Quantitative Assessment of Mechanical Laxity in the Functionally Unstable Ankle


Medicine & Science in Sports & Exercise: May 2004 - Volume 36 - Issue 5 - pp 760-766
CLINICAL SCIENCES: Clinical Investigations

HUBBARD, T. J., T. W. KAMINSKI, R. A. VANDER GRIEND, and J. E. KOVALESKI. Quantitative Assessment of Mechanical Laxity in the Functionally Unstable Ankle. Med. Sci. Sports Exerc., Vol. 36, No. 5, pp. 760–766, 2004.

Purpose: Delineating between functional and mechanical instability in those with chronic ankle dysfunction is a challenging task. Current methods of assessing ankle ligamentous laxity are subjective in nature and limit our ability to identify the site and extent of instability; therefore, a need exists for objective laxity measurements. The purpose of this study was to determine whether subjects with self-reported, functional ankle instability (FAI) demonstrated increased mechanical laxity when tested with instrumented arthrometry and stress radiography.

Methods: Both ankles were tested in 51 subjects with self-reported unilateral FAI. An instrumented ankle arthrometer measured ankle-subtalar joint motion for total anteroposterior (AP) displacement (mm) during loading at 125 N and total inversion-eversion (I-E) rotation (degrees of ROM) during loading at 4 N·m. The Telos GA-II/E device provided either anterior or lateral stress (15 kp) while fluoroscopic radiographs were recorded for anterior displacement (mm) and talar tilt (degrees).

Results: The arthrometry measurements of anterior and total AP displacement and the radiographic measurements of anterior displacement were greater (P < 0.05) in the FAI ankles when compared with the uninjured ankles. There were no differences in total I-E rotation, inversion rotation, or talar tilt between ankles when analyzed with either measurement technique.

Conclusion: The ability to objectively measure mechanical instability in the functionally unstable ankle is important to understanding the nature and cause of the instability. Ankle arthrometry and stress radiographic measurements are objective assessment tools for mechanical laxity. Despite finding greater laxity in the functionally unstable ankle, the clinical significance of the observed displacement remains unanswered. Further research is needed to determine the amount of laxity that constitutes mechanical instability and how this relates to FAI.

1Pennsylvania State University, State College, PA; 2University of Delaware, Newark, DE; 3University of Florida, Gainesville, FL; and 4University of South Alabama, Mobile, AL

Address for correspondence: Thomas W. Kaminski, Ph.D., ATC, FACSM, University of Delaware, Dept. of Health, Nutrition & Exercise Science, Human Performance Laboratory, Fred Rust Ice Arena, 541 South College Avenue, Newark, DE 19716; E-mail: kaminski@udel.edu.

Submitted for publication April 2003.

Accepted for publication December 2003.

Article Outline

Chronic ankle instability is a common problem after acute lateral ankle sprain, manifested by recurring injuries that often limit sports participation. Whether instability is the result of injury with or without demonstrable ankle ligament laxity is unclear. The prevalence of residual symptoms (pain and swelling) and disability (“giving-way” and weakness) after initial ankle sprain are common. Seventy-three percent of ankle sprain patients seen in a regional primary care facility over a 1-yr period reported residual symptoms 6–18 months after medical evaluation (2). Yeung and colleagues (31) reported that 59% of 380 athletes presented significant disability and residual symptoms that led to impairment in performance.

To explain this phenomenon, the concepts of functional and mechanical ankle instability have been introduced. Functional ankle instability (FAI) is characterized by a history of insecurity (27) and “giving-way” (5) of the ankle during activity. Tropp et al. (29) described FAI as joint motion beyond voluntary control but not necessarily exceeding the physiologic range-of-motion or producing significant ligamentous laxity. Several causes of FAI have been postulated and include nerve injury within or proximal to the lateral ligaments, (5,17,21) peroneal muscle weakness (1,28), and delayed peroneal muscle activation (18). Tropp defined mechanical ankle instability (MAI) as ankle joint motion that exceeds the physiologic range (29). MAI also includes laxity of the ankle resulting from structural impairments to the ligamentous tissue that supports the joint (9,10). In addition to pathological laxity, MAI can be attributed to arthrokinematic restrictions and synovial and degenerative changes. Hertel hypothesized that MAI can affect the talocrural, subtalar, and/or inferior tibiofibular joints after lateral ankle sprains (9). Not all individuals experiencing chronic ankle instability after an ankle sprain report both mechanical and functional problems. FAI occurs in approximately 25–50% of those affected by grade II or III ankle sprains within 6 months after injury (23). Reports also suggest those experiencing FAI have no associated ligamentous laxity (23,25), whereas others suggest that those with MAI do present with associated functional defects (1,5,8). If a relationship exists between the two it has been difficult to ascertain.

