Introduction

• The isolated spine is an unstable structure. It has a critical buckling load of around 350 N, 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 a 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 multidirectional compression. The cancellous core carries around 55% of the load through a motion segment dropping to 35% with age and osteoporosis. 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 (75–89%), type II collagen and proteoglycan. 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 30° orientation to the end plate. Again the alternating arrangement of fibres allows annulus to resist loading from all directions. When loaded the disc acts as a thick walled pressure vessel

Spinal Stability (White & Panjabi) (1)

• Ability of spine to resist displacement under physiologic loading in order to protect the neural structures from damage and prevent deformity and pain

Denis 3 column theory (2)

• 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.

Figure 1. Denis three column theory

Motion Segment

• This is also called a functtional spinal unit or articular triad
• A spinal motion segment is two adjacent vertebrae,the intervertebral disc,facet joints and all intervening soft tissues(ligaments) but excludes muscle
• The functional spinal unit is the smallest physiological unit of the spine that exhibits biomechanical properties similar to that of the entire spine.

Spinal stability

Although a number of theories for spinal stability exist the model described by Panjabi has gained widespread use

Spinal stability is conferred by the interaction of 3 subsystems

1. Passive musculoskeletal system

2. Active musculoskeletal system

3. Neural system

The three subsystems work together to provide the overall stability of the spinal system

1. Passive musculoskeletal system

Includes vertebrae, intervertebral discs, facet Joints, ligaments, joint capsules

Vertebrae

• 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
• S shape in sagittal plane allows body weight to be away from the central axis of rotation, which aids balance and gait
• Axis of rotation goes through posterior part of vertebral body

Figure 2. Facet joint orientation 

Figure 3. IVD with compression

Figure 4. Face joint orientation

Figure 5 (a). Rotation mid thoracic facet joints

Figure 5(b). Rotation thoracolumbar junction facet joints

Incorporation of systems in spinal stability

Spinal range of motion has a neutral zone and an elastic zone

Neutral zone

• Initial zone of high flexibility where the active and neural systems predominate to give proprioceptive feedback controlling motion
• The motion region of the joint which functions independently of the osseoligamentous complex with a relatively small load producing a large displacement

Elastic zone

Towards the end range of motion where the passive system predominates to limit movement and prevent damage to the neural structures (stability)

Spinal Disease or Injury

In degenerative disease or burst fracture:

• The size of the neutral zone is increased
• It is increased to greater than the total physiologic motion of the spine
• The stabilizing systems do not reach the stiffening phase of the force displacement curve
• This means there is a much wider zone of movement where neurologic injury may occur

Figure 6. The load-displacement curve of the spine is generally nonlinear. At small loads, there is relatively large displacement; at larger loads, there is relatively less displacement. This load-displacement curve may be divided into physiologic and traumatic ranges. The physiologic ROM may be further divided into two parts(1)NZ, which is the displacement beyond the neutral position due to application of a small force,(2) EZ, more load is required per unit displacement. The traumatic range is defined by the PZ; it is the displacement beyond the EZ to failure. 

Classic reference (3)

Denis F. The three column spine and its significance in the classification of acute thoracolumbar spinal injuries. Spine. 1983 Nov 1;8(8): 817-31

Key message

The middle column is important in determining both the mechanical and neurological stability of spinal fractures affecting the thoracic and lumbar spine.

Why it’s important

Minor spinal fractures are relatively common injuries and most can be managed with analgesia.  However, it is well recognised that more severe injuries can lead to devastating complications.  The classification and treatment of spinal fractures remains controversial.  As severe neurological complications are seldom reversible, it is beholden on the treating surgeon to ensure that they do not develop as the result of treatment or preventable secondary injury.  In this paper Francis Denis presents his attempt to try and classify acute spinal fractures in order to detect those fractures that are at high risk of neurological complications and those in which intervention might prevent secondary deterioration.  The paper seeks to integrate, the basic science (biomechanics), the mechanism of injury, the radiological appearance and outcome.  By classifying fractures into different subtypes it seeks to facilitate communication between surgeons and comparison of results between different treatments and different centres.  It highlights burst fractures as at risk of subsequent neurological deterioration. 

Strengths

This was a large study rooted in what had already been described in terms of the basic science and previous classification systems. The descriptions of the mechanism of injury and associated radiological appearances make the reader relatively confident in their subsequently ability to identify thoracolumbar fractures in which the middle column has been disrupted.

