34th Annual Scientific Meeting proceedings

Introduction
The carpal sheath of the digital flexor tendons extends from the junction of the middle and distal one thirds of the radius to the middle third of the metacarpus. Notionally, it is sometimes descriptively subdivided into proximal (radial), carpal and distal (metacarpal) regions. The carpal canal is enclosed by the carpal flexor retinaculum and consequently is less voluminous than its proximal and distal counterparts. Proximally the sheath contains the myotendinous junctions of superficial and deep digital flexors (SDF & DDF): distally it terminates at the junction of the DDF tendon (DDFT) with its accessory ligament (ALDDF) (Sisson 1975; Leach et al 1981). Medially the DDFT and SDF tendon (SDFT) are connected by a common mesotenon from the caudal sheath wall. This creates a cul-de-sac between the tendons and precludes circumferential tenoscopic evaluation medially. The tendon of insertion and variable amounts of the muscle belly of the radial head of the DDF are also intrathecal. The tendon has a mesotenon from the caudomedial sheath wall and  joins the principal tendon (from its humeral and ulnar heads) in the distal antebrachium.

The proximal medial sheath is bounded by the accessory ligament of the SDF (ALSDF) and tendon of insertion of flexor carpi radialis (FCR). Both are covered by predominantly villous synovium. The median artery and nerve are medial to the caudal ALSDF. The remaining synovium in the proximal recess is villous.

The palmar carpal ligament forms the dorsal boundary of the carpal portion of the sheath and this blends laterally with the axial fibrocartilage of the accessory carpal bone. The carpal flexor retinaculum extends dorsally from this to form the medial boundary where proximally this blends imperceptibly with the distal ALSDF. The synovium in this region is largely avillous.

In the proximal metacarpus the synovium is again villous. The dorsal sheath is bounded by the ALDDF. The medial palmar artery and nerve are subsynovial in the mesotenon.

The universal portal for comprehensive tenoscopic evaluation and most surgical procedures is proximolateral (Southwood et al 1998; McIlraith et al 2014) but insertion of the arthroscope into the metacarpal sheath (Cauvin et al 1997) can occasionally be of benefit.

The importance of comprehensive tenoscopic evaluation of every case cannot be overstated. This should be systematic and repeated on all occasions. Surgeons are encouraged initially to follow recommended guidelines (McIlwraith et al 2014) but with experience can modify these to create their own routine. It generally takes only 1-2 minutes to visualize all of the accessible structures and surfaces. Dorsal recumbency is strongly recommended.

Bullet points for the presenter’s most common tenoscopic procedures follow. There is substantial commonality in presenting signs (lameness, distension, acute phase intrathecal haemorrhage). Differentiation is generally possible with radiography and ultrasonography but additional and complex lesions are common; thorough tenoscopic evaluation therefore remains important.

Osteochondromas

• Monostotic
• Generally considered to result from translocation of physeal chondrocytes
• Radiography is a good predictor of size and shape
• Clinical signs principally from impingement on DDFT but dynamic relationship with other sheath structures occasionally places the ALSDF and adjacent median neurovascular bundle at risk
• Damage to DDFT and degree of lameness do not correlate with osteochondroma size
• Surgery includes removal of lacerated tendon tissue
• Technique for amputation of the osteochondroma is determined by its size
• Osseous debridement is conservative
• Recurrence is rare
• Wright & Minshall (2012a)

Tears of the radial head of the DDF

• Injuries appear (endoscopically) to be avulsions from the principal (conjoined humeral and ulnar) tendon
• Ultrasonographic predictability is good
• Surgical removal of torn tissue aims to reduce exposure of disrupted collagen tissue and create a suitable environment for second intention healing
• Prognosis for athletic work is good
• Minshall G J & Wright I M (2012b)

