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Motion of the Ankle in a Simulated Supination-External Rotation Fracture Model*

JAMES D. MICHELSEN, M.D., URI M. AHN, M.D. and STEPHEN L. HELGEMO, M.D., BALTIMORE, MARYLAND

Investigation performed at the Department of Orthopaedic Surgery, The Johns Hopkins School of Medicine, Baltimore.

	Abstract

Top Abstract Introduction Materials and Methods Results Discussion References

 
An experimental study was undertaken with use of axially loaded, unconstrained cadaver ankles to determine the motion patterns seen with progressive stages of the supination-external rotation type of fracture. As described by Lauge-Hansen, these fractures were modeled by transection of the anterior aspect of the capsule and the anterior tibiofibular ligament (stage I), followed by oblique fibular osteotomy ending at the level of the ankle joint (stage II), transection of the posterior aspect of the capsule (stage III), and sequential sectioning of the superficial and deep fibers of the deltoid ligament (stage IV). Thirteen specimens were tested on an apparatus that allowed for controlled loading while the ankle was passed through a physiological range of dorsiflexion and plantar flexion. The ankles were unconstrained about the axial (internal and external rotation) and coronal (varus and valgus angulation) axes. Measurements were made throughout the range of motion in these axes in order to define the kinematic behavior. In the intact specimens, maximum plantar flexion was associated with a mean (and standard deviation) of 1.9 ± 4.12 degrees of internal rotation of the talus and maximum dorsiflexion, with a mean of 7.2 ± 3.88 degrees of external rotation. Varus angulation increased slightly with plantar flexion compared with the value in dorsiflexion (2.4 ± 2.40 compared with 0.3 ± 1.96 degrees). Internal and external rotation was not affected by fibular osteotomy or by transection of the superficial fibers of the deltoid ligament. Transection of the deep fibers of the deltoid ligament caused a significant (p < 0.02) increase in external rotation of the talus at maximum plantar flexion; this was corrected incompletely by insertion of an anatomical fibular plate. With the numbers available for study, we could not show that varus or valgus angulation was significantly affected by any combination of sectioning of the deltoid ligament and fibular osteotomy. These experiments were repeated with the addition of fixation of the subtalar joint with a talocalcaneal screw. With the number of specimens available, we could detect no significant difference, with respect to axial rotation, due to fixation of the subtalar joint. However, along the coronal axis, increased valgus angulation (p < 0.02) was seen during plantar flexion when either the deep or the superficial fibers of the deltoid ligament had been cut. CLINICAL SIGNIFICANCE: These results indicate that stability of the loaded ankle is primarily due to the deltoid ligament, which exerts a restraining influence on external rotation of the talus. Complete fibular osteotomy did not cause abnormal motion of the ankle in the absence of a medial injury. In the presence of a complete injury, lateral reconstruction only partially restored the mechanical integrity of the ankle. The results provide justification for the non-operative treatment of isolated fractures of the lateral malleolus. The data also suggest that a lateral fracture associated with a major injury of the deltoid ligament should be treated with anatomical lateral fixation followed by immobilization without early motion, to allow adequate healing of the deltoid ligament at its resting length.

	Introduction

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The treatment of fractures of the ankle is based on the concept that if the fracture is sufficiently severe and is left unreduced there is a higher propensity for premature post-traumatic osteoarthrosis. Malunion of a fracture of the ankle results in derangement of motion, leading to abnormal distributions of pressure within the ankle joint^13,21. Clinical studies of bimalleolar fractures have reinforced this concept with the consistent finding that anatomical reduction through internal fixation yields results that are superior to those obtained with non-anatomical closed treatment^3,8,22.

Studies of lateral malleolar fractures without discernible medial injury have been less conclusive. Although experiments have shown that lateral malleolar displacement, under some circumstances, leads to abnormal mechanics of the ankle²¹, clinical studies of such fractures have almost universally demonstrated equivalent results for open anatomical and closed non-anatomical treatment^3,8,22. The missing piece of information linking the configuration of the fracture to the clinical outcome is the relationship between the pattern of injury and the resultant derangement of the mechanics of the ankle.

