Prospective Validation of a Weightbearing CT Stability Threshold for Subtle Lisfranc Injuries.

BACKGROUND

Accurate evaluation can be challenging in subtle Lisfranc injuries. Although weightbearing computed tomography (WBCT) provides 3-dimensional assessment under physiological load, its role in diagnosing subtle Lisfranc instability and guiding treatment thresholds remains undefined. Currently there is no consensus on the optimal timing, measurement protocol, or stability threshold with WBCT.

METHODS

We prospectively recruited patients with nondisplaced (<2 mm), intra-articular fractures and/or avulsion fractures in the tarsometatarsal 1-3 area. To assess Lisfranc joint stability, patients underwent bilateral, single-leg WBCT scans. Medial cuneiform (C1) and second metatarsal (M2) measurements, evaluating the integrity of both the dorsal and interosseous Lisfranc ligament, were combined and compared to the contralateral, healthy side to create a difference score. Threshold of instability was defined as >3 mm C1-M2 difference score. Measurements were tested for agreement using the intraclass correlation coefficient (ICC).

RESULTS

38 patients were included in the study and were able to fully weightbear after a median of 9 days postinjury. Eight patients (21%) had a C1-M2 difference score >3 mm (95% CI, 3.8-5.8) while loading. Instability was confirmed in all 8 patients during a fluoroscopic stress test. The remaining 30 patients, with a C1-M2 difference score <3 mm (95% CI, 0.6-1.1), were classified as stable and received conservative treatment. These patients were followed up with an additional WBCT scan after 12 weeks, and none had signs of Lisfranc instability with a median different score 0.6 mm (95% CI, 0.4-0.7). Our measurement method demonstrated excellent interrater (ICC 0.95, 95% CI, 0.93-0.96) and intrarater agreement (ICC 0.97, 95% CI, 0.96-0.98).

CONCLUSION

For subtle Lisfranc injuries, adequate WBCT can be performed 9 days postinjury. In our study population, using our dual-measuring method, a C1-M2 difference >3 mm on WBCT reliably identified subtle Lisfranc instability.