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III ASSESSING FRACTURE HEALING Few would quarrel with the proposition that anatomic restoration is the radiographic hallmark of fracture healing, but is this goal realistic when assessing frac- tures? Of course not. Full restoration may require months or even years to complete, and who would monitor the radiographic progress of an uncompli- cated fracture for that long? So what is a practical objective in evaluating fracture healing, and how can it be determined radiographically? Before going further with this train of thought, some background information is necessary; namely, what are the expectations for any individual fracture: how long will it take to heal or, in the medical parlance of the day, what is the expected outcome? Determinants of Fracture Healing The following factors influence fracture healing: The severity of the fracture Which bone is fractured Where in a particular bone the fracture occurs Whether or not the fracture is fresh Whether or not the fracture is open The age of the horse Whether any other serious injuries or preexisting disease are present How the fracture is repaired The skill of the surgeon The quality of the aftercare Fracture Severity. Generally, comminuted and multi- ple fractures require more time to heal than simple two-piece fractures, assuming equal degrees of reduc- tion and stabilization. The greater the degree of fragment displacement, the greater the amount of secondary muscle and vascular damage—both impor- tant factors in callus formation. Fracture Location. Fractures that occur at the distal end of long bones often cannot be compressed because Chapter 2 Fracture Healing and Other Forms of Bone Remodeling there is insufficient room for a bone plate and screws. Distal limb fractures are also surrounded by less muscle than proximal fractures and thus have less potential collateral circulation and soft-tissue support. Fracture Age. Fresh fractures are easier to work with, and the surrounding muscles, nerves, lymphatics, and vasculature are in better condition. Subacute fractures are not only often overridden but are beginning to form a primitive callus, which, along with lacerated and bruised muscles, makes fragment manipulation very difficult. Unstabilized limb fractures move regu- larly, leading to varying degrees of secondary frag- mentation, especially at the ends of the damaged bones. A similar fate awaits horses that must be trans- ported more than a short distance to the hospital, so-called transport fractures. Open Fractures. Open fractures (also termed com- pound fractures) are often infected and as such carry a greater potential for nonunion than closed fractures. Age of the Horse. In people, pets, and cattle, the young heal more rapidly and with fewer complica- tions than do the old. Thus foals, yearlings, and young adults usually heal more rapidly than older horses, assuming a comparable degree of injury and similar treatment. Concomitant Injury. Concomitant injuries place an additional demand on the body’s resources, especially the immune system, compromising healing to varying extents. Method of Repair. All things equal, a plated fracture will heal more rapidly and with less callus than one that is pinned. Surgical Skill. The skill of the surgeon may be the single most important variable in predicting the outcome of orthopedic procedures. 23

Veterinary Diagnostic Imaging: The Horse || Fracture Healing and Other Forms of Bone Remodeling

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III ASSESSING FRACTURE HEALING

Few would quarrel with the proposition that anatomicrestoration is the radiographic hallmark of fracturehealing, but is this goal realistic when assessing frac-tures? Of course not. Full restoration may requiremonths or even years to complete, and who wouldmonitor the radiographic progress of an uncompli-cated fracture for that long? So what is a practicalobjective in evaluating fracture healing, and how canit be determined radiographically?

Before going further with this train of thought, somebackground information is necessary; namely, whatare the expectations for any individual fracture: howlong will it take to heal or, in the medical parlance ofthe day, what is the expected outcome?

Determinants of Fracture Healing

The following factors influence fracture healing:

∑ The severity of the fracture∑ Which bone is fractured∑ Where in a particular bone the fracture occurs∑ Whether or not the fracture is fresh∑ Whether or not the fracture is open∑ The age of the horse∑ Whether any other serious injuries or preexisting

disease are present∑ How the fracture is repaired∑ The skill of the surgeon∑ The quality of the aftercare

Fracture Severity. Generally, comminuted and multi-ple fractures require more time to heal than simpletwo-piece fractures, assuming equal degrees of reduc-tion and stabilization. The greater the degree of fragment displacement, the greater the amount of secondary muscle and vascular damage—both impor-tant factors in callus formation.

Fracture Location. Fractures that occur at the distalend of long bones often cannot be compressed because

C h a p t e r 2

Fracture Healing and Other Forms of Bone Remodeling

there is insufficient room for a bone plate and screws.Distal limb fractures are also surrounded by lessmuscle than proximal fractures and thus have lesspotential collateral circulation and soft-tissue support.

Fracture Age. Fresh fractures are easier to work with,and the surrounding muscles, nerves, lymphatics, andvasculature are in better condition. Subacute fracturesare not only often overridden but are beginning toform a primitive callus, which, along with laceratedand bruised muscles, makes fragment manipulationvery difficult. Unstabilized limb fractures move regu-larly, leading to varying degrees of secondary frag-mentation, especially at the ends of the damagedbones. A similar fate awaits horses that must be trans-ported more than a short distance to the hospital, so-called transport fractures.

Open Fractures. Open fractures (also termed com-pound fractures) are often infected and as such carry agreater potential for nonunion than closed fractures.

Age of the Horse. In people, pets, and cattle, theyoung heal more rapidly and with fewer complica-tions than do the old. Thus foals, yearlings, and youngadults usually heal more rapidly than older horses,assuming a comparable degree of injury and similartreatment.

Concomitant Injury. Concomitant injuries place anadditional demand on the body’s resources, especiallythe immune system, compromising healing to varyingextents.

Method of Repair. All things equal, a plated fracturewill heal more rapidly and with less callus than onethat is pinned.