Clinical diagnosis of FAI is predicated on the patient’s self-reported episodes of “giving-way” and residual symptoms of pain and swelling. Manual examination and stress radiographic measurements of joint translation and increased range-of-motion are useful in the clinical diagnosis of MAI. The inherent subjectivity with manual exam techniques in differentiating the degree of lateral ankle ligament stability renders manual stress tests inaccurate for diagnosing specific ligament involvement. Stress radiographs are obtained using an application of inversion or anterior stress to the ankle. Stress can be applied either manually or with the help of a custom-built device that allows for reproducible force application and consistent positioning. The amount of laxity (displacement and rotation) is then subsequently measured from the radiographic image. The major problems associated with stress radiography are that it does not provide direct linear and angular measurements and has been reported as an unreliable measurement technique (3,6,22,24). The exact relationship between FAI and MAI may not be known, due in part to the discrepancies reported in measuring mechanical instability with stress radiography (15,24,30).

In an attempt to improve objective ankle evaluation, instrumented ankle arthrometry has been developed as a diagnostic tool for measuring ankle-subtalar joint complex laxity (19,20). Ankle arthrometry incorporates a six-degrees-of-freedom spatial kinematic linkage system that quantifies the anteroposterior (AP) load-displacement and inversion-eversion (I-E) rotational laxity characteristics of the ankle-subtalar joint complex. Quantitative and reliable measurements of mechanical laxity after ankle sprain could provide greater understanding of the relationship between FAI and MAI (10).

We are unaware of studies that have used both instrumented arthrometry and stress radiography to assess lateral ankle ligamentous laxity in the functionally unstable ankle. The purpose of this study was to determine differences in mechanical laxity between the unstable and uninjured ankles in subjects with self-reported FAI. We hypothesized that ankles identified as functionally unstable would not demonstrate mechanical laxity when measured arthrometrically or radiographically.

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Fifty-one subjects (22 males and 29 females; age = 20.6 ± 1.2 yr., mass = 72.6 ± 12.7 kg, ht = 170.5 ± 7.4 cm) with self-reported unilateral FAI were enrolled to participate. Subjects were recruited from a large university population using advertisements and brochures. Potential subjects completed a FAI questionnaire (Table 1) that was modified from the one developed by Hubbard and Kaminski (12). All subjects signed an informed consent agreement approved by the university’s institutional review board (IRB # 265-2001). Subjects were not compensated. One subject did not participate in the radiographic portion of the study, and his data were not included in the statistical analysis for that portion of the study.

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Laxity (mechanical instability) in the functionally unstable ankle and contralateral (normal) ankle was determined using stress radiographic and instrumented arthrometric measurements.

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Stress radiography.

The Telos GA-II/E device (Austin & Associates, Inc, Fallston, MD) provided graded anterior and inversion stress to the ankle during the radiographic examination. This device uses a threaded shaft attached to a pressure plate allowing for a continuous readout of displacement forces. Displacement of the talus on the tibia was measured. A Mini 6600 Fluoroscope (OEC Medical Systems, Inc., Salt Lake City, UT) with a digital mobile C-arm recorded the stress images for anterior displacement (mm) and talar tilt (degrees) for each ankle (Fig. 1).

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Ankle arthrometer.

