AB0855 THE “HORSE SADDLE” SIGN: A NEW ULTRASOUND SIGN FOR OSTEOARTHRITIS
Authors: G. Añez, E. Trallero, A. Erra, M. Barceló-Bru, M. Lopez Lasanta, J. J. De Agustin
Journal: Annals of the Rheumatic Diseases (2020)

Background: Hand Osteoarthritis is one of the most prevalent rheumatic diseases that can give early visible findings by ultrasound where synovial hypertrophy, effusion, osteophytes and articular cartilage decrease stand out. However, these findings, although sensitive, may not be very specific since they are also observed in inflammatory arthritis. Throughout our clinical practice in osteoarthritis, we have seen, over repeated examinations, an specific morphological change of bone not previously described in the literature. It is a bone extension in the head of the phalanx of the finger joints that causes a deformity that we have called “horse saddle” and that is typically located in the proximal and distal interphalangeal joints of the fingers. This sign can be seen in the longitudinal exploration of the palm of the hand by grayscale ultrasound. This sign that we have not found specifically described in the literature reviewed to date is considered to be useful for the diagnosis of osteoarthritis. Objectives: To Assess the sensitivity and specificity of the “horse saddle” sign in the diagnosis of osteoarthritis. Methods: An exploratory clinical comparative cross-sectional study where an ultrasound of the hands and comparative radiographs in PA view were performed on patients with osteoarthritis, inflammatory arthritis and healthy patients seen in the Rheumatology clinic of the Vall de Hebron Hospital. Age, sex and time of evolution of the disease were collected as clinical variables. The MCP, PIP and DIP joints from the second to the fifth finger of both hands were viewed with grayscale in longitudinal and transverse plane of both the dorsal and palmar face, assessing for osteophytes, synovitis and the horse saddle sign. A General Electric Logiq S8 machine was used with an 8-13 MHz linear probe. All patients signed an informed consent and approval was obtained from the hospital ethics committee. The statistical analysis was carried out with Stata 15.1. Results: A total of 38 patients with osteoarthritis, 20 patients with inflammatory arthritis (8 psoriatic, 9 RA, 1 LES, 1 PMR and 1 Sjögren) and 2 healthy patients were assessed. It was found that the horse saddle sign had a sensitivity of 66.7% and specificity of 86.4% in osteoarthritis showing a p-value of 0.052 by means of the chi-square test. 87% of patients with the horse saddle sign had osteoarthritis and only in 2 patients with RA and in the patient with LES. In contrast for osteophytes a sensitivity of 100% was observed with a specificity of 45.45% (p of 0.001) and for synovitis a sensitivity of 53.3% and specificity of 77.27% was observed with p of 0.039. Conclusion: The horse saddle sign is an ultrasound sign with good sensitivity and specificity for the diagnosis of hand osteoarthritis comparable to other classic ultrasound signs such as osteophytes and synovitis. References: [1]Iagnococo A, Palombi G, Valesini G. Role of ultrasound in osteoarthritis. Rev Esp Reumatol. 2001; 28: 301-6. [2]Abraham AM, Pearce MS, Mann KD, Francis RM, Birell F. Population prevalence of ultrasound features of osteoarthritis in the hand, knee and hip at age 63 years: the Newcastle thousand families birth cohort. BMC Musculoskelet Disord. 2014; 15: 162. DOI: 10.1186/1471-2474-15-162. Disclosure of Interests: None declared