1. Basic Anatomy

INTRODUCTION

Mechanically, the healthy human skeleton is an optimal structure which has adapted itsform in response to its function. In order to perform any biomechanical analysis of themusculoskeletal system, it is necessary to understand the underlying anatomy since thiscomprises the geometry, material properties, and some of the boundary conditions of theproblem. Our approach will be to learn enough anatomy to enable us to perform ouranalyses. Depending on the goal of the particular analysis, a more detailed treatment ofthe anatomy may be necessary. For example, in some cases, it may be sufficient to modelthe femoral diaphysis as a hollow circular cylinder, in other cases it may be necessary toaccount for its non-circular, asymmetric cross-section. This decision is up to the analystand that decision making process is a major challenge to the bioengineer.

Treating the musculoskeletal system as a biomechanical system, we begin with a briefoverview of the four main functions of the skeleton. Second, we examine the structure ofthe skeleton, those regions that will be of special interest to us, and the four mainclassifications of bones. Once we have developed a basic understanding of the skeletalsystem, we will describe some important anatomic terms and the anatomic planes, thevarious types of joints, the structure of the hip, knee, and spine, and some of theimportant muscle groups. Students requiring a more in-depth treatment of this subjectshould consult an anatomy text.

THE FUNCTIONS OF THE SKELETON

The skeletal system consists of bone, the passive soft tissues (tendon, ligament, cartilage,meniscus, joint capsules), muscles, and nerves. It has four main functions, the first two ofwhich are mechanical in nature, the second two, physiological:

• support and motion• protection of the vital organs• mineral storage• hematopoiesis.

Support and Motion: Probably the most important function of the skeleton from anevolutionary perspective, the relatively rigid bones which articulate at the synovial jointsenable the body to move quickly and in a relatively agile manner. The bones and jointsoperate together as levers, with the muscles providing the active torque about the joint.When the muscles contract they produce forces which cause a bone or an entire limb torotate about a joint, thereby generating movement. For example, when the biceps contract,the lower arm rotates about the elbow joint, an action known as flexion. Since muscles are

usually attached (or inserted) quite close to the joint, there is a mechanical disadvantage atmost joints, i.e. the muscle must pull with a force that is much larger than the externalload. However, the advantage of this is that small contractions of the muscle will producelarge motions at the end of the limb, which may be advantageous from an evolutionaryperspective.

Protection of the vital organs: In order to protect the vital organs such as the brain, heart,spinal cord and lungs, the skeleton has developed various structures that allow it toabsorb large amounts of energy yet remain lightweight. For example, the cranial bones ofthe skull have a sandwich construction consisting of a stiff cortical shell surrounding arelatively compliant trabecular bone core (Figure 1.1). The outer cortical shell distributesexternal forces evenly to the underlying trabecular bone which absorbs most of the energyon compression. The ribs and sternum protect the lungs and heart in a similar manner asdo the spine and pelvis their respective soft tissue organs.

Figure 1.1: Cross-sectional view of the human skull (Atlas of the Human Body. HarperPerennial, 1989).

Mineral Storage: The skeletal system also has an important physiological function in thatit acts as a mineral bank, especially for calcium and phosphorous. Bone is made upprimarily of a mixture of collagen (a compliant and ductile protein polymer) andhydroxyapatite (a brittle calcium phosphate ceramic). Approximately 99% of the calciumin the human body is stored in the skeleton. One of the ways that the body regulates thelevel of these minerals in the bloodstream is by a continuous process of remodeling (theresorption and formation of bone tissue). If the body falls short of its daily calcium intakevia the gut, it will turn to the bones to get what it needs. Thus, individuals with calciumdeficient diets are at risk of losing bone mass, which in turn would lead to weakening oftheir bones. Since calcium absorption decreases with aging, elderly people are advised toincrease their daily intake of calcium to help reduce the risk of osteoporotic bonefractures.

