Biomechanics of lower limb
Introduction to biomechanics
The study of biomechanics ranges from the inner workings of a cell to the movement and development of limbs, the vasculature, and bones. As we develop a greater understanding of the physiological behavior of living tissues, researchers are able to advance the field of tissue engineering, as well as develop improved treatments for a wide array of pathologies.
Biomechanics as a sports science, kinesiology, applies the laws of mechanics and physics to human performance in order to gain a greater understanding of performance in athletic events through modeling, simulation, and measurement.
Biomechanics is the science concerned with the internal and external forces acting on the human body and the effects produced by these forces. At the highest levels of sports in which techniques play a major role, improvement comes so often from careful attention to detail that no coach can afford to leave these details to chance or guesswork. For such coaches knowledge of biomechanics might be regarded as essential
It is often appropriate to model living tissues as continuous media. For example, at the tissue level, the arterial wall can be modeled as a continuum. This assumption breaks down when the length scales of interest approach the order of the microstructural details of the material. The basic postulates of continuum mechanics are conservation of linear and angular momentum, conservation of mass, conservation of energy, and the entropy inequality. Solids are usually modeled using “reference” or “Lagrangian” coordinates, whereas fluids are often modeled using “spatial” or “Eulerian” coordinates. Using these postulates and some assumptions regarding the particular problem at hand, a set of equilibrium equations can be established. The kinematics and constitutive relations are also needed to model a continuum.
Second and fourth order tensors are crucial in representing many quantities in biomechanics. In practice, however, the full tensor form of a fourth-order constitutive matrix is rarely used. Instead, simplifications such as isotropy, transverse isotropy, and incompressibility reduce the number of independent components. Commonly-used second-order tensors include the Cauchy stress tensor, the second Piola-Kirchhoff stress tensor, the deformation gradient tensor, and the Green strain tensor. A reader of the biomechanics literature would be well-advised to note precisely the definitions of the various tensors which are being used in a particular work.
Biomechanics of circulation
Under most circumstances, blood flow can be modeled by the Navier-Stokes equations. Whole blood can often be assumed to be an incompressible Newtonian fluid. However, this assumption fails when considering flows within arterioles. At this scale, the effects of individual red blood cells become significant, and whole blood can no longer be modeled as a continuum.
Biomechanics of the bones
Bones are anisotropic but are approximately transversely isotropic. In other words, bones are stronger along one axis than across that axis, and are approximately the same strength no matter how they are rotated around that axis.
The stress-strain relations of bones can be modeled using Hooke’s Law, in which they are related by linear constants known as the Young’s modulus or the elastic modulus, and the shear modulus and Poisson’s ratio, collectively known as the Lamé constants. The constitutive matrix, a fourth order tensor, depends on the isotropy of the bone.
There are three main types of muscles:
Skeletal muscle (striated): Unlike cardiac muscle, skeletal muscle can develop a sustained condition known as tetany through high frequency stimulation, resulting in overlapping twitches and a phenomenon known as wave summation. At a sufficiently high frequency, tetany occurs, and the contracticle force appears constant through time. This allows skeletal muscle to develop a wide variety of forces. This muscle type can be voluntary controlled. Hill’s Model is the most popular model used to study muscle.
Cardiac muscle (striated):
Cardiomyocytes are a highly specialized cell type. These involuntarily contracted cells are located in the heart wall and operate in concert to develop synchronized beats. This is attributable to a refractory period between twitches.
Smooth muscle (smooth – lacking striations): The stomach, vasculature, and most of the digestive tract are largely composed of smooth muscle. This muscle type is involuntary and is controlled by the enteric nervous system.
Soft tissues such as tendon, ligament and cartilage are combinations of matrix proteins and fluid. In each of these tissues the main strength bearing element is collagen, although the amount and type of collagen varies according to the function each tissue must perform. Elastin is also a major load-bearing constituent within skin, the vasculature, and connective tissues. The function of tendons is to connect muscle with bone and is subjected to tensile loads. Tendons must be strong to facilitate movement of the body while at the same time remaining compliant to prevent damage to the muscle tissues. Ligaments connect bone to bone and therefore are stiffer than tendons but are relatively close in their tensile strength. Cartilage, on the other hand, is primarily loaded in compression and acts as a cushion in the joints to distribute loads between bones. The compressive strength of cartilage is derived mainly from collagen as in tendons and ligaments, however because collagen is comparable to a “wet noodle” it must be supported by cross-links of glycosaminoglycans that also attract water and create a nearly incompressible tissue capable of supporting compressive loads.
Recently, research is growing on the biomechanics of other types of soft tissues such as skin and internal organs. This interest is spurred by the need for realism in the development of medical simulation.