Instrumented measurement of ankle-subtalar joint stability was performed using a portable ankle arthrometer (Blue Bay Research, Inc., Milton, FL). The arthrometer consists of an adjustable plate that is fixed to the foot, a load-measuring handle that is attached to the footplate through which the load is applied, and a tibial pad attached to the tibia. A six-degrees-of-freedom spatial kinematic linkage connects the tibial pad to the footplate that measures all components of motion (three rotations and three translations) of the footplate relative to the tibial pad (Fig. 2) (19,20). Measurements quantify the AP load-displacement and I-E rotational laxity characteristics of the ankle-subtalar joint complex (talocrural and subtalar joints). During measurement, the force and torque loads produced via the arthrometer’s loading handle are transferred to the skeletal and soft tissues of the ankle-subtalar joint complex. The spatial kinematic linkage of the arthrometer measures the relative motion between the arthrometer footplate and the reference pad attached onto the tibia.

A computer with an analog to digital converter was used to simultaneously calculate and record data into a custom software program written in Lab View 5.1 (National Instruments, Austin, TX). The resulting AP and anterior displacement (millimeters) and I-E and inversion rotation (degrees of ROM), along with the corresponding AP load and I-E torque, were recorded. In addition, anterior displacement and inversion rotation were recorded. Based on the manufacturer’s recommendations, AP loading was performed first, followed by I-E loading. Before testing, the reliability of the ankle arthrometer was derived from tests on 20 healthy ankles by one investigator (TJH). Laxity testing was performed on two different days so that test-retest reliability could be assessed.

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Test procedures.

At session 1, instrumented arthrometry assessed the laxity of the functionally unstable and normal ankles using testing procedures previously described (11,19,20). The order of testing was randomly assigned between ankles and one examiner (TJH), who was blinded throughout as to the subject’s affected ankle. The PI (TJH) obtained all measurements. Subjects were positioned lying supine on the treatment table. The foot was positioned so it was extended over the edge of the table. A restraining strap was wrapped around the distal lower leg 1 cm above the malleoli to prevent lower-leg movement during testing. The examiner secured the arthrometer to the foot by placing the bottom of the foot onto the footplate and adjusting the heel and dorsal clamps. The heel clamp prevented the device from rotating on the calcaneus, while the dorsal clamp secured the foot to the footplate.

The tibial pad was then positioned 5 cm above the ankle malleoli and secured to the lower leg. To minimize variation, the arthrometer was oriented and positioned in a similar manner for all tests. The force loads administered by the examiner were applied through the load handle in-line with the footplate. A lower-leg support held the leg in a firm, extended position and did not allow for extraneous leg movement.

The ankles were positioned at neutral (0° of flexion) using a goniometer centered over the talocrural joint axis of rotation. This neutral position was defined as the measurement reference position (11,19,20). Motion anterior to the reference position was defined as anterior displacement, whereas motion in a posterior direction as posterior displacement. Total AP displacement was the sum of the anterior and posterior motion at a given force load. Rotation internal or external to the reference position was defined as inversion and eversion rotation, respectively. The I-E rotation is occurring about an AP axis. Total I-E rotation was the sum of the inversion and eversion rotation for a given torque load. Thus, total AP displacement and total I-E rotation are recorded as ankle-subtalar joint complex laxity.

To record total AP displacement, the ankle was loaded with 125 newtons (N) of anterior and posterior force (12.74 kilopond [kp]). Starting at the neutral position, an anterior load was applied initially, followed by a posterior load. Total AP displacement between the calcaneus and talus in an AP direction was recorded and defined as AP displacement. For I-E rotation, the ankles were loaded to 4 N·m (0.41 kp· m−1) of inversion and eversion torque. Starting at the neutral position, inversion loading was applied first, followed by eversion loading. Total I-E rotation of the foot was recorded and defined as I-E angular displacement. The computer monitor was visualized to control the amount of force required to obtain the maximum load of 125 N for AP displacement and 4 N·m for I-E rotation. The force loads were applied in a consistent manner and were identical to those previously used (19). To prevent any unwanted lower leg muscle activation or guarding, we carefully visualized the calf region for muscle contraction. Trials were repeated if a visible muscle contraction occurred. This, however, was a rare occurrence as most subjects were comfortable with the movement and remained relaxed throughout the trial.

