Musculoskeletal Disorders. Sean Gallagher
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in the middle (diaphysis, also known as shaft of the bone), and a transitional zone between them (metaphyses) (Figure 3.18). The epiphysis is the expanded end of the bone that is covered by articular cartilage. The metaphysis is the junctional region between the epiphysis and the diaphysis and includes the growth plate (physis). The diaphysis is the shaft of long bones and is located in the region between metaphyses. The growth plate is a zone of endochondral ossification (cartilage‐to‐bone conversion) that mediates growth in bone length in an actively growing cartilage‐to‐bone region (Yang, 2010).
Bone tissue can also be classified by texture, matrix arrangement, maturity, or developmental origin (Yang, 2010). There are two main subtypes of bone: cortical and trabecular bone. Both types are chemically identical, but differ in terms of their structure, arrangement, and cell density. Approximately 80% of bone is cortical bone, with the remainder as trabecular bone (Carter & Hayes, 1977).
Trabecular bone (also known as cancellous or spongy bone) has numerous cavities (Figure 3.18). It is found mainly at the ends of many long bones and in areas like the ears and nose (Cooper, Milgram, & Robinson, 1966). Individual trabeculae are extensively connected and are oriented along the lines of mechanical stress on the bone in question. Trabecular bone is more metabolically active than cortical bone because of its much larger surface area for remodeling.
Cortical bone is dense in texture with few or no cavities, although it does contain pores for blood vessels, for example (Figures 3.18 and 3.19). Cortical bone is typically seen as the hard outer shell that surrounds trabecular bone or the centrally located marrow cavity. It is surrounded externally and internally by a periosteum and endosteum, respectively (Yang, 2010). Cortical bone is organized into Haversian systems or osteons (Figure 3.19), in which the lamellae are concentrically organized around a vascular canal, the Haversian canal. The blood supply of cortical bone enters from the periosteum via Volkmann canals, which also connect Haversian canals with each other (Brooks, 1963).
Lamellar bone is mature bone in which collagen fibers are arranged in parallel. It is located in both trabecular bone and cortical bone, the latter concentrically organized around a vascular canal.
Figure 3.18 The organization of long bones. (a) Three‐dimensional micro computed tomographic image of a rat’s radius and ulna bones at the level of the wrist. (b) Staining a section of the radius with safranin O (Saf O) shows the location of cartilage in the epiphyseal plate (growth plate) and trabecular bone). (c) Hematoxylin and eosin staining showing the location of the growth plate proximal to the epiphysis, trabecular bone within the bone marrow region, and denser cortical bone on the outer edge of the bone. (d) Von Kossa staining of calcified trabecular and cortical bone.
Figure 3.19 Osteons in cortical bone. (a) The osteocytes canaliculi are visible in the osteon. (b) Complete osteons (complete circles) and partial osteons left from past remodeling events are shown.
Bone Function
Bones serve numerous functions in the human body. Bones provide strength and structural stability to the body and provide a means by which loads can be transferred from one part of the body to another. Biomechanically, they are critical structures in that they provide levers and points of attachment for tendons (driven by muscle contraction) that are essential in permitting movement of the human body. Bones also play an important structural protective role for important organs of the body (most notably as the skull around the brain and the rib cage around the heart and lungs).
However, there is much more to bone than its structural and protective roles. Bones themselves are living dynamic organs and are quite complex in nature. These organs are constantly changing and remodeling to adapt to the stresses imposed them. This attribute permits both restorative and/or adaptive repair and helps to prevent fracture development when damage accumulation is not excessively rapid. Bone remodeling and maintenance is a normal, homeostatic process mediated by chondrocytes, bone‐forming osteoblasts, bone‐resorbing osteoclasts, and the mechanosensing osteocytes. Bone remodeling is triggered by growth, mechanical loading (muscular and/or cyclical), injuries (microcracks or fractures), and various local or systemic cytokines, chemokines, and hormones. During active remodeling, bone matrix is resorbed and replaced where needed in response to the loading demands on the bone. Many studies have demonstrated an anabolic effect of loading on bones in case studies or animal models (Schriefer, Warden, Saxon, Robling, & Turner, 2005; Srinivasan et al., 2003; Warden, Fuchs, Castillo, Nelson, & Turner, 2007; Zhang, Sun, Turner, & Yokota, 2007). Other studies show that depending on the intensity, frequency, and form of loading, persistent intense bone loading can also result in long‐term damage, excessive resorption, and detrimental changes to the cartilage and bone [reviewed in (Barbe & Popoff, 2020)]. This topic will be explored further in Chapter 11.
Bone is also the site of a number of hematopoietic cells and blood cell formation—all essential to the body’s nutrition and protection against infection and damaged tissue. Bones are a major site for the storage of minerals, especially calcium and phosphorus. Lastly, bone plays a critical role in storing nutrients and lipids that serve as an energy reserve for the body.
Ligament
Ligaments are structures that hold bones together at joints (Figure 3.20). They are made up of dense regularly arranged connective tissues with a predominance of collagenous fibers arranged into bundles, and a small number of fibroblastic‐like or synoviocyte‐like cells. Ligaments can be either extrinsic ligaments located outside a synovial joint or strategic reinforcing thickenings of a joint capsule (intrinsic ligaments). These features are summarized in Table 3.6.
Ligament Structure
Ligaments have similarities yet differences from tendons in important respects. Ligaments have a similar hierarchical organization of collagen structures as tendons, although the collagen fibers tend to be more loosely packed in ligaments (the collagen fibrils are slightly less in volume fraction and organization than tendon). However, ligaments have a higher percentage of proteoglycan matrix than tendons. Another significant difference in structure between ligaments and tendons is the amount of elastin found in ligaments. Elastin comprises less than 4% of the dry weight of tendons; in contrast, typical ligaments exhibit 4–9% elastin as dry weight, while some highly extensible ligaments have over 70% elastin as dry weight. The elastin network in the ligament resides along and between the collagen fibers (Zitnay & Weiss, 2018).
Figure 3.20 Ligaments. (a) A diagram showing the ligament of the knee: lateral collateral ligament (LCL), medial collateral ligament (MCL), anterior and posterior cruciate ligaments (ACL and PCL), and several other tendons. Sensory receptors present in the ligaments are listed and include free nerve endings (pain endings), Ruffini endings (pressure