Hematopoiesis: Finally, trabecular bone, the spongy highly porous bone found at the endsof the long bones, the vertebrae, and several other locations (skull, pelvis, sternum)

provides sites for the formation of red blood cells, a process known as hematopoiesis.This occurs only in the red bone marrow. Yellow bone marrow, which is found in themiddle (or diaphysis) of most long bones, serves primarily as a storage area for fat cells.

BONES

There are 206 bones in the human skeleton (Figure 1.2a). Eighty of these bones are foundin the axial skeleton (torso) while the remaining 126 comprise the appendicular skeleton(head and limbs). These bones are often divided into four categories based on their shape:long, short, flat, and irregular. The long bones include the femur, tibia, and humerus. Themetacarpals and vertebral bodies are short bones. The ilium, cranium, and scapula are flatbones. Irregular bones are those that do not fit neatly into the first three categories andinclude the wrist bones (carpals) and the posterior vertebral elements. Many of ouranalyses will deal with the mechanics of long bones. These bones are extremely importantin the study of fracture fixation and total joint replacements. We will also examine indetail the vertebral body, a short bone, since it plays a major role in osteoporosis of thespine. Another bone of particular interest is the irregularly-shaped top (or proximal) partof the femur, since it is also important in age-related hip fractures.

Because the spine is an important area of concern in orthopaedic biomechanics, somebasic knowledge of the vertebral column is necessary. The spine consists of 33 vertebrae(Figure 1.2b) in three sections: seven in the cervical spine (the neck), 12 vertebrae in thethoracic spine (surrounding the chest and rib cage), five in the lumbar spine (the lowerback), five in the sacral spine (fused to the pelvis), and four vertebrae are in the coccygealregion, which is the tail for many animals. Usually the sacral and coccygeal vertebrae arefused, i.e. allow no relative motion. Consequently they are seldom injured and we concernourselves only with the cervical, thoracic, and lumbar regions. These latter regions havelimited ability to articulate via the intervertebral disk on the front (or anterior) side, andthe facet joints on the back (or posterior) side. Most of the compressive load goesthrough the disk, i.e. the facet joints allow twisting motions and limit our ability to bendbackwards (extension of the spine).

a: b:

Figure 1.2: a: the human skeleton; b: the vertebral column (Netter, Frank H. The CIBACollection of Medical Illustrations. Vol. 8, Part 1, 1987).

ANATOMIC TERMS AND PLANES OF MOTION

When we discuss the bones of the human skeleton it is often useful to distinguishbetween different regions of the bone. Thus, the following terms are frequently used:• Proximal aspect: Nearest to the top of the body. Usually only used in conjunctionwith the bones of the appendicular skeleton. Thus, we talk of the proximal femur, whichis at the hip joint.• Distal aspect: The opposite of proximal: nearest the bottom of the body. Again, thisterm is normally used in conjunction with the bones of the appendicular skeleton. Thisdistal femur, for example, is at the knee joint.• Inferior: Beneath or lower. Used to denote the bottom or underside of a tissue orstructure. Especially important when discussing bones of the axial skeleton.• Superior: Opposite of inferior; same rules of usage.• Lateral: The part closest to the outside of the body or furthest from the body’smidline. So the lateral aspect of the femur is on the outside of your (left or right) thigh.• Medial: Opposite of lateral: the part closest to the inside or midline of the body.Note lateral and medial are referenced to the mid-line of the body, not to either the left orright sides.

• Anterior: Before or in front.• Posterior: Behind or in back.

The above terms allow us to describe structures of the skeleton. In order to describemotions of the body we must first define the anatomic position and the three anatomicplanes. In the anatomic position, the individual is standing with head and palms facingforward. The frontal (coronal) plane divides the skeleton front-back, the sagittal planedivides the skeleton left-right and the transverse plane divides the skeleton top-bottom.For example, a biceps curl is a motion in the sagittal plane; twisting one’s head to the sideis a motion in the transverse plane. In order to describe motions in these planes we usethe following terms:

• Flexion: A folding movement in which the anterior angle between two bones isdecreased (except the knee and toes in which case the angle is measured posteriorly).It generally means that you are moving a bone closer to the body with respect to itsanatomical position.

