Cross-sectional Area of the Achilles Tendon in a Cohort of Elite Military Warriors Using Standard Ultrasound Techniques

Cross-sectional Area of the Achilles Tendon in a Cohort of Elite Military Warriors Using Standard... Abstract Background The prevalence of Achilles tendon (AT) pathology is common and can result in disability. Understanding normal AT properties can improve our ability to prevent AT injuries. We examined the cross-sectional area of the AT at multiple levels in an asymptomatic population of Army Rangers. Methods This is a prospective cohort study composed of 41 voluntarily recruited United States Army Rangers deployed in a combat theater. All subjects were members of the Ranger Regiment participating in more than 20 h of intense bipedal non-sport weekly training with no history of AT pathology. While standing, each subject had bilateral AT calcaneal tuberosity insertions (0 cm) marked, along with skin markings made at 2 cm, 4 cm, and 6 cm superior to the AT insertion. AT diameter was measured at each level in the coronal and sagittal planes using ultrasound. Results Mean sagittal diameter of the AT was 4.4 mm, 4.3 mm, 4.2 mm, and 3.9 mm at 0 cm, 2 cm, 4 cm, and 6 cm, respectively. Mean coronal diameter of the AT was 19.3 mm, 14.7 mm, 13.8 mm, and 14.5 mm at 0 cm, 2 cm, 4 cm, and 6 cm, respectively. The cross-sectional area was calculated as 0.66 cm2, 0.5 cm2, 0.46 cm2, and 0.44 cm2 at 0 cm, 2 cm, 4 cm, and 6 cm, respectively. Conclusion Our data suggest that increased non-sport activity may not increase the cross-sectional area of the AT. Identifying the normal diameter at multiple levels throughout the most commonly injured area may improve the provider’s ability to identify early disease processes and apply targeted interventions to help slow or prevent progression and possible rupture. Level of Evidence Level III–V. INTRODUCTION The Achilles tendon (AT) is the largest tendon in the body.1 The gastrocnemius is the largest and most superficial muscle within the superficial posterior compartment. It has medial and lateral heads originating from the corresponding femoral condyles crossing the knee posteriorly. The soleus muscle lies deep into the gastrocnemius and is broad and flat originating from the posterior aspect of the tibia and fibula. The gastrocnemius and soleus muscles glide independently with the gastrocnemius tendinous portion being two to three times longer than that of the soleus.2 The two tendons converge into the AT approximately 5–6 cm from its calcaneal tuberosity insertion. The tendon then rotates 90 degrees in the axial plane so that the medial fibers end posteriorly. The spiraling fibers create an energy-efficient elastic recoil, which also creates a stress riser 2–5 cm proximal to the insertion.3 This area of the tendon also coincides with the vascular watershed that accounts for 75% of acute Achilles ruptures.3 In the United States Military and competitive sports, cardiovascular endurance and strength are paramount to mission success. The benefits of intense training and tissue adaptation must also be weighed against the physiological changes to the musculoskeletal system. Elite American military units often train more than 20 h/wk while carrying an excess of 60–80 lb of additional combat equipment. These conditions substantially change the cyclical load and elastic recoil of the AT, which may cause secondary micro-tears and tissue changes resulting in tendon hypertrophy. Monitoring these changes using ultrasound-measured cross-sectional area (CSA) has been described, but the effects of physical activity on CSA remain unclear with conflicting outcomes of increased size and decreased size.4–7 Continued research to increase our knowledge base of normal AT properties can improve our ability to reduce and prevent future AT injuries. Surveillance and monitoring of the AT using ultrasound CSA could potentially allow customized training regimens while preventing costly overuse injuries, tendinopathies, and ruptures. The purpose of this study is to examine the CSA of the AT at multiple levels in an asymptomatic population of elite American military service members to determine the adaptive changes that occur with their intense training program. MATERIALS AND METHODS This study is a prospective cohort of 41 active duty United States Army Rangers from within the same organization who were voluntarily recruited to participate while deployed in a combat theater. The US Army Medical Research and Material Command Institutional Review Board approved the study. All subjects had a brief history performed by the senior author and were eligible if they were a member of the United States Army Ranger Regiment for at least 3 yr participating in more than 20 h of intense weekly training, passed a military physical fitness test within the past 6 mo, and passed a pre-deployment health assessment physical. Subjects were excluded if they had a history of Achilles tendinopathy, rupture, or fracture of the lower extremity as these subjects are three times more likely to develop Achilles tendinopathies.1 Subjects were also excluded if they had a history of metabolic conditions or inflammatory diseases, which may alter the AT structure.8 Our subjects had a standardized 2-h