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Treatment of Dorsal Intercalated Segment Instability (DISI) of the Wrist by "Overstuffing" the Scaphoid in a Cadaveric Model
Anthony E. Capito, MD; James P. Higgins, MD
Curtis National Hand Center, Union Memorial Hospital, Baltimore, MD

Purpose: Options are limited for scaphoid nonunions of the proximal pole where there is a deficiency of bone. The free medial femoral trochlea (MFT) flap has been described for this situation. The flap replaces the proximal pole of the scaphoid with articular cartilage without scapholunate (SL) ligament reconstruction. The reported procedure maintained normal carpal alignment radiographically without development of carpal instability. Long term results, however, are unknown and may lead to carpal instability with resultant degenerative arthritis. The procedure entails placement of a larger graft which may alter the kinematics of the carpus and prevent carpal malalignment. The purpose of this study was to evaluate whether expanding i.e. “overstuffing” the scaphoid can correct a dorsal intercalated segmental instability (DISI) deformity of the wrist in a cadaveric model radiographically.

Methods: Five fresh cadaver wrists were used for this study. The radiolunate angle was calculated, with the SL ligament and scaphoid intact, for each wrist in six positions: neutral, flexion, extension, radial and ulnar deviation, and clenched fist. The SL interval was measured in neutral, ulnar deviation, and clenched fist views only. The SL interosseous ligament was then completely incised and an osteotomy was performed to excise the proximal third of the scaphoid to simulate a proximal pole deficiency nonunion and create a DISI deformity. The radiographic measurements were repeated. The proximal pole of the scaphoid was then replaced with its original piece of bone and radiographic measurements were then repeated without SL ligament repair. Finally, the osteotomy site was overstuffed with a 4mm sawbone spacer without SL ligament repair and radiographs were obtained. Four test conditions were developed: intact, excised, replaced, and overstuffed.

Results: Sectioning the SL ligament and removing the proximal pole of the scaphoid resulted in radiographic evidence of DISI deformity. The deformity persisted with replacement of the proximal pole with k-wire fixation without ligament repair. Overstuffing corrected the radiolunate angle in all positions and was significant in neutral and clenched fist views. The SL interval significantly improved on clenched fist view between the overstuffed specimen and replaced specimen with the SL ligament divided.

Conclusions: The results demonstrate that in this cadaver model, “overstuffing” the scaphoid can correct a DISI deformity without SL ligament reconstruction. This overcorrection might correlate with improved radiographic measurements when performing a scaphoid nonunion with humpback deformity reconstruction. This also provides an explanation of how the MFT can work without SL reconstruction.

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