Abstract
Aim: Tendon xanthomas are part of the clinical triad of diagnostic criteria for familial hypercholesterolemia (FH) in Japan. The Achilles tendon generally has a twisted structure, and we investigated the impact of torsion on Achilles tendon thickness (ATT) assessment.
Methods: In this single-center retrospective study, 61 acute coronary syndrome (ACS) patients who underwent ATT assessment using radiography (ATT-Xp) and ultrasonography (ATT-US) were analyzed. Ultrasonographic ATT assessment used two axes - antero-posterior axis (ATT-US (AP)) and corrected axis according to Achilles tendon torsion (ATT-US (correct)) - and the torsion angle was measured. The association of torsion with each ATT assessment was investigated.
Results: The torsion angle of the Achilles tendon varied widely. Both ATT-US (AP) and ATT-US (correct) were significantly correlated with ATT-Xp, although the correlation between ATT-Xp and ATT-US (correct) was modest compared to the correlation with ATT-US (AP) (ATT-US (AP)-Right: r= 0.91,p＜0.001, Left: r= 0.91,p＜0.001; ATT-US (correct)-Right: r = 0.82,p＜0.001, Left: r = 0.76,p＜0.001, respectively). Torsion angle was well correlated with the differences in ATT between ATT-Xp and ATT-US (correct) (Right: r= 0.62,p＜0.001, Left: r= 0.66,p＜0.001). There were no independent factors associated with Achilles tendon torsion.
Conclusion: This is the first study to quantitatively evaluate the three-dimensional twisted structure of the Achilles tendon and demonstrate that Achilles tendon torsion is associated with the difference between ATT-Xp and ATT-US (correct). Torsion of the Achilles tendon should be considered in Achilles tendon assessment, particularly radiographical assessment.
Keywords: Achilles tendon thickness, Achilles tendon torsion, Ultrasonography, Axis correction, Familial hypercholesterolemia
See editorial vol. 33: 382-383
Abbreviations: ACS: Acute coronary syndrome, ASCVD: Atherosclerotic cardiovascular disease, ATT: Achilles tendon thickness, ATT-US (AP): Ultrasonographic ATT with antero-posterior axis, ATT-US (correct): Ultrasonographic ATT with corrected axis, ATT-Xp: Radiographic assessment of ATT, BMI: Body mass index, CKD: Chronic kidney disease, FH: Familial hypercholesterolemia, HDL-C: High density lipoprotein cholesterol, ICC: Intraclass correlation coefficient, JAS: Japan Atherosclerosis Society, LDL-C: Low-density lipoprotein cholesterol, Lp(a): Lipoprotein(a), NSTEMI: Non–ST-elevation myocardial infarction, ROC: Receiver-operating characteristic , STEMI: ST-elevation myocardial infarction, UAP: Unstable angina pectoris, US: Ultrasonography, Xp: Radiography
Background
Tendon xanthomas are specific characteristic manifestations of familial hypercholesterolemia (FH), a common genetic cause of premature atherosclerotic cardiovascular disease (ASCVD) due to lifelong elevated plasma low-density lipoprotein cholesterol (LDL-C) concentrations. The Achilles tendon is the most typical location for tendon xanthomas to develop, which result from lipid accumulation in peripheral tissues, where long-term exposure to elevated LDL-C from early childhood increases cholesterol deposition in tendons, particularly those subject to mechanical stress and inflammation 1 , 2) . Given that Achilles tendon xanthomas are recognized as an important diagnostic feature, the Dutch Lipid Clinic Network criteria 3) , Simon-Broome diagnostic criteria 4) and Make Early Diagnosis to Prevent Early Death 5) include tendon xanthomas in their diagnostic criteria for FH. Achilles tendon thickness (ATT) has been commonly assessed by radiography in the diagnostic evaluation of FH 1 , 6) , whereas Achilles tendon assessment using ultrasonography has emerged recently and been increasingly reported on 7 - 10) . Achilles tendon assessment using ultrasonography is now included in the Japan Atherosclerosis Society FH guideline (JAS guideline) as a diagnostic criterion 11) .
