Chapter 35 The Proximal and Middle Phalanges and Proximal Interphalangeal Joint
The term pastern originated from the shackle that was secured below the metacarpophalangeal or metatarsophalangeal joints to tether a horse to the pasture.1 The proximal interphalangeal (PIP) joint or pastern joint is a diarthrodial joint, which is formed from the distal aspect of the proximal phalanx and the proximal aspect of the middle phalanx. The pastern region is bounded dorsally by the common digital extensor tendon and on the palmar/plantar border by the distal sesamoidean ligaments, digital flexor tendons, digital flexor tendon sheath (DFTS), and the proximal and distal digital annular ligaments. The superficial digital flexor tendon (SDFT) inserts on the distal palmar/plantar aspect of the proximal phalanx and the proximal palmar/plantar aspect of the middle phalanx. The straight sesamoidean ligament arises from the base of the proximal sesamoid bones (PSBs) and extends distally to insert on the proximal palmar/plantar aspect of the middle phalanx. The oblique (middle) sesamoidean ligaments arise from the base of the PSBs and attach to the triangular region on the palmar/plantar region of the middle portion of the proximal phalanx. The medial and lateral collateral ligaments of the PIP joint provide support in the sagittal plane and attach to the collateral tubercles of the distal aspect of the proximal phalanx and proximal palmar/plantar aspect of the middle phalanx. The paired smaller abaxial and axial ligaments are located just palmar/plantar to the collateral ligaments. The neurovascular bundle of the digit runs just abaxial to the deep digital flexor tendon on the medial and lateral sides.
The palmar/plantar eminence of the middle phalanx extends proximal to the horizontal axis of the joint surface and blends proximally with a fibrocartilaginous cap at the most proximal extent of the middle phalanx. In the adult horse this is dense, and it is important to engage this portion of the bone during internal fixation of fractures or arthrodesis procedures.
The proximity of the pastern region to the ground, the paucity of soft tissue coverage over the dorsum and sides, and the important soft tissue structures on the palmar/plantar surfaces make it especially vulnerable to external trauma. Wounds in this region may involve the DFTS and warrant careful investigation. Angular deformities from growth disturbances at the pastern are uncommon but do occur. More commonly, deformities arise from physeal abnormalities of the distal aspect of the third metacarpal/metatarsal bones affecting the metacarpophalangeal/metatarsophalangeal joints. Radiological closure of the proximal physis of the proximal and middle phalanges occurs by 6 to 9 months of age, but functional closure is earlier, usually by 8 weeks of age. Radiological closure of the distal physis of the proximal phalanx occurs by 1 month of age. Radiological closure of the distal physis of the middle phalanx occurs by the time of birth.2
Physical derangements usually are obvious because of minimal soft tissue coverage of the pastern. Phalangeal fractures usually are associated with soft tissue swelling and focal pain. Effusion of the DFTS is typical of penetrating wounds of the sheath, damage to the enclosed digital flexor tendons, or injury of the sesamoidean ligaments located dorsal to the sheath. In horses with osteoarthritis (OA), new bone formation on the dorsomedial and dorsolateral aspects of the PIP joint often occurs, causing obvious swelling around the joint. Angular deformity of the digit with limb shortening often is present in horses with complex fractures of the proximal or middle phalanx. Effusion in the metacarpophalangeal or metatarsophalangeal joint is common in horses with fractures of the proximal articular surface of the proximal phalanx.
Lameness from the pastern region ranges from severe to subtle depending on the injury. Generally, disorders involving the PIP joint or DFTS cause obvious lameness, but lameness from early OA of the PIP joint may be mild, and the clinical signs and blocking pattern may be confused with foot or occasionally fetlock lameness. Pain in the pastern region may be exacerbated by distal limb flexion. There is often a weight-bearing lameness that is worse if a horse is lunged or trotted with the affected limb on the inside of a circle.
Response to intraarticular analgesia varies depending on the injury, but improvement of lameness by 50% or more implicates the PIP joint as an important source of pain. Entry into the PIP joint can be difficult, especially if new bone is present as a result of OA. Techniques for intraarticular and perineural analgesia are described in Chapter 10. Complete analgesia of the PIP joint is not always accomplished by perineural analgesia of the palmar nerves at the level of the PSBs, and a low four-point (palmar and palmar metacarpal nerves) block may be necessary. However, in some horses lameness improves with palmar/plantar digital analgesia. Intraarticular analgesia of the PIP joint should be considered in a horse with suspected metacarpophalangeal or metatarsophalangeal joint pain that has not responded to treatment. Complete analgesia of the DFTS requires intrasynovial or a low four-point block. Localization by perineural analgesia to the pastern joint does not limit the source of the lameness to the joint itself. Injury to the soft tissue structures should be carefully evaluated if radiological examination reveals no clinically significant abnormalities.
