By Rachel Tucker BSc MVetMed DipECVS MRCVS 

There are many conditions affecting tendon and ligaments which cause lameness in the horse. We often focus our attention on injuries affecting performance animals, such as superficial digital flexor tendinitis in race horses, or proximal suspensory ligament desmitis in sport horses. In the webinar called Tendon and Ligament injuries in the non-athletic horse, I will discuss some common soft tissue causes of lameness which can occur in any equine, irrespective of their level of use. We will also discuss some conditions which are more commonly observed in the older animal. 

 1) Digital flexor tendon sheath pathology 

2) Superficial digital flexor tendinopathy 

3) Check ligament desmitis (ALDDFT) 

4) Luxation of the superficial digital flexor tendon from the calcaneus 

5) Peroneus tertius rupture 

6) Suspensory apparatus degeneration 

1) Digital flexor tendon sheath pathology 

The digital sheath extends from the distal metacarpal/metatarsal region to the foot on the palmar/plantar aspect of the limb. It encompasses the superficial and deep digital flexor tendons as they past over the palmar/plantar aspect of the fetlock. A number of soft tissue lesion can cause lameness within the sheath.  

Tendon sheath lameness particularly affects middle age and older, pony and cob breeds, although all breeds and ages are represented. Lameness is usually moderate in severity with a positive response to distal limb flexion. Tenosynovitis is present and a pain response is often elicited on palpation of the digital sheath. Firm, asymmetric thickening may be palpated. Diagnosis is based on clinical presentation, diagnostic analgesia and ultrasound findings. Positive contrast tenography has also been used to diagnoses specific soft tissue lesions. 

Synovial effusion enables detailed ultrasonographic assessment of soft tissue structures within the digital sheath. The palmar/plantar annular ligament is imaged at the level of the sesamoid bones and is difficult to image in the normal horse due to its small size (usually 0.6-1.0 mm). Visualisation is most easily achieved by moving the ultrasound probe medially or laterally in transverse section to image the ligament close to its attachment on the sesamoid bone. Features of a tendon border tear within the sheath include the presence of soft tissue material within the sheath, visualisation of a tear, irregular tendon margin and irregularities of the tendon when imaged in a non weightbearing position. There will be concurrent tenosynovitis. Mesotenon attachments at the lateral and medial margin of the DDFT should not be mistaken for adhesions. The manica flexoria can be imaged as a thin band enveloping the dorsal border of the DDFT, just proximal to the palmar/plantar annular ligament. Tearing and displacement of the manica flexoria may be identified ultrasonographically. Tendon damage can be difficult to image and a negative finding does not rule out the presence of a tear. Tenoscopy should be considered in cases which fail to respond to conservative management or if positive findings are identified.  

Seignour M., Coudry V., Norris R. and Denoix J-M (2012) Ultrasonographic examination of the palmar/plantar aspect of the fetlock in the horse: Technique and normal images. Eq. Vet. Edu. 24(1) 19-29. 

Fiske Jackson A.R., Barker W.H.J. Eliashar E., Foy, K., Smith R.K.W. (2013) The use of intrathecal analgesia and contrast radiography as preoperative diagnostic methods for digital flexor tendon sheath pathology. Eq. Vet. J 45(1) 36-40. 

Findley J.A., Oliveira F.De, Bladon B. (2012) Tenoscopic surgical treatment of tears of the manica flexoria in 53 horses. Vet. Surg. 41(8) 924-930. 

Arensburg L., Wilderjans H., Simon O., DeWulf J. and Boussauw B. (2011) Nonseptic tenosynovitis of the digital flexor tendon sheath caused by longitudinal tears in the digital flexor tendons: A retrospective study of 135 tenoscopic procedures Eq. Vet. J. 43(6) 660-668. 

Smith M.R.W. and Wright I.M. (2006) Noninfected tenosynovitis of the digital flexor tendon sheath: a retrospective analysis of 76 cases. Eq. Vet. J. 38(2) 134-141. 

2)Superficial digital flexor (SDF) tendinopathy 

This condition is not confined to athletic equines performing fast work. Severe SDF tendinopathy is seen in geriatric equines with no known inciting cause. Injury can occur in the field or following light work, commonly presenting with marked lameness and significant local soft tissue swelling. Tendon pathology is usually severe, often at the level of the proximal metacarpus, and may extend into the carpal sheath. Conservative treatment is indicated, with box rest and NSAID in the first instance. I find application of a Robert Jones bandage helpful to improve comfort. Ultrasound examination at 7 days post injury reveals the full extent of the tendon pathology and provides comparison for ongoing monitoring. Prognosis for ridden work is guarded and prolonged rehabilitation is required. Reinjury is common in my experience.  

