Altered patterns of displacement within the Achilles tendon following surgical repair

An Optimization Approach for Creating Application-specific Ultrasound Speckle Tracking Algorithms

OBJECTIVE

Ultrasound speckle tracking enables in vivo measurement of soft tissue deformation or strain, providing a non-invasive diagnostic tool to quantify tissue health. However, adoption into new fields is challenging since algorithms need to be tuned with gold-standard reference data that are expensive or impractical to acquire. Here, we present a novel optimization approach that only requires repeated measurements, which can be acquired for new applications where reference data might not be readily available or difficult to get hold of.

METHODS

Soft tissue motion was captured using ultrasound for the medial collateral ligament (MCL) of three quasi-statically loaded porcine stifle joints, and medial ligamentous structures of a dynamically loaded human cadaveric knee joint. Using a training subset, custom speckle tracking algorithms were created for the porcine and human ligaments using surrogate optimization, which aimed to maximize repeatability by minimizing the normalized standard deviation of calculated strain maps for repeat measurements. An unseen test subset was then used to validate the tuned algorithms by comparing the ultrasound strains to digital image correlation (DIC) surface strains (porcine specimens) and length change values of the optically tracked ligament attachments (human specimens).

RESULTS

After 1500 iterations, the optimization routine based on the porcine and human training data converged to similar values of normalized standard deviations of repeat strain maps (porcine: 0.19, human: 0.26). Ultrasound strains calculated for the independent test sets using the tuned algorithms closely matched the DIC measurements for the porcine quasi-static measurements (R > 0.99, RMSE < 0.59%) and the length change between the tracked ligament attachments for the dynamic human dataset (RMSE < 6.28%). Furthermore, strains in the medial ligamentous structures of the human specimen during flexion showed a strong correlation with anterior/posterior position on the ligaments (R > 0.91).

CONCLUSION

Adjusting ultrasound speckle tracking algorithms using an optimization routine based on repeatability led to robust and reliable results with low RMSE for the medial ligamentous structures of the knee. This tool may be equally beneficial in other soft-tissue displacement or strain measurement applications and can assist in the development of novel ultrasonic diagnostic tools to assess soft tissue biomechanics.

Ultrasound Measurement of Local Deformation in the Human Free Achilles Tendon Produced by Dynamic Muscle-Induced Loading: A Systematic Review

Achilles tendinopathy is the most prevalent lower limb tendinopathy, yet it remains poorly understood, with mismatches between observed structure and reported function. Recent studies have hypothesised that Achilles tendon (AT) healthy function is associated with variable deformation across the tendon width during use, focusing on quantifying sub-tendon deformation. Here, the aim of this work was to synthesise recent advances exploring human free AT tissue-level deformation during use. Following PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines, PubMed, Embase, Scopus and Web of Science were systematically searched. Study quality and risk of bias were assessed. Thirteen articles were retained, yielding data on free AT deformation patterns. Seven were categorised as high-quality and six as medium-quality studies. Evidence consistently reports that healthy and young tendons deform non-uniformly, with the deeper layer displacing 18%-80% more than the superficial layer. Non-uniformity decreased by 12%-85% with increasing age and by 42%-91% in the presence of injury. There is limited evidence of large effect that AT deformation patterns during dynamic loading are non-uniform and may act as a biomarker of tendon health, risk of injury and rehabilitation impact. Better considered participant recruitment and improved measurement procedures would particularly improve study quality, to explore links between tendon structure, function, aging and disease in distinct populations.

Exploration of muscle-tendon biomechanics one year after Achilles tendon rupture and the compensatory role of flexor hallucis longus

