Elsevier

Foot and Ankle Surgery

Volume 17, Issue 4, December 2011, Pages 252-255
Foot and Ankle Surgery

The role of Plantaris Longus in Achilles tendinopathy: A biomechanical study

https://doi.org/10.1016/j.fas.2010.08.004Get rights and content

Abstract

Background

The Plantaris Longus Tendon (PLT) may be implicated in Achilles (AT) tendinopathy. Different mechanical characteristics may be the cause. This study is designed to measure these.

Methods

Six PLT and six AT were harvested from frozen cadavers (aged 65–88). Samples were stretched to failure using a Minimat 2000™ (Rheometric Scientific Inc.). Force and elongation were recorded. Calculated tangent stiffness, failure stress and strain were obtained. Averaged mechanical properties were compared using paired, one-tailed t-tests.

Results

Mean stiffness was higher (p < 0.001) in the PLT, measuring 5.71 N/mm (4.68–6.64), compared with 1.73 N/mm (1.40–2.22) in AT. Failure stress was also higher (p < 0.01) in PLT: 1.42 N/mm2 (0.86–2.23) AT: 0.20 N/mm2 (0.16–0.25). Failure strain was less (p < 0.05) in PLT: 14.1% (11.5–16.8) than AT: 21.8% (14.9–37.9).

Conclusions

The PLT is stiffer, stronger than AT, demonstrating potential for relative movement under load. The stiffer PLT could tether AT and initiate an inflammatory response.

Introduction

The Plantaris Longus Tendon (PLT) runs alongside the gastro-soleus complex, and continues along the medial aspect of the Achilles tendon (AT) before inserting onto the greater tuberosity of the calcaneus [1]. It is therefore situated within the peritendinous tissues which have been implicated in the pathogenesis of Achilles tendon [2], [3]. It has also been observed that PLT is preserved during most acute AT ruptures suggesting that the two structures respond differently to a similar stress [4], [5], [6]. Little is known about how PLT might itself contribute to the mechanisms which trigger tendinopathy.

Theories as to the aetiology of Achilles tendinopathy include the effect of neighboring structures such as an accessory soleus muscle [7], and in other sets of synergic tendons different mechanical properties have been demonstrate [8]. Similarly, it has been reported that shear stress could cause inflammation of the peritenon [9].

Although the mechanical properties under load of the Achilles tendon have been previously studied [10], the differential mechanical properties under similar stress in paired Achilles and Plantaris tendons have to our knowledge not previously been reported.

The aim of this study was to describe the comparative mechanical properties in matched pairs of AT and PLT samples with the hypothesis that Plantaris is stiffer and stronger than the Achilles tendon. This variation in response to tensile stress could lead to differential movements between the two structures and therefore be a potential mechanism through which an inflammatory response is created around the Achilles tendons. Furthermore, this study will bring insight into the biomechanical properties of the PLT which to our knowledge has never been reported.

Section snippets

Cadaveric material and preparation of tendon samples for testing

Six human calf specimens were disjointed at knee level from 4 frozen human cadavers, all male. The age of the specimens ranged from 65 to 88 years (mean 80.5). None of the subjects had died of a condition known to affect tendon metabolism and none of them had sustained an injury or had a surgical procedure to the gastro-soleus complex. The Achilles (AT) and Plantaris Longus (PLT) tendons were dissected. Samples measuring 5–10 cm long of the entire PLT were obtained. Samples of the larger AT were

Sample characteristics

Specimen sizes differed between the two tendons. Cross-sectional area averaged 6.3 mm2 (SD 2.1) for PLT specimens and 14.3 mm2 (SD 3.0) for AT. Average specimen lengths were 15.4 mm for PLT and 13.8 mm for AT.

Force–elongation graphs

Typical force–elongation graphs (Fig. 2) showed a linear region followed by a turning point (zero gradient) which marked the ultimate tensile strength (UTS). Minimat software was used to calculate tissue stiffness (N/mm) as the gradient of the linear region of the graph, using linear

Discussion

Plantaris is a stronger, stiffer and less extensible tendon than the adjacent Achilles (Fig. 3), suggesting potential differential movement between the two structures under load. This may play a role in the pathogenesis of the inflammatory processes. The mechanical properties of Plantaris have the potential to create repeated shear stresses to the peritendinous tissues, and/or to the Achilles tendon itself. Our results demonstrate that, under similar tensile stresses, the Achilles elongates a

Conclusions

The Plantaris Longus and Achilles tendons demonstrate different mechanical properties in response to similar tensile stress. Differential movement and shear stress between them could trigger an inflammatory response in or near the Achilles tendon.

Conflict of interest statement

The authors report no conflict of interest and disclose any financial and personal relationships with other people or organisations that could inappropriately influence (bias) their work.

Acknowledgements

Funding was provided by the Avon Orthopaedic Center Research Fund for use of the facilities in Bristol University Anatomy Department, including logistic help and cadavers samples.

The authors state that each of them was actively involved in the process of the present work.

References (19)

There are more references available in the full text version of this article.

Cited by (40)

  • Achilles Paratenon Needle Tendoscopy in the Office Setting

    2022, Arthroscopy Techniques
    Citation Excerpt :

    They reported that the procedure was safe and yielded satisfactory results with lower morbidity than other reported techniques. The plantaris tendon has been identified as a potential cause of midportion Achilles tendinopathy owing to differential movement and shear stress between the tendon and Achilles itself, leading to an inflammatory response in or near the Achilles tendon.20,21 In a prospective study by Bedi et al.,22 elite athletes underwent open ventral paratenon scraping and plantaris release for non-insertional Achilles tendinopathy.15

  • Tendoscopies in the foot and ankle

    2021, Fuss und Sprunggelenk
  • Noninsertional Tendinopathy of the Achilles

    2017, Foot and Ankle Clinics
    Citation Excerpt :

    This muscle can become adherent to the medial aspect of the Achilles tendon, initiating an inflammatory response and localized tendinopathy. Studies have looked at combining the mini–open scraping with stripping of the plantaris tendon and its potential to improve tendon structure and reduce pain.96,97 A few small studies, looking at release of the plantaris tendon, resulted in good outcomes with significant reduction in pain and improved function at 2 years.98,99

  • Differential Plantaris-Achilles Tendon Motion: A Sonographic and Cadaveric Investigation

    2017, PM and R
    Citation Excerpt :

    Although previous researchers have hypothesized that some degree of differential PT-AT motion is a normal occurrence, and symptomatic PT snapping has been reported in a single individual, the current study is the first to objectively establish the existence of differential PT-AT motion in nonpathologic states [6-11,13,14,17-19]. Clinically, our results support current theories hypothesizing that repetitive soft-tissue stresses within the PT-AT interval may induce pathologic changes within the intervening fibrofatty tissues or the adjacent tendons, and therefore contribute to the development of clinical symptoms [6-10,13,14,16-19,29]. Several observations are worthy of further discussion.

View all citing articles on Scopus

The authors state that the use of human cadaver material for this research was submitted to and approved by the National Health Service North Bristol Trust Research Ethics Committee.

View full text