Page 546 - Adams and Stashak's Lameness in Horses, 7th Edition

512 Chapter 4

THE PASTERN

VetBooks.ir ashlEE E. Watts and gaRy M. BaxtER

The proximal interphalangeal (PIP) joint or pastern OSTEOARTHRITIS (OA) OF THE PIP JOINT
joint is a diarthrodial joint, which is formed from the (HIGH RINGBONE)
distal aspect of the proximal phalanx (P1) and the
proximal aspect of the middle phalanx (P2). The pas- The term “high ringbone” is often used synonymously
tern region is bounded dorsally by the common or long with OA of the PIP joint and “low ringbone” with OA of
digital extensor tendon together with the dorsal the distal interphalangeal joint. Most cases of ringbone
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branches of the suspensory ligament. Palmar/plantar present with both joint and periarticular pathology. OA
support structures of the pastern region are formed by or degenerative joint disease of the PIP joint is an impor-
the distal sesamoidean ligaments (DSLs) (straight, tant and common cause of lameness in virtually all
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oblique, cruciate, and short), superficial digital flexor breeds and ages of horses. Older horses appear to be at
tendon (SDFT), deep digital flexor tendon (DDFT), and greater risk and the forelimbs are more frequently
proximal and distal digital annular ligaments within affected than the hindlimbs. Secondary OA from P2 frac-
the digital flexor tendon sheath (DFTS). The medial and tures (particularly palmar/plantar eminence fractures) or
lateral collateral ligaments provide support in the sagit- OC occurs more commonly in the hindlimbs. 31,71
tal plane.
Abnormalities such as dorsal swelling or bony Etiology
enlargement in the pastern region are often obvious due
to minimal soft tissue in the area. The severity of lame- Chronic repetitive trauma of the PIP joint and surround-
ness from the pastern region ranges from subtle to severe ing structures is thought to be the most common cause of
depending on the injury. Generally, injuries such as frac- PIP joint OA. Inherent conformational traits and the type of
tures that involve the PIP joint or tendinitis within the work the horse performs may also contribute to problems
DFTS cause obvious lameness, whereas lameness due to in the PIP joint. For example, horses that are base narrow
early osteoarthritis (OA) of the PIP joint or strains of the and toe out are thought to be predisposed to OA on the
DSLs may be mild. Pain and lameness in the pastern lateral side of the joint, whereas horses that are base wide
region is often exacerbated by distal limb flexion or and toe in are believed to be predisposed to injury on the
lunging the horse with the affected limb on the inside of medial side of the joint. Pasterns that are overly upright
the circle. may also result in increased concussion to the PIP joint
Complete analgesia of the pastern region by peri-
neural analgesia is variable. Occasionally, horses will
have complete analgesia after a palmar digital (PD)
nerve block, most will not have complete analgesia
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until a basisesamoid or abaxial nerve block has been
performed, and still others may require a low four‐
point nerve block. When blocking the pastern region,
horses with suspected stress fractures of P1 should
not be blocked to avoid further displacement of the
fracture.
Radiography is important for the initial characteri-
zation of the injury. Ultrasonographic evaluation of
the pastern is an integral part of characterizing the
extent of any soft tissue injury. Additionally, nuclear
scintigraphy, computed tomography (CT), magnetic
resonance imaging (MRI), or tenoscopy of the DFTS
may all be important to provide additional informa-
tion necessary for a complete and accurate diagnosis
and prognosis.
Differential diagnoses for disorders of the pastern
region include PIP joint OA, osteochondrosis (OC),
fractures, bone bruises, luxation/subluxation of the PIP
joint, infection, lacerations, and soft tissue injuries;
however, the types of injuries are often breed or use
specific. For the purposes of this discussion, conditions
of the pastern will include bone and joint abnormali-
ties of P1 and P2 and the PIP joint and soft tissue inju-
ries of the palmar/plantar aspect of the pastern
including the DSLs, digital annular ligaments, distal
branches of the SDFT, and the DDFT within the distal Figure 4.85. An upright pastern conformation predisposes to
aspect of the DFTS. pastern OA.