physical fitness regimen 6 d/wk, while participating in standardized daily military duties, which included an additional 8 h of intense physical activities. As an elite infantry unit, daily activities included, but were not limited to, carrying heavy body armor and backpacks (50–80 lb additional to bodyweight) while climbing and performing tactical drills. Physical activity level is paramount in evaluating the AT size and characteristics as subjects with frequent exercise have been previously found to have larger ATs.9,10 Outcomes were predetermined to include the sonographic measurement of the sagittal and coronal diameter of the AT bilaterally at the bone tendon interface (insertion 0 cm) 2 cm, 4 cm, and 6 cm proximally along the tendon. The subjects also obtained standard demographic information, which included age, height, weight, and dominant foot. The dominant foot was identified by asking the subjects which foot they would kick a ball with and that foot was chosen as the dominant foot. The dominant foot was identified as it has been previously reported that AT ruptures are more common on the left side, the planted side with right footed kicking.10,11 All ultrasound examinations were performed using the SonoSite NanoMaxx with a 10- to 5-MHz linear transducer (FUJIFILM SonoSite, Inc., Bothel, Washington, DC, USA) between 03:00 h and 04:00 h on non-mission days. All ultrasound examinations were performed in the standing position to ensure consistency in tendon tension, position, and to aid in measurements. Throughout all testing, the ultrasound depth and penetration was left on the factory setting for musculoskeletal imagining to ensure consistency. The Achilles bone tendon interface (insertion 0 cm) was marked and identified followed by identification of the 2 cm, 4 cm, and 6 cm points proximally along the tendon bilaterally (Fig. 1). The AT was initially scanned transversely with the transducer perpendicular to the tendon measuring the coronal diameter at each of the given levels (Fig. 2A & B). Then the transducer was aligned longitudinally, to the tendon allowing maximum echogenicity while measuring the anteroposterior diameter (Fig. 3A & B). This technique also allowed for a secondary verification of the proximity along the tendon using ultrasound measurements. The judgment of transverse and perpendicular transducer placement was based on visual alignment and clinical judgment of the senior author. Examination utilized standard ultrasound gel. A spacer pad was not used as it is not a standard practice within the orthopedic community. Figure 1. View largeDownload slide The insertion (0) was marked and identified followed by identification of the 2 cm, 4 cm, and 6 cm points proximally along the tendon; this was done bilaterally. Figure 1. View largeDownload slide The insertion (0) was marked and identified followed by identification of the 2 cm, 4 cm, and 6 cm points proximally along the tendon; this was done bilaterally. Figure 2. View largeDownload slide (A) The transducer was positioned perpendicular to the tendon, here at the 2-cm position and the coronal plane of the tendon was measured. (B) Utilizing the ultrasound’s ability to measure between two points the measurement in the coronal plane was made. Figure 2. View largeDownload slide (A) The transducer was positioned perpendicular to the tendon, here at the 2-cm position and the coronal plane of the tendon was measured. (B) Utilizing the ultrasound’s ability to measure between two points the measurement in the coronal plane was made. Figure 3. View largeDownload slide (A) The transducer is positioned parallel to the tendon to allow measurement of the tendon in the sagittal plane. (B) Utilizing the ultrasound’s ability to measure between two points the measurement in the sagittal plane was made. Figure 3. View largeDownload slide (A) The transducer is positioned parallel to the tendon to allow measurement of the tendon in the sagittal plane. (B) Utilizing the ultrasound’s ability to measure between two points the measurement in the sagittal plane was made. All examinations were performed by the same fellowship trained orthopedic foot and ankle surgeon and verified by an emergency medicine physician specially trained in ultrasound. To ensure consistency and accuracy of measurement, a third provider assisted to create a consensus on each measurement, a technique previously validated.10 The study was conducted over a 2-mo period during which time all subjects continued physical training and combat-related tasks. The combat environment and strict unit regulations also prevented any alcohol consumption within 30 d of the examination, as a previous study has demonstrated alcohol use as a preventable risk factor in AT tendinopathies.1 All subjects were on malaria prophylaxis, but no subjects were on fluoroquinolones during the testing. RESULTS In 41 male subjects, a total of 82 ATs were examined. The mean age of the cohort was 26 yr, 70 in. tall, with a mean weight of 187 pounds. The mean sagittal and coronal diameter of the AT is depicted in Table I along with the mean CSA. Mean sagittal diameter of the AT was 4.4 mm, 4.3 mm, 3.9 mm, and 4 mm at 0 cm, 2 cm, 4 cm, and 6 cm, respectively. Mean coronal diameter of the AT was 