Surgical Skill. The skill of the surgeon may be thesingle most important variable in predicting theoutcome of orthopedic procedures.

23

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24 SECTION I III The Extremities

A B

C DFigure 2-1 • Radiographs of a horse that stepped in a hole and dislocated its pastern joint approximately a year ago. Theanimal is now mildly to moderately lame, especially after exercise. Close-up lateral (A) and dorsopalmar (B) views of thepastern joint show advanced osteoarthritic remodeling as evidenced by (1) symmetric periarticular and extraarticular newbone deposition, (2) a narrowed cartilage space, and (3) subchondral sclerosis. Similar views (C, D) of the opposite normalpastern are provided for comparison.

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CHAPTER 2 III Fracture Healing and Other Forms of Bone Remodeling 25

A B

C DFigure 2-2 • Radiographs of a horse that completely severed its flexor tendons 2 to 3 years previously. As a result, bothproximal sesamoids are severely deformed, as seen in close-up dorsopalmar (A), true lateral (B), lateral oblique (C), andmedial oblique (D) views, resembling what is often observed following displaced apical or body fractures. Adaptive remodel-ing of this sort is rarely documented radiographically because many horses with such injuries are destroyed.

Postoperative Aftercare. The type, amount, andquality of postsurgical aftercare strongly influencehealing time. This is especially true of physiotherapy.

III BONE GRAFTS

Although it is theoretically possible radiographicallyto identify and monitor the progress of autogenouscancellous bone grafts, in fact, such exercises provefutile more often than not.1 On the other hand, crushedand stave-type cortical bone grafts can usually be

readily recognized. Examples of short- and long-termfracture healing are presented in the chapters thatfollow.

III BONE REMODELING

Exercise Induced

Sprint and endurance training in racehorses causesvarying degrees of bone remodeling, typically takingthe form of increased radiographic density and

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26 SECTION I III The Extremities

Figure 2-3 • Lateral view of the fetlock of a horse that completelysevered its right front flexor tendons 20 years ago. As a result, themetacarpal condyle has “drifted” toward the palmar aspect of the proxi-mal phalanx, which now has a flatter articular surface, to better match thedorsal half of the overlying condyle, with which it now articulates. Fortheir part, the sesamoids are fully (and for the most part) articulating withthe palmar aspect of the canon bone, accommodating its forward inclina-tion and the hyperflexed attitude of the fetlock. The flattened mound ofnew bone on the dorsal surface of the distal metacarpal metaphysis and asimilar deposit on the underlying phalanx are impingement exostoses, aconsequence of one surface striking the other.

A BFigure 2-4 • A, What makes the previous case (Figure 2-3) an even better example of remodeling are the secondary changesto the opposite fetlock, which in many respects are more pronounced than those in the injured limb. Note the greater degreeof hyperextension, and the extensive dystrophic calcification in the suspensory field. B, A normal lateral view is provided forcomparison.

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CHAPTER 2 III Fracture Healing and Other Forms of Bone Remodeling 27

increased scintigraphic uptake. The most commonlyaffected bones in the horse are the third carpal, proxi-mal sesamoids, and distal metacarpus/metatarsus.2

The described bony alterations are often quite subtle, requiring technically comparable, high-quality, serial examinations to appreciate any radiographic differences.

Traumatically Induced

Perhaps the best and most obvious example of skele-tal remodeling is the fracture callus. Take the case of asimple two-piece distal extremity fracture treated witha cast. The ends of the broken bone are first physio-logically fixed by a combination of clot and connectivetissue, the so-called soft callus. Then, through a sur-prisingly sophisticated process, the soft callus is trans-formed into a lump of very primitive bone, which notonly joins but also immobilizes the fracture fragments.The process of remodeling begins shortly thereafter.

Thought of most simply, fracture remodeling is arestorative process. Seemingly, the broken bone isattempting, not just to repair itself, but ideally toregain fully its original appearance: a normal cortex, anormal medulla, a normal length and width, and nodeformity. Like any restoration, the process of fracturehealing is often slow and sometimes tedious, a little bitof bone added here, a little bit subtracted there. In mostinstances, however, the bone succeeds in its restorativeefforts, so much so that some months later it may bedifficult or impossible to identify the original injury.

Accommodative

Remodeling may take another form, one predicated onthe concept of accommodation. A good example of

accommodative remodeling is the sprain-fracture-dislocation. In this often painful and debilitatinginjury, one or more joint surfaces are often fractured,producing large gaps in the articular cartilage and sub-chondral bone, which fill eventually with new bone, asort of interior callus. The result is incongruency, onejoint surface no longer matching the other.

Triggered by an articular mismatch, and to a lesserextent by any associated instability, each of theinvolved bone surfaces, injured and uninjured alike,attempts to reach an anatomic accommodationthrough the process of remodeling. In other words,each surface tries to match that of the other, althoughusually not with the usual accoutrements of a normaljoint, such as articular cartilage.

Accommodative remodeling also can occur in unin-jured parts, especially joints in the opposite leg.Secondary remodeling is induced by a variety of bio-mechanical factors but most importantly by a combi-nation of overwork and overload. Examples ofprimary and secondary accommodative remodelingare shown in Figures 2-1 to 2-4.

References

1. Kold SE, Hickman J, Melson F: Qualitative aspects of theincorporation of equine cancellous bone grafts, Equine VetJ 19:111, 1987.

2. Ehrlich PJ, Dohoo IR, O’Callaghan MW: Results of bonescintigraphy in racing standard bred horses: 64 cases(1992-1994), J Am Vet Med Assoc 215:982, 1999.

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