Musculoskeletal Fundamentals: Bone & Cartilage

Bone and cartilage are two key elements of the musculoskeletal system and are fundamental components that provide structure and fluidity to the body and its ability to move. Contained within bone and cartilage is connective tissue. This connective tissue is surrounded by a collection of fibers which defines strength and flexibility within the tissue itself. Comprised within the connective tissue is a mixture of both elastic and collagen fibers. Elastic fibers are flexible and are exceptionally resilient and while collagen fibers are extremely strong they have almost no give so it’s important that there is a balance of both fibers in order to maintain proper symmetry within the body’s structural support system.

Bone, also known as osseous tissue, provides structural support to the human body and protects vital organs like the heart and lungs by encasing them in cage-like structures. Osteocytes, also referred to as mature bone cells, lie within a collection of collagen fibers which are very strong. Bones are classified into several categories; the long bones, like the leg and arm; short bones, located in the hands and feet which are used as levers to transmit force and for manual handling; flat bones, including the scapula and pelvis which are broad housings that protect underlying soft tissue in addition to aiding movement and irregular bones, which are found in the spine and skull.

Calcium, a mineral often associated with bone growth and development provides bones with extra rigidity and durability and is an essential nutrient during bone growth and developmental stages. During adolescence, most bones in the body will stop growing, but there are some, such as the lower jaw and nose, that will continue to grow throughout one’s life. Bone in general is considered a very vascular tissue, meaning it has an abundant blood supply. Under normal conditions bone has an excellent chance to repair and heal itself. 

Although most bone growth stops by early adulthood, bones are constantly undergoing a regenerative process whereby bone reabsorption or bone loss is balanced by new bone deposition. Essentially, bone is always under construction as it is constantly regenerating. Exercise and proper conditioning help aid daily bone regeneration.

When a bone is exposed to stress, it will speed up the regeneration process and respond to that stress by increasing the rate at which it produces or lays down a new layer of bone. When bones are exposed to vigorous exercise they will increase in density. Runners who put great demands on their feet have been shown to increase the density in the bones of their feet.

The increase in bone density is a response to stress, but doesn’t happen instantly. If the exercise and or training program is too rigorous, stressful or even sudden the regeneration process will not be able to keep up with your level of exertion which may cause a stress fracture. Continued training with a stress fracture can eventually develop into a full-fledged fracture.

Conversely, the regeneration process works in reverse in the presence of inactivity. Thus, a lack of stress to weight-bearing bones of the body can cause bone reabsorption to occur faster than bone deposition, resulting in weak or brittle bones. 

Stress and trauma are the main criteria for bones fractures. Stress fractures begin as tiny cracks along the surface of a bone and can be caused by overtraining. A common site of stress fractures is the tibia, or shin bone, which are located in the lower leg; but stress fractures can occur on any bone, including the body’s strongest bone, the femur or thigh.

Fractures caused by trauma or impact are either closed or open, meaning they are contained beneath the surface of the skin or pierce the skin open. An open fracture is known as a compound fracture. 

Cartilage is comprised of cells called chondrocytes. Chondrocytes are primarily embedded in collagen fibers which are composed of approximately 80% water. Cartilage has wonderful shock absorption properties and functions to resist compressive forces which occur during movement and activity like running, jumping, exercise and even walking. During youth the body is springy and resilient; however with age, the water content of cartilage decreases causing a corresponding decrease in the ability of cartilage to absorb shock. Compounding the problem, cartilage unlike bone, is avascular, meaning it doesn’t have its own blood supply and therefore does not heal well. Thus, cartilage gets its nutrition from nearby blood supplies serving adjacent bone. An injury to cartilage combined with aging can limit performance substantially.

There are 2 types of cartilage, hyaline cartilage and fibrocartilage. Hyaline cartilage also known as articular cartilage is the most abundant type cartilage in the human body. It lines the ends of bones that come together to form joints and absorbs shock to protect the bones. The foot and ankle are comprised of hyaline cartilage like other key joints, but are subject to the most impact during movement as shock moves up the body. This makes the toes, foot and ankle joint vulnerable to soreness, pain and degeneration. This vulnerability is well recognized and various products like insoles, orthotics, supports and other foot appliances have been designed to combat symptoms and problems stemming from poor shock absorption.   

Unfortunately, when hyaline cartilage wears away as in osteoarthritis, it can no longer act as a shock absorber and all of the shock is transmitted to underlying bone. Fibrocartilage, while similar to articular cartilage, is slightly more spongy. It is located in the areas of the body that need additional shock absorption capability like the knee joint and spine which are subject to added forces.

While cartilage’s regenerative capacity is low it does have some adaptive ability. When exercising the cells in cartilage increase in size, making them thicker and stronger. A 10 minute warm-up can cause cartilage to swell with protective fluids by as much as 15%, making daily exercise and activity an essential part of today’s lifestyle and an excellent way to help maintain and restore the body’s natural healing capacity.