In the present study, we examined the changing biomechanical behavior of the ankle joint when it was subjected to a simulated supination-external rotation fracture progressing from stage I through stage IV, as defined by Lauge-Hansen. This fracture pattern is characterized by disruption of the anterior tibiofibular ligament (stage I), with subsequent progression by means of an oblique spiral fracture of the lateral malleolus (stage II), a tear of the posterior aspect of the capsule (stage III), and, finally, a medial malleolar fracture or disruption of the deltoid ligament (stage IV). The study was conducted with use of an axially loaded ankle preparation, which was put through a physiological range of dorsiflexion and plantar flexion while unconstrained axial (internal and external) rotation and coronal (varus and valgus) angulation were allowed. Associated motions about the axial and coronal axes were measured to determine the abnormal rotatory and angulatory motions that occur under these circumstances. In an additional experiment, the subtalar joint was rigidly stabilized in an effort to distinguish between its contributions and those of the ankle to the over-all pattern of motion of the leg and foot. The goal of the study was to rationalize the treatment of fractures of the ankle through a more detailed knowledge of the instability associated with specific fracture patterns.

	Materials and Methods

Top Abstract Introduction Materials and Methods Results Discussion References

 
The study was performed on thirteen below-the-knee amputation specimens (five from the left side and eight from the right) that had been obtained from thirteen cadavera and frozen until the time of testing. No specimen had any evidence of previous operative treatment of the ankle or of grossly abnormal motion. Although the ages of the donors were not always available, all appeared to have been elderly.

After removal of the skin and subcutaneous tissues, the foot was fixed to a mounting board by means of anchoring screws placed through the first and fifth metatarsals and the calcaneus. This arrangement prevented motion of all of the joints of the foot but permitted unrestricted motion of the ankle and subtalar joint. The proximal parts of the tibia and fibula were potted in low-melting-point alloy, with care being taken not to disturb the tibiofibular relationship. The entire lateral surface of the fibula and the medial surface of the tibia were then exposed, with care being taken to leave the ligamentous structures intact. Neutral alignment was determined according to the anterior tibial cortex and the malleolar prominences; the transverse bimalleolar axis served as the axis of rotation for plantar flexion and dorsiflexion. The ankle was therefore aligned so that this motion coincided with one of the primary axes of rotation of the testing machine.

The specimens were then inverted and placed in the testing apparatus (Fig. 1). The apparatus comprised a standing metal housing enclosing pneumatic, hydraulic, and electrical systems for the application of loads and the collection of data (Experimental Engineering, Little Rock, Arkansas)¹⁰. The lower arm of the apparatus moved in the vertical plane to apply a maximum load of 900 newtons to the specimen. The system was operated in load control, with the load-cell output serving as the feedback signal. The upper arm was constrained from rotating in the sagittal axis in order to simulate the ground on which the foot was placed.

View larger version (29K): [in this window] [in a new window]   Fig. 1 Diagram of the testing apparatus with the cadaver ankle. The proximal parts of the tibia and fibula were potted into a gimbal that can rotate in three planes, while the foot was anchored onto a stable platform. The ankle was loaded axially and cycled through dorsiflexion-plantar flexion. The bottom of the gimbal was instrumented to measure all rotational motions, which are equal to the motions that occur at the ankle-subtalar complex.

The proximal part of the tibia was displaced along the lower arm of the apparatus (to force motion of the ankle about the sagittal axis) by engaging a worm drive at the base of the mount. During the experiment, the operator of the device turned the drive wheel, moving the proximal part of the tibia anteriorly or posteriorly, which caused the ankle to dorsiflex or plantar flex. The proximal part of the tibia was free to rotate about the axial and coronal axes, reflecting motion at the ankle joint. These axes correspond to internal-external rotation and varus-valgus angulation at the ankle and subtalar joint. Each rotation and angulation was measured with potentiometers embedded in the mobile mount, and the data were stored in the computer. Therefore, the controlled plantar flexion-dorsiflexion was measured, along with the internal-external rotation and varus-valgus angulation, at the ankle. Experimental measurements were calibrated to measurements obtained at 0 degrees (defined by a plumb line) and 90 degrees (determined with a mechanical goniometer) to convert the output of the potentiometer to degrees of motion. On the basis of repeated measurements and calibration data, maximum experimental error was calculated to be ±0.25 degree around any axis.

Data were collected every five seconds, at approximately every 5 degrees of the sagittal arc. The specimen was taken through a range of motion, and the data were stored on a spreadsheet for later analysis. The measurements that were obtained were for motion of the proximal part of the leg at the mobile articulation of the testing machine. Motion at this articulation was possible because of equal compensatory motion at the ankle. Therefore, these measurements reflect motion at the ankle. For clarity, the motions that are reported here are those for the ankle joint. Because measurements were not made at precisely every 5 degrees of sagittal motion, there was interexperimental variation in the maximum dorsiflexion and plantar flexion. The mean maximum dorsiflexion (and standard deviation) was 28.8 ± 3.17 degrees, and the mean maximum plantar flexion was 36.5 ± 4.47 degrees. There were no significant differences among the ankles with respect to maximum dorsiflexion or plantar flexion (p > 0.1, analysis of variance).