Viscoelasticity is readily evident in many soft tissues, where there is energy dissipation, or hysteresis, between the loading and unloading of the tissue during mechanical tests. Some soft tissues can be preconditioned by repetitive cyclic loading to the extent where the stress-strain curves for the loading and unloading portions of the tests nearly overlap. The most commonly used model for viscoelasticity is the Quasilinear Viscoelasticity theory (QLV). In addition, soft tissues exhibit other viscoelastic properties, including creep, stress relaxation, and preconditioning.
Kinematics is the branch of biomechanics concerned with the study of movement with reference to the amount of time taken to carry out the activity.
Distance and displacement
Distance and displacement are quantities used to describe the extent of a body’s motion. Distance is the length of the path a body follows and displacement is the length of a straight line joining the start and finish points e.g. in a 400m race on a track the length of the path the athlete follows (distance) is 400m but their displacement will be zero metres (they finish where they start).
Speed and velocity
Speed and velocity describe the rate at which a body moves from one location to another. These two terms are often thought, incorrectly, to be the same. Average speed of a body is obtained by dividing the distance by the time taken where as the average Velocity is obtained by dividing the displacement by the time taken e.g. consider a swimmer in a 50m race in a 25m length pool who completes the race in 60 seconds – distance is 50m and displacement is 0m (swimmer is back where they started) so speed is 50/60= 0.83m/s and velocity is 0/60=0 m/s
Speed and Velocity = distance traveled ÷ time taken
Acceleration is defined as the rate at which velocity changes with respect to time.
average acceleration = (final velocity – initial velocity) ÷ elapsed time
From Newton’s 2nd law:
Force = Mass x Acceleration
Acceleration = Force ÷ Mass
If the mass of a sprinter is 70kg and the force exerted on the starting blocks is 700N then acceleration = 700 ÷ 70 = 10 msec²
Acceleration due to gravity
Whilst a body is in the air it is subject to a downward acceleration, due to gravity, of approximately 9.81m/s²
Vectors and scalars
Distance and speed can be described in terms of magnitude and are known as scalars. Displacement, velocity and acceleration that require magnitude and direction are known as vectors.
Components of a vector
|Figure 1||Figure 2|
Let us consider the horizontal and vertical components of velocity of the shot in Figure 1.
Figure 2 indicates the angle of release of the shot at 35° and the velocity at release as 12 m/sec.
Vertical component Vv = 12 x sin 35° = 6.88 m/sec
Horizontal component Vh = 12 x cos 35° = 9.82 m/sec
Let us now consider the distance the shot will travel horizontally (its displacement).
Range (R) = ((v² × sinØ × cosØ) + (v × cosØ × sqrt((v × sinØ)² + 2gh))) ÷ g
Where v = 12, Ø = 35, h = 2m (height of the shot above the ground at release) and g = 9.81
R = ((12² × sin35 × cos35) + (12 × cos35 × sqrt((12 × sin35)² + 2×9.81×2))) ÷ 9.81
R = 16.22m
The time of flight of the shot can be determined from the equation:
Time of flight = Distance (R) ÷ velocity (Vh)
Time of flight = 16.22 ÷ 9.82 = 1.65 seconds
Uniformly accelerated motion
When a body experiences the same acceleration throughout some interval of time, its acceleration is said to be constant or uniform. In these circumstances, the following equations apply:
Final velocity = initial velocity + (acceleration x time)
Distance = (initial velocity x time) + (½ x acceleration x time²)
Moment of force (torque)
The moment of force or torque is defined as the application of a force at a perpendicular distance to a joint or point of rotation.
Angular distance and displacement
When a rotating body moves from one position to another, the angular distance through which it moves is equal to the length of the angular path. The angular displacement that a rotating body experiences is equal in magnitude to the angle between the initial and final position of the body.
Angular movement is usually expressed in radians where 1 radian = 57.3°
Angular speed, velocity and acceleration
Angular speed = angular displacement ÷ time
Angular velocity = angular displacement ÷ time
Angular acceleration = (final angular velocity – initial angular velocity) ÷ time
Angular momentum is defined as: angular velocity x moment of inertia
The angular momentum of a system remains constant throughout a movement provided nothing outside the system acts with a turning moment on it. This is known as the Law Conservation of Angular Momentum. In simple terms, this means that if a skater, when already spinning, changes their moment of inertia (they move their arms out to the side) then the rate of spin will change but the angular momentum will stay the same.
Kinetics is concerned with what causes a body to move the way it does.
