Painful disorders of the Achilles and patellar tendons are a major problem in competitive and recreational sports (10). Achilles tendinopathy and patellar tendinopathy occur in athletic patients of similar age groups. However, the first occurs in the main body of the tendon, the latter adjacent to the bone-tendon interface. It is unclear on a clinical basis whether the pathologic process would be similar. Histopathology studies in symptomatic Achilles tendinopathy reveal degeneration and disordered arrangement of collagen fibers and increased vascularity. Although at least six different types of collagen degeneration have been described (8,9), Achilles tendon degeneration is usually either “mucoid” or “lipoid” (8,9). Light microscopy reveals collagen fibers thinner than normal, with large interfibrillar mucoid patches and vacuoles. The characteristic hierarchical structure is also lost. Vascularity is increased, and blood vessels are randomly oriented. Inflammatory lesions and granulation tissue are not a feature of the condition (20). There is an increase in the number of tenocytes with rounded nuclei, even in patients in whom symptoms had only been present for a relatively short time.
Macroscopically, the patellar tendons of patients with patellar tendinopathy contain soft, yellow-brown, and disorganized tissue, with “mucoid” degeneration (5,7). Occasionally, “hyaline” degeneration can be present (24,28), with hardening of the tendon (8,9). At light microscopy, the collagen fibers are separated by increased mucoid ground substance that gives them a disorganized and discontinuous appearance, with variable fibrosis and neovascularization. Clefts in collagen and occasional necrotic fibers may suggest microtearing (23), with loss of the characteristic reflective polarized light appearance (12). The tenocytes themselves lose their fine spindle shape, and the rounded nuclei suggest fibrocartilaginous metaplasia (3,4). Small vessel ingrowth can also be evident (4). Tenocytes are more conspicuous and common than in normal tendon (25), but inflammatory cells are absent (12).
Therefore, it appears that tendinosis underlies both Achilles and patellar tendinopathy (14,19). However, to our knowledge, no study has ascertained whether there are histological differences between tendinopathic Achilles and patellar tendons. It could be possible that two separate pathological processes occur in athletic patients who suffer from chronic overuse pain in their Achilles or patellar tendons.
Preliminary exam of our database ascertained a male/female ratio of ~6.5:1 to start with. When we tried to match the women using the procedure described hereinafter, only five women satisfied the criteria. Hence, we decided to restrict this study only to male athletes. All procedures were approved by the local ethics committee. All patients gave written informed consent that surgery and the histological procedures described in this investigation could be carried out.
All patients with a diagnosis of Achilles or patellar tendinopathy were part of a longitudinal multicenter study on tendinopathy. In the period in question (April 1992 to December 2001), the surgeons in the centers involved in the study operated on 652 patients with Achilles tendinopathy and on 246 patients with patellar tendinopathy. We included in the study only male patients aged 18–45 with a diagnosis of unilateral exercise-induced tendinopathy of the main body of the Achilles or of “classical” jumper’s knee (tendinopathy at the interface between the lower pole of the patella and the patellar tendon) proven by clinical and imaging (high resolution real time ultrasonography or MRI) findings, and postoperative histological examination. We included in this study only patients who were originally referred to our care after a 3- to 6-month history of unilateral Achilles or patellar tendinopathy, and in whom in house conservative management for 3–6 months had failed. We excluded patients who had received peritendinous corticosteroids injections and patients who had undergone previous tendon surgery in either of the tendons under study. We matched each of the 28 patients with patellar tendinopathy who satisfied the above criteria with a patient with Achilles tendinopathy who practiced the same sport and who was within 2 yr of age at operation.
Achilles (N = 28; average age 34.1 yr, SD 16.3; range 18–45) or patellar (N = 28; average age 32.1 yr, SD 12.4; range 18–45) tendons were obtained from patients undergoing surgical exploration (16,17). During the operation, a sample 3 × 3 × 3 mm was removed within the area of degeneration.
Achilles tendon from deceased patients with no known tendon pathology.