• Extension: The opposite of flexion: an increase in the anterior angle between twobones (except the knee and toes in which case the angle is measured posteriorly).

• Abduction: Movement away from the midline of the body, usually in the frontalplane.

• Adduction: Movement towards the midline of the body, usually in the frontal plane.• Hyperextension: Continuation of motion beyond the anatomic position.• Lateral flexion: Movement of the spine to the right or left, in the frontal plane.• Supination: A movement of the forearm to rotate the hand into the anatomic

position. For example, this would be a clockwise rotation of the right forearm (lookingdown the arm).

• Pronation: Opposite of supination: a movement of the forearm to rotate the hand sothat the palm faces backwards.

• Dorsiflexion: Rotation of the ankle about a transverse axis so that the toes moveupwards (away from the ground) in the sagittal plane. Used only for the ankle.

• Plantar flexion: Opposite of dorsiflexion: rotation of the ankle so that the toes movetoward the ground. Used only for the ankle.

JOINTS OF THE BODY

There are two ways to classify joints, functionally, and structurally. The functionalclassification is based on the amount of relative motion permitted by the joint. One thatallows no relative motion between the bones is called synarthrosis. If the joint allowsslight motion, it is called an amphiarthrosis. Finally, a joint which allows large relativemotions is called a diarthrosis or a diarthrodial joint.

Because we will focus on the mechanics of various types of joints, it is often more usefulto employ the structural classifications. Fibrous joints and cartilaginous joints are heldtogether by fibrous connective tissue or cartilage as their names imply. For analysis of

most human motion and the design of total joint replacements, the joints of most interestare the synovial joints, a subset of diarthrodial joints. The bones forming a synovial jointare held together by a fibrous joint capsule which may contain connective ligaments. Thejoint also contains a cavity which is filled with synovial fluid, a highly viscous fluid thathelps provide lubrication between the bones in the joint. The synovial fluid is secreted bya thin layer of synovial cells (the synovium) that line the inside of the joint capsule.Human synovial joints possess an extremely small coefficient of friction, lower in factthan virtually any man-made bearing surfaces (Table 1.1). The other main characteristic ofsynovial joints is that each end of the bone is covered with a thin layer of articularcartilage.

Table 1.1: Coefficients of Friction for Various Joints and Common Bearing Materials

(Mow, V.C. et al. “Cartilage and diarthrodial joints as paradigms for hierarchical materialsand structures.” Biomaterials. Vol. 13, No. 2, pp. 67-97, 1992).

Joint / Materials Coefficient ofFriction

Investigator

Human knee 0.005-0.02 J. Charnley (1960)Human hip 0.01-0.04 A. Unsworth (1975)

Canine ankle 0.005-0.01 F.C. Linn (1968)Porcine Shoulder 0.02-0.35 C.W. McCutchenBovine Shoulder 0.002-0.03 L.L. Malcolm (1976)

Gold on gold 2.8Aluminum on aluminum 1.9

Silver on silver 1.5Steel on steel 0.6-0.8Brass on steel 0.35Glass on glass 0.9Wood on wood 0.25-0.5Nylon on nylon 0.2Graphite on steel 0.1Ice on ice at 0oC 0.01-0.1

Deterioration of the joints occurs to some degree in nearly all individuals. Osteoarthritisoccurs when some combination of mechanical wear and biochemical degradation erodesthe cartilage on each covering each bone. It is a localized effect and is most common in theknee and hip since these joints bear the largest loads in the skeleton. Rheumatoid arthritisis a systemic condition where the immune system attacks the joints. The articularcartilage swells, deteriorates, limits mobility, and puts pressure on the nerve endings inthe underlying bone. Surrounding soft tissues such as the ligaments and tendons are alsoaffected. Typically this disease starts in the hands and spreads to the back and limbs.

