Chapter 33 The Distal Phalanx and Distal Interphalangeal Joint
Primary Pain Associated with the Distal Interphalangeal Joint
Functional Anatomy
The distal interphalangeal (DIP) joint is a complex structure comprising not only the articulation between the middle and distal phalanges, with supporting collateral ligaments, but also the articulation with the navicular bone. It has a close relationship with the distal sesamoidean impar ligament (DSIL) and the collateral sesamoidean ligaments. The DSIL consists of bundles of longitudinally orientated collagen fibers, interspersed by synovial invaginations from the DIP joint and penetrating blood vessels.
The DIP joint can move in three planes with flexion and extension movements in the sagittal plane, lateromedial movements in the frontal plane, and rotation and sliding in the transverse plane.1,2 During normal load bearing and propulsion on a flat, level surface, DIP joint movement is principally flexion and extension. On an uneven surface and a circle, or if the foot is unbalanced, passive movements result in the distal phalanx sliding and twisting relative to the middle phalanx. Movement is restricted by the collateral ligaments of the DIP joint, the deep digital flexor tendon (DDFT), the distal digital annular ligament, the DSIL, and collateral sesamoidean ligaments. The degree of sliding and axial rotation within the DIP joint3 may predispose the horse to DIP joint injury and may explain why lameness associated with the DIP joint frequently is accentuated on a circle.
History
Lameness associated with the DIP joint may be acute or insidious in onset and unilateral or bilateral. It is more common in forelimbs than in hindlimbs but does occur in both. Lameness in horses with unilateral disease tends to be sudden in onset, but those with bilateral lameness may be evaluated because of poor performance (e.g., shortened stride, unwillingness to jump drop fences, or unwillingness to land with one forelimb leading).
Clinical Signs
The DIP joint capsule often is distended unilaterally or bilaterally. Distention of the joint capsule also can occur in clinically sound horses; therefore this finding is not pathognomonic for DIP joint pain. However, chronic distention does reflect synovitis, and treatment may reduce the risk of future problems. It is possible that chronic distention can predispose to low-grade instability of the joint. The dorsal proximal outpouching of the joint capsule can be palpated on the distal dorsal aspect of the pastern. Distention may result in obvious swelling, but it may be difficult to appreciate unless the horse has a fine hair coat. When distention of the joint capsule is present, ballottement of fluid from medial to lateral of the dorsal midline should be possible. The degree of distention may vary according to the recent work history.
Pain may be present on flexion or rotation of the distal limb joints. However, a marked reaction to distal limb flexion more likely reflects metacarpophalangeal joint pain. Mediolateral, dorsopalmar, or both types of foot imbalance are frequent findings and are considered important predisposing factors for the development of DIP joint pain. The degree of lameness varies depending on the nature of the underlying pathological change, recent work history, and whether lameness is unilateral or bilateral. With bilateral DIP joint pain the horse may move just a bit “flat” with a slightly shorter than normal stride. Severe lameness may reflect trauma to one of the supporting soft tissue structures of the joint. Lameness may be accentuated by distal limb flexion or rotation of the distal limb joints, but the response is variable. Lameness often is worse on a circle, especially on a hard surface, with the lamest limb either on the inside or outside of the circle.
Diagnosis
Local Analgesia
Pain associated with the DIP joint often improves after perineural analgesia of the palmar digital nerves and sometimes is alleviated fully. However, in some horses perineural analgesia of the palmar nerves at the level of the base of the proximal sesamoid bones is required to completely eliminate lameness.
