Download PL- Patellar Luxation

RESEARCH PROJECT 2 REPORT
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Candidate number:
S425
RP2 project title:
Prevalence and risk factors for patellar luxation (PL) in dogs
attending veterinary practices in England
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1|Page
Contents
Abstract… 3
Abbreviations… 4
1.0 Introduction…5
1.1 Overview…5
1.2 Importance of Patella luxation…5
1.3 Current data analysis…5
1.4 Anatomy and physiology …6
1.5 Diagnosis…7
1.6 Study Aim…7
2.0 Methods and materials …8
3.0 Results…10
3.1 Risk Factor – Adult Breed size associations…10
3.2 Risk Factor- Breed classification associations…10
3.3 Risk Factor- Gender associations…10
3.4 Risk Factor- Age associations…11
Table of results…12
4.0 Discussion…13
4.1 Data Analysis of hypothesis…13
4.2 Risk Factors of PL…13
4.2.1 Association of breed…13
4.2.2 Influence of gender…14
4.2.3 Influence of age…14
4.2.4 Insurance status of affected patients…14
4.2.5 Limb association…15
4.3 Limitations of study…15
5 Conclusion….16
References…17
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Prevalence and risk factors for patellar luxation (PL) in dogs attending veterinary
practices in England
Abstract
Patella luxation is a common orthopaedic condition of the canine, characterised by either
medial or lateral displacement of the patella out of the patella sulcus. This study aimed to
elicit a prevalence and common risk factors associated with patella luxation in England.
Primary veterinary care data obtained from the VetCompass Animal Surveillance Project
and was used for this study. Prevalence was estimate from the overall canine cohort,
associated risk factor analysis used a case-control design with univariable logistic
regression. There were 849 cases of patella luxation verified case, from 2836 clinical
recorded out of 6,334 potential cases identified from 235,384 dogs attending 120 practices
from the study period September 1st 2009 and August 31st 2014. Cases were aged 2 months
to 17 years at age if diagnosis. 54.5% of cases and 48.1% of controls were female, 79.95%
of case and 79.4% of controls were purebreds. The apparent prevalence was 0.8% (95% CI
0.77–0.84%), with the estimated prevalence in small breed 4.03% (95% CI 3.28-4.95%).
Yorkshire Terrier (OR 3.75, (95% CI 2.49-5.63, p= <0.0001), Cavalier King Charles (OR
4.00, (95% CI 2.16-7.40, p= <0.0001), and Pomeranian (OR 5.25, (95% CI 1.79-15.4, p=
<0.0001), Small breed (OR 2.27 95% CI 1.91-2.71) and dogs of 1-2 years if age (OR 1.45
95% CI 1.12-1.89) were all at increased odds of patella luxation. This study highlights
patella luxation as a prominent canine disease in England and reports increased odds of
diagnosis in certain breeds and sizes of dogs.
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Abbreviations
CI – Confidence interval
DoB – Date of Birth
EPR - electronic patient records
IQR –Interquartile range
PL- Patellar Luxation
PMS – Practice management system
OR – odds ratio
DoB – Date of Birth
YER -years
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1.0 Introduction
1.1 Overview
Patellar luxation (PL) is a common condition of the dog, being the seventh most commonly
diagnosed orthopaedic disorder 1. Patella luxation is also considered one of the most important
hereditary defects of the domestic canid 2, however the mode of inheritance is yet unknown3 nor
has the mechanical mechanism be truly elucidated 4 Recent studies have suggested that the
luxation occurs due to skeletal limb abnormalities that effect the whole stifle extensor
mechanism5. It is accepted that a luxated patella is defined as ‘a displacement of the patella from
the trochlear sulcus’ 6.