19.3 mm, 14.7 mm, 13.8 mm, and 14.5 mm at 0 cm, 2 cm, 4 cm, and 6 cm, respectively. The CSA was calculated at each respective level utilizing the following equation:   CSAofanOval=pi(a)(b)where a is the radius of the tendon in the sagittal plane and b is the radius in the coronal plane. The CSA was calculated as 66 ± 12 mm2, 50 ± 8 mm2, 46 ± 8 mm2, and 44 ± 8 mm2 at 0 cm, 2 cm, 4 cm, and 6 cm, respectively. The non-dominant ankle was slightly larger at each level, but the difference was not statistically significant. Table I. Mean Sagittal Diameter, Mean Coronal Diameter, and Mean Calculated CSA of the ATs Measured by Ultrasound. Distance from Insertion (cm)  Mean Sagittal Diameter (mm)  Mean Coronal Diameter (mm)  Mean CSA (mm2)   0  4.4 ± 0.6  19.3 ± 1.6  66 ± 12  2  4.3 ± 0.5  14.7 ± 1.3  50 ± 8  4  4.2 ± 0.6  13.8 ± 1.2  46 ± 8  6  3.9 ± 0.6  14.5 ± 1.6  44 ± 8  Distance from Insertion (cm)  Mean Sagittal Diameter (mm)  Mean Coronal Diameter (mm)  Mean CSA (mm2)   0  4.4 ± 0.6  19.3 ± 1.6  66 ± 12  2  4.3 ± 0.5  14.7 ± 1.3  50 ± 8  4  4.2 ± 0.6  13.8 ± 1.2  46 ± 8  6  3.9 ± 0.6  14.5 ± 1.6  44 ± 8  Table I. Mean Sagittal Diameter, Mean Coronal Diameter, and Mean Calculated CSA of the ATs Measured by Ultrasound. Distance from Insertion (cm)  Mean Sagittal Diameter (mm)  Mean Coronal Diameter (mm)  Mean CSA (mm2)   0  4.4 ± 0.6  19.3 ± 1.6  66 ± 12  2  4.3 ± 0.5  14.7 ± 1.3  50 ± 8  4  4.2 ± 0.6  13.8 ± 1.2  46 ± 8  6  3.9 ± 0.6  14.5 ± 1.6  44 ± 8  Distance from Insertion (cm)  Mean Sagittal Diameter (mm)  Mean Coronal Diameter (mm)  Mean CSA (mm2)   0  4.4 ± 0.6  19.3 ± 1.6  66 ± 12  2  4.3 ± 0.5  14.7 ± 1.3  50 ± 8  4  4.2 ± 0.6  13.8 ± 1.2  46 ± 8  6  3.9 ± 0.6  14.5 ± 1.6  44 ± 8  DISCUSSION The AT endures substantial force during running and jumping, reaching loads in excess of 3,500 N.12 The stress (N/m2) imposed on the tendon is the force transmitted to the tendon divided by its CSA. These calculations demonstrate the extreme stress a high demand, and high-activity elite soldier applies to the AT while carrying a typical combat load ranging from 60 to 80 lb. Our subjects performed consistent physical activity over a minimum 3-yr period but did not show AT hypertrophy as compared with historic normal values.10 The current study is the first to our knowledge to demonstrate that elite soldier athletes participating in more than 20 h/wk of strenuous activity often with combat loads did not demonstrate increased CSA of the AT. The results of this study are nearly equivocal to those found in a comparable cohort of college athletes from North Carolina State University, USA, with similar BMI, which demonstrated an average CSA throughout the AT watershed of 42 ± 8 mm2.4 Our study results are also similar to a study of collegiate level, elite, hand ball players in Denmark with similar heights, which demonstrated an average AT diameter of 5 mm at 2 cm superior to the insertion.5 In a comparable study of college athletes, Chinese subjects who exercised more frequently (6 h/wk) had significantly larger CSA of the AT than those who exercised infrequently (1 h/wk).10 The authors also noted that the dominant AT had a larger although not significantly larger CSA than the non-dominant side. The authors concluded that the mean diameter of the AT at the level of the medial malleolus was 5.2 mm. In the current study, our results are similar but surprisingly smaller with our subjects’ dominant AT averaging 4.4 mm in diameter 4 cm above the insertion and our non-dominant side averaging 4.6 mm. The authors did not provide BMI nor height but do describe their subjects as having a smaller body build than Caucasians. These studies vary in population, activity level, and the methods in which ultrasound was used to measure AT CSA but demonstrate a difference of less than 1 mm in CSA. There is evidence that hypertrophy does occur in military recruits after their first 6 mo of training.13 This study utilized military recruits and measured CSA at 2.5 cm from the insertion before and after 6 mo of high-load military training. They found a statistically significant increase in CSA of the military recruits after training. With this in mind, our study may not have found hypertrophied ATs in our population because the population had already gone through their initial training. The correlation between activity level and AT CSA is called into question given the results of the current study. Our subject population has a regimented consistent training schedule from initial selection training to daily physical fitness challenges. Our subjects, unlike soccer players, consistently train in a bipedal manner. Soccer players have a significant difference between the dominant (kicking foot) and non-dominant AT (planting foot) with the non-dominant AT being thicker. In our population, there is no significant difference between the dominant and non-dominant ATs, which is likely a function of equal amounts of stress being put on their ATs. Additionally, our subjects apply larger amounts of consistent prolonged stress to the AT secondary to marching long distances while carrying heavy loads. We feel that the current study results more directly reflect that of bipedal sports allowing practitioners to monitor AT pathology and