Objective:
Dorsal sagittal angulation malunion of the proximal phalanx (P1) is not an unusual observation after vascularized toe joint transfer (VJT) for the reconstruction of the proximal interphalangeal joint (PIPJ). The main purpose of the study is to determine the possible factors accounting for P1 malunion in VJT, and, the effect of the P1 malunion on surgical outcomes.

Materials and Methods:
We performed a retrospective review of patients undergoing vascularized PIPJ transfer from 2008 to 2021. Measurements of the dorsal sagittal angulation angle were done early post-operatively (<1 month), and later after bone union (>3 months). An angulation for more than 10 degrees is defined as malunion.
Post-operative skin flap laser doppler flux% measurements were utilized as an indicator of the transferred joint perfusion status. Our primary endpoint is post-operative PIPJ range of motion (ROM), especially the extension lag.

Results:
A sample of 57 transferred joints were included, on average achieving PIPJ ROM of 58 degrees post-operatively. From early post-op to P1 bone union, an average 4.87 degree increase in dorsal sagittal angulation was found.
Parallel cerclage interosseous wire with an additional 90-degree wire has a tendency of less malunion compare to fixation only with parallel cerclage wires (14.3% vs. 35%, p=0.159). We also find a tendency of poorer laser doppler flux% in the later malunion cases (-22.7% vs. -3.24%, p=0.484). A significant correlation of poorer laser doppler flux% with increased dorsal sagittal angulation was noted (p=0.046).
For malunion cases, we did not find a significant difference in surgical outcomes, including extension lag and total ROM.

Conclusion:
The shape discrepancy between the P1 shaft of finger and toe accounts for challenging stable osteosynthesis during VJT, while the inherent nature of the intrinsic muscles makes it more susceptible to malunion. In this study, we showed that fixation method and transferred joint perfusion may have an important role leading to malunion.
There is no clinically evident cause-and effect relationship found of the P1 malunion with surgical outcomes. The decision of performing corrective osteotomy should still take other clinical findings into consideration.

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