1.2 Importance of patella luxation
Although it’s been reported that dogs with congenital PL seldom feel pain or develop
degenerative joint disease7, a recent Belgium study of 141 dogs demonstrated gross cartilage
erosions. An intra-operative assessment was made and crossed with 13 digitally captured and
analysed photographs before being assigned a grade. Each grade demonstrated dogs with areas
of erosion, and an increase in grade was correlated to an increase in cartilage erosion.8. Further
pathology is often associated with patients suffering from chronic medial patella luxation, the
anatomical abnormalities increase the strain on the cranial cruciate ligament in the stifle that may
lead to ligament rupture4.
1.3 Current data analysis
Data is limited on the frequency of patella luxation, with studies reporting in different ways, such
as on breed prevalence, or percentage of dog’s affected and relative risk, a 2009 UK study
demonstrates the Labrador at 21% to be the most commonly affected breed from a study of 155
dogs with 92% of cases medial. 9 Another recent study of PL in Thailand, showed that the medial
and lateral luxation prevalence was 87% and 13% respectively in affected small dogs 3.
An incidence study performed in South Korea between 2000-2005 experienced medial and
lateral luxation in 95% and 5% of cases respectively. Sixty nine percent of affected patients met
the study criteria for ‘small breed’10.
Predispositions have been mainly reported in small breeds such as the Pomeranian, Chihuahua,
Boston Terrier, Miniature Poodle and Yorkshire Terrier11 . Pomeranians seem to be particularly
affected in the USA with 42.4% of the population affected3. Denny (1985) also reported
Cavalier King Charles spaniels to be more at risk. Recent studies have reported predispositions
specifically in larger breeds, with the Labrador Retrievers and Staffordshire Bull Terriers being
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more at risk 12. Furthermore a study performed by W.Prister in USA and Canada, between 1964
and 1969, concluded that small breed dogs in general are 12 times more likely to suffer from
medial PL. However the study failed to compare affected dogs against the non-affected
population and obtain a perspective of the size of dogs owned during the study period. The
study observed 542 clinically affected dogs, of these females were 1.5 times more likely to be
effected in one study 2.
The median age has been reported for lateral PL cases based upon the grading of the disorder,
put grade I PL at 16.5 months, II 12 months, III 7 months, IV 6months. A younger diagnosis
correlated with a more server grade of PL. The majority of cases were recorded at 12 months of
ages at grade II13. Other reports stat that the abnormalities of the stifle progress with age along
with the degree of luxation14. Foremost PL is classified as a congenital disorder 12,14,15.
The role of oestrogens has also been suggested in having a pathological role. Experimental
induction of oestradiol causes development of a shallower trochlear sulcus, thus propagating the
idea that patella luxation may be partly under hormonal control16. Moreover this could also
explain, why females were 1.5 times more likely to be affected 2.
1.4 Anatomy and pathophysiology
PL is associated with several anatomical abnormalities including, coxofemoral joint
conformation; shallow trochlear groove; femoral torsion and angulation; deviation of the tibial
crest; and tibial torsion (see figure 1). Many of these abnormalities are involved in the stifle
extensor mechanism5, thus suggesting that misalignment of the quadriceps mechanism along
No.
Location
Abnormality
1
Trochlear
sulcus
Shallow
trochlear
groove
2
Tibial
diaphysis
Tibial torsion
medial or
laterally
3
Tibial
crest
Deviation of
the tibial crest
4
Femoral
angulation
Angle of
inclination
femoral neck
5
Femora
diaphysis
femoral
torsion medial
or laterally
Figure 1- Visually highlights the anatomical abnormalities associated with PL, according to
with
a shallow
sulcus
may be anthe
important
factorplanes
in patella
luxation17
.
previous
data. trochlear
The arrowed
demonstrate
approximate
of rotation,
torsion
or site of
deviation.
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1.5 Diagnosis
PL is characterised by either medial or lateral displacement of the patellar, which is either
permanent or intermittent, giving rise to Singletons 4 tier grading system:
Grade I: Intermittent Patella luxation, can be manually luxated but returns to normal on its own;
Grade II: Frequent luxation associated with 15-30⁰ tibial crest deviation;
Grade III: Permanent luxation associated with 30-60⁰ tibial crest deviation;
Grade IV: Permanent luxation associated with 60-90⁰ tibial crest deviation7.