Achilles tendon (AT) rupture leads to long-term structural and functional impairments. Currently, the predictors of good recovery after rupture are poorly known. Thus, we aimed to explore the interconnections between structural, mechanical, and neuromuscular parameters and their associations with factors that could explain good recovery in patients with non-surgically treated AT rupture. A total of 35 patients with unilateral rupture (6 females) participated in this study. Muscle-tendon structural, mechanical, and neuromuscular parameters were measured 1-year after rupture. Interconnections between the inter-limb differences (Δ) were explored using partial correlations, followed by multivariable linear regression to find associations between the measured factors and the following markers that indicate good recovery: 1) tendon length, 2) tendon non-uniform displacement, and 3) flexor hallucis longus (FHL) normalized EMG amplitude difference between limbs. Δmedial gastrocnemius (MG) (β = -0.12, p = 0.007) and Δlateral gastrocnemius (β = -0.086, p = 0.030) subtendon lengths were associated with MG tendon Δstiffness. MG (β = 11.56, p = 0.003) and soleus (β = 2.18, p = 0.040) Δsubtendon lengths explained 48 % of variance in FHL EMG amplitude. Regression models for tendon length and non-uniform displacement were not significant. Smaller inter-limb differences in Achilles subtendon lengths were associated with smaller differences in the AT stiffness between limbs, and a smaller contribution of FHL muscle to the plantarflexion torque. In the injured limb, the increased contribution of FHL appears to partially counteract a smaller contribution from MG due to the elongated tendon, however the role of FHL should not be emphasized during rehabilitation to allow recovery of the TS muscles.

Towards modern understanding of the Achilles tendon properties in human movement research

The Achilles tendon (AT) is the strongest tendon in humans, yet it often suffers from injury. The mechanical properties of the AT afford efficient movement, power amplification and power attenuation during locomotor tasks. The properties and the unique structure of the AT as a common tendon for three muscles have been studied frequently in humans using in vivo methods since 1990's. As a part of the celebration of 50 years history of the International Society of Biomechanics, this paper reviews the history of the AT research focusing on its mechanical properties in humans. The questions addressed are: What are the most important mechanical properties of the Achilles tendon, how are they studied, what is their significance to human movement, and how do they adapt? We foresee that the ongoing developments in experimental methods and modeling can provide ways to advance knowledge of the complex three-dimensional structure and properties of the Achilles tendon in vivo, and to enable monitoring of the loading and recovery for optimizing individual adaptations.

In vivo localised gastrocnemius subtendon representation within the healthy and ruptured human Achilles tendon

The Achilles tendon (AT) is composed of three distinct in-series elastic subtendons, arising from different muscles in the triceps surae. Independent activation of any of these muscles is thought to induce sliding between the adjacent AT subtendons. We aimed to investigate displacement patterns during voluntary contraction (VOL) and selective transcutaneous stimulation of medial (MGstim) and lateral (LGstim) gastrocnemius between ruptured and healthy tendons, and to examine the representative areas of AT subtendons. Twenty-eight patients with unilateral AT rupture performed bilateral VOL at 30% of the maximal isometric un-injured plantarflexion torque. AT displacement was analysed from sagittal B-mode ultrasonography images during VOL, MG_stim and LG_stim. Three-way ANOVA revealed a significant two-way interaction of contraction type*location on the tendon displacement (F(10-815)=3.72, p<0.001). The subsequent two-way analysis revealed a significant contraction type*location interaction for tendon displacement (F(10-410)=3.79, p<0.001) in the un-injured limb only, where LGstim displacement pattern was significantly different from MG_stim (p=0.008) and VOL (p=0.005). When comparing contraction types between limbs the there were no difference in the displacement patterns, but displacement amplitudes differed. There was no significant difference in the location of maximum or minimum displacement between limbs. The displacement pattern was not different in non-surgically treated compared to un-injured tendons one-year post rupture. Our results suggest that near the calcaneus, LG subtendon is located in the most anterior region adjacent to medial gastrocnemius. However, free tendon stiffness seems to be lower in the injured AT, leading to more displacement during electrically-induced contractions compared to the un-injured.

An equine tendon model for studying intra-tendinous shear in tendons that have more than one muscle contribution

