Plantar Flexion Influences Radiographic Measurements of the Ankle Mortise

Saldua, Nelson S. MD; Harris, James F. MD; LeClere, Lance E. MD; Girard, Paul J. MD; Carney, Joseph R. MD

Journal of Bone & Joint Surgery - American Volume:
doi: 10.2106/JBJS.I.00084
Scientific Articles
Abstract

Background: The treatment of ankle fractures often depends on the integrity of the deltoid ligament. Diagnosis of a deltoid ligament tear depends on the measurement of the medial clear space. We sought to evaluate the impact of ankle plantar flexion on the medial clear space.

Methods: Mortise radiographs were made for twenty-five healthy volunteers, with the ankle in four positions of plantar flexion (0°, 15°, 30°, and 45°). Four observers measured the medial clear space and the superior clear space on each radiograph. The mean medial clear space at 0° was defined as the control, and the deviation of the medial clear space from the control value was calculated at 15°, 30°, and 45° of plantar flexion. The ratio of the medial clear space to the superior clear space was determined on all radiographs, and ratios that were false-positive for a deltoid ligament injury were identified.

Results: Fourteen male and eleven female volunteers were evaluated. The average increase in the medial clear space when ankle plantar flexion was increased from 0° to 45° was 0.38 mm (95% confidence interval, 0.18 to 0.58 mm). This increase was significant (p = 0.005). The average increase in the medial clear space was 0.04 mm when ankle plantar flexion was increased from 0° to 15° and 0.22 mm when it was increased from 0° to 30°. Neither of these changes was significant (p = 0.99 and 0.20). The prevalence of false-positive findings of deltoid injury based on the ratio of the medial clear space to the superior clear space increased as ankle plantar flexion increased, but this increase did not reach significance in our study group (p = 0.18).

Conclusions: Plantar flexion of the ankle produces changes in radiographic measurements of the medial clear space. The potential for false-positive findings of deltoid disruption increases with increasing ankle plantar flexion.

Clinical Relevance: This study highlights the importance of the ankle being in a neutral position when radiographs are made to measure the medial clear space to assist in the decision of whether to perform nonoperative or operative treatment.

Author Information

1Department of Orthopaedic Surgery, Naval Medical Center San Diego, 34800 Bob Wilson Drive, Suite 112, San Diego, CA 92134. E-mail address for N.S. Saldua: nelson.saldua@med.navy.mil

2Department of Orthopaedic Surgery, University of California San Diego Medical Center North, 200 West Arbor Drive, San Diego, CA 92103

Article Outline

The decision to treat an ankle fracture either nonoperatively or operatively is based on ankle stability. Unstable ankle fractures are typically managed with surgical fixation, while stable ankle fractures can be managed nonoperatively with good results1,2. The determination of ankle stability is relatively straightforward when a fracture involves both the medial and the lateral malleolus. As a bimalleolar ankle fracture is inherently unstable, operative fixation is typically needed to maintain the tibiotalar joint in a reduced position.

Determination of ankle stability is less obvious when there is a medial injury involving the deltoid ligament3. Physical examination findings of medial tenderness on palpation, swelling, and ecchymosis have been reported in the literature to have low sensitivity for the detection of deltoid ligament injuries4,5. Other modalities, such as ultrasonography, magnetic resonance imaging, and even arthroscopy, have been described as additional methods for diagnosing a deltoid ligament tear, but they may be impractical in some clinical settings6-8. The diagnosis of a deltoid ligament tear often rests on an interpretation of radiographs. The integrity of the deltoid ligament, particularly the deep deltoid ligament, in association with a fracture of the lateral malleolus often has implications with regard to the decision whether to perform surgical fixation of the lateral malleolus or nonoperative management of the fracture9.

