Ankle fracture classification systems have been developed by Lauge-Hansen1, Weber2,3, and the Orthopaedic Trauma Association Committee for Coding and Classification (OTA/AO)4,5. These classifications are based on non-stress, standard radiographs, and they are useful for describing fracture patterns. However, the reliabilities or prognostic values of these classifications have not been proven to be consistent6. Stability-based diagnostic algorithms have been developed in the last few decades, because of a fairly comprehensive consensus opinion that fractures with a stable ankle mortise can be treated conservatively, but unstable fractures are expected to have a better outcome with operative treatment6,7. The exclusion of dynamic instability is thought to be particularly important when the most common type of ankle fracture, known as a supination-external rotation fracture1 and classified as OTA/AO 44-B4,5, represents an isolated lateral malleolar fracture without medial widening of the ankle mortise6,7.
The deep deltoid ligament and its osseous insertion (medial malleolus) are considered the main stabilizers of the ankle mortise8,9. A displaced fibular fracture is thought to be of minor importance in affecting this stability6. However, clinical signs (medial swelling, tenderness, and ecchymosis) regarding the deltoid ligament are generally considered insufficient for a reliable prediction of ankle mortise stability10-12. Consequently, further imaging, either static or dynamic, has been advised. Magnetic resonance imaging (MRI) studies have confirmed that the deltoid ligament is always injured to some degree in supination-external rotation-type fractures13-15. However, because of its moderate reliability, MRI offers limited value to clinical decision-making15. Alternatively, manual external-rotation stress radiographs10-19, gravity stress tests16-19, or weight-bearing radiographs20-22 have been used to quantify possible dynamic instability. Computed tomography (CT) has mainly been used in research settings23,24.
There is no gold-standard method for evaluating the stability of the ankle mortise; however, external-rotation stress radiographs have been studied most extensively6,7,10-21. External-rotation stress testing typically requires experience19, and weight-bearing radiographs may not be possible initially because of pain. Therefore, an early control visit within 2 weeks is often needed20-22. Stress test examinations are costly, increase exposure to radiation, and may cause pain to patients19. Therefore, a less intensive diagnostic protocol would be beneficial.
To our knowledge, no previous study has evaluated morphological factors from standard radiographs that might indicate the stability of the ankle mortise in patients with isolated supination-external rotation-type lateral malleolar fractures. This study aimed to identify and assess these factors from standard radiographs, using external-rotation stress radiographs as the reference for stability. Our hypothesis was that ankles with minimally displaced fractures are stable and, therefore, do not require further stress testing.
Materials and Methods
The local ethics review board approved the study protocol. Informed consent was obtained from each patient for study participation.
We screened 308 prospectively collected non-stress radiographs of consecutive, skeletally mature patients (≥16 years of age) with unilateral, Lauge-Hansen1 supination-external rotation (OTA/AO 44-B)4,5 ankle fractures. The fracture had to show no indication of medial widening or incongruity on standard ankle radiographs. We included only patients with typical supination-external rotation oblique fractures that originated <2 cm above the talar dome. Patients were treated within a week after injury at 1 of 2 main trauma centers: the Oulu University Hospital, between March 2012 and May 2015, and the Tampere University Hospital, between February 2013 and August 2014. Fractures were classified according to the Lauge-Hansen classification system1. An external-rotation stress test15 was performed by a senior orthopaedic trauma surgeon at the Oulu University Hospital, and by the surgeon on call at the Tampere University Hospital. The manual external-rotation stress test was the standard protocol at both trauma centers; therefore, all examiners were trained and familiar with performing the test. Patients were excluded when they had a pathologic fracture (n = 0), concomitant fractures that contraindicated external-rotation stress (n = 1), or a previous notable ankle injury (n = 21).
The final study group comprised 286 consecutive patients who met inclusion criteria (mean age, 45 years [range, 16 to 85 years]), including 144 female patients (mean age, 50 years [range, 17 to 85 years]) and 142 male patients (mean age, 40 years [range, 16 to 84 years]). The mean time delay from the injury to a clinical evaluation and stress test was 2.4 days (range, 0 to 6 days).