Radiographical assessment can also be performed to evaluate PL, whereby mediolateral views of
both femurs and ventrodosal projections of the pelvis to the bottom 1/3 of the tibia have been
described 11.
1.6 Study aim
The majority of texts relating to PL draw on methods of surgical correction and diagnostic
imaging. Despite epidemiological reports that study the prevalence of patella luxation, the author
can find no recent controlled studies of the population in England. The current study aims to
evaluate the prevalence of PL and evaluate the associations of the possible risk factors of breed,
size, and gender predispositions from a study control population from English first opinion
practice. It was hypothesised that smaller breed dogs have a higher prevalence of PL,
additionally it was hypothesised that females may be more affected.
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2.0 Methods and Materials
Data collected as part of the VetCompass Animal Surveillance project18, a project of the Royal
Veterinary College that collaborates data from first-opinion Practice Management Systems
(PMS), from willing participation practices. 19.Practitioners routinely record the clinical records
and select an appropriate summary diagnostic term(s) from a terminology list embedded within
the software database, known as VeNom codes20 De-identified data are shared from over 120
clinics across England, and is relevant to the owned canine population. The data includes free
text clinical notes, (summery diagnostic terms, treatments and deceased status) and demographic
data on Breed, Gender, and date of birth recorded (DoB), neuters status, weight and insurance
status. Data downloaded from the PMSs following relevant clinical queries data was uploaded to
the secure VetCompass mySQL database.
The cohort study of dogs attending participating practices during the study period was evaluated
for potential PL, VeNom codes where use to searched for coded cases. Free text clinical notes
were also assessed for evidence of potential cases from free text, multiple field terms where
used: patella, MPL, LPL, PL, slipping pat, floating pat, trochlear groove, kneecap lux, femoral
groove, floating kneecap, slipping kneecap, kneecap disloc. Patients identified from the VeNom
search where grouped, and those with missing data or duplications where removed. In order to
confirm the diagnosis selected by the VeNom query, the full clinical histories were examined
thoroughly, to sort differentials from a definitive diagnosis. Case definition for PL required a
definitive diagnosis based on either conscious physical exam findings (luxated or ability to
luxate), sedated physical examination findings, or radiological findings. For each case there had
to be no evidence of trauma that could have caused the PL.
Patients that met case criteria had further data coding, to identify date of diagnosis, incident or
pre-existing, and if surgery was performed, whether there was case referral for surgical treatment
(within the organisation or separate referral centre), and the limbs affected were also recorded.
To evaluate risk factors for a diagnosis of PL a case control study was nested within the cohort
of dogs attending practices. The control population was selected at random, through a random
number generator 21, from the denominator of dogs, patients with a suggestive history of PL
where excluded from the control group.
Data sets once selected and coded from the VetCompass, where extracted to WSP Spreadsheets22
for data cleaning. Data cleaning involved verification checks to eliminate any cases of missing
data, and data classification. Classification included standardisation of breed into three groups of
adult target weight as determined by The Kennel Club23, 1) <10kg Small 2) 10-20kg Medium 3)
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>20kg Large, or unknown which consisted of unidentified crossbreeds. The pedigree variable
was categorised into three. Dogs where recorded as either ‘recognised pedigree breed or pure
breed’ such as Pomeranian, ‘designer breed’ such as ‘Cavapoo’ or ‘non-recognised’ such as a
cross breed of unknown mix, (Not including ‘Jack Russell Terrier’ or Parsons Jack Russell). Sex
was categorised into two, of ‘male’ and ‘female’. Finally limbs affected was categorised into
four, ‘Bilaterally affected’ ‘left side only’ ‘right side only’ or ‘non-recorded’. Age for the
confirmed case population is considered the date of confirmed diagnosis according to electronic
patient records (EPRs), minus the date of birth. The age was summarised to years, those below a
year are recorded as 0 years. Control patient ages were calculated by the centre point between the
date of birth and final EPR entry.