Momentum, inertia, mass, weight and force
Momentum: mass x velocity
Inertia: the resistance to acceleration – reluctance of a body to change whatever it is doing
Mass: the quantity of matter of which a body is composed of – not affected by gravity – measured in kilograms (kg)
Weight: force due to gravity – is mass x gravity (9.81m/s²)
Force: a pushing a pulling action that causes a change of state (rest/motion) of a body – is proportional to mass x acceleration – is measured in Newtons (N) where 1N is the force that will produce an acceleration of 1 m/s² in a body of 1kg mass
The classification of forces, external or internal, depends on the definition of the ‘system’. In biomechanics, the body is seen as the ‘system’ so any force exerted by one part of the system on another is known as an internal force all other forces are external.
Newton’s Laws of Motion
First Law: Every body continues in its state of rest or motion in a straight line unless compelled to change that state by external forces exerted upon it.
Second Law: The rate of change of momentum of a body is proportional to the force causing it and the change takes place in the direction in which the force acts
Third Law: To every action there is an equal and opposite reaction OR for every force that is exerted by one body on another there is an equal and opposite force exerted by the second body on the first
Newton’s law of gravitation
Any two particles of matter attract one another with a force directly proportional to the product of their masses and inversely proportional to the square of the distance between them
Work, Energy and Power
Kinetic energy is mechanical energy possessed by any moving object. An equation for Kinetic Energy can be derived from the work definition:
Work = force x distance moved in the direction of the force
Kinetic Energy = ½ x mass x velocity² (result is in joules)
Power is defined as the rate at which energy is used or created from other forms
Power = energy used ÷ time taken
Power = (force x distance) ÷ time taken
Power = force x velocity
Translation and couple
A force that acts through the centre of a body result in only translation. A force whose line of action does not pass through the body’s centre of gravity is called an eccentric force and results in translation and rotation.
Example – if you push through the centre of an object it will move forward in the direction of the force (translation) if you push to one side of the object (eccentric force) it will move forward and rotate.
A couple is an arrangement of two equal and opposite forces that cause a body to rotate.
A lever is a rigid structure, hinged at one point and to which forces are applied at two other points. The hinge or pivot point is known as the fulcrum. One of the forces that act on the lever is known as the weight that opposes movement and the other is the force that causes movement. For more details see the page on Levers.
Lift forces interact with objects in flight and are caused by the aerodynamic shape of the object. If an object has a curved top and flat bottom (wing of an aircraft), the air will have further to travel over the top than the bottom. For the two airflows to reach the back of the object at the same time the air flowing over the top of the object will have to flow faster. This means that there will be less pressure above the object (air is thinner) than below it and the object will lift. This is often referred to as the Bernoulli effect.
Introduction to hip joint anatomy
The hip joint is a ball and socket joint, formed by the head of the Femur (thigh bone) and the acetabulum of the pelvis. The dome-shaped head of the femur forms the ball, which fits snuggly into the concave socket of the acetabulum. The hip joint is a very sturdy joint, due to the tight fitting of the bones and the strong surrounding ligaments and muscles.
Understanding how the different layers of the hip are built and connected can help you understand how the hip works, how it can be injured, and how challenging recovery can be when this joint is injured. The deepest layer of the hip includes the bones and the joints. The next layer is made up of the ligaments of the joint capsule. The tendons and the muscles come next.
The important structures of the hip can be divided into several categories.
· bones and joints
· ligaments and tendons
· blood vessels
Bones of the hip joint
The femur is the longest bone in the body which forms the thigh. The part which articulates with the pelvis to form the hip joint is known as the head of the femur. This is a round, dome shaped protrusion. Close to the top of the femur are two other protrusions, known as the greater and lesser trochanters. The main function of the trochanters is for muscle attachment.
The pelvis is actually two large bones which connect at the front by the pubis symphesis (a cartilage disc) and at the back by the Sacrum. The Sacrum is part of the spine and consists of 4 fused vertebrae which do not move independently of one another. The joints formed by either side of the Sacrum and the two pelvic bones are the Sacroiliac joints (SIJ).
The surfaces of both the head of the femur and the acetabulum are covered with a thin layer of hyaline cartilage which acts to allow smooth movement of the joint.
Bones and Joints
The bones of the hip are the femur (the thighbone) and the pelvis. The top end of the femur is shaped like a ball. This ball is called the femoral head. The femoral head fits into a round socket on the side of the pelvis. This socket is called the acetabulum.
The femoral head is attached to the rest of the femur by a short section of bone called the femoral neck. A large bump juts outward from the top of the femur, next to the femoral neck. This bump,
called the greater trochanter, can be felt along the side of your hip. Large and important muscles connect to the greater trochanter. One muscle is the gluteus medius. It is a key muscle for keeping the pelvis level as you walk.