THE HIP

Both the hip and knee joints are synovial joints. The hip joint is a relatively simple ball-and-socket joint in which the head of the femur rotates relative to the fixed acetabulum(Figure 1.4). The most common reason for hip replacements is to restore the range ofmotion and eliminate the pain caused by osteoarthritis. When the head of the femur isreplaced, the natural load transfer paths are interrupted and we must examine the stressesin various regions of the bone. The easiest place to start is the diaphysis because it canoften be approximated by a hollow circular beam. The metaphysis (the region where theshaft starts to expand, remember “meta” means “change”) is more difficult to analyzesince the load transfer to the surrounding bone is more complicated and we must oftenresort to finite element studies. Also important to the load distribution and overallmechanical analysis are the ephiphysis and the linea aspera. The ephiphyses (one at eachend of the bone) contain spongy trabecular bone and are two of the principal sites forhematopoiesis. All bone growth during maturation occurs at the epiphyseal plate, a softcartilaginous tissue that eventually fuses and turns into bone. Fractures across theepiphyseal plate in children are therefore dangerous since they can interrupt or eventerminate bone growth. The linea aspera is a raised bony ridge on the posterior side of thefemur to which many of the muscle groups attach. The reason for the raised ridge willbecome apparent when we discuss the adaptation of bone to applied loads (boneremodeling). Other examples of specialized muscle attachment points are the greater andlesser trochanters on the proximal femur (Figure 1.3) and the calcaneus, which forms thepart of the ankle joint. Biomechanically, these ridges create an increased moment arm forthe muscles with respect to the joint center.

Figure 1.3: The right femur. (a): anterior view; (b): posterior view.

THE KNEE

The knee joint (Figure 1.4) is a condyloid joint that allows the femur and tibia to rotate,twist, and slide relative to one another. Each type of motion is important to the stabilityof the joint and must be reproduced in an artificial knee replacement. Otherwise abnormalforces can develop on the cartilage or in the ligaments, which can lead to theirdeterioration. This intimate relationship between the kinematics and loads is acharacteristic of most synovial joints. Among the most important structures in the kneeare the medial collateral ligament (MCL), the lateral collateral ligament (LCL), quadricepstendon, patellar ligament, anterior cruciate ligament (ACL), and the posterior cruciateligament (PCL). Athletes, especially football players and gymnasts, are well acquaintedwith the MCL, LCL, ACL, and the pain associated with injuries to these structures. TheACL and PCL lie within the joint capsule; the other ligaments are outside. The ACLattaches at the anterior side of the proximal tibia and the middle, bottom surface of thedistal femur. Similarly, the PCL connects the posterior side of the proximal tibia and themiddle, bottom surface of the distal femur (Figure 1.4b). It should be noted here that thereis an inherent difference between ligaments and tendons. Ligaments connect one bone toanother while tendons connect bone to muscle. Another important structure in the knee isthe meniscus. This crescent-shaped pad helps distribute the loads from the femoralcondyles evenly over the surface of the tibia.

a: b:

Figure 1.4: (a) Side and (b) anterior views of the knee joint. Note that the fibular collateralligament is also known as the lateral collateral ligament and runs along the lateral aspect ofthe joint. (Tortora, G. J. Principles of Human Anatomy. Harper and Row, Publishers,New York, 1983.)

THE SPINE

One of the few cartilaginous joints that we will study is the anterior part of theintervertebral joint (Figure 1.5a). In fact the connective tissue is not cartilage but afibrocartilage structure known as the intervertebral disc. The disc is analogous to aninflated tire and is the largest avascular tissue in the body (Figure 1.5b). The outside is aconcentric ring of collagen sheets (the annulus fibrosis) while the center is filled with ahighly viscous gel (the nucleus pulposis). With aging, this gel solidifies, which can havesignificant biomechanical consequences. This particular joint allows small motionsbetween the bones comprising it and is often injured, hence its significance to theorthopaedic biomechanics community. On the posterior side, there is the synovial typefacet joint. This controls lateral twisting and helps limit extension in the spine.

a: b:

Figure 1.5: The intervertebral joints (a) and disk (b). (Tortora, G. J. Principles of HumanAnatomy. Harper and Row, Publishers, New York, 1983.)

