Stijn Bogaerts

Strain mapping in the Achilles tendon – A systematic review

Achilles tendinopathy remains one of the most prevalent overuse injuries in elite as well as recreational athletes. Regardless of the fact that the aetiology of tendinopathy has not been fully understood, therapeutic mechanical loading programs have emerged as being the treatment of choice. In this light, mechanical properties of the tendon and their response to changes in loading or unloading have been the subject of many previous investigations. One of these properties often investigated is strain, a measure of relative deformation. By means of a systematic review, an overview was given of research in this field, with a primary objective to list the methods used and secondary aim to synthesize data on strain mapping in the Achilles tendon. Following the guidelines of the PRISMA statement, 47 articles were found appropriate for qualitative assessment. Achilles tendon strain has been investigated across a variety of contexts, including the response to exercise, walking, unloading, ageing, hormonal changes and weight. Only three studies investigated the effect of the presence of tendinopathy on strain. Ultrasound was the most often used imaging modality to measure or estimate strain. Further methodological parameters, e.g. the location of measurement, differed greatly between all different studies. Nearly all studies considered global strain. Some studies investigated the transverse strain response of the Achilles tendon. Recently, however, the role of local - intratendinous - strain distribution has been found to be of critical importance and further studies should focus on imaging modalities to investigate these local changes.

Evidence of patellar tendon buckling during passive knee extension

PURPOSE The purpose of this study was to investigate and describe buckling of the patellar tendon. METHODS Healthy young adults (28±4years, 10F/10M) underwent passive knee flexion/extension during the simultaneous measurement of knee angle and collection of cine ultrasound from the patellar tendon. Patellar tendon buckling was observed visually in ultrasound images, and the corresponding knee angle at which evidence of buckling disappeared was identified. RESULTS All subjects showed evidence of distal buckling which occurred on average at 23±8° flexion. Proximal buckling was only observed in fourteen subjects (10F/4M) at an average of 15±8°. Buckling patterns varied between subjects, but with high within-subject consistency. Buckling magnitude increased with age (p=0.03) and decreased with more weekly exercise (p=0.02). DISCUSSION The patellar tendon exhibited significant buckling in knee extension suggesting that buckling is a component of healthy knee function. Like tendon crimp, buckling may serve as a protective mechanism, allowing the tissue to unwrinkle prior to undergoing pure strain. The links between increased buckling magnitude and both age and reduced activity suggest that excessive buckling may be maladaptive, though future work is necessary to elucidate this relationship. Buckling is relevant to consider when estimating tendon length, as buckling can lead to significant underestimation of resting length and thus overestimation of strain. CONCLUSION This study demonstrates the complexity of tendon behavior even in healthy adults undergoing passive motion, suggesting that buckling may be relevant to an improved understanding of tendon health and pathology.

Non-uniform 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.

Evaluation of tissue displacement and regional strain in the Achilles tendon using quantitative high-frequency ultrasound

The Achilles tendon has a unique structure-function relationship thanks to its innate hierarchical architecture in combination with the rotational anatomy of the sub-tendons from the triceps surae muscles. Previous research has provided valuable insight in global Achilles tendon mechanics, but limitations with the technique used remain. Furthermore, given the global approach evaluating muscle-tendon junction to insertion, regional differences in tendon mechanical properties might be overlooked. However, recent advancements in the field of ultrasound imaging in combination with speckle tracking have made an intratendinous evaluation possible. This study uses high-frequency ultrasound to allow for quantification of regional tendon deformation. Also, an interactive application was developed to improve clinical applicability. A dynamic ultrasound of both Achilles tendons of ten asymptomatic subjects was taken. The displacement and regional strain in the superficial, middle and deep layer were evaluated during passive elongation and isometric contraction. Building on previous research, results showed that the Achilles tendon displaces non-uniformly with a higher displacement found in the deep layer of the tendon. Adding to this, a non-uniform regional strain behavior was found in the Achilles tendon during passive elongation, with the highest strain in the superficial layer. Further exploration of tendon mechanics will improve the knowledge on etiology of tendinopathy and provide options to optimize existing therapeutic loading programs.

3D tendon strain estimation on high-frequency 3D ultrasound images a simulation and phantom study

Tendon strain is a topic of interest within the orthopaedics and sports medicine community. If accurately estimated, it can improve existing treatment and rehabilitation protocols and aid in detection of presymptomatic abnormalities. This paper presents a novel US-based strain estimation framework that integrates an affine image registration approach to quantify tendon strain with a high-resolution 3D US imaging system. Validation of this framework was performed on simulated and phantom data. An accuracy test of the acquisition system and the performance of 3D and 2D strain estimations were evaluated. Results show that attention should be paid to the acquisition protocol, best accuracy is obtained for simulation data and along the major deformation direction and 3D strain estimations seems to reduce out-of-plane effect. By using this technique, it is expected that clinicians expand knowledge on aetiology of tendinopathy and optimize the existing therapeutic programs. Furthermore this technique can be extrapolated to other tendons and ligaments that are vulnerable to overuse.