Articular cartilage is the material that covers the ends of the bones of any joint. Articular cartilage is about one-quarter of an inch thick in the large, weight-bearing joints like the hip. Articular cartilage is white and shiny and has a rubbery consistency. It is slippery, which allows the joint surfaces to slide against one another without causing any damage. The function of articular cartilage is to absorb shock and provide an extremely smooth surface to make motion easier. We have articular cartilage essentially everywhere that two bony surfaces move against one another, or articulate.
In the hip, articular cartilage covers the end of the femur and the socket portion of the acetabulum in the pelvis. The cartilage is especially thick in the back part of the socket, as this is where most of the force occurs during walking and running.
The hip joint is a synovial joint formed by the articulation of the rounded head of the femur and the cup-like acetabulum of the pelvis. It forms the primary connection between the bones of the lower limb and the axial skeleton of the trunk and pelvis. Both joint surfaces are covered with a strong but lubricated layer called articular hyaline cartilage. The cuplike acetabulum forms at the union of three pelvic bones — the ilium, pubis, and ischium.<href=”#cite_note-Faller-174-4″> The Y-shaped growth plate that separates them, the triradiate cartilage, is fused definitively at ages 14-16.<href=”#cite_note-Thieme-Atlas-365-5″> It is a special type of spheroidal or ball and socket joint where the roughly spherical femoral head is largely contained within the acetabulum and has an average radius of curvature of 2.5 cm.<href=”#cite_note-Thieme-Atlas-378-6″> The acetabulum grasps almost half the femoral ball, a grip augmented by a ring-shaped fibrocartilaginous lip, the acetabular labrum, which extends the joint beyond the equator.<href=”#cite_note-Faller-174-4″> The head of the femur is attached to the shaft by a thin neck region that is often prone to fracture in the elderly, which is mainly due to the degenerative effects of osteoporosis.
Transverse and sagittal angles of acetabular inlet plane.
The acetabulum is oriented inferiorly, laterally and anteriorly, while the femoral neck is directed superiorly, medially, and anteriorly.
The transverse angle of the acetabular inlet can be determined by measuring the angle between a line passing from the superior to the inferior acetabular rim and the horizontal plane; an angle which normally measures 51° at birth and 40° in adults, and which affects the acetabular lateral coverage of the femoral head and several other parameters. The sagittal angle of the acetabular inlet measures 7° at birth and increases to 17° in adults.<href=”#cite_note-Thieme-Atlas-379-7″>
Femoral neck angle
The angle between the longitudinal axes of the femoral neck and shaft, called the caput-collum-diaphyseal angle or CCD angle, normally measures approximately 150° in newborn and 126° in adults (coxa norma).<href=”#cite_note-Thieme-Atlas-367-8″> An abnormally small angle is known as coxa vara and an abnormally large angle as coxa valga. Because changes in shape of the femur naturally affects the knee, coxa valga is often combined with genu varum (bow-leggedness), while coxa vara leads to genu valgum (knock-knees). <href=”#cite_note-Platzer-196-9″>
Changes in trabecular patterns due to altered CCD angle. Coxa valga leads to more compression trabeculae, coxa vara to more tension trabeculae.<href=”#cite_note-Thieme-Atlas-367-8″>
Changes in CCD angle is the result of changes in the stress patterns applied to the hip joint. Such changes, caused for example by a dislocation, changes the trabecular patterns inside the bones. Two continuous trabecular systems emerging on auricular surface of the sacroiliac joint meander and criss-cross each other down through the hip bone, the femoral head, neck, and shaft.
In the hip bone, one system arises on the upper part of auricular surface to converge onto the posterior surface of the greater sciatic notch, from where its trabeculae are reflected to the inferior part of the acetabulum. The other system emerges on the lower part of the auricular surface, converges at the level of the superior gluteal line, and is reflected laterally onto the upper part of the acetabulum.
In the femur, the first system lines up with a system arising from the lateral part of the femoral shaft to stretch to the inferior portion of the femoral neck and head. The other system lines up with a system in the femur stretching from the medial part of the femoral shaft to the superior part of the femoral head.<href=”#cite_note-Palastanga-353-10″>
On the lateral side of the hip joint the fascia lata is strengthened to form the iliotibial tract which functions as a tension band and reduces the bending loads on the proximal part of the femur.<href=”#cite_note-Thieme-Atlas-367-8″>
Where friction occurs between muscles, tendons, and bones there is usually a structure called a bursa. A bursa is a thin sac of tissue that contains fluid to lubricate the area and reduce friction. The bursa is a normal structure. The body will even produce a bursa in response to friction.
