Magnetic Resonance and CadavericFindings of the "Watershed Band" of theAchilles Tendon

Amol Saxena, DPM, FACFAS,1 and Daniel Bareither, PhD2

A review of 63 magnetic resonance images of 86 ankles was performed. (There were 23 bilateralstudies.) A contiguous fascial band from the deep posterior compartment to the Achilles tendonwatershed region was noted in 83 ankles. This fascial band enveloped the watershed region, coursingfrom the flexor (Iacinate) ligament medially, to the peroneal retinaculum laterally. This band was absentin three patients who were scanned, two of whom sustained a complete Achilles tendon rupture, whilethe third had previously undergone peritenolysis. Forty cadaveric specimens with no known Achillestendon pathology were evaluated for presence of this structure. An organized fascial band existed in allof these specimens. This previously undescribed structure is termed the "watershed band" and mayhave surgical significance. (The Journal of Foot & Ankle Surgery 40(3):132-136, 2001)

Key words: Achilles tendon, Achilles tendonopathy, paratendonosis, peritenolysis, watershed region

The Achilles tendon is the thickest and strongest tendonin the body (1). It is formed from the gastrocnemiusand soleus muscles in the superficial compartment of theposterior leg (1-6). Unlike other tendons in the leg, theAchilles tendon lacks a synovial sheath. Instead, it has aparatenon, which is an array of thin, filmy, fibrous tissue,containing blood vessels from the mesotenon (Fig. I, Aand B). The paratenon is considered a continuation ofthe crural fascial layer (1, 2). The anterior aspect of thisstructure contains the most abundant vascular supply tothe tendon (1).

Additional blood supply to the tendon comes from thecalcaneus at its insertion, and from the muscle fibers of thesoleus proximally. The confluence of this blood supply isoften incomplete. This arrangement results in an area ofthe tendon that can be compromised and is known as thewatershed area. The vascular watershed region is locatedapproximately 2-6 em proximal to the Achilles' insertionon the posterior calcaneus.

The deep posterior and lateral leg compartments areeach enveloped by their own fascial components. Theflexor or lacinate retinaculum is a specialized thickeningof the deep posterior compartment fascia and courses

From l Palo Alto Medical Foundation, Palo Alto, CA and 2SchollCollege of Podiatric Medicine, Chicago, IL. Address correspondenceto: Amol Saxena, DPM, FACFAS, Department of Sports Medicine, PaloAlto Medical Foundation, 795 EI Camino Real, Palo Alto, CA 94301;e-mail: HeySax@AOL.com.

Received for publication January 23, 1999; accepted in revised formfor publication December 12, 2000.The Journal of Foot & Ankle Surgery 1067-2516/01/4003-0132$4.00/0Copyright © 2001 by the American College of Foot and Ankle Surgeons

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posteriorly and inferiorly from the medial malleolus. Simi­larly, the superior peroneal retinaculum is a specializedthickening of the lateral compartment fascia and coursesposteroinferiorly from the lateral malleolus. Both thesuperior peroneal and the lacinate retinaculi attach byblending with the calcaneal periosteum (2-7). Althoughthese thickenings of the fascia have been well docu­mented by anatomical texts (2-7), there is no descrip­tion in the literature of these retinaculi having involve­ment with Achilles tendon. However, two anatomical textscontain diagrams or drawings of the deep fascia of theleg that show the retinaculi to envelope the watershedregion (7, 8).

Chronic Achilles tendon pathology often includesparatendonosis and tendonosis (9-11). Paratendosis (alsoknown as stenosing tenovaginitis and peritendonitis) isdefined as inflammation of the paratenon itself, whichis essentially an organized layer of fascia (and isoften termed "fascia cruris") (12, 13). More commonin athletes, paratendonosis is manifested by diffuseswelling and crepitus in the Achilles tendon region (9,II). Magnetic resonance imaging (MRI) often confirmsthe diagnosis with the "halo-sign" and increased signalaround the Achilles tendon is noted on axial and sagittalviews (Fig. 2) (10, II, 13, 14). Surgical removal of thisstructure in recalcitrant cases has been described byseveral authors (10, 13, 16-19).

Tendonosis of the Achilles is defined as chronic inflam­mation, with or without actual tendon degeneration (9, 16,19) Abnormal MRI signal can be noted within andaround the tendon (14, IS). Some surgeons recommenddebridement with peritenolysis for recalcitrant cases. Insome reports, the excised paratenon showed pathologic

FIGURE 1 A, Paratenon of the Achilles tendon (cadaver); B, close-up.

FIGURE 2 "Halo" sign in axial MRI image of a patient with chronicAchilles tendonosis.

mucoid degeneration (Fig. 3) (l0, 13, 16-19). If theretinaculi were contiguous with the Achilles paratenon,a relationship may exist between thickening of thesestructures and chronic paratendonosis and tendonosis.

Due to frequency of pathology of the Achilles tendonaround the watershed region and corresponding MRIabnormalities, the authors sought to investigate theanatomy in this region. Because peritenolysis has beenrecommended for patients with chronic Achilles tendonpathology, it was proposed that the anatomy in this regionmay play a role in causing the abnormal signal withMRI studies. The purpose of this study was to determinevia MRI and cadaveric dissection if the paratenon iscontiguous with portions of the deep fascia of the leg,and not as an isolated structure that previous anatomicaldescriptions have portrayed.

