The Effect of Capitate Position on Coronal-Plane Wrist Motion after Simulated Four-Corner Fusion

Background: Scaphoid excision with four-corner fusion (4CF) is often performed as a salvage procedure when the radiolunate articulation is spared in conditions such as scapholunate advanced collapse (SLAC) and scaphoid nonunion advanced collapse (SNAC). In particular, the position of the fused lunate in the sagittal plane is a determining factor of wrist flexion and extension and is considered by some to be the most important step of the procedure. To our knowledge, however, the coronal relationship between the lunate and capitate and its effect on wrist motion has not been reported. The objective of this study was to examine the effect of altering the capitolunate relationship on coronal-plane wrist motion after scaphoidectomy and 4CF. We hypothesized that wrist resting posture would be altered and radial-ulnar motion would diminish following fusion in the “capitate covered” position compared with normal wrists.

Materials and Methods: Eight human cadaveric limbs were disarticulated at the elbow and mounted on a custom jig. The baseline position of the wrist was recorded with no load applied, followed by sequential loads of 44.4 N, 89 N, and 125 N applied to the flexor carpi radialis (FCR), extensor carpi radialis longus (ECRL), and extensor carpi radialis brevis (ECRB) tendons to simulate maximal radial deviation (RD) and to the flexor carpi ulnaris (FCU) and extensor carpi ulnaris (ECU) tendons to simulate ulnar deviation (UD). A scaphoidectomy was then performed and the two methods of 4CF (anatomic position and “capitate covered” position; Fig. 1) were studied in random order. Range of motion (ROM) was compared using one-way analysis of variance (ANOVA) and Bonferroni correction.

Results: The resting position of the “capitate covered” wrist was in significantly greater radial deviation than the preoperative wrist position (p < 0.01). Under 44.4-N radial load, the “capitate covered” position had significantly greater radial deviation than the preoperative wrist at the same radial load (p = 0.01). The wrist fused in anatomic position did not differ significantly from the preoperative wrist at any position.

Conclusions: To our knowledge, this is the first study to evaluate the coronal position of the capitolunate articulation following scaphoid excision and 4CF. In this cadaveric model, the resting position of the wrist, as well as the coronal arc of motion, was not significantly changed from the preoperative state when the anatomic carpal bone positions were maintained after scaphoid excision. In contrast, complete covering of the capitate head by the lunate, as recommended in the literature, placed the wrist in excessive radial deviation. The increased radial posture and motion are likely due to changes induced in the resting tension of the extrinsic wrist ligaments and may affect postoperative appearance and function.

Fig. 1 (a) Normal wrist. (b) Anatomic position of 4CF. (c) “Capitate covered” position of 4CF.