Chapter 36 The Metacarpophalangeal Joint
Imaging Considerations
Ultrasonographic evaluation of the fetlock can be valuable for evaluation of tendon and ligament injuries around the fetlock, especially the digital flexor tendons, the palmar annular ligament (see Chapter 74), and the dorsal synovial pad/plica, as well as the bone margins.10,11 Ultrasonography (96%) was more accurate than radiology (44%) in predicting the number and location of osseous fragments on the dorsal aspect of the metacarpophalangeal and metatarsophalangeal joints identified using arthroscopy.12 Three-dimensional imaging modalities such as computed tomography (CT)13-15 (see Chapter 20) or magnetic resonance imaging (MRI) (see Chapter 21) can also be very valuable in the fetlock, especially in areas like the distal palmar McIII condyle, the axial aspect of the PSBs, and the periarticular soft tissues.16-19
Types of Fetlock Joint Lameness
Most fetlock joint lamenesses can be categorized into one of three types:
Conditions specific to the metatarsophalangeal joint and a detailed description of sagittal fractures of the proximal phalanx are discussed elsewhere (see Chapters 35 and 42). Injuries of the digital flexor tendon sheath, superficial and deep digital flexor tendons, the palmar annular ligament, and the proximal digital annular ligament are discussed in Chapter 74. Suspensory ligament (SL) branch injuries are discussed in Chapter 72. Injuries of the distal sesamoidean ligaments are discussed in Chapter 82. Traumatic disruption of the suspensory apparatus is discussed in Chapter 104.
Acute or Repetitive Overload Injuries
Chronic Proliferative (Villonodular) Synovitis
Diagnosis
The diagnosis of proliferative synovitis is based on physical examination, radiography, ultrasonography, or a combination of these modalities. The most common radiological sign of the lesion is a crescent-shaped, radiolucent “cut-out” on the dorsal aspect of the McIII at the level of the joint capsule attachment (Figure 36-2). The proliferative lesion may undergo dystrophic mineralization and be radiologically visible. Radiographic contrast studies can be used, but ultrasonography is simpler and more reliable (Figure 36-3). Normal thickness on ultrasonographic examination has been described as less than 2 mm,10,11 but mere identification of a slightly thicker than average structure is certainly not an indication for surgical excision. Many older racehorses have a substantially thicker synovial pad without any associated discomfort. Horses with severe OA have proliferative synovitis in the palmar pouch, and a large concave outline of the distal palmar aspect of the McIII is seen proximal to the PSBs (Figure 36-4 and Figure 34-17), which is associated with a poor prognosis.