Background/Purpose:

The forefoot is described as the most common site of symptoms in the foot and ankle of patients with rheumatoid arthritis (RA).  Pressure under the forefoot area is significantly increased in patients with RA compared to normal subjects and forefoot peak pressures have been reported to correlate with pain, damage and higher erosion scores at the MTP joints.  Damage to plantar structures such as the capsule and plantar plate of the MTP joints in patients with RA is associated with bone erosion, suggesting altered biomechanics or mechanical effects due to capsule or plantar plate abnormalities may cause bone changes.  While a tendency towards a lateral distribution has been noted in the hands, the location of erosions at the MTP joints has not been previously reported in patients with RA.  This exploratory study hypothesised that the majority of erosions at the MTP joints in patients with RA are on the plantar aspect, contributing to pain, damage and increased pressure.

Methods:

In 24 patients with RA the more symptomatic forefoot was imaged using 3Tesla MRI.  T1 weighted sagittal and short axis post gadolinium sequences were acquired through the MTP joints.  Images were scored for bone erosion in the distal and proximal part of the MTP joints using the RAMRIS system.  The base of the proximal phalanx and the head of the metatarsal were divided into quadrants to determine the location of erosions in dorsal-medial, dorsal lateral, plantar medial and plantar lateral regions.

Results:

Seventeen females and seven  males with a mean age of 55.5 years, disease duration 10.6 years (range 0.6 – 36) and self reported foot pain measured by 100mm visual analogue scale (VAS) of 43.4 (SD 27.9) took part in the study.  Eighteen patients were rheumatoid factor positive, the mean (SD) DAS44 (CRP) and DAS44 (ESR) were 2.5 (0.8) and 2.6 (0.9) respectively.  The location of erosions at the MTP joints in patients with RA is shown in the table below.  The majority of erosions are reported on the plantar aspect of the MTP joints.

	Dorsal Medial	Dorsal Lateral	Plantar Medial	Plantar Lateral
1st Proximal Phalanx	7	3	5	7
1st Metatarsal	6	4	13	19
2nd Proximal Phalanx	0	1	5	8
2nd  Metatarsal	6	5	9	6
3rd Proximal Phalanx	1	1	6	6
3rd Metatarsal	3	4	10	8
4th Proximal Phalanx	1	1	6	6
4th Metatarsal	6	3	12	6
5th Proximal Phalanx	2	4	9	5
5th Metatarsal	10	11	14	14
Total	42	37	89	75

Conclusion:

This is the first study to report the location of erosions at the MTP joints in patients with RA and forefoot pain.  The results confirm the hypothesis of this exploratory study; erosions were more commonly reported on the plantar aspect of the distal and proximal parts of the MTP joints, often in sites not readily imaged using standard ultrasound protocols.  Further work is required to determine the relationship between forefoot pain, deformity and raised plantar pressures with the location of erosions at the MTP joints in patients with RA.  Furthermore, these study findings highlight the need to image the dorsal and plantar aspect of the MTP joint when using ultrasound alone to detect erosions in RA.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/location-of-erosions-at-the-metatarsophalangeal-joints-in-patients-with-rheumatoid-arthritis/