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Concept: Tendon


While the predominant function of all tendons is to transfer force from muscle to bone and position the limbs, some tendons additionally function as energy stores, reducing the cost of locomotion. Energy storing tendons experience extremely high strains and need to be able to recoil efficiently for maximum energy storage and return. In the equine forelimb, the energy storing superficial digital flexor tendon (SDFT) has much higher failure strains than the positional common digital extensor tendon (CDET). However, we have previously shown that this is not due to differences in the properties of the SDFT and CDET fascicles (the largest tendon subunits). Instead, there is a greater capacity for interfascicular sliding in the SDFT which facilitates the greater extensions in this particular tendon (Thorpe et al., 2012). In the current study, we exposed fascicles and interfascicular matrix (IFM) from the SDFT and CDET to cyclic loading followed by a test to failure. The results show that IFM mechanical behaviour is not a result of irreversible deformation, but the IFM is able to withstand cyclic loading, and is more elastic in the SDFT than in the CDET. We also assessed the effect of ageing on IFM properties, demonstrating that the IFM is less able to resist repetitive loading as it ages, becoming stiffer with increasing age in the SDFT. These results provide further indications that the IFM is important for efficient function in energy storing tendons, and age-related alterations to the IFM may compromise function and predispose older tendons to injury.

Concepts: Collagen, Muscle, Cartilage, Tendon


BackgroundLess invasive percutaneous acute Achilles tendon rupture repair techniques gain popularity because of lower risk of surgical wound complications. But these approaches have an increased risk of sural nerve iatrogenic injury as this sensory nerve is usually not visualized during minimally invasive operative procedures. We compared standard percutaneous Bunnell type and our proposed modified-medialized percutaneous technique in a cadaver study to evaluate potential advantages. Methods10 pairs of fresh frozen specimens were divided into two groups for comparative anatomical study. Tenotomies of Achilles tendons were made and wounds sutured. 10 standard and 10 modified-medialized repairs were applied for artificially performed ruptures. All sutured tendons were dissected meticulously. We carefully looked at repaired Achilles tendon end-to-end contact and adaptation, distance from Achilles insertion in calcaneal tubercle to place where sural nerve crosses lateral border of the Achilles tendon and possible sural nerve and vein entrapment. Groups were compared using Fisher’s exact and Student-T tests.ResultsAll ends of sharply dissected tendons in both groups were in sufficient contact. No measurable diastasis between tendon ends was found in all cases. No entrapment of sural nerve or vein was found in modified percutaneous Bunnell suture technique group. Whereas 7 of 10 sural nerves and 9 small saphenous veins were entrapped when using standard percutaneous Bunnell type technique. Average distance from Achilles tendon insertion in tuber calcanei to sural nerve crossing the lateral border of Achilles was 93 mm.ConclusionMedialization of percutaneous suture in acute Achilles tendon rupture repair show clear advantages compared to standard non medialized technique ensuring a possible lower incidence of sural nerve entrapment injury. Our modified percutaneous Bunnell type technique allows sufficient adaptation of ruptured Achilles tendon.

Concepts: Muscle, Wound, Tendon, Achilles tendon, Tendons, Small saphenous vein, Sural nerve, Achilles tendon rupture


Autologous adipose stromal vascular fractions (SVFs) containing adipose tissue-derived stem cells (ASCs) are currently being used in clinical settings for various orthopedic applications for human patients. Due to its potential capability of regenerating cartilage, bone, and tendons, autologous adipose SVFs are being tried in treating patients with osteoarthritis (OA), chondromalacia, meniscus tear, osteonecrosis of the femoral head, and tendon injuries. Here, we have reviewed available human clinical studies with regard to patient applications of autologous adipose SVF containing ASCs, specifically assessing effectiveness and safety in the field of orthopedic disorders. All studies reviewed in this article presents potential benefits of autologous adipose SVF in various orthopedic applications without any serious side effects.

Concepts: Bone, Clinical trial, Collagen, Extracellular matrix, Effectiveness, Cartilage, Ligament, Tendon


With efficiencies derived from evolution, growth and learning, humans are very well-tuned for locomotion. Metabolic energy used during walking can be partly replaced by power input from an exoskeleton, but is it possible to reduce metabolic rate without providing an additional energy source? This would require an improvement in the efficiency of the human-machine system as a whole, and would be remarkable given the apparent optimality of human gait. Here we show that the metabolic rate of human walking can be reduced by an unpowered ankle exoskeleton. We built a lightweight elastic device that acts in parallel with the user’s calf muscles, off-loading muscle force and thereby reducing the metabolic energy consumed in contractions. The device uses a mechanical clutch to hold a spring as it is stretched and relaxed by ankle movements when the foot is on the ground, helping to fulfil one function of the calf muscles and Achilles tendon. Unlike muscles, however, the clutch sustains force passively. The exoskeleton consumes no chemical or electrical energy and delivers no net positive mechanical work, yet reduces the metabolic cost of walking by 7.2 ± 2.6% for healthy human users under natural conditions, comparable to savings with powered devices. Improving upon walking economy in this way is analogous to altering the structure of the body such that it is more energy-effective at walking. While strong natural pressures have already shaped human locomotion, improvements in efficiency are still possible. Much remains to be learned about this seemingly simple behaviour.