All measurements were performed with digital imaging and a high-resolution diagnostic monitor. The measurements were calibrated with a standard 30-mm calibration disc and/or a constant 115-cm source-to-detector distance and were recorded to the nearest millimeter. Mortise radiographs were measured to determine the tibiofibular clear space at the level of the epiphyseal scar on the distal part of the tibia, the maximum width of the fracture line (lateral diastasis), and the distance between the distal tip of the fracture and the talar dome (distal fracture height) (Fig. 1). Measurements on lateral radiographs included the maximum width of the fracture line (posterior diastasis); the anterior and posterior fracture heights, measured perpendicular to the level of the talar dome; and the obtuse angle between the fracture line and the axis of the fibula (fracture line angle) (Fig. 2). The fracture fragments were counted, except for the common anteroinferior tibiofibular ligament-avulsion fragment. The first author measured the medial clear space (the distance between the lateral border of the medial malleolus and the medial border of the talus, at the level of the talar dome) from external-rotation stress radiographs25. The evaluator was blinded to the measurements of standard radiographs. The ankle mortise was considered unstable when the medial clear space was ≥5 mm, and at least 1 mm larger than the superior tibiotalar clear space15.
Two senior orthopaedic trauma surgeons analyzed all standard mortise and lateral non-stress radiographs in separate sessions and were blinded to each other’s measurements. The interobserver reliabilities were determined with intraclass correlation coefficients26. Interobserver reliabilities and 95% confidence intervals (CIs) were excellent for all variables: 0.967 (95% CI, 0.957 to 0.975) for tibiofibular clear space, 0.974 (95% CI, 0.965 to 0.980) for lateral diastasis, 0.981 (95% CI, 0.977 to 0.985) for distal fracture height, 0.975 (95% CI, 0.968 to 0.980) for posterior diastasis, 0.983 (95% CI, 0.979 to 0.987) for anterior fracture height, 0.995 (95% CI, 0.993 to 0.996) for posterior fracture height, and 0.972 (95% CI, 0.964 to 0.977) for fracture line angle. Assessment of the number of fracture fragments (2 or >2) was in perfect agreement between the 2 observers.
The summary measurements are presented as the mean and standard deviations or as proportions, unless otherwise stated. Comparisons between stable and unstable ankle mortise groups were conducted with t tests for continuous variables and chi-square tests for categorical variables. Two-tailed p values of <0.05 were considered significant.
Variables with p values of <0.2 were manually entered, one by one, into the multiple logistic regression analysis to model the stability of the ankle mortise. A variable remained in the model when its p value was <0.05 or when it had a significant impact on the −2 log likelihood value. The logistic regression model results are presented as odds ratios (ORs) with 95% CIs.
Receiver operating characteristic (ROC) analyses were performed for continuous variables to test for diagnostic accuracy and to determine optimal thresholds. Variables with an area under the ROC curve (AUC) of >0.75 were considered adequate for further investigation. A sensitivity of >90% was used as the criterion for an optimal threshold.
The probability of ankle mortise stability was calculated in different situations with a logit function (logit = ln[odds]) (see Appendix).
The group of 217 patients with stable ankles (75.9%) had a mean medial clear space (and standard deviation) of 3.5 ± 0.6 mm (range, 2.0 to 4.0 mm). The group of 69 patients with unstable ankles had a mean medial clear space of 5.7 ± 1.1 mm (range, 5.0 to 11.0 mm).
These groups showed significant differences in patient sex (p = 0.001), tibiofibular clear space (p < 0.001), posterior diastasis (p < 0.001), posterior fracture height (p = 0.031), fracture line angle (p = 0.017), and the number of fragments (p = 0.001) (Table I). The multiple logistic regression analysis showed that female sex (OR, 2.5 [95% CI, 1.4 to 4.6]), a posterior diastasis of <2 mm (OR, 10.8 [95% CI, 3.7 to 31.5]), and only 2 fracture fragments (OR, 7.3 [95% CI, 2.1 to 26.3]) were independent factors for predicting a stable ankle mortise (Table II).
The only continuous radiographic variable with an adequate AUC value (0.78 [95% CI, 0.71 to 0.84]) in the ROC analysis was the posterior diastasis, measured on lateral radiographs (see Appendix). The selected threshold of <2 mm for posterior diastasis corresponded with a sensitivity of 0.94 and a specificity of 0.39 (Table III).
Among the 85 patients with fractures that had a posterior diastasis of <2 mm and only 2 fracture fragments, the probability of a stable ankle mortise was 0.98 for 48 female patients (16.8% of all patients) and 0.94 for 37 male patients (12.9% of all patients). In contrast, among 6 male patients (2.1%) with fractures that had a posterior diastasis of ≥2 mm and 3 fracture fragments, the probability of an unstable ankle mortise was 0.83 (Table IV).
