The musculoskeletal system translates ideas into action. Nerve impulses in the brain speed down neurologic conduits to create the finely tuned symphony of chemical reactions that translates into physical movements. These movements allow the body to negotiate our ever-changing environment. The skin, connective tissue, bones, and joints provide the scaffolding to interconnect the cells and provide a protective and nourishing environment for cellular function.

A solid understanding of the skin, connective tissue, bone, and joints, as well as a comprehensive knowledge of the interplay of tissues and joints above and below, provides the framework for building a concise history and physical examination for each patient. This first step guides further testing, imaging, and treatment. This framework also allows the practitioner the opportunity to minimize unnecessary medical costs and patient disability. These skills take years to master and, so far, have not been usurped by advances in technology. This framework is also the art by which the science of musculoskeletal medicine has advanced.

Connective Tissue

Connective tissue provides the tensile strength, elasticity, substance, and nutrients to the multitude of cells that make up the human body. One of the primary actions of connective tissue is to transmit forces for movement by reducing friction between structures and protecting the neurovascular bundles from injury. Connective tissue is made of collagen fibers and amorphous ground substance of extracellular matrix interspersed with cells (Fig. 6–1).

The extracellular matrix describes the noncellular component of tissues and comprises collagen fibers and amorphous ground substance. The extracellular matrix provides the fluid substrate and cellular scaffolding and initiates the cues for tissue adaptation.¹ Fibroblasts, macrophages, and mast cells exist within the extracellular matrix and are essential for maintaining homeostasis and tissue repair.²

Collagen is the primary structural element of connective tissue that includes bone, cartilage, fascia, tendons, and ligaments. It provides tissues with a great deal of tensile strength and inextensibility. Collagen makes up 30% of the total protein mass of the extracellular matrix.¹ Fibroblasts produce collagen, which organizes into fibrils, cords and sheets in line with the direction of force.¹ A special property of collagen is crimp, which allows for elongation of the collagen without damage, providing shock absorption.³ Collagen fibers eventually crosslink; the greater the crosslinking, the stronger the structure, allowing for the tremendous characteristic strength of collagen structures.⁴ The most common collagen is known as the mature, type I collagen, which is abundant in tissues that are subject to tensile forces. (Fig. 6–2) Immature type III collagen is the earliest collagen placed during tissue healing.

Elastin is found in the skin and some connective tissues. It gives tissue the ability to recoil from repeated stretch back to its original length. There is limited stretch capacity due to closely associated collagen fibrils,¹ similar to the ligamentum flavum of the spine (Fig. 6–3).

Amorphous ground substance, or proteoglycans (protein-carbohydrate structures), attract water to create a hydrated gel, which supports and facilitates the diffusion of materials between cells. This hydrated gel creates the specific compliance and mobility of individual tissues.¹

Skin

The epidermis, dermis, and superficial fascia form the skin. It is considered the largest organ of the body and facilitates mobility of the underlying tissues.

The epidermis is the outermost layer that protects the body from the harsh external environment, including dehydration and infectious organisms. This layer is made primarily of keratinocytes that mainly produce keratin.⁵

The dermis is the next layer below, which is primarily made up of collagen. The collagen, elastin, proteoglycans, water, and salt provide significant density within this layer to provide cushioning. Blood vessels, nerves, and other glands exist within this layer as well⁶ (Fig. 6–4).

The superficial fascia, also known as the hypodermis, lies beneath the dermis and is primarily responsible for storing fat. The superficial fascia connects to the overlying dermis and deep fascia via fibrous septa, which create a flexible, resilient structure that resists multiaxial forces.⁷

Bone

Two hundred and six bones compose the adult human skeleton. Bone is made up of a collagen matrix that provides areas for hydroxyapatite crystals to adhere, providing the bone with its characteristic strength. The small number of cells within the bone include osteoblasts, osteoclasts, and osteocytes. The diaphysis makes up the tubular column in a long bone. Next distally, the metaphysis is the location of the growth plate. Finally, the wider areas at the distal bone structures are called the epiphysis, which forms the articular surfaces of joints.⁸

With increasing age, bone increases in diameter. Osteoblasts deposit bone in the periosteum (appositional growth), while bone is removed by osteoclasts in the medullary cavity. Radial bone growth is characterized by concurrent growth at the periosteal surface and removal at the endosteal surface, forming a large medullary cavity (Fig. 6–5).

A fibrous membrane called the periosteum envelopes bone. It covers the entire bony surface except at the areas covered by articular cartilage. The periosteum contains a large number of nerves and blood vessels and is also the site of tendons and ligament attachment to bone.