Intraarticular analgesia of the DIP joint is not specific for pain that affects the joint itself. The potential exists for relief of pain from the navicular bone, the DSIL and the DDFT and their insertions on the distal phalanx, the palmar processes of the distal phalanx,4 and the sole, even at the toe.5,6 A standard approach is suggested to aid in interpretation of the response. A maximum volume of 6 mL of local anesthetic solution should be used. After injection through a dorsal midline approach, the horse should stand still until reassessment 5 minutes after injection. The clinician should ascertain whether sensation remains around the coronary band and whether any response to hoof testers has been eliminated to determine the specificity of the block. Lameness caused by primary DIP joint pain usually improves rapidly and substantially after intraarticular analgesia. If the lameness persists, the veterinarian should reassess the horse after an additional 5 minutes. If lameness is still apparent, the DIP joint is not a likely primary source of pain. However, after this time the block still could result in further improvement in lameness as the local anesthetic solution diffuses to adjacent structures, thereby potentially confounding the response to any other block performed at this stage.4,7 Intraarticular analgesia of the DIP joint can substantially improve lameness associated with navicular disease within 5 minutes of injection,8,9 although 20% of horses with navicular bone pain had a negative response to intraarticular analgesia of the DIP joint.8 Therefore the result of intraarticular analgesia of the DIP joint is best interpreted compared with the response to analgesia of the navicular bursa.7,8 Intraarticular analgesia of the DIP joint may not relieve pain associated with a primary injury of one of the collateral ligaments, although improvement may be seen if there is concurrent synovitis or osteoarthritis (OA).
A slight or negative response to intraarticular analgesia of the DIP joint does not eliminate completely a response to treatment of the joint, especially if the joint capsule is distended. For example, medication of the joint with hyaluronan and triamcinolone acetonide (10 mg) sometimes may resolve lameness that was not altered by intraarticular analgesia.
Retrieval of synovial fluid from the DIP joint depends on the synovial fluid pressure and position of the needle. Using a dorsal approach to the joint and a 20-gauge needle, synovial fluid usually appears spontaneously in the needle hub and, if the joint capsule is distended, may flow out under pressure. Relief of this pressure may help to resolve lameness. However, not all horses with considerable pressure within the DIP joint respond to intraarticular analgesia or medication. The pressure within the DIP joint also increases if the contralateral limb is picked up; therefore if the contralateral limb is picked up to aid restraint of the horse for injection of the DIP joint of the ipsilateral limb, a tendency for backflow through the needle puncture site may occur until the contralateral limb is placed on the ground.
Imaging Techniques
Radiography
Comprehensive radiographic examination of the DIP joint should include weight-bearing dorsopalmar, lateromedial, dorsoproximal-palmarodistal oblique, and flexed dorsolateral-palmaromedial and dorsomedial-palmarolateral oblique images of the interphalangeal joints.10 Because intraarticular analgesia may influence pain associated with the palmar processes of the distal phalanx and the navicular bone, these structures also should be evaluated carefully.
The shape of the extensor process of the distal phalanx varies considerably among horses on lateromedial images (Figure 33-1), but the shape usually is bilaterally symmetrical.10 Modeling changes of the extensor process can be present without associated lameness. Care should be taken in interpretation of the bony prominences on the distal medial and lateral aspects of the middle phalanx, which always appear larger in bigger-boned horses. Entheseous new bone at the origins of the collateral ligaments can be seen as an incidental finding. The DIP joint should be inspected carefully for recognition of small osteophytes on the distal palmar aspect of the middle phalanx and the dorsoproximal aspect of the navicular bone. Joint space congruity and the shape of the proximal articular surface of the distal phalanx should be assessed carefully. A smoothly outlined depression sometimes is seen in the middle of the proximal articular surface of the distal phalanx in clinically normal horses.
Fig. 33-1 Lateromedial radiographic images of the extensor process of normal distal phalanges. Note the variability in shape.
The flexed oblique radiographic images—dorsal 60° lateral-palmaromedial oblique and dorsal 60° medial-palmarolateral oblique—enhance detection of periarticular new bone (Figure 33-2) modeling of the distal aspect of the middle phalanx and entheseous new bone at the origin of the collateral ligaments of the DIP joint. Care is needed to differentiate between periarticular osteophytes and entheseous new bone at the insertion of the digital extensor tendon. Evaluation of the integrity and thickness of the subchondral bone plate of the middle and distal phalanges is important. Discontinuity of the subchondral bone plate may be the first radiological sign of the development of an osseous cystlike lesion. Increased thickness of the subchondral bone plate may occur with OA.
Fig. 33-2 Flexed dorsal 60° lateral-palmaromedial oblique radiographic image of the distal interphalangeal joint. There are modeling changes of the articular margins of both the distal interphalangeal and proximal interphalangeal joints.
New bone also may develop on the dorsal aspect of the diaphysis of the middle phalanx. Smoothly outlined new bone usually is subsynovial and clinically insignificant, whereas active-appearing, pallisading new bone usually is associated with lameness. Palisading is one of the earliest radiological signs but often is missed. The dorsal cortex of the middle phalanx may develop increased radiopacity (thicker), and early proliferative new bone may be seen.