Human Achilles tendon is composed of three smaller sub-tendons and exhibits non-uniform internal displacements, which decline with age and after injury, suggesting a potential role in the development of tendinopathies. Studying internal sliding behaviour is therefore important but difficult in human Achilles tendon. Here we propose the equine deep digital flexor tendon (DDFT) and its accessory ligament (AL) as a model to understand the sliding mechanism. The AL-DDFT has a comparable sub-bundle structure, is subjected to high and frequent asymmetric loads and is a natural site of injury similar to human Achilles tendons. Equine AL-DDFT were collected and underwent whole tendon level (n=7) and fascicle level (n=7) quasi-static mechanical testing. Whole tendon level testing was performed by sequentially loading through the proximal AL and subsequently through the proximal DDFT and recording regional strain in the free structures and joined DDFT and AL. Fascicle level testing was performed with focus on the inter-sub-bundle matrix between the two structures at the junction. Our results demonstrate a significant difference in the regional strain between the joined DDFT and AL and a greater transmission of force from the AL to the DDFT than vice versa. These results can be partially explained by the mechanical properties and geometry of the two structures and by differences in the properties of the interfascicular matrices. In conclusion, this tendon model successfully demonstrates that high displacement discrepancy occurs between the two structures and can be used as an easy-access model for studying intra-tendinous shear mechanics at the sub-tendon level. STATEMENT OF SIGNIFICANCE: Our study provides a naturally occurring and easily accessible equine model to study the complex behaviour of sub-tendons within the human Achilles tendon, which is likely to play a critical role in the pathogenesis of tendon disease. Our results demonstrate that the difference in material stiffness between the equine AL and DDFT stems largely from differences in the inter-fascicular matrix and furthermore that differences in strain are maintained in distal parts of the tightly joined structure. Furthermore, our results suggest that distribution of load between sub-structures is highly dependent on the morphological relationship between them; a finding that has important implications for understanding Achilles tendon mechanical behaviour, injury mechanisms and rehabilitation.

Individual variation in Achilles tendon morphology and geometry changes susceptibility to injury

The unique structure of the Achilles tendon, combining three smaller sub-tendons, enhances movement efficiency by allowing individual control from connected muscles. This requires compliant interfaces between sub-tendons, but compliance decreases with age and may account for increased injury frequency. Current understanding of sub-tendon sliding and its role in the whole Achilles tendon function is limited. Here we show changing the degree of sliding greatly affects the tendon mechanical behaviour. Our in vitro testing discovered distinct sub-tendon mechanical properties in keeping with their mechanical demands. In silico study based on measured properties, subject-specific tendon geometry, and modified sliding capacity demonstrated age-related displacement reduction similar to our in vivo ultrasonography measurements. Peak stress magnitude and distribution within the whole Achilles tendon are affected by individual tendon geometries, the sliding capacity between sub-tendons, and different muscle loading conditions. These results suggest clinical possibilities to identify patients at risk and design personalised rehabilitation protocols.

Non-uniform displacement within ruptured Achilles tendon during isometric contraction

The purpose of this study was investigate tendon displacement patterns in non-surgically treated patients 14 months after acute Achilles tendon rupture (ATR) and to classify patients into groups based on their Achilles tendon (AT) displacement patterns. Twenty patients were tested. Sagittal images of AT were acquired using B-mode ultrasonography during ramp contractions at a torque level corresponding to 30% of the maximal isometric plantarflexion torque of the uninjured limb. A speckle tracking algorithm was used to track proximal-distal movement of the tendon tissue at 6 antero-posterior locations. Two-way repeated measures ANOVA for peak tendon displacement was performed. K-means clustering was used to classify patients according to AT displacement patterns. The difference in peak relative displacement across locations was larger in the uninjured (1.29 ± 0.87 mm) than the injured limb (0.69 ± 0.68 mm), with a mean difference (95% CI) of 0.60 mm (0.14-1.05 mm, P < .001) between limbs. For the uninjured limb, cluster analysis formed 3 groups, while 2 groups were formed for the injured limb. The three distinct patterns of AT displacement during isometric plantarflexion in the uninjured limb may arise from subject-specific anatomical variations of AT sub-tendons, while the two patterns in the injured limb may reflect differential recovery after ATR with non-surgical treatment. Subject-specific tendon characteristics are a vital determinant of stress distribution across the tendon. Changes in stress distribution may lead to variation in the location and magnitude of peak displacement within the free AT. Quantifying internal tendon displacement patterns after ATR provides new insights into AT recovery.