identify early hypertrophy in these populations. Early identification will allow for prophylactic activity modification and therapy striving to decrease chronic pain and the devastating consequences of acute rupture. The strengths of this study include sample size and consistency of participants: all men of similar stature participating in the same regimented exercise program weekly. Although the consistency of participants is a strength of this study, it is also a weakness in that this population does not represent the general population. An additional weakness of the study is the lack of a control group that either do not exercise or exercise in a non-bipedal manner. Our study also lacks intra–inter observer reliability as reported results were based on a consensus of three experts at the time of measurement, a study design previously used.10 In conclusion, this study provides baseline AT size at various locations while supporting the idea that hypertrophy may be pathologic rather than a normal physiologic response to stress. This is useful to practitioners in guiding the management of patient workload in those at high risk of AT tendinopathy. Further prospective research is necessary to evaluate this idea and should include a wider variety of participants to better represent the general population. Acknowledgements We would like to thank all the Rangers and CPT Frank Brown for his help and support. Presentation This research was presented as a podium presentation at the Society of Military Orthopaedic Surgeons (SOMOS) annual meeting in Scottsdale, Arizona, between December 11, 2017, and December 15, 2017; and as an ePoster at the American Orthopaedic Foot and Ankle Society's (AOFAS) annual meeting in Seattle, Washington, between July 12, 2017 and July 15, 2017. REFERENCES 1 Owens BD, Wolf JM, Seelig AD, et al.  : Risk factors for lower extremity tendinopathies in military personnel. Orthop J Sports Med  2013; 1( 1): 2325967113492707. Google Scholar CrossRef Search ADS PubMed  2 Cummins EJ, Anson BJ: The structure of the calcaneal tendon (of Achilles) in relation to orthopedic surgery, with additional observations on the plantaris muscle. Surg Gynecol Obstet  1946; 83: 107– 16. Google Scholar PubMed  3 Gwynne-Jones D, Sims M, Handcock D: Epidemiology and outcomes of acute Achilles tendon rupture with operative or nonoperative treatment using an identical functional bracing protocol. Foot Ankle Int  2011; 32: 337– 43. doi:10.3113/FAI.2011.0337. Google Scholar CrossRef Search ADS PubMed  4 Farris DJ, Trewartha G, McGuigan MP: Could intra-tendinous hyperthermia during running explain chronic injury of the human Achilles tendon? J Biomech  2011; 44: 822– 6. Google Scholar CrossRef Search ADS PubMed  5 Fredberg U, Bolvig L, Lauridsen A, Stengaard-Pedersen K: Influence of acute physical activity immediately before ultrasonographic measurement of Achilles tendon thickness. Scand J Rheumatol  2007; 36( 6): 488– 9. DOI:10.1080/03009740701607059. Google Scholar CrossRef Search ADS PubMed  6 Emerson C, Morrissey D, Perry M, Jalan R: Ultrasonographically detected changes in Achilles tendons and self reported symptoms in elite gymnasts compared with controls – An observational study. Manual Therapy  2010; 15: 37– 42. Google Scholar CrossRef Search ADS PubMed  7 Tardioli A, Malliaras P, Maffulli N: Immediate and short-term effects of exercise on tendon structure: biochemical, biomechanical and imaging responses. Br Med Bull  2012; 103: 169– 202. Google Scholar CrossRef Search ADS PubMed  8 Langberg H, Skovgaard D, Karamouzis M, Bulow J, Kjaer M: Metabolism and inflammatory mediators in the peritendinous space measured by microdialysis during intermittent isometric exercise in humans. J Physiol  1999; 515( Pt 3): 919– 27. Google Scholar CrossRef Search ADS PubMed  9 Eriksen H, Pajala A, Leppilahti J, Risteli J: Increased content of type III collagen at the rupture site of human Achilles tendon. J Orthop Res  2002; 20( 6): 1352– 7. Google Scholar CrossRef Search ADS PubMed  10 Ying M, Yeung E, Li B, Lui M, Tsoi CW: Sonographic evaluation of the size of Achilles tendon: the effect of exercise and dominance of the ankle. Ultrasound Med Biol  2003; 29: 637– 42. Google Scholar CrossRef Search ADS PubMed  11 Maffulli N, Waterston SW, Squair J, Reaper J, Douglas AS: Changing incidence of Achilles tendon rupture in Scotland: a 15-year study. Clin J Sport Med  1999; 9( 3): 157– 60. Google Scholar CrossRef Search ADS PubMed  12 Freedman B, Gordon J: Soslowsky. The Achilles tendon: fundamental properties and mechanisms governing healing. Muscles Ligaments Tendons J  2014; 4( 2): 245– 55. Google Scholar PubMed  13 Milgrom Y, Milgrom C, Altaras T, et al.  : Achilles tendons hypertrophy in response to high loading training. Foot Ankle Int  2014; 35( 12): 1303– 8. Google Scholar CrossRef Search ADS PubMed  Author notes The views expressed are solely those of the authors and do not reflect the official policy or position of the US Army, US Navy, US Air Force, the Department of Defense, or the US Government. © Association of Military Surgeons of the United States 2018. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Military Medicine Oxford University Press

