Research has elucidated new information about the disease processes that contribute to Achilles tendinopathy and the relative effectiveness of therapies such as heavy-load eccentric exercise and injections of corticosteroids, dextrose, and other solutions.
by Michael Ryan PhD, CPed(C)
Achilles tendinopathy predominantly refers to an overuse injury to the Achilles tendon occurring in either of two distinct anatomical areas: the tendon mid-substance or its insertion on the calcaneal tubercle. Individuals in running-based sports appear particularly susceptible. In fact, the occurrence of Achilles tendinopathy is higher in elite than recreational distance runners, owing to differences in foot strike patterns and soft-tissue load distribution throughout the lower extremity.1,2 In a survey of more than 2000 running injuries seen at a referral-based sports medicine clinic, Achilles tendinopathy was the seventh most common injury.2 However, sedentary individuals may also experience Achilles tendon pain (either from increased loading associated with increases to body mass and/or as a result of stress shielding negative adaption) with a lifetime incidence of 5.9% in this population.3,4
Approximately 66% of overuse injuries to the Achilles tendon occur at the mid-substance location, which is why the bulk of the related research is devoted to pain at this location.5 While it may be tempting to extrapolate results from the mid-portion literature to applications at the Achilles insertion site, certain considerations should be made, most notably with regard to the pathomechanism of injury. Repetitive tensile overload remains the prevailing mechanical cause for overuse injury at the Achilles mid-substance; however, evidence exists that suggests compressive forces on the deep ventral fibers at the enthesis site may be responsible for injury at the Achilles insertion (a commonality between most insertional tendinopathies at the knee, elbow and shoulder).6,7
Ultrasound imaging has confirmed differences in the structural presentation of injury at these two sites, particularly with regard to hypoechoic regions (regions on ultrasound appearing darker than expected as a result of changes to the tissue properties that affect the reflection of sound waves). In the case of tendinopathy, hypoechoic tissue changes are degenerative in nature. Mid-substance cases typically present with more extensive intratendinous tearing (albeit with some hypoechoic regions), while insertional cases are defined more from hypoechoic and calcific changes to the tendon enthesis and cortical bone (Figures 1 and 2).8 This may help explain why certain exercise-based treatments that were originally designed – and were successful – for a population with mid-substance injury could not be replicated with a population with insertional injury.9 In a study of 78 patients with chronic mid-portion Achilles tendinosis and 31 patients with chronic insertional Achilles tendon pain, Fahlstrom et al found that 12 weeks of eccentric calf muscle training significantly decreased pain and improved activity levels in 89% of the mid-substance group but only 32% of the insertional group.9
It is generally accepted that Achilles tendon pain (at either location) may be more neurogenic than inflammatory in nature; reports indicate a lack of inflammatory markers (prostaglandin E2 (PGE2)) and an increase in the neurotransmitter glutamate, glutamate’s NMDR1 receptor, and the neuropeptide substance P.10,11 The role of inflammation in initiating the degenerative process(es) in tendinopathy, however, should not be overlooked – particularly in light of the evidence from laboratory studies on the negative effect of chronic exposure to biochemical markers of inflammation (PGE2 and IL6) on tendon fibroblasts and the continued clinical success of corticosteroid interventions.12,13 Advances in our understanding of the pathomechanics of overuse tendinopathies have impacted greatly on modernizing treatment approaches.
In approximately 24% to 45% of cases, conservative measures for Achilles tendinopathy—such as rest, ice, activity medication and non-steroidal anti-inflammatory drugs (NSAIDs)–are unsuccessful, so that surgery or other more aggressive treatments must be considered.14 Heavy load eccentric exercise and injection-based treatments are alternatives for tendinopathy management that have received considerable attention in the literature15,16 and have helped change treatment objectives to emphasize restoration of mechanical properties and tendon function as outcomes, rather than simply addressing symptomatic relief.
Tendon fibroblasts are known to up-regulate the production of collagen and improve the structural properties of tendon when exposed to moderate levels of tensile load.17 Although moderate levels of concentric or eccentric training in Achilles tendon patients have produced equivocal results in terms of self-reported pain outcomes,18 patients suffering from long-standing painful Achilles mid-portion tendinopathy have shown tremendous improvements in pain and return to function with a heavy-load eccentric only heel drop regimen.19-21 The heavy-load protocol involved subjects standing on a step with heels over the edge, using the muscles of the uninjured leg to (concentrically) elevate the body until the heels were above the step, then transferring the weight to the injured limb and eccentrically lowering the body until the heels were once again below the level of the step.
