Lameness associated with non-septic tenosynovitis of the digital flexor tendon sheath (DFTS) is a common diagnosis in equine sports medicine practice. The following article will discuss identification of lameness associated with the DFTS and common pathologies.
Digital flexor tendon sheath anatomy
The DFTS is found in the fetlock region, running from the lower third of the cannon bone or metacarpal/tarsal III, and ending just proximal to the navicular bursa. This thin-walled, synovial structure contains the superficial (SDFT) and deep digital (DDFT) flexor tendons as they pass around the palmar/plantar aspect of the metacarpophalangeal joint (in the interest of ease, hereafter termed palmar but information also applies to the hindlimb). The rostral aspect is composed of the metacarpophalangeal joint bones and the intersesamoidian ligament lying between medial and lateral sesamoid bones.
The DFTS proximal to the fetlock is bordered palmarly by the palmar annular ligament (PAL), and distally by the proximal and distal digital annular ligaments. Within the proximal aspect of this structure, the manica flexoria originates from the medial and lateral aspects of the SDFT and wraps around the DDFT. The vinculae or mesotenon supply blood and nutrients to the tendons within the DFTS (Schramme and Smith, 2011).
Pathology within the DFTS can result in acute or chronic lameness. Mild, bilateral and soft fluid-filled “windgalls” are commonly identified and are rarely likely to herald pathology, although they should be considered as a sign of mild chronic tenosynovitis (Schramme and Smith, 2011). When the DFTS is unilaterally inflamed, with clearly delineated swelling and occasionally pain on palpation of the structures within, the symptoms are much more likely to be implicated in lameness.
Confirmation of the DFTS as the origin of the lameness can be achieved using either low 4- or low 6-point nerve block or infusing local anaesthetic directly into the DFTS using an intra-thecal technique. Although logical to assume DFTS-associated lameness should significantly improve with intra-thecal anaesthesia, it is important to remember that some pathologies will not. Abaxial sesamoid perineural anaesthesia may also partially improve lameness associated with DFTS pathology.
There are two commonly used sites for intra-thecal anaesthesia: either a lateral or medial approach, proximal to the palmar annular ligament or between the proximal and distal digital annular ligaments in the palmar pastern region (Schramme and Smith, 2011). Use of the proximal site may be complicated by villous hypertrophy, making fluid retrieval and confirmation of successful needle placement challenging.
After localisation of the lameness to the DFTS has been achieved, diagnostic imaging should be undertaken. Although the DFTS is primarily a soft tissue structure, screening radiography to ascertain absence of fetlock or sesamoid bone pathology is an important aspect of any investigation.
Contrast radiography (tenography) together with a lateral to medial radiographic projection is being used increasingly routinely to identify abnormal contrast patterns which can be consistent with pathologies such as manica flexoria tears (Kent et al., 2020; Figure 1).
This is the first step in assessment of the DFTS. It is cost effective and easily achieved in first opinion and referral practice. Horses and ponies with thick skin can benefit from soaking the leg in water or ultrasonographic gel and wrapping with cling film for a number of hours before attempting to ultrasound. Assessment of each of the soft tissue structures, particularly fibre pattern in the SDFT and DDFT, thickening of the PAL and integrity of the manica flexoria (Figure 2), should be completed in a methodical fashion. Diagnosis may be enhanced by using non-weight-bearing and dynamic ultrasonography (Garcia da Fonseca et al., 2019), although this requires practice to confidently assess. Some pathologies such as longitudinal DDFT tears and manica flexoria tears can be difficult to visualise and other imaging methods may add further information and help guide treatment plans and prognosis.
Magnetic resonance imaging (MRI)
MRI can provide additional information about the soft tissue structures within the DFTS and their interaction with the associated bony structures. This is particularly useful to identify injuries to the digital flexor tendons, intersesamoidian ligament or the peri-thecal distal sesamoidian ligaments. Information gained can then be used to help plan treatment: it is unlikely that tenoscopy will be successful if there is concurrent severe pathology in the DDFT, distal sesamoidian ligaments or manica flexoria which have not been identified. MRI can help in this identification and enable owners to understand the prognosis of the injury prior to the horse undergoing tenoscopy.
