Ashley Davidoff MD Greogory Waryasz MD

The Common Vein Copyright 2011

Introduction

Bone is a specialised form of connective tissue that serves both as a connective tissue as well as an organ.

As a connective tissue it is composed of the three basic elements that comprise all connective tissues; cells, fibers and an extracellular matrix. In this realm its function is to connect body tissues with each other, provide a framework for the body, protect the organs, and store energy.

It is also considered an organ since it is made up of a collection of tissues joined together in a structural unit to perform a function.  These functions include hematopoeietic function, protection, mobility, and mineral homeostasis.  It stores minerals such as calcium and phosphorus and participates in acid base metabolism by absorbing or releasing alkaline salts to maintain and sustain a stable PH environment.

Its dominant structural characteristics are that it is hard and lightweight.  Its character has been described as being similar to a tree trunk – strong and sponge like.

The cellular component is made of osteoblasts, (generate bone) osteoclasts (resorb bone), and osteocytes (bone-maintaining cells).  Osteocytes are inactive osteoblasts trapped in the extracellular matrix.

The extracellular matrix is responsible for the strength of the bone.  It contains organic elements and minerals. The organic component is made of proteins mostly of type I collagen providing resilience and tensile strength. The mineral component is composed of calcium, phosphate, and hydroxyl ions.  Together the minerals form a compound called hydroxyapatite (Ca5(PO4)3(OH)). The hydroxyapatite provides hardness, strength and rigidity for the bone.

The mineral (hydroxyapatite) represents 70%, the proteins (type 1 collagen) 22% and water 8% by weight

The mineral component resists compression but has poor ability to withstand tensile loads.  In essence it is hard but brittle and acts like glass when subjected to a force – ie it cracks. In contrast the collagen arranged at oblique angles in the lamellae affords tensile strength.  This provides a rubbery and to some degree elastic nature. Overall however bone acts more like glass than like rubber.

Stiffness of bone therefore depends on mineral content while toughness depends on collagen (Augat).  The combination of the hydroxyapatite and collagen provides bone with a high compressive strength, but poor tensile strength and very low shear stress strength.  This infers that it resists pushing forces but not pulling or torsional forces.  

There are 206 bones in the adult human body grouped as being either part of the axial skeleton (80) or the appendicular skeleton (126). The component bones of the skeleton include the following:

skull (28):

vertebral column (26),

hyoid bone, (1)

sternum and ribs (25),

upper extremities (64),

lower extremities (62)

Of the sesamoid bones only the patellae are included in the above evaluation, but the smaller sesamoid bones are not

Compact Bone and Spongy Bone

There are two types of bone tissue; compact bone and spongy bone.  They are very different entities and have different functions bur are interdependent and collaborative. Compact bone is hard and dense while spongy bone is softer and porous.

Compact bone (aka cortical bone) is the hard outer shell of bone.  A cortical layer surrounds all bone.  The shafts of bone have thicker layers of compact bone.  It is called cortical bone  because it surrounds the bone as a hard external frame and cover.  The word cortex derives from the Latin word “cortex” which means bark of a tree, and it is therefore used in describing an outer shell or husk of a structure.  The cortex of the kidney is the outer part of the kidney for example.

Structurally compact bone is hard strong and stiff and represents 80% of the weight of the skeletal system.  The long bones contain most of the compact bone in the body. Compact bone forms an intricate system around vessels known as Haversian systems or osteons.  Volkmann’s canals help to connect the Haversian systems. At a microscopic level the osteon is the unit (brick equivalent) of compact bone. It has a lamellar structure, which implies that it is made of thin disc like layers of tissue.

Functionally compact bone facilitates support of the body in general, provides anchorage for muscles and ligaments, functions as lever for movement, stores calcium and other chemicals that are used in neural transmission and muscles contraction.

 Compact bone and spongy bone are so diverse in structure and function that they should really be considered two distinct entities.  For example when their turnover/year is considered compact bone has about a 2% change whereas spongy bone has a turnover of 25%.

Osteon – Haversian System

The osteon (aka Haversian system) is the morphological unit that makes up compact bone.

Structurally it is described as a cylindrical structure that is made up from plates of tissue called lamellae.  The osteon has a central canal that contains the lifelines for the bone including the arteries veins and nerves. The osteon measures about 200 micrometers in diameter (.2mm) and may be up to 10mms tall. A group of osteons form a series of columns along the long axis of bone.