One month later the subjects reported to have their ankle laxity assessed using stress radiography. During this session the Telos GA-II/E device provided either an anterior or lateral stress while the radiographs were obtained using a digital fluoroscope. For anterior drawer testing, the subject was situated side-lying so that the ankle was placed in the device enabling an unrestricted radiographic view of the ankle mortise. The heel was positioned firmly in the center of the foot holder and the front cushion of the Telos device placed 2 cm above the medial malleolus. The anterior stress load was then applied to the ankle at 15 kp and a fluoroscopic image recorded.

Inversion talar tilt radiographs were obtained with the subject positioned supine on the table. The knee was flexed at 20° using a bolster and the heel firmly fixed in the footplate. The front cushion of the pressure device was placed 5 cm above the medial malleolus. The inversion stress load was applied to the ankle at 15 kp while another fluoroscopic image was recorded.

Anterior talar displacement from the anterior drawer stress radiograph was measured using the shortest distance from the posterior articular surface of the tibia to the talar dome (4). The talar tilt stress radiographs were measured by calculating the total talar tilt angle between the tibia and talus. The talar tilt angle refers to the angle between two lines drawn on the tibial plafond (horizontal distal tibial articular surface) and the talar dome (26). An orthopedic foot and ankle specialist (RVG), who was blinded to the FAI status of the subject, performed all radiographic measurements.

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Statistical analysis.

Intratester reliability for AP displacement and I-E rotation was evaluated by calculating intraclass correlation coefficients (ICC [2,1]). The SEM was calculated as an additional measure of precision. A dependent t-test was conducted to test for differences in laxity measurements between test-retest trials.

Dependent t-tests were used to determine differences in joint laxity between the functionally unstable and uninjured ankles for both the arthrometric and stress radiographic measurements. An a priori level of significance was set at P < 0.05 for all comparisons.

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Instrumented measurement reliability.

Intratester reliability for the AP displacement (ICC [2,1] = 0.91; SEM = 0.88 mm) and I-E rotation (ICC [2,1] = 0.99; SEM = 0.97°) measurements were high (Table 2). No significant differences between trials for AP displacement (t (19) = 1.50, P = 0.149) or I-E rotation (t (19) = 0.43, P = 0.673) were observed.

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Arthrometric measurements.

Total AP displacement in the FAI ankle (19.8 ± 5.1 mm) compared with the uninjured ankle (18.3 ± 4.4 mm) was significantly greater (t (50) = 2.10, P = 0.040). Anterior displacement was also significantly greater (t (50) = 2.17, P = 0.035) in the FAI ankle (12.1 ± 3.1 mm) versus the uninjured ankle (11.1 ± 3.2 mm). No significant differences were observed between the FAI and uninjured ankles for total I-E rotation (60.6 ± 7.7° vs. 59.6 ± 7.5°; t (50) = 1.04, P = 0.288) or inversion rotation (35.7 ± 6.1° vs. 34.8 ± 6.1°; t (50) = 1.14, P = 0.259). Effect sizes for AP and anterior displacement are presented in Table 3.

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Stress radiographic measurements.

Significantly greater anterior displacement (t (49) = 2.05, P = 0.045) in the FAI ankles (6.9 ± 2.5 mm) was present when compared with the uninjured ankles (6.2 ± 2.2 mm). There were no significant differences (t (49) = 1.47, P = 1.47) between injured (3.3 ± 3.0°) and uninjured (2.5 ± 2.3°) ankles for inversion talar tilt angle. The effect size for anterior displacement is presented in Table 3.

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This study was unique in that it examined if individuals with self-reported unilateral FAI also demonstrated mechanical laxity when evaluated by both instrumented arthrometry and stress radiography. The results illustrate increased joint laxity in the sagittal plane for the functionally unstable ankle when measured by both instrumented ankle arthrometry and stress radiography. This difference in anterior and AP translation between ankles was in contrast to our original research hypothesis that the functionally unstable ankle would not demonstrate increased mechanical laxity.