Tenoscopy is carried out under general anaesthesia and despite the perianaesthetic risks, tenoscopy enables the veterinary surgeon to provide a clearer diagnosis and also enables therapeutic intervention. As well as visualising the structures of the DFTS, probes can be used to assess the structural integrity of the tissue and then resection of any damaged or pathological tissue can be done within the same procedure. The palmar annular ligament can also be sectioned longitudinally if it is contributing to the lameness, or if it is so constrictive that a full tenoscopic assessment cannot be completed.
Linear tears of the DDFT are the most common, non-septic cause of tenosynovitis associated with the DFTS. These can be difficult to detect on ultrasound, with a quoted 75 percent specificity and 63 percent sensitivity for ultrasonographic detection of longitudinal DDFT tears (Arensburg et al., 2011). This paper also described a higher prevalence of forelimb rather than hindlimb injuries in showjumpers. Treatment for these lesions include tenoscopic debridement, removal of any adhesions and lavage of the structure to remove inflammatory mediators. Reported success rates for return to athletic function range from 38 to 54 percent (Arensburg et al., 2011; Kent et al., 2020).
Manica flexoria tears
Manica flexoria tears are another common cause of non-septic tenosynovitis of the DFTS. These are more often found in the hindlimb and usually on the medial aspect of the attachment to the SDFT (Findley, 2012; Kent et al., 2020). Often concurrent adhesions are found between the manica flexoria and the DFTS, especially if the condition has been of a chronic nature. Interestingly, ponies and cobs seem to be over-represented. Tenoscopic resection of the manica flexoria can yield a favourable prognosis of 67 to 78 percent return to athletic function (Findley, 2012; Kent et al., 2020).
Palmar annular ligament (PAL) pathology
The PAL is usually found to be less than 2mm thick on ultrasound examination. In some cob types, it can be 2 to 4mm without being the inciting cause of the tenosynovitis so interpretation must be undertaken with care in these breeds (Findley, 2012). If the PAL is thickened and causing constriction of the DFTS structures, it will usually be found in the hindlimbs and often shows a characteristic “notch” in the proximal DFTS when viewed from a lateral to medial angle (Arensburg et al., 2011). Resolution is achieved by surgically sectioning the ligament. This can be done using an open or closed technique and has been suggested to have a success rate of 69 percent if it is a primary injury (Findley, 2012), although others quote poorer prognoses. Concurrent DDFT pathology will understandably reduce the prognosis.
Acute primary tenosynovitis
This does occur and can be rapidly resolved with rest, anti-inflammatory therapy and potentially even intra-thecal medication. It is imperative to assess the integrity of the structures within the DFTS before diagnosing acute primary tenosynovitis as any intra-thecal medication is likely to be detrimental to healing if tendinous or ligamentous structures are damaged (Schramme and Smith, 2011).
SDFT injuries within the DFTS do occasionally occur; however, these are much less frequent. It is important not to confuse these lesions with the normal mesotenon or vinculae when undertaking ultrasonographic examination (Findley, 2012).
Diagnosis of septic tenosynovitis should be aided by the presence of a wound or penetrating injury. The horse would be expected to show acute onset severe lameness with obvious distension of the DFTS if the wound is not continuing to leak synovial fluid. Rapid diagnosis and treatment are vital to ensure the best chance of return to full athletic function and minimise adhesion formation post-surgical treatment.
Diagnosis of sepsis can be confirmed by an increased nucleated cell count and total protein within the synovial fluid, confirmation of fluid leaking from the DFTS when injected at a distant site, or contrast radiography (Schramme and Smith, 2011). Treatment involves copious lavage, either via needle flush or tenoscopy under general anaesthesia. Tenoscopy allows better access, improved flushing and also the chance to visualise and remove adhesions or foreign material within the DFTS.