Data was statistically analysed with IBM SPSS V.20.24 Categorical data were summarised with
number (%). Categorical data included variables ‘Breed Size’, ‘Pedigree, ‘Sex’, and ‘Limb(s)
affected’. Quantitative data were assessed graphically for normality and summarised with or
median and range, normal distributions were summarised with mean and standard deviation.
Skewed distributions were summarised with median, range and interquartile range, The MannWhitney U test used to analyse as data was skewed. Analysis were carried out using the Pearson
Chi-Square or fishers exact test as appropriate for categorical data25, Univariable binary logistic
regression was used to quantify the association with odds ratios confidence intervals of 95%26.
Statistical significant set at 5%.
Prevalence was estimated by using the number of confirmed cases as a proportion percentage to
all possible cases, the population was scaled up using proportion of the cases sampled to estimate
the prevalence in the study population demographic, and not just analysed records. A small breed
specific prevalence was calculated for study comparison, using prevalence of breed size categories
for the study population. The study control was proportionally up scaled, to be used as
denominator for the study population. 95% CI where calculated in Microsoft Excel.
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3.0 Results
Query search delivered a possible 6,334 patella luxation cases. Following manual verification,
of a random 2836 cases, 849 confirmed cases where identified that met the case definition.
Others could not be included into PL group due to lack of definitive diagnosis, or other patella
abnormalities. Overall study data set comprised 235,394 dogs, from 120 clinics across England.
During the study period September 1st 2009 and August 31st 2014, 849 cases were identified
from a sample group of 2836 giving an apparent prevalence of as 0.81 per cent (95% CI 0.77–
0.84%). This study demonstrates 30% of England’s canine population to be of small breed, it
can thus be calculated that 4.03% (95% CI 3.28-4.95%) of the small breed population is affected
by PL. The 849 cases where possible were classified according to limbs affected. Bilat-306 LHS260 RHS-220 Not recorded- 63 patients, P-value 0.807.
3.1 Risk Factor - Adult breed size associations
In the case control study 849 cases (PL group) were compared with 850 randomised control
dogs. The PL group consisted of 68.3% (n=580); small breed dogs; 9.7% Medium breed (n=82);
3.8% (n=32) large breed and 18.3% (n=155) unknown. The controls consisted of 30% (n=255)
small breeds; 27.4% (n=233) medium breeds; 25.6% (n=218) large breeds and 16.9% (n=144)
unknown breed sizes (P value of <0.001), Small breed dogs were 2.2 times more at odd to be
diagnosed as PL case (95%CI 1.91-2.71) (Table1) than medium breeds P value (<0.001), large
breeds are 0.3 times at odds to have PL (95% CI 0.27-0.46) more at odds (P value <0.001).
The Yorkshire terrier appeared the most affected in this study, they account for 13.7% of our
affected case population, comparable with and estimated population in England of 3.6% from the
control, the Yorkshire terrier appeared approximately 3.8 time more at odds of having PL (95%
CI- 2.49-5.63) than the base of crossbred.
3.2 Risk Factor - Breed classification associations
79.95%, (n=677) affected dogs were regarded as Kennel club registered breeds, a further 1.4%
(n=12) were identified as ‘designer’ breeds, 18.8% (n=160) were classified as crossbreeds (not
including the Jack Russell Terrier). Control analysis highlighted population groups as 79.4%
(n=675) pure breeds, and 3.6% (n=31), 16.9% (n=144), designer and crossbred respectively, (P =
0.010). Comparably pure breeds where 2.5 times more at odds to be affected by PL that others,
(P value <0.005).