After the specimen had been mounted in the apparatus, neutral alignment of the tibia was obtained about the sagittal and coronal axes with use of a plumb line to align the leg vertically about both axes; the centers of the proximal parts of the tibia and ankle were used as the reference points. The foot-mount was horizontal in both planes, as restricted by the testing apparatus. All subsequent measurements were derived from this neutral alignment.

The initial part of the testing routine consisted of applying a 300-newton vertical load component and then moving the specimen through one complete motion cycle (neutral to maximum plantar flexion, back to neutral, to maximum dorsiflexion, and back to neutral) in approximately 5-degree increments. The component of the compressive loading in the vertical direction remained constant, even when the tibia and fibula were not vertical during testing. Therefore, the compression force coincident with the tibial axis was increased when the ankle was moved away from the neutral position. This was calculated to be a maximum of 320 newtons at maximum dorsiflexion and 370 newtons at maximum plantar flexion. The calculated sagittal moments exerted on the ankle joint were thirty-one and fifty-two newton-meters at these positions, respectively. With the number of specimens tested in earlier, pilot studies, we could not show that motion of the intact ankle varied significantly between axial loads of 150 and 750 newtons. Because of the extreme instability of the ankles that were tested in the current study, 300 newtons of loading was used to limit potential uncontrolled damage to the structures of the ankle that would have prevented repetitive trial runs. Neutral was a stable alignment in all specimens under all conditions. All data reflect measurements that were obtained after two loaded pre-cycling trials, as preliminary tests showed that pre-cycling of each specimen twice yielded consistent data in subsequent cycles.

Experiment 1
The first set of specimens (seven ankles) was initially tested with all ligaments and osseous structures intact. This was followed by use of six different alterations: (1) transection of the anterior tibiofibular ligament and oblique fibular osteotomy, beginning at the level of the tibial plafond anteriorly and extending proximally and in a posterior oblique direction (to simulate a supination-external rotation stage-II fibular fracture⁹); (2) anatomical repair of the fibular osteotomy with a lateral fibular plate fixed with three cortical-bone screws proximally and three trabecular screws distally; (3) transection of the superficial fibers of the deltoid ligament, with the lateral fibular plate left in place; (4) removal of the lateral fibular plate, with the superficial fibers of the deltoid ligament remaining disrupted; (5) transection of the deep fibers of the deltoid ligament, without the lateral fibular plate; and (6) replacement of the lateral fibular plate, with the deep fibers of the deltoid ligament remaining disrupted. With these alterations performed in this order, the fibular plate was screwed onto the fibula only twice and removed once, decreasing the likelihood of osseous wear at the site of the screws and thus reducing the chance that the plate would loosen.

Experiment 2
The second set of specimens (six ankles) was initially tested in the intact state, as described for Experiment 1, and then was retested with the subtalar joint rigidly stabilized by means of a 6.5-millimeter cancellous-bone compression screw, directed at a 45-degree angle downward from the talus toward the heel in the sagittal plane and approximately 15 degrees medially in the transverse plane into the body of the calcaneus. The fixation of the subtalar joint was performed with the specimen in the neutral position. The six alterations in the experimental conditions were carried out as described for Experiment 1, with testing repeated after each alteration. These ankles were dissected after the experiment had been completed. Direct observation confirmed that motion of the subtalar joint was completely inhibited by the screw.

Statistical Analysis
In Experiment 1, the values for internal-external rotation and varus-valgus angulation obtained at maximum dorsiflexion and at maximum plantar flexion after the six alterations were compared with those values for the specimens in the intact state. For Experiment 2, the same comparison was made between the values after the six alterations and those for the intact ankles with the subtalar screw in place. In Experiment 2, the values for the intact ankles with the screw in place were also compared with the values for the same intact ankles without the screw, to determine the contribution of the subtalar joint to axially loaded motion of the ankle and hindfoot.

All results are expressed as the mean and standard deviation. Statistical analysis was performed with two or three-way analysis of variance, controlling for the specimen, the testing condition, and the presence of fixation of the subtalar joint. A difference was considered significant if p was less than 0.05.

