|skeletal properties|Shoulder|stability1|2|wrist|hip|Knee|ankle|Foot|Spine|                   |joint lubrication| patello-femoral biomechanics| 

Skeletal Tissue properties:

Bone: We have seen before that bone is uniquely structured to be lightweight and strong at the same time. This purposeful construction to efficiency is evident the more we look into it. Biomechanically, bone is akin to a biphasic composite structure, of organic and inorganic phases. Organic component gives it flexibility and the inroganic component provides rigidity. The composite structure is far stronger than the original constituents alone. Microscopically bone is composed of cortical and cancellous bone. Cortical bone is stiffer than cancellous bone. 

Bone being viscoelastic shows loading rate sensitivity and becomes stiffer, stronger and more brittle at higher loading rates. Bone responds to load by deformation. This ability depends on its ability to store energy. Older bone has low ductility, is less able to store energy than a young one and fails at a lower level of load. 

Low energy loading results in clean cut fracture. High energy loading results in comminuted fracture.

Muscle contraction is important to regulate bone loading. They can neutralise tensile load and allow bone to carry increased load. Fatigue failure can result from muscle weakness. 

Bone behaviour is also influenced by its geometry. Bone response to bending and torsion follows the same principle, so distribution of bone mass away from the neutral axis is helpful. This is why long bones are tubular in shape. Bending moment is also influenced by its length. The longer a bone, the more its bending moment and resultant stress. 

A long bone can act as a column, supporting compressive load along its long axis. or as a shaft, to resist torsion or as a beam , resisting bending moments. Tibia acts as a column to support body weight , neck of femur resists bending moments. When the foot is twisted, tibia resists torsion. 

Bone structure is dissimilar in longitudinal and transverse directions, this results in anisotropic behaviour. 

Bone behaves differently under different loading conditions. It is stronger in compression than tension than shear. Different types of loading also produces different types of fracture.

Wolff's Law: Bone is a living structure and adapts itself to its suroundings and demands placed on it. This was elegantly described by Julian Wolff in 1892. In simple terms, bone grows in response to mechanical stress. The law helps us understand how to encourage bone remodelling and avoid bone stock loss. 

Bone undergoes regular remodelling to recover from loading, but if the amount of repeated loading is far above power of remodelling , then this results in fatigue or stress fracture. 

Muscle contraction is important to regulate bone loading. They can neutralise tensile load and allow bone to carry increased load. 

Bone behaviour is also influenced by its geometry. Bone response to bending and torsion follows the same principle, so distribution of bone mass away from the neutral axis is helpful. Bending moment is also influenced by its length. The longer a bone, the more its bending moment and resultant stress. 

Cartilage: We would only consider Hyaline cartilage which is the articular cartilage of concern. Hyaline cartilage is present in synovial joint and is a very specialised tissue well suited to the sustained loads. Its ability to absorb shock and distribute high joint loads evenly across the joint results from multiphasic structure of cartilage. The extracellular matrix of cartilage can be thought of as a tri-phasic structure, composed of a charged solid phase, a fluid phase and an ionic phase. Mechanical behaviour of cartilage depends on the interaction between these three phases. Solid phase consists of collagen fibres and aggrecan molecules- responsible for tensile and compressive behaviour of cartilage. Fluid is water, 30% within the intrafibrillar space. Water content is governed by the fixed charge density (FCD) of proteoglycans (PG). FCD also determines the collagen fibril diameter. PG is closely packed .Their negative charge brings together electro-positive ions to maintain electro-neutrality- the third phase of our multi-phasic structure. This gives rise to the swelling pressure according to the Donnan osmotic pressure law. The solid matrix and the fluid are both incompressible. So, when cartilage is compressed fluid is exuded. But the flow of fluid out of the tissues and the drag this creates on the solid phase determines the compressive behaviour of cartilage. 