3.3 Risk Factor - Gender associations
The gender distribution placed 54.5% (n=463) of affected dogs within female and 45.5%
(n=386) males’ categories respectively, including neutered canines. 48.1% (n=409) of controls
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where female, and 51.2% (n=435) male, with 6 control cases missing due to lack of original
practise data. (P value 0.002). Females where 1.14% (95% CI 0.97-1.35%) more at odds of
having patella luxation than males, (P value 0.087).
3.4 Risk Factor - Age associations
The age of entire case population ranges from 0.2-17.6 years, with a Median age of 4.0years
(Interquartile range 1.7-7.5 yrs), Entire control patient population age at the centre point ranges
from 0.1-30.7, with multiple outliers. Median age of patients was 3.8 years (IQR 1.2-7.9 yrs) (Pvalue 0.93). Most PL cases are of 2-4 years at 21.1% (n=181) of the affected population (PValue= 0.0001) (Table1),however based on ORs, the odds of dogs at 1-2years of age were most
at odds 1.45 (CI 1.12-1.89%).
375 (44.2%) of the patella luxation cases where insured patients, conversely comparable to noncases with 161 (18.9%) patients insured (P value = .005). 266 (31.3%) cases where uninsured the
further 208 (24.5%) patients where unknown. Control patients had 552 (32.5%) uninsured
patients with a further 403 (47.4%) unknown.
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Table 1- Results of univariable logistic regression analysis of individual risk factors, or
interquartile range where appropriate, for canine PL
Variable
Category
Case
Non
Case
OR
95% CI
Sex
P-value
0.087
Male
Female
386
463
435
404
1 (base)
1.14
Crossbred
Designer
Purebred
160
12
677
144
31
675
1 (base)
0.38
1.00
0.97-1.35
Pure Bred status
<0.005
0.19-0.75
0.87-1.58
Common Breeds
<0.0001
Crossbred
Yorkshire Terrier
Jack Russell Terrier
Chihuahua
Cavalier King Charles
Terrier - West
Highland White
Bichon
Staffordshire Bull
Terrier
Pomeranian
Pug
Lhasa Apso
Shih-tzu
French Bull dog
160
116
71
69
52
40
144
31
50
22
13
18
1 (base)
3.75
1.42
3.14
4.00
2.22
2.49-563
0.97-2.06
1.93-5.12
2.16-7.40
1.26-3.91
31
26
15
75
2.07
0.35
1.10-3.86
0.22-0.54
21
21
20
18
16
4
9
7
21
3
5.25
2.34
2.86
0.85
5.33
1.79-15.4
1.06-5.13
1.20-6.80
0.45-1.62
1.55-18.4
Unknown Crosses
Small <10kg
Medium 10-20kg
Large >20kg
155
580
82
32
144
255
233
218
1 (base)
2.27
0.352
0.48
Uninsured
Insured
Unknown
266
375
208
552
161
403
1 (base)
2.33
0.52
Median (IQR)
4 (1.77.5)
3.8 (1.27.9)
<1year
1-2years
2-4 years
4-6years
6-8years
8-10 years
10> years
83
158
181
140
106
75
110
180
109
128
98
93
61
143
Adult Size Category
<0.0001
1.91-2.71
0.27-0.46
0.40-0.57
Insurance Status
<0.005
1.89-2.87
0.43-0.63
Age Continuous
0.93
4.84-5.25
Age categorical
<0.0001
1 (base)
1.45
1.42
1.43
1.14
1.23
0.77
1.12-1.89
1.11-1.81
1.08-1.88
0.85-1.53
0.86-1.74
0.59-1.00
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4.0 Discussion
The aim of this retrospective study was to assess the prevalence and risk factors of PL patients
across England, using EPR data from multiple first opinion practices.