Frequently quoted by residents, registrars and pocket books, this paper has led to a classification system that is easy to learn and quick to apply.  Using this classification system it is relatively quick and easy to grasp enough information in order to classify a fracture as stable or potentially unstable.  It has become almost ubiquitous.

This paper has resulted in terminology that is frequently used and it has therefore achieved its self-assigned goal of facilitating communication between different centres and surgeons.

Weaknesses

The main weakness of this study is that it is limited by its retrospective, observational design.  The three-column theory represents the (expert) opinion of a single author.  The goal is clearly to improve the outcome in patients with thoracolumbar fractures but a good outcome is not clearly defined.

• The shoulder 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° and medially inclined 45° 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 glenohumeral 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 glenohumeral 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 scapulothoracic 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.

Figure 7. Anterior view of the right shoulder showing the force-couple between the deltoid and the rotator cuff muscles during active shoulder abduction.

Figure 8. Compressions into the glenoid concavity. The rotator cuff compresses the humeral articular convexity into the glenoid concavity.

The glenohumeral joint depends on both static and dynamic stabilizing factors to stabilise the shoulder

Static factors

• Labrum
• Articular geometry
• GH ligaments and capsule
• Negative intra articular pressure
• Surface area mismatch

Labrum

• Triangular cross section similar to the meniscus
• Responsible for 20% of stability on its own
• Deepens the glenoid concavity
• Allows for attachment of the biceps and GH ligaments
• Deepens glenoid by 9mm supero-inferiorly and 5mm antero-posteriorly
• Increases humeral head coverage and also the contact surface area of the glenohumeral joint
• Has suction effect on the humeral head like a plunger
• Acts like “Chock block” to humeral head movement

Articular geometry

Glenoid version

• 7º retroversion
• 5º superior tilt

Humeral version

• Neck/shaft angle 135º.
• Head retroverted 20º

Glenohumeral ligaments and capsule

IGHL

• Primary stabiliser of the abducted shoulder
• Anterior band tightens with ER preventing anterior and inferior translation humeral head
• Posterior band tightens with IR
• Forms a hammock around humeral head (axilliary pouch) that supports humeral head
• Inserts primarily on labrum and blends with the capsule

Key point: The IGHL is the primary stabiliser limiting anterior movement of the 90º abducted shoulder. It is the most frequently injured portion of the capsule that can result in instability

MGHL

• Arises from the supraglenoid tubercle and Anterior superior labrum

• Blends with Subscap and inserts onto Lesser Tuberosity

• Contributes to anterior stability below 90 degrees abduction

SGHL

• Arises from the Supragenoid tubercle and attaches to Lesser Tuberosity

• Intimately linked to biceps tendon and coracohumeral ligament

• Lies within the rotator interval

Intra-articular pressure

The negative intra-articular pressure inside the joint has the effect of sucking the humeral head onto the glenoid

Surface area mismatch

Because humeral head is bigger than glenoid joint reaction force is elevated

This increases stability by compression of the humeral head into the glenoid

Dynamic stabilizers

• Rotator cuff
• Proprioception
• Long head of biceps
• Deltoid
• Scapula rotators

Rotator cuff

• Joint compression created by coordinated activity of rotator cuff is the primary dynamic stabilizer of the GH joint
• Subscapularis muscle provides stability at lower degrees of abduction but contributes little when shoulder is in 90^o abduction
• Rotator cuff compresses humeral head into glenolabral socket, contributing stability, especially in middle ROM when ligaments are lax
• Superior migration of the humeral head may occur with large cuff tears

Proprioception

• Vital for shoulder stability
• In patients with MDI reduced capacity to recruit propriocetors 

Long head of biceps

• Biceps contributes significantly to anterior, inferior and posterior stability. Its role becomes very important in an unstable shoulder.

Deltoid

• In abduction its direction of fibres contributes to concavity compression

Scapula rotators

• Levator scapulae, Rhomboids, Trapezius, Serratus anterior
• The scapular rotators work together to establish synchronous motion between the scapula to conserve joint alignment throughout shoulder range of movement
• Disruption of any group leads to failure of this function and may cause instability

Dynamic stabilizers are more important in the mid range of movement whilst the static stabilizers are most important at the end ranges of movement

• The elbow joint 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 get reduced in flexion.
• Elbow has a carrying angle, more prominent in women to accommodate the broader pelvis. This should be 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.

Free body diagram

Force produced by brachialis (B) acts at a distance 5cm from the instantaneous of rotation of the joint(IAR). Force produced by the weight of the forearm taken to be 15N acts at a distance of 15cm from the  IAR. The force produced by the weight held in the hand acts at a distance of 30cm from the axis of rotation.

