561057
research-article2014
FAIXXX10.1177/1071100714561057Foot & Ankle InternationalBrown et al
Article
Ankle Ligament Laxity and Stiffness in Chronic Ankle Instability
Foot & Ankle International® 1–8 © The Author(s) 2014 Reprints and permissions: sagepub.com/journalsPermissions.nav DOI: 10.1177/1071100714561057 fai.sagepub.com
Cathleen N. Brown, PhD1, Adam B. Rosen, PhD2, and Jupil Ko, MS1
Abstract Background: The contribution of mechanical laxity and ligament stiffness to chronic ankle instability is unclear, particularly when using the inversion laxity test, and may have implications for diagnosis, prognosis, and treatment. Our purpose was to determine if individuals with chronic ankle instability demonstrate greater mechanical ligament laxity and altered stiffness compared to controls and copers (those with a healed sprain) during an instrumented arthrometer inversion stress test. Methods: Recreationally active individuals were classified as those with chronic ankle instability (n = 16), copers (n = 16), or controls (n = 16) based on injury history and self-reported score on the Cumberland Ankle Instability Tool (CAIT). Three trials of an inversion stress test were applied with an instrumented arthrometer utilizing a reliable tester. Talocrural inversion (degrees) and stiffness values were extracted. One-way ANOVAs were calculated, and Tukey post hoc testing was applied (α ≤ .05). Results: Groups were not different in age, height, or weight. The chronic ankle instability group (19 ± 6) had significantly lower CAIT scores than the control (30 ± 1) and coper (29 ± 1) groups (P < .001). The chronic ankle instability group (23 ± 12 degrees) demonstrated significantly greater inversion than the controls (13 ± 9 degrees) (P = .04) but was not significantly different than the copers (17 ± 10 degrees). No significant differences were detected in stiffness between the groups. Conclusion: The chronic ankle instability group demonstrated decreased self-reported ankle function and increased mechanical laxity utilizing an instrumented arthrometer for inversion compared to the control group but not the coper group. Laxity, but not stiffness, may be a factor affecting chronic ankle instability and self-reported function. Level of Evidence: Level III, comparative study. Keywords: arthrometer, coper, inversion Lateral ankle sprains are one of the most common sportsrelated injuries,4,5,18 and a large percentage of individuals who suffer lateral ankle sprains go on to develop chronic ankle instability.41 Repeated sprains and instability have been linked to an increased risk of osteoarthritis of the ankle,36,37 and individuals with instability may not return to their previous levels of activity.13,26 A population, termed “copers,” is one that experienced an initial ankle sprain but then recovered and did not go on to develop chronic instability. Thus, they had a “healed sprain” and were able to “cope” with their injury, recover, and return to activity. Copers have been identified as a comparison group that could offer insight into why some individuals develop chronic ankle instability and others do not.11,38 A number of factors that contribute to and perpetuate chronic ankle instability have been identified, and one possible factor may be the tissue quality of lateral ligaments after injuries, specifically mechanical laxity and stiffness.1,22 Ligamentous mechanical ankle laxity is defined as range of motion that is excessive beyond the normal physiological range of motion and may be caused by changes to
ligamentous tissue or altered arthrokinematics.7 Specifically, mechanical laxity of the ankle associated with chronic ankle instability may be characterized by excess inversion of the hindfoot assessed using an instrumented arthrometer or manual stress testing.2,10 Some research has used the anterior drawer test, which focuses on the anterior talofibular ligament.28,29,31,40 Others have used the talar tilt or ankle inversion test, which may focus on the calcaneofibular ligament.9,19,21,29,30 A positive inversion test result, or laxity in the frontal plane with inversion, may be a better indicator of multiligamentous laxity6 and mechanical laxity.7 1
Biomechanics Laboratory, Department of Kinesiology, University of Georgia, Athens, Georgia, USA 2 Biomechanics Research Building, School of Health, Physical Education and Recreation, University of Nebraska, Omaha, Nebraska, USA Corresponding Author: Cathleen N. Brown, PhD, Biomechanics Laboratory, Department of Kinesiology, University of Georgia, 330 River Road, Athens, GA 30602, USA Email: [email protected]
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Table 1. Participant Demographics and Injury History With 1-Way Analysis of Variance Results. Variable Sex, n Mean age, y Mean height, cm Mean weight, kg Mean CAIT score No. of sprains reported, mean (range)
Chronic Ankle Instability
Coper
Control
P Value
16 (12 M, 4 F) 21 ± 2 174 ± 9 76 ± 14 19 ± 6a 6 ± 5 (2-21)
16 (12 M, 4 F) 22 ± 4 175 ± 11 75 ± 13 29 ± 1 2 ± 1 (1-2)
16 (12 M, 4 F) 21 ± 1 172 ± 8 72 ± 11 30 ± 1 0
NT NS NS NS .05) (Table 2).