The Achilles tendon is anatomically known to have a three-dimensional torsional structure and an oval shape 12 - 14) . Therefore, proper assessment of the Achilles tendon using the correct axis is essential since inappropriate assessment can lead to over- or under-diagnosis of FH. In addition, the JAS FH diagnostic criteria state differences in the thresholds for ATT between radiography and ultrasonography, where ATT is defined as ≥ 8 mm for males and ≥ 7.5 mm for females for radiography and ≥ 6 mm for males and ≥ 5.5 mm for females for ultrasonography. Thus, there is a 2 mm discordance in the diagnostic cutoff values between radiography and ultrasonography for Achilles tendon assessment 8) . This may be due to an unclear boundary between the Achilles tendon and skin in assessment using radiography, in spite of adjusting voltage and other parameters to increase contrast. Hence, skin and subcutaneous tissue can be included in ATT assessment using radiography. Another reason is twisting or torsion of the Achilles tendon between the calcaneus and soleus muscle. Considering that ultrasonography enables detection and assessment of the Achilles tendon perpendicularly, ultrasonography would potentially be more accurate than radiography in assessment.
If there is some degree of Achilles tendon torsion, a lateral shadow in assessment using radiography may lead to overestimation of thickness. On the other hand, if there is no torsion of the Achilles tendon, the greater threshold for ATT with radiography stated in the guideline could result in underdiagnosis. Therefore, we speculated that torsion of the Achilles tendon partially contributes to the differences in ATT between radiography and ultrasonography. In the current study, we evaluated ATT using radiography and ultrasonography, and investigated the impact of Achilles tendon torsion.
Methods
Study Population and Definitions
A total of 61 patients with acute coronary syndrome (ACS) who underwent Achilles tendon measurement using both radiography and ultrasonography between November 2024 and June 2025 at Osaka Medical and Pharmaceutical University Hospital were retrospectively investigated in the current study. ACS was defined as ST-elevation myocardial infarction (STEMI), non–ST-elevation myocardial infarction (NSTEMI), or unstable angina pectoris (UAP). FH was diagnosed according to the 2022 criteria of the Japan Atherosclerosis Society 11) . They include the following: subjects who fulfill at least 2 of the clinical characteristics among (1) untreated LDL-C level ≥ 180 mg/dL, (2) tendon xanthoma (tendon xanthoma on the dorsal hands, elbows, and knees, or Achilles tendon thickening) or nodular xanthoma on the skin, and (3) a history of familial hypercholesterolemia or premature CAD within first degree relatives. Based on the guideline, ATT was defined as ≥ 8.0 mm in men or ≥ 7.5 mm in women on radiography, or ≥ 6.0 mm in men or ≥ 5.5 mm in women on ultrasonography. Patients with systolic blood pressure of ≥ 140 mmHg, and/or diastolic blood pressure ≥ 90 mmHg, or use of antihypertensive medications were considered to have hypertension. Diabetes was defined based on the Japan Diabetes Society’s definition of diabetes 15) , or the use of diabetes medication. Current smoking habit assessed by medical interview was considered as smoking status. This study was approved by the Ethics Review Board of Osaka Medical and Pharmaceutical University (2024-170). Written informed consent was not mandatory for this observational and retrospective study. All procedures were conducted in accordance with the ethical standards of the responsible committee on human experimentation (institutional and national) and the 1975 Declaration of Helsinki, revised in 2008.