Two major issues must be examined before the relationship between FAI and MAI in the individual experiencing chronic ankle instability can be defined. The first involves identifying the criteria that defines FAI, whereas the second involves obtaining valid and reliable measurements of mechanical laxity. The criteria used to define FAI vary greatly in the literature (9,12,14). We attempted to improve upon earlier studies by using a consistent set of inclusion criteria to identify the functionally unstable ankle (12). The subject answered specific questions (Table 1) related to their history of ankle sprain and instability. This approach ensures that ankles are identified using a consistent set of inclusion criteria that classifies them as functionally unstable and excludes those that are not. We did not assess our subject’s functional status; decisions on inclusion were based on the subjective answers of the participants. There is a void in the literature of any consistent effort on the part of researchers’ to objectively evaluate and document mechanical laxity in the ankle suspected of being functionally unstable. A wrong conclusion from studies that have not identified the presence or absence of mechanical instability is the inference of little or no association between FAI and MAI. If the presence (or lack thereof) of mechanical instability is important in determining the functional instability status, then a valid and reliable measurement of laxity needs to be obtained. To examine this relationship, we objectively evaluated the functionally unstable ankle using both stress radiography and instrumented ankle arthrometry.

The ankle arthrometer was developed as an assessment tool to provide objective and quantifiable assessment of ankle laxity. The use of the ankle arthrometer to quantify ankle-subtalar joint complex laxity is possible due to the development of an instrumented spatial linkage system that measures three-dimensional motion within the anatomical coordinates of the joint (11). Both high validity (Pearson r = 0.88) and reliability (ICC [2,1] = 0.80–0.97) using ankle arthrometry for ankle-subtalar joint complex laxity have been reported (13,19,20). When we examine our reliability measurements (ICC [2,1] = 0.91–0.99) with those previously reported, we are confident that reliable measurements of ankle-subtalar joint laxity were acquired.

The results of the arthrometric measurements from this study demonstrated significantly greater anterior and AP displacement in the functionally unstable ankle when compared with the uninjured ankle. Direct comparison of the data for the functionally unstable ankle with laxity values reported in the literature is not possible, as no published values exist for ankle laxity using a similar device under comparable injury or testing conditions. The mean AP displacement of the uninjured ankles in the present study (18.3 ± 4.4 mm) compares favorably with the mean AP displacement (18.5 ± 5.1 mm) of uninjured ankles previously reported (19). This same study also reported no significant dominant-to-nondominant side differences for either AP displacement or I-E rotation (19). The relatively large standard deviations observed in this study and those reported in previous studies indicate sizable variation in laxity among normal ankles. This is important to point out because ankle-subtalar joint complex laxity of the normal ankle exists in a large range.

Stress radiography using a calibrated loading device allows for reproducible force application and patient positioning, but does not provide direct linear and angular measurements. In addition, this method has been shown to be unreliable despite being used in numerous ankle ligament injury studies (3,6,22,24). Despite these reports, we included stress radiography measurements since many clinicians traditionally consider this technique as the “gold standard” for measuring ankle laxity or instability (3,16). What makes our application of stress radiography unique is that we examined the subjects under the Telos stress using a portable fluoroscope.

Similar to the anterior and AP displacement measurements derived using instrumented arthrometry, anterior displacement in the FAI ankle measured radiographically was significantly greater than the uninjured ankle. The mean anterior laxity difference between ankles was only 0.70 mm. This is considerably less than the 3 mm side-to-side difference previously reported that define mechanical instabilities using stress radiographic measurements (15,17). Konradsen et al. (17) and Karlsson et al. (15) reported that anterior translation unilaterally of 10 mm or greater or a difference between ankles of 3 mm or greater is considered pathological, whereas an inversion talar tilt of more than 9° on one side or a side-to-side difference of 3° or more is considered pathological when measured from stress radiographs (23). In addition, Freeman defined a mechanically unstable ankle as one that on a stress x-ray yields a side-to-side difference in talar tilt of 6° or more (5). The mean inversion talar tilt difference between ankles observed in this study was only 0.8°, which was not statistically significant. It is important to note, the measurements reported from the stress radiographs are much less then those reported from the arthrometer. Stress radiographs examine bony displacement of the talocrural joint. The arthrometer examines the load-displacement response at both the talocrural and subtalar joints.