Think of a bursa like this. If you press your hands together and slide them against one another, you produce some friction. In fact, when your hands are cold you may rub them together briskly to create heat from the friction. Now imagine that you hold in your hands a small plastic sack that contains a few drops of salad oil. This sack would let your hands glide freely against each other without a lot of friction.
A bursa that sometimes causes problems in the hip is sandwiched between the bump on the outer hip (the greater trochanter) and the muscles and tendons that cross over the bump. This bursa, called the greater trochanteric bursa, can get irritated if the iliotibial band (discussed earlier) is tight. Another bursa sits between the iliopsoas muscle where it passes in front of the hip joint. Bursitis here is called iliopsoas bursitis. A third bursa is over the ischial tuberosity, the bump of bone in your buttocks that you sit on.>
The capsule attaches to the hip bone outside the acetabular lip which thus projects into the capsular space. On the femoral side, the distance between the head’s cartilaginous rim and the capsular attachment at the base of the neck is constant, which leaves a wider extracapsular part of the neck at the back than at the front<href=”#cite_note-11″>. <href=”#cite_note-Platzer-198-12″> The strong but loose fibrous capsule of the hip joint permits the hip joint to have the second largest range of movement (second only to the shoulder) and yet support the weight of the body, arms and head.
The capsule has two sets of fibers: longitudinal and circular.
The circular fibers form a collar around the femoral neck called the zona orbicularis.
The longitudinal retinacular fibers travel along the neck and carry blood vessels.
Ligaments of the hip joint
The stability of the hip owes greatly to the presence of its ligaments.
Iliofemoral ligament: This is a strong ligament which connects the pelvis to the femur at the front of the joint. It resembles a Y in shape and stabilises the hip by limiting hyperextension
Pubofemoral ligament: The pubofemoral ligament attaches the part of the pelvis known as the pubis (most forward part, either side of the pubic symphesis) to the femur
Ischiofemoral ligament: This is a ligament which reinforces the posterior aspect of the capsule, attaching to the ischium and between the two trochanters of the femur.
Ligaments and Tendons
There are several important ligaments in the hip. Ligaments are soft tissue structures that connect bones to bones. A joint capsule is a watertight sac that surrounds a joint. In the hip, the joint capsule is formed by a group of three strong ligaments that connect the femoral head to the acetabulum. These ligaments are the main source of stability for the hip. They help hold the hip in place.
A small ligament connects the very tip of the femoral head to the acetabulum. This ligament, called the ligamentum teres, doesn’t play a role in controlling hip movement like the main hip ligaments. It does, however, have a small artery within the ligament that brings a very small blood supply to part of the femoral head.
A long tendon band runs alongside the femur from the hip to the knee. This is the iliotibial band. It gives a connecting point for several hip muscles. A tight iliotibial band can cause hip and knee problems.
A special type of ligament forms a unique structure inside the hip called the labrum. The labrum is attached almost completely around the edge of the acetabulum. The shape and the way the labrum is attached create a deeper cup for the acetabulum socket. This small rim of cartilage can be injured and cause pain and clicking in the hip.
Labrum of the hip joint
Just like the ball and socket joint of the shoulder, the hip joint has a labrum. This is a circular layer of cartilage which surrounds the outer part of the acetabulum effectively making the socket deeper and so helping provide more stability. Labrum tears are a common injury to the hip joint.
Muscle Groups surrounding the hip joint
There are numerous muscles which attach to or cover the hip joint:
Gluteals: Gluteus Maximus, Gluteus Minimus and Gluteus Medius are the three muscles referred to as the gluteals. They all attach to the posterior surface of the large flat area of the pelvis (Ilium) and travel laterally to insert into the greater trochanter of the femur. Medius and Minimus are responsible for abducting and medially rotating the hip joint, as well as stabilising the pelvis. Gluteus maximus extends and laterally rotates the hip joint.
Quadriceps: The four Quadricep muscles (Vastus lateralis, medialis, intermedius and Rectus femoris) all attach inferiorly to the tibial tuberosity of the shin. Rectus femoris originates at the Anterior Inferior Iliac Spine (AIIS – protrusion at the front of the ilium) and acts to flex the hip. The 3 other Quad muscles do not cross the hip joint, and attach around the greater trochanter and just below it.
Iliopsoas: The is the primary hip flexor muscle which consists of 3 parts. Together they attach superiorly to the lower part of the spine and the inside of the ilium (flat upper part of the pelvis). They then cross the hip joint and insert to the lesser trochanter of the femur.
Hamstrings: The hamstrings are three muscles which form the back of the thigh. They all attach superiorly to the ischial tuberosity (lowest part of the pelvis, sometimes referred to as the sitting bone!) and cause hip extension.