Intrathecal disruption of the ALSDF

• Originates from an irregular ridge between the second and third quadrants of the caudal medial radial diaphysis. The proximal portion is extrathecal but the bulk of the ligament is subsynovial
• It is thickest proximally where there is frequently a large perforating artery which is a short direct branch of the median artery
• Other branches of the palmar carpal rete pervade the ALSDF and are also thought to be responsible for intrathecal haemorrhage that often accompanies injury
• Proximity and location of the median artery and nerve are critical to surgical interference
• Ultrasonography is usually diagnostic but does not predict the location(s) of thecal communication
• Tears into the sheath are most common cranial to the DDFT (when the carpus is slightly flexed at surgery) but can also occur in the cul de sac between the SDFT & DDFT or caudomedial to the SDFT
• Surgery involves removal of torn, extruded tissue
• Caution is critical; iatrogenic damage to the median artery and/or nerve are serious but avoidable complications
• Large tears may benefit from concurrent desmotomy
• Published results (Minshall & Wright 2015) understate return to work

Fractures of the accessory carpal bone

• Most dorsal (frontal) plane fractures occur in the middle third and therefore communicate with carpal sheath 
• Comminution is common; displaced fragments can displace medially (intrathecal) or laterally (extrathecal); smaller fragments and fracture debris can also migrate into the most distal recess of the sheath in the middle of the metacarpus
• Carpal flexion results in distraction and displacement
• Although desirable (de Preux et al 2022; Fürst & Jackson 2022; Aßmann et al 2022) reduction and repair are commonly not possible
• Instability  frequently results in axial rotation of the principal dorsal fragment, sometimes with associated comminution, to impinge and lacerate the lateral margin of the DDFT
• Fibrous nonunion of the fracture can be adequate for return to soundness but damage to and continued impingement of the DDFT  frequently results in lameness, restricted flexion and persistent distension
• Surgery involves removal of protruding bone and/or fragments and debridement of lacerated tendon
• Intra-operative dynamic assessment of carpal canal clearance is critical to success
• Minshall & Wright (2014)                                            

References

1. Sisson S (1975) Equine myology In: The Anatomy of the Domestic Animals 5^th edn Ed: R G Getty W B Saunders, Philadelphia pp 376 – 453
2. Leach D, Harland R & Burko B (1981) The anatomy of the carpal sheath of the horse J Anat 133 301 -307
3. Southwood L L, Stashak T S & Kainer R A (1998) Tenoscopic anatomy of the equine carpal flexor synovial sheath Vet Surg 27 150 – 157
4. McIlwraith C W, Nixon A J & Wright I M (2014) Diagnostic and Surgical Arthroscopy in the Horse 4^th edn Elsevier pp 344 – 386
5. Cauvin E R J, Munroe G A & Boyd J S (1997) Endoscopic examination of the carpal flexor tendon sheath in horses. Equine Vet J 29 459 – 466
6. Wright I M & Minshall G J (2012a) Clinical, radiological and ultrasonographic features, treatment and outcome in 22 horses with caudal distal radial osteochondromata. Equine Vet J 44 319 – 324
7. Minshall G J & Wright I M (2012b) Tenosynovitis of the carpal sheath of the digital flexor tendons associated with tears of the radial head of the deep digital flexor: observations in 11 horses. Equine Vet J 44 76 -78
8. Minshall G J & Wright I M (2015) Diagnosis and treatment of intrathecal tears of the accessory ligament of the superficial digital flexor. Equine Vet J 47 48 – 53
9. de Preux, M., Van der Vekens, E., Racine, J., Sangiorgio, D., Klopfenstein Bregger, M.D., Brünisholz, H.P. et al. (2022) Accessory carpal bone fracture repair by means of computer-assisted orthopaedic surgery in a Warmblood stallion. Equine Vet Educ, 34, 478–484.
10. Fürst  A & Jackson M (2022) Fracture of the accessory carpal bone – conservative or surgical therapy? Equine Vet Educ 34 566 – 568
11. Aßmann A D, Fürst A E, Bischofberger A S (2022) Standing osteosynthesis of an accessory carpal bone fracture in a Warmblood mare with a 6-hole 3.5 talonavicular fusion plate and 3.5-mm screw https://doi.org/10.1111/eve.137
12. Minshall G J & Wright I M (2014) Frontal plane fractures of the accessory carpal bone and implications for the carpal sheath of the digital flexor tendons. Equine Vet J 46 579 - 584