Achilles Tendon Thickness Measurement
ATT was measured using radiography and ultrasonography. For radiographic ATT assessment (ATT-Xp), standard lateral ankle radiographs were obtained with the ankle in a neutral position. On these lateral images, ATT-Xp was defined as the maximal antero-posterior tendon thickness. The ultrasonographic ATT assessment (ATT-US) was performed using an ultrasound system (5500CV; Koninklijke Philips N.V., the Netherlands) with a linear transducer (L12-5; 12–5 MHz) by 2 methods: (1) ultrasonographic ATT using antero-posterior axis [ATT-US (AP)] ( Fig.1a, b ) and (2) ultrasonographic ATT using corrected axis [ATT-US (correct)] ( Fig.1c ) . Participants were examined in the prone position with the ankle in neutral. The transducer was positioned perpendicular to the Achilles tendon to obtain maximally magnified, transversely oriented cross-sectional images. The imaging depth was set to the shallowest available setting, the overall gain was fixed at 50%, and a single focal zone was placed at the mid-tendon. All other presets (including harmonic/compound imaging and speckle reduction) were left at the system defaults. These parameters were kept constant for all acquisitions. ATT-US (AP) was acquired under these conditions but without axis correction, and measured the distance between the Achilles tendon’s upper and lower edges at the thickest portion. ATT-US (correct) was obtained by rotating the probe to align the imaging plane to be perpendicular to the tendon long-axis (axis correction), measuring the maximal tendon thickness on a short-axis image at the thickest portion ( Fig.1c ) . In a subset of the patients, ATT measurement was not performed due to logistical or clinical factors, including inability to assume the prone position, moderate-to-severe cognitive impairment or early discharge upon patient request. Torsion of the Achilles tendon was defined as the deviation between the antero-posterior axis and corrected axis. The angle of the torsion was calculated using software (NazcaView; Nihon Kohden Corporation, Tokyo, Japan). All measurements were performed by two trained and certified physicians to ensure consistency and minimize inter-observer variability. Inter-observer agreement for ATT-US and Achilles tendon torsion were high, with an intraclass correlation coefficient (ICC) of 0.92 and 0.83, respectively between the two examiners.
Fig.1. Achilles tendon assessment using radiography and ultrasonography.
(a): Representative case of Achilles tendon without torsion. Achilles tendon thickness (ATT) measurement using radiography is the same as that for ultrasonography.
(b): Representative case of Achilles tendon with torsion. Ultrasonography device is positioned perpendicular to foot.
(c): Representative case of Achilles tendon with torsion. Ultrasonography is positioned perpendicular to Achilles tendon. Owing to torsion of the Achilles tendon, (ATT) using radiography (ATT-Xp) is greater than ATT using ultrasonography with corrected axis (ATT-US (correct)).
Statistical Analysis
All statistical analyses were performed using JMP Pro version 18.2.2 (SAS Institute Inc., Cary, NC, USA).
Categorical data were summarized as frequencies and percentages and compared using the Pearson χ2 test. Continuous variables were summarized as mean±SD if normally distributed, or as median (interquartile range) if non-normally distributed, and compared using the Mann–Whitney U test. Multivariable linear regression was used to calculate standardized β after controlling simultaneously for potential confounders with the continuous torsion angle as the dependent variable. All statistically significant variables in univariable linear regression were considered for inclusion and a backwards selection method at alpha=0.05 was used to keep the variables in the final multivariable models. To examine the ability of torsion angle to predict a 2 mm difference in ATT measurement between radiography and ultrasonography, receiver-operating characteristic (ROC) curve analyses and calculations of sensitivity and specificity were performed. The best cutoff value for torsion angle was determined using the Youden index method.
Results
Patient Characteristics and Achilles Tendon Measurements
A total of 61 subjects with ACS were evaluated in the current study. Patient characteristics are summarized in Table 1 : 80.3% were male, and the mean age was 68.5 years. Five patients were clinically diagnosed with FH based on the JAS guideline 11) . The Achilles tendon measurements acquired by radiography and ultrasonography are presented in Table 2 . Mean ATT measured by radiography and ultrasonography (AP and correct) were 6.6±1.1 mm on the left and 6.5±1.1 mm on the right (ATT-Xp), 6.7±1.3 mm on the left and 6.6±1.2 mm on the right. (ATT-US (AP)), and 5.1±1.1 mm on the left and 5.0±1.0 mm on the right (ATT-US (correct)), respectively. The mean torsion angle of the Achilles tendon was 14.4° (range: 4–31°) on the left and 14.8° (range: 4–40°) on the right. The distribution of Achilles tendon torsion was demonstrated in the Supplementary Fig.1 .