Articular cartilage is also visco-elastic. This can be explained from the multi=phasic interaction. Creep is due to fluid exudation and it ceases when fluid loss increases the swelling pressure enough to counter the applied external pressure. Stress relaxation is caused by fluid redistribution within the extracellular matrix. 

Cartilage also exhibits inhomogenecity and anisotropic behaviour. This is due to varying collagen and PG concentration. 

Tendon/Ligament: these are dense connective tissue. They contain a lot of collagen fibrils but unlike cartilage, they also have elastin which gives them different mechanical properties. They show non-linear load deformation behaviour.

The toe region is due to "uncrimoing" of the collagen fibrils- a lot of elasticity as they are unfurled. Then it decreases to the linear region and as the fibrils start to fail we have the yield region.

They are also visco-elastic and show creep, stress relaxation and loading rate sensitivity. 

Shoulder Joint: this joint hangs free from the axial skeleton and gets its mobility due to absence of bony constraints. The result is a very unstable joint. It is a ball-socket joint. But the glenoid socket contains only one-third of the humeral head. The humeral head is retroverted 30^o and medially inclined 45^o compared to long axis of the humerus. So, we have an upper limb well forward and away from body plane. The glenoid is slightly superiorly tilted and is thought to contribute to stability of the joint by preventing inferior subluxation of the humerus.

As the glenoid is rather shallow, we need additional supports to keep the joint together. The  role of glenoid labrum and gleno-humeral ligaments is discussed in the power point presentation. Glenoid labrum (GL) increases the depth of glenoid by 50%. Superiorly the long head of Biceps is attached and it can be disrupted here along with the GL in SLAP lesion. 

The gleno-humeral ligaments (GHL) are condensations of the joint capsule but provide good stability to the joint. Inferior GHL is the most important and is the primary anterior stabiliser of the abducted shoulder. 

Movement in the joint is mainly rotational, with little translation. It is helpful to remember that movement at the shoulder is not isolated but accompanied by movements at the scapulo-thoracic joints and the spine.

The clearest example is abduction. This movement is accompanied from the start by lateral rotation of scapula, produced by upper and lower fibres of trapezius and lower fibres of serratus anterior. This places the glenoid cavity superiorly and allows the humerus to be placed still higher. Shoulder abduction is a function of supraspinatus and deltoid. There is also synergistic contraction of teres minor (TM) and infraspinatus (IS). Isolated deltoid pull would have pulled the humerus right against the acromion. This is prevented by TM and IS contraction. They do not prevent abduction as they act along the axis of abduction. 

Elbow joint: this is a three-joints complex. There is disagreement as to what type of joint it is, but the mainstream opinion is that it is a trochleoginglymoid -a modified hinge joint. Rotation is afforded at the proximal radio-ulnar  and humero-ulnar joints. Centre of rotation moves during flexion-extension , suggesting a complex hinge motion. 

There is misconception that this is a NON-weight bearing joint. Actually, when an object is held in hand away from the elbow, significant joint reaction forces can develop at the elbow joint. Because of poor mechanical leverage, joint forces are high nearer extension and gets reduced in flexion.

A 1 kg weight is held on the palm of the hand at a distance of 40 cm away from centre of rotation (CoR) of elbow jt. Forearm weight of 20 N acts 15 cm away from CoR.Flexors pull on forearm 5 cm away from CoR. So, for equilibrium: M=0, F=0

F×5-(40×10)-(20×15)=0 ,   F=140 N

F-J-20N-10N=0,                 J=110 N

Elbow has a carrying angle , more prominent in women to accommodate the broader pelvis. This should taken into account when designing elbow replacement prosthesis. 

Joint stability is shared equally by bony and muscular factors. Anterior band of the medial collateral ligament (MCL) is the primary stabiliser against valgus force . Radial head acts as a secondary stabiliser. Ulno-humeral joint and LCL provide equal stability to varus stress. Damage to LCL results in posterolateral instability of the elbow. 

