Keywords avulsion - fracture - orthopaedic - epidemiology - dog
Introduction
An avulsion fracture occurs when a bone fragment detaches from the main part of bone
at the attachment site of tendon, ligament or joint capsule as a result of physical
trauma. Tensile forces often cause an avulsion fracture, and shear forces also play
an important role in its mechanism of injury. An avulsion fracture is considered uncommon
compared with all other types of fractures; however, when they occur, they still cause
severe pain and functional disability in dogs. An avulsion fracture commonly occurs
near the joints and may involve the physis in growing animals, which generally requires
immediate surgical intervention.[1 ]
[2 ]
[3 ]
[4 ]
[5 ] Despite its clinical significance, little is known about the epidemiological aspects
of avulsion fractures in dogs.
Current textbooks describe avulsion fractures of the following anatomical locations,
including the acromion process (origin of the acromial part of deltoid muscle), supraglenoid
tubercle (origin of biceps brachii muscle), greater tubercle (insertion of supraspinatus
muscle), olecranon (insertion of triceps brachii muscle), ulnar styloid (lateral collateral
ligament), radial styloid (medial collateral ligament), accessory carpal bone (carpal
ligaments and insertion of flexor carpi ulnaris), ischial tuberosity (origins of biceps
femoris, semitendinosus and semimembranosus muscles), femoral head (round ligament
of the femur), greater trochanter (insertions of gluteal muscles), femoral origin
of gastrocnemius muscle, femoral and tibial insertions of cranial cruciate ligament,
femoral insertion of medial collateral ligament, femoral origin of long digital extensor
muscle, tibial tuberosity (quadriceps femoris), medial malleolus (medial collateral
ligament), lateral malleolus (lateral collateral ligament) and calcaneus (insertion
of gastrocnemius muscle).[3 ]
[4 ]
[5 ] The diagnosis and treatment outcomes of avulsion fractures have been sparsely reported
in veterinary literature and most of them are single case reports. At present, clinical
characteristics and rate of incidence of avulsion fracture in dogs are not well documented.
The goal of this study is to analyse the epidemiological data of canine appendicular
avulsion fractures from three large referral centres in the United States. Epidemiological
studies may provide insight to understand the manifestation of an injury and to help
construct a practical differential list for dogs with a history of trauma and lameness
in daily practice. In this study, we analysed data on avulsion fracture type, breed,
sex, weight, age and affected side from three academic veterinary hospitals across
the United States (East Coast, Midwest and West Coast).
Materials and Methods
A keyword search with avulsion fracture was conducted on available medical database
at Cornell University College of Veterinary Medicine (Cornell; Universal Veterinary
Information System), University of California Davis (UCD; Veterinary Hospital Information
System), and Michigan State University (MSU; Vetstar) for the period of 2008 to 2018.
Inclusion criteria were canine patients of all breeds, who were diagnosed with an
appendicular avulsion fracture by imaging (radiographs, computed tomography, magnetic
resonance imaging) interpreted at the time by a board-certified radiologist or by
a board-certified surgeon. Breed, sex, body weight, age, side of the lesion and type
of fractures were recorded. Avulsion fractures in the carpal area were grouped into
one ‘carpal’ avulsion fracture category. Avulsion fractures in the tarsus were grouped
into one ‘tarsal’ avulsion fracture category, except for malleolar fractures which
were grouped into a separate group of ‘malleolus’ avulsion fracture category. Patients
whose records and/or imaging were insufficient were excluded from the study as well
as dogs that underwent surgery prior to fracture. Dogs that had a cranial cruciate
ligament avulsion fracture were excluded, as the definitive diagnosis was not obtained
in many cases. Neuter status could not be determined due to the inconsistency of recording
style in the database.
The difference of types of avulsion fracture, breed, sex, injured side, age and weight,
among three study centres (CU, UCD, MSU) was examined by chi-squared tests. The difference
of age and weight among the study centres was compared by analysis of variance. Kruskal–Wallis
test and Steel–Dwass test were applied for the difference of age and weight between
six common avulsion types. Spearman's rank correlation coefficient was calculated
to analyse the correlation of age and weight. All statistical analyses were performed
by the use of statistical software (RStudio, version 1.1.463 for Mac, Boston, Massachusetts,
United States). The significance level was set to be 0.05.