Cross-sectional Area of the Achilles Tendon in a Cohort of Elite Military Warriors Using Standard Ultrasound Techniques

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Abstract

Abstract Background The prevalence of Achilles tendon (AT) pathology is common and can result in disability. Understanding normal AT properties can improve our ability to prevent AT injuries. We examined the cross-sectional area of the AT at multiple levels in an asymptomatic population of Army Rangers. Methods This is a prospective cohort study composed of 41 voluntarily recruited United States Army Rangers deployed in a combat theater. All subjects were members of the Ranger Regiment participating in more than 20 h of intense bipedal non-sport weekly training with no history of AT pathology. While standing, each subject had bilateral AT calcaneal tuberosity insertions (0 cm) marked, along with skin markings made at 2 cm, 4 cm, and 6 cm superior to the AT insertion. AT diameter was measured at each level in the coronal and sagittal planes using ultrasound. Results Mean sagittal diameter of the AT was 4.4 mm, 4.3 mm, 4.2 mm, and 3.9 mm at 0 cm, 2 cm, 4 cm, and 6 cm, respectively. Mean coronal diameter of the AT was 19.3 mm, 14.7 mm, 13.8 mm, and 14.5 mm at 0 cm, 2 cm, 4 cm, and 6 cm, respectively. The cross-sectional area was calculated as 0.66 cm2, 0.5 cm2, 0.46 cm2, and 0.44 cm2 at 0 cm, 2 cm, 4 cm, and 6 cm, respectively. Conclusion Our data suggest that increased non-sport activity may not increase the cross-sectional area of the AT. Identifying the normal diameter at multiple levels throughout the most commonly injured area may improve the provider’s ability to identify early disease processes and apply targeted interventions to help slow or prevent progression and possible rupture. Level of Evidence Level III–V. INTRODUCTION The Achilles tendon (AT) is the largest tendon in the body.1 The gastrocnemius is the largest and most superficial muscle within the superficial posterior compartment. It has medial and lateral heads originating from the corresponding femoral condyles crossing the knee posteriorly. The soleus muscle lies deep into the gastrocnemius and is broad and flat originating from the posterior aspect of the tibia and fibula. The gastrocnemius and soleus muscles glide independently with the gastrocnemius tendinous portion being two to three times longer than that of the soleus.2 The two tendons converge into the AT approximately 5–6 cm from its calcaneal tuberosity insertion. The tendon then rotates 90 degrees in the axial plane so that the medial fibers end posteriorly. The spiraling fibers create an energy-efficient elastic recoil, which also creates a stress riser 2–5 cm proximal to the insertion.3 This area of the tendon also coincides with the vascular watershed that accounts for 75% of acute Achilles ruptures.3 In the United States Military and competitive sports, cardiovascular endurance and strength are paramount to mission success. The benefits of intense training and tissue adaptation must also be weighed against the physiological changes to the musculoskeletal system. Elite American military units often train more than 20 h/wk while carrying an excess of 60–80 lb of additional combat equipment. These conditions substantially change the cyclical load and elastic recoil of the AT, which may cause secondary micro-tears and tissue changes resulting in tendon hypertrophy. Monitoring these changes using ultrasound-measured cross-sectional area (CSA) has been described, but the effects of physical activity on CSA remain unclear with conflicting outcomes of increased size and decreased size.4–7 Continued research to increase our knowledge base of normal AT properties can improve our ability to reduce and prevent future AT injuries. Surveillance and monitoring of the AT using ultrasound CSA could potentially allow customized training regimens while preventing costly overuse injuries, tendinopathies, and ruptures. The purpose of this study is to examine the CSA of the AT at multiple levels in an asymptomatic population of elite American military service members to determine the adaptive changes that occur with their intense training program. MATERIALS AND METHODS This study is a prospective cohort of 41 active duty United States Army Rangers from within the same organization who were voluntarily recruited to participate while deployed in a combat theater. The US Army Medical Research and Material Command Institutional Review Board approved the study. All subjects had a brief history performed by the senior author and were eligible if they were a member of the United States Army Ranger Regiment for at least 3 yr participating in more than 20 h of intense weekly training, passed a military physical fitness test within the past 6 mo, and passed a pre-deployment health assessment physical. Subjects were excluded if they had a history of Achilles tendinopathy, rupture, or fracture of the lower extremity as these subjects are three times more likely to develop Achilles tendinopathies.1 Subjects were also excluded if they had a history of metabolic conditions or inflammatory diseases, which may alter the AT structure.8 Our subjects had a standardized 2-h physical fitness regimen 6 d/wk, while participating in standardized daily military