Histology and Pathology

Inflammation

As mentioned previously inflammation plays an initial role in normal tendon healing, however histological studies have failed to categorically identify the presence of the inflammatory cells (Neutrophils and macrophages) in tendinopathy. Recent advances in technology have enabled greater specificity and accuracy when detecting the presence of inflammatory cells within tendons. A number of studies have shown the presence of elements of an inflammatory reaction within established tendinopathy (Rees et al 2013).

Schubert et al (2005) for example analysed the composition of 10 TA samples, from patients with a history of tendinopathy. They demonstrated the presence of macrophages, along with T and B lymphocytes, was significantly higher in this sample. In conjunction with this, Schubert et al (2005) also examined 10 samples of spontaneously ruptured tendons. They identified the presence of a large numbers of granulocytes but did not see significant numbers of macrophages or T/B lymphocytes. Concurrently, samples of granulation tissue from patients with AT frequently contained haemosiderophages. This granulation tissue consisted of groupings of capillaries embedded in a fibroblast rich stroma with evidence of macrophage and B/T lymphocyte infiltration.

Hyperplasic and hypertrophied tenocytes have also been discovered in pathological tendons, which may provide indirect evidence of up-regulated inflammatory mediators (Scott et al 2007). Proliferation and increased metabolic activity of tenocytes is known to occur in the presence of cytokines and growth factors which are part of the inflammatory response (Rees et al 2013). A drawback to the study by Scott et al (2007) is that it was completed in an animal population and the results of this study cannot be extracted directly across to a human population.

However, studies by Kragsnaes et al (2014) and Dean et al (2014) have explored the presence of inflammatory cells, in human populations, in achilles and rotator cuff tendons respectively. Kragsnaes et al (2014) compared tissue samples between groups of non-ruptured chronic TA tendinopathy and a healthy population. All biopsies were analysed immunohistochemically for the presence of macrophages, hemosiderophages, T lymphocytes, B lymphocytes, natural killer cells, schwann cells and endothelial cells. Significantly greater numbers of macrophages and endothelial cells were observed in the symptomatic population, versus the healthy population. Similarly, Dean et al (2014) compared inflammation cell prevalence in healthy samples versus those presenting with rotator cuff tendinopathy. Findings of significantly higher presence of leucocytes and macrophages in the tendinopathy group were reported. The identifying and quantifying of the presence of these cells is not an admission of their activity however (Kragsnaes et al 2014).

While the area of inflammation in tendinopathy is poorly defined, over time our understanding of the pathological process may resemble the current understanding of the process of osteoarthritis; degeneration and mechanical overload being the key driver of pathology with elements being mediated through inflammatory response (Rees et al 2013).

It is currently unclear the exact role inflammation may or may not play in tendinopathy; either way with pathology the degenerative process soon supersedes this (Rees et al 2009). Disorganised healing, intra-tendinous degeneration, hyper cellularity and fibre disorientation are the primary histological findings in symptomatic tendons (Sharma and Maffuli 2005).

Matrix Changes

It is now understood that the swollen appearance of pathological tendons, namely achilles and patellar, is not as a result of inflammatory process as suggested initially. It is secondary to altered cell permeability and increased production of large proteoglycans (PGs) (Rio et al 2013). Large PGs, especially aggrecan, attract and bind H2O which results in causing the tendon to swell; thus this is completely devoid of an inflammatory response. It has been suggested that PGs may not only cause tendons to swell, but may also have a role in cell-matrix interference and tendon pain (Rios et al 2013).

For example, the swelling of the tendon will stimulate local c-fibres and increase hydrogen and potassium concentrations. This may influence ion channel activation and, in turn, stimulate nociceptors and be received as pain (Rio et al 2013). It also have been suggested that large PGs may disrupt communication between cells, resulting in a loss of gap junctions between parallel rows of tenocytes. The disruption of gap junctions could influence homeostasis sufficiently to bring about a nocioceptive response (Rios et al 2013). On the other hand, impaired gap junction function could act to protect the tendon by isolating the area of damage and prevent toxic communication between cells (Rios et al 2013)

Tendon Appearance

Tendons which, usually appear white with a fibroelastic texture to the naked eye, appear grey/yellowish-brown and display a soft, fragile and oedematous texture (Kaux et al 2011). This is described as mucoid degeneration; commonly seen in TA, patellar tendinopathy and rotator cuff tendinopathies (Sharma and Maffuli 2005). The presence of large mucoid patches and vacuoles between collagen fibres is commonly associated with this form of degeneration. Lipoid degeneration, also seen in TA tendinopathy, refers to the abnormal presence of intratendinous lipid with a consequent disruption of collagen structure (Sharma and Maffuli 2005).

