Tenoscopy of the Digital Flexor Tendon Sheath

Non-infected tenosynovitis of the digital flexor tendon sheath (DFTS) is a common finding in warmblood horses used as a dressage horse or as a show jumper. Chronic inflammation of the DFTS can lead to constriction of the annular ligament. The underlying cause, in many of these horses suffering from chronic distension of the DFTS, is a longitudinal tear or marginal cleft in the border of the deep digital flexor tendon.
The superficial digital flexor tendon (SDFT) and suspensory ligament are a more commonly injured in performance horses in comparison to the deep digital flexor tendon (DDFT).
Lesions in one of the lobes of the DDFT are a common diagnosis following MRI examination of the front foot.
DDFT lesions within the digital flexor tendon sheath (DFTS) are almost always associated with chronic (non-infected) tenosynovitis. The most common presentation of DDFT lesions within the DFTS, are longitudinal tears in the lateral border of the DDFT (Wright and McMahon 1999, Wilderjans et al. 2003, Smith and Wright 2006). Central core lesions, dorsal or palmar/plantar lesion in the DDFT are seen but are less common. Manica flexoria (MF) tears, longitudinal and branch tears of the SDFT, desmitis of the palmar annular ligament (PAL) and tears in the DFTS itself can also result in a chronic tenosynovitis of the DFTS.

Anatomy of the digital flexor tendon sheath and its content:
The DFTS surrounds the SDFT and DDFT palmar or plantar to the fetlock joint. The DFTS begins 4 to 7 cm proximal to the proximal sesamoid bones and extends distally to the middle third of the middle phalanx. At this level a thin wall separates the DFTS from the proximal recess of the podotrochlear bursa and the proximopalmar recess of the distal interphalangeal joint (Denoix JM 1994). The DFTS is surrounded by the PAL, the proximal and the distal digital annular ligament. The PAL attaches on the palmar/plantar aspect of the sesamoid bones and creates an inelastic canal between the sesamoid bones, intersesamoidean ligament and the PAL. The proximal digital annular ligament is a thinner quadrilateral sheet located over the palmar/plantar aspect of the proximal phalanx. This ligament is mostly adherent to, and very difficult to differentiate from, the DFTS in normal limbs. The distal digital annular ligament is located distally in the pastern and is adherent to the palmar/plantar surface of the distal part of the DFTS (Denoix JM 1994). Just proximal to the proximal sesamoid bones the SDFT encircles the DDFT forming a ring called the MF. The distal aspect of the MF is located underneath the PAL (personal observation). Proximal to the MF the DDFT is attached to the DFTS by a medial and lateral band. This band is called the mesotendon. It can easily be recognised on a transverse ultrasound image especially if the tendon sheath is distended (Dik et al, 1995). On the palmar aspect of the fetlock, the SDFT is also attached sagitaly (palmar/plantar midline of SDFT) with a mesotendon to the DFTS (Dik et al, 1995; Nixon 1990). This band can clearly be visualised on an ultrasound image of a distended DFTS when there is no important constriction of the PAL (personal observation). The mesotendon of the SDFT, both medial and lateral mesotendon of the DDFT and the MF can also clearly be visualised by tenoscopy of the DFTS (Wright and McMahon 1999, Wilderjans et al. 2003).  [...]