Radiographic Scaphoid Waist Index and Nonunion After Scaphocapitate Fusion for Kienböck Disease

To evaluate whether lower radiographic scaphoid waist index (WI), measured on routine preoperative lateral wrist radiographs, is associated with nonunion after scaphocapitate fusion for Kienböck disease.

We performed a retrospective chart and imaging review of patients who underwent scaphocapitate fusion for Kienböck disease within a single integrated multicampus health system (1996-2025). The primary outcome was nonunion. Symptomatic adjacent-joint osteoarthritis within 1 year was evaluated separately as a descriptive secondary outcome. WI was measured independently by two observers on preoperative lateral radiographs. Rater-averaged WI was analyzed as a continuous variable. A conservative dichotomized scaphoid type analysis was performed exploratorily, with type II assigned only when both observers measured WI < 0.40.

Of 98 scaphocapitate fusions identified, 22 met inclusion criteria after excluding cases with nonretrievable archived imaging or follow-up <1 year. Median available postoperative follow-up was 23.7 months (interquartile range [IQR]: 14.4-41.2 months); range, 12.0-185.4 months. Nonunion occurred in 5 of 22 wrists (23%). Rater-averaged WI was numerically lower in wrists with nonunion than in wrists without nonunion: median 0.365 (IQR: 0.364-0.365) versus 0.429 (IQR: 0.375-0.452). The Hodges-Lehmann median difference was -0.0568, with a bootstrap 95% CI: -0.0981 to 0.0087. In the exploratory conservative dichotomized analysis, nonunion occurred in four of nine scaphoid type II wrists and 1 of 13 scaphoid type I wrists; the odds ratio estimate was imprecise and included the null (OR: 9.60; exact 95% CI: 0.64-510.98). Symptomatic adjacent-joint osteoarthritis occurred in two wrists.

In this small, selected cohort, rater-averaged radiographic WI was numerically lower in wrists with nonunion, suggesting a preliminary, hypothesis-generating WI-nonunion signal. The conservative scaphoid type I versus type II dichotomization was imprecise and should not be interpreted as definitive. Larger studies with more complete follow-up and adjustment for technical and clinical confounders are needed.

Prognostic III.