4.1 Data analysis of hypothesis
The findings of this study demonstrate the estimated prevalence of canine PL across the study to
be 0.8% (95% CI 0.77% – 0.84%). Calculating that 30% of the English canine population is to
be small breed, it can be calculated that 6.83% (95% CI 6.62% - 7.04%) is the total prevalence in
small breeds across England, Conversely it can be calculated that 0.3% of large breed dogs are
affected in the general population, meaning that small breed dogs are 22 times more likely to be
effected. Although the prevalence percentage is low among small breeds, the comparative odds
are about twice that previously recorded at 12 times 2. The author is unaware of any similar
reports on the whole canine population in England, nor any recorded prevalence percentage in
England.
4.2 Risk factors of PL
4.2.1 Association of breed
The current study found that small breeds were associated with a greater risk of PL than both
medium and large breeds. The over-representation of small breeds at 68.3% of the affected dog
population, is a common theme among many studies including Priester (1972) and Wangdee
(2005). Priester demonstrated that small breeds were 12 times more likely to be affected, from a
population of 542 cases, from a study performed across the USA in 1969. This report concurs
with the association of small breeds, however places them at 22 times more likely to be affected.
The current study and Priesters differ in definition of small breed, whereby Priester states small
breeds to weigh less than 9kg, compared to 10kg in the current. The classification of body
weight obtained by Priesters is the adult weight guidelines from the American Kennel Club,
conversely the current study uses The Kennel Club. Predicted adults weights are no longer
published by the American Kennel Club27, however disparity between the two groups over 44
years may explain some of the difference between the breed associations with PL. Furthermore
the suggested adult weight of breeds may hold little relation to actual patient weight, depending
on environmental factors, breed linage, and accuracy of predicted adult weight classifications.
The Yorkshire terrier, Jack Russell terrier, Pomeranian and Cavalier King Charles showed
increased risk of diagnosis with PL with approximately 13.7%; 8.4%; 6.1% and 8.1%
respectively of the case group, with the Pomeranian most at odd. This concurs from Priesters in
1969 study that place Pomeranians as the most at risk accounting for 8.1% of the effected
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population. The current study also concurs with reports that Yorkshire terrier, Chihuahua and
Cavalier King Charles are frequently affected by PL 7. Moreover at the time of Denny’s report
the Jack Russell terrier was not a Kennel Club recognised breed, therefore may have classified as
crossbreed, thus not occur in Denny’s study.
Pure breeds or recognised pedigrees accounted for the majority of affected dogs, and are
significantly more at risk. However the control group mirrors the percentage of the pure breed
population. It could be argued that they are more at risk due to the numbers of these dogs in the
general population, rather than associations with pedigree.
4.2.2 Influence of gender
The finding that females were at an increased risk of PL, compared to males is in agreement with
previous work from Bound and others (2009), which reported 54% of dogs as female, this study
finds similar evidence of a female bias at 54.5%. Evidence of increased risk may link to
previously work suggesting a hormonal influence 16. The population of females shown in this
study control group is lower than male at 48.1%, 51.2% respectively.
4.2.3 Influence of age
This study elicits the median age for 4 years with the greatest risk to dogs at 1-2 years of age,
preceded 2-4years. Least at risk dogs ages 8-10years. The control population demonstrates only a
minimal difference compared to the PL cases, except from the population below a year of age,
where the control population is over twice that of the PL cases. Previous reports designate PL as
a congenital disorder2, further works suggest that PL cases are most commonly observed at 4
years of age 28, while other studies give median age that varies with disease progression,
suggesting that clinical signs and diagnosis increases with severity. Retroductive reasoning
suggests that animals would be more commonly presented to a veterinarian with a greater degree
of PL. The current study demonstrates a binary effect in age of diagnosis, both in young adult
mature dogs and older dogs. Theorising could suggest that young adult become skeletally mature
and demonstrate clinical signs, while older dogs loose muscle tone and favour of the quadriceps
mechanism.