Table 1. Baseline patient characteristics.
| Total (n = 61) | |
|---|---|
| Age (years) | 68.5±11.6 |
| Male, n (%) | 49 (80.3) |
| Height (cm) | 164.4±9.0 |
| Weight (kg) | 66.3±14.4 |
| BMI (kg/m2) | 24.4±4.1 |
| Hypertension, n (%) | 39 (63.9) |
| Dyslipidemia, n (%) | 20 (32.8) |
| Diabetes, n (%) | 19 (31.1) |
| CKD, n (%) | 6 (9.8) |
| Smoking, n (%) | 46 (75.4) |
| FH, n (%) | 5 (8.2) |
| Diagnosis | |
| STEMI, n (%) | 34 (55.8) |
| NSTEMI, n (%) | 21 (34.4) |
| UAP, n (%) | 6 (9.8) |
| Biochemistry data | |
| LDL-C (mg/dL) | 106.5±35.2 |
| HDL-C (mg/dL) | 51.0±13.8 |
| Total cholesterol (mg/dL) | 178.8±38.7 |
| Triglycerides (mg/dL) | 100.0 (71.5, 127.0) |
| Lp(a) (mg/dL) | 15 (7, 29) |
| HbA1c (%) | 6.2±0.8 |
Values are expressed mean±SD or median (interquartile range)
BMI, body mass index; CKD, chronic kidney disease; FH, familial hypercholesterolemia; LDL-C, low
density lipoprotein cholesterol; HDL-C, high density lipoprotein cholesterol; Lp(a), lipoprotein (a)
Table 2. The Achilles tendon measurements.
| Total (n = 61) | |
|---|---|
| Right side | |
| ATT-Xp (mm) | 6.5±1.1 |
| ATT-US (AP) (mm) | 6.6±1.2 |
| ATT-US (correct) (mm) | 5.0±1.0 |
| ΔATT between Xp and US (AP) (mm) | 0.4±0.3 |
| ΔATT between Xp and US (correct) (mm) | 1.6±0.7 |
| Torsion angle (°) | 14.8±7.3 |
| Left side | |
| ATT-Xp (mm) | 6.6±1.1 |
| ATT-US (AP) (mm) | 6.7±1.3 |
| ATT-US (correct) (mm) | 5.1±1.1 |
| ΔATT between Xp and US (AP) (mm) | 0.4±0.3 |
| ΔATT between Xp and US (correct) (mm) | 1.5±0.7 |
| Torsion angle (°) | 14.4±6.2 |
AT, Achilles tendon; ATT, Achilles tendon thickness; X-ray, radiography; US, ultrasonography
Supplementary Fig.1. Distribution of Achilles tendon torsion.
(a) Right side (b) Left side
Correlations between ATT-Xray and ATT-US
On both the right and left sides, ATT-Xp was significantly correlated with ATT-US (AP) (Rt: r = 0.91, p＜0.001, Lt: r = 0.91, p＜0.001, respectively) and ATT-US (correct) (Rt: r = 0.82, p＜0.001, Lt: r = 0.76, p＜0.001, respectively). However, the correlation between ATT-Xp and ATT-US (correct) was modest compared to that for ATT-US (AP) ( Fig.2 ) . Fig.3 shows the correlation between the torsion angle of the Achilles tendon and the differences in ATT between ATT-Xp and ATT-US (correct). The torsion angle of the Achilles tendon was well correlated with the differences in ATT between ATT-Xp and ATT-US (correct) (Rt: r = 0.62, p＜0.001, Lt: r = 0.66, p＜0.001)). ROC curve analysis further demonstrated that a torsion angle of 19° was the threshold for predicting the ≥ 2 mm difference between ATT-Xp and ATT-US (correct) defined by the Japanese Atherosclerosis Society. The AUC was 0.87, with a sensitivity of 0.73 and specificity of 0.91 for the left side, and 0.82 with a sensitivity of 0.73 and specificity of 0.87 for the right side ( Supplementary Fig.2 ) . Supplementary Fig.3 shows a Venn diagram illustrating the detection of Achilles tendon thickening using radiography and ultrasonography. In total, Achilles tendon thickening was detected in 17 tendons: 7 by both radiography and ultrasonography, 3 by radiography alone, and 7 by ultrasonography alone.