Discussion The most important result was that the chronic ankle instability group demonstrated greater laxity than the control group on inversion with an instrumented arthrometer, as we hypothesized. However, the chronic ankle instability group did not display greater laxity than the coper group, which did not support our hypothesis. There was a high degree of variability in laxity among groups. Stiffness did not appear to be different between groups.
The chronic ankle instability group demonstrated increased laxity, or greater inversion, on the instrumented arthrometer compared to the control group but not the coper group. Based on these means, the control and coper groups were not statistically different. A systematic review concluded that unstable ankle groups demonstrated more inversion compared to healthy controls; however, the standardized effect sizes were often small and crossed zero, indicating limited differences.1 This supports our results in which only the chronic ankle instability–control comparison had a large effect size (0.87), while the chronic ankle instability–coper comparison had a moderate effect size (0.49), and the control-coper effect size was small (0.39). In the systematic review, the authors concluded that there was a lack of statistical differences between the chronic ankle instability and control groups, which does not support our results. We may have found differences by using stricter inclusion criteria than the studies included in the review. Using 2 more recent studies not included in the systematic review, we calculated the effect size and power for chronic ankle instability–control inversion laxity comparisons using tabled data. In both studies, the chronic ankle instability group demonstrated greater laxity than the control group, with effect sizes of 0.75 to 1.5 and powers of .53 to .99,20,30 which more closely mirror our results. The difference in methods, including the type of arthrometer used,3,15,20,24,25,30 positioning of the foot/
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Brown et al Table 2. Mean Values and 1-Way Analysis of Variance Results for Instrumented Arthrometer–Dependent Variables.
Variable
Effect Size for Multiple Comparisons Mean ± SD 95% CI for Mean P Value Power (1 – β) Effect Size (ηp2) (Cohen d)
Inversion talar tilt, deg Chronic ankle 22 ± 12 instability Coper 17 ± 10 Control 13 ± 9 Linear stiffness (40-60 N), N/mm Chronic ankle 3±1 instability Coper 4±1
17 to 28
.57
0.12
.21
.32
0.07
.41
.20
0.04
12 to 22 8 to 18 3 to 4 4 to 4
Control 4±1 Linear stiffness (125-150 N), N/mm 7±1 Chronic ankle instability Coper 7±1
6 to 7
Control
6 to 7
7±1
.05
3 to 4
7 to 8
95% CI for Effect Size (Cohen d)
Chronic ankle –0.23 to 1.18 instability–coper: 0.49 0.12 to 1.57 Chronic ankle instability–controla: 0.87 Coper-control: 0.39 –0.32 to 1.07 Chronic ankle –0.13 to 1.28 instability–coper: 0.59 Chronic ankle –0.56 to 0.83 instability–control: 0.14 Coper-control: 0.44 –0.27 to 1.13 Chronic ankle –0.28 to 1.12 instability–coper: 0.43 Chronic ankle –0.60 to 0.78 instability–control: 0.09 Coper-control: 0.33 –0.38 to 1.01
Abbreviations: CI, confidence interval; SD, standard deviation. a Post hoc Tukey honestly significant difference indicates significant difference between groups at P < .05.
