Time Zero

The Forces at Work

The Common Vein Copyright 2011

Ashley Davidoff MD

“breaks like a lifeless stick if overloaded..” (Wojnar)

 

Introduction

Time zero is the moment of impact when two fundamental forces are in play;

the imposing force itself and

the resistance of the bone

The result initially is quite simple – “yes fracture” or “no fracture”

This module expands the “yes fracture” route and explores the variety of imposing forces and the nature of specific bones under the circumstances and the consequent appearance of the of the fracture.

High Velocity Low Flying Halloween Trauma

The image on the left shows a witch who flew into a telephone pole.   It  is the closest available image depicting time zero. The impact of the accident is sustained by her forehead on the pole. A depressed comminuted fracture of the frontal bone is seen on the right side of the image.  This injury is often the result of a high speed decelerating injury caused by impaction of the patient’s forehead on the steering wheel or the front wind screen. In such an accident associated injuries such as whiplash events to the cervical spine or aortic tears can be life threatening. Courtesy Ashley Davidoff MD and Philips Medical Systems Copyright 2011 65840c04.8s

Types of Forces

Thus the type of forces that can lead to a fracture relate to

the type of instrument generating the force; blunt vs penetrating

the magnitude of the force; velocity, duration, surface area of contact, and

the vector of the force;  longitudinal, transverse, spiral, twisting.

Most fractures arise from blunt trauma.  Less common but often more devastating are the injuries that relate to high velocity and penetrating injuries such as bullet wounds and knife injuries.  When the skin and soft tissues are breached, the risk of visceral injury in the path of the penetration is high and therefore meticulous evaluation is required often involving a CT scan.  Also the risk of infection becomes high and therefore management of penetrating injuries requires meticulous debridement of the wound and long term care of the wound.  Most fractures resulting from blunt trauma are closed (aka simple) fractures implying there is no breach of the soft tissues.  Blunt trauma less commonly does cause a breach of the skin and they are then classified as open or compound fractures.  These fractures therefore require meticulous debridement of the wound as well as meticulous wound care since there is a high risk of infection.

 

The Direction of the Force

The forces may come from one of many different angles at the moment of impact, but we will discuss three major forces exemplified by their impact on the shaft of long bones. The basic forces include; right angled forces, those traveling along the long axis of the bone and those that have a twisting component.

Force at Right Angles to the Long Axis of the Bone

The following diagram describes shows the effect of an excessive force when the force is at right angles to a long bone.

 

When the force at right angles has a higher impact and also if the area is located where ligamentous attachments anchor the bone to multiple other bones the injury can have significant ripple down effects.  The example below is also a blunt injury to the lateral malleolus but the site of impact is slightly higher on the fibula, and perhaps the velocity force weight and area of contact were more excessive and so the injury is far more serious.

Sudden Force Imposed at Right Angles to the Lateral Malleolus

This X-ray on the antero-posterior (A-P) projection shows the effects of a sudden force exerted on the lateral aspect of the left ankle of a 36 year old male (red arrow b). The result at first appears to be a simple displaced oblique fracture of the distal shaft of the fibula However two additional observations are warranted. The first is the presence of significant, medial displacement of the proximal fragment of the fibula (white arrow in b) so there is virtually no bone on bone alignment with the distal fragment of the fibula. This morphology is not conducive to satisfactory healing and likely would require open reduction. In addition to the fracture there has been significant damage to soft tissues and the mortise is not intact with medial displacement and dislocation of the tibia in relation to the talus (yellow arrow) suggesting rupture of at least the deltoid ligament (interrupted blue line with blue arrow). So the findings are consistent with a fracture dislocation and the implications are significant ligamentous injury and indication for surgical repair. Ashley Davidoff TheCommonVein.net  99854.6b01c02L.8

Forces along the Long Axis of a Long Bone or a Series of Bones (Spine)

Impaction and Compression Fractures

 An impaction fracture occurs when one fragment is forcibly driven or telescoped into an adjacent fragment and or are compressed against one another.

A compression fracture is a type of impaction fracture where a flat surface of one bone forces an adjacent flat surface to compress.  This is best exemplified in vertebral body fractures.

This type of force often will cause an impaction and or compression fracture at the weakest point of the bone, or a fracture of that area that will receive the largest impact of the force.  A fall on an outstretched hand will usually direct the force along the diaphysis of the radius (which has a thick cortical rim) to the region of the metaphysis which is an area where the cortical (compact bone) thins and the fracture will usually be at the interface of the diaphysis with the metaphysis.  This fracture is common and characteristic in location and is called a Colle’s fracture.

The following example describes the compressive forces on the radius and ulna when the mechanism of the injury is a fall on the outstretched palm of the hand with outstretched forearm.  There is a natural reflex to open the palm of the outstretched forearm in order to break the fall and prevent the face and head from incurring the injury. Tripping is a frequent day to day event as people go about their business mostly looking forward as they walk and not down. An unexpected elevation in the pavement or sidewalk form a root popping up, or a slight elevation or crack in the cement causes the trip and fall. In the winter slippery “black” ice, which is mostly invisible is a major cause of tripping injuries, particularly in the elderly.