duties, which included an additional 8 h of intense physical activities. As an elite infantry unit, daily activities included, but were not limited to, carrying heavy body armor and backpacks (50–80 lb additional to bodyweight) while climbing and performing tactical drills. Physical activity level is paramount in evaluating the AT size and characteristics as subjects with frequent exercise have been previously found to have larger ATs.9,10 Outcomes were predetermined to include the sonographic measurement of the sagittal and coronal diameter of the AT bilaterally at the bone tendon interface (insertion 0 cm) 2 cm, 4 cm, and 6 cm proximally along the tendon. The subjects also obtained standard demographic information, which included age, height, weight, and dominant foot. The dominant foot was identified by asking the subjects which foot they would kick a ball with and that foot was chosen as the dominant foot. The dominant foot was identified as it has been previously reported that AT ruptures are more common on the left side, the planted side with right footed kicking.10,11 All ultrasound examinations were performed using the SonoSite NanoMaxx with a 10- to 5-MHz linear transducer (FUJIFILM SonoSite, Inc., Bothel, Washington, DC, USA) between 03:00 h and 04:00 h on non-mission days. All ultrasound examinations were performed in the standing position to ensure consistency in tendon tension, position, and to aid in measurements. Throughout all testing, the ultrasound depth and penetration was left on the factory setting for musculoskeletal imagining to ensure consistency. The Achilles bone tendon interface (insertion 0 cm) was marked and identified followed by identification of the 2 cm, 4 cm, and 6 cm points proximally along the tendon bilaterally (Fig. 1). The AT was initially scanned transversely with the transducer perpendicular to the tendon measuring the coronal diameter at each of the given levels (Fig. 2A & B). Then the transducer was aligned longitudinally, to the tendon allowing maximum echogenicity while measuring the anteroposterior diameter (Fig. 3A & B). This technique also allowed for a secondary verification of the proximity along the tendon using ultrasound measurements. The judgment of transverse and perpendicular transducer placement was based on visual alignment and clinical judgment of the senior author. Examination utilized standard ultrasound gel. A spacer pad was not used as it is not a standard practice within the orthopedic community. Figure 1. View largeDownload slide The insertion (0) was marked and identified followed by identification of the 2 cm, 4 cm, and 6 cm points proximally along the tendon; this was done bilaterally. Figure 1. View largeDownload slide The insertion (0) was marked and identified followed by identification of the 2 cm, 4 cm, and 6 cm points proximally along the tendon; this was done bilaterally. Figure 2. View largeDownload slide (A) The transducer was positioned perpendicular to the tendon, here at the 2-cm position and the coronal plane of the tendon was measured. (B) Utilizing the ultrasound’s ability to measure between two points the measurement in the coronal plane was made. Figure 2. View largeDownload slide (A) The transducer was positioned perpendicular to the tendon, here at the 2-cm position and the coronal plane of the tendon was measured. (B) Utilizing the ultrasound’s ability to measure between two points the measurement in the coronal plane was made. Figure 3. View largeDownload slide (A) The transducer is positioned parallel to the tendon to allow measurement of the tendon in the sagittal plane. (B) Utilizing the ultrasound’s ability to measure between two points the measurement in the sagittal plane was made. Figure 3. View largeDownload slide (A) The transducer is positioned parallel to the tendon to allow measurement of the tendon in the sagittal plane. (B) Utilizing the ultrasound’s ability to measure between two points the measurement in the sagittal plane was made. All examinations were performed by the same fellowship trained orthopedic foot and ankle surgeon and verified by an emergency medicine physician specially trained in ultrasound. To ensure consistency and accuracy of measurement, a third provider assisted to create a consensus on each measurement, a technique previously validated.10 The study was conducted over a 2-mo period during which time all subjects continued physical training and combat-related tasks. The combat environment and strict unit regulations also prevented any alcohol consumption within 30 d of the examination, as a previous study has demonstrated alcohol use as a preventable risk factor in AT tendinopathies.1 All subjects were on malaria prophylaxis, but no subjects were on fluoroquinolones during the testing. RESULTS In 41 male subjects, a total of 82 ATs were examined. The mean age of the cohort was 26 yr, 70 in. tall, with a mean weight of 187 pounds. The mean sagittal and coronal diameter of the AT is depicted in Table I along with the mean CSA. Mean sagittal diameter of the AT was 4.4 mm, 4.3 mm, 3.9 mm, and 4 mm at 0 cm, 2 cm, 4 cm, and 6 cm, respectively. Mean coronal diameter of the AT was 19.3 mm, 14.7 mm, 13.8 mm, and 14.5 mm at 0 cm, 2 cm, 4 cm, and 6 cm, respectively. The