The word lamella is the diminutive of the Latin word lamina which means “thin plate”, and lamella therefore means a small thin plate. The lamellae consist of concentric layers of collagen, bone cells and the hydroxyapatite matrix.  Most of the lamellae are arranged around the central canal as plates of tissue shaped like thick old fashioned records.  Each lamella is between 3 and 7 µm in width, and each osteon has between 4 and 20 lamellae. (Wojnar)

The collagen in the lamellae is organized so that for a given lamella the collagen project in one direction but the next layer has the collagen vector in the opposite direction creating a zig-zag pattern.

The lacunae are spaces in the osteon that contain the osteocytes which communicate with each other via the canaliculi.  The canaliculi (small canals) are small communicating channels carved out in the hard bone. The osteocytes are the cells which are housed in the lacunae.

Applied Anatomy

Compact bone is hard and brittle and can fracture, but will usually fracture at its thinnest and weakest point unless the force is directly imposed on the thickest portion.  Since the middle of the shaft of long bones contains the thickest compact bone fractures are not as commonly seen in this region as they are at the distal ends of the long bones near the metaphyses.

Spongy Bone (aka Cancellous Bone Trabecular Bone)

Spongy bone (aka trabecular bone and cancellous bone) is the porous and softer trabeculated bone that has a spiculated appearance. It is found in the epiphyses and metatphysis of long bones and forms the matrix of the vertebra, short bones, flat bones of the ribs and the pelvis.

Structurally it is characterized by its spongy appearance (aka trabecular and cancellous appearance). The word trabecular derives from the Latin trabēcula, which is the diminutive of trabs meaning a small beam, rod or strut since the bone is marked with small vertical bars and horizontal cross bars.  The word cancellous originates from the Latin word cancellus that means a lattice which also describes its morphological appearance.   The structure is not random.  Rather it is organized with bars of bone oriented along the lines of physiological loads.  The morphology is dependent on the loads exerted on the bone, and this depends on its position in the body, and  the unique load imposed on the bone.  The morphology serves to dissipate the force as well as to provide support.

 

Functionally spongy bone creates a dynamic infrastructure within the relatively adynamic compact bone and responds rapidly to changing forces on the bone.  It also houses hematopoietic tissue – aka the red marrow.

There is a significant difference in the activity of spongy bone and compact bone.  The latter is slow and steady and is not subject to day to day change whilst spongy bone is a dynamic tissue always responding to the day to day stresses on the bone.

 

Applied Anatomy of the Spongy Bone

Since spongy bone is softer than compact bone it will fracture more easily

The normal turnover of cancellous bone is far quicker than compact bone and therefore by implication is far more relevant in the healing process than is compact bone.  When a patient fractures a bone attention is paid to the amount of “bone on bone” contact that is retained.  This “bone on bone” contact is directed to the evaluation of the more voluminous spongy bone.  When the fragments are anatomically aligned and there is good “bone on bone” contact then the bone will heal well without intervention.  On the other hand when the fragments are significantly distracted, angulated or rotated then the patient requires surgical intervention.

Poor Bone on Bone Contact

 

Interaction of Spongy and Compact Bone

The interaction of spongy and compact bone is a dynamic relationship that evolves to strengthen the bone in response to forces imposed on them by body weight and the forces of movement that they are subjected to on a day to day basis.

The head and neck of the femur for example are exposed to the primary vertical force of the body transferred via the spinal column and pelvis, and in response develop spongy bone called primary trabeculae that have a vertical orientation (see diagram below).  The primary vertical  trabeculae in turn act as a bridge to dissipate the force to the compact bone on the medial border of the shaft of the femur.

The secondary horizontal trabeculae respond to the horizontal forces imposed on the femur and transfer these forces to the compact bone on the lateral aspect of the shaft of the femur.

As the force of gravity with the weight of the body progresses down the leg and through the knee, it needs to be transferred to the ankle and foot which are at right angles to the force.

Examination of the trabeculae of the foot reveals to some extent the manner with which the body handles the dissipation of the full body weight to the foot.

The Periosteum

The periosteum is a tough double layer of “skin” that surrounds bone except at the level of the joint surfaces.

The periosteum consists of an outer fibrous layer and an inner cellular layer that has the capacity to produce bone.  Tendons that attach to bone traverse the periosteum and insert into the compact bone via Sharpey’s fibers.

It is structurally characterized by its component two layers.   The outer layer consists of dense fibrous tissue containing fibroblasts and the inner layer is an osteogenic layer that contains progenitor cells that can develop into osteoblasts.  There are blood vessels in the periosteum which act as a source of blood supply and venous drainage for the outer parts of the bone but also carry the nutrient artery that penetrates the bone.  There are pain sensitive fibers in the periosteum.