ankle,28 and inclusion/exclusion criteria,2 may have contributed to the inconsistency of results. There were no group differences in linear stiffness in the 40- to 60-N low-load range, or the 125- to 150-N high-load range, during inversion. While the chronic ankle instability group had the lowest mean stiffness in both regions, the means and 95% confidence intervals reported were not statistically different from the other groups and were quite comparable to the control group. The coper group was the stiffest and had 95% confidence intervals that were shifted higher but still demonstrated overlap with the chronic ankle instability and control groups. Group comparisons indicated only small to moderate effect sizes, with all 95% confidence intervals for the effect size crossing zero. The low power may be attributable to the small sample size. With the means reported, our study appears to support no differences between groups in terms of stiffness, but there is limited literature on the stiffness of lateral ligaments in chronic ankle instability populations.40 One study reported no group differences between the chronic ankle instability, control, and coper groups in stiffness,9 supporting our results. Decreased stiffness was found in cadaveric specimens after ligament sectioning32 and in people with clinically evident mechanical instability compared to a mechanically stable group at 40 to 60 N and 200 N.31 However, another study reported increased stiffness in
the chronic ankle instability and coper groups to anterior drawer compared to uninjured controls.40 As a static joint stabilizer, a stiffer ligament could theoretically better respond to sudden inversion. Even after an injury and a substantial decrease in self-reported function, changes in stiffness were not detected with the means available. Our stiffness values were greater than those reported in a previous study that used the same units but a different arthrometer.29 Some authors calculated stiffness in the first 30% to 40% of the test,31,32 or at different loads,31 while another calculated it at the end range,28 and others did not specify.9,29,40 Additionally, comparisons of stiffness using the anterior drawer test,28,31,32 as opposed to inversion,9,29 may not be appropriate. Finally, the rate of loading in a portable arthrometer does not approximate a potential realworld mechanism of injury. It is unclear if stiffness is a factor contributing to chronic ankle instability. Our results indicate that it is not. However, differences in acute treatment, immobilization, and rehabilitation after injuries could affect stiffness values. Prospective research following initial injuries is necessary to determine the clinical course of chronic ankle instability. Only a few studies to date have incorporated a coper group into comparisons of laxity, but they may better represent a clinically useful pathway from injury to healing.38,39 Separating out copers from controls may improve the effect
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size of group comparisons. Our results indicate that copers appear to exist on a continuum, somewhere between those with chronic ankle instability and controls. Using previous studies’ tabled data, chronic ankle instability groups were more lax to inversion rotation than coper groups, with large effect sizes (1.03-1.4) and powers (.81-.96) in studies utilizing a different arthrometer.19,30 With the means reported, we did not find significant differences between the chronic ankle instability and coper groups in laxity and stiffness, but they had moderate effect sizes, indicating possible clinical relevance.16 Based on our means, it appears that the coper group falls in between the chronic ankle instability and control groups in terms of laxity but may skew stiffer. Copers’ mean stiffness was higher at low- and high-load regions of the force-displacement curve but not significantly so (P > .05). Comparisons between coper and control groups are also mixed. Our coper group’s mean inversion value was larger than that of the control group, but the means were not statistically different, with only a small to moderate effect size (0.39). Utilizing tabled data, a control-coper comparison of mean inversion demonstrated a small effect size (0.16) and low power (.07) in one study,30 while another study reported no control-coper group differences.9 Both of these studies support our results. Our small effect size and overlap in 95% confidence intervals do not support differences in stiffness between copers and controls. Little is known about copers following an injury. They may demonstrate better tissue response, especially at low loads where others have found differences31 in being able to respond appropriately and moderate inversion forces. Currently, it appears that copers overlap substantially in terms of laxity and stiffness with both the chronic ankle instability and control groups. Thus, mechanical laxity and stiffness do not appear to be contributing factors driving individuals toward or away from chronic ankle instability after an initial sprain. Copers reported better function than those in the chronic ankle instability group and similar function to controls but had a wide range of laxity and stiffness values, some of which were comparable to participants with chronic ankle instability. These results also provide evidence for a model of chronic ankle instability with more subgroups12 in which mechanical laxity is separated out from perceived instability and recurrent sprains while also addressing the combined effects of those characteristics. There are several limitations to this study. The first is that it was cross-sectional; thus, we were unable to determine if the laxity and stiffness values noted are results of ankle sprains and chronic ankle instability or if pre-existing laxity and stiffness caused the participants to develop chronic ankle instability or be copers. There was no specific “numerical” indicator of instability, as we utilized a selfreport questionnaire and an instrumented arthrometer to perform talocrural inversion rather than radiological
measures. The talocrural inversion was used because it is the recommended test with the arthrometer and software rather than the anterior drawer test. Thus, rotational instability was tested, and some instability may not be picked up with any method of testing, as it does not replicate sporting or daily living conditions. Additionally, participant differences in neuromuscular control, severity of the initial injury, rehabilitation, and involvement of the anterior talofibular versus calcaneofibular ligament were not determined but could have affected results. We did not use electromyography and could not tell if the ankle musculature was truly relaxed during arthrometry, which may have influenced laxity and stiffness values. Preliminary testing and previous studies31,40 indicated that the first data collection trial was not reliable and was lower than subsequent trials. We discarded this trial, but its lack of inclusion could have affected our results. We calculated linear stiffness based on available data, but rotary stiffness may present different results. An a priori power analysis determined the sample size, but the stiffness measures were underpowered. A larger sample size would likely increase power.