One Achilles tendon was obtained from 21 male patients (average age 61.8 yr, SD 11.2; range 49–73) who had died of cardiovascular accidents in the period April 1996 to April 2001. Consulting the hospital notes and from direct questioning of the families, no patient had ever sustained an acute or overuse injury to their Achilles tendon, had taken corticosteroids for the past 5 yr, or had taken fluoroquinolones over the course of the 24 months preceding their death. The Achilles tendon was harvested through a medial approach under sterile conditions at post mortem within 24 h of death. The tendon was freed from surrounding tissue, and as much muscle and fat as possible were removed. The tendon was cut horizontally at the superior and inferior ends. The specimen was pinned to a piece of cork to prevent contraction and distortion of the tissue during the fixation process (10% neutral buffered formalin for 24 h). For this study, a 3 × 3 × 3 mm sample was excised from the main body of one Achilles tendon 4 cm proximal to the distal end of the tendon, the region where most tendinopathies occur (19). The histopathological appearance of the Achilles tendons from deceased individuals has been reported in previous studies from our laboratories (15,18,27).
Patellar tendon from patients undergoing anterior cruciate ligament reconstruction (N = 15 tendons).
A 3 × 3 × 3 mm sample was excised from the proximal most part of the lateral portion of the medial third of the patellar tendon from 15 male patients (average age 28.3 yr, SD 7.2; range 19–43) undergoing routine reconstruction for symptomatic anterior cruciate ligament reconstruction surgery. Care was taken to excise the portion of the tendon as close as possible to the lower pole of the patella. No patients presented clinical signs and symptoms of patellar tendinopathy, as ascertained by history taking, physical examination and preoperative MRI scanning.
Preparation of slides.
The specimens obtained were stained using hematoxylin and eosin according to established techniques routinely in use in our setting (2,15,26,27).
Assessment of tendon degeneration.
Per each tendon sample, three slides were randomly selected and examined using light microscopy (× 600, SM-LUX, Leitz, Wetzlar, Germany). The identification number on each slide was covered with a removable sticker, and each slide was numbered using random generated numbers. After one of the authors (N.M.) interpreted all the slides once, the stickers were removed, a new sticker was applied, and the slides were renumbered using a new series of random-generated numbers. The degree of staining was reassessed by the same author and the two results compared. If an inconsistency (more than one grade on the scoring system described below) existed between the two results, the slides were reassessed, and a final mark decided upon.
The area of each specimen showing the most advanced pathological changes was selected, and the worst possible results (i.e., the highest) for each slide was used in this study. The criteria used to score the slides were adapted from a semiquantitative grading scale (21,22) routinely in use in our setting (15,26,27). Using this method, we assessed 1) fiber structure, 2) fiber arrangement, 3) rounding of the nuclei, 4) regional variations in cellularity, 5) increased vascularity, 6) decreased collagen stainability, and 7) hyalinization. The slides were used to assess the seven variables, using a four-point scoring system: 0 indicates a normal appearance, 1 slightly abnormal, 2 moderately abnormal, and 3 markedly abnormal. The following scheme was used: 1) fiber structure (0 = linear, no interruption, 3 = short with early truncation); 2) fiber arrangement (0 = well ordered and regular, 3 = no pattern identified); 3) appearance of nuclei (0 = flat, 3 = rounded); 4) regional variations in cellularity (0 = uniform; 3 = high regional variation); 5) vascularity (0 = absent, 3 = high); 6) collagen stainability (0 = vivid, 3 = pale); and 7) hyalinization (0 = absent, 3 = high). Overall, the total score for a given slide could vary between 0 (normal tendon) and 21 (most severely degeneration detectable).
Per each slide and per each reading, the interpreter stated whether he believed the sample to be from an Achilles or a patellar tendon.
Kappa statistics was used to analyze the intraobserver reproducibility of the classification of the tendon appearance. Differences in the pathological variables, comparing all groups, were analyzed using the chi-square test. Logistic regression was used to determine which of the semiquantitative grading scale criteria was better able to predict correctly whether a tendinopathic tendon was a patellar or an Achilles tendon. For this analysis, however, we excluded the data on hyalinization, as kappa statistics demonstrated poor intratester reproducibility agreement (see Results). Each variable was entered separately in the logistic regression analysis, and we also entered all the possible couples of variables, again excluding hyalinization. The SPSS (release 9.0.1. standard version, Chicago, IL) statistical package was used to analyze the results. A probability level of P < 0.05 was considered significant.
Histopathological appearance and reproducibility of grading.