Comparing Absorbable and Nonabsorbable Suture Materials for Repair of Achilles Tendon Rupture: A Magnetic Resonance Imaging-Based Study

We aimed to compare magnetic resonance imaging (MRI) findings and corresponding clinical outcomes of repaired Achilles tendons using absorbable and nonabsorbable sutures. Patients who underwent Achilles tendon repair were divided into 2 groups, with 11 in the absorbable group (group A) and 11 in the nonabsorbable group (group B). For all patients, MRI findings taken 6 months postoperatively were evaluated for morphological changes in the tendon. Concurrently, the American Orthopedic Foot and Ankle Society (AOFAS) ankle-hind foot score and incidence of postoperative complications were evaluated. Regarding MRI findings, the extent to which the cross-sectional area of the repaired tendon was thicker than that of the preoperative tendon was significantly greater in group B than in group A (p = 0.0012). Notably, more stitches remained within the tendon in group B than in group A (p = 0.0063). No other MRI findings showed a significant difference between the two groups. No significant difference was observed in the AOFAS score, and there was one re-rupture each in both groups. Because nonabsorbable suture material in the treatment of Achilles tendon rupture yielded a thicker postoperative MRI cross-sectional area, enhanced rehabilitation is recommended in order to prevent scar formation.

Achilles tendon elongation after acute rupture: is it a problem? A systematic review

PURPOSE

Rupture of the Achilles tendon (AT) is a common injury. Strength deficits may persist over the long term, possibly owing to elongation of the tendon or inferior mechanical properties. This study aimed to provide a systematic review of the literature on the prevalence and consequences of tendon elongation in patients after acute AT rupture treatment. It was hypothesized that an elongated tendon would be associated with a worse clinical outcome.

METHODS

The databases for MEDLINE, CENTRAL and Web of Science were searched. Clinical studies related to AT rupture reporting tendon elongation and clinical or functional outcomes, with a minimum follow-up of 6 months, were eligible for inclusion. Only studies testing for statistical correlations (SCs) between AT elongation and other outcomes were eligible, with the exception of biomechanical studies in which statistically significant AT elongation was found to be a generalized finding in the study group. For these studies to be eligible, the study group had to be compared with a healthy control group, or the injured limb compared with the uninjured limb, regarding biomechanical parameters.

RESULTS

Twenty-eight papers were selected for inclusion. Mean AT elongation measured with imaging techniques ranged from 0.15 to 3.1 cm (n = 17). Ten studies investigated SCs with Patient Reported Outcome Measures (PROMs), in which two found SCs with tendon elongation. Five studies reported strength and power evaluations and their correlation with AT elongation, with two having found SCs between decreased strength and tendon elongation. In ten studies reporting data on biomechanical tests, nine found influence of tendon elongation. In this group, four out of five studies found SCs with biomechanical parameters.

CONCLUSION

Fair evidence of the influence of tendon elongation in biomechanical parameters was found. In a general population, evidence of a detrimental effect of tendon elongation on PROMs or functional strength at follow-up was not found in this review.

LEVEL OF EVIDENCE

Level IV.

Running after cycling induces inter-limb differences in muscle activation but not in kinetics or kinematics

Overuse injuries are a common problem to triathletes' population. Overuse injuries may arise from inter-limb biomechanical differences during running, but the literature lacks information regarding inter-limb differences in triathletes. In this study inter-limb differences were investigated in injury-free triathletes during the running portion of a simulated cycle-run transition. Thirteen triathletes performed a 5 km run preceded by a 20 min cycling trial at 70% of maximal power output. During the Start, Mid and End stages of running, kinetic, kinematic and muscle activation variables were compared between the preferred and non-preferred limbs across the stance phase. A statistical parametric mapping analysis showed no differences between limbs when considering kinetic and kinematic variables (p > 0.05, ES<0.60). A lower soleus activation was observed in the preferred limb (p < 0.05, ES>0.60) from 53.40-75.9% of the stance phase at the End stage of running. In conclusion, inter-limb differences in kinetic or kinematic variables may not represent a risk for overloading in triathletes. However, inter-limb differences in triceps surae activation during running after cycling may represent one potential factor leading to overuse injuries in triathletes and should be further investigated.

The Effect of Ankle Foot Orthosis' Design and Degree of Dorsiflexion on Achilles Tendon Biomechanics—Tendon Displacement, Lower Leg Muscle Activation, and Plantar Pressure During Walking

Background: Following an Achilles tendon rupture, ankle foot orthoses (AFO) of different designs are used to protect the healing tendon. They are generally designed to protect against re-rupture by preventing undesired dorsiflexion and to prevent elongation by achieving plantarflexion in the ankle. There is limited knowledge of the biomechanical effects of different AFO designs and ankle angles on the tendon and lower leg muscles.

