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

doi:10.1016/j.fas.2010.08.004

Foot and Ankle Surgery

Volume 17, Issue 4, December 2011, Pages 252-255

https://doi.org/10.1016/j.fas.2010.08.004 Get 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.

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The plantaris tendon has been described as a potential cause of midportion Achilles tendinopathy. One cadaveric study showed that the plantaris tendon is stiffer than the Achilles tendon, potentially leading to compression of the Achilles tendon and inflammation.36 Plantaris involvement may explain continued medial-sided symptoms and may lead to continued symptoms if not addressed at the time of percutaneous stripping.37–39
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The most important finding of the current investigation is that differential PT-AT motion appears to be a normal phenomenon during dorsiflexion and plantarflexion. 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].
Differential motion between the plantaris and Achilles tendons has been hypothesized to contribute to pain in some patients presenting with Achilles tendinopathy. However, objective evidence of differential Achilles-plantaris motion is currently lacking from the literature.
To determine whether differential, multidirectional motion exists between the plantaris tendon (PT) and Achilles tendon (AT) as documented by dynamic ultrasound (US) and postdissection examination in an unembalmed cadaveric model.
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Twenty unembalmed knee-ankle-foot specimens (9 right, 11 left) obtained from 6 male and 10 female donors ages 55-96 years (mean 80 years) with body mass indices of 14.1-33.2 kg/m² (mean 22.5 kg/m²).
A single, experienced operator used high-resolution dynamic US to qualitatively document differential PT-AT motion during passive ankle dorsiflexion-plantarflexion. Specimens were then dissected and passive dorsiflexion-plantarflexion was repeated while differential PT-AT motion was visualized directly.
Presence or absence of multidirectional differential PT-AT motion.
All 20 specimens exhibited smooth but variable amplitude multidirectional differential PT-AT motion. Whereas US readily demonstrated medial-lateral and anterior-posterior PT motion relative to the AT, differential longitudinal motion was only appreciated on dissection and direct inspection. Many specimens exhibited partial or complete encasement of the PT between the gastrocnemius portion of the AT and the soleus aponeurosis.
Some degree of multidirectional differential PT-AT motion appears to be a normal phenomenon, and PT motion can be evaluated sonographically in both the medial-lateral and anterior-posterior directions. The existence of normal differential PT-AT motion suggests that alterations in PT motion or repetitive stress within the PT-AT interval may produce symptoms in some patients presenting with Achilles region pain syndromes. The PT should be evaluated in all patients presenting with Achilles, plantaris, or calf pain syndromes. Future research would benefit from the development of a sonographic classification system for PT anatomy and motion with the goal of differentiating normal versus pathologic states and identifying risk factors for symptom development.
IV
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This hypothesis is supported by studies demonstrating multi-planar motion differential between the plantaris and Achilles tendons during passive plantarflexion and dorsiflexion22 and increased compressive forces between the two tendons at different ankle joint positions.23 In addition, the plantaris tendon has a higher intrinsic stiffness than the Achilles tendon in normal cadaveric specimens24 further supporting the potential for tethering of the plantaris against the Achilles tendon under tension. As well as the differences in biomechanical properties, anatomical variations in the course and insertion of the plantaris tendon could contribute to the development of pathology.
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El tendón del plantar delgado está implicado en algunos casos de tendinopatía de Aquiles resistente a la carga. El tendón se desplaza cerca de la porción media del tendón de Aquiles antes de su inserción en la región medial del calcáneo, aunque existen variaciones anatómicas en el curso y la inserción. La influencia del plantar delgado en la tendinopatía de Aquiles debe sospecharse ante un dolor en la región medial del Aquiles persistente y que no responde a un programa de rehabilitación y en el que las pruebas de imagen revelan un tendón plantar engrosado y / o tendinosos medial focal del tendón de Aquiles. Los mecanismos potenciales incluyen fuerzas de compresión o cizallamiento entre el tendón de Aquiles y el plantar delgado. El tratamiento inicial debe consistir en un programa de carga modificado que evite la carga en el tramo final. Los casos resistentes pueden ser susceptibles a la extirpación quirúrgica del plantar delgado que ha demostrado buenos resultados clínicos a corto y largo plazo, aunque la evidencia se limita a series de casos. Los métodos percutáneos son prometedores, pero se requieren más estudios que demuestren su validez real.

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^☆: 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.

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

Abstract

Background

Methods

Results

Conclusions

Introduction

Section snippets

Cadaveric material and preparation of tendon samples for testing

Sample characteristics

Force–elongation graphs

Discussion

Conclusions

Conflict of interest statement

Acknowledgements

Clin Biomech (Bristol, Avon)

J Biomech

J Biomech

Anatomical variations of the plantaris muscle and a potential role in patellofemoral pain syndrome

Clin Anat

Surgical treatment for chronic disease and disorders of the Achilles tendon

J Am Acad Orthop Surg

A classification of Achilles tendon disease

Am J Sports Med

Neglected Achilles tendon rupture with central insertional plantaris tendon hypertrophy: two cases

Strategies Trauma Limb Reconstr

Occurrence of the plantaris tendon in patients sustaining subcutaneous rupture of the Achilles tendon

Foot Ankle

Concurrent rupture of the Achilles and plantaris tendons

Am J Orthop (Belle Mead NJ)