Materials and Methods

Sixty-three magnetic resonance studies of 86 anklesscanned between 1992 and 1998 were available forreview. There were 23 bilateral studies. All of thesepatients were from the senior author's (A.S.) private prac­tice and were scanned for a variety of ankle pathologies.Inclusion criteria were axial images that included viewsthrough the watershed region. Specifically, the T1 and T2axial images of 5 mm or smaller were reviewed to see

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FIGURE 3 Intraoperative photo of mucoid degeneration of para­tenon ("watershed band").

FIGURE 4 Preoperative MRI of a patient with intratendinousdegeneration. Note "watershed band" (contiguous fascia from deepposterior leg which envelopes Achilles).

if the deep posterior leg compartment's fascia continuedposteriorward to encompass the paratenon of the Achillestendon in the watershed region (Fig. 4). If this was visual­ized to be present, it was termed a "positive." If the fasciawas not contiguous, it was termed a "negative." Analysiswas performed by the senior author and a board certi­fied musculoskeletal radiologist to determine whether thedeep fascia was contiguous or disrupted in the watershedregion, as it related to the Achilles tendon.

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FIGURE 5 Artist's depiction of "watershed band" laterally,enveloping the Achilles tendon. (Drawing courtesy of Maria Bidny,DPM)

Forty cadaver limbs, with an equal number of "rights"and "lefts" were dissected by the junior author (D.B.).None of the cadavers had known Achilles pathologyprior to expiring. (Information such as surgical historyand cause of death was available to the junior author.)Dissection was carried down to the level of the paratenonof each specimen, and then under direct visualization,verification was made if the deep fascia of the posteriorleg was contiguous with the paratenon. Specifically, if thedeep fascia of the posterior leg was directly contiguousto the paratenon, it was recorded as "positive" for the"watershed band" (Fig. 5).

Results

MRI studies of 86 ankles were reviewed. There werean equal number of right and left ankles. The deep fasciaof the posterior leg compartment was contiguous with thewatershed region in 83 ankles. In patients without thisfascial band, two had sustained previous ruptures, and onehad a previous peritenolysis (and was being scanned for"academic" purposes) (Fig. 6).

Of the 40 cadaveric limbs that were dissected, thedeep fascia of the deep posterior leg compartment wasfound to be contiguous around the watershed region in allspecimens.

FIGURE 6 Same patient (now asymptomatic) as in Figure 4 whounderwent peritenolysis and debridement. Note lack of deep fasciaconnecting to the Achilles region, minimal "high" signal and absenceof intratendinous degeneration, all of which are better seen with thisT-2 image.

Discussion

The findings of this study indicate that the deep fasciaof the posterior leg is contiguous with watershed region ofthe Achilles tendon. Being a bandlike retinacular structure,we propose the term "watershed band." We were able todemonstrate that the watershed band was present in allcadaveric specimens.

The incidence of the deep fascial band continuingposteriorly beyond the deep posterior leg compartmentto encompass the Achilles watershed region could havesignificant clinical relevance. Achilles tendon debride­ment, decompressions, peritenolysis, and so forth, allinvolve surgery in this region. Though specific mention of"releasing" this structure has not been made previously,it appears that studies with favorable results for surgeryof Achilles tendonosis/paratendonosis advocated only re­approximation of subcutaneous tissue and skin (not theparatenon) (10, 16-18).

Re-approximation of the paratenon may not allow forpostoperative swelling that occurs around the tendon. Inaddition, if the paratenon is abnormal, it may lead toadhesions. Several authors have revealed that debridementof this tissue is needed for successful peritenolysis (10,15-18). Surgeons who performed open peritenolysis havedemonstrated superior results to those who have used a

closed, percutaneous longitudinal tenotomy (15). Perhapsby not re-approximating the deep fascia, true decompres­sion of the Achilles tendon can be achieved.

The absence of the watershed band in patients withprevious ruptures may indicate that this fascia was adher­ent to the Achilles tendon, and then became disruptedwith the rupture. Although our sample size was small,we think this substantiates our findings: the deep fasciaof the posterior leg is contiguous with the paratenon.Future MRI studies on patients with Achilles ruptures mayconfirm this. Perhaps anthropologic studies comparing theevolution of other retinacular structures could show ifthe watershed band had a benefit of keeping the Achillestendon from "bowstringing."

Treatment of patients with Achilles paratendonosisoften includes massage of the deep posterior leg muscles(11, 20, 21). It may be that inflammation of these corre­sponding leg muscles places tension on the deep fasciaand retinaculi, and subsequently promotes tension on theparatenon. Because these are thickened structures, repet­itive gliding of the paratenon may cause irritation to theAchilles tendon. This may cause adhesions and secondarytendonosis. This sequence of paratendonosis developinginto tendonosis has been described by Puddu et al. (9).

Conversely, damage to the Achilles tendon can resultin adhesions of the paratenon (16-19). Perhaps serialMRI evaluation can reveal which occurs first, the "halo­sign" indicating mucoid degeneration of the paratenon,or intrasubstance degeneration of the Achilles tendonitself. We are proposing that either sequence may occur.Regardless, abnormal pathologic changes occur within theparatenon, leading to symptomatology.

Conclusion

The authors demonstrated through MRI and cadavericevaluation that the deep posterior leg fascia forms an orga­nized band that courses posteriorly to the Achilles tendonparatenon. This fascia envelopes the watershed region andis termed the "watershed band." This previously unde­scribed anatomical finding and its significance to Achillestendon pathology should be studied in more detail.

Acknowledgments

The authors would like to thank Robin Lie, DPM, EricJamrok, DPM, and Julie Clark for their contributions tothis article.

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