Our findings suggest that rheumatoid hand phenotypes and disease progression represent several specific patterns (Fig. 4). In the early phase, hands with type 1 thumb deformity were the most common phenotype, and patients had minimal disability, implying that type 1 thumb deformity is the mildest and first hand deformity we encounter clinically. Other hands developed type 3 or 4 thumb deformity first. In the middle phase, several progression paths appeared. Some hands with type 1 thumb deformity progressed to type 2, some hands developed severe boutonnière deformity, and other hands developed severe swan-neck deformity. In the terminal phase, despite the fact that no hands progressed to cluster 7 from other clusters, in this study, if disease activity remains high, any thumb deformity would progress to type 6.
Originally, the Nalebuff classification for thumb deformity divided thumbs into six types by the initially affected joint and its appearance[14, 15], and type change over time was not considered. Additionally, to our knowledge, no studies have compared hand function by type and none have quantified the impact of deformity type on hand function. Our results showed that type 1, 3, and 4 are the primary phenotypes of the thumb deformity, and type 2 is a secondary lesion of type 1. In type 2, the carpometacarpal joint was initially involved, and flexion contracture of the metacarpophalangeal joint was secondary; however, type 3 also initially involves the carpometacarpal joint. The underlying mechanisms influencing this phenotype difference are unknown. Some hands with type 1 thumb deformity progressed to type 2 in this study, suggesting that type 2 involves the metacarpophalangeal joint, first, followed by the carpometacarpal joint.
Quantifying finger deformities, specifically swan-neck deformity and boutonnière deformity, was challenging in this study. Anatomically, the index to little fingers have different roles. The index and middle fingers are mainly used in extension for reach behaviours, in contrast with the ring and little fingers, which work in flexion while grasping. Therefore, the affected finger should be considered when interpreting our results. A previous study evaluated each affected finger separately, but the authors did not evaluate the proximal interphalangeal joint and did not describe the finger deformity phenotypes. Another study reported the results of a stratified analysis by finger among patients who underwent surgery with silicone arthroplasty. The authors reported that the ring and little finger had larger extension lags, but the authors did not describe hand function. A study evaluating finger deformity separately showed an almost even distribution for the characteristic finger deformities from the index to little fingers. However, to our knowledge, no patient-rated subjective indicator evaluating hand function assesses fingers separately; therefore, the absence of weighting impact on function difference by each finger would have minimal impact on the results.
In this study, the swan-neck deformity scores and boutonnière deformity scores were treated equally and entered into the cluster analysis. Several studies have shown that swan-neck deformity indicates more severe disability than boutonnière deformity[1, 2, 18]. This suggests that scores from swan-neck deformity should be weighted; however, our previous study showed that both deformities contribute equally to hand function. Therefore, we used the same quantification method, in this study.
The ulnar drift scores were equally dispersed across the clusters except for cluster 3, indicating that ulnar drift may occur independently from other deformities. Ulnar drift arises mainly from metacarpophalangeal joint involvement, but clinically, many other causes dictate its course. As a result, ulnar drift should be managed separately from other deformities when rheumatologists consider referring a patient for surgery. Silicone arthroplasty for ulnar drift can provide excellent relief from the cosmetic effects and poor hand function related to rheumatoid arthritis[17, 20].
Our stratified analysis showed that hands affected for ≤ 10 years constituted only approximately 10% of clusters 3 or higher (middle or terminal phase). In contrast, hands affected for more than 10 years constituted 30% of clusters 3 and higher, suggesting that a 10-year affected period could be a meaningful threshold indicating increasing risk of rapid disability progression.
This study includes several limitations. First, our cluster analysis prioritized the impact of thumb deformity to determine the seven clusters. Therefore, the characteristics of all clusters were influenced by thumb deformity. Given that thumb deformity provides the main impact on disability, our cluster classification could be very useful in determining daily medication therapy, but it remains unclear which deformities and what degree of severity influence disability. Second, the cluster analysis assigned hands to each cluster group retrospectively. Therefore, our clusters are explanatory research and cannot necessarily be applied to new single hands; additional studies are needed. Third, we used the Kapandji index as a functional evaluation, and our results should be verified using patient-reported outcome measures.