Non-uniformity in pre-insertional Achilles tendon is not influenced by changing knee angle during isometric contractions

Achilles tendinopathy remains a prevalent condition among recreational and high‐level athletes. Mechanical loading has become the gold standard in managing these injuries, but exercises are often generic and prescribed in a “one‐size‐fits‐all” principle. The aim of this study was to evaluate the impact of knee angle changes and different levels of force production on the non‐uniform behavior in the Achilles tendon during isometric contractions. It was hypothesized that a flexed knee position would lead to a more distinct non‐uniform behavior, due to greater differential loading of soleus vs gastrocnemius, and that this effect would be attenuated by higher levels of force production. Contrary to the hypotheses, it was found that the non‐uniform deformation, that is, superficial‐to‐deep variation in displacement with highest displacement in the deep layer, is consistently present, irrespective of the level of force production and knee angle (n = 19; mean normalized displacement ratio 6.32%, 4.88%, and 4.09% with extended knee vs 5.47%, 2.56%, and 6.01% with flexed knee, at 25%, 50%, and 75% MVC, respectively; P > .05). From tendon perspective, aside from the influence on muscle behavior, this might question the mechanical rationale for a change in knee angle during eccentric heel drops. Additionally, despite reaching high levels of plantar flexion force, the relative contribution of the AT sometimes appears to be decreased, potentially due to compensatory actions by agonist muscle groups. These results are relevant for optimizing AT rehabilitation as the goal is to reach specific local tendon loading.

Non-uniformity in the healthy patellar tendon is greater in males and similar in different age groups

There is increasing evidence that tendons are heterogeneous and take advantage of structural mechanisms to enhance performance and reduce injury. Fascicle-sliding, for example, is used by energy-storing tendons to enable them to undergo large extensions while protecting the fascicles from damage. Reductions in fascicle-sliding capacity may thus predispose certain populations to tendinopathy. Evidence from the Achilles tendon of significant superficial-to-deep non-uniformity that is reduced with age supports this theory. Similar patellar tendon non-uniformity has been observed, but the effects of age and sex have yet to be assessed. Healthy adults (n = 50, 25M/25F) from a broad range of ages (23-80) were recruited and non-uniformity was quantified using ultrasound speckle-tracking during passive knee extension. Significant superficial-to-deep non-uniformity and proximal/distal variations were observed. No effect of age was found, but males exhibited significantly greater non-uniformity than females (p < 0.05). The results contrast with previous findings in the Achilles tendon; in this study, tendons and tendon regions at high risk for tendinopathy (i.e. males and proximal regions, respectively) exhibited greater non-uniformity, whereas high-risk Achilles tendons (i.e. older adults) previously showed reduced non-uniformity. This suggests that non-uniformity may be dominated by factors other than fascicle-sliding. Anatomically, the varied proximal attachment of the patellar tendon may influence non-uniformity, with quadriceps passive resistance limiting superficial tendon movement, thus linking flexibility, non-uniformity and injury risk. This study also provides evidence of a differential effect of aging on the patellar tendon compared with evidence from prior studies on other tendons necessitating further study to elucidate links between non-uniformity and injury.

18 Strain Mapping In The Achilles Tendon

Introduction Achilles tendinopathies continue to present in epidemic-like proportions. Due to its multifactorial aetiology and its fragmentarily understood pathophysiology, the management of Achilles tendinopathy is time-consuming and therefore often frustrating. Nevertheless, eccentric exercises have emerged as the key element in the treatment of these tendinopathies. However, to date, there is no general agreement on the optimal eccentric exercise protocol as subregional strain differences within the Achilles tendon are not taken into account. The purpose of this paper is therefore to critically review the published literature that currently exists on Achilles tendon strain in normal Achilles tendons as well as in Achilles tendons with tendinosis. Knowledge on subregional Achilles tendon strain differences may enhance our understanding of the aetiology, pathophysiology and management of Achilles tendinopathy. Methods The following three databases were examined on November 1st 2012 using strict search criteria: PubMed, Cochrane Library and CINAHL. The following main keywords were used: “Achilles tendon”, “Tendinopathy” and “Strain”. These keywords were then expanded to aim for an inclusion of all the relevant literature. Results Abstract 18 Figure 1 Flowchart Discussion All included studies in this review concerned global 2D strain measurements. Great variation exists in the measurement technique (ultrasound versus MRI), the location of measurement and the procedure of activation. In the vast majority of the cases, strain is measured in a longitudinal direction over the entire length of the Achilles tendon. Only three studies investigated Achilles tendon strain in transverse direction over the entire thickness of the tendon [Grigg, 2012; Wearing, 2011; Park, 2011]. Values for strain found in literature ranged from 2.1 to 9.2% in the longitudinal direction and from -19.5 to -8.3% in the transverse direction. Moreover, global 2D in vivo Achilles tendon strain in response to Achilles tendinopathy is investigated in only three studies [Grigg, 2012; Arya 2010; Child 2010]. Two-dimensional ultrasound imaging is the preferred imaging technique and maximum voluntary isometric contraction testing is the preferred measurement procedure to assess Achilles tendon strain. Taking into account that strain is a three-dimensional parameter as indicated by subregional strain differences on cadaveric Achilles tendons [Lersch, 2012; Wren, 2003; Lyman, 2004] and considering that Achilles tendinosis can occur at different subregional locations, it is clear that further research regarding non-invasive in vivo intratendinous strain mapping of the healthy and sick Achilles tendon is warranted. References Arya et al. J Appl Physiol. 2010;108:670–675 Child et al. Am J Sports Med. 201038:1885–1893 Grigg et al. Med Sci Sports Exerc. 2012;44:12–17 Lersch et al. Clin Biomech. 2012;27:955–961 Lyman et al. Am J Sports Med. 2004;32:457–461 Park et al. Foot Ankle Int. 32:407–413 Wearing et al. J Appl Physiol. 2011;110:1384–89 Wren et al. Ann Biomed Eng. 2003;31:710–717