Impaction Force on the Distal Radius – Fall on the Outstretched Hand Full Weight of the Body on the Distal Radius

The image demonstrates the position of the wrist when a person falls forward and the reflex is to flex the wrist and open the palm of the hand to break the fall. The pressure is placed on the distal radius and ulna, and if excessive will fracture one or both bones. This is an example of a compressive force, (red arrow) resulting in an impaction force, where the shaft of the radius is impacted on the broader but weaker metaphysis of the radius. The fracture is called a Colle’s fracture. Courtesy Ashley Davidoff Copyright 2011 61876c02.8s

Vulnerable Locations in the Long Bones

A typical long bone consists of a shaft and two ends.  As noted in the section on the structure of bone, the compact bone is thickest around the diaphysis and thins at the junction of the diaphysis with the metaphysis where spongy bone becomes dominant.  The Colle’s fracture described above occurs at this weak point.

A Weak Spot in the Long Bones Junction of the Diaphysis and Metaphysis

The diagram shows the basic anatomy of the distal end of a long bone such as the humerus or femur which consists of a shaft (diaphysis) and an expanded end each containing an epiphyses (pink), the growth plate in children (purple line) or epiphyseal line in adults, and the metaphysis (red). The shaft (diaphysis) consists of a thick outer layer of compact bone (white) which thins significantly as it progresses to the metaphysis (green arrow). The metaphysis and epiphyses contain spongy bone (trabecular or cancellous bone) which is more porous and softer.  The fracture of the Colle’s fractures for example, occurs at this juncture. Courtesy Ashley Davidoff Copyright 2011 104007b06c19L.8s

The following example shows CT scan of a fracture at the diaphyseal-metaphyseal junction of the proximal humerus .

CT Scan Shoulder The Weak Spot

The reconstructed CT scan of the right shoulder exemplifies the pathogenesis of a fracture along the vulnerable part of the bone where the shaft (diaphysis) meets the metaphysis (neck) indicated by a green arrow. The most important aspect to appreciate is the manner in which the strong compact bone tapers as it progresses from the middle of the diaphysis to the junction of the diaphysis and metaphysis also marked by the arrow. The spongy resides in the epiphysis (salmon pink) and metaphysis (red). Courtesy Philips Medical Systems and Ashley Davidoff MD 88367bc03.8s

Compressive and Impaction Forces on the Axial Skeleton

Compressive and impaction fractures of the spine send frightening chills down the spine since the consequences can be so devastating and are usually injuries of young active people.  Athletic injuries such as in football and ice hockey where forces may be projected along the axial skeleton are not uncommon.  It is also a common injury in people falling from a height, or from diving accidents where the brunt of the force is borne by the spine.

The vertebral bodies are composed dominantly of spongy bone and are therefore relatively weak.  Sometimes the fracture is a mere compression but with more forceful vectors the vertebral body can be shattered.  In addition to the primary force and the reactive forces there are a host of chain reactions that follow time zero at the time of the main reaction.

A compression fracture of the spine is the most common fracture of the spine and can manifest with a simple fracture with no neurological sequelae, but on the other extreme there can be retropulsion of fragments into the spinal cord with potentially devastating and life threatening sequelae.

Diving injuries where the compression extends from the cranium down the spine has resulted in many quadriplegic injuries with unimaginable lifelong morbidities.

C1/C2 fractures may result in loss of breathing, C3 injuries may result in loss of diaphragmatic movement and hence loss of spontaneous respiration, and all complete injuries above C7 will preclude the ability for normal independent daily living.

Vertical Forces on the Cervical Spine During Diving

The image shows a young man diving off a high board into the sea and will be gaining significant downward, and vertical gravitational forces (red arrow) as he straightens out for his entry into the water. If he had the terrible misfortune to hit any shallow rocks, a resistant force of the rocks would be equal in force but in the opposite direction (green arrow, b) with no “give” to the opposing force that would have been afforded by water. The opposing forces exerted on his spine would result in a compressive injury on his cervical spine with potential devastating consequences including quadriplegia or even death. . Courtesy Ashley Davidoff Copyright 2011 98186pb03.8s

The following X-Ray shows the appearance of a burst compression fracture of the C5 vertebral body with anterior and posterior displacements of fragments.

Pulling Forces: The Avulsion Fracture

Sometimes the force is a “pulling” force rather than a pushing force.  In the section on structural principles we learned that the combination of the hydroxyapatite and collagen provided bone with a high compressive strength, but poor tensile strength and very low shear stress strength.  This infers that it resists pushing forces but not pulling or torsional forces.

Pulling forces are usually secondary to a pushing force with secondary traction (pulling) on another bone by ligaments or tendons. The result is an avulsion fracture.

Spiral Forces

Spiral fractures occur when torsional forces of both compressive and tensile nature exceed the limits of brittleness and toughness of bone and the resulting fracture has a spiral pattern.   Spiral fractures may also occur when both pulling and pushing forces act in opposing vector lines and the net vector is oblique or twisted.