4.2.4 Insurance status of affected patients
Insured dogs showed increased odds of diagnosis with PL compared with uninsured dogs,
however a large proportion of the study control had unknown states of insurance. Raising the
question of EPRs data input. In the authors opinion many veterinarians may only request the
insurance information upon the diagnosis of a disease process that may require funding.
Furthermore insured animals are likely to be observed by veterinarians more frequently. It can
14 | P a g e
also be considered that some veterinarians will only record a diagnosis of PL with supportive
diagnostic imaging, that some uninsured animals may not have access too.
4.2.5 Limb Association
The current report did not detect statistical evidence for limb association of occurrence for PL,
although more dogs where recorded bilaterally than either the right or the left. In agreements
with other reports 8 bilaterally affected patients are observed more frequently. Conversely other
reports suggest that because PL may be due to asymmetric growth of the femoral physes17, that
would correlate more to a single limb affected.
4.3 Limitations of study
The study prevalence estimates relied on the accuracy that 29.98% of VeNom queries used
highlight true PLs (849 of 2836), the possibility for further cases using different terms may be
applicable, however it allows a retroductive reasoning to estimate total population figures. In the
author’s opinion, the 2836 EPRs verified is minimal considering potential size of population.
The accuracy of the prevalence could be increased with a greater size of study group, the major
factor in this being time available to manually verify each record.
The nature of data collection from first opinion PMSs, and EPRs is such that the study relies on
data that was not collected for the sole purpose of statistical analysis. As such, data entry for the
current reports, noted primary veterinarians did not always record the grade of luxation, nor
whether it was medial or lateral. Other fields had missing data that could elicit errors in the
analysis of risk factors. Furthermore VeNom codes were used to identify potential PL cases, the
accuracy of data sets is reliant on the VeNom codes highlighting all potential PLs. A further
issue of data collection is the location of data sources during the study period. In the author’s
option the location of a clinic can greatly change the diversity of breed seen, unless geographical
population equalling is performed data may be subject to spatial variation.
Limitations of specific risk factors, such as weight as a continuous variable are apparent through
lack of data, due to numerous missing weight records. Resulting in lack of accurate accusations.
Furthermore current study developed an estimate of age of diagnosis for cases by subtracting the
date of birth (DOB) from the date of PL notes in the EPRs. However the author noted many
EPRs where the diagnosis may have been made prior to the current recording in the clinical
history. Moreover incidence where by the DOB recording may have been incorrect are apparent,
one cases recorded DOB was 23/09/1983, ageing the patient at 30 years. Although PL is a
congenital disease process, the age of diagnosis varies considerably from different reports, it
15 | P a g e
suggests that practitioners may not be testing for PL during routine physical examination, or
triggering factors are still to be elucidated.
The population size observed in the current study is reduced due to time constraints at the level
of manual verification of individual EPR entries. In the authors opinion a greater population size
would reflect on the report, specifically in relation to total population prevalence.
5.0 Conclusion
Overall this retrospective cohort study provides evidence on the major risk factors of PL,
supporting similar reports from other nations. In this study the total population prevalence was
0.8% for patella luxation in England, with small breeds 22 time more likely to be affected at
6.68% of the whole population over 0.3% large breed, a figure that surpasses a similar study
from the 1960s. The major risk factors elucidated from this study remain in agreements with
other studies from other nations, with small breeds and females being at highest risk. The
Yorkshire terrier and Jack Russell terrier were the two most common breeds affected from this
study from number of dogs affected, but the Pomeranians was most as odds of PL, however the
study also places other breeds recorded ‘most at risk’ from other reports within the top raking.
Future work on PL may provide a more definitive prevalence, and work on breed and gender in
correlation to anatomical measurements may prove an exciting addition to the literature.
Word Count (excluding abstract, references, table/ figures and title) – 3963
Abstract word count – 245
Acknowledgments
The author would like to thank the VetCompass team, with a special thanks to Dan O’Neill for
data handling from the VetCompass database, and David Brodbelt for supervision of the
VetCompass analytics.
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