Wrist joint: as the joint connecting hand to the forearm, wrist is the key to hand function and its positioning in space. It is a biaxial synovial joint. The concave distal radial ellipsoid articular surface articulates with convex proximal carpal row. It has a short radius for flexion/extension antero-posteriorly and long radius transversely for ABduction/ADduction. These movements are accompanied by movement at the intercarpal joints. 60% of wrist flexion and 40% of extension occurs at the metacarpal joint. Rotation is not allowed because of two different curvatures at right angles to each other. Physiologically, isolated movements do not occur. Wrist extension is accompanied by a degree of ADduction and flexion by ABduction. 

There are two basic types of hand grip: power grip and precision grip. Power grip involves flexion of digits and thumb to grasp object between palm and digits. Whereas power grip involves the whole digit precision grip emphasises the involvement of the digital pulp. Digits are flexed and thumb is opposed  and palmarly adducted. the Thumb opposition is an important requirement of hand function. The saddle shaped CMC joint of the thumb allows much wider movement than the other CMC joints. Along with the thumb CMC  joint, the CMC joints of the 4th and 5th ray allow some movement. This is how we can cup our hands. The IP joints are bicondylar hinge. They are vary stable and only allow movement in one plane.  

Wrist and hand function synergistically to increase mechanical leverage. Wrist extension allows full finger flexion and flexion tenses the digital extensor tendons and aids full finger extension. For a strong grip wrist needs to be stable and slightly extended. 

Hip joint: this is a ball-socket joint. It is a very stable joint- at the expense of some reduction in mobility. However, we still have more movement than we need. To perform ADL , we need 120⁰ of flexion and 20⁰ of ER and ABduction. The load is transmitted to the femoral head mainly via superior quadrant of the acetabulum. Proximal femur has trabeculae oriented to help transmit load to diaphysis. Normal neck-shaft angle is 125⁰. Lower and higher angle gives rise to coxa-vara and valga respectively.

Surface motion is mainly gliding .This is tangential to the articular surface. If there is any joint incongruity, then this is lost and random gliding damages the articular cartilage.

Hip joint reaction force depends on the ratio of lever arm of abductor muscle force and gravity. Because the centre of gravity lies posterior to the joint axis, body weight also creates a bending moment , increased with hip flexion. 

Lever arm of muscles are three times less than that of body weight leading to large joint reaction force. In quiet normal standing the load on each hip is equal to one-third of body weight . This increases with muscle activation (eg. non- weight bearing) and reaches ten times the body weight in running, jumping etc. Body weight lever arm can be reduced by tilting the trunk over the supporting hip joint. People with weak abductor and painful hip instinctively use this technique.

Use of a walking stick on the side opposite the painful hip can reduce muscle activation and JRF. 

Ankle joint: it is a hinge joint with a changing axis of motion between DF and PF. Talus also rotates during ankle motion. So, it is not a simple hinge. It is a very strong joint and stability depends on bony and soft tissue factors. Bony stability is provided by the shape of the ankle, built like a mortice. This is more important when load bearing as the bony congruency provides much of the stability. Fibula migrates inferiorly upto 1 mm during loading to deepen the mortise and increase stability. Lateral ligament complex resists INversion and IR. Anterior talo-fibular lig prevents anterior talar displacement (commonest ligament to be injured) and IR of talus. Posterior talo-fibular ligament limits ER of talus. Deltoid resists ER and Eversion. It is the strongest of the ligaments and is key to preventing lateral talar shift. 

Ankle joint has the largest load bearing surface. The joint is also subjected to substantial shear and very large axial loading, many times the body weight. Load is mainly carried by the tibia , with fibula transmitting 17% of the total load. 

The foot: powerpoint presentation

Spine: the isolated spine is an unstable structure. It has a critical buckling load of around 350N, roughly the weight of head and upper torso. Yet the spine does not collapse, due to the stabilising role of the spinal muscles. 

Spinal ligaments are viscoelastic and loading rate sensitive. This allows effortless spinal movement in physiological range but provides a very rigid structure against high loading. 

