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Introduction: The normal trapezium is 'saddle-shaped': a biconcave-convex configuration. Contradictory reports of articular wear in thumb carpometacarpal (CMC-I) arthritis include preferential volar wear (Pelligrini 1991), radial wear (Nufer 2008), and dorsal-ulnar sparing (Xu 2008). We hypothesize that volar trapezial wear predominates in patients undergoing trapeziectomy for advanced CMC-I arthritis.

Methods:  A single surgeon excised 39 trapeziae from 37 patients for CMC arthroplasty procedures over 1.5 years, with radiographic Eaton stage 2-4 disease.  Explanted specimens represented 27 female (69%), 12 male (31%); average age at surgery was 62 (30-76).  These included 25 Right trapeziae and 14 Left.  The trapezial CMC-I articular surfaces were inspected and photographed.  Three discrete but consistent patterns of wear were collectively identified: 1) retained saddle, 2) dish shape, and 3) 'cirque' shape.  The dish shape resembles a mortar and pestle pattern of degeneration, with the trapezium as the recipient mortar. A 'cirque' shape was defined as preferential pattern of volar wear, creating a separate concave facet of the volar half in sharp contrast to retained convexity dorsally, similar to a glacial mountain formation. Subsequently, 2 independent experts evaluated and classified each specimen twice, while blinded to patient identity and the other's categorization. Percentage of eburnation of other trapezial articular surfaces (scaphoid, trapezoid, and 2nd metacarpal), preferential wear patterns, and osteophyte locations were documented. Micro Computed Topography (uCT) provided further evaluation of the articular contour of representative saddles, dishes, and cirques.

Results: The 39 classified trapezial specimens fell into three discrete patterns: 20 (51%) retained saddle, 12 (31%) dish, and 7 (18%) cirque.  Intra-rater reliability was 0.96 and inter-rater reliability of the second round of classification was 0.94. Full eburnation (Outerbridge grade 4) was found in 15 (43%). Eleven (31%) had no eburnation of the scaphoid, lunate, or 2nd metacarpal articulations. Volar osteophyte formation at the metacarpal beak articulation was identified in all cirque specimens; the osteophyte location and extent was variable. uCT slices in the volar-dorsal and radial-ulnar orientations illustrate the three discrete patterns of wear (Fig 1 and Fig 2). 

Discussion and Conclusions: Surgically excised trapeziae demonstrate three distinct patterns of wear; we believe the dish and cirque represent divergent patterns from the original saddle shape, rather than a progression of severity. This refined characterization addresses the discrepancy in the literature and may provide new insight of the role of abnormal mechanical forces in thumb CMC arthritis.