3) Check ligament desmitis (ALDDFT) 

The accessory ligament of the deep digital flexor (ALDDFT or inferior check ligament) arises from the palmar carpal ligament at the palmar aspect of the 3rd carpal bone, where it lies on the dorsal surface of the carpal sheath. It runs distally to join the dorsal surface of the deep digital flexor tendon (DDFT) at the level of the mid metacarpus and is of a similar size to the DDFT. The ligament is slightly laterally positioned and runs slightly obliquely as compared to the flexor tendons making on incidence ultrasound imaging of the ligament at a slightly different orientation to that of the flexor tendons. The ligament shares the load of the DDFT in mid-stance and prevents overstretching of the DDFT during maximal extension of the joints of the distal limb.  In the hindlimb, the ALDDFT is small and occasionally absent. Pathology in the hindlimb is rare but can occur. Palpable abnormality may be absent due to the prominent splint bones and flexural deformity of the metatarsophalangeal joint is often a feature.  

Degenerative aging changes occur in the ALDDFT and force to failure of the ligament is lower in older horses compared to younger animals. Injury is more prevalent in the mature animal. Desmitis is usually associated with sudden onset lameness of moderate severity, with local heat, swelling and pain on palpation. Local swelling often persists long term. Ultrasound is recommended for diagnosis and monitoring of healing. Prognosis is good in uncomplicated cases, with 10/13 horses returning to full work in one study within 5-9 months (Dyson 1991). Adhesions can form between the ALDDFT and adjacent soft tissue structures and if severe, this causes flexion of the metacarpophalangeal joint and elevation of the heel. Controlled exercise is therefore an important part of rehabilitation, however uncontrolled turnout is considered detrimental to healing.  Concurrent SDFT injury also significantly lowers prognosis and subsequent injury of the contralateral check ligament may also occur.  

Dyson S. (1991) Desmitis of the accessory ligament of the deep digital flexor tendon: 27 cases (1986-1990) 23(6) 438-444. 

4) Luxation of the superficial digital flexor tendon from the calcaneus (tuber calcanei) 

In the hindlimb, the superficial digital flexor originates from the supracondylar fossa of the distal femur and extends down the caudal aspect of the limb to insert via the plantar fibrocartilage at the proximal phalanx and distal phalanx. In the distal part of the crus the SDF winds around the medial aspect of the gastrocnemius tendon to lie on its caudal surface. It widens to form a fibrocartilaginous cap as it runs over the apex of the calcaneus, which is attached medially and laterally to the calcaneus via a thick, ligamentous retinaculum. This attachment maintains the position of the SDF in this location and enables it to function as part of the reciprocal (stay) apparatus.  

Luxation or subluxation of the SDFT from the point of the hock can occur secondary to a retinacular tear or a defect in the SDFT fibrocartilage cap. The most common presentation is lateral displacement of the tendon due to a medial retinacular tear. The tendon may lie permanently lateral to the calcaneus or may subluxate from a normal position at rest (hock extended) to a displaced position during limb flexion. Initially, lameness is severe and the horse may be distressed, particularly if the tendon is unstable during movement of the limb. Diagnosis is based on clinical examination. Ultrasound imaging is useful, although note that the tendon may be normally positioned at rest. If the tendon is permanently luxated in a lateral position, conservative management is warranted, with horses reported to return to full athletic function, including jumping. Residual mechanical lameness will remain however, limiting some athletic ability, particularly in dressage. Unstable subluxation causes persistent lameness and is best treated surgically. Tenoscopic transection and resection of the torn fibrocartilage causes permanent stable luxation of the SDF. A few cases of repair have been reported but this is difficult to achieve and requires placement of a full limb cast.  

Wright I.M. and Minshall G.J. (2012) Injuries of the calcaneal insertions of the superficial digital flexor tendon in 19 horses Eq. Vet. J. 44 136-142. 

5) Peroneus tertius rupture 

The peroneus tertius (fibularis tertius) is a tendinous band which forms the cranial portion of the reciprocal apparatus in the hindlimb. It originates at the extensor fossa of the distal femur and inserts on the dorsoproximal aspect of the third metatarsus. Any traumatic event which causes forced extension of the hindlimb when the hock is fixed can cause traumatic rupture of this structure. Rupture usually occurs in the mid portion of the tendon at the level of the mid crus, but can occur more distally. A wound to the dorsal hock can cause transection of the tendon. With complete rupture the stifle can be flexed with the hock in extension – n.b. this may not be clear at walk or trot and the limb must be manipulated to test this. Three months box rest with slow resumption of work is usually curative and healing can be monitored ultrasonographically. Koening et al (2002) report15/21 horses returning to full athletic function with a mean convalescent period of 41 weeks. 

Koening J, Cruz A, Genovese R, et al: Rupture of the peroneus tertius tendon in 25 horses, Proc Am Assoc Equine Pract 48:326, 2002.  

6) Suspensory apparatus degeneration 

This condition is most commonly seen in geriatric retired sport horses and broodmares. Previous proximal suspensory desmitis may have been a feature and a straight hock conformation is common. There is progressive hyperextension of the hind fetlock joints, with the dorsal pastern becoming parallel to the ground. The condition is bilateral and usually results in poor hindlimb action rather than overt lameness. The condition is generally progressive and horses are usually considered pasture sound but not fit for proper athletic function.