Approaches to the distended digital flexor tendon sheath

During this lecture the most common findings during tenoscopy of the digital flexor tendon will be discussed. In warmblood horses the most common lesion is a longitudinal tear in the lateral border of the deep digital flexor tendon (DDFT). We will discuss the treatment, the outcome and problems we are facing in the healing of those lesions. The prognosis for full athletic recovery long term is still poor (< 50 %) despite more than 10 years experience in diagnosing and treating them. We will discuss where we are now in the treatment and where we want to go in the future.

Non-infected tenosynovitis of the digital flexor tendon sheath (DFTS) is a common finding in warmblood horses especially in show jumpers. Chronic inflammation of the DFTS can lead to constriction of the annular ligament. In our hospital case-load, almost 80% of horses suffering from a unilateral chronic distension of the DFTS in a front limb, a longitudinal tear or marginal cleft in the border of the deep digital flexor tendon is the primary source of inflammation and subsequent distension.

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 EVJ, 1999, Wilderjans et al. EVJ, 2003, Smith and Wright EVJ 2006, Arensburg and Wilderjans 2011 EVJ Equine vet. J. (2011) 43 (6) 660-668). Central core lesions, dorsal or palmar/plantar lesion in the DDFT are also diagnosed but are less common.

Other less common lesions that can cause tenosynovitis of the digital flexor tendon sheath are: Manica flexoria (MF) tears, longitudinal and branch tears of the SDFT, desmitis of the palmar annular ligament (PAL) and tears in the DFTS.

Anatomy of the digital flexor tendon sheath

The DFTS surrounds the SDFT and DDFT palmar or plantar to the fetlock joint. The DFTS starts 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 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 digital manica flexoria is a thinner sheet located in the pastern, dorsal to the DDFT and palmar/plantar to the straight sesamoidean ligament. There is a membranous and a tendinous type. Pathology is uncommon but a few cases are described and clinicians should be aware of the variations in order to perform a correct interpretations during ultrasound and tenoscopy.

More proximal in the fetlock canal, the SDFT encircles the DDFT forming a ring called the manica flexoria (MF). The distal aspect of the MF is located underneath the PAL.

Proximal in the DFTS and underneath the MF the DDFT is attached to the tendon sheath wall by a medial and lateral band. This band is called the mesotendon of the DDFT (lateral and medial mesotendon). It can easily be recognised on a transverse ultrasound image especially if the tendon sheath is distended. On the palmar aspect of the fetlock, the SDFT is also attached sagittaly (palmar/plantar midline of SDFT) with a mesotendon to the DFTS (Dik et al, 1995; Nixon 1990). This band, the mesotendon of the SDFT, can clearly be visualised on an ultrasound image of a distended DFTS when there is no important constriction of the PAL. 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 EVJ 1999, Wilderjans et al. EVJ 2003) and are important reference points/landmarks when performing a tenoscopic inspections of the DFTS.

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