Groin muscles: There are three main groin muscles, which are anatomically termed the adductor muscles. They all attach superiorly to the pubis and travel down the inside of the thigh. Their action is hip adduction.
The hip is surrounded by thick muscles. The gluteals make up the muscles of the buttocks on the back of the hip. The inner thigh is formed by the adductor muscles. The main action of the adductors is to pull the leg inward toward the other leg. The muscles that flex the hip are in front of the hip joint. These include the iliopsoas muscle. This deep muscle begins in the low back and pelvis and connects on the inside edge of the upper femur. Another large hip flexor is the rectus femoris. The rectus femoris is one of the quadriceps muscles, the largest group of muscles on the front of the thigh. Smaller muscles going from the pelvis to the hip help to stabilize and rotate the hip.
Finally, the hamstring muscles that run down the back of the thigh start on the bottom of the pelvis. Because the hamstrings cross the back of the hip joint on their way to the knee, they help to extend the hip, pulling it backwards.
All of the nerves that travel down the thigh pass by the hip. The main nerves are the femoral nerve in front and the sciatic nerve in back of the hip. A smaller nerve, called the obturator nerve, also goes to the hip.
These nerves carry the signals from the brain to the muscles that move the hip. The nerves also carry signals back to the brain about sensations such as touch, pain, and temperature.
Traveling along with the nerves are the large vessels that supply the lower limb with blood. The large femoral artery begins deep within the pelvis. It passes by the front of the hip area and goes down toward the inner edge of the knee. If you place your hand on the front of your upper thigh you may be able to feel the pulsing of this large artery.
The femoral artery has a deep branch, called the profunda femoris (profunda means deep). The profunda femoris sends two vessels that go through the hip joint capsule. These vessels are the main blood supply for the femoral head. As mentioned earlier, the ligamentum teres contains a small blood vessel that gives a very small supply of blood to the top of the femoral head.
Other small vessels form within the pelvis and supply the back portion of the buttocks and hip.
The movements of the hip joint is thus performed by a series of muscles which are here presented in order of importance<href=”#cite_note-Platzer-244-17″> with the range of motion from the neutral zero-degree position<href=”#cite_note-Thieme-Atlas-386-16″> indicated:
Lateral or external rotation (30° with the hip extended, 50° with the hip flexed): gluteus maximus; quadratus femoris; obturator internus; dorsal fibers of gluteus medius and minimus; iliopsoas (including psoas major from the vertebral column); obturator externus; adductor magnus, longus, brevis, and minimus; piriformis; and sartorius.
Medial or internal rotation (40°): anterior fibers of gluteus medius and minimus; tensor fascia latae; the part of adductor magnus inserted into the adductor tubercle; and, with the leg abducted also the pectineus.
Extension or retroversion (20°): gluteus maximus (if put out of action, active standing from a sitting position is not possible, but standing and walking on a flat surface is); dorsal fibers of gluteus medius and minimus; adductor magnus; and piriformis. Additionally, the following thigh muscles extend the hip: semimembranosus, semitendinosus, and long head of biceps femoris.
Flexion or anteversion (140°): iliopsoas (with psoas major from vertebral column); tensor fascia latae, pectineus, adductor longus, adductor brevis, and gracilis. Thigh muscles acting as hip flexors: rectus femoris and sartorius.
Abduction (50° with hip extended, 80° with hip flexed): gluteus medius; tensor fascia latae; gluteus maximus with its attachment at the fascia lata; gluteus minimus; piriformis; and obturator internus.
Adduction (30° with hip extended, 20° with hip flexed): adductor magnus with adductor minimus; adductor longus, adductor brevis, gluteus maximus with its attachment at the gluteal tuberosity; gracilis (extends to the tibia); pectineus, quadratus femoris; and obturator externus. Of the thigh muscles, semitendinosus is especially involved in hip adduction.
The hip joint has the following normal ranges of movement: Flexion, Extension, Adduction, Abduction, Medial Rotation and Lateral Rotation.
Planes and Axes
Joint actions are described in relation to the anatomical position. Movement is defined by referring to the three planes and the three axis. (see diagram below)
|The Three Planes|
Sagittal Plane – a vertical plane which passes from front to rear dividing the body into two symmetrical halves
Frontal Plane – which passes from side to side at right angles to the sagittal plane
Transverse Plane – any horizontal plane which is parallel to the diaphragm
The Three Axis
Frontal Axis – passes horizontally from side to side at right angles to the sagittal plane
Sagittal Axis – passes from front to rear lying at right angles to the frontal plane
Longitudinal Axis – passes from head to foot at right angles to the transverse plane
HIP JOINT BIOMECHANICS
A 3D model of the hip and surrounding soft tissue has been developed and is being used to investigate the effects of implant geometry on muscle and joint reaction forces. The indeterminate system of mechanical equations was solved by assuming that the body selects muscle forces in a way which minimises some measure of effort or work. Nonlinear optimisation criteria are known to give better results, but have been considered too complex and time consuming to apply on a large scale. An algorithm has been developed to apply such a criterion efficiently so that a large number of geometries could be investigated.