Considering the rotational equilibrium of the forearm about IAR, summation of all moments about O will be zero

• The wrist joint connects the hand to the forearm and 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 anteroposteriorly 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. 
• 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 a bicondylar hinge. They are very 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.

• The hip joint 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 120° of flexion and 20° 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 the diaphysis. Normal neck–shaft angle is 125°. 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 is 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 (e.g. 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.

Free body diagram hip

Specific assumptions about the FBD of the hip joint

• Single leg stance
• Weight of one leg is 1/6th total body weight

Assume BW=600N(1 X limb=1/6BW)

a=0.05m

b=0.15m

Clockwise moment=Anticlockwise moment

500 x 0.15 = FAB x 0.05

FAB=1500N

A force triangle is then drawn to calculate JRF-estimated by length of the limbs or by trigonometry

JRF=2000N

5/6BW=500N

FAB=1500N

Introduction of a stick

• Introducing a stick on the opposite side adds another anticlockwise moment.
• The effect is to lower the body weight force and aid the abductors
• As the moment arm on the stick is large there is a significant reduction in JRF

Figure 11. FBD Hip

Anatomy

• Femorotibial surfaces.
• Largest joint of the body, condyloid type articulation:
• Femoral condyles are described as “cam-shaped” in lateral profile. Their cross-section is generally oval, being described as part of a helix. However, the posterior part, the part that meets the tibia in deep flexion is circular.
• Medial condyle has a larger radius of curvature than the lateral. Medial compartment is concave and stabilised on weight bearing. It also extends distal to the lateral on AP view.
• Lateral condyle may be smaller, but projects further anteriorly to act as a buttress against lateral patellar dislocation. In contrast to the medial condyle, the anterior part of the lateral condyle is rather flat and is in contact with the anterior horn and the anterior part of the tibial articular surface in full extension. Lateral compartment is less stable compared to the medial one.
• Tibial plateau has a 30° lateral inclination and 90° of posterior slope.
• Medial tibial plateau is posteriorly flat where it is in contact with flat posterior horn of the menisci and the posterior femoral facet. Anteriorly it slopes upwards and forwards. Here, the anterior part meets the anterior circular facet of the femur in extension.
• Lateral tibial plateau is centrally flat but curves downwards in front and behind to provide area for the menisci.
• Contact area of medial plateau is 50% larger than the lateral one and sustains higher forces than the lateral.
• Tibiofemoral contact is made more congruous by the presence of the menisci.
• Between the two articular surfaces is the intercondylar eminence. It is like a raised inverted cone. The eminence is the pivot around which the femur rotates. It is straight medially and convex laterally; suggesting that as the condyles move around the eminence, medial condyle translates in a straight line, while the lateral condyle has a somewhat curved excursion.

 The menisci

• Triangular shaped on cross-section.
• They increase the congruence of the femorotibial joint surface and increase the available surface area for transfer of force from femur to tibia. So, they are load bearing to some extent.
• They act as a mechanical block to joint dislocation (in extreme flexion of the knee, the femoral surface is actually in contact with the posterior horn of the menisci and not the tibia).
• MM provides greater restraint to ant translation. LM is more mobile and the popliteus tendon guides its movement. MM is more constrained and more prone to injury.
• They also help in shock absorption.
• Menisci can transmit up to 50% of compressive load in extension and 90% in flexion. Medial menisectomy can decrease contact area by 70%.

The collateral ligaments

• Provide side-side stability. MCL is assisted by contraction of pes anserinus. LCL is assisted by ilio-tibial tract, tensed by tensor fasciae latae.
• They are taut in extension and lax in flexion.
• Superficial part of MCL is the more important contributor to stability. Cutting the superficial part results in joint widening. But if the deep part is cut keeping the superficial part intact then there is very little laxity.
• Posteromedial capsule gets tight with increasing extension of the knee and provides some stability. MCL becomes important to stability with increased flexion.
• LCL resists half of varus load in full extension and is the primary restraint to varus load in ROM of flexion. LCL is tight in extension and get increasingly lax at more than 30° of flexion. With increasing flexion, biceps femoris provides continuous tension to LCL and helps to maintain its role as a stabiliser against varus load.

