How do free tendon and intramuscular​ ​tendon differ in structure, function and pathology?

Structurally, ​ ​the​ ​cross-sectional area ​of​ ​the​ isolated​ intramuscular​ ​tendon ​is​ ​substantially ​smaller​ ​than the free tendon it contributes to. At​ ​a histological​ ​level​,​​ ​it​ ​is​ ​unlikely​ ​to​ ​have​ ​the well-aligned fascicular ​bundles​ ​and the specialised​ ​interfascicular matrix seen​ ​in​ ​free​ ​tendon. The intramuscular​ ​tendon is composed primarily of type 1 collagen, organisationally more reticular at the endomyseal, perimyseal and epimyseal levels as it accumulates muscle forces from varying angles of pennation.6

Functionally, the free tendon can ​tolerate​ ​high​ ​strain​ ​rates​ ​to​ ​store​ ​and​ ​release​ ​energy. Resultant strains​ ​of up​ ​to​ ​8%–10% are ​mostly​ derived from movement ​between​ ​the​ ​fascicle​ ​bundles​ ​in the interfascicular matrix, ​ ​rather than​​ ​of tendon​ ​collagen​ ​fibres themselves​7​​.​ In contrast, the intramuscular​ ​tendon in an activated musculo-tendinous unit​ is considerably stiffer, with ​strains of ​ ​2%–2.6%6 reported for triceps surae. Further, there is reported variability in longitudinal strain (some areas may shorten) and lateral expansion of up to 5%; considered a result of oblique tension from pennate contributing fibrils as well as expansion of the whole muscle as it shortens.6 These properties may exist within other muscle groups. Ultimately, the lack of fascicular bundles and interfascicular matrix result in a functionally stiffer intramuscular​ ​tendon, which cannot ​store​ ​and​ ​release​ ​energy​ ​like​​ ​free​ tendon.

Pathologically, free​ ​tendon​ ​succumbs​ ​to​ ​an overuse​ ​tendon​ ​pathology, ​ ​eventually​ ​becoming​ ​​degenerative in nature with little capacity to repair ​as​ ​there​ ​is​ ​little or no bleeding. ​ Over time, the collateral regions of the tendon appear to remodel and increase tendon diameter to share load.8 Rarely, ruptures occur in the degenerative region, where an ​inflammation, ​ ​proliferation​ ​and​ ​maturation​ response is triggered, resulting in a new matrix ​ and considerably larger free​ ​tendon​.

It is unlikely that intramuscular tendons have an overuse pathology and a pre-existing degenerative pathology as they have a higher vascular perfusion than free tendon. However, unlike the satellite cell response of myotendinous repair and similar to rupture in free tendon, intramuscular tendon ruptures result​ ​in​ ​bleeding​ ​and an inflammation, proliferation and maturation response​,​ resulting in formation of hypertrophic intramuscular tendon scar tissue. Retraction of the stumps​ of intramuscular tendon lesions is generally limited as the​ ​surrounding​ ​​ ​muscle acts as a splint, which differs in free tendon ruptures. As such, surgery is rarely indicated.

Pain, generally localised at the tendon bone junction, is the presenting clinical feature of free tendon injury; this differs somewhat from recurrent structural failure and variable pain in IMT injury. While the sensory nerve supply of free tendons principally resides in the peritendon supplying the periphery of the tendon only, very​ ​little​ ​is​ ​understood ​about​ ​the neural​ ​supply of the intramuscular tendon; ​in​ ​particular, those​ ​Iintramuscular tendons​ ​that​ ​are​ ​deep​ ​within​ ​a​ ​muscle,​ ​as​ ​opposed​ ​to​ ​the​ ​more superficial​ ​aponeuroses.​ ​Clinically,​​ ​particularly​ ​in intramuscular tendon strains of soleus and hamstring,​ ​ ​presenting symptoms​ may be of ​​ ​progressive​ ​tightness​ ​or​ ​acute​ ​pain,​ ​which​ ​further clouds​ ​clinical ​assessment​ ​and​ ​prognoses.

This editorial highlights why intramuscular tendon does not behave like a free tendon either functionally or when injured. While many of the exercise progressions may be similar, there are symptomatic, structural and mechanical differences that should influence early and late phase rehabilitation principles. Perhaps the most appropriate term for the pathology should not include ‘tendon’ as it misleads our understanding of both the pathology and its management. ‘Intramuscular aponeurosis’ or ‘intramuscular connective tissue’ are appropriate terms that reflect the unique structural, behavioural and pathological properties.