The mean pathology score for the Achilles tendons from deceased patients was 5.8 ± 2.1 and 6.0 ± 2.2. Kappa statistics showed good reproducibility of the mean pathology sum score (0.71, P = 0.04). The mean pathology score for the patellar tendons from the patients undergoing anterior cruciate ligament reconstruction was 6.3 ± 1.8 and 6.7 ± 2.1. Kappa statistics showed good reproducibility of the mean pathology sum score (0.7, P = 0.04). The mean pathology score for the tendinopathic Achilles tendons was 10.8 ± 4.9 and 11.6 ± 5 for the first and second reading, respectively (kappa: 0.67; P = 0.041). The mean values for the tendinopathic patellar tendons were 10.4 ± 3 and 10.2 ± 4.1 for the first and second reading, respectively (kappa: 0.77; P = 0.038) (Table 1). The intraobserver reproducibility of the scores was evaluated using kappa statistics and ranged from 0.174 (poor agreement when assessing hyalinization) to 0.760 (good to excellent agreement when assessing cellularity). The chi-square test showed no significant differences between tendinopathic Achilles and patellar tendons in any of the criteria examined, except for the significantly greater increased vascularity shown by the tendinopathic Achilles tendons (Table 2).
Results of logistic regression.
Logistic regression showed that no single histopathological variable (fiber structure, disordered fiber arrangement, rounding of the nuclei, increased regional variations in cellularity, increased vascularity, or decreased collagen stainability) was able to predict correctly whether a tendinopathic tendon was a patellar or an Achilles tendon. When couples of variables were entered in the analysis, the combination of more disordered fiber arrangement and increased vascularity would predict the specimen was from a patient with Achilles ten- dinopathy (Table 3). In the patellar tendon, the combination of increased regional variations in cellularity and increased vascularity (P = 0.04), and increased vascularity and decreased collagen stainability (P = 0.042) would predict that the specimen came from a patient with patellar tendinopathy.
Identification of Achilles or patellar tendon.
The kappa statistics showed that the ability to ascertain whether a specimen originated from an Achilles or from a patellar tendon was low (kappa = 0.23 for Achilles tendon, correctly identified in 6 of 21 control tendons, and in 7 of 28 tendinopathic tendons; 0.20 for patellar tendon, correctly identified in 5 of 15 control tendons, and in 11 of 28 tendinopathic tendons) (Table 4).
To our knowledge, this is the first study that has shown that samples of tendinopathic Achilles and patellar tendons exhibit similar histopathological changes. These changes are consistent with those described by ourselves and other authors in chronic tendinopathy of the Achilles and of the patellar tendon (10–12,15,18). In both tendinopathic Achilles and patellar tendons, the tendon fibers showed waviness, separation, and, in some cases, a complete loss of structure and hyalinization. In the tendons from patients with no known tendon pathology, fibers are arranged parallel to each other. In the tendinopathic samples, the arrangement was haphazardous. In the tendons from patients with no known tendon pathology, the tenocyte nuclei are flattened and spindle shaped, sometimes arranged in rows. In the tendinopathic samples, the nuclei were rounded. The whole area of the slide was assessed for variations in cellularity. The tendons from patients with no known tendon pathology show little variation in cellularity. In those specimens with the highest evidence of degeneration, there were areas of densely packed cells compared to the tendons from patients with no known tendon pathology. In both the tendinopathic Achilles and patellar tendons, there was a generalized increase in cellularity as a whole, with focal areas of cellular proliferation. In some instances, there was random blood vessel formation throughout the section, increasing with the degeneration of the tendon. In the tendons from patients with no known tendon pathology, these vascular bundles run parallel to the collagen fibers. Collagen fibers stain a deep color. The tendinopathic samples appeared of a paler pink, showing decreased collagen stainability. Only few specimens showed any evidence of hyalinization, and this histopathological criterion was poorly reproducible.
As expected, the most prominent features present in the tendinopathic specimens study were collagen degeneration and disorganization, increased cellularity and rounding of the nuclei, and, in some instances, hypervascularity, with no significant differences between the two tendons. The increase in extracellular matrix, coupled with the decrease in collagen fibers, shows an imbalance between the two structural components of the tendon tissue. It is not certain which process precedes the other. Tendinopathic tendons show increase in GAG content (27), possibly a result of mechanical overloading, and this, in its turn, may affect the fiber structure and arrangement leading to a reparative response with neovascularization. This imbalance between injury and repair may produce tissue damage (1).