Small, well-rounded mineralized opacities on the dorsoproximal aspect of the distal phalanx are not uncommon and may be present unassociated with clinical signs. Large mobile pieces are more likely to be associated with lameness.
The distal border of the navicular bone also should be evaluated carefully because the radiolucent zones along the distal border represent synovial invaginations from the DIP joint. An increase in size and number of these lucent zones has been observed with chronic synovitis of the DIP joint.
Ultrasonography
Diagnostic ultrasonography with a 7.5- to 13-MHz transducer and a standoff is invaluable for assessment of the dorsal pouch of the DIP joint, the amount of fluid within the joint, and the presence of synovial proliferation. Assessment of the palmar pouch is much more difficult, and evaluation of the articular cartilage is extremely limited. The structure of the proximal aspect of the collateral ligaments of the DIP joint proximal to the hoof capsule can be assessed,11,12 as well as the chondrocompedal, chondrocoronal, and distal digital annular ligaments. Using a transcuneal approach, the insertion of the DSIL can be assessed.13
Nuclear Scintigraphy
Nuclear scintigraphy has been useful in the identification of horses with DIP joint capsule and subchondral bone trauma. It appears to be rather insensitive to the identification of OA unless the disease is advanced.14 Increased radiopharmaceutical uptake (IRU) is best detected in lateral images.
Diagnostic Arthroscopy
The dorsal and palmar pouches of the DIP joint may be inspected arthroscopically; however, the view of joint surfaces is limited, and complete assessment of the integrity of the articular cartilage is not possible (see Figure 23-5). Access may be enhanced after joint trauma with resultant instability of the joint. Affected horses usually develop long-term lameness problems. Lavage of the joint may be beneficial therapeutically in some horses with chronic DIP joint pain without joint instability. A limited view of the DSIL can be seen from the navicular bursa.
Magnetic Resonance Imaging
Sagittal, frontal, and transverse magnetic resonance (MR) images of the DIP joint permit excellent evaluation of the articular cartilage and subchondral bone of the joint and the dorsal and palmar pouches of the DIP joint capsule (Figure 33-3). The DSIL, DDFT, collateral sesamoidean ligaments, and the navicular bone and bursa also may be assessed. Magnetic resonance imaging (MRI) is the imaging modality of choice for horses with chronic DIP joint pain that does not respond adequately to medical treatment.
Fig. 33-3 Dorsal three-dimensional T2* gradient-echo magnetic resonance image of the left hind foot of a riding horse with lameness of 3 months’ duration. The lameness was completely resolved by intraarticular analgesia. Radiological examination was negative; scintigraphy revealed focal increased radiopharmaceutical uptake in the proximal aspect of the distal phalanx. A focal lesion is present in the subchondral bone plate of the distal phalanx (arrow).
Differential Diagnosis of Primary Distal Interphalangeal Joint Pain
Synovitis
The most common cause of DIP joint pain is synovitis, which may occur unilaterally or bilaterally. Lameness is mild to moderate in degree, and palpable distention of the DIP joint capsule usually is present. Intraarticular analgesia generally resolves the lameness. Treatment should be directed to identification of any predisposing causes. Corrective trimming to restore correct foot balance and appropriate shoeing are essential for successful management. Horses with a collapsed heel usually benefit substantially from properly fitted egg bar shoes (see Figure 30-18). The timing of trimming and shoeing can be crucial: if the feet are allowed to get too long, soreness may return.
Care is necessary during injection of the DIP joint to avoid puncturing the large vessels on the distodorsal aspect of the pastern. Puncture tends to cause localized fibrosis, and future injections are more difficult. With excellent technique, the DIP joint will tolerate well many injections, and the prognosis for future soundness is good.
Osteoarthritis
Osteoarthritis without Radiological Abnormalities
Scintigraphy may be useful in the diagnosis of early subchondral lesions associated with OA. The definitive diagnosis of OA without radiological abnormalities is possible premortem only by MRI (Figure 33-4). Horses may have signs similar to those of primary synovitis, but the degree of lameness may be more severe, especially if the horse is exerted maximally, and the response to intraarticular medication tends to be shorter and less complete. MRI may show a reduced signal intensity within the articular cartilage of the DIP joint, in addition to irregularity in the cartilage surface, with or without concurrent abnormalities in the subchondral bone. The prognosis for sustained future soundness is guarded.
