Osteoarthritis in feline coxofemoral joints : a comparison of diagnostic imaging and histologic findings

Abstract: Osteoarthritis (OA) is a progressive, low-grade inflammatory disease of synovial joints. It is characterized by the degradation of articular cartilage but involves the whole joint including the subchondral bone and soft tissues. In cats, it is a frequent disease of appendicular joints and the coxofemoral joint is amongst the most frequently affected. Feline OA may be challenging to diagnose, as cats may not show typical clinical signs of joint-associated pain. There is a lack of knowledge regarding which mild joint changes indicate coxofemoral OA compared to normal anatomical variations in coxofemoral joint shape, margins and density. As computed tomography (CT) has several advantages over radiography (e.g. capacity to detect mild changes in density, providing a three-dimensional image and thus the ability to reconstruct the image in any plane), it would be of interest to know whether CT is a useful tool in diagnosing feline OA. The aims of this study were to investigate whether osseous changes in feline coxofemoral joints detected by CT are associated with histologic findings of OA, as well as to describe the changes seen with CT and histology. An additional aim was to compare the frequency of detection of feline coxofemoral joint OA by CT and radiography. The study material included whole-body CT studies and histological samples from 21 euthanized cats (29 coxofemoral joints). Eight cats also had radiographs. Some of the samples had been collected previously but all histologic sampling done in 2018 was CT-guided (20 joints from 12 cats), i.e. areas of specific interest in CT images were sampled using subjective guidance from CT images. CT and radiography images of joints were graded for lesions suspected to indicate OA. Histologically, a system based on Pritzker et al. (2006) for grading cartilage lesions was used and other lesions such as osteophytes were recorded. After grading of the joints, statistical analysis of association between classifications of joints as either OA positive or OA negative (between histology and CT) was performed using Fisher’s exact test. To investigate associations between grades (Pritzker and CT) and age, and between CT grades and radiography grades Spearman rho was used. The result of this study shows that osseous changes detected by CT are associated to histologic OA lesions in feline coxofemoral joints. However, the results need to be interpreted with care, as the majority of joints were histologically OA positive (27 of 29 joints) and the statistical analysis of the data was limited. Of the 27 histological OA positive joints, 23 joints (85%) were diagnosed as OA positive by CT. The frequency of detection of coxofemoral joint OA for CT and radiography is about the same (9/14 joints for radiography, 10/14 joints for CT). No certain conclusion could be made about CT’s ability to detect histologically normal joints, as there were few joints without histologic cartilage lesions in the study. There were eleven joints that were diagnosed as OA positive by CT based on the presence of osteophytes, where osteophytes could not be verified histologically. Whether there were no osteophytes in the joints (false positive CT diagnosis) or whether the suspected osteophyte was not included in the histology sample is not possible to determine. Nevertheless, the majority of these eleven joints had other bone shape features seen histologically which were suspected to explain the ‘osteophyte’ suspected in the CT image. Further research is needed to determine whether these shapes are normal variation or osseous features associated to OA.