Hypothesis: The hypothesis was that non-uniform displacement in the Achilles tendon, lower leg muscle activity, and plantar pressure distribution would be affected differently in different designs of AFO and by varying the degree of dorsiflexion limitation.

Study Design: Controlled laboratory study.

Methods: Ultrasound of the Achilles tendon, EMG of the lower leg muscles and plantar pressure distribution were recorded in 16 healthy subjects during walking on a treadmill unbraced and wearing three designs of AFO. Ultrasound speckle tracking was used to estimate motion within the tendon. The tested AFO designs were a rigid AFO and a dorsal brace used together with wedges and an AFO with an adjustable ankle angle restricting dorsiflexion to various degrees.

Results: There were no significant differences in non-uniform tendon displacement or muscle activity between the different designs of AFO. For the rigid AFO and the adjustable AFO there was a significant reduction in non-uniform displacement within the tendon and soleus muscle activity as restriction in dorsiflexion increased.

Conclusion: The degree of dorsiflexion allowed within an AFO had greater effects on Achilles tendon displacement patterns and muscle activity in the calf than differences in AFO design. AFO settings that allowed ankle dorsiflexion to neutral resulted in displacement patterns in the Achilles tendon and muscle activity in the lower leg which were close to those observed during unbraced walking.

Reliability of ultrasound speckle tracking with singular value decomposition for quantifying displacement in the carpal tunnel

Inhibited movement patterns of carpal tunnel structures have been found in carpal tunnel syndrome (CTS) patients. Motion analysis on ultrasound images allows us to non-invasively study the (relative) movement of carpal tunnel structures and recently a speckle tracking method using singular value decomposition (SVD) has been proposed to optimize this tracking. This study aims to assess the reliability of longitudinal speckle tracking with SVD in both healthy volunteers and patients with CTS. Images from sixteen healthy volunteers and twenty-two CTS patients were used. Ultrasound clips of the third superficial flexor tendon and surrounding subsynovial connective tissue (SSCT) were acquired during finger flexion-extension. A custom made tracking algorithm was used for the analysis. Intra-class correlation coefficients (ICCs) were calculated using a single measure, two-way random model with absolute agreement and Bland-Altman plots were added for graphical representation. ICC values varied between 0.73 and 0.95 in the control group and 0.66-0.98 in the CTS patients, with the majority of the results classified as good to excellent. Tendon tracking showed higher reliability values compared to the SSCT, but values between the control and CTS groups were comparable. Speckle tracking with SVD can reliably be used to analyze longitudinal movement of anatomical structures with different sizes and compositions within the context of the carpal tunnel in both a healthy as well as a pathological state. Based on these results, this technique also holds relevant potential for areas where ultrasound based dynamic imaging requires quantification of motion.

Biplanar ultrasound investigation of in vivo Achilles tendon displacement non-uniformity

The Achilles tendon is a common tendon for the medial and lateral gastrocnemius and soleus muscles. Non-uniform Achilles tendon regional displacements have been observed in vivo which may result from non-uniform muscle loading and intra-tendinous shearing. However, prior observations are limited to the sagittal plane. This study investigated Achilles tendon tissue displacement patterns during isometric plantarflexor contractions in the coronal and sagittal planes. Fourteen subjects (5 female, 9 male, 26±3 yr) performed maximal isometric plantarflexor contractions with the knee in full extension and flexed to 110°. An ultrasound transducer positioned over the free Achilles tendon collected beam formed radio frequency (RF) data at 70 frames/s. Localized tissue displacements were analyzed using a speckle tracking algorithm. We observed non-uniform Achilles tendon tissue displacements in both imaging planes. Knee joint posture had no significant effect on tissue displacement patterns in either imaging plane. The non-uniform Achilles tendon tissue displacements during loading may arise from the anatomical organization of the sub-tendons associated with the three heads of the triceps surae. The biplanar investigation suggests that greatest displacements are localized to tissue likely to belong to soleus sub-tendon. This study adds novel information with possible implications for muscle coordination, function and muscle-tendon injury mechanisms.

