7 Ultrastructure of Bones and Joints
7.1 Bone
7.1.1 Subchondral Bone Plate
The subchondral bone plate underlies the articular cartilage and is composed of three to five layers of bone. The layers are bridged by trabecular struts and, in some cases, plates of bone, thus separating the layers with small spherical voids (▶Figs. 7.1 and 7.2). This arrangement is referred to as the subchondral multilaminar plate. The thickness of the subchondral multilaminar plate and the number of laminations increase in the zones of highest stress. Typically, in the center of the joint there are up to five laminations, while away from the center of the joint there are often two or three laminations.
Immediately below the subchondral bone plate is the metaphysis. This area consists of trabecular bone with a complex arrangement that takes the physiological loads from the articular surface and spreads them down to the metaphyseal and diaphyseal regions of the bone. The subchondral bone has spherical voids between the trabeculae; however, further away from the subchondral bone plate, the voids become ellipsoid.
The structure of the cortical and trabecular bone is determined by genetic factors and by Wolff’s law. As can be seen in ▶Fig. 7.3, thick struts of bone extend down from the volar aspect of the lunate facet to the volar cortex of the distal radius.
7.1.2 Corner Zones
There is a concentration of cancellous bone at the corner areas of the distal radial metaphysis. These are located at the dorsal, radiopalmar, and ulnopalmar corners and consist mainly of platelike trabeculae. On serial sectioning, an appreciation can be gained for this distribution (▶Fig. 7.4). We propose that these concentrations of trabeculae merge and reinforce the cortical pillars.
7.1.3 Physeal Scar
There is an axially oriented condensation of trabeculae spanning the entire width of the distal radius. This is the physeal scar, the remnant of the epiphyseal growth plate. The trabeculae of the transition zone merge with this layer distally and with the trabecular vault proximally (▶Fig. 7.5).
7.1.4 Trabecular Vault
The metaphyseal region of the distal radius has a relatively thin cortex and contains a concentration of cancellous bone arranged in a series of arches. We have called this structure the trabecular vault. The apex of the arch lies deep to the physeal scar and extends down toward the thicker cortex of the metaphyseal–diaphyseal junction (▶Figs. 7.5 and 7.6). This arrangement is very similar to that seen in the arches of a gothic cathedral.
The cancellous bone in this region consists principally of platelike trabeculae in a longitudinal arrangement. The plates of trabeculae are interconnected by smaller struts (or rods) of cancellous bone, features that have been previously described. 2
The trabecular vault underlying the lunate facet typically has its apex located eccentrically toward the volar lip of the distal radius to accommodate the volar projection of the lunate facet (▶Fig. 7.6). There is typically trabecular reinforcement of the volar aspect of the lunate facet (▶Fig. 7.3). The apex of the trabecular vault underlying the scaphoid facet is centrally rather than eccentrically located (▶Fig. 7.5).
7.1.5 Pillars
We identified the presence of three cortical thickenings representing the three pillars of the distal radius. These are located on the radiopalmar, ulnopalmar, and dorsal aspects of the metaphyseal–diaphyseal junction of the distal radius. It can be clearly seen that the relative density of the cortex is greater at the three pillars of the metaphyseal–diaphyseal junction. It is our belief that these pillars are the principal areas of force transmission from the articular surface, through the trabecular vault to the shaft of the radius.
7.1.6 Diaphysis
The endosteal surface of the metaphyseal–diaphyseal junction and the diaphysis are lined by longitudinally arranged ridges of bone (▶Fig. 7.7). Individual trabeculae of the trabecular vault can be seen to insert into the endosteal surface of the bone, contributing to the formation of these longitudinal ridges. The longitudinal ridges then continue into the diaphyseal region.
Cartilage
The articular cartilage of the joint is an important anatomical structure. It reduces friction and allows smooth, painless movement between joint surfaces. The chondrocyte is a highly specialized cell and is the only cell type found in cartilage. It is found within the matrix, which also consists of collagen, proteoglycans, and noncollagenous proteins. In hyaline cartilage, 95% of the collagen is type II collagen, which is very stable and has a half-life of 25 years. Collagen arcades are important for resisting tensile forces within the cartilage. The chondral matrix has a high water content (up to 80% of cartilage weight), which is important because the hydrostatic pressures within the cartilage can absorb stresses that are placed onto it. Proteoglycans are produced by the chondrocytes and are made up of glycosaminoglycans, of which chondroitin sulfate and keratin sulfate are the predominant types. Proteoglycans retain and regulate the water content in cartilage and so play an important role in the age-related degeneration of articular cartilage. 3 The chondrocytes within the articular cartilage have a low metabolic rate, and it is for this reason that they are able to survive only on the oxygenation from the synovial fluid. The articular surface is covered with synovial fluid. Due to the hydrophobic chains, a barrier layer is created that is important for minimizing friction at the articular surface. There are no blood vessels, lymphatic vessels, or nerves in cartilage. 4
The ability of cartilage to heal is limited by a lack of vascularity and undifferentiated cells that can migrate to, and participate in, the repair process. Superficial cartilage injuries have a very limited inflammatory response due the avascularity of the area and do not heal. Deeper injuries that penetrate the subchondral bone incite a classic inflammatory reaction, with cells entering the defect from the bone marrow space. 5 Mesenchymal cells then differentiate to form fibrocartilage and repair the defect. However, this type of cartilage is inferior to the native hyaline cartilage and is only half as thick.
Immobilization of a joint has a profound effect on cartilage. It has been shown in animal and human studies that there is significant cartilage thinning in response to immobilization and lack of weight bearing. 6 It has also been shown that cartilage’s biochemical and biomechanical properties do not return to normal levels even after long-term follow-up, although the clinical significance of this is uncertain. 7
Within the joint, structures such as the synovial fringes or plicae are common. These assist in distribution of the synovial fluid throughout the joint. Capsular tissues that take a load will tend to form fibrocartilage or even bone. Good examples of this are the patella, the volar plate with the sesamoids, and the dorsal plate of the proximal interphalangeal (PIP) joint.
Related posts:
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