Figure 6 provides two views of a lumbar vertebra, the largest and strongest in thevertebral column. The principal load bearing region is the vertebral body. The cortex isonly 200-350 mm thick and the interior is comprised of some of the lowest densitytrabecular bone in the body, particularly in the elderly. The articular processes (alsoknown as facet joints) connect the spinous process of one vertebral body to those aboveand below. The foramen (derived from the Latin for “hole”) houses the spinal cord. Theentire structure then, serves to support the upper body and protect the spinal cord fromtrauma. When analyzing the mechanics of the lower spine it is often assumed that theback muscles (the erector spinae among others) act at a distance of 5 cm posterior to thecenter of the vertebral body, although this tends to be an oversimplification. The center ofrotation of the joint for sagittal bending is within the disk, although this can vary withaging.

Figure 1.6: Lumbar vertebra: superior (A) and right lateral (B) views.

MAJOR MUSCLE GROUPS

There are approximately 700 different muscles in the human body and they are dividedinto three different types, skeletal, cardiac, and smooth or visceral muscles. Skeletalmuscle is voluntary and striated and makes up approximately 36% of the total bodyweight in women and 42% in men. The cardiac muscle is also striated but is aninvoluntary muscle. Smooth muscle tissue involuntary and is not striated.

We will be concerned almost exclusively with skeletal muscles and because many commonmovements are coordinated by muscles acting in groups, we will often consider them assuch. For instance, the quadriceps (the “thigh muscles”) are made up of the rectusfemoris, vastus medialis, vastus lateralis, and the vastus intermedius, but they all acttogether to extend the knee. The main condition for lumping muscles together in thisfashion is that they all have a common insertion point on the bone, thereby creating nomoment about that point. For example, the quadriceps all come together at the patella viathe single quadriceps tendon and both the long and short heads of the biceps attach to theradius through a single tendon. A summary of the major muscles or muscle groups isprovided in Table 1.2 along with the action that they effect.

Table 1.2: Major muscle groups and their actions on the knee, hip, lumbar spine, andelbow.

Joint Action Muscles/ Muscle GroupsKnee Flexion Hamstrings*

Extension Quadriceps**Hip Flexion Iliacus, Psoas Major

Extension Hamstrings*, Gluteus MaximusAbduction AbductorsAdduction Adductors

Lumbar Spine Flexion Rectus Abdominis, Internal and External ObliquesExtension Erector Spinae

Elbow Flexion Brachialis, Biceps BrachiiExtension Triceps Brachii

* The hamstrings consists of three different muscles, the semitendinosus,semimembranosus, and biceps femoris.** The quadriceps consists of four different muscles, the rectus femoris, vastus lateralis,vastus medialis, and vastus intermedius.

Acknowledgements: Written with Eric Nauman, Ph.D.REFERENCES

Charnley, J. The lubrication of animal joints. In: Symposium on Biomechanics, pp. 12-22Institute of Mechanical Engineers, London, 1959.

Linn, F. C. Lubrication of animal joints: II. The mechanism. J. Biomech., 1: 193-205,1968.

Malcolm, L. L. An experimental investigation of the frictional and deformationalresponses of articular cartilage interfaces to static and dynamic loading. Ph.D.Thesis, University of California, San Diego, 1976.

McCutchen, C.W. The frictional properties of animal joints. Wear, 5: 1-17, 1962.Netter, Frank H. The CIBA Collection of Medical Illustrations. Vol. 8, Part 1, 1987Atlas of the Human Body. Harper Perennial, 1989Tortora, G. J. Principles of Human Anatomy. Harper and Row, Publishers, New York,

1983Unsworth, A., Dowson, D. and Wright, V. The frictional behavior of human synovial

joints: I. Natural joints. J. Lubr. Technol., 97:360-376, 1975.

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