Conclusion The chronic ankle instability group demonstrated greater inversion mechanical laxity to the instrumented arthrometer compared to the control group as well as decreased selfreported function compared to the control and coper groups. However, the coper group was not more lax than the chronic ankle instability or control groups. Increased mechanical laxity appears to exist in the chronic ankle instability group compared to the control group. Copers’ laxity appears to exist on a continuum between the chronic ankle instability and control groups. There were no group differences in linear stiffness, and tissue response to the applied load may not be a factor contributing to chronic ankle instability. Future prospective studies should determine the role of mechanical laxity in developing and perpetuating chronic ankle instability. Declaration of Conflicting Interests The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: University of Georgia College of Education.
References 1. Cordova ML, Sefton JM, Hubbard TJ. Mechanical joint laxity associated with chronic ankle instability: a systematic review. Sports Health. 2010;2:452-459.
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Brown et al 2. Delahunt E, Coughlan GF, Caulfield B, Nightingale EJ, Lin CW, Hiller CE. Inclusion criteria when investigating insufficiencies in chronic ankle instability. Med Sci Sports Exerc. 2010;42:2106-2121. 3. Docherty CL, Rybak-Webb K. Reliability of the anterior drawer and talar tilt tests using the ligmaster joint arthrometer. J Sport Rehabil. 2009;18:389-397. 4. Doherty C, Delahunt E, Caulfield B, Hertel J, Ryan J, Bleakley C. The incidence and prevalence of ankle sprain injury: a systematic review and meta-analysis of prospective epidemiological studies. Sports Med. 2014;44:123-140. 5. Fong DT, Hong Y, Chan LK, Yung PS, Chan KM. A systematic review on ankle injury and ankle sprain in sports. Sports Med. 2007;37:73-94. 6. Fujii T, Luo ZP, Kitaoka HB, An KN. The manual stress test may not be sufficient to differentiate ankle ligament injuries. Clin Biomech. 2000;15:619-623. 7. Gehring D, Faschian K, Lauber B, Lohrer H, Nauck T, Gollhofer A. Mechanical instability destabilises the ankle joint directly in the ankle-sprain mechanism. Br J Sports Med. 2014;48:377-382. 8. Gribble PA, Delahunt E, Bleakley C, et al. Selection criteria for patients with chronic ankle instability in controlled research: a position statement of the international ankle consortium. J Orthop Sports Phys Ther. 2013;43:585-591. 9. Gutierrez GM, Knight CA, Swanik CB, et al. Examining neuromuscular control during landings on a supinating platform in persons with and without ankle instability. Am J Sports Med. 2012;40:193-201. 10. Hertel J. Functional anatomy, pathomechanics, and pathophysiology of lateral ankle instability. J Athl Train. 2002;37:364-375. 11. Hertel J, Kaminski TW. Second international ankle sym posium summary statement. J Orthop Sports Phys Ther. 2005;35:A2-A6. 12. Hiller CE, Kilbreath SL, Refshauge KM. Chronic ankle instability: evolution of the model. J Athl Train. 2011;46:133-141. 13. Hiller CE, Nightingale EJ, Raymond J, et al. Prevalence and impact of chronic musculoskeletal ankle disorders in the community. Arch Phys Med Rehabil. 2012;93:1801-1807. 14. Hiller CE, Refshauge KM, Bundy AC, Herbert RD, Kilbreath SL. The Cumberland Ankle Instability Tool: a report of validity and reliability testing. Arch Phys Med Rehabil. 2006;87:1235-1241. 15. Hirai D, Docherty CL, Schrader J. Severity of functional and mechanical ankle instability in an active population. Foot Ankle Int. 2009;30:1071-1077. 16. Hoch MC, Mullineaux DR, Andreatta RD, et al. Effect of a 2-week joint mobilization intervention on single-limb balance and ankle arthrokinematics in those with chronic ankle instability. J Sport Rehabil. 2014;23:18-26. 17. Hoffman M, Schrader J, Applegate T, Koceja D. Unilateral postural control of the functionally dominant and nondominant extremities of healthy subjects. J Athl Train. 1998;33:319-322. 18. Hootman JM, Dick R, Agel J. Epidemiology of collegiate injuries for 15 sports: summary and recommendations for injury prevention initiatives. J Athl Train. 2007;42:311-319.