Fig.2. Correlation between ATT-Xp, and ATT-US (AP) and ATT-US (correct).
In the right foot, both ATT-US (AP) and ATT-US (correct) were correlated with ATT-Xp (ATT-US (AP): r = 0.91, p＜0.001; ATT-US (correct): r = 0.76, p＜0.001, respectively). In the left foot, both ATT-US (AP) and ATT-US (correct) were correlated with ATT-Xp (ATT-US (AP): r = 0.91, p＜0.001; ATT-US (correct): r = 0.82, p＜0.001, respectively).
Fig.3. Relationship between torsion angle and differences between ATT-Xp and ATT-US (correct).
Torsion angle was significantly correlated with the differences between ATT-Xp and ATT-US (correct) (Right: r = 0.66, p＜0.001, Left: r = 0.62, p＜0.001, respectively).
Supplementary Fig.2.
Receiver operating curve (ROC) analysis further demonstrated that the cut-off value of torsion angle for predicting a ≥ 2 mm difference between ATT-Xp and ATT-US (correct) was 19° for both sides
Supplementary Fig.3. Venn diagram showing detection of Achilles tendon thickening using radiography and ultrasonography based on the JAS FH criteria.
Achilles tendon thickening was detected in 17 tendons: 7 by both radiography and ultrasonography, 3 by radiography alone, and 7 by ultrasonography alone. In subjects without any torsion of the Achilles tendon, ATT-Xp and ATT-US should theoretically be the same. Using the JAS FH criteria, a non-thickened AT could be identified by radiographical assessment when there was no torsion, particularly in subjects with an ATT of 5.5 – 7.5 mm for women or 6.0 – 8.0 mm for men. However, as the torsion of the Achilles tendon becomes greater, the difference between ATT-Xp and ATT-US would increase and by radiography assessment, the ATT would tend to be overestimated.
Uni- and Multivariable Linear Regression Analysis for Determinants of Achilles Tendon Torsion Angle
On univariable linear regression, age (β = −0.30, p = 0.02), sex (female) (β = −0.29, p = 0.02), and triglycerides (β = 0.27, p = 0.03) were significantly associated with the torsion angle. The multivariable linear model with simultaneous adjustment for covariates revealed that there were no factors independently associated with torsion angle (age: β = −0.20, p = 0.15; sex (female): β = −0.21, p = 0.11; BMI: β = −0.05, p = 0.72; HT: β = 0.10, p = 0.46, Triglycerides: β = 0.24, p = 0.06) ( Table 3 ) .
Table 3. Uni- and Multivariable linear regression analysis for determinants of the Achilles tendon torsion angle.
| Univariate Analysis | Multivariate Analysis | |||
|---|---|---|---|---|
| β | p value | β | p value | |
| Age | -0.30 | 0.02 | -0.20 | 0.15 |
| Sex (Female) | -0.29 | 0.02 | -0.21 | 0.11 |
| Height | 0.19 | 0.15 | ||
| Weight | 0.14 | 0.27 | ||
| BMI | 0.08 | 0.56 | -0.05 | 0.72 |
| Hypertension | 0.19 | 0.14 | 0.10 | 0.46 |
| Dyslipidemia | 0.20 | 0.12 | ||
| Diabetes | 0.01 | 0.91 | ||
| CKD | 0.05 | 0.73 | ||
| Smoking | 0.07 | 0.62 | ||
| FH | 0.18 | 0.16 | ||
| LDL-C | 0.14 | 0.29 | ||
| HDL-C | 0.13 | 0.34 | ||
| Total cholesterol | 0.24 | 0.06 | ||
| Triglycerides | 0.27 | 0.03 | 0.24 | 0.06 |
| Lp(a) | 0.05 | 0.72 | ||
β, standardized regression coefficient; BMI, body mass index; CKD, chronic kidney disease; FH, familial hypercholesterolemia; HDL-C, high density lipoprotein cholesterol; LDL-C, low density lipoprotein cholesterol; Lp(a), lipoprotein(a)
Discussion
Achilles tendon thickening, a highly specific clinical finding in FH, is crucial for the diagnosis of FH, as well as for assessment of cardiovascular risk 16 , 17) . However, the absolute differences in the ATT thresholds between radiography and ultrasonography in the JAS FH criteria need to be considered. In the current study, we quantitatively assessed the three-dimensional torsional structure of the Achilles tendon using radiography and ultrasonography, and investigated the effect of torsion on the difference in ATT measurements between radiography and ultrasonography. Our results showed that ATT-US (AP) was significantly correlated with ATT-Xp, whereas the correlation between ATT-US (correct) and ATT-Xp was modest compared with that for ATT-US (AP). The differences between ATT-Xp and ATT-US (correct) were proportional to the torsion angle of the Achilles tendon. An Achilles tendon torsion angle of 19° was the threshold for predicting a 2 mm-difference between ATT-Xp and ATT-US (correct). There were no independent predictors of acquired factors associated with torsion of the Achilles tendon.