Results
A total of 114 avulsion fractures met the inclusion criteria ([Table 1 ]). The overall mean age was 1.6 years old (±2.3 standard deviation) with an overall
mean weight of 18.48 kg (±11.48 standard deviation). No significant difference was
observed in type of avulsion fracture, breed, sex, injured side, age or body weight
among the three study centres. Tibial tuberosity fractures were the most common avulsion
fracture at every study centre, overall comprising 54% of all avulsion fractures ([Fig. 1 ]). The most common types of avulsion fractures were in order of frequency: tibial
tuberosity (62/114), malleolar (10/114), carpal (6/114), supraglenoid tubercle (5/114)
and greater trochanter (4/114). Breeds most commonly affected were mixed breed (26/114),
Labrador Retriever (15/114), American Pit Bull Terrier (5/114), Australian Shepherd
(5/114), French Bulldog (5/114) and Rottweiler (5/114). Among the 114 dogs, 60 were
female (53%) and 54 were male (47%). Sixty-one cases had left-sided fractures (53%),
and fifty-one had right-sided fractures (45%) and two were affected bilaterally (2%).
Fig. 1 Frequency of occurrence of different types of avulsion fracture at referral centres.
MSU, Michigan State University; UCD, University of California, Davis.
Table 1
Summary of sample characteristics of avulsion fractures at three study centres
Avulsion summary
Study centre
All
Cornell
UCD
MSU
Number
114
31
30
53
Gender (M/F)
54/60
11/20
13/17
30/23
Age (y ± SD)
1.60 ± 2.30
1.72 ± 2.42
1.85 ± 2.48
1.39 ± 2.15
Weight (kg ± SD)
18.48 ± 11.48
18.55 ± 11.82
21.68 ± 11.20
16.64 ± 11.25
Side (L/R/Bilat)
61/51/2
16/15/0
17/13/0
28/23/2
Type
Tibial tuberosity 62
Tibial tuberosity 15
Tibial tuberosity 16
Tibial tuberosity 31
Malleolus 10
Malleolus 4
Malleolus 3
Malleolus 3
Carpal 6
Supraglenoid tubercle 2
Carpal 3
Carpal 2
Supraglenoid tubercle 5
Carpal 1
Supraglenoid tubercle 1
Supraglenoid tubercle 2
Greater trochanter 4
Greater trochanter 1
Greater trochanter 1
Greater trochanter 2
Others 27
Others 8
Others 6
Tarsal 2
Others 11
Breed
Mix 26
Mix 7
Mix 7
Mix 12
Labrador Retriever 15
Labrador Retriever 7
Labrador Retriever 4
Labrador Retriever 4
PitBull Terrier 5
German Shepherd 2
PitBull Terrier 3
French Bulldog 4
Australian Shepherd 5
Australian Shepherd 1
Rottweiler 2
Australian Sheperd 3
French Bulldog 5
Rottweiler 1
Australian Shepherd 1
PitBull Terrier 2
Rottweiler 5
PitBull Terrier 0
French Bulldog 1
Rottweiler 2
Others 53
French Bulldog 0
Others 12
Others 26
Others 13
Abbreviations: MSU, Michigan State University; SD, standard deviation; UCD, University
of California, Davis.
The distribution of age and weight was significantly different between the types of
avulsion fractures (p < 0.001; [Figs. 2 ] and [3 ]). Carpal avulsion fractures tended to be observed in older dogs (p < 0.10). Tibial tuberosity fractures occurred in younger dogs (p < 0.001). Tibial tuberosity fractures were also observed in lighter dogs, but a confounding
effect due to age needs to be considered. In fact, age and weight were significantly
correlated with each other (r = 0.424, p < 0.001). Therefore, the true factor, which can be different in fracture type, might
be age.
Fig. 2 Box plot of the effect of age on types of avulsion fracture. Strong significance
(p < 0.001) and tendency (p < 0.10) are shown by *** and †, respectively. Heavy lines represent medians, boxes
are the range from the first quantile (Q1 ) to the third quantile (Q3 ) and whiskers show the maximum and minimum data points in ± 1.5 (Q3 –Q1 ) from the edges of the box.
Fig. 3 The effect of body weight on types of avulsion fracture. Strong (p < 0.001) and standard significance (p < 0.05) are shown by *** and *, respectively. Heavy lines represent medians, boxes
are the range from the first quantile (Q1 ) to the third quantile (Q3 ), and whiskers show the maximum and minimum data points in ± 1.5 (Q3 –Q1 ) from the edges of the box.
Discussion
There are no recent comprehensive studies analysing avulsion fracture in dogs. This
study evaluated the epidemiological characteristics of all types of appendicular avulsion
fractures in three regions of the United States, by analysing cases seen at three
academic institutions in the past 10 years. Avulsion fractures appeared to be much
less common compared with other types of fractures at these study centres (<5% of
all fractures, unpublished data). The most commonly diagnosed avulsion fracture was
tibial tuberosity fracture.
The results found in this study regarding tibial tuberosity fractures are similar
to those in previous studies where most of these fractures are reported in young animals.