CSA was calculated at each respective level utilizing the following equation:   CSAofanOval=pi(a)(b)where a is the radius of the tendon in the sagittal plane and b is the radius in the coronal plane. The CSA was calculated as 66 ± 12 mm2, 50 ± 8 mm2, 46 ± 8 mm2, and 44 ± 8 mm2 at 0 cm, 2 cm, 4 cm, and 6 cm, respectively. The non-dominant ankle was slightly larger at each level, but the difference was not statistically significant. Table I. Mean Sagittal Diameter, Mean Coronal Diameter, and Mean Calculated CSA of the ATs Measured by Ultrasound. Distance from Insertion (cm)  Mean Sagittal Diameter (mm)  Mean Coronal Diameter (mm)  Mean CSA (mm2)   0  4.4 ± 0.6  19.3 ± 1.6  66 ± 12  2  4.3 ± 0.5  14.7 ± 1.3  50 ± 8  4  4.2 ± 0.6  13.8 ± 1.2  46 ± 8  6  3.9 ± 0.6  14.5 ± 1.6  44 ± 8  Distance from Insertion (cm)  Mean Sagittal Diameter (mm)  Mean Coronal Diameter (mm)  Mean CSA (mm2)   0  4.4 ± 0.6  19.3 ± 1.6  66 ± 12  2  4.3 ± 0.5  14.7 ± 1.3  50 ± 8  4  4.2 ± 0.6  13.8 ± 1.2  46 ± 8  6  3.9 ± 0.6  14.5 ± 1.6  44 ± 8  Table I. Mean Sagittal Diameter, Mean Coronal Diameter, and Mean Calculated CSA of the ATs Measured by Ultrasound. Distance from Insertion (cm)  Mean Sagittal Diameter (mm)  Mean Coronal Diameter (mm)  Mean CSA (mm2)   0  4.4 ± 0.6  19.3 ± 1.6  66 ± 12  2  4.3 ± 0.5  14.7 ± 1.3  50 ± 8  4  4.2 ± 0.6  13.8 ± 1.2  46 ± 8  6  3.9 ± 0.6  14.5 ± 1.6  44 ± 8  Distance from Insertion (cm)  Mean Sagittal Diameter (mm)  Mean Coronal Diameter (mm)  Mean CSA (mm2)   0  4.4 ± 0.6  19.3 ± 1.6  66 ± 12  2  4.3 ± 0.5  14.7 ± 1.3  50 ± 8  4  4.2 ± 0.6  13.8 ± 1.2  46 ± 8  6  3.9 ± 0.6  14.5 ± 1.6  44 ± 8  DISCUSSION The AT endures substantial force during running and jumping, reaching loads in excess of 3,500 N.12 The stress (N/m2) imposed on the tendon is the force transmitted to the tendon divided by its CSA. These calculations demonstrate the extreme stress a high demand, and high-activity elite soldier applies to the AT while carrying a typical combat load ranging from 60 to 80 lb. Our subjects performed consistent physical activity over a minimum 3-yr period but did not show AT hypertrophy as compared with historic normal values.10 The current study is the first to our knowledge to demonstrate that elite soldier athletes participating in more than 20 h/wk of strenuous activity often with combat loads did not demonstrate increased CSA of the AT. The results of this study are nearly equivocal to those found in a comparable cohort of college athletes from North Carolina State University, USA, with similar BMI, which demonstrated an average CSA throughout the AT watershed of 42 ± 8 mm2.4 Our study results are also similar to a study of collegiate level, elite, hand ball players in Denmark with similar heights, which demonstrated an average AT diameter of 5 mm at 2 cm superior to the insertion.5 In a comparable study of college athletes, Chinese subjects who exercised more frequently (6 h/wk) had significantly larger CSA of the AT than those who exercised infrequently (1 h/wk).10 The authors also noted that the dominant AT had a larger although not significantly larger CSA than the non-dominant side. The authors concluded that the mean diameter of the AT at the level of the medial malleolus was 5.2 mm. In the current study, our results are similar but surprisingly smaller with our subjects’ dominant AT averaging 4.4 mm in diameter 4 cm above the insertion and our non-dominant side averaging 4.6 mm. The authors did not provide BMI nor height but do describe their subjects as having a smaller body build than Caucasians. These studies vary in population, activity level, and the methods in which ultrasound was used to measure AT CSA but demonstrate a difference of less than 1 mm in CSA. There is evidence that hypertrophy does occur in military recruits after their first 6 mo of training.13 This study utilized military recruits and measured CSA at 2.5 cm from the insertion before and after 6 mo of high-load military training. They found a statistically significant increase in CSA of the military recruits after training. With this in mind, our study may not have found hypertrophied ATs in our population because the population had already gone through their initial training. The correlation between activity level and AT CSA is called into question given the results of the current study. Our subject population has a regimented consistent training schedule from initial selection training to daily physical fitness challenges. Our subjects, unlike soccer players, consistently train in a bipedal manner. Soccer players have a significant difference between the dominant (kicking foot) and non-dominant AT (planting foot) with the non-dominant AT being thicker. In our population, there is no significant difference between the dominant and non-dominant ATs, which is likely a function of equal amounts of stress being put on their ATs. Additionally, our subjects apply larger amounts of consistent prolonged stress to the AT secondary to marching long distances while carrying heavy loads. We feel that the current study results more directly reflect that of bipedal sports allowing practitioners to monitor AT pathology and identify early hypertrophy in these populations. Early identification will allow for