 This spiral  force is  usually generated when the the person trips and the limb is anchored for example in a pothole or in ski bindings, and the weight and center of gravity of the person creates a second  vector line causing the whole axis to spiral. 

Spiral Fracture of the Humerus

The X-ray of the right humerus in A-P projection is from a 76 year old male and shows comminuted spiral fracture consisting of three fragments with a middle triangular or butterfly fragment. (dark green). There is reasonable anatomic alignment with excellent bone on bone contact. The fracture was treated with a sling and the follow up 4 months later showed excellent healing . Ashley Davidoff TheCommonVein.net 103417c03.8s

Segmental Forces and Fractures

A segmental fracture occurs when a fracture along a long bone occurs at different levels creating at least three distinct segments.

This fracture may occur with transverse forces across the bone such as outlined below, or by a cascade of forces as the primary force is transferred and reflected between bones or between bone and other anchoring forces such as ligaments, tendons and muscles.  An example of bone to bone transfer is described below exemplified by a fall on the flexed elbow and consequent vertical forces on the humerus.

Fall on the Elbow

The image demonstrates the position of the elbow when a person falls sideways and the reflex in this instance is to flex the elbow to break the fall. The downward pressure of the weight of the individual (red arrow) is placed on the proximal ulna and specifically on the olecranon process. The resistive force is the hard ground (green arrow) and the upward pressure is then a vertical force along the shaft of the humerus. The site of the fracture will be located where the force overcomes the strength of the bone or commonly at a weak point of the shaft ie the junction of the metaphysis with the diaphysis. If the force is sufficiently strong it will continue along the long axis of the shaft and either cause a second fracture along another weak point in the bone or together with the resistance of the scapula cause a second downward resistive force creating the second fracture. Courtesy Ashley Davidoff Copyright 2011 61321cL01.8s

Segmental Fractures of the Humerus at the Elbow and Shoulder

The lateral X-Ray of the left humerus demonstrates the theoretical to and fro of forces on the forearm as a person falls to the ground on the elbow. Image a is the X-ray as it presents to you. In image (b) the force 1 (red arrow) represents the downward pressure of the weight of the individual which terminates on the proximal ulna and specifically on the olecranon process as the person hits the ground. The resistive force of the hard ground (green arrow 2) and the upward pressure is then a vertical force along the shaft of the humerus. In this instance the olecronon pushes on the elbow joint and shears off a condyle of the humerus (3 green lightning arrow) In image c, the resistive force (green arrow 4) transfers the force back along the shaft If the force is sufficiently strong it will continue along the long axis of the shaft and will meet the resistance of the scapula. (orange arrow 5) and in this instance cause a second downward resistive force creating the second fracture at the weak point of the metaphyseal-diaphyseal junction (lightning arrow 6). Courtesy Ashley Davidoff Copyright 2011 88797c08.8

 Shearing Forces

A shearing force is a force acting on a bone in a direction at right angles to the protuberance of the bone and a shearing fracture will occur if this force causes the projection of bone to break.  

All long bones have protuberances at either end.  Thus if the  vector of the force is along one of these protuberances rather than on the long axis of the shaft, a shearing fracture could occur.  The distal end of the humerus for example, has prominent medial and lateral epicondyles and in the example shown above the mechanism of the fracture of the medial epicondyle was via shearing forces.  The force shears off the bony extension since that part of the bone is unable to transmit the force along the axis. The fracture line will run parallel to the vector of the applied force. Thus shearing forces result in the fracture of bony prominences not placed along the direct axis of a diaphysis or the main body of any other bone.

 

Shear Fracture and Dislocation at the Elbow

The lateral X-Ray of the left elbow demonstrates a fracture dislocation of the elbow of a 78 year old male after falling on his elbow. The injury demonstrates a shearing injury and fracture of the medial epicondyle of the humerus. Image a is the lateral X-ray. In image (b) the primary force (red arrow 1) represents the downward pressure of the weight of the individual with an anterior and vertical vector. It terminates on the proximal ulna and specifically on the olecranon process as the person hits the ground. The resistive force of the hard ground (green arrow 2) and the upward pressure is then translated into a vertical force along the elbow with the result that the ulna and radius are thrust upward. This force causes dislocation of the ulna and radius from the humerus and because the vector is along the medial malleolus there is a shearing force imposed on the protuberant medial epicondyle which sustains a shearing fracture (red edge along the epicondyle which is overlaid in teal) that runs parallel with the vector of the force (green lightning arrow in b). Courtesy Ashley Davidoff Copyright 2011 88797c01d07.8

 

 The vectors of fractures become complicated when multidirectional forces are at play.  For example a person who while running accidentally steps into a pothole and as a result of gravity falls sideways.  A twisting or rotational  force vs the bone ensues and the resulting fracture will reflect this as spiral fracture.

References

Newton C Etiology Classsification and Diagnosis of Fractures U Penn School of veterinary Medicine

Rixford E  Theory and Treatment of Spiral Fractures Annals of Surgery Vol 8 (1) 1925

Rooney J DVM  Fractures of Long Bones (Horses)

Summer Smith   Bone in Clinical Orthopedics

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