The cruciates: anatomy and biomechanics

• ACL: although a single ligament, it is arranged into distinct bands. It has ant-medial and post-lateral bands: named from their tibial origins. Ant-med band is taut in flexion, the post-lat band is taut in extension.
• The bands change in length through knee movement. Both anatomically and functionally, the ligament is a collection of distinct bands and is never truly ISO METRIC. It is flat in extension, but twists 90° on itself on increasing flexion.
• The ligament becomes more horizontal with flexion and acts as a primary restraint to anterior tibial translation.
• It also acts as a secondary restraint to IR, varus/valgus strain and hyperextension.
• Apart from anterior translation of tibia, ACL deficiency also results in loss of roll-glide mechanism at the joint. Rolling predominates in initial flexion, followed by sudden posterior shift of contact point (this is the basis for the pivot shift test). This abnormal movement also increases the risk of meniscal damage.
• Anterior draw of tibia is accompanied by coupled IR of tibia and posterior draw by tibial ER.
• PCL: also arranged in two bands – ant-lat and post-medial. No part of PCL is truly isometric during ROM.
• It is a primary restraint to post translation of the tibia on the femur and secondary restraint to varus/valgus and ER. It causes couples ER of tibia during posterior translation.
• ACL and PCL are commonly described as being analogous to a crossed four bar linkage system. It accounts for the changing roll-glide ratio with knee movement.

Figure 13. Representation of the human knee joint composed of a four-bar linkage. Illustration of the posterior translation of the contact point between the femur and the tibia.

The popliteus and screw-home mechanism

• Popliteus has important role in screw home mechanism.
• Screw-home mechanism is essentially an automatic rotation of tibia between 0 and 20° of knee flexion.
• Helped by:
• Larger medial femoral condyle.
• Longer AP dimension of tibial medial condyle.
• Adds stabiliy to knee in extension.
• From 20° of flexion to full extension.
• Anterior tibial glide.
• Tibia rotates externally.
• This is the STABLE position of the knee.
• Reverses in knee flexion.
• From full extension to 20° of flexion.
• Posterior tibial glide.
• Internal rotation of tibia.
• Popliteus complex consists of a dynamic component: which is the popliteus muscle and a static complex: which is the popliteal ligamentous complex. It originated from posteromedial part of tibia and is directed obliquely outwards to lateral femoral condyle.
• It is unique in being the only structure positioned obliquely across the posterolateral corner of the knee and is well suited to prevent tibial ER with increasing knee flexion. Its fibres are tensed by tibial ER. Sectioning of popliteus results in marked increase in tibial ER in 90° flexion.
• Popliteus is active in screw-home mechanism in terminal knee extension.
• It initiates knee flexion by unscrewing the locked knee.
• It also retracts the lateral meniscus.
• Popliteofibular ligament is an important restraint to tibial ER at all ranges of flexion.

Figure 13. During the last 20 degrees of knee extension the anterior tibial glide persists on the tibia's medial condyle because its articular surface is longer in that dimension than the lateral condyle's. Prolonged anterior glide on the medial side produces external tibial rotation, the "screw-home" mechanism.