Tendon xanthomas occur most frequently in areas subject to mechanical stress, in particular elbows, knees, wrists and buttocks. Given that the Achilles tendon is the strongest and thickest tendon in the body, it is the most typical location for tendon xanthomas to develop. The FH diagnostic criteria in the JAS guideline are the only ones in the world to include Achilles tendon thickening in addition to presence of tendon xanthomas. In the clinical setting, ATT has been assessed by radiography in the diagnostic evaluation of FH for decades 18) . Recently, ultrasonographic assessment has also been introduced and demonstrated its potential to detect tendon xanthomas more accurately compared to radiography 19) . Subsequently, ultrasonographic ATT thresholds for the diagnosis of FH were investigated and adopted in the JAS FH criteria 20 , 21) . However, in the JAS FH criteria, the diagnostic cutoffs for FH differ between these two modalities. Specifically, ATT is defined as ≥ 8 mm for males and ≥ 7.5 mm for females on radiography or ≥ 6 mm for males and ≥ 5.5 mm for females on ultrasonography, and a recent study found that the ATT assessment by radiography was in fact greater than that by ultrasonography 22) .
There are several reasons for this discrepancy. One possibility is an unclear boundary between the Achilles tendon and skin in assessment using radiography, in spite of adjusting voltage and other parameters to increase contrast. Therefore, skin and subcutaneous tissue could be included in radiographical ATT assessments, which would lead to overestimation of ATT. Another possibility is twisting or distortion of the Achilles tendon between the calcaneus and soleus muscle. Given that the Achilles tendon generally presents an oval shape, inappropriate assessment can also lead to overestimation of ATT. As radiographical images for assessment of the Achilles tendon are taken laterally along the side of the foot, assessment by radiography does not always identify the axis perpendicular to the Achilles tendon correctly, particularly in the case of distortion of the Achilles tendon. In the current study, the correlation of ATT-Xp was greater with ATT-US (AP) than ATT-US (correct), suggesting that ultrasonographical assessment using an uncorrected antero-posterior axis measures a similar segment of the Achilles tendon to that for radiography ( Fig.1a and b ) . However, ultrasonography using a corrected axis enabled us to observe the Achilles tendon perpendicularly and measure the Achilles tendon segment more appropriately ( Fig.1c ) . Furthermore, the torsion angle of the Achilles tendon was proportional to the differences between ATT-Xp and ATT-US (correct), which further supports our speculation that torsion of the Achilles tendon could partially contribute to overestimation in radiographical ATT assessment. Therefore, it is crucial to evaluate the Achilles tendon perpendicularly to obtain the correct ATT.