19. Hubbard TJ. Ligament laxity following inversion injury with and without chronic ankle instability. Foot Ankle Int. 2008;29:305-311. 20. Hubbard TJ, Cordova M. Effect of ankle taping on mechanical laxity in chronic ankle instability. Foot Ankle Int. 2010;31:499-504. 21. Hubbard TJ, Cordova M. Mechanical instability after an acute lateral ankle sprain. Arch Phys Med Rehabil. 2009;90: 1142-1146. 22. Hubbard TJ, Hertel J. Mechanical contributions to chronic lateral ankle instability. Sports Med. 2006;36:263-277. 23. Hubbard TJ, Hicks-Little CA. Ankle ligament healing after an acute ankle sprain: an evidence-based approach. J Athl Train. 2008;43:523-529. 24. Hubbard TJ, Kaminski TW, Vander Griend RA, Kovaleski JE. Quantitative assessment of mechanical laxity in the functionally unstable ankle. Med Sci Sports Exerc. 2004;36:760-766. 25. Hubbard-Turner T. Relationship between mechanical ankle joint laxity and subjective function. Foot Ankle Int. 2012;33:852-856. 26. Konradsen L, Bech L, Ehrenbjerg M, Nickelsen T. Seven years follow-up after ankle inversion trauma. Scand J Med Sci Sports. 2002;12:129-135. 27. Kovaleski JE, Hollis JM, Heitman RJ, Gurchiek LR, Pearsall AW. Assessment of ankle-subtalar-joint-complex laxity using an instrumented ankle arthrometer: an experimental cadaveric investigation. J Athl Train. 2002;37:467-474. 28. Kovaleski JE, Norrell PM, Heitman RJ, Hollis JM, Pearsall AW. Knee and ankle position, anterior drawer laxity, and stiffness of the ankle complex. J Athl Train. 2008;43:242-248. 29. McKeon PO, Paolini G, Ingersoll CD, et al. Effects of balance training on gait parameters in patients with chronic ankle instability: a randomized controlled trial. Clin Rehabil. 2009;23:609-621. 30. Miller H, Needle AR, Swanik CB, Gustavsen GA, Kaminski TW. Role of external prophylactic support in restricting accessory ankle motion after exercise. Foot Ankle Int. 2012;33:862-869. 31. Nauck T, Lohrer H, Gollhofer A. Clinical evaluation of a new noninvasive ankle arthrometer. Phys Sportsmed. 2010;38:55-61. 32. Nauck T, Lohrer H, Gollhofer A. Evaluation of arthrometer for ankle instability: a cadaveric study. Foot Ankle Int. 2010;31:612-618. 33. Pearsall AW, Kovaleski JE, Heitman RJ, Gurchiek LR, Hollis JM. The relationships between instrumented measurements of ankle and knee ligamentous laxity and generalized joint laxity. J Sports Med Phys Fitness. 2006;46:104-110. 34. Rijke AM, Jones B, Vierhout PA. Stress examination of traumatized lateral ligaments of the ankle. Clin Orthop Relat Res. 1986;(210):143-151. 35. Shrout PE, Fleiss JL. Intraclass correlations: uses in assessing rater reliability. Psychol Bull. 1979;86:420-428. 36. Valderrabano V, Hintermann B, Horisberger M, Fung TS. Ligamentous posttraumatic ankle osteoarthritis. Am J Sports Med. 2006;34:612-620. 37. Valderrabano V, Horisberger M, Russell I, Dougall H, Hintermann B. Etiology of ankle osteoarthritis. Clin Orthop Relat Res. 2009;467:1800-1806.
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38. Wikstrom EA, Brown CN. Minimum reporting standards for copers in chronic ankle instability research. Sports Med. 2014;44:251-268. 39. Wikstrom EA, Tillman MD, Chmielewski TL, Cauraugh JH, Naugle KE, Borsa PA. Discriminating between copers and people with chronic ankle instability. J Athl Train. 2012;47:136-142.
40. Wikstrom EA, Tillman MD, Chmielewski TL, Cauraugh JH, Naugle KE, Borsa PA. Dynamic postural control but not mechanical stability differs among those with and without chronic ankle instability. Scan J Med Sci Sports. 2010;20:e137-e144. 41. Yeung MS, Chan KM, So CH, Yuan WY. An epidemiological survey on ankle sprain. Brit J Sports Med. 1994;28:112-116.