	Results

Top Abstract Introduction Materials and Methods Results Discussion References

 
The characteristic pattern for all of the intact ankles that were tested was external rotation with maximum dorsiflexion and relative internal rotation with plantar flexion (Fig. 2). There was a plateau, with little change in axial rotation, around the neutral position, and there was additional internal rotation as plantar flexion was increased. Relative to the neutral position, there was 1.9 ± 4.12 degrees of internal rotation at maximum plantar flexion and 7.2 ± 3.88 degrees of external rotation at maximum dorsiflexion. In the coronal plane, with the number of specimens tested we could detect no significant difference in the amount of varus-valgus motion in maximum dorsiflexion (0.3 ± 1.96 degrees) compared with that in maximum plantar flexion (2.4 ± 2.40 degrees) (p > 0.05) (Fig. 3). In Experiment 2, fixation of the subtalar joint did not result in a significant difference in either coronal angulation or axial rotation.

View larger version (36K): [in this window] [in a new window]   Figs. 2, 3, and 4: Graphs showing the motion, in degrees, associated with dorsiflexion and plantar flexion. After axial loading at 300 ± 10 newtons, each specimen was cycled through a range of dorsiflexion-plantar flexion with simultaneous measurement of internal and external rotation or varus and valgus angulation. st = subtalar. Fig. 2: Internal and external rotation in an intact specimen, with and without fixation of the subtalar joint.

View larger version (23K): [in this window] [in a new window]   Fig. 3 Varus and valgus angulation in an intact specimen, with and without fixation of the subtalar joint.

View larger version (26K): [in this window] [in a new window]   Fig. 4 Internal and external rotation in an intact specimen, after fibular osteotomy and transection of the deep fibers of the deltoid ligament (dd), and after insertion of a fibular plate and transection of the deep fibers of the deltoid ligament.

The presence or absence of lateral instability (fibular osteotomy or a fibular plate) had no demonstrable effect on the varus or valgus angulation of the ankle (Table II). Transection of the superficial or deep fibers of the deltoid ligament, again with or without lateral instability, had a similarly negligible effect in Experiment 1. As mentioned, the only destabilization maneuver that led to a significant change in varus-valgus angulation was disruption of the deltoid ligament in association with fixation of the subtalar joint (range, -1.3 ± 0.70 to -2.1 ± 1.43).

	Discussion

Top Abstract Introduction Materials and Methods Results Discussion References

 
Since the publication of the reports by Ramsey and Hamilton as well as Yablon et al., the guiding principle of operative treatment for fractures of the ankle has been the anatomical reduction of the lateral malleolus. The supposition extrapolated from those studies has been that the mechanical behavior of the ankle is determined primarily by the lateral structures. However, neither of these seminal studies included direct investigation of the alteration in the motion of the tibiotalar joint that was assumed to occur.

In a subsequent study, it was suggested that motion of the talus in the presence of an isolated fracture of the lateral malleolus was not substantially different from that of an intact talus⁴. In that study, the dynamic tibiotalar contact area (determined with use of an unconstrained ankle preparation that was taken through a physiological range of motion) did not decrease in association with a displaced fracture of the lateral malleolus; however, transection of the deltoid ligament significantly decreased the dynamic contact area by more than 15 per cent (p < 0.05). This result is consistent with the observation that the pattern of instability of the talus was not straight lateral translation but, rather, anterolateral rotation underneath the tibial plafond^4,10. The present study was designed to examine more thoroughly the kinematics of the ankle complex during progressive stages of a Lauge-Hansen supination-external rotation fracture. Presumably, the decision-making process regarding the treatment of such a fracture would be aided by a more detailed understanding of the specific patterns of kinematic instability of the fracture.

The normal motion of the tibiotalar joint is not of the hinge type but, rather, includes coupled rotations in the axial and coronal planes as the ankle moves from dorsiflexion to plantar flexion^11,16,18. The consequence of increased tension on the deltoid ligament with plantar flexion can also be seen in the coupled rotation in the coronal plane: with increased plantar flexion, there was increased varus motion of the ankle. This finding is consistent with those of studies in which the deltoid ligament was found to resist eversion of the ankle^6,14,15,19. Plantar flexion of the ankle joint results in internal rotation of the talus (Fig. 2).

The addition of lateral instability by means of a fibular osteotomy did not substantially change the pattern of internal-external rotation during dorsiflexion-plantar flexion. This observation provides evidence against a primary stabilizing role of the lateral malleolus with respect to the talus.

The axial kinematics of the ankle were not altered after the superficial fibers of the deltoid ligament had been transected. Significant changes (p < 0.05) in internal and external rotation were seen after the deep fibers of the deltoid ligament had been transected (Fig. 4). With disruption of the deep fibers, the primary abnormal motion was increased external rotation of the talus during plantar flexion. Motion during the range of dorsiflexion was not appreciably altered. Anatomical fixation of the lateral malleolus only partially alleviated the abnormal axial rotation, which remained significant (p < 0.05).

