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We read with interest the short case study by Huntley and Howie concerning the potential role of mechanical factors in joint destruction of the rheumatoid hand.1 Their report indicated severe bilateral destruction of all metacarpophalangeal (MCP) joints except those in the congenitally short 4th metacarpal bone.
In a search of roughly 3500 posteroanterior views of hand radiographs by Picture Archiving and Communication System from May 2003 to June 2004, we identified 12 patients with rheumatoid arthritis (RA) among the 27 patients who had unilateral or bilateral short metacarpal(s). An experienced bone and joint radiologist (KBJ), who was unaware of our review, scored the erosion and joint space narrowing of their MCP joints according to the Sharp method modified by van der Heijde.2 We found that short metacarpals were confined to the 4th and/or 5th fingers and that MCP joints in short metacarpals seemed to show less destruction than the MCP joints in natural metacarpals (table 1). This is in accord with the report by Huntley and Howie.
We further wondered if the MCP joints of the 4th and 5th metacarpals had less destruction than those of the 2nd and 3rd metacarpals in cases of RA. We evaluated hand radiographs in 50 consecutive patients with erosive RA whose metacarpals were of normal length, using the same method and radiologist as in the previous review. The modified Sharp scores for erosion of left and right hands were, respectively 0.7 (1.4), 1.1 (1.7) (I); 1.6 (2.0), 1.6 (2.2) (II); 0.8 (1.6), 1.5 (2.0) (III); 0.7 (1.7), 1.2 (2.0) (IV); and 0.8 (1.6), 1.0 (1.8) (V). The scores for joint space narrowing were 0.8 (1.3), 1.0 (1.3) (I); 1.0 (1.4), 1.4 (1.6) (II); 0.6 (1.2), 1.2 (1.5) (III); 0.4 (1.0), 0.8 (1.3) (IV); and 0.6 (1.2), 0.8 (1.3) (V). The paired t test was applied to compare means between the sum of the scores of the 2nd+3rd MCP joints and of the 4th+5th MCP joints for bony erosion and joint space narrowing. We found that the 4th and 5th MCP joints in RA showed less joint destruction than the 2nd and 3rd MCP joints (table 2). We found more evidence for this conclusion in the articles by Belt et al3,4 and Mulherin et al,5 in which we noticed that 4th and 5th MCP joints were less involved in destructive changes from RA than the 2nd and 3rd MCP joints, although those authors dealt with other subjects in their articles.
From these reports, one concludes that the protective role of a short metacarpal bone to MCP joints in a rheumatoid hand might be due in part to the anatomical position because short metacarpals were only found in the 4th and 5th fingers in our large scale search and because 4th and 5th MCP joints were less affected by bony destruction caused by RA. An analysis of raw data from previously published, large scale studies dealing with radiological scoring of hand radiographs by various methods would clarify this matter.
We thank Jun et al for examining the hypothesis advanced in our case report.1 They suggest that the metacarpophalangeal (MCP) joint of the fourth metacarpal is intrinsically less susceptible to the changes of rheumatoid arthritis (RA) (as evidenced by radiography)—and that this in part accounts for the observed sparing of MCP joints of congenitally short metacarpals.
Jun et al identified 12 patients with RA and short MCP joints (table 1, above). However, only one of these (patient 11) had a configuration that would result in radioulnar splintage of the MCP joint (the other patients had either short fourth and fifth metacarpals, or only a short fifth metacarpal). On the basis of the data in their table 2, Jun et al suggest that the 4th and 5th MCP joints are generally spared, relative to the 2nd and 3rd MCP joints. Confusingly, there also appear to be marked differences in bony erosion and joint space narrowing when the left and right hands are compared. Explanations for relative sparing of particular MCP joints have been proposed by other authors, including: (a) a role for different intra-articular pressures,2 and (b) an effect of wearing gold rings.3,4
Our report1 was couched in terms of speculation, and was necessarily constrained by space. Two further features strengthen the argument about the importance of mechanical factors in the progression of joint deterioration:
In our case, of all the proximal interphalangeal (PIP) joints, it is those of the ring fingers that are worst affected (both clinically and radiologically; see fig 1 in ref 1). Given stabilising splintage of the fourth MCP joint, the destruction at the PIP joint of the ring finger can also be accounted for in mechanical terms—it is likely to be subjected to adverse splintage by the ulnar drift of the long and small digits.
Our patient never wore gold on the right hand, and only for 1 year on the left hand, this being 8 years before he developed RA. In this case, therefore, the hypothesis about locally worn gold is effectively precluded.
We agree that the explanation of Jun et al may have a minor contributory role, but the degree of sparing is so marked that our mechanical explanation—MCP joint splintage by substantially longer neighbouring metacarpals—probably defines the substantive mechanism.