. It is performed by intravenous administration of technetium-labeled phosphate compounds (99mTc-pyrophosphate, 99mTc-diphosphonate, etc.). The intensity and rate of RFP incorporation into bone tissue depend on two main factors - the amount of blood flow and the intensity of metabolic processes in the bone. Both an increase and a decrease in blood circulation and metabolism inevitably affect the level of RFP incorporation into bone tissue, and therefore are reflected in scintigrams.

If it is necessary to conduct a study of the vascular component, a three-stage method is used. At the 1st minute after the intravenous injection of the radiopharmaceutical, the arterial circulation phase is recorded in the computer memory, and from the 2nd to the 4th minute, a dynamic series of the "blood pool" follows. This is the phase of general vascularization. After 3 hours, a scintigram is produced, which is a "metabolic" image of the skeleton.

In a healthy person, the radiopharmaceutical accumulates relatively evenly and symmetrically in the skeleton. Its concentration is higher in the growth zones of bones and the area of articular surfaces. In addition, the shadow of the kidneys and bladder appears on scintigrams, since about 50% of the radiopharmaceutical is excreted in the same period through the urinary tract. A decrease in the concentration of the radiopharmaceutical in bones is observed in case of skeletal development anomalies and metabolic disorders. Individual areas of weak accumulation ("cold" foci) are found in the area of bone infarctions and aseptic necrosis of bone tissue.

Local increase in the concentration of radiopharmaceuticals in the bone ("hot" foci) is observed in a number of pathological processes - fractures, osteomyelitis, arthritis, tumors, but without taking into account the anamnesis and clinical picture of the disease, it is usually impossible to decipher the nature of the "hot" focus. Thus, the osteoscintigraphy technique is characterized by high sensitivity, but low specificity.

In conclusion, it should be noted that in recent years, radiation methods have been widely used as a component of interventional procedures. These include bone and joint biopsy, including biopsy of intervertebral discs, sacroiliac joint, peripheral bones, synovial membranes, periarticular soft tissues, as well as injections of medicinal preparations into joints, bone cysts, hemangiomas, aspiration of calcification from mucous bags, embolization of vessels in primary and metastatic bone tumors.