prophylactic activity modification and therapy striving to decrease chronic pain and the devastating consequences of acute rupture. The strengths of this study include sample size and consistency of participants: all men of similar stature participating in the same regimented exercise program weekly. Although the consistency of participants is a strength of this study, it is also a weakness in that this population does not represent the general population. An additional weakness of the study is the lack of a control group that either do not exercise or exercise in a non-bipedal manner. Our study also lacks intra–inter observer reliability as reported results were based on a consensus of three experts at the time of measurement, a study design previously used.10 In conclusion, this study provides baseline AT size at various locations while supporting the idea that hypertrophy may be pathologic rather than a normal physiologic response to stress. This is useful to practitioners in guiding the management of patient workload in those at high risk of AT tendinopathy. Further prospective research is necessary to evaluate this idea and should include a wider variety of participants to better represent the general population. Acknowledgements We would like to thank all the Rangers and CPT Frank Brown for his help and support. Presentation This research was presented as a podium presentation at the Society of Military Orthopaedic Surgeons (SOMOS) annual meeting in Scottsdale, Arizona, between December 11, 2017, and December 15, 2017; and as an ePoster at the American Orthopaedic Foot and Ankle Society's (AOFAS) annual meeting in Seattle, Washington, between July 12, 2017 and July 15, 2017. REFERENCES 1 Owens BD, Wolf JM, Seelig AD, et al.  : Risk factors for lower extremity tendinopathies in military personnel. Orthop J Sports Med  2013; 1( 1): 2325967113492707. Google Scholar CrossRef Search ADS PubMed  2 Cummins EJ, Anson BJ: The structure of the calcaneal tendon (of Achilles) in relation to orthopedic surgery, with additional observations on the plantaris muscle. Surg Gynecol Obstet  1946; 83: 107– 16. Google Scholar PubMed  3 Gwynne-Jones D, Sims M, Handcock D: Epidemiology and outcomes of acute Achilles tendon rupture with operative or nonoperative treatment using an identical functional bracing protocol. Foot Ankle Int  2011; 32: 337– 43. doi:10.3113/FAI.2011.0337. Google Scholar CrossRef Search ADS PubMed  4 Farris DJ, Trewartha G, McGuigan MP: Could intra-tendinous hyperthermia during running explain chronic injury of the human Achilles tendon? J Biomech  2011; 44: 822– 6. Google Scholar CrossRef Search ADS PubMed  5 Fredberg U, Bolvig L, Lauridsen A, Stengaard-Pedersen K: Influence of acute physical activity immediately before ultrasonographic measurement of Achilles tendon thickness. Scand J Rheumatol  2007; 36( 6): 488– 9. DOI:10.1080/03009740701607059. Google Scholar CrossRef Search ADS PubMed  6 Emerson C, Morrissey D, Perry M, Jalan R: Ultrasonographically detected changes in Achilles tendons and self reported symptoms in elite gymnasts compared with controls – An observational study. Manual Therapy  2010; 15: 37– 42. Google Scholar CrossRef Search ADS PubMed  7 Tardioli A, Malliaras P, Maffulli N: Immediate and short-term effects of exercise on tendon structure: biochemical, biomechanical and imaging responses. Br Med Bull  2012; 103: 169– 202. Google Scholar CrossRef Search ADS PubMed  8 Langberg H, Skovgaard D, Karamouzis M, Bulow J, Kjaer M: Metabolism and inflammatory mediators in the peritendinous space measured by microdialysis during intermittent isometric exercise in humans. J Physiol  1999; 515( Pt 3): 919– 27. Google Scholar CrossRef Search ADS PubMed  9 Eriksen H, Pajala A, Leppilahti J, Risteli J: Increased content of type III collagen at the rupture site of human Achilles tendon. J Orthop Res  2002; 20( 6): 1352– 7. Google Scholar CrossRef Search ADS PubMed  10 Ying M, Yeung E, Li B, Lui M, Tsoi CW: Sonographic evaluation of the size of Achilles tendon: the effect of exercise and dominance of the ankle. Ultrasound Med Biol  2003; 29: 637– 42. Google Scholar CrossRef Search ADS PubMed  11 Maffulli N, Waterston SW, Squair J, Reaper J, Douglas AS: Changing incidence of Achilles tendon rupture in Scotland: a 15-year study. Clin J Sport Med  1999; 9( 3): 157– 60. Google Scholar CrossRef Search ADS PubMed  12 Freedman B, Gordon J: Soslowsky. The Achilles tendon: fundamental properties and mechanisms governing healing. Muscles Ligaments Tendons J  2014; 4( 2): 245– 55. Google Scholar PubMed  13 Milgrom Y, Milgrom C, Altaras T, et al.  : Achilles tendons hypertrophy in response to high loading training. Foot Ankle Int  2014; 35( 12): 1303– 8. Google Scholar CrossRef Search ADS PubMed  Author notes The views expressed are solely those of the authors and do not reflect the official policy or position of the US Army, US Navy, US Air Force, the Department of Defense, or the US Government. © Association of Military Surgeons of the United States 2018. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices)

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Military MedicineOxford University Press

Published: Mar 14, 2018

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