To our knowledge, this study is the first to show that the posterior diastasis in a lateral non-stress radiograph was an independent risk factor for instability in supination-external rotation-type isolated lateral malleolar fractures without medial widening of the ankle mortise. Previously, an analysis of standard, non-stress radiographs alone has been considered insufficient to determine the stability of the ankle mortise in patients with this type of isolated supination-external rotation fracture10-22. We also found that female sex and only 2 fracture fragments were independent factors for predicting ankle mortise stability. According to our results, the ankle mortise can be diagnosed as stable when the posterior diastasis of the fracture is <2 mm on lateral radiographs. However, further stress testing would be recommended when the posterior diastasis is ≥2 mm or the fracture is comminuted. The interrater reliability of our measurements was excellent.
Biomechanical1 and MRI studies have shown that the anteroinferior tibiofibular ligament is the first structure to break in supination-external rotation-type injuries. Moreover, the anteroinferior tibiofibular ligament seems to be injured in every case14,15, even though the posteroinferior tibiofibular ligament might be intact14. According to the Lauge-Hansen classification, a supination-external rotation injury starts anteriorly and progresses posteriorly, because of increasing external rotation of the talus1. It is also shown that the distal fracture fragment follows the talus, as it rotates externally, and the proximal part of the fibula is medialized and is internally rotated24. Our results support this description, because the diastasis in mortise radiographs did not correspond well with the stability of the ankle mortise; instead, instability increased with the fracture line width in the lateral radiographs, which increased as the distal part of the fibula rotated externally with the talus.
It can be assumed that younger male patients have better bone density than older male patients and female patients; thus, with high-energy trauma, younger male patients are likely to sustain more severe ligamentous injuries prior to a fracture of the lateral malleolus23. We did not systematically record the trauma mechanism or the level of dislocating force in our study population, but male patients were 10 years younger than female patients on average, they had more comminuted fractures, and a larger proportion had unstable ankles, even though comminution was a rather rare occurrence (5.2%). Therefore, male sex may be a surrogate factor for high-energy trauma and not a true independent risk factor for instability of the ankle mortise.
The 2 primary strengths of our study are the relatively large number of prospectively collected radiographs of consecutive patients from 2 main trauma centers and the blinded analysis of standard radiographs by 2 experienced orthopaedic surgeons, which provided excellent interobserver reliability. The large number of patients included and the limited number of variables analyzed enabled multiple regression analyses with quite narrow confidence intervals. External-rotation stress radiographs were analyzed by only 1 examiner, but we did not consider this a major limitation, because a prior study showed that the reliability of the medial clear space analysis was nearly perfect (kappa coefficient of >0.8)19.
The prognostic value of the stability assessment of an isolated supination-external rotation-type lateral malleolar fracture without widening of the medial clear space remains controversial. There is rather solid evidence that stable ankle fractures can be treated nonoperatively with excellent results6,7. However, we lack high-quality prospective clinical studies with long-term follow-up to confirm the prognostic value of any of the currently available tests for dynamic incongruity. Future studies should address this problem; to our knowledge, a study by Sanders et al.27 remains the only investigation of unstable fractures (according to external-rotation stress radiographs) randomized to operative and nonoperative groups. They found no difference between groups in functional outcomes, but the risk of displacement and problems with union were substantially lower in the operative group than in the nonoperative group. Furthermore, the follow-up time of 12 months in the study by Sanders et al. was too short to assess the prognostic value of external-rotation stress radiographs.
In conclusion, we identified 3 independent factors for predicting stable supination-external rotation-type lateral malleolar ankle fractures: a posterior diastasis of <2 mm on lateral radiographs, only 2 fracture fragments, and female sex. Further stability testing of a noncomminuted lateral malleolar fracture is not necessary when the posterior fracture line width is <2 mm.
Descriptions of the relationship between probability and odds and the use of logit function to calculate the posttest probability and a figure showing the ROC curves of the measurements on non-stress radiographs showing their ability to distinguish between the stable and unstable groups are available with the online version of this article as a data supplement at jbjs.org (http://links.lww.com/JBJS/A146).
Investigation performed at the Department of Surgery, Oulu University Hospital, Oulu, Finland, and the Division of Orthopaedics and Traumatology, Department of Trauma, Musculoskeletal Surgery and Rehabilitation, Tampere University Hospital, Tampere, Finland
A commentary by J. Lawrence Marsh, MD, is linked to the online version of this article at jbjs.org.
Disclosure: There was no outside source of funding for this study. The Disclosure of Potential Conflicts of Interest forms are provided with the online version of the article (http://links.lww.com/JBJS/A145).
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