Rather than seeing a fracture line involving the medial malleolus, an examiner making a radiographic diagnosis of a deltoid ligament tear must depend on measurements of radiographic parameters such as the medial clear space. The medial clear space is defined as the distance between the medial aspect of the talus and the lateral intra-articular portion of the medial malleolus. Deltoid ligament tears are diagnosed radiographically when the medial clear space exceeds 5 mm or when it exceeds the width of the superior clear space10-12. The superior clear space is defined as the distance between the superior aspect of the talus and the tibial plafond. These measurements are often subtle and sometimes require the use of contralateral ankle radiographs for comparison, as can be the case in the diagnosis of other injuries of the foot and ankle such as those of the Lisfranc joint13. Determination of the competence of the deltoid ligament often requires an applied force, as radiographs without the application of force may still show a reduced ankle mortise when there is a deltoid ligament tear6. When a fibular fracture is found with a concomitant deltoid ligament injury, the surgical treatment involves rigid fixation of the fibula only. The deltoid ligament is allowed to heal without surgical repair14,15.

Inman described the talus as a cross section of a cone with the medial side oriented toward the apex of the cone and the lateral side oriented toward the base of the cone16. In this configuration, the deltoid ligament complex is responsible for the stability of the medial side; thus, an injury of the deltoid ligament with a concomitant injury of the lateral malleolus can result in an ankle with poor biomechanical stability. In the more recent literature, it has been suggested that, in some patients, the cross section of the talus is trapezoidal in shape, in that it is wider anteriorly than it is posteriorly17,18; accordingly, the position of the ankle in the sagittal plane may affect measurements of the medial clear space. Our hypothesis was that, when the ankle is plantar flexed, a narrower portion of the talus occupies the tibiotalar joint, and a mortise radiograph made with the ankle in this position can be falsely positive for a deltoid ligament injury. The purpose of this study was to evaluate the effect of increasing ankle plantar flexion on the medial clear space when no rotational stress is applied to the ankle.

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Materials and Methods

A power analysis was performed before this institutional review board-approved study was begun. We calculated that a sample of twenty-three subjects would be required to detect a deviation of 1 mm in the medial clear space with an alpha of 0.05, a power of 0.80, and a standard deviation of 1.2. Subsequently, twenty-five healthy volunteers without a history of ankle injury were recruited. Four separate lower-extremity braces were constructed from injected, molded plastic, with a half-shell formed to contain the posterior aspect of the calf and a second half-shell formed to contain the sole of the foot. The shells were then bonded together at four different flexion angles: 90°, 105°, 120°, and 135°. Velcro straps were configured to the shells to securely strap the calf and foot of the subject so that the four different braces produced four different fixed ankle flexion angles: neutral, 15° of plantar flexion, 30° of plantar flexion, and 45° of plantar flexion. These ankle flexion angles were confirmed with use of a goniometer. Each brace was fixed to a rectangular board with the foot portion of the brace oriented perpendicular to the board. Mortise radiographs of the ankle were made with the subject's leg and foot strapped into the brace and the board placed on the radiographic examination table. A 15° foam wedge was placed under the board to ensure an ankle joint orientation of 15° of internal rotation relative to a radiograph beam that was directed perpendicular to the table at the level of the medial malleolus.

The subjects were randomized to have either the left or the right ankle studied. Four ankle mortise radiographs, one at each position of ankle plantar flexion, were made for each study subject. The brace itself was not fixed to the examination table; rather, the entire brace was internally rotated 15° to 20° relative to the x-ray beam in order to obtain a mortise view of the ankle18. The four radiographs were performed in random order for each patient. While the radiographs were being made, one of the investigators ensured that the subject's heel was well-seated in the brace to allow a consistent ankle position for all radiographic images.

Four of the authors—two orthopaedic surgery residents (N.S.S. and L.E.L.) and two orthopaedic surgery staff (P.J.G. and J.R.C.)—examined all 100 radiographs in a blinded fashion. All radiographs were processed with use of a PACS system (IMPAX; Agfa-Gevaert Group, Mortsel, Belgium), and measurements were made with a digital goniometer. The observers were allowed to manipulate the magnification of the images as needed. Each observer was blinded to the identity of the study subject and the specific position of ankle plantar flexion used for each radiograph. Each observer measured the medial clear space and the superior clear space with the method described by Beumer et al.11. The observers were instructed to measure the medial clear space 5 mm beneath the talar dome along a line parallel to the talar dome.