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research-article2014
FAIXXX10.1177/1071100714561057Foot & Ankle InternationalBrown et al
Article
Ankle Ligament Laxity and Stiffness in Chronic Ankle Instability
Foot & Ankle International® 1–8 © The Author(s) 2014 Reprints and permissions: sagepub.com/journalsPermissions.nav DOI: 10.1177/1071100714561057 fai.sagepub.com
Cathleen N. Brown, PhD1, Adam B. Rosen, PhD2, and Jupil Ko, MS1
Abstract Background: The contribution of mechanical laxity and ligament stiffness to chronic ankle instability is unclear, particularly when using the inversion laxity test, and may have implications for diagnosis, prognosis, and treatment. Our purpose was to determine if individuals with chronic ankle instability demonstrate greater mechanical ligament laxity and altered stiffness compared to controls and copers (those with a healed sprain) during an instrumented arthrometer inversion stress test. Methods: Recreationally active individuals were classified as those with chronic ankle instability (n = 16), copers (n = 16), or controls (n = 16) based on injury history and self-reported score on the Cumberland Ankle Instability Tool (CAIT). Three trials of an inversion stress test were applied with an instrumented arthrometer utilizing a reliable tester. Talocrural inversion (degrees) and stiffness values were extracted. One-way ANOVAs were calculated, and Tukey post hoc testing was applied (α ≤ .05). Results: Groups were not different in age, height, or weight. The chronic ankle instability group (19 ± 6) had significantly lower CAIT scores than the control (30 ± 1) and coper (29 ± 1) groups (P < .001). The chronic ankle instability group (23 ± 12 degrees) demonstrated significantly greater inversion than the controls (13 ± 9 degrees) (P = .04) but was not significantly different than the copers (17 ± 10 degrees). No significant differences were detected in stiffness between the groups. Conclusion: The chronic ankle instability group demonstrated decreased self-reported ankle function and increased mechanical laxity utilizing an instrumented arthrometer for inversion compared to the control group but not the coper group. Laxity, but not stiffness, may be a factor affecting chronic ankle instability and self-reported function. Level of Evidence: Level III, comparative study. Keywords: arthrometer, coper, inversion Lateral ankle sprains are one of the most common sportsrelated injuries,4,5,18 and a large percentage of individuals who suffer lateral ankle sprains go on to develop chronic ankle instability.41 Repeated sprains and instability have been linked to an increased risk of osteoarthritis of the ankle,36,37 and individuals with instability may not return to their previous levels of activity.13,26 A population, termed “copers,” is one that experienced an initial ankle sprain but then recovered and did not go on to develop chronic instability. Thus, they had a “healed sprain” and were able to “cope” with their injury, recover, and return to activity. Copers have been identified as a comparison group that could offer insight into why some individuals develop chronic ankle instability and others do not.11,38 A number of factors that contribute to and perpetuate chronic ankle instability have been identified, and one possible factor may be the tissue quality of lateral ligaments after injuries, specifically mechanical laxity and stiffness.1,22 Ligamentous mechanical ankle laxity is defined as range of motion that is excessive beyond the normal physiological range of motion and may be caused by changes to
ligamentous tissue or altered arthrokinematics.7 Specifically, mechanical laxity of the ankle associated with chronic ankle instability may be characterized by excess inversion of the hindfoot assessed using an instrumented arthrometer or manual stress testing.2,10 Some research has used the anterior drawer test, which focuses on the anterior talofibular ligament.28,29,31,40 Others have used the talar tilt or ankle inversion test, which may focus on the calcaneofibular ligament.9,19,21,29,30 A positive inversion test result, or laxity in the frontal plane with inversion, may be a better indicator of multiligamentous laxity6 and mechanical laxity.7 1
Biomechanics Laboratory, Department of Kinesiology, University of Georgia, Athens, Georgia, USA 2 Biomechanics Research Building, School of Health, Physical Education and Recreation, University of Nebraska, Omaha, Nebraska, USA Corresponding Author: Cathleen N. Brown, PhD, Biomechanics Laboratory, Department of Kinesiology, University of Georgia, 330 River Road, Athens, GA 30602, USA Email: [email protected]
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Table 1. Participant Demographics and Injury History With 1-Way Analysis of Variance Results. Variable Sex, n Mean age, y Mean height, cm Mean weight, kg Mean CAIT score No. of sprains reported, mean (range)
Chronic Ankle Instability
Coper
Control
P Value
16 (12 M, 4 F) 21 ± 2 174 ± 9 76 ± 14 19 ± 6a 6 ± 5 (2-21)
16 (12 M, 4 F) 22 ± 4 175 ± 11 75 ± 13 29 ± 1 2 ± 1 (1-2)
16 (12 M, 4 F) 21 ± 1 172 ± 8 72 ± 11 30 ± 1 0
NT NS NS NS .05) (Table 2).
Discussion The most important result was that the chronic ankle instability group demonstrated greater laxity than the control group on inversion with an instrumented arthrometer, as we hypothesized. However, the chronic ankle instability group did not display greater laxity than the coper group, which did not support our hypothesis. There was a high degree of variability in laxity among groups. Stiffness did not appear to be different between groups.
