Researchers have provided insight into the existence of 2 distinct forms of foot osteoarthritis (OA)—isolated first metatarsophalangeal (MTP) joint OA and polyarticular foot OA—according to research published in Arthritis Care & Research.

“The pattern and location of joint involvement have played a fundamental role in shaping the current understanding of osteoarthritis,” the authors wrote. “We present [the] first empirical evidence for the separation of first metatarsophalangeal joint OA from a form of multijoint ‘polyarticular foot OA’ on the basis of patterning of joint involvement on plain radiographs.”

In order to investigate the existence of distinct foot OA phenotypes, Trishna Rathod, MSc, from the Arthritis Research UK Primary Care Centre at the Research Institute for Primary Care & Health Sciences in Keele, Staffordshire, and colleagues recruited 533 participants (mean age 64.9 years, 55.9% women) who reported foot pain in the previous year.


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Radiographs were scored for OA in the first MTP joint, as well as in the first and second cuneometatarsal, navicular first cuneiform, and talonavicular joints. The researchers also investigated distinct underlying classes of foot OA by latent class analysis and assessed their association with symptoms and risk factors.

The researchers found that in all of the participants, radiographic OA was clustered across both feet (P < .001) and was highly symmetrical (adjusted odds ratio, 3.0; 95% confidence interval, 2.1-4.2). Latent class analysis also identified 3 distinct classes of foot OA: no or minimal foot OA (64%), isolated first MTP joint OA (22%), and polyarticular foot OA (15%).

After adjusting for age and sex, polyarticular foot OA was found to be associated with a greater symptomatic burden—nodal OA, increased body mass index, and more pain and functional limitation—compared with the other classes.

“We have demonstrated that, as is the case for OA at other small joint sites (particularly the hands), patterning of individual joint involvement in radiographic foot OA is polyarticular and strongly symmetrical,” the authors wrote. “Patterns of joint involvement in radiographic foot OA have indicated a distinction between individuals with isolated first MTP joint OA and those with a more widespread form of OA labeled ‘polyarticular foot osteoarthritis,’ but that also includes one or both first MTP joints.”

Although it was not statistically significant, there was a slight increase in the probability that those with OA in the first MTP joint had worn high- or very high-heeled shoes between the ages of 20 and 49 years, “which is consistent with a previous study that found high-heeled footwear to be associated specifically with disorders of the forefoot and toes.”

The researchers noted that in order to extend our understanding of foot OA and how it should be best managed, further investigation is needed to examine if these subgroups differ in their foot-specific symptoms, clinical presentation, diagnostic imaging, and the symptomatic course over time.

Summary and Clinical Applicability

In the first detailed analysis of the pattern of multiple-joint involvement in foot OA, researchers have provided insight into the existence of 2 distinct forms of foot OA: isolated first MTP joint OA and polyarticular foot OA.

These findings identify possible causes of foot OA, including a joint-specific predilection for OA at the first MTP joint and possible systemic risk factors and mechanical mechanisms, leading to a more generalized presentation of OA that includes the midfoot.

The identification of a subgroup of OA involving only the first MTP joint may suggest that some people have a predilection for developing OA in this joint, perhaps due to altered foot structure or inappropriate footwear.

Reference

Rathod T, Marshall M, Thomas MJ, et al. Investigations of potential phenotypes of foot osteoarthritis: cross-sectional analysis from the clinical assessment study of the foot. Arthritis Care Res. 2016;68(2):217-227. doi:10.1002/acr.22677.