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Statistical Methods

The mean medial clear space at each position of ankle plantar flexion was calculated by averaging the measurements of all four observers. The medial clear space at neutral plantar flexion was defined as the baseline value for each patient. Deviations in the medial clear space from the value at neutral were then calculated at 15°, 30°, and 45° of ankle plantar flexion for each patient, and the difference between the value at neutral and that at each ankle flexion position was analyzed with a one-way analysis of variance with multiple comparisons (the Scheffé method). The ratio of the mean medial clear space to the superior clear space was also determined at each ankle position. The ratio was considered to be false-positive for a deltoid ligament injury when the medial clear space exceeded the superior clear space. The prevalence of ratios that were false-positive for a deltoid ligament tear was analyzed with a Fisher exact test. For this study, significance was defined as a p value of <0.05.

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Source of Funding

No external funding was provided for this study.

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Results

Fourteen male and eleven female volunteers were evaluated. The subjects’ mean age was twenty-seven years (range, twenty-two to forty-seven years). Fourteen left and eleven right ankles were examined. The average deviation in the medial clear space, from that in the neutral position, was 0.04 mm (95% confidence interval, –0.09 to 0.17 mm) at a plantar flexion angle of 15° (p = 0.99; Scheffé pairwise comparison) and 0.22 mm (95% confidence interval, 0.068 to 0.38 mm) at a plantar flexion angle of 30° (p = 0.20). A larger increase in the medial clear space (0.38 mm [95% confidence interval, 0.18 to 0.58 mm]) was seen when the measurement at neutral was compared with that at 45°, and this difference was found to be significant (p = 0.005). The deviations in the medial clear space at the various flexion angles are summarized in Figure 1.

With increased ankle plantar flexion, there was an increased prevalence of false-positive ratios (defined as a medial clear space that was larger than the superior clear space in our healthy study population). There were seven false-positive results at 15° of ankle plantar flexion, ten at 30°, and fourteen at 45° (Fig. 2). With the number of subjects studied, the increase in false-positive rates was not found to be significant (p = 0.18).

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Discussion

Supination-external rotation ankle fractures are the most common rotational ankle fractures19. Stable supination-external rotation ankle fractures can be managed nonoperatively with a cast or walking boot, while unstable fractures are best managed operatively. Regardless of the treatment that is chosen, the success depends on the ability to maintain a well-reduced tibiotalar joint in which the talus is centered in the ankle mortise. Biomechanical studies have shown that a malreduced fibula leads to a malreduced tibiotalar joint, which in turn results in increased joint-contact pressures20,21.

When a distal fibular supination-external rotation fracture pattern is found with a concomitant deltoid ligament injury, the injury is often deemed unstable and is treated with operative fixation of the distal part of the fibula14,15,22-24. Medial ankle tenderness on palpation does not correlate well with the diagnosis of a deltoid ligament tear; as a result, radiographs are often employed to discern medial clear space widening, which is associated with deltoid ligament injury5,11,25.

In this study, we evaluated the effect of ankle plantar flexion on measurements of the medial clear space when no stress was applied to the ankle. We found that, even without the application of stress, increased ankle plantar flexion is associated with deviations in the medial clear space and can lead to an increase in false-positive radiographic findings of deltoid ligament tears. It should be pointed out that, although the deviation in the measured medial clear space was significant, with a p value of <0.05, when ankle plantar flexion reached 45°, the actual increase was quite small (average, 0.38 mm). It can be argued that this small measured increase is not clinically relevant when the medial clear space is used alone to identify a deltoid ligament tear. However, these small increases in the medial clear space can have larger effects when the ratio of the medial clear space to the superior clear space is used to establish the presence of a deltoid ligament injury. While the increase in false-positive ratios with increasing plantar flexion was not found to be significant in this study, the fact that >25% of the tests were false-positive with just 15° of plantar flexion is a concern in a clinical setting in which one is deciding between operative and nonoperative treatment. Furthermore, it is important to note that no rotational stress was applied during the radiographic measurements. Rotational stress studies are often employed to identify deltoid ligament injuries when surgery is being considered, and we expect application of stress to the ankle to further increase the medial clear space even without a deltoid ligament injury9,10,22,26.