The chronic ankle instability group demonstrated increased laxity, or greater inversion, on the instrumented arthrometer compared to the control group but not the coper group. Based on these means, the control and coper groups were not statistically different. A systematic review concluded that unstable ankle groups demonstrated more inversion compared to healthy controls; however, the standardized effect sizes were often small and crossed zero, indicating limited differences.1 This supports our results in which only the chronic ankle instability–control comparison had a large effect size (0.87), while the chronic ankle instability–coper comparison had a moderate effect size (0.49), and the control-coper effect size was small (0.39). In the systematic review, the authors concluded that there was a lack of statistical differences between the chronic ankle instability and control groups, which does not support our results. We may have found differences by using stricter inclusion criteria than the studies included in the review. Using 2 more recent studies not included in the systematic review, we calculated the effect size and power for chronic ankle instability–control inversion laxity comparisons using tabled data. In both studies, the chronic ankle instability group demonstrated greater laxity than the control group, with effect sizes of 0.75 to 1.5 and powers of .53 to .99,20,30 which more closely mirror our results. The difference in methods, including the type of arthrometer used,3,15,20,24,25,30 positioning of the foot/
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Brown et al Table 2. Mean Values and 1-Way Analysis of Variance Results for Instrumented Arthrometer–Dependent Variables.
Variable
Effect Size for Multiple Comparisons Mean ± SD 95% CI for Mean P Value Power (1 – β) Effect Size (ηp2) (Cohen d)
Inversion talar tilt, deg Chronic ankle 22 ± 12 instability Coper 17 ± 10 Control 13 ± 9 Linear stiffness (40-60 N), N/mm Chronic ankle 3±1 instability Coper 4±1
17 to 28
.57
0.12
.21
.32
0.07
.41
.20
0.04
12 to 22 8 to 18 3 to 4 4 to 4
Control 4±1 Linear stiffness (125-150 N), N/mm 7±1 Chronic ankle instability Coper 7±1
6 to 7
Control
6 to 7
7±1
.05
3 to 4
7 to 8
95% CI for Effect Size (Cohen d)
Chronic ankle –0.23 to 1.18 instability–coper: 0.49 0.12 to 1.57 Chronic ankle instability–controla: 0.87 Coper-control: 0.39 –0.32 to 1.07 Chronic ankle –0.13 to 1.28 instability–coper: 0.59 Chronic ankle –0.56 to 0.83 instability–control: 0.14 Coper-control: 0.44 –0.27 to 1.13 Chronic ankle –0.28 to 1.12 instability–coper: 0.43 Chronic ankle –0.60 to 0.78 instability–control: 0.09 Coper-control: 0.33 –0.38 to 1.01
Abbreviations: CI, confidence interval; SD, standard deviation. a Post hoc Tukey honestly significant difference indicates significant difference between groups at P < .05.
ankle,28 and inclusion/exclusion criteria,2 may have contributed to the inconsistency of results. There were no group differences in linear stiffness in the 40- to 60-N low-load range, or the 125- to 150-N high-load range, during inversion. While the chronic ankle instability group had the lowest mean stiffness in both regions, the means and 95% confidence intervals reported were not statistically different from the other groups and were quite comparable to the control group. The coper group was the stiffest and had 95% confidence intervals that were shifted higher but still demonstrated overlap with the chronic ankle instability and control groups. Group comparisons indicated only small to moderate effect sizes, with all 95% confidence intervals for the effect size crossing zero. The low power may be attributable to the small sample size. With the means reported, our study appears to support no differences between groups in terms of stiffness, but there is limited literature on the stiffness of lateral ligaments in chronic ankle instability populations.40 One study reported no group differences between the chronic ankle instability, control, and coper groups in stiffness,9 supporting our results. Decreased stiffness was found in cadaveric specimens after ligament sectioning32 and in people with clinically evident mechanical instability compared to a mechanically stable group at 40 to 60 N and 200 N.31 However, another study reported increased stiffness in
the chronic ankle instability and coper groups to anterior drawer compared to uninjured controls.40 As a static joint stabilizer, a stiffer ligament could theoretically better respond to sudden inversion. Even after an injury and a substantial decrease in self-reported function, changes in stiffness were not detected with the means available. Our stiffness values were greater than those reported in a previous study that used the same units but a different arthrometer.29 Some authors calculated stiffness in the first 30% to 40% of the test,31,32 or at different loads,31 while another calculated it at the end range,28 and others did not specify.9,29,40 Additionally, comparisons of stiffness using the anterior drawer test,28,31,32 as opposed to inversion,9,29 may not be appropriate. Finally, the rate of loading in a portable arthrometer does not approximate a potential realworld mechanism of injury. It is unclear if stiffness is a factor contributing to chronic ankle instability. Our results indicate that it is not. However, differences in acute treatment, immobilization, and rehabilitation after injuries could affect stiffness values. Prospective research following initial injuries is necessary to determine the clinical course of chronic ankle instability. Only a few studies to date have incorporated a coper group into comparisons of laxity, but they may better represent a clinically useful pathway from injury to healing.38,39 Separating out copers from controls may improve the effect