Functionally the periosteum is responsible for providing access to the compact bone for the attachment of muscles, tendons and ligaments with the assistance of Sharpey’s fibers. The inner layer is responsible for circumferential growth.  The osteogenic nature and prominent vascularity make the periosteum an important structure in providing assistance in the healing of fractures.

In children the periosteum is thicker, metabolically more active, more vascular and not as firmly attached to the periosteum.  In an acute fracture subperiosteal hematoma is usually more prominent in the children.  All these factors enable more rapid repair and accelerated healing of pediatric fractures.

Endosteum

The endosteum is considered a layer of “resting” bone marrow that abuts the inner surface of the bone.

Blood Supply

Bone has a rich blood supply and receives 10-20% of the cardiac output.  The nutrient artery enters the diaphysis of the long bone and divides into an ascending and descending branch and supplies the inner 2/3 of the cortex and the medullary cavity.  The metaphysis and epiphyses are supplied by vessels that supply the joints.  The nutrient arteries and arteries around the joints anastomose with each other.  The periosteum has its own blood supply and these vessels also supply the outer 1/3 of the cortical compact bone. (Bonakdarpour)

Bone Marrow

Bone marrow is a soft tissue that acts as a packing within the bone, but more importantly has hematopoietic function.  There are two types of marrow; yellow marrow and red marrow.  The yellow marrow is composed mostly of fat and found in the medullary cavity of the diaphysis of long bones in adults.  The red marrow which has hematopoietic function is found in the cancellous bone of the metaphysis and epiphyses of long bones as well as in the flat bones.  The bone marrow produces 2.5 billion red cells per kg/day, 2.5billion platelets per kg/day and 1 billion white cells per kg/day.  For a 70kg man that is more than 400 billion blood cells ( Bakitas, Erslev).  There are in comparison 7 billion people on earth.  Imagine the enormity of the function of the bone marrow!

Thus bone is thus intimately associated with the hematopoietic system and they share cells and local factors that regulate both of them.

Classification of the Bones Based on Size and Shape

The classification that revolves around the size and shape of bones is the most common method of classifying bones.  It has proved to be practical, since the classification allows one to predict the compact /spongy bone content, the type of function the bone fulfils, and the type of fractures they may sustain.

There are basically 5 types of bones in this classification

long bones eg femur tibia and phalanges

short bones eg carpals and tarsals of the wrist and ankle

flat bones eg skull sternum and ribs

irregular bones eg vertebra and pelvis

sesamoid bones– eg patella

Long Bones

The long bones will be discussed in detail at this point since they demonstrate examples of both compact and spongy bone and also demonstrate the physiological changes of growth exemplifying the role of the metaphysis growth plate, and epiphysis.

The long bones such as the femur, humerus and phalanges are characterized by having a shaft and two expanded extremities which usually function as two articulating surfaces.

The shaft has the thickest compact bone of all bones in the body while the proximal and distal ends have spongy bone surrounded by a thin layer of compact bone.

The shaft is also called the diaphysis.  The word diaphysis originates from the Greek words “dia” meaning across or through, and physis meaning growth and it is therefore the part of the long bone that grows between or across the two growth areas.  It is the first part of the bone to ossify during growth.  It consists of an outer cortical layer and an inner medullary cavity that contains yellow marrow in the adult.

The ends of the long bone house the metaphysis, growth plate (subsequently becoming the growth line) and the epiphysis.

The metaphysis and epiphysis contain spongy bone that usually contains red marrow.

From Fetus to Adult – From Cartilage to Bone

In the 3-5 week fetus the long bones are made entirely from cartilage.  By 6 weeks a primary ossification center appears in the shaft and is surrounded by a collar of compact bone.  At birth the shaft is fully ossified. Between 2-5 years a marrow cavity is formed in the shaft and secondary ossification centers appear at the ends of the shafts consisting of a core of spongy bone and an outer rim of cartilage.  By adulthood (approximately 18-25 years) the cartilage in the epiphysis and metaphysis disappear and the secondary ossification center merges with the diaphysis.

The first phase of ossification is the appearance of the primary ossification center which usually occurs in the prenatal period and for long bones is the diaphysis and for irregular bones is in the main body. There is usually only one primary ossification center per bone.

The secondary ossification centers represent the second site in bone that starts to ossify.  At birth in general there is a paucity of secondary ossification centers and often none are present.  Those that may be present at birth include the center at the distal femur, proximal tibia, calcaneus, talus, and cuboid.

