Weightbearing radiographs accurately predict deep deltoid ligament injury in Weber B ankle fractures – a minimally invasive arthroscopic evaluation

Abstract

Background

Differentiating Weber B/SER4a from SER4b fractures relies on weightbearing radiographs and gravity stress tests, assumed to reflect deep posterior tibiotalar ligament (dPTTL) integrity. This study assessed their diagnostic accuracy for dPTTL injury, with arthroscopy as the reference standard.

Methods

We prospectively assessed 20 Weber B/SER4a-b fractures with a medial clear space ≥ 5.0 mm. Arthroscopy under local anesthesia evaluated dPTTL integrity, defined as tensioning and medial joint space closure during dorsiflexion.

Results

Radiographically, 15 fractures were classified as SER4a, and five as SER4b. Arthroscopy revealed intact dPTTL in 14 of 15 SER4a cases, while all SER4b cases had ruptures. Weightbearing radiographs showed 83.3% sensitivity and 100.0% specificity for detecting dPTTL injury; the gravity stress test had 100.0% sensitivity but 0.0% specificity.

Conclusions

Stable weightbearing radiographs reliably indicate dPTTL integrity, while instability suggests dPTTL rupture.

Level of evidence

Level II, Cross-sectional study.

1 Introduction

Accurate assessment of ankle stability is essential in managing Weber B/supination–external rotation (SER) fractures ^,^,^,^,^,^, . The treatment decision hinges on differentiating stable injuries suitable for nonoperative care from unstable fractures requiring surgery. Weightbearing radiographs have become central to this decision-making, as they assess mortise congruency under physiological load and often support functional treatment, even when gravity stress radiographs suggest instability ^,^, .

A functional subclassification has been proposed: SER4a injuries widen on gravity stress but remain congruent on weightbearing radiographs, suggesting partial deltoid disruption with preserved deep posterior tibiotalar ligament (dPTTL) function . SER4b injuries show incongruity in both modalities, indicating complete dPTTL rupture . This distinction is clinically pivotal—classifying an injury as SER4a can spare patients unnecessary surgery, whereas missing an SER4b can lead to chronic instability and arthritis.

Despite this, the central assumption—that weightbearing radiographs accurately reflect dPTTL integrity—has never been prospectively validated. Evidence to date relies on MRI correlations, cadaveric models, or indirect clinical inference ^,^, .

To address this critical gap, we performed a prospective arthroscopy-controlled study to test the diagnostic accuracy of weightbearing radiographs in Weber B fractures. By validating whether stable weightbearing imaging corresponds to an intact dPTTL, this study aims to strengthen the evidence base for nonoperative treatment decisions and reduce both overtreatment and undertreatment in this typical fracture pattern.

2 Materials and methods

The study was conducted at the Østfold Hospital Trust, Norway, from May 2020 to February 2022. It was prospectively registered in ClinicalTrials.gov (NCT04674046) and received approval from the Regional Committee for Medical and Health Research (Ref. 123245) and the Østfold Hospital Trust Institutional Review Board. Reporting followed the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) checklist . This study was funded by the Østfold Hospital Trust and the South-Eastern Norway Regional Health Authority (grant 2023014).

2.1 Participants

Adults aged 18–80 years presenting to the Østfold Hospital Trust with a supination–external rotation (SER) type Weber B ankle fracture were prospectively screened. Initial non–weightbearing mortise radiographs needed to show a medial clear space (MCS) < 7 mm, as fractures with an MCS ≥ 7 mm are widely regarded as unequivocally unstable and were therefore excluded from further assessment ^,^, . Dynamic instability was defined as an MCS ≥ 5 mm on either gravity-stress or weightbearing views; only fractures meeting this threshold were considered for inclusion, aligning with previous reports ^, . Participants were required to have sufficient proficiency in Norwegian to complete patient-reported outcome measures. Additional exclusion criteria included prior fracture or major surgical procedure involving either the ankle or hindfoot, pre-injury reliance on walking aids, peripheral neuropathy or other sensory deficits affecting gait, cognitive impairment that prevented informed consent, and any condition likely to hinder adherence to the study protocol.

2.2 Procedure

Following a Weber B/supination-external rotation (SER) ankle fracture diagnosis, a below-knee cast was applied. Patients were instructed to wear the cast until a final assessment of ankle stability could be performed in the outpatient clinic 3–7 days after the injury. This assessment involved bilateral weightbearing and gravity stress radiographs. Before weightbearing radiography, the cast was removed, and patients were told to stand with equal weight on both feet. The injured foot was required to be plantigrade and to bear at least 50% of the patient’s body weight. To ensure adequate loading, patients’ ability to meet this criterion was verified using a bathroom scale, as previous studies have highlighted variability in force application during imaging. For gravity stress testing, patients were positioned laterally on the side of the injured ankle on a flat examination table. The foot and distal half of the leg extended beyond the edge of the table, and patients were instructed to relax the foot during imaging. Radiographs were analyzed using the Sectra Picture Archiving and Communication System (Sectra AB, Linköping, Sweden). Ankle stability was assessed by measuring the MCS, which indirectly indicates tibiotalar congruence. The MCS was defined as the distance between the medial border of the talus and the lateral border of the medial malleolus, measured along a line parallel to and 5.0 mm below the talar dome on mortise-view radiographs. This measurement method has demonstrated high intra- and interrater reliability in previous studies. An ankle was classified as stable if the MCS was less than 5.0 mm. Instability was defined as an MCS of ≥ 5.0 mm, with a difference of ≥ 1.0 mm compared to the contralateral, uninjured ankle. These thresholds were applied to weightbearing and gravity stress radiographs to determine ankle stability.

2.3 Arthroscopic evaluation

The surgeons were permitted to review plain, non-weightbearing radiographs but were blinded to weightbearing radiographs and gravity stress test results. Arthroscopies were performed in the outpatient clinic. The standard anterior approach for ankle arthroscopy was used. Local anesthesia was administered at the planned portal sites and intra-articularly. No tourniquet was applied. The anteromedial portal was placed just medial to the tibialis anterior tendon at the level of the ankle joint. A minimal skin incision was made, and an Arthrex Nanoscope (Arthrex GmbH, Naples, Florida, USA) was inserted into the joint for visualization. An anterolateral portal was established lateral to the peroneus tertius tendon or the extensor digitorum longus tendon if the peroneus tertius was not easily palpable. The latter portal was used for distention and irrigation. First, the anteromedial gutter of the ankle joint was visualized. A dynamic stability assessment was performed by dorsiflexing the ankle from a neutral position, during which talar motion and changes in the medial joint space were evaluated. With an intact deep posterior deltoid ligament, dorsiflexion resulted in physiologic medial talar translation and tightening of the medial joint space, with maintained congruent apposition between the talar dome and the medial malleolus.

The arthroscope was then advanced into the medial gutter to visualize the deep posterior deltoid ligament, which was assessed for continuity and tension. Preserved ligament morphology with increased or maintained tension during dorsiflexion, together with medial talar translation, was classified as an SER4a injury pattern. In contrast, ligament disruption, or loss of tension, particularly when accompanied by absence of medial talar translation or paradoxical lateral talar translation during dorsiflexion, was classified as an SER4b injury pattern.

Figs. 1 and 2 show representative radiographic and arthroscopic findings for an SER 4a and an SER 4b injury, respectively.