Biomechanics

• Open and closed kinematic chain: body joints are likened to mechanical chains. In open chain movement, the proximal component is fixed and the distal component is mobile. Example: kicking a ball. Distal component is fixed in closed chain movement. Example: getting up from a chair.
• This has important function implications. Bone movements are obviously dictated by which part is fixed. Example: with open chain exercise, tibia is mobile and glides anteriorly on femur, but in closed chain exercise tibia is fixed, so femur has to yield and slide in posterior direction.
• Recent MRI studies of the articular surfaces (Martelli and Pinskerova, 2002)⁷ have revealed some differences in femorotibial articulating surfaces and challenged some old conceptions. In the functional range, the medial part of femur is spherical and contained in a somewhat conforming tibial socket. This would suggest that the medial femoral condyle would have six degrees of freedom to rotate on tibia but little ability to translate and only in anteroposterior direction. It has been likened to a ball–socket articulation like the hip. There is little backward movement of the condyle, but the contact area between femur–tibia does translate posteriorly.
• Over the functional range, femur rests on a largely flat tibial surface on the lateral side. Femur is spherical posteriorly but flat distally where it meets the intercondylar eminence. This would allow free anteroposterior translation but little rotation. Lateral condyle has been likened to a “wheel of a sidecar on ice.” It rolls and slides in ant-post direction. So, laterally both the femoral condyle and femorotibial contact area moves posteriorly.
• The lateral condyle translates much more in posterior direction than the medial one. As a result there is femoral ER with flexion.
• This behaviour is true for the functional range. Beyond 120° flexion, both condyles present circular surfaces and roll back posteriorly onto the posterior horns of the menisci and the joint is in a subluxed state.
• The axis of rotation of the femoral condyles is through the medial spine. This is because of the shape of the spines. Medial edge of the medial spine is almost straight in anteroposterior direction, while the lateral edge of the lateral spine is convex.
• The medialisation of the axis means that the lateral condyle has to travel more than the medial one.
• Femorotibial roll-glide is dependent on many factors. Knee extensors actively cause gliding when they pull the tibia anteriorly under the femur during knee extension. Conversely, flexors cause tibia to glide posteriorly during flexion.
• The cruciates have a passive role in roll-slide. ACL pulls on femur and causes it to slide anteriorly during knee flexion while it rolls in a posterior direction.
• During extension, PCL causes the femoral condyle to slide posteriorly while it rolls in an anterior direction.
• As a whole, the cruciates ensure ant-post stability while allowing hinge movements of the knee. ACL is stretched by extension and helps to limit hyperextension. PCL is stretched by flexion.
• The knee joint has 6 degrees of freedom, but predominant movement occurs in the sagittal plane.
• For the sake of simplicity, the joint can be thought of as a hinge.
• Normal functional range of knee movement is 200–120°. Flexion beyond 120° is entirely passive. Active flexion can be increased beyond 120° by hip flexion as the hamstrings are relaxed and become more efficient knee flexors.
• The knee is stable in extension and unstable in flexion. Knee can rotate on flexion, this is maximal at 90° of flexion. Knee flexion, while allowing knee rotation, prevents hip rotation.
• Knee ROM can be divided into three distinct stages. Between 0 and 20° is the range of “screw-home movement.” 200–120° is the active functional range of movement. Between 120° and 160° is the range of arc of passive flexion. These three different arcs of movement have differing kinematic properties.
• Knee flexes about 20° during stance phase, increasing to maximum 60° during swing phase for foot clearance. Flexion up to 120° is used for stair climbing and getting off chairs. In this active range longitudinal rotation of tibia can occur independent of flexion and vice versa.
• Flexion beyond 120° is passive. The knee gets subluxed, femoral condyles lose contact with tibial surface and are in contact with posterior horn of the menisci.
• The relative movement of the condyles and the tibiofemoral contact surfaces are rather complex. Femur both slides and rolls on tibia. If it only rolled then femoral condyle would have completely dislocated from the tibial platform (it does to some extent) as the length of circumference of femoral condyle is twice the length of the tibial condyle.
• If it only slides, then flexion would have been prematurely stopped by impact of femur on the posterior border of the tibial condyle.
• In the arc of active flexion, the lateral femoral condyle translates posteriorly much more (15 mm) compared to the medial (2 mm). This is by a combination of roll and slide. The result is about 30° external rotation of femur around a medial axis in this range.
• It was thought that both condyles rollback and slide. Recent MRI evidence would suggest that the medial femorotibial articular surfaces is somewhat congruent to allow any effective sliding and as such the medial condyle can only rotate like the ball in a socket and produce flexion and longitudinal rotation. On the other hand, as the lateral surface is comparatively flat, it allows the lateral condyle to roll and slide (Martelli and Pinskerova, 2002).⁷
• It is now felt that the medial condyle flexes and rotates longitudinally, whereas the lateral condyle undergoes both roll and glide.
• The medial tibi-femoral contact area transfers from anterior to posterior through increasing flexion.
• The joint reaction force shifts from medial to lateral throughout the gait cycle. Medial plateau sustains the force in stance phase when the force is the highest.
• Femoral axis runs inferomedially. So, the applied force F on tibia has a vertical component V and a horizontal component T. The horizontal component is resisted by the medial soft tissue constraints. If there was no medial soft tissue balance, then the femur would tilt medially (broken blue femur).

Patellofemoral joint

• Patella is an ovoid sesamoid bone embedded in the quadriceps muscle. Patellofemoral joint is part of the knee joint. But functionally the joint is distinct from the condyloid tibiofemoral articulation.
• Anatomy of the patella:
• Patella is triangular with apex directed downwards.
• Anterior surface of the patella is gently convex.
• Deep surface is only partially articulating and changes throughout the ROM of the knee.
• The joint surfaces are not very congruent. Upper three-quarters part of the posterior surface is articular articular surface has a convex medial facet and a concave lateral facet.
• Medial facet is divided into a medial odd facet and a lateral middle facet, lateral facet has a larger contact area than the medial facet.
• Articular cartilage of the patella is the thickest of the body, signifying the significant stress that the joint goes through. The cartilage does not follow the contour of the underlying subchondral bone. Cartilaginous medial ridge does not accurately follow the bony medial ridge of the patella.
• Anatomy of the distal femur:
• Anterior trochlear surface of distal femur is notched to accommodate patella.
• Viewed end-on, the lateral femoral condyle projects more anteriorly and acts as a buttress to prevent lateral subluxation of patella.
• Mean angle of the femoral sulcus is 137°.

