Asymmetric Lambda Sign on Axial Weightbearing CT Scans of the Foot: An Indicator to Aid in the Diagnosis of Subtle Lisfranc Instability?

Abstract

Category: Trauma; Midfoot/Forefoot Introduction/Purpose: Subtle Lisfranc instability, a spectrum of conditions resulting from trauma of individual structures within the Lisfranc ligamentous complex (LLC) including the dorsal- (DLL), interosseous- (ILL), and plantar Lisfranc ligaments (PLL) remain challenging to effectively diagnose. The current standard to asses these injuries is through bilateral weightbearing radiography. However, weight bearing computed tomography (WBCT), which provides a clearer visualization of osseous structures, joint spaces, and instability could also be utilized for evaluation. This study aims to define and report the occurrence rate of an ‘asymmetric lambda sign’ observed on axial WBCT imaging secondary to simulated purely ligamentous Lisfranc injuries of various severities subjected to increasing magnitudes of weightbearing conditions. It is hypothesized that this sign will be a reliable aid in detecting subtle Lisfranc instability, clinically. Methods: The asymmetric lambda sign was assessed on 24 match-paired cadaveric legs (mean age, 46.0 +- 14.8 years; mean weight, 80.2 +- 17.4 kg; mean body mass index, 25.3 +- 4.2 kg/m2). Dissection groups were described as follows: control intact LLC (group 1), dissection of the DLL (group 2), dissection of the DLL and ILL (group 3), dissection of the DLL, ILL, and PLL (group 4). After each dissection, CT scans were acquired in non- (NWB, 0 kg), partial- (PWB, 40 kgs), and full-weightbearing (FWB, 80 kgs) conditions. In a standardized axial view, the lambda sign was appreciated by visualizing the joint spaces between the medial cuneiform and the second metatarsal base (C1-M2), the medial and middle cuneiform (C1-C2), and the second metatarsal base and middle cuneiform (M2-C2). The asymmetric lambda sign was defined by a widening of the C1-M2 joint space relative to the C1-C2 or M2-C2 joint space (Fig). Results: The asymmetric lambda sign was observed in 25.6% (221/864) of all studies. This sign was not found in any intact specimens regardless of the weightbearing status. In group 2, this sign was observed in 4.2% of NWB, 15.3% of PWB, and 16.7% of FWB conditions. With the DLL and ILL dissected, simulating a more developed subtle LLC injury (group 3), this sign was observed in 15.3%, 27.8%, and 38.9% in NWB, PWB and FWB conditions, respectively. Additionally, the fully dissected specimens (group 4), demonstrated this specific sign in 33.3%, 72.2%, and 83.3% in NWB, PWB, and FWB conditions, respectively. The inter- and intra- observer reliability was calculated to a kappa value of .843 and .912. Conclusion: An asymmetric lambda sign viewed through axial WBCT imaging is a reliable indicator in the diagnosis of a complete Lisfranc ligamentous injury with secondary joint instability in partial and full weightbearing conditions as demonstrated using a cadaveric model. Clinically, this simple sign can be utilized as an aid to accurately evaluate and diagnose patients presenting with subtle Lisfranc instability.