Concepts: Metabolism, Energy, Muscle, Force, Human anatomy, Potential energy, Walking, Tendon


Tendons are often injured and heal poorly. Whether this is caused by a slow tissue turnover is unknown, since existing data provide diverging estimates of tendon protein half-life that range from 2 mo to 200 yr. With the purpose of determining life-long turnover of human tendon tissue, we used the (14)C bomb-pulse method. This method takes advantage of the dramatic increase in atmospheric levels of (14)C, produced by nuclear bomb tests in 1955-1963, which is reflected in all living organisms. Levels of (14)C were measured in 28 forensic samples of Achilles tendon core and 4 skeletal muscle samples (donor birth years 1945-1983) with accelerator mass spectrometry (AMS) and compared to known atmospheric levels to estimate tissue turnover. We found that Achilles tendon tissue retained levels of (14)C corresponding to atmospheric levels several decades before tissue sampling, demonstrating a very limited tissue turnover. The tendon concentrations of (14)C approximately reflected the atmospheric levels present during the first 17 yr of life, indicating that the tendon core is formed during height growth and is essentially not renewed thereafter. In contrast, (14)C levels in muscle indicated continuous turnover. Our observation provides a fundamental premise for understanding tendon function and pathology, and likely explains the poor regenerative capacity of tendon tissue.-Heinemeier, K. M., Schjerling, P., Heinemeier, J., Magnusson, S. P., Kjaer, M. Lack of tissue renewal in human adult Achilles tendon is revealed by nuclear bomb (14)C.

Concepts: Mass spectrometry, Biology, Tendon, Achilles tendon, Nuclear weapon, Tendons, Carbon-14, Accelerator mass spectrometry


Aim: The purpose of this study was to clarify training-related risk factors for overuse injuries. Methods: This was twelve-month retrospective study which was done by self-reported postal questionnaire. The study group consisted of 446 men and women top-level Finnish athletes representing three different endurance sports (cross-country skiing, swimming, long-distance running) between the ages of 15-35. Self-reported anthropometric and training-related variables (such as starting age of training, years of active training, hours trained yearly, competition hours and weekly resting days) and occurrence of overuse injuries. Results: Athletes with less than 2 rest days per week during the training season had 5.2-fold risk (95% confidence intervals [CI] 1.89-14.06, P=0.001) for an overuse injury, and athletes who trained more than 700 hours during a year had 2.1-fold risk (95% CI 1.21-3.61, P=0.008) for an overuse injury compared to the others. Athletes who reported a tendon injury were on average two years older than athletes without such an injury (P<0.001). Conclusion: We found that low number of recovery days and a high amount of training are training-related risk factors for overuse injuries in top-level endurance athletes. The higher number of tendon overuse injuries in older than younger athletes may indicate that age-related degeneration plays an important role in the etiology of tendon injuries. These findings should be taken into account when planning exercise programs for endurance athletes.

Concepts: Training, Injuries, Injury, Confidence interval, The Higher, Tendon


INTRODUCTION: Achilles and patellar tendinopathy are overuse injuries that are common among athletes. Isolated eccentric muscle training has become the dominant conservative management strategy for Achilles and patellar tendinopathy but, in some cases, up to 45 % of patients may not respond. Eccentric-concentric progressing to eccentric (Silbernagel combined) and eccentric-concentric isotonic (heavy-slow resistance; HSR) loading have also been investigated. In order for clinicians to make informed decisions, they need to be aware of the loading options and comparative evidence. The mechanisms of loading also need to be elucidated in order to focus treatment to patient deficits and refine loading programmes in future studies. OBJECTIVES: The objectives of this review are to evaluate the evidence in studies that compare two or more loading programmes in Achilles and patellar tendinopathy, and to review the non-clinical outcomes (potential mechanisms), such as improved imaging outcomes, associated with clinical outcomes. METHODS: Comprehensive searching (MEDLINE, EMBASE, CINAHL, Current Contents and SPORTDiscus(™)) identified 403 studies. Two authors independently reviewed studies for inclusion and quality. The final yield included 32 studies; ten compared loading programmes and 28 investigated at least one potential mechanism (six studies compared loading programmes and investigated potential mechanisms). RESULTS: This review has identified limited (Achilles) and conflicting (patellar) evidence that clinical outcomes are superior with eccentric loading compared with other loading programmes, questioning the currently entrenched clinical approach to these injuries. There is equivalent evidence for Silbernagel combined (Achilles) and greater evidence for HSR loading (patellar). The only potential mechanism that was consistently associated with improved clinical outcomes in both Achilles and patellar tendon rehabilitation was improved neuromuscular performance (e.g. torque, work, endurance), and Silbernagel-combined (Achilles) HSR loading (patellar) had an equivalent or higher level of evidence than isolated eccentric loading. In the Achilles tendon, a majority of studies did not find an association between improved imaging (e.g. reduced anteroposterior diameter, proportion of tendons with Doppler signal) and clinical outcomes, including all high-quality studies. In contrast, HSR loading in the patellar tendon was associated with reduced Doppler area and anteroposterior diameter, as well as greater evidence of collagen turnover, and this was not seen following eccentric loading. HSR seems more likely to lead to tendon adaptation and warrants further investigation. Improved jump performance was associated with Achilles but not patellar tendon clinical outcomes. The mechanisms associated with clinical benefit may vary between loading interventions and tendons. CONCLUSION: There is little clinical or mechanistic evidence for isolating the eccentric component, although it should be made clear that there is a paucity of good quality evidence and several potential mechanisms have not been investigated, such as neural adaptation and central nervous system changes (e.g. cortical reorganization). Clinicians should consider eccentric-concentric loading alongside or instead of eccentric loading in Achilles and patellar tendinopathy. Good-quality studies comparing loading programmes and evaluating clinical and mechanistic outcomes are needed in both Achilles and patellar tendinopathy rehabilitation.