Figure 15  Contact area patterns on the patella as a function of the knee flexion angle. In general, contact area increases with increasing knee flexion.

Biomechanics of PFJ

• Patellar articular surface has shifting area of contact throughout the ROM of the knee.
• From full extension to full flexion, patella glides 7 cm in a caudal direction on the femoral condyles.
• Lower third of the patella articulates with the trochlear surface of the femur in a relaxed knee.
• With increased flexion the articulating surface migrates superiorly on the patella.
• Whole patella moves superolaterally on quads contraction along the line of pull of quads- this is patellar tracking. The femoral articular cartilage extends more proximally on the lateral side to accommodate this movement.
• In flexion: medial odd facet articulates with the patellar surface of medial femoral condyle and middle facet sits in the intercondylar notch.
• In extension: odd facet becomes non-articular and the middle facet contacts with the intercondylar notch.
• Quads force increases with increased knee flexion.
• This is because as the knee flexes, the centre of gravity shifts posteriorly and increases the flexion moment on the knee, so the quads have to increase force to counter the flexion moment.
• This results in increased joint reaction force. Patellofemoral contact area increases from 0° to 60° flexion and helps to limit rise in compressive stress.
• Joint reaction force can be double in a knee flexed 90° compared to a knee flexed to 5°. This is only applicable for a closed chain exercise.
• Force is dissipated as it rounds the patella. This is because the quads tendon is attached to patella nearer to joint surface than the patellar tendon.
• Patella moves laterally with increased flexion, increased flexion also results in internal tibial rotation and Q angle to disappear. So, lateral subluxation of patella occurs in early flexion.
• The lever arm of the patellofemoral joint is maximal at 15–20° of flexion, when the tibiofemoral contact area moves posteriorly.
• Patellofemoral contact pressure is maximum at 80–90° of flexion.
• The joint contact area also reaches a maximum at 90° flexion, thereby reducing the contact stress at deep flexion.
• It is also notable that with deep flexion the patellofemoral contact area shifts proximally where the cartilage is thickest.
• So, we have highest contact area and highest cartilage thickness at highest compressive load.

Implications

• Patello-femoral pain:
• For normal patello-femoral function, we need normal patellar tracking.
• Normal pressure on patellar articulating surface.
• Patellofemoral function also depends on:
• Overall limb alignment.
• Tibiofemoral alignment.
• Rotational position of the femur.

Patellar tracking

• Normal patellar tracking is a fine balance between lateral and medial patellar constraints.
• Patella has both dynamic and static constraints.
• Important static constraint is the medial patellofemoral ligament.
• Important dynamic constraint is the VMO (vastus medialis obliquous). VMO holds the patella flush against femur even in extension and neutralises the lateral pull.
• VMO weakness would result in lateral subluxation of patella in early flexion. As most of the cases of patellar dislocation takes place in early flexion, it is likely that weak quads is a very important contributing factor.
• Oblique quads force has an important lateral vector.
• An imbalance of the constraints results in increased lateral pull and patellar mal tracking examples include:
• Increased Q angle, female sex (broad pelvis).
• Patella alta: essentially superior patellar subluxation, less patellofemoral congruence, loss of buttressing effect of prominent lateral femoral condyle and lateral subluxation.
• Genu recurvatum: patella is lifted off the femoral surface, again the buttressing effect is lost, predisposing to instability.
• Lateral femoral dysplasia would also increase patellar instability due to loss of lateral buttress effect.
• Q angle could also be increased due to femoral neck anteversion, external tibial torsion, and subtalar pronation.

Figure 16 The patellofemoral joint reaction force increases as a function of quadriceps force and knee angle. F_q, Force of the quadriceps tendon; F_p, force of the patella tendon; R, patellofemoral joint reaction force

• 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 up to 1 mm during loading to deepen the mortise and increase stability. Lateral ligament complex resists INversion and IR. Anterior talo-fibular ligament prevents anterior talar displacement (commonest ligament to be injured) and IR of talus. Posterior talofibular 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.

Figure 17. Ankle motice