Fig. 33-4 Sagittal three-dimensional T2* gradient-echo magnetic resonance image of the foot of a Grand Prix show jumper. There is loss of the normal homogeneous signal in the cartilage of the distal interphalangeal joint and irregularities in the subchondral bone in the center of the distal phalanx (arrow). Mild periarticular osteophyte formation was evident on radiographs. Scintigraphic examination was unremarkable.
Osteoarthritis with Radiological Abnormalities
Correlation is lacking between modeling in the region of the extensor process of the distal phalanx and lameness associated with the DIP joint. Enthesophytes at the site of insertion of the common digital extensor tendon should be differentiated from osteophytes. Enthesophytes may not be associated with current lameness, but may reflect chronic instability of the joint. The presence of periarticular osteophytes on the distodorsal and palmar aspects of the middle phalanx and the proximal articular surface of the navicular bone is more likely to be associated with lameness (Figures 33-5 and 33-6). Radiological evidence of OA of the DIP joint can be seen with other causes of lameness, such as navicular disease. Horses with primary OA of the DIP joint may respond better to serial treatments with PSGAG than treatment with hyaluronan and corticosteroids.15,16 However, if concurrent severe synovitis is present, primary treatment with triamcinolone acetonide and hyaluronan, followed by intraarticular PSGAG, may yield the best results. In my experience, better results are achieved in horses that are sound after intraarticular analgesia compared with horses that show partial improvement in lameness. Prognosis usually is inversely related to the severity of the radiological abnormalities.7
Fig. 33-5 Lateromedial radiographic image of the distal interphalangeal joint of a 7-year-old pleasure horse. Marked enthesophyte formation is present on the dorsoproximal aspect of the distal phalanx at the insertion of the common digital extensor tendon (arrow). In addition, there is a well-rounded fragment on the dorsal aspect of the distal interphalangeal joint (arrowhead) and modeling of the extensor process of the distal phalanx.
Fig. 33-6 A, Dorsolateral-palmaromedial oblique radiographic image of a flexed distal interphalangeal joint of an 8-year-old show jumper with bilateral forelimb lameness improved by intraarticular analgesia of the distal interphalangeal joints. There is modeling of the proximal articular margin of the distal phalanx (arrow), radiological evidence of osteoarthritis (compare with Figure 33-2). There is also enthesophyte formation on the dorsomedial aspect of the middle phalanx at the origin of the medial collateral ligament of the distal interphalangeal joint. No radiological abnormalities were seen in lateromedial or dorsopalmar radiographic images. B, Lateromedial radiographic image of the left front foot of a riding horse with lameness improved by intraarticular analgesia of the distal interphalangeal joint. An articular osteophyte is seen on the dorsoproximal aspect of the navicular bone (arrow), and there is a fragment on the dorsal aspect of the distal interphalangeal joint (arrowhead).
Traumatic Damage to Articular Cartilage
Sudden onset of unilateral lameness may be related to traumatic damage to the articular cartilage of the DIP joint, with or without other concurrent soft tissue damage. Definitive diagnosis is possible only with MRI. Intraarticular medication may provide temporary relief of clinical signs, but the long-term prognosis is guarded.
Joint Capsule Trauma
Traumatic damage to the joint capsule, with or without subchondral bone trauma, usually results in sudden-onset, severe lameness that persists despite rest. Lameness may be accentuated markedly when the horse turns. In the acute stage, no abnormalities are detected on radiographic examination. However, periarticular new bone may develop after several weeks (Figure 33-7). Nuclear scintigraphic examination may show generalized increased radiopharmaceutical uptake in the region of the DIP joint (Figure 33-8). Arthroscopic evaluation in these horses has been unrewarding. The response to intraarticular medication has been poor, and the prognosis for return to athletic function despite prolonged rest is guarded.
Fig. 33-7 Lateromedial radiographic image of the left front foot of a 3-year-old Thoroughbred filly with severe lameness that improved substantially after intraarticular analgesia of the distal interphalangeal joint. Entheseous new bone is seen on the dorsal aspect of the middle phalanx, in addition to the small fragment at the extensor process of the distal phalanx. Nuclear scintigraphic examination had revealed a similar pattern of uptake of the radiopharmaceutical to that seen in Figure 33-8.
