Introduction

Feline osteoarthritis (OA) is now well recognized as a significant clinical problem causing pain and disability, although these signs are often hidden in cats, being revealed to veterinarians only by proactive questioning about changes to the cat’s behavior (e.g., mobility, activity, grooming, and temperament).¹ “It is a poor reflection on the veterinary profession that osteoarthritis of the cat has been unrecognised for so long.”¹ Whether feline osteoarthritis was always present at current levels and unnoticed by the veterinary profession or whether it has increased in prevalence in recent decades is a question that might provide clues to its currently unknown aetiology.²

Interest in feline osteoarthritis was first shown in anecdotal reports.^3–11 Scientific studies then followed.^12–18 The prevalence was initially reported only from retrospective examinations of radiological archives. Hardie et al. reported a 20% prevalence in cats over 12 yr, similar to the reported prevalences of 22 and 30% by Godfrey and 17% by Clarke et al.^12–14,16 Two prospective cross-sectional studies showed higher prevalences—91 and 61%.^17,18

This study’s aim was to obtain evidence whether feline OA is a novel condition by finding the prevalence of radiographic OA in a population of cats living 30–50 yr earlier than previously studied populations.

Materials and Methods

Cats euthanized at the Beaumont Animal Hospital (now Beaumont Sainsbury Animal Hospital), Royal Veterinary College, University of London, United Kingdom, between 1972 and 1973, and whose owners permitted, had a set of radiographs taken post mortem. This hospital offered free veterinary care to the local inner-city population. The collection of material was continued until 100 sets of images were produced, and the images were stored in film envelopes until 2007. Each set was intended to include a lateral view of the head and neck; a lateral view of the thorax including both front legs with the two legs separated, offering a mediolateral view of one shoulder and elbow and a lateromedial view of the others (also some carpi); a ventrodorsal view of the pelvis that usually included craniocaudal views of both stifles and some tarsi; and a lateral view of the abdomen including both hind legs with the legs separated, to show a mediolateral view of one stifle and a lateromedial view of the other (also some tarsi). Some sets were incomplete, and a minority included additional views.

Aspects of clinical data were recorded on each film envelope. These included the cat’s given age, breed, sex, neuter status, and the reasons for presentation to the hospital and the decision to euthanize. The cats were placed in one of four age groups to allow a direct comparison with the prospective study of Lascelles et al.¹⁷

The radiographs were reviewed by the two authors. An agreement was made for the presence or absence of any of the following features of OA: osteophyte, increased bone opacity, enthesiophyte, remodeling, joint effusion, and bone cysts. Mineralizations closely associated with the joint were also used as an inclusion criterion.¹⁴ The shoulder, elbow, carpal, hip, stifle, and tarsal joints were considered. If one or more joints had radiographic signs of OA, that cat was said to have OA. The femoral-tibial and femoral-patellar joints, the carpus, and the tarsus were each considered a single entity. Left and right joints were counted separately. Other skeletal abnormalities recorded included meniscal mineralization (MM), sesamoid bone of the supinator muscle (SS), and hip dysplasia (HD). Hip dysplasia was diagnosed if the center of the femoral head was considered outside the encompassment of the dorsal edge of the acetabulum.^14,19

Data were analyzed using analytical software^a,b,c. The ages, sexes, and neutered status of cats with and without OA, MM, SS, and HD were compared. Mann-Whitney U tests were used to compare the medians as the data were nonparametric. χ² tests were used to compare the prevalences of the sexes and the numbers of neutered and entire cats. Significance was set at P < .05. Prevalences were calculated for the test population, for each joint, and for the presence of bilateral OA.

Results

The prevalence of radiographic OA in the 100 cats was 74% (67% with MM disregarded; Table 1). Some images were missing, so this was a minimum figure. Although mean (7.8 yr) and median (6 yr) ages were similar, the data were left-skewed. Five cats were of unknown age. The prevalence of radiographic OA in the different age ranges was as follows: 5 yr and under (n = 46) 46%; 6–10 yr (n = 18) 89%; 11–15 yr (n = 22) 91%; 16–22 yr (n = 9) 100%. Of the 74 cats with OA, 47 (64%) affected cats had bilateral disease. The median number of joints affected was two (range 0–9).

TABLE 1 The Proportions, Percentages, and 95% Confidence Intervals of Radiographic Osteoarthritis in Each Joint with Comparisons from the Literature

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The median age of the 74 cats with OA was 10 yr (range 1–22), and the median age of the 26 cats without OA was 2 yr. The difference between these median ages was significant at P < .001. Of the 96 cats with a known sex, 33 (34%) were entire males, 35 (36%) neutered males, 18 (19%) entire females, and 10 (10%) neutered females. No significant differences were seen in the numbers of each sex or in neutered status between the cats with and without OA.