Concepts: Central nervous system, Nervous system, Collagen, Evidence-based medicine, Tendon, Achilles tendon, Tendinopathy, Tendons


Rotator cuff (RC) tears represent a large proportion of musculoskeletal injuries attended to at the clinic and thereby make RC repair surgeries one of the most widely performed musculoskeletal procedures. Despite the high incidence rate of RC tears, operative treatments have provided minimal functional gains and suffer from high re-tear rates. The hypocellular nature of tendon tissue poses a limited capacity for regeneration. In recent years, great strides have been made in the area of tendonogenesis and differentiation towards tendon cells due to a greater understanding of the tendon stem cell niche, development of advanced materials, improved scaffold fabrication techniques, and delineation of the phenotype development process. Though in vitro models for tendonogenesis have shown promising results, in vivo models have been less successful. The present work investigates structured matrices mimicking the tendon microenvironment as cell delivery vehicles in a rat RC tear model. RC injuries augmented with a matrix delivering rat mesenchymal stem cells (rMSCs) showed enhanced regeneration over suture repair alone or repair with augmentation, at 6 and 12-weeks post-surgery. The local delivery of rMSCs led to increased mechanical properties and improved tissue morphology. We hypothesize that the mesenchymal stem cells function to modulate the local immune and bioactivity environment through autocrine/paracrine and/or cell homing mechanisms. This study provides evidence for improved tendon healing with biomimetic matrices and delivered MSCs with the potential for translation to larger, clinical animal models. The enhanced regenerative healing response with stem cell delivering biomimetic matrices may represent a new treatment paradigm for massive RC tendon tears.

Concepts: Developmental biology, Cell division, Stem cell, Mesenchymal stem cell, Bone marrow, Stem cells, Cellular differentiation, Tendon


BACKGROUND: Bilateral midportion Achilles tendinopathy/tendinosis is not unusual, and treatment of both sides is often carried out. Experiments in animals suggest of the potential involvement of central neuronal mechanisms in Achilles tendinosis. OBJECTIVES: To evaluate the outcome of surgery for Achilles tendinopathy. METHODS: This observational study included 13 patients (7 men and 6 women, mean age 53 years) with a long duration (6-120 months) of chronic painful bilateral midportion Achilles tendinopathy. The most painful side at the time for investigation was selected to be operated on first. Treatment was ultrasound-guided and Doppler-guided scraping procedure outside the ventral part of the tendon under local anaesthetic. The patients started walking on the first day after surgery. Follow-ups were conducted and the primary outcome was pain by visual analogue scale. In an additional part of the study, specimens from Achilles and plantaris tendons in three patients with bilateral Achilles tendinosis were examined. RESULTS: Short-term follow-ups showed postoperative improvement on the non-operated side as well as the operated side in 11 of 13 patients. Final follow-up after 37 (mean) months showed significant pain relief and patient satisfaction on both sides for these 11 patients. In 2 of 13 patients operation on the other, initially non-operated side, was instituted due to persisting pain. Morphologically, it was found that there were similar morphological effects, and immunohistochemical patterns of enzyme involved in signal substance production, bilaterally. CONCLUSION: Unilateral treatment with a scraping operation can have benefits contralaterally; the clinical implication is that unilateral surgery may be a logical first treatment in cases of bilateral Achilles tendinopathy.

Concepts: Anesthesia, Pain, Suffering, Tendon, Pain management, Tendinosis, Tendinopathy, Tendinitis


Ultrasound (US) imaging is commonly used to visualise tendon structure. It is not clear whether the presence of structural abnormalities in asymptomatic tendons predicts the development of future tendon symptoms in the Achilles or patellar tendon.

Concepts: Tendon, Tendinopathy