Standard radiographic examination for evaluation of the proximal phalanx includes lateromedial, dorsopalmar (plantar), dorsal 45° lateral-palmaromedial oblique, and dorsal 45° medial-palmarolateral oblique images. Oblique radiographs that are angled distally are helpful to identify osteochondral fragments on the proximal palmar/plantar aspect of the proximal phalanx.3 If a sagittal plane fracture of the proximal phalanx is suspected, dorsopalmar/plantar radiographs that are 5 degrees medial or lateral to the midsagittal plane are useful. Slight underexposure of a lateromedial image helps to identify callus on the proximodorsal aspect of the proximal phalanx associated with chronic, short, incomplete fractures. Radiographic examination of the PIP joint includes lateromedial, dorsopalmar/plantar, and oblique images. Flexed oblique images are particularly helpful for evaluation of the articular margins.3 A well-positioned and exposed dorsopalmar/plantar image may reveal subtle joint space narrowing and increased radiopacity of the subchondral bone consistent with early OA. Comparative views of the contralateral limb may be helpful.
Contrast radiography may be used to determine the course of fistulous tracts or to identify communication between wounds and the DFTS or areas of proliferative synovium within the DFTS. Contrast radiography is performed by placement of a 50% solution of diatrizoate preglumine and diatrizoate sodium (Hypaque-76, Nyromed, Princeton, New Jersey, United States) within a draining tract or directly into the synovial structure after aseptic preparation. Standard radiographs are obtained to identify communication with draining tracts or filling defects.
Computed or digital radiography is helpful in detecting subtle changes in trabecular bone architecture and opacity and is the present state of the art for radiographic examination. Computed tomography (CT) of the distal aspect of the limb offers detailed information regarding structural injury to the bony column of the digit that may not be apparent on radiographic examination.
Magnetic resonance imaging (MRI) of the pastern region is indicated in horses that have pain localized to the pastern region without radiological or ultrasonographic evidence of disease and may reveal soft tissue, bone, and/or cartilage pathology.5 A more detailed discussion of MRI is found in Chapter 21. Benign osseous cystlike lesions are identified more commonly with MRI than with radiology in the proximal and middle phalanges but are frequently not associated with pain and lameness, and their significance should not be overinterpreted.
Nuclear scintigraphy is useful in identifying incomplete fractures of the proximal phalanx. Incomplete fractures of the proximal articular margin of the proximal phalanx can occur in all breeds, but they are most commonly seen in Standardbred (STB) and Thoroughbred (TB) racehorses. Clinical signs and response to perineural analgesia vary. Nuclear scintigraphy should be considered to eliminate fracture in horses with lameness referable to the fetlock and clinical signs consistent with an incomplete fracture of the proximal phalanx. Nuclear scintigraphy can be helpful to identify bone modeling in the early phases of OA of the PIP joint. Other bone-related abnormalities in the pastern region often are evident radiologically, and scintigraphy is unnecessary.
Differential diagnoses of disorders of the pastern region of all breeds include OA, osteochondrosis, fractures, and infection. The types of fractures tend to be specific to breed or use. Fractures of the proximal phalanx occur most often in racing breeds, with the STB most often affected. Fractures of the middle phalanx occur most commonly in horses used for Western-type activities, such as reining, but are seen in all breeds during lunging or after kicks. Osteochondrosis of the proximal aspect of the proximal phalanx is common in yearling and 2-year-old TB and STB horses. Osteochondrosis of the distal aspect of the proximal phalanx and proximal aspect of the middle phalanx is less common with no breed predilection. OA of the PIP joint occurs most often in older horses used for riding or Western-type activities. It may be seen in young horses secondary to osteochondrosis.
Fractures of the proximal phalanx are important causes of lameness in all breeds. Racing breeds are particularly prone. Clinical signs vary from subtle to obvious. In horses with complete fractures, lameness is severe, and limb swelling and deformity may be present. In horses with incomplete fractures, lameness may be subtle, and careful clinical and radiographic examination may be necessary to define the fracture. Serial radiographs may be needed to detect radiolucency and callus formation associated with short incomplete fractures of the proximal phalanx. Nuclear scintigraphy may be helpful.