Background/Purpose: Joint erosions in RA correlate with structural damage progression and functional capacity. Therefore, detection and the follow-up of erosions are of paramount importance for RA diagnosis, monitoring and prognostication. The 5^th MTP is usually the first and most commonly destructed joint in RA. Ultrasound (US) is a useful and as good tool as MRI to detect 5^th MTP erosions especially in early RA patients. Although a few MRI studies revelaed that most of the MTP erosions were located at the plantar aspect of the joint, it is unknown whether plantar or dorsal or lateral plane US better detects 5^th MTP erosions.  In this study we aimed to determine the best ultrasonographic plane for detection of 5^th MTP joint erosion in RA patients and to assess clinical characteristics of patients with 5^thMTP erosions.

Methods: The 5^thMTP of 92 feet of 48 RA patients were evaluated by B-mode and power- Doppler US for signs of erosion. US images were obtained from 3 different aspects, the dorsal, lateral and the plantar surface of the foot, in longitudinal and tranverse scans.  The presence of erosion was determined according to OMERACT definition. Patients were also assessed clinically (tender/swollen joint count, DAS28, HAQ scores) along with disease characteristics. Each erosion in each aspect were recorded seperately. 

Results: The study cohort consisted of 48 RA patients (F/M=35/13, mean age 50.7±12.3 years, mean DAS28 score 4.04±1.47, HAQ score 0.94±0.88) with mean disease duration of 8.2±7.3 years. RF and anti- CCP positivity were 66.7% and 60.4% respectively. The 5^th MTP erosions were detected in 35 of 48 patients (72.9%) and 62 of 92 feet (67.4%). Of the erosions 15 (16.3%) were observed at dorsal plane whereas plantar and lateral planes revelaed majority of the erosions, 53 (57.6%) and 52 (56.5%), respectively.  In 26 feet (28.3%), erosions were observed on both plantar and lateral planes, in 15 feet (16.3%) erosions were observed in all three planes. None of the patients had erosion just in dorsal plane examination of the 5^th MTP.  The presence of 5^th MTP erosion in plantar aspect of foot was significantly higher than both lateral and dorsal aspect (P<0.0001, P=0.002, respectively). Patients with dorsal erosion had significantly higher disease duration (13.8 years vs 6.5 years, P=0.005). Patients with erosions in any aspects have similar disease characteristics including RF, anti-CCP positivity, disease duration, HAQ scores and current biologic requirement, when compared to patients without erosions. 

Conclusion: The 5^th MTP erosions in RA patients can be better detected with plantar plane US than dorsal and lateral aspect evaluations. These data also indicate that dorsal aspect US of foot may miss erosions in early disease and insufficient when performed solely.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/sonographic-evaluation-of-the-fifth-metatarsophalangeal-joint-erosion-in-rheumatoid-arthritis/