Alfredson et al postulated that excessive tensile loading of tendon tissue, such as in a loaded heel drop, obliterates vascular and neural in-growth, thereby improving both symptomatic and functional outcomes.22 Other research centers, however, have been unable to replicate the success of this heavy load exercise regimen in patients with mid-portion Achilles tendinopathy, possibly as a result of extraneous variables accounting for the variation in patient outcomes, most notably female gender, pre-treatment tendon thickness, and previous activity level.23 In fact, a 2009 report from Knobloch et al indicates that women do not appear to benefit as much as men when undergoing eccentric training protocols for tendinopathy from the standpoint of pain reduction and return to activity.24
While there is speculation as to the specifics of how to apply eccentric movements for therapeutic relief of tendinopathy, it is becoming clear that incorporating eccentric exercise into treatment regimens results in superior outcomes compared to concentric-based programs.16 Allison and Purdam suggest that the role eccentric exercise plays in the treatment of chronic tendon injury is likely not through optimizing strengthening elements, but rather there may be a unique stretching stimulus that results in an improved homogeneity of passive structures, modulation of the neurological stretch responses back to normal, and/or increasing shear forces between the tendon and peritendinous structures that may interfere with vascular infiltration associated with neurogenic pain.25 Eccentric-based programs have also been shown to result in a greater decrease in tendon thickness than concentrically-based exercises for the calf and Achilles tendon, presumably as a result of greater extravasation of water attributed to an increase in tendon crimp strengthening, combined with greater collagen stretch and realignment.26 However, it remains to be determined whether this process is specifically related to positive remodeling.
Injection-based treatments for chronic tendinosis are quickly becoming a popular trend for addressing cases of Achilles tendinopathy (or most other tendinopathies) that are recalcitrant to even eccentric-based exercise protocols, and provide a suitably less-invasive approach than surgery (Figure 3). In fact, Yelland et al reported that, compared to eccentric exercises, hypertonic dextrose injections resulted in a greater reduction in stiffness and return to activity in a population with chronic Achilles tendinopathy.27 No fewer than seven separate solution-based treatments, delivered with or without image guidance, for chronic tendon pain have been documented in the literature within the past 10 years: corticosteroid (betamethasone), polidocanol, prolotherapy solution (sodium morrhuate 14.7% and dextrose 10.7%), dextrose (25%), the matrix metalloproteinases (MMP)-inhibitor aprotinin, whole autologous blood ,and platelet-rich preparations (PRP).28-34 Interestingly, a procedure involving only dry-needling and no injectable solution resulted in clinically significant improvements in pain in patients with calcific shoulder tendinosis, and there was no significant difference between a steroid treatment group versus placebo in a study of subjects with rotator cuff tendinopathy.29,35 Based on this circumstantial evidence, it is reasonable to speculate that there is a common underlying treatment effect across these injectable treatments that is likely stemming, at least partially, from the needle stick injury.
To counteract the degenerative process, external stimuli delivered via the injectant may be necessary to assist in the healing response of the tendon. Intratendinous injections of either irritant materials – such as dextrose alone or in combination with sodium morrhuate – or whole blood or blood-borne bioactive solutions (e.g., PRP) are understood to deliver directly to the injury site pro-wound healing cytokines and platelet-born growth factors, as well as creating significant fibril disruption, allowing greater blood permeation and healing stimulus.36-38 The vascular sclerosing effect of peritendinous polidocanol injections has been well documented by Alfredson and Öhberg as interfering with an apparent neurogenic source of pain in patients with chronic Achilles and infrapatellar tendinopathies.39-44 As previous research has uncovered the role of MMPs in the pathogenesis of tendinosis, selective use of MMP inhibitors, such as aprotinin, has become documented.
Current management strategies for chronic Achilles tendinopathy have advanced remarkably during the preceding decade. Treatment options are becoming increasingly varied, with many approaches claiming clinical success. Therefore, researchers and clinicians in this area need to work together in developing pragmatic treatment algorithms. One such algorithm proposed by Alfredson and Cook45 starts with six to 12 weeks of heavy-load eccentric training, then suggests training modifications and treatment alternatives (topical glyceryl trinitrate, extracorporeal shock wave therapy, corticosteroid injection) in those who do not initially respond to the training protocol. Practitioners and patients will benefit from these types of algorithms, which synthesize results from the basic science literature into a cohesive and clinically useful tool.
Michael Ryan PhD, CPed(C), is a post-doctoral fellow in the department of orthopedics and rehabilitation at the University of Wisconsin-Madison.
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