Tendon and Muscle

Tendons provide tensile force transmission as well as storage and release of elastic energy with locomotion.⁹ Tendons are composed of dense parallel fibers, oriented in the direction of the force and can withstand greater tensile forces than ligaments.¹⁰ Absorption and release of elastic energy occur via the associated muscle bellies.⁹

Skeletal muscle is surrounded by epimysium, which is a dense connective tissue continuous with bone. Furthermore, muscle fibers are wrapped by a layer of connective tissue called the perimysium (Fig. 6–6).

There is minimal space for cells and vascular components interspersed between the collagen fibers of the tendons. The epitenon (surface of the tendon) contains cells and the vascular supply. The endotenon is a thin and loose connective tissue that facilitates the movement of adjacent fascicles and creates a conduit for blood vessels.¹⁰

Some tendons have a paratenon, which is a fibroelastic sleeve that permits free movement of tendons within tissue structures. The epitenon lies beneath the paratenon and is continuous with the endotenon. The paratenon of the Achilles tendon is commonly described, but the term is used inconsistently, leading to confusion.¹¹

The enthesis (osteotendinous and osteoligamentous structures) typically dissipate forces into adjacent structures such as ligaments and fascia.¹² Tendon entheses demonstrate similar stratification or zonular zones, as found within ligament anchor sites progressing from tendinous fiber to progressively increasing calcification. Bony nodules can exist within tendinous structures such as the sesamoid bones within the foot¹³ and fibrocartilaginous tendon attachment sites.¹⁴

Ligaments and Joint Capsules

The joint capsule and adjacent ligaments facilitate, restrain, and stabilize articular motion. The joint capsule is made up of an outer fibrous cuff of collagen oriented in a weave pattern, which limits the gliding of individual fibers. Thickened capsular ligaments create additional stability, such as the anterior cruciate ligament within the knee capsule. Ligaments of the joint capsule are made of 80% collagen with up to 5% elastic fibers.¹⁵ The inner synovial membrane of the joint capsule is made of a loose connective tissue membrane that covers the entire capsule except for the weight-bearing articular surfaces. The inner synovial membrane is also elastic to prevent the pinching of local tissue during joint movement. It contains synoviocytes and a capillary network, which produce synovial fluid to lubricate the ligaments and provide nutrition to the weight-bearing cartilage, labrum, and menisci. The flow of synovial fluid is aided by movement¹⁵ (Fig. 6–7).

Cartilage

Cartilage is an avascular weight-bearing connective tissue that is resistant to compression. It is composed of hyaline cartilage, elastic cartilage, and fibrocartilage. Hyaline cartilage is the weakest type of cartilage. It has a glassy appearance and is found in the articular weight-bearing surface as well as the nose, ribs, and trachea. Hyaline cartilage is made up of a gel-like matrix consisting of type II collagen fibers and amorphous ground substance. The proteoglycans of the amorphous ground substance trap and retain water, providing resilience against compression.¹⁶ Hyaline articular cartilage is typically 2 to 4 mm thick and is devoid of blood vessels and nerves.¹⁷ A balance between weight bearing and nonweight bearing is necessary for the diffusion of nutrients through the cartilage layers. Elastic cartilage is made up of resilient elastic fibers that retain their shape (e.g., external ear and larynx). Fibrocartilage is the strongest type of cartilage. The tremendous tensile strength is due to a significant amount of type I collagen alternating with hyaline cartilage. Fibrocartilage is located at the transition between hyaline cartilage and ligament and tendon fibers (e.g., labrum, meniscus, annulus fibrosis).

Clinical Reasoning

Patients most commonly complain of pain, stiffness, weakness, numbness, and tingling. The behavior associated with pain is one of the most common reasons for assessing the musculoskeletal system. Assessment is defined as a pinpoint clinical diagnosis that guides effective treatment. It is critical to have knowledge of functional anatomy, an understanding of pain behavior, and an appreciation of the psychological factors that may contribute to changes in the perception of pain. Musculoskeletal pain is often localized and referred from the ipsilateral side of the painful stimuli. Patients sometimes give only a vague description of their symptoms, making the diagnosis more difficult.

The greater the stimulus, the further the symptoms will refer distally within the dermatome. Proximal structures can refer pain and tenderness more distantly to the end of the dermatome. Hence, structures at the distal end of the dermatome are easier to localize.¹⁸ Although pain is precisely localizable on the skin, it is more difficult to identify in deeper structures, including pain conditions caused by visceral structures^18–20 (Fig. 6–8).