Logistic regression showed that no single histopathological variable was able to predict correctly whether a tendinopathic tendon was a patellar or an Achilles tendon. On the other hand, alterations in fiber arrangement, cellularity, and vascularity, when considered together, were significantly more likely to be present in tendinopathic Achilles tendons. In tendinopathic patellar tendons, changes in regional variations in cellularity, increased vascularity and decreased collagen stainability were significantly more likely to be present. It therefore seems that some histopathological characteristics are different between tendinopathic Achilles and patellar tendons, but only when taken together. This may be consequent to the fact that the two pathologies do not arise from homologous tendon regions.
Reliability of histopathological investigations.
In this study, each slide was scored twice by the same experienced investigator. The kappa statistics assessed the measure of agreement between the two scores, showing how difficult it is to recognize specific patterns in histology, and the importance of having well trained individuals to interpret the slides. To improve upon these kappa statistics, the assessment could be repeated several more times. Also, using another observer would decrease observer bias. However, the whole procedure would become unyielding, cumbersome, and uneconomical.
As shown in previous investigations, aging in and by itself does not produce evident degenerative changes in the Achilles tendon, as reflected by the age of the patients from whom the Achilles tendon samples were obtained. For the patellar tendon, we harvested a sample from athletic patients of a comparable age with that of the patients undergoing surgery for patellar tendinopathy. We acknowledge, however, that some physically active individuals may present histological evidence of patellar tendon degeneration in the absence of clinical and imaging signs. Given our selection criteria and the histological picture presented by our subjects, though, we believe that this population was free of patellar tendinopathy (6).
Limitations of the present study.
We acknowledge that our study population is relatively small and that we were unable to study the bone-tendon interface, which may be involved in patellar tendinopathy. However, the specimens were harvested from a relatively homogeneous population of male athletes with closely matched ages practicing the same sports who had experienced symptoms for similar lengths of time and had undergone similar conservative management. Also, we only used one staining method. Obviously, extra lipids, calcium deposits, collagen denaturation, pathological tenocyte metabolism, collagen types, and foreign materials could have been detected using more advanced histochemical and immunohistochemical techniques (16,18). However, the staining technique employed in the present study is widely available, is cost effective, requires little technical abilities, and most pathologists are familiar with it, being used to interpret a variety of specimens stained in this fashion.
We did not perform a formal power analysis to calculate the number of patients needed to ascertain a priori the number of samples necessary to achieve statistically significant differences in the outcome measures used in the present investigation. However, the results of our study are univocal, and, given the accurate matching of patients with Achilles and patellar tendinopathy that we were striving to obtain, we believe that our findings are both clinically and scientifically true and relevant. Also, the results of our semiquantitative assessment of control and tendinopathic tendon matches closely what we found in our previous studies in this field (18,27), and this strengthens both our findings and the conclusions drawn from them.
We have recently shown that tenocytes from ruptured Achilles tendons produce greater quantities of Type III collagen than tenocytes from normal Achilles tendons (18). This altered production of collagen may be one reason for the histopathological alterations described in this study and may result in the tendinopathic tendons being less resistant to tensile forces, and thus at increased risk of micro- and macroscopic changes. However, it still remains unclear how these noninflammatory changes result in the pain that induces patients to alter their athletic participation and to consult a physician (11,13).
In males, tendinopathic Achilles and patellar tendons exhibit similar histological characteristic features under light microscopy after hematoxylin and eosin staining. Either a common pathological mechanism has acted on both tendon populations, or the histological appearance represents the final common pathway of different tendinopathic processes. On the basis of the staining method used and of the general appearance at light microscopy, we were not able to distinguish between Achilles and patellar tendons, and only statistical analysis applied to a cohort of tendinopathic tendons would allow a greater degree of certainty regarding the provenience of a sample. Within the limitation of the relatively crude staining methods used, the general pattern of degeneration in male patients was similar in the two anatomical locations. It is therefore possible that there is a common, as yet unidentified, pathological mechanism that has acted on both of these tendon populations, and that the histological appearance described in the present study represents the final common pathway of potentially different tendinopathic processes, as the biopsies were obtained from male patients with chronic, end-stage tendinopathy.
Many thanks are given to Miss Linda Lothian for her help with the manuscript.
No benefit in any form has been received or will be received from a commercial party related directly or indirectly to the subject of this article.
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