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size of group comparisons. Our results indicate that copers appear to exist on a continuum, somewhere between those with chronic ankle instability and controls. Using previous studies’ tabled data, chronic ankle instability groups were more lax to inversion rotation than coper groups, with large effect sizes (1.03-1.4) and powers (.81-.96) in studies utilizing a different arthrometer.19,30 With the means reported, we did not find significant differences between the chronic ankle instability and coper groups in laxity and stiffness, but they had moderate effect sizes, indicating possible clinical relevance.16 Based on our means, it appears that the coper group falls in between the chronic ankle instability and control groups in terms of laxity but may skew stiffer. Copers’ mean stiffness was higher at low- and high-load regions of the force-displacement curve but not significantly so (P > .05). Comparisons between coper and control groups are also mixed. Our coper group’s mean inversion value was larger than that of the control group, but the means were not statistically different, with only a small to moderate effect size (0.39). Utilizing tabled data, a control-coper comparison of mean inversion demonstrated a small effect size (0.16) and low power (.07) in one study,30 while another study reported no control-coper group differences.9 Both of these studies support our results. Our small effect size and overlap in 95% confidence intervals do not support differences in stiffness between copers and controls. Little is known about copers following an injury. They may demonstrate better tissue response, especially at low loads where others have found differences31 in being able to respond appropriately and moderate inversion forces. Currently, it appears that copers overlap substantially in terms of laxity and stiffness with both the chronic ankle instability and control groups. Thus, mechanical laxity and stiffness do not appear to be contributing factors driving individuals toward or away from chronic ankle instability after an initial sprain. Copers reported better function than those in the chronic ankle instability group and similar function to controls but had a wide range of laxity and stiffness values, some of which were comparable to participants with chronic ankle instability. These results also provide evidence for a model of chronic ankle instability with more subgroups12 in which mechanical laxity is separated out from perceived instability and recurrent sprains while also addressing the combined effects of those characteristics. There are several limitations to this study. The first is that it was cross-sectional; thus, we were unable to determine if the laxity and stiffness values noted are results of ankle sprains and chronic ankle instability or if pre-existing laxity and stiffness caused the participants to develop chronic ankle instability or be copers. There was no specific “numerical” indicator of instability, as we utilized a selfreport questionnaire and an instrumented arthrometer to perform talocrural inversion rather than radiological
measures. The talocrural inversion was used because it is the recommended test with the arthrometer and software rather than the anterior drawer test. Thus, rotational instability was tested, and some instability may not be picked up with any method of testing, as it does not replicate sporting or daily living conditions. Additionally, participant differences in neuromuscular control, severity of the initial injury, rehabilitation, and involvement of the anterior talofibular versus calcaneofibular ligament were not determined but could have affected results. We did not use electromyography and could not tell if the ankle musculature was truly relaxed during arthrometry, which may have influenced laxity and stiffness values. Preliminary testing and previous studies31,40 indicated that the first data collection trial was not reliable and was lower than subsequent trials. We discarded this trial, but its lack of inclusion could have affected our results. We calculated linear stiffness based on available data, but rotary stiffness may present different results. An a priori power analysis determined the sample size, but the stiffness measures were underpowered. A larger sample size would likely increase power.
Conclusion The chronic ankle instability group demonstrated greater inversion mechanical laxity to the instrumented arthrometer compared to the control group as well as decreased selfreported function compared to the control and coper groups. However, the coper group was not more lax than the chronic ankle instability or control groups. Increased mechanical laxity appears to exist in the chronic ankle instability group compared to the control group. Copers’ laxity appears to exist on a continuum between the chronic ankle instability and control groups. There were no group differences in linear stiffness, and tissue response to the applied load may not be a factor contributing to chronic ankle instability. Future prospective studies should determine the role of mechanical laxity in developing and perpetuating chronic ankle instability. Declaration of Conflicting Interests The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: University of Georgia College of Education.
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