Using the current cut-off values of radiographical ATT assessment in the JAS FH criteria, subjects without torsion - in particular those with an ATT of 6.0 – 8.0 mm for men and 5.5 – 7.5 mm for women -could be determined to be non-thickening cases of the Achilles tendon, whereas for individuals with torsion of the Achilles tendon, as it becomes greater, the difference between ATT-Xp and ATT-US would increase and radiographical assessment would tend to overestimate ATT. Our findings demonstrated that there were several discordances in the diagnosis of Achilles tendon thickening (Suppl 2). Thus, Achilles tendon thickening identified by only radiography has the potential for overdiagnosis in subjects with torsion, whereas for subjects with no torsion, those determined to be ATT thickening cases by ultrasonography could be determined to be non-thickening cases by radiography. These findings underscore a risk of misclassification when relying on radiography alone and are consistent with a previous study demonstrating that ultrasonography increases the detection of tendon xanthomas in HeFH 7) . Therefore, ultrasonography may be a promising alternative in Achilles tendon assessment, for improving accuracy and avoiding mis-diagnosis of FH as the impact of Achilles tendon torsion on variability in ATT assessment can be eliminated by ultrasonography. Another notable advantage of ultrasonography is that it does not require radiation. Furthermore, the present study clearly demonstrated the limitations of X-ray-based assessment for the Achilles tendon. However, further investigation including a larger population would be of value in elucidating the impact of the Achilles tendon torsion on ATT assessment.
The Achilles tendon, which consists of fascicles that originate from the medial head of the gastrocnemius, lateral head of the gastrocnemius and soleus muscle, is generally thought to have a twisted structure and the fascicles become fused in twisting. Previously, the twisted structure of the Achilles tendon was reported to be already seen in the second trimester of the fetus, and was similar to that seen in adults 23 - 25) . This indicates that, unlike Achilles tendon thickening, its torsion is not affected by acquired factors, such as age, sex, BMI, mechanical stress or LDL-C, so torsion of the Achilles tendon is a relatively stable individual characteristic. In the current study, there were no independent factors associated with the degree of torsion of the Achilles tendon. Therefore, we should definitely take torsion of the Achilles tendon into consideration, particularly when assessing it with radiography.
Limitations
The current study has several limitations. First, it involved a retrospective, observational analysis performed at a single center and the number of subjects was relatively small. Second, it included only Japanese patients with ACS. Whether this analysis can be translated to non-Japanese or a healthy population warrants further investigation. Third, structural changes of the Achilles tendon, particularly softness and calcification, were not assessed in this study. These factors may or may not affect torsion of the Achilles tendon, hence these perspectives need to be investigated to elucidate associations in future research. Finally, the current study population was limited in only ACS patients. The further investigation including larger population with other conditions, particularly non-ACS, pure FH, healthy individuals and general outpatient population is required to investigate the associations. Nevertheless, these limitations are considered unlikely to materially affect our conclusion that ultrasonography may mitigate torsion-related overestimation with radiography.
Conclusions
This is the first study to quantitatively evaluate the three-dimensional twisted structure of the Achilles tendon and demonstrate the impact of the Achilles tendon torsion on variability in radiographical ATT assessment. Torsion of the Achilles tendon should be considered when assessing the Achilles tendon with radiography and ultrasonography.
Declaration of Interest
M.H-S. holds stock of Lipid Pharmaceuticals, and has received speaking honoraria from Amgen, MEDPACE, Kowa, BML, Protosera and Novartis. The other authors have no conflict of interest to declare.
Funding
No funding.
Author Contribution Statement
Kenta Sakaguchi, Masahito Michikura, Shimpei Fujioka, Yumiko Kanzaki: Writing – review & editing; Daisuke Shishikura: Writing – review & editing, Project administration, Supervision, Writing; Mariko Harada-Shiba: Funding acquisition, Project administration, Supervision, Writing – review & editing: Hideaki Morita: Supervision, Writing – review & editing
Use of AI and AI-Assisted Technologies Statement
AI and AI-assisted technologies have not been used during the writing process.
Ethical Statement
The Ethics Review Board of Osaka Medical and Pharmaceutical University approved this retrospective study and waived the requirement for informed consent (2024-170). All procedures were conducted in accordance with the ethical standards of the responsible committee on human experimentation (institutional and national) and the 1975 Declaration of Helsinki, revised in 2008.
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