The number of cats with an SS was 9/95 (10%). Of these, two had bilateral SSs and four had other signs of elbow osteoarthritis. The median age of cats with and without an SS was 8 yr (range 2–22 yr) and 6 yr, respectively. The number of cats with HD was 11/98 (11%), and 7/11 had OA in the hips. The median age of cats with and without HD was 7 and 6 yr, respectively. None of these differences were significant.

There were 42/100 cats with MM. Cats with an MM (median age 12 yr) were significantly older than those without (median age 4 yr; P < .01). Of these 42, 17 had concurrent stifle OA.

Discussion

The prevalence of OA per individual rather than per joint, (74%) was higher than the previous retrospective studies (17–30%) and also higher than the 61% level found prospectively by Slingerland et al.^{12–14,16,18} Two aspects of their study may explain the even higher prevalence (91%) found by Lascelles et al.¹⁷ Their population’s mean age was 9.4 yr—higher than the current study—and, as further evidenced in the present study, OA is more common with advancing age.^13,16 But they also found an 80% prevalence of OA in cats 6 mo to 5 yr old, whereas this study found 43% in cats less than 6 yr, and the median number of affected joints was five compared with two in this study.¹⁷ Our study may be underreporting OA in younger cats due to its less rigorous radiographic technique. The prevalence of radiological OA given here can only be an underestimate (as with any retrospective study of archived radiographs). These sets of images were incomplete. Craniocaudal views of the shoulder, elbow, and carpus were routinely absent because of the difficulty of postmortem positioning, and some individual views were absent for an unknown reason. Also, oblique views rather than direct mediolateral or lateromedial were often seen in the more caudally placed forelimb, making evaluation of that elbow joint more difficult. The carpus appears protected from radiographic OA in this and previous studies (Table 1). There may be a reason for feline carpi being protected from OA, and a detailed study would be welcome.

The prevalence of appendicular OA varies according to the method and criteria for diagnosis. Here, only radiology was considered, and individuals with OA found on gross postmortem examination, histopathology, and/or clinical examination, or where the diagnosis is made from clinical signs that resolve with analgesic treatment, may have no radiographic features of the disease as assessed by standard radiography.^{15,17,19–22} It is common for cats to have radiological OA without clinical signs, and the present study did not investigate resulting clinical signs.^12,15,16,23

Hip dysplasia has been reported in 7–32% of cats.^24,25 Clarke and others found HD in 10/16 cats with hip OA.¹⁴ It was one of only two underlying causes of OA identified in this study. The presence of an SS has been reported in 40% of cats.²⁶ And Clarke and others found an SS in 7/13 elbows with OA and no elbows without.¹⁴ Godfrey found an SS in 12% of cats with elbow OA.¹⁶ It is likely that some of the cats in the present study would have had signs of osteoarthritis identified had a cranio-caudal view been available; also, some SS might have been hidden in the somewhat oblique view common on the caudally placed elbow. Meniscal mineralization is common in cats. The mean age of cats with MM has previously been suggested to be younger than that of cats with stifle OA diagnosed using only other radiographic changes.^14,16 The present study added to these observations, but the significance here was marginal and does not disprove the recent assertion that stifle osteoarthritis is positively correlated with the presence of MMs.^17,27

Some age data were missing, and the reliability of the female neuter status and the age data must be questionable given the population examined. Nevertheless, the study provides further evidence that feline osteoarthritis is more likely in older cats.^{13,14,16–18} No evidence was found for sex or neuter status being a risk factor for OA. The apparent bias for males in this total population of euthanized cats was of unknown cause and significance. There was no apparent bias in selection of cats for radiography post mortem. The male bias must therefore have reflected cats being presented for untreatable conditions or directly for euthanasia by their owners. Territorial marking, trauma, and feline immunodeficiency virus are recorded as more common in male cats.^28,29 The pattern of joint disease suggests the same problem as in contemporary cats, a condition that is often a polyarthropathy and is more common with progressing age but is otherwise considered primary (idiopathic).^13–17

Conclusion

This study showed feline appendicular radiological OA, with a pattern similar to the current disease, was very common in urban cats in the United Kingdom in the early 1970s, suggesting that an increase in awareness rather than disease prevalence has occurred. The study also provides further evidence of the increasing prevalence of OA with ageing and that the feline carpus may be protected from the disease.