Lunate Shift Index (LSI): A New Parameter for the Evaluation of Residual Ulnar Side Wrist Pain in Patients with Wrist Osteoarthritis Undergoing Three-Corners Arthrodesis vs. Four-Corners Arthrodesis—A Retrospective Comparative Study with Minimum 2 Years of Follow-up

Purpose Both scaphoid non-union advanced collapse wrist (SNAC) and scapho-lunate advanced collapse wrist (SLAC) at stage II–III are common indications for limited wrist fusions including four-corners fusion (4CF) and three-corners fusion (3CF). The aim of this study was to assess the clinical and radiological outcomes in patients undergoing 3CF vs. 4CF. A new radiological index called Lunate Shift Index (LSI) was devised to evaluate the importance of the lunate displacement relative to the radiolunate joint. Materials and Methods Twenty-eight patients undergoing 3CF and 40 patients undergoing 4CF were clinically evaluated. The radiolunate angle, the carpal height, and the LSI were recorded radiographically. The LSI corresponds to the ratio between the distance from the lunate centre to the middle of the intermediate column and the length of the intermediate column of the distal radius. Results A statistically significant correlation was observed between LSI and clinical outcomes. The lunate displacement was associated with an increased incidence of wrist ulnar pain. No statistically significant differences were observed between 3 and 4CF in all parameters compared. Conclusions The osteoarthritis of piso-triquetral joint has been identified as the cause of wrist ulnar pain in patients undergoing 4CF. The lunate correct positioning allows to maintain the carpal height and to increase the contact area at the level of the radiolunate joint. A good reduction of the lunate could be obtained with the 3CF compared to 4CF. This study showed how proper realignment of the lunate following midcarpal arthrodesis correlates with a better clinical outcome. Level of Evidence Level III, Retrospective comparative study.