Lower material stiffness in rupture-repaired Achilles tendon during walking: transmission-mode ultrasound for post-surgical tendon evaluation

PURPOSE

This cross-sectional study used transmission-mode ultrasound to evaluate dynamic tendon properties during walking in surgically repaired and contralateral Achilles tendon (AT), with a median (range) post-operative period of 22 (4-58) months. It was hypothesised that the axial transmission speed of ultrasound (TSOU) during walking would be slower, indicating lower material stiffness in repaired compared with contralateral AT.

METHODS

Ten patients [median (range) age 47 (37-69) years; height 180 (170-189) cm; weight 93 (62-119) kg], who had undergone open surgical repair of the AT and were clinically recovered according to their treating clinicians, walked barefoot on a treadmill at self-selected speed (1.0 ± 0.2 m/s). Synchronous measures of TSOU, sagittal ankle motion, vertical ground reaction force (GRF), and spatiotemporal gait parameters were recorded during 20 s of steady-state walking. Paired t tests were used to evaluate potential between-limb differences in TSOU, GRF, ankle motion, and spatiotemporal gait parameters.

RESULTS

TSOU was significantly lower (≈175 m/s) in the repaired than in the contralateral AT over the entire gait cycle (P < 0.05). Sagittal ankle motion was significantly greater (≈3°) in the repaired than in the contralateral limb (P = 0.036). There were no significant differences in GRF or spatiotemporal parameters between limbs.

CONCLUSIONS

Repaired AT was characterised by a lower TSOU, reflecting a lower material stiffness in the repaired tendon than in the contralateral tendon. A lower material stiffness may underpin greater ankle joint motion of the repaired limb during walking and long-term deficits in the muscle-tendon unit reported with AT repair. Treatment and rehabilitation approaches that focus on increasing the material stiffness of the repaired AT may be clinically beneficial. Transmission-mode ultrasound would seem useful for quantifying tendon properties post AT rupture repair and may have the potential to individually guide rehabilitation programmes, thereby aiding safer return to physical activity.

LEVEL OF EVIDENCE

II.

Nonuniform Deformation of the Patellar Tendon During Passive Knee Flexion

The purpose of this study was to evaluate localized patterns of patellar tendon deformation during passive knee flexion. Ultrasound radiofrequency data were collected from the patellar tendons of 20 healthy young adults during knee flexion over a range of motion of 50°-90° of flexion. A speckle tracking approach was used to compute proximal and distal tendon displacements and elongations. Nonuniform tissue displacements were visible in the proximal tendon (P < .001), with the deep tendon undergoing more distal displacement than the superficial tendon. In the distal tendon, more uniform tendon motion was observed. Spatial variations in percent elongation were also observed, but these varied along the length of the tendon (P < .002), with the proximal tendon remaining fairly isometric while the distal tendon underwent slight elongation. These results suggest that even during passive flexion the tendon undergoes complex patterns of deformation. Proximal tendon nonuniformity may arise from its complex anatomy where the deep tendon inserts onto the patella and the superficial tendon extends to the quadriceps tendon. Such heterogeneity is not captured in whole tendon average assessments, emphasizing the relevance of considering localized tendon mechanics, which may be key to understanding tendon behavior and precursors to injury and disease.

The challenges of measuring in vivo knee collateral ligament strains using ultrasound

Ultrasound-based methods have shown promise in their ability to characterize non-uniform deformations in large energy-storing tendons such as the Achilles and patellar tendons, yet applications to other areas of the body have been largely unexplored. The noninvasive quantification of collateral ligament strain could provide an important clinical metric of knee frontal plane stability, which is relevant in ligament injury and for measuring outcomes following total knee arthroplasty. In this pilot cadaveric experiment, we investigated the possibility of measuring collateral ligament strain with our previously validated speckle-tracking approach, but encountered a number of challenges during both data acquisition and processing. Given the clinical interest in this type of tool, and the fact that this is a developing area of research, the goal of this article is to transparently describe this pilot study, both in terms of methods and results, while also identifying specific challenges to this work and areas for future study. Some challenges faced relate generally to speckle-tracking of soft tissues (e.g. the limitations of using a 2D imaging modality to characterize 3D motion), while others are specific to this application (e.g. the small size and complex anatomy of the collateral ligaments). This work illustrates a clear need for additional studies, particularly relating to the collection of ground-truth data and more thorough validation work. These steps will be critical prior to the translation of ultrasound-based measures of collateral ligament strains into the clinic.