Fig. 33-8 A, Dorsal nuclear scintigraphic image of the front feet of a 6-year-old show jumper with sudden-onset, severe lameness markedly improved by intraarticular analgesia of the distal interphalangeal joint. The left forelimb is on the right. Radiopharmaceutical uptake is increased in the subchondral bone of the distal interphalangeal joint. B, Lateral scintigraphic image of the left front foot shows increased uptake of the radiopharmaceutical centered on the distal interphalangeal joint (arrow). Slight periarticular new bone formation was evident radiologically. Arthroscopic evaluation revealed synovial proliferation. The horse remained lame.
Subchondral Bone Trauma
Subchondral bone trauma may be focal or more generalized and usually is associated with unilateral lameness, which responds poorly to intraarticular medication and short periods of rest. No detectable radiological abnormalities may be apparent in the acute stage, although Ross14 described subtle proliferative changes on the distal aspect of the middle phalanx and the proximal aspect of the distal phalanx and a variable degree of subchondral lucency in the proximal aspect of the distal phalanx. Nuclear scintigraphy may be helpful.14 Several horses have been examined with acute-onset severe and persistent lameness, which was partially improved by intraarticular analgesia of the DIP joint. MRI has revealed a focal lesion in the proximal subchondral bone plate of the distal phalanx, usually axial and toward the palmar or plantar aspect (see Figure 33-3). Response to conservative management, with or without intraarticular medication or systemic treatment with tiludronate, has been poor.
Osseous Cystlike Lesions
Osseous cystlike lesions occur most often in the center of the proximal aspect of the distal phalanx, midway dorsal to palmar and axial.17,18 They vary in size and the presence of visible communication with the DIP joint. Not all are detectable radiologically; some have been identified only either using MRI or at postmortem examination after a poor response to medical treatment of the DIP joint.16 Horses with large osseous cystlike lesions may be asymptomatic but be lame at a later date. Lameness usually is unilateral, sudden in onset, and moderate to severe. Occasionally lameness is sporadic, but when present it is severe. Osseous cystlike lesions may occur in young immature horses and mature athletes. Not all osseous cystlike lesions that cause pain have active bone turnover; therefore nuclear scintigraphy may not be helpful in determining whether a long-standing osseous cystlike lesion is the current cause of lameness. However, currently or recently developing osseous cystlike lesions usually are associated with marked increased bone activity and may be evident on nuclear scintigraphic scans before radiological evidence is evident. Conservative treatment may result in spontaneous resolution of the lameness, with or without resolution of the cyst, but in some horses lameness persists, with or without enlargement of the cyst. Arthroscopic access to a cyst usually may be limited if it is in the central portion of the distal phalanx. Therefore surgical debridement of a well-defined osseous cystlike lesion usually is performed through the hoof wall. Less commonly small osseous cystlike lesions are seen on the dorsoproximal aspect of the distal phalanx, palmar to the extensor process. Such cysts located more dorsally can be debrided arthroscopically. Prognosis depends in part on the integrity of the overlying articular cartilage, and lameness persists in some horses.19,20
Palisading New Bone on Dorsal Aspect of Middle Phalanx
Pain associated with the DIP joint occasionally is associated with new bone formation on the dorsal aspect of the middle phalanx. The origin of this new bone is unknown. Arthroscopic evaluation of the joint may reveal crumbly bone, which is easily debrided, with resolution of lameness in some horses.21
Osseous Fragments on the Dorsal Aspect of the Distal Interphalangeal Joint
Small osseous fragments on the dorsoproximal aspect of the extensor process of the distal phalanx may be seen in clinically normal horses. Some are pointed proximally with a flat base and look like the tip of the extensor process, whereas others are well rounded. These fragments may represent separate centers of ossification or may be a manifestation of osteochondrosis and can be incidental radiological findings. If the fragments are seen in association with DIP joint pain, the clinical significance should be interpreted with care. Arthroscopic removal should be considered only if medical therapy of the joint fails.
Fracture of Extensor Process of Distal Phalanx
Fractures of the extensor process of the distal phalanx are discussed in the next section.
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