Changes in force with geometry are significant. An F.E. model of the construct (bone and implant) is being used to study the effects of changing force on stress, strain and related quantities.
Introduction to knee joint anatomy
The knee joint is the largest joint in the body, consisting of 4 bones and an extensive network of ligaments and muscles. Injuries to the knee joint are amongst the most common in sporting activities and understanding the anatomy of the joint is fundamental in understanding any subsequent pathology.
To better understand how knee problems occur, it is important to understand some of the anatomy of the knee joint and how the parts of the knee work together to maintain normal function.
First, we will define some common anatomic terms as they relate to the knee. This will make it clearer as we talk about the structures later.
Many parts of the body have duplicates. So it is common to describe parts of the body using terms that define where the part is in relation to an imaginary line drawn through the middle of the body. For example, medial means closer to the midline. So the medial side of the knee is the side that is closest to the other knee. The lateral side of the knee is the side that is away from the other knee. Structures on the medial side usually have medial as part of their name, such as the medial meniscus. The term anterior refers to the front of the knee, while the term posterior refers to the back of the knee. So the anterior cruciate ligament is in front of the posterior cruciate ligament.
The important parts of the knee include
bones and joints
ligaments and tendons
Bones and Joints
The knee is the meeting place of two important bones in the leg, the femur (the thighbone) and the tibia (the shinbone). The patella (or kneecap, as it is commonly called) is made of bone and sits in front of the knee.
The knee joint is a synovial joint. Synovial joints are enclosed by a ligament capsule and contain a fluid, called synovial fluid, that lubricates the joint.
The end of the femur joins the top of the tibia to create the knee joint. Two round knobs called femoral condyles are found on the end of the femur. These condyles rest on the top surface of the tibia. This surface is called the tibial plateau. The outside half (farthest away from the other knee) is called the lateral tibial plateau, and the inside half (closest to the other knee) is called the medial tibial plateau.
The patella glides through a special groove formed by the two femoral condyles called the patellofemoral groove.
The smaller bone of the lower leg, the fibula, never really enters the knee joint. It does have a small joint that connects it to the side of the tibia. This joint normally moves very little.
Articular cartilage is the material that covers the ends of the bones of any joint. This material is about one-quarter of an inch thick in most large joints. It is white and shiny with a rubbery consistency. Articular cartilage is a slippery substance that allows the surfaces to slide against one another without damage to either surface. The function of articular cartilage is to absorb shock and provide an extremely smooth surface to facilitate motion. We have articular cartilage essentially everywhere that two bony surfaces move against one another, or articulate. In the knee, articular cartilage covers the ends of the femur, the top of the tibia, and the back of the patella.
This is a hinge type of synovial joint that permits some rotation.
Its structure is complicated because it consists of three articulations: an intermediate one between the patella and femur and lateral and medial ones between the femoral and tibial condyles.
Articular Surfaces of the Knee Joint
The bones involved are the femur, tibia, and patella.
The articular surfaces are the large curved condyles of the femur, the flattened condyles of the tibia, and the facets of the patella.
The knee joint is relatively weak mechanically because of the configurations of its articular surfaces. It relies on the ligaments that bind the femur to the tibia for strength.
On the superior surface of each tibial condyle, there is an articular area for the corresponding femoral condyle.
These areas, commonly referred to as the medial and lateral tibial plateaux, are separated from each other by a narrow, nonarticular area, which widens anteriorly and posteriorly into anterior and posterior intercondylar areas, respectively.
Surface Anatomy of the Knee Joint
This joint may be felt as a slight gap on each side between the corresponding femoral and tibial condyles. When the leg is flexed or extended, a depression appears on each side of the patellar ligament.
The articular capsule is very superficial in these depressions. The knee joint lies deep to the apex of the patella.
Movements of the Knee Joint
The principal movements occurring at this joint are flexion and extension of the leg, but some rotation also occurs in the flexed position.
Flexion and extension of the knee joint are very free movements.
Flexion normally stops when the calf contacts the thigh. The ligaments of the knee stop extension of the leg.
When the knee is fully extended, the skin anterior to the patella is loose and can easily be picked up. This laxity of the skin helps flexion to occur.
The knee “locks” owing to medial rotation of the femur on the tibia. This makes the lower limb a solid column and more adapted for weight bearing. To “unlock” the knee the popliteus muscle contracts, thereby rotating the femur laterally so that flexion of the knee can occur.