Our findings in healthy, unsedated patients with stable ankles are consistent with those of Kragh and Ward, who studied stable and unstable ankles with manual stress testing under sedation27. They found that, in both the stable and the unstable ankles, the measurements of the medial clear space increased with increasing ankle plantar flexion when patients were sedated. Thus, we believe that it is reasonable and prudent to assume that the position of the ankle during preoperative evaluation of the medial clear space should be considered when making surgical treatment decisions.

Our study had limitations. Radiographs were performed mostly on young patients; hence, they may not be consistent with findings in older patients. Also, the plantar flexion angles that we studied may not duplicate exactly the ankle positions most commonly encountered in the clinical setting. It has been established that the normal resting plantar flexion position of the ankle is 37°28, and our measurements were performed at 15°, 30°, and 45° of ankle plantar flexion. While the ankle positions in this study were not 37°, we did measure beyond this value; thus, gradual progression of ankle plantar flexion served to elicit a pattern of increasing medial clear space distance as the plantar flexion angle increased.

This study was performed without the application of rotational stress on the ankle. When a deltoid ligament injury is suspected, rotational stress radiographs are often employed to aid in the identification of an abnormal medial clear space representing deltoid ligament insufficiency9,10,22,26. However, the findings in our study have implications for the performance of stress testing even without the application of rotational stress to the ankle to evaluate whether deltoid ligament insufficiency requires surgical treatment. When rotational stress testing is performed, the ankle is often left free to assume its resting position, and then the rotational force is applied. This flexed resting position can leave the subject at risk for a false-positive finding of medial clear space widening, as was evident in the current study, and any subsequent rotational force applied can further contribute to the potential “normal” widening of the medial clear space with plantar flexion of the ankle. To our knowledge, the additional contribution of medial clear space widening resulting from rotational stress in stable ankles has not been specifically quantified; nonetheless, the findings of Kragh and Ward clearly revealed that manual rotational stress applied to stable plantar flexed ankles of sedated subjects resulted in increasing medial clear space measurements27. As a consequence, we believe that it is prudent to attempt to maintain the ankle near neutral when performing rotational ankle stress testing to determine the need for surgery.

In conclusion, this study demonstrated that increasing plantar flexion of the ankle results in deviations of medial clear space measurements on mortise radiographs and increases the potential for false-positive findings of deltoid ligament disruption. Thus, ankle plantar flexion can potentially indicate to the surgeon the need for surgical treatment when nonoperative treatment is more appropriate. In clinical situations in which surgical decision-making depends on the determination of medial clear space widening, we recommend that the ankle be maintained near a neutral position to avoid any potential false-positive findings of deltoid ligament insufficiency.

NOTE: The authors thank Dr. Robert Riffenburgh, PhD, for his assistance with the statistical analysis for this paper and Ms. Waine Macallister for her assistance with the preparation of this manuscript.

Investigation performed at the Department of Orthopaedic Surgery, Naval Medical Center San Diego, San Diego, California

Disclaimer: The views expressed in this article are those of the authors and do not reflect the official policy of the Department of the Navy, the Department of Defense, or the United States Government.

Read in part at the Annual Meetings of the Orthopaedic Trauma Association, Denver, Colorado, October 15-18, 2008; Society of Military Orthopaedic Surgeons, Las Vegas, Nevada, December 8-13, 2008; and American Academy of Orthopaedic Surgeons, Las Vegas, Nevada, February 25-28, 2009.

Disclosure: The authors did not receive any outside funding or grants in support of their research for or preparation of this work. Neither they nor a member of their immediate families received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity.

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