Under light microscopy tendinopathy also shows a number of histological changes:

Disorganised and disrupted collagen with a loss in typical hierarchical structure (Riley 2008).
Abnormally increased production of type III collagen, commonly associated with wound healing, by tenocytes local to the area of the tendinopathy (Cook et al 2002).
Increased ground substance along with higher concentrations of proteins such as glycosaminoglycans and proteoglycans (Rees et al 2009). Increased turnover of proteoglycans results in alterations in tendon homeostasis and contribute to tissue dysfunction (Rees et al 2009).
Cellularity changes with altered tenocyte concentration and appearance (Cook et al 2002).
Loss of cellular homeostatic tension (Cook et al 2002) and increased apoptosis rates which may be related to oxidative stress (Millar et al 2009).
Neovascularisation, as demonstrated on colour and power Doppler ultrasound (Alfredson et al 2006).

Cellular Appearance

Tenocytes react and respond to their environment, be that ionic, mechanical or osmotic. In tendinopathy, tenocytes begin to proliferate, become rounded in shape and have a greater proportion of protein-producing organelles. The changes appear to increase the production of receptors and substances involved in pain (Rios et al 2013); along with impacting the communication between cells though gap junctions as outlined previously.

Neovessels

Altered blood flow, dissimilar to that of a healthy tendon, is a common finding in Doppler ultrasound (Rees et al 2013). This change in blood flow is referred to as neovascularisation (Rees et al 2013). Angiogenesis, or the sprouting of new blood vessels, is a common histological finding in studies examining chronic tendinopathy (Rees et al 2013). It is hypothesized that this process of neovascularisation may be linked to tendon repair (Alfredson et al 2009) or the development of chronic pain (Knobloch 2008). Nerves and receptors, such as adrenoreceptors, which are found in vessel walls in tendinopathy are unlikely to be associated with pain; these are likely associated with angiogenesis (Rios et al 2013).

Along with angiogenesis, a body of literature also suggests neo-innervation accompanies the presence of neovessels. It is hypothesised that neural sprouting may be a cause or contributor to pain within the pathological tendon (Alfredson et al 2003). However there is currently limited evidence to support this hypothesis as presence of neural sprouting is not correlated with painful tendons (Rio et al 2013). This neural ingrowth comprises mainly of sympathetic nerves whose function is to regulate neovessels, these nerve fibres are not sensory in nature (Lawerence 2014)

A study by Sengkerij et al (2009) demonstrated the presence of neovascularization in the majority of the symptomatic TAs examined (N=33). However this study also reported that the degree of the presence of neovascularisation was not collated with the severity of the symptoms. Although neovascularisation has been associated with tendinopathy, not all painful tendons display increased vascularity (Cook et al 1998). It is also true that not all tendons with increased vascularity are painful (Cook et al 2004).

While it is fair to say that tendinopathy commonly displays features of neovascularisation and neural spouting, it is not accurate to correlate these changes with increased pain across all pathological presentations (Rios et al 2013).

Biochemical Changes

A myriad of biochemical changes occur in tendinopathy, however none of these would fully explain or account for tendon pain. It is likely that substances which have both pro- and anti-inflammatory effects play a role in mediating tendon pain (Rios et al 2013). This would include chemicals such as cytokines, interleukin and signalling molecules such as calcium (Rios et al 2013).

Cytokines [Tumour necrosis factor-alpha (TNK-a)] and interleukin [Interleukin-1-Beta (Il-1b)] for example are implicated in tendinopathy (Rios et al 2013). TNK-A is capable of matrix structure changes and inducing apoptosis. While Il-1b, along with inducing apoptosis, can cause cell proliferation (Rios et al 2013). Other cytokines, such as glial cells, have not been examined in tendinopathy but could impact communication of tendon pain as they are crucial in synaptic transmission (Rios et al 2013).

A neuropeptide known as substance-p (SP) is another chemical implicated in tendinopathy (Rios et al 2013). SP causes vasodilation and protein extravasion in the surrounding tissue in a process called peptidergic inflammation. This process is initiated by nocioceptor activation in the tissue which could be brought about by the increased tenocyte metabolism and proliferation caused by SP. Potentially this could result in a painful stimulus being received from the tendon.