We individually examined the stress radiographs of the functionally unstable ankle for differences using the side-to-side laxity criteria (15,17). Seven of 50 (14%) ankles presented with a positive talar tilt (>3°), whereas 6 of 50 (12%) ankles demonstrated a positive anterior displacement (>3 mm). In addition, three of the functionally unstable ankles measured positive for both anterior drawer and talar tilt. These findings indicate that refinement of the anterior-displacement and inversion-tilt criteria used to determine the amount of laxity that constitutes mechanical instability is needed when radiographic measurements are used to determine ankle mechanical instability.

Manual stress exams are often used by clinicians to assess ankle instability. Although we did not examine ankle laxity using manual examination, future studies should be performed that validate manual stress tests with instrumented arthrometry and stress radiography. Because most clinicians may not have direct access to arthrometry or stress radiographic techniques, it is important to show the validity and reliability of the manual exam for ruling out MAI in the functionally unstable ankle. This may be important because manual techniques for assessing ankle laxity involve application of unknown forces and moments to the ankle and observation of the displacements and rotations that result. Although many clinicians become skilled in evaluating ligamentous injury, the examination procedure is largely subjective and may not be sufficient to differentiate ankle ligament injuries (7).

The classic work of Freeman (5) examined if MAI of the ankle caused FAI. Studying a small group of subjects (N = 14), Freeman reported MAI was rarely the initial cause of FAI. He concluded that repeated ankle sprain caused by the tendency of the foot to “give-way” could produce MAI. This description explains an association between MAI and FAI (5). It is interesting to note that Freeman’s terminology described mechanical instability involving the ankle and functional instability involving the foot. Since his initial report in 1965, this description of ankle instability has been absent in the literature and replaced with MAI and FAI, with both entities being attributed to the ankle. Also of interest is that Freeman’s initial work was later refuted by Tropp et al., (29) who found an association between FAI and MAI, even though more than half of the functionally unstable ankles were reported to be mechanically stable. Tropp et al. (29) further described MAI as a parallel phenomenon to FAI that produced other lesions of the ankle such as secondary degenerative arthritis. One should be careful in applying these findings to certain patient populations because Tropp and his colleagues (29) studied a select group of soccer players who displayed mild to moderate ankle sprain symptoms and who were participating in competition. In addition, perhaps a strong relationship between MAI and FAI only exists in cases of severe functional disability that may be present after grade II or III ankle sprains and that precludes one from participating in competition (23). Recently, Hertel (10) reported that mechanical and functional insufficiencies are probably not mutually exclusive entities but more likely form a continuum of pathologic contributors to chronic ankle instability (Fig. 3). Additional research is necessary to further examine the enigmatic relationship between functional and mechanical instabilities of the ankle and foot.

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Functional and mechanical ankle instabilities are defined as two separate entities in the literature. Our results demonstrate that ankles identified as functionally unstable may also have associated mechanical instability. Although greater values for anterior and AP displacement were observed, further study is needed to determine the clinical significance of such findings. The present study further confounds the relationship between MAI and FAI. Additional research using instrumented arthrometry should be helpful in determining the amount of ankle-subtalar joint complex laxity that constitutes mechanical instability. As normative data are derived, normal and abnormal laxity ranges can be identified to classify various grades of MAI and any association to FAI.

Clinically, it is important to assess mechanical instability in those patients experiencing FAI. After lateral ankle sprain, there sometimes is a complex interaction between ligamentous laxity and functional impairment. In patients where function is limited due to MAI, stress radiographs or ankle arthrometry may be used to determine the exact amount of instability present. As our understanding of ankle instability grows and normative laxity data are established, instrumented ankle arthrometry should become a clinically accepted tool, much like the measurements used to assess knee joint laxity. The clinician would be able to accurately measure ankle laxity after injury and throughout the rehabilitation process to assist in making return to play decisions.

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