Lameness arising from the digital flexor tendon sheath

Lameness associated with the digital flexor tendon sheath (DFTS) is common and usually presents as asymmetric synovial effusion, often accompanied by pain on palpation in the acute stages. Diagnostic analgesia is used to confirm the location of the lameness to the DFTS by the instillation of local anaesthetic solution via one of the four access sites of the sheath. The easiest and most reliable access can be gained via the distal palmar pouch of the sheath, which extends between the two distal branches of the SDFT and between the two digital annular ligaments, along the palmar surface of the deep digital flexor tendon (DDFT). The needle can be introduced through the skin to one side of the midline and, to avoid iatrogenic damage to the deep digital flexor tendon, gently and slowly advanced at approximately 45 degrees to the skin surface until synovial fluid is seen at the needle hub. 7-10mls of local anaesthetic solution is injected for adequate desensitization of the DFTS. However, it is now routine to combine diagnostic analgesia with contrast tenography because it can indicate injuries to specific structures within the DFTS and indicate possible palmar/plantar annular ligament constriction. For this, 5mls of a water-soluble iodine-containing contrast agent is combined with 10mls local anaesthetic and a lateromedial radiography obtained after walking the horse for a few steps.

Ultrasonography is effective at identifying mid-substance pathology within a tendon sheath. However, intra-thecal DDFT tears are more difficult to identify and it can also be challenging to determine whether mid-substance injuries communicate with the synovial environment. Improvement in identifying tendon tears is achieved by using non-weight-bearing views and assessing the DDFT in oblique images obtained just distal to the proximal sesamoid bones, and displacement and thickening of the manica flexoria in longitudinal (and transverse) mid-line scans at the level of the apices of the proximal sesamoid bones. Movement of the tendons can also be assessed in non-weight bearing views where gapping between the tendons can signify adhesion formation or a fully torn manica flexoria.

Magnetic resonance imaging has superior soft tissue contrast and may be of use to identify occult lesions not seen radiographically or ultrasonographically. However, the standing MRI images may have inferior quality, especially in hindlimbs, because of greater movement artefacts which makes the diagnosis of intra-thecal tendon tears difficult. Tenoscopy is the best imaging modality for evaluating the internal structures of tendon sheaths.

Specific conditions and treatments

1. Primary tenosynovitis While a common diagnosis in the past, better imaging and the advent of tenoscopy has revealed that the majority of tenosynovitis cases are secondary to an intra- or peri-thecal soft tissue injury. Traumatic tearing of the wall of the tendon sheath does occur [1]. Most of these will heal spontaneously with rest but a small proportion can progress to the formation of a synovial outpouching or synoviocoele. They are not always significant but can cause pain and lameness if non-compressible when the limb is raised. Treatment is best achieved by tenoscopic enlargement of the synovioceole opening using ultrasound guidance [2].
2. Palmar/Plantar annular ligament syndrome Reduced contrast within the fetlock canal and the ease of passage of the arthroscope through the fetlock canal are the best ways of determining constriction by the palmar/plantar annular ligament. Constriction of the PAL is usually secondary to other pathology within the digital sheath and hence transection should only be performed with concurrent tenoscopic evaluation of the sheath.
3. Mid-substance tendon lesions Mid-substance lesions are those within the superficial digital tendon tendon (SDFT) or DDFT within the extent of the DFTS. They can be managed conservatively but, as they are contained, they lend themselves to intra-tendinous administration using the orthobiologicals. Ideally, in this location, such intra-tendinous treatments should be performed under combined tenoscopic and ultrasonographic control because of the difficulty in identifying any surface defects pre-operatively through which the product could leak, thereby reducing its potential benefit.
4. Tendon/ligament tears which communicate with the synovial cavity

a) Deep digital flexor tendon tears The deep digital flexor tendon can be injured throughout the length of the digital sheath. These injuries can be either mid-substance tears where only hyperaemia may be evident tenoscopically or have marginal tears which are usually where the tendon is under maximal compression, at the level of the fetlock joint. Treatment of these injuries involves debridement of the tear and removal of the torn fibres using a synovial resector and/or arthroscopic scissors or suction punch rongeurs. However, it is rarely possible to leave the defect completely free of prolapsed tendon fibres and subsequent healing of the defect is limited within the synovial environment resulting is a guarded prognosis, varying from 18%-40% depending on extent [3, 4].

b) Manica flexoria tears The manica flexoria is a thin loop of tendon tissue that is attached to both medial and lateral borders of the superficial digital flexor tendon and surrounds the deep digital flexor tendon within the proximal digital sheath. With the advent of tenoscopy, tearing of one or both of the attachments of this structure to the superficial digital flexor tendon has been found to be a common cause of digital sheath tenosynovitis and lameness, most commonly in the hindlimb. The tear will not heal and recurrent lameness is common with conservative management. Hence the best treatment is for the manica flexoria to be removed, which can be done tenoscopically. In contrast to the deep digital flexor tendon tears, these cases carry a good prognosis of approximately 80% returning to the same level of performance after surgery [3, 5].

c) Ligament tears into the digital sheath The straight and oblique, distal sesamoidean ligaments lie on the dorsal border of the DFTS and injuries frequently blocked to DFTS diagnostic analgesia [6]. Most of these injuries are managed conservatively but when they communicate with the digital sheath cavity and so are amenable to tenoscopic debridement. These lesions can be debrided as for DDFFT tears. The proximal scutum can also tear into the sheath and be managed in the same way tenoscopically.

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