With the numbers available for study, complete disruption of the deltoid ligament did not result in a detectable change in varus-valgus angulation except when the subtalar joint was immobilized. This was probably related to the increased stability of the ankle joint caused by axial loading, which also changes the pattern of mechanical constraints^10,11,19. The articular geometry became the primary restraint to varus-valgus angulation when the ankle was placed under axial load. Attempts to induce abnormal varus or valgus angulation were prevented by direct impingement between the talar dome and the tibial plafond. The apparent compensatory motion of the subtalar joint that contributed to this phenomenon was reflected by the increased valgus angulation of the ankle that occurred when the subtalar joint was stabilized with a screw. In contrast, axial rotation was not primarily restricted by the articular anatomy¹⁹. Consequently, in an experiment involving axial loading, limited coronal instability would be expected, even in the presence of considerable axial rotatory abnormalities.

None of the mentioned reports, including the present one, contain a description of the role of dynamic stabilizers, such as muscle-tendon units; only the passive constraints to motion of the ankle were modeled. Although it is generally thought that passive constraints are the primary stabilizers of the ankle, the extent to which dynamic elements contribute to stability is unknown. In addition, the present study involved rigid proximal fixation of both the fibula and the tibia. Since previous investigators have noted motion of the proximal part of the fibula as the ankle dorsiflexes²⁰, preliminary experiments were performed to determine the effect of fixation of the tibiofibular joint on measured motion of the ankle. Comparison of motion of the ankle with the lateral malleolus intact with that after an osteotomy proximal to the tibial plafond revealed no differences between the two groups of ankle specimens.

The pattern of instability in the current study—external rotation during maximum plantar flexion—was similar to that reported by others^4,11,16,17. The deltoid ligament, specifically the deep component, appeared to act as a checkrein, preventing external rotation of the talus during plantar flexion. This concept is consistent with our observations of the kinematics of the intact ankle as well as with others' observations of the mechanics of the deltoid ligament—that is, with plantar flexion the deltoid became taut¹⁵ and began to limit external rotation of the talus^4,6,14,15. In the current study, as plantar flexion increased, the resulting tension of the deltoid ligament served as an internal rotatory force acting on the talus. This led to the progressive internal rotation that was seen at the extremes of plantar flexion in the intact ankle. It is interesting that anatomical rigid fixation of the fibula limited the external rotation of the talus in the presence of a complete rupture of the deltoid ligament, as this suggests that the lateral structures provide a degree of secondary stabilization of the ankle. However, their influence on motion of the ankle was significant only when the medial structures were incompetent (p < 0.05).

These experimental results might lead to a recommendation for medial repair of a ruptured deltoid ligament when open reduction and internal fixation of a displaced fracture of the lateral malleolus is undertaken. However, several clinical studies clearly demonstrated that repair of the deltoid ligament was not necessary in such patients^1,2,5,7,21. Critical to those studies was the fact that the ankle was treated with a cast or splint, in a neutral position, after fixation of the lateral malleolar fracture. This allowed the deltoid ligament to reapproximate at its resting length and to heal at that length. While these earlier results are not inconsistent with our data, they serve to emphasize the importance of immobilization with a cast when the medial structures are not repaired.

There has been some controversy with regard to the treatment of lateral malleolar fractures in the absence of a medial injury. Our data indicate that such a fracture does not result in abnormal kinematics. Consequently, there is very little reason to perform an operation that presumably is directed at the correction of abnormal mechanical behavior. This position was substantiated by several long-term studies. After a maximum of thirty years, patients who had had a displaced, isolated fracture of the lateral malleolus and had been managed non-operatively had an excellent result, provided that the talus had been adequately reduced underneath the tibial plafond^3,8,22. In contrast, patients who had had a bimalleolar fracture had a substantially better outcome after anatomical open reduction and fixation^12,23. The problem that can arise is how to ascertain when medial tenderness, in association with a lateral malleolar fracture but without talar shift, indicates a complete disruption of the deltoid ligament. This combination of injuries is mechanically equivalent to a bimalleolar fracture and warrants operative treatment. Decision-making with regard to these injuries remains a clinical challenge.

	Footnotes

 
*No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. No funds were received in support of this study.

Department of Orthopaedic Surgery, The Johns Hopkins Out-patient Center, 601 North Caroline Street, Baltimore, Maryland 21287-0881.

	References

Top Abstract Introduction Materials and Methods Results Discussion References

 

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