Vertebral bodies support the weight and gradually increase in bulk as we go down. They are made of a thick cortical shell filled with cancellous bone. The cancellous trabeculae are oriented in both vertical as well as transverse directions to resist multi-directional compression. Cancellous bone resists compressive load and cortical shell provides stiffness against torsional and bending load. This composite structure is best suited to keep weight at the minimum while resisting loads from different directions. 

Intervertebral discs are composed of a nucleus pulposus core enveloped by annulus fibres. Nucleus is mainly water , with type II collagen. This allows the nucleus to act like a hydrostatic buffer during loading- redistributing load more uniformly and storing energy. Annulus has type I collagen. The fibres in annulus are arranged in bands at alternative 30⁰ orientation to the end plate. Again the alternating arrangement of fibres allows annulus to resist loading from all directions. 

Denis described the three column concept to describe stability of spinal injury. The posterior column consists of the posterior ligamentous complex. The middle column includes the posterior longitudinal ligament, posterior annulus fibrosus, and posterior wall of the vertebral body. The anterior column consists of the anterior vertebral body, anterior annulus fibrosus, and anterior longitudinal ligament. An injury with two or more column involvement is unstable. 

The facet joints are part of the posterior column. They are load bearing and could carry 1/3rd of total spinal load. Spinal movement in each segment is guided by the orientation of the facet joints. Vertebrae have six degrees of freedom of movement. All types of cervical motions are allowed, thoracic segment allows flexion/extension, lateral flexion and rotation ;lumbar motions are flexion/extension, lateral flexion but hardly any rotation. 

During quiet standing the centre of gravity goes anterior to the vertebral body, the spine is subject to a forward bending moment so the spinal muscles are constantly active. Any forward bending would increase the moment arm and increase load. 

Synovial joint lubrication: in spite of the massive loads generated in them, synovial joints are efficient bearings with very low friction. The coefficient of friction of a synovial joint is around 0.02. This compares to 0.03 for ice sliding on ice. A coefficient of friction of 0.01 means that a load of 100 lb could be made to slide by applying a force of 1lb. Joint lubrication is the key to reduced friction. So, it is helpful to understand them in order to better understand and treat joint wear. It is still unclear how lubrication works, but there are many theories, based on man-made ball-bearings. What is clear is that no single mechanism is responsible and different modes of lubrication work at different stages of joint function. 

The joint is lined by wear resistant hyaline cartilage and is bathed by synovial fluid. Unlike a typical newtonian fluid synovial fluid has a viscosity that decreases with increasing shear rate. The function of a lubricant is to provide an intermediate layer with low shear resistance in between the two sliding surfaces to reduce friction. A thixotropic fluid would fit the bill perfectly. 

Basic lubrication is of two types: fluid-film, boundary and mixed. 

Fluid film : a thin fluid film separates the bearing surfaces. Of two types: hydrodynamic and squeeze film. Hydrodynamic lubrication is unlikely to be feasible in vivo as the sliding velocity of joints are too low to generate a substantial fluid film. Squeeze film lubrication takes place by the production of a fluid film under pressure as the two bearing surfaces move perpendicularly towards each other. Fluid film and resultant load bearing capacity depends on fluid viscosity. It could explain lubrication under sudden loading but is not suitable for prolong loading conditions.

Boundary: the bearing surfaces come to contact with each other, but "lubricin" from synovial fluid is attached to the cartilage surface and offers an interposed layer which when rubbed provides less resistance to shear.

Mixed: weeping lubrication: on load application synovial fluid is released or "wept" from articular cartilage. It separates the two bearing surfaces and reduces friction due to the hydrostatic pressure. On unloading the fluid is squeezed back in. This mechanism is not dependent on sliding speed .

Boosted lubrication: is almost the same as weeping type. As fluid is wept between cartilage linings, the ultrafiltrate can move freely ioto cartilage , leaving a concentrated gel of HA complex which reduces friction and helps to carry the load.