The Articular Capsule of the Knee
The fibrous capsule is strong, especially where local thickenings of it form ligaments.
Superiorly, the fibrous capsule is attached to the femur, just proximal to the articular margins of the condyles and to the intercondylar line posteriorly.
It is deficient on the lateral condyle, which allows the tendon of the popliteus muscle to pass out of the joint and insert into the tibia.
Inferiorly the fibrous capsule is attached to the articular margin of the tibia, except where the tendon of the popliteus muscle crosses the bone.
Here the fibrous capsule is prolonged inferolaterally over the popliteus to the head of the fibula, forming the arcuate popliteal ligament.
The fibrous capsule is supplemented and strengthened by five intrinsic ligaments; patellar ligament, fibular collateral ligament, tibial collateral ligament, oblique popliteal ligament, and arcuate popliteal ligament.
These are often called the external ligaments to differentiate them from the internal ligaments (e.g., the cruciate ligaments, which are internal to the fibrous capsule).
This very strong, thick band is the continuation of the tendon of the quadriceps femoris muscle.
The patella is a sesamoid bone in this tendon.
The patella is continuous with the fibrous capsule of the knee joint and is most easily felt when the leg is extended.
The superior part of its deep surface is separated from the synovial membrane of the knee joint by a mass of loose fatty tissue called the infrapatellar fatpad. The inferior part of the patellar ligament is separated from the anterior surface of the tibia by the deep infrapatellar bursa.
With the leg flexed, the patellar ligament is struck to elicit a knee jerk. This patellar reflex or knee reflex results in the extension of the leg. This reflex is blocked by damage to the femoral nerve, which supplies the quadriceps muscle. Similarly, damage to the reflex centres in the spinal cord (L2, L3, and L4) will affect the patellar reflex.
The Fibular Collateral Ligament
The fibular collateral ligament (lateral ligament) is a round pencil-like cord about 5 cm long.
It extends inferiorly from the lateral epicondyle of the femur to the lateral surface of the head of the fibula.
The tendon of the popliteus muscle passes deep to the fibular collateral ligament, separating it from the lateral meniscus.
The biceps femoris muscle is also split into two parts by this ligament.
The fibular collateral ligament is fused with the fibrous capsule of the knee joint superiorly; hence, this part of it is an intrinsic ligament.
Inferiorly the fibular collateral ligament is separated from the fibrous capsule by fatty tissue; hence this part of it is an extrinsic ligament.
The Tibial Collateral Ligament
This ligament (also known as the medial ligament) is a strong, flat band, 8 to 9 cm long, which extends from the medial epicondyle of the femur to the medial condyle and superior part of the medial surface of the tibia.
It is a thickening of the fibrous capsule of the knee joint and is partly continuous with the tendon of the adductor magnus muscle.
The medial inferior genicular vessels and nerve separate the inferior end of the ligament from the tibia.
The deep fibres of the tibial collateral ligament are firmly attached to the medial meniscus and the fibrous capsule of the knee.
Injuries, the collateral ligaments and the knee joint
The tibial and fibular collateral ligaments normally prevent disruption of the sides of the knee joint.
They are tightly stretched when the leg is extended and prevent the rotation of the tibia laterally or the femur medially.
As the collateral ligaments are slack during flexion of the leg, they permit some rotation of the tibia on the femur in this position.
The fibular collateral ligament is not commonly torn because it is very strong. However, lesions (e.g., strains or tears) or the fibular collateral ligament can have serious consequences.
Usually, it is the distal end of the ligament that tears, and sometimes the head of the fibular is pulled off because the ligament is stronger than the bone. Complete tears are associated with stretching of the common fibular (peroneal) nerve. This affects the muscles of the anterior and lateral compartments of the leg and may produce foot-drop owing to paralysis of the dorsiflexor and eversion muscles of the foot.
The firm attachment of the tibial collateral ligament to the medial meniscus is of considerable clinical significance because injury to the tibial collateral ligament frequent results in concomitant injury to the medial meniscus.
Rupture of the tibial collateral ligament, often associated with tearing of the medial meniscus and anterior cruciate ligament, is a common type of football injury. The damage is frequently caused by a blow to the lateral side of the knee.
When considering soft tissue injuries of the knee, always think of the three Cs which indicate those structures that may be damaged: Collateral ligaments, Cruciate ligaments, and Cartilage (menisci).
The Oblique Popliteal Ligament
The broad band is an expansion of the tendon of the semimembranosus muscle. The oblique popliteal ligament strengthens the fibrous capsule of the knee joint posteriorly.