 

Over the next few years all the other secondary ossification centers develop progressively, each bone evolving at a different time.  There is usually more than one secondary ossification center per bone. These centers of secondary ossification occur in the epiphyses at both ends of long bones.

The proximal femoral secondary ossification center appears at about 6 months.  The ossification center for the capitellum appears by about 8 months, the radial head at about age 3-4, the medial epicondyle at about age 5, the trochlea at about age 7, the olecranon at about age 9, and the lateral epicondyle at about age 11.

With time the cartilage progressively ossifies and the growth centre – epiphyseal plate – remain as a linear stripe between the metaphysis and epiphysis.

The Epiphyseal Plate – Active Child – Sports and Fractures

As the child grows stronger and more active, physical activity becomes a large part of the child’s day.  Many of the bones will have lost almost all the cartilage except for the growth plate.  The physical activities put the weakest part of the bone at risk. A fracture in the epiphyseal plate may have significant implications on the child’s growth.

The focus of the Salter Harris classification of pediatric fractures is the method used to predict the implications of the fracture on the epiphyseal plate and on growth.

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Once the growth plate closes the only remaining cartilage in the bone is at the articular surface.  The epiphyseal plate becomes a non active sclerotic line and the concerns for injury of the plate specifically are no longer viable.

Short Bones

Short bones are roughly cube shaped, consisting primarily of spongy bone covered by a thin layer of compact bone. The carpals and tarsal bones are examples of short bones.

Flat Bones

Flat bones such as the skull, ribs and sternum are thin, flattened and curved with the cortical have the two cortical margins running closely parallel to each other.

 

Irregular Bones

Irregular bones that have an unusual shape such as the vertebra and some of the bones in the cranium such as the sphenoid bones and bones of the middle ear.  They have a thin cortical margin and are mostly composed of spongy bone.

Sesamoid Bones

Sesamoid bones are characterized by their location within  a tendon.  A classical example is the patella which is found between the quadriceps femoris tendon, the tendons of the vastus muscles and the ligament of the patella which runs inferiorly.

Character of Bone and Aging

In the pediatric population the bone is less brittle and more elastic. Hence bending fractures or greenstick fractures are more common and exclusive to the pediatric population. Additionally the periosteum is not as adherent to the bone in children and hence stripping of the periosteum by the hematoma is more common.

At the other end of the spectrum the bones become more brittle with age as osteopenia and osteoporosis evolve.

Thus this section is a preamble to the section on fractures.  It is important at this stage to understand that all bone is not created equal.  Most important is the distinction between compact bone and spongy bone.  Age and time has significant implications as the bone starts out as a purely cartilagenous tissue and slowly ossifies as it matures by about age 16-18years. Each bone and individual has a time line that should fit within a bell curve for maturation.  In the context of fractures, cartilage cannot normally be visualized by plain film examination and thus some fractures can be invisible.  Health care givers should pursue the diagnosis of a fracture in patients where there is a high index of clinical suspicion despite a negative radiology report.  At the other extreme the same vigilance has to be paid to the elderly in whom fractures may be invisible because of the loss of density of bone.

References

Augat P, Schorlemmer S The Role of Cortical Bone in Bone Strength Age and Ageing 35 S-2 ii 27-ii312006

Bakitas M and Wujcik D  Blood and Marrow Stem Cell Transplanation : Principles, Practice and Nursing Insights Jones and Bartlett Learning  Second Edition 1997 Sudbury Massachusetts and London

Bonakdarpour Akbar, Reinus WWilliam R, Khurana Javsvir S Diagnostic Imaging of Musculoskeletal Diseases First Edition 2010 Springer New York Dordrecht Heidelberg London

Erslev, A.J., Weiss, L. Structure and Function of Marrow in Williams W Beutler E, Erslev AJ Eds Hematology 1983 New York Mc Graw-Hill

Fleisch H Bisphosphonates in Bone Disease: from the laboratory to the patient  2000 4th Edition  Academic Press London

Hall M Trabecular Patterns of the head of the Femur Canad. M A. J. Nov 1961 Vol 85 pages 1141-1143.

Wojnar R   Bone and Cartilage – Its Structure and Physical Properties From
Biomechanics of Hard Tissues: Modeling, Testing, and Materials.  Edited by Andreas Ochsner and Waqar Ahmed ¨  Copyright 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Links

Bone Development and Growth  SEER Training Modules

University of Michigan School of Engineering