There have been several studies analysing the pathology of tibial tuberosity avulsion
fractures specifically. In a study by Skelly et al, tibial tuberosity avulsion fractures
in a litter of greyhound puppies were found in six out of seven puppies, some bilaterally.
The study noted histopathological changes suggestive of osteochondritis.[6 ] Another study by Gower et al revealed that 86% of affected animals (51/59) were
Staffordshire Bull Terriers with a mean age of 5 months.[7 ] These studies (from Ireland and the United Kingdom) allude at higher incidence on
certain breeds, whereas our study from three remote locations in the United States
suggested that there was no breed predominance in the United States. The previous
results might be skewed based on region and general breed predominance on a particular
area.
Interestingly, recent studies reported atypical clinical presentation of tibial tuberosity
fractures, proposing atraumatic pathology.[8 ]
[9 ] Namely, von Pfeil et al study discusses the relationship between predisposing diseases
(such as osteochondritis and Osgood–Schlatter disease) and tibial tuberosity avulsion
in dogs.[8 ] However, our study did not evaluate aetiology of avulsion fractures. Particularly
for tibial tuberosity avulsion fractures, assessment of traumatic versus atraumatic
events would add substantial value, and future studies should focus on aetiology.
Beside tibial tuberosity fractures, other less common avulsion fractures have been
only sparsely reported in dogs. Previous studies described radiographic appearance
of intra-articular stifle avulsion fractures (cranial cruciate ligament and long digital
extensor tendon).[10 ]
[11 ] More recent case studies described diagnostic procedures and treatment outcome of
avulsion fractures near the shoulder joint (infraspinatus and supraspinatus)[12 ] and supraglenoid tuberosity,[13 ] elbow joint (biceps and triceps)[14 ]
[15 ]
[16 ] and hip joint (lesser trochanter).[17 ] Many of the reports are single cases in the format of “What is Your Diagnosis” in
the Journal of American Veterinary Medical Association . These case reports include avulsion fractures of the caudal cruciate ligament, origins
of gastrocnemius and popliteal muscles and various ligaments in carpal and tarsal
regions.[18 ]
[19 ]
[20 ]
[21 ]
[22 ]
[23 ]
Biomechanical factors related to avulsion fractures in dogs have so far not been discussed
in different facture locations. Although tensile forces produced by muscle contraction
is classically attributed to cause avulsion fractures, shear forces also play an important
role in their mechanism of injury.[3 ]
[4 ]
[5 ] As shown in this study, malleolar and carpal avulsion fractures were the second
and third most common avulsion fractures in dogs in the United States. These types
of fractures are considered to occur from trauma; hence, shear forces must play an
important role in its injury mechanism. The tibial tuberosity serves as the insertion
site of the quadriceps femoris muscle, and avulsion of the tuberosity can result from
contraction of the muscle, while the stifle joint is flexed and the foot is set firmly
on the ground.[5 ] Fracture of the medial or lateral malleolus often occurs with shearing injury but
may occur as an isolated fracture.[5 ]
Analysis of aetiology, as well as other confounding factors such as body condition
score, obesity, activity level, possibly diet, rate of growth, lifestyle (i.e. working
dog versus solely companion animal), region (countryside versus urban area), injury
mechanisms and type of trauma (atraumatic incident versus low energy versus high energy
trauma) would provide with pertinent information that may help decreasing the incidence
of this type of injury in the future.
In this study, there is a relationship between age and body weight, but they were
significantly correlated with each other and a true factor was not clearly identified.
In addition, correlating body weight to standard body weights to evaluate role of
obesity with avulsion fractures was not possible, given we did not collect data on
body condition scores nor the numbers of one specific breed were sufficient to make
the analysis significant.
Thus, a limitation of the study is the lack of information on aetiology of avulsion
fractures, as discussed above. Another limitation is the uncertainty of true incidence
of avulsion fractures in comparison to other orthopaedic conditions in dogs. For example,
odds ratio statistics could have enhanced this study. We also did not evaluate the
effect of neutering on the incidence of fractures, since our study did not evaluate
factors related to aetiology; further investigation might be beneficial to determine
a way to prevent its occurrence. Hereditary and developmental factors might be associated
with these types of injury of young dogs, especially with tibial tuberosity avulsion
fractures. Our study is comprised of only small numbers of other avulsion fracture
types; hence, our results regarding other fractures might not be statistically meaningful
due to the limited sample size. Other specific types of avulsion fractures might warrant
further investigation. Notably, taking in consideration our finding that carpal fractures
tend to occur in older animals, it might indicate that not all avulsion fractures
have same or similar aetiologies.
Conclusion
The most common avulsion fractures were tibial tuberosity fractures in young and lighter
dogs. There were no other clinically relevant trends in avulsion fracture type based
on geographical location, breed, sex, body weight and affected side.
