Plantaris Frictional Syndrome: An Underappreciated Cause of Midportion Achilles Tendinopathy

For many clinicians, the plantaris muscle and tendon may be somewhat of an afterthought.¹ Despite being present in more than 92 percent of the population,^2,3 the plantaris has long been regarded as having little clinical relevance. More recently, however, the the literature has begun to recognize the plantaris as much more than a vestigial muscle, trivia question for medical students, or autograft donor site.³

As with any tendon in the extremities, painful tendinitis or rupture of the plantaris is certainly possible.^3,4 Furthermore, emerging literature suggests that the plantaris is involved, and likely neglected, in many cases of Achilles midportion tendinopathy.^3,5-7

The literature widely discusses the role of the plantaris tendon in Achilles midportion tendinopathy.^3,5,8-9 Differential motion between the plantaris and Achilles can create localized shear forces between the two tendons, resulting in an inflammatory response with associated remodeling and pain.^9,10 Some fittingly refer to this phenomenon as a “frictional syndrome” of the plantaris.^3,9

With continued friction, focal thickening may take place and the tendon may take on a fusiform shape. Interposition of fibrous tissue, adhesions, fat, and neurovasculature may also develop at the contact surfaces between the two tendons.^1,11 This abnormal contact results in increased shear and compression of the peritendinous structures of the plantaris and Achilles tendons resulting in chronic pain.^3,10

Histological studies have verified the abnormal architecture of the plantaris and peritendinous tissues in cases of midportion Achilles tendinopathy.¹ Doppler studies also suggest that abnormal vascular flow in the peritendinous fat may be a hallmark of this syndrome.¹ Ultrasound and intraoperative investigations reveal that anatomic variants such as the “invaginated” plantaris tendon may be implicated in midportion Achilles tendinopathy.^6-8 However, the diagnosis is largely based on clinical observations and patient history.

How Might Plantaris-Associated Achilles Tendinopathy Present Clinically?

Consider the following potential example: a young, active patient presents with symptoms of heel cord pain that appears consistent with midportion Achilles tendinitis. Treatment includes numerous conservative measures such as icing, discontinuation of sport, immobilization, bracing, physical therapy, dry needling, manual soft tissue mobilization, and eccentric exercise. The reported pain may decrease somewhat following this treatment. However, the patient continues to experience chronic pain localized to the medial aspect of the heel cord 2 to 7 cm above the posterior calcaneal tuberosity.⁷

Despite frustration with the lingering pain, the patient-athlete may ultimately decide to return to sport. Upon resumption of this strenuous activity, severe pain recurs. The patient once again undergoes treatment with conservative measures using a multi-modal approach, but residual focal pain over the medial aspect of the Achilles persists. Again, the pain worsens with increased physical activity, and the cyclical pattern of recalcitrant pain continues.¹⁰ This recurrent, “acute-on-chronic” presentation suggests development of some persistent architectural abnormality or pathologic process, and one should have some suspicion for frictional plantaris syndrome.¹⁰

At this juncture, there is promising evidence that stripping and resection of the plantaris tendon with scraping of the ventral Achilles is a viable and effective treatment for midportion Achilles tendinitis.^5,7,11-12 In a recent study by Masci and colleagues published in 2021, all 16 patients with diagnosed mid-portion Achilles tendinopathy expressed satisfaction following plantaris excision and ventral scraping.¹¹ Fourteen of the 16 patients in this series were able to return to their prior activity levels at 24 month follow up.¹¹

Another case series showed significant improvement in pain following excision of the plantaris in situations where the plantaris was adhered or invaginated into the Achilles tendon.¹⁰ Upon histologic examination, 13 out of 16 excised tendons lacked signs of plantaris tendinopathy, suggesting that the disease was frictional or compressive in nature.¹⁰ Other novel treatments, such as ultrasound-guided stripping and electrochemical ablation are also currently under investigation.^6,13

With a keen eye and an open mind, one should give the diagnosis of frictional plantaris syndrome or plantaris- related midportion Achilles tendinopathy serious consideration if the clinical picture suggests it. In the end, refinement of effective treatment techniques can only take place with increasing recognition, discussion, and debate of this pathology.

Mr. Anjum is a fourth-year student at AZCPM, Midwestern  University.

Dr. McAlister is a Fellow of the American College of Foot and Ankle Surgeons and practices in Phoenix and Scottsdale, AZ.

Disclaimer: The views and opinions expressed are those of the author(s) and do not necessarily reflect the official policy or position of Podiatry Today or HMP Global, their employees and affiliates. Any content provided by our bloggers or authors are of their opinion and are not intended to malign any religion, ethnic group, club, association, organization, company, individual, anyone or anything.

References

1.     Spang C, Alfredson H, Ferguson M, Roos B, Bagge J, Forsgren S. The plantaris tendon in association with mid-portion Achilles tendinosis: tendinosis-like morphological features and presence of a non-neuronal cholinergic system. Histol Histopathol. 2013;28(5):623- 632.

2.     Kelikian AS, Sarrafian SK: Myology. In: Kelikian AS, Sarrafian SK, (eds). Sarrafian's anatomy of the foot and ankle: descriptive, topographical, functional, Ch. 5, LWW, 2011, pp.223-290.

3.     Spang C, Alfredson H, Docking SI, Masci L, Andersson G. The plantaris tendon: a narrative review focusing on anatomical features and clinical importance. Bone Joint J. 2016;98-B(10):1312-1319.

4.     Zickmantel B Jr, Krause F, Frauchiger L. Isolated rupture of the distal plantaris muscle. J Foot Ankle Surg. 2018;57(5):995-996.

5.     Alfredson H. Midportion Achilles tendinosis and the plantaris tendon. Br J Sports Med. 2011;45(13):1023-1025.

6.     Masci L, Spang C, van Schie HT, Alfredson H. How to diagnose plantaris tendon involvement in midportion Achilles tendinopathy - clinical and imaging findings. BMC Musculoskelet Disord. 2016;17:97.

7.     Olewnik L, Wysiadecki G, Polguj M, Topol M. Anatomic study suggests that the morphology of the plantaris tendon may be related to Achilles tendonitis. Surg Radiol Anat. 2017;39(1):69-75.

8.     Alfredson H, Masci L, Spang C. Sharp pain in a normal Achilles tendon of a professional female football player was related to a plantaris tendon in a rare position: a case report. J Med Case Rep. 2021;15(1):513.

9.     Stephen JM, Marsland D, Masci L, Calder JDF, Daou HE. Differential motion and compression between the plantaris and Achilles tendons: a contributing factor to midportion Achilles tendinopathy? Am J Sports Med. 2018;46(4):955-960.

10.  Calder JD, Stephen JM, van Dijk CN. Plantaris excision reduces pain in midportion Achilles tendinopathy even in the absence of plantaris tendinosis. Orthop J Sports Med. 2016;4(12):2325967116673978.

11.  Bedi HS, Jowett C, Ristanis S, Docking S, Cook J. Plantaris excision and ventral paratendinous scraping for Achilles tendinopathy in an athletic population. Foot Ankle Int. 2016;37(4):386-393.

12.  Masci L, Neal BS, Wynter Bee W, Spang C, Alfredson H. Achilles scraping and plantaris tendon removal improves pain and tendon structure in patients with mid-portion Achilles tendinopathy - a 24 month follow-up case series. J Clin Med. 2021;10(12):2695.

13.  Mattiussi G, Moreno C. Percutaneous electrochemical debridement of the plantaris tendon. J Am Podiatr Med Assoc. 2018;108(5):437-441.