Chapter 82Soft Tissue Injuries of the Pastern
Injuries to the digital flexor tendons and ligaments in the pastern are a common cause of lameness in horses.1-6 Injuries to the collateral ligaments or the palmar or plantar ligaments of the proximal interphalangeal joint are a less frequent cause of lameness.1,7-9 Swelling, heat, and sensitivity of the affected tendon or ligament to palpation often accompany lameness. Ultrasonographic evaluation of the pastern is indicated when local swelling, heat, and sensitivity are detected, or when effusion occurs in the digital flexor tendon sheath (DFTS).10 The cause of the swelling can be determined by ultrasonography, and the severity of the injury can be characterized. The clinician should keep in mind that peripheral longitudinal splits in the digital flexor tendons within the DFTS are difficult to detect and can be missed ultrasonographically.10 If injury occurs to the superficial or deep digital flexor tendons (SDFT, DDFT) in the metacarpal or metatarsal region, ultrasonographic examination should include an evaluation of these structures in the pastern. Lameness associated with soft tissue injuries of the pastern also can occur without localized soft tissue swelling, and ultrasonographic examination is indicated if pain is localized to the region using diagnostic analgesia and no radiological abnormality is detected, or entheseous new bone is seen. The clinician should bear in mind that intraarticular analgesia of the metacarpophalangeal or metatarsophalangeal joints, the proximal interphalangeal joint, and the DFTS is not necessarily specific and may influence closely related structures such as the distal sesamoidean ligaments and the palmar ligaments of the proximal interphalangeal joint. It also may be important to use diagnostic analgesia to determine whether the injury causing soft tissue swelling in the pastern region is the source of the lameness. Nuclear scintigraphy may help to determine whether entheseous new bone is active, and magnetic resonance imaging (MRI) has the potential to provide additional information. This chapter focuses on lesions diagnosed using ultrasonography, but the absence of detectable ultrasonographic abnormalities does not preclude soft tissue pathology causing pain.
Most of the soft tissue structures in the pastern are on the palmar or plantar aspects and are similar in the forelimbs and hindlimbs. The following describes the forelimb but applies equally to the hindlimb. The SDFT forms a thin ring around the DDFT at the ergot and in the proximal-most portion of the pastern and then bifurcates into medial and lateral branches. The origin of the branches has a teardrop shape. The cross-sectional area (CSA) of each SDFT branch gradually enlarges as the branch extends distally along the palmarolateral and palmaromedial aspects of the pastern, until the branches insert on the distal aspect of the proximal phalanx and on the proximal aspect of the middle phalanx. The DDFT lies immediately dorsal to the SDFT and extends along the midline to its insertion on the distal phalanx.1-6,11-15 The DDFT has a bilobed shape in the pastern and is surrounded by the DFTS. The oblique (middle) sesamoidean ligaments (OSLs) originate from the base of the lateral and medial proximal sesamoid bones (PSBs) as two large, round to oval branches. These branches become smaller in CSA as they extend distally. The branches join in the proximal to mid aspect of the proximal phalanx and insert as a broad band on the palmar aspect of the middle of the proximal phalanx. The straight sesamoidean ligament (SSL) also has its origin at the base of the PSBs and the palmar ligament and extends distally in the midline, palmar to the OSL, to insert on the scutum medium of the middle phalanx.1-6,11-13,16 The SSL lies dorsal to the DDFT and has an hourglass shape, larger proximally and distally, and narrowest in the middle.
The DFTS surrounds the SDFT and DDFT throughout the proximal aspect of the proximal phalanx to the bifurcation of the SDFT.1-6,11-15,17 The entire length of the DDFT is included in the DFTS, except for a small area in the distal palmar aspect of the pastern, just proximal to the bulbs of the heel. The dorsal aspect of the DFTS extends farther distally than its palmar aspect. The proximal digital annular ligament is adhered closely to the palmar aspect of the SDFT in the proximal aspect of the pastern.1,3-5,18 The distal digital annular ligament forms a sling over the distal part of the DDFT. These two structures are thin in normal horses.
The abaxial and axial palmar ligaments of the proximal interphalangeal joint originate in pairs from the medial and lateral aspects, respectively, of the middle of the proximal phalanx and dorsal to the SDFT branches. They insert on the scutum medium abaxial to the branches of the SDFT (abaxial palmar/plantar ligament) and between the SSL and the branches of the SDFT (axial palmar/plantar ligament). These are large, round to oval ligaments that extend in a diagonal direction from the origin to the insertion. The collateral ligaments of the proximal interphalangeal joint originate from a small eminence on the lateral and medial aspects of the proximal phalanx, distal to the origin of the palmar ligaments, and arc across the joint to insert on a small eminence on the lateral and medial aspects, respectively, of the proximal aspect of the middle phalanx.1,3,7,11 The proximal interphalangeal joint has a closely adhered joint capsule.
The common digital extensor tendon is located on the dorsal aspect of the pastern.1 The extensor branches of the suspensory ligament (SL) join the common digital extensor tendon in the distal part of the proximal phalanx. The main insertion of the common digital extensor tendon is on the extensor process of the distal phalanx, but there are also areas of insertion onto the proximal and middle phalanges. A bursa is present between the tendon and the proximal interphalangeal joint.
In the proximal aspect of the pastern (zone P1A), the SDFT is imaged from the palmar aspect and is homogeneously echogenic and has a thin, half-moon shape in the transverse plane.1-6,12-14 In longitudinal images the SDFT has a parallel fiber pattern and a triangular shape along the midline in zone P1A because its thickness decreases distally. In normal horses, distinguishing the proximal digital annular ligament from the DFTS and the palmar border of the SDFT is difficult. The proximal digital annular ligament can be identified if thickened by following it medially and laterally to its attachment to the proximal aspect of the proximal phalanx. The body of the SDFT ranges in thickness (palmar to dorsal) from 2 to 6 mm in P1A to 1 to 4 mm over the middle of the proximal phalanx.3 The teardrop-shaped branches in the proximal to mid pastern region (at the junction of zones P1A and P1B) are imaged from the palmaromedial and palmarolateral aspects and are followed individually to triangular-shaped insertions. The SDFT branches are similarly homogeneously echogenic, with a parallel fiber pattern throughout. The branches of the SDFT range in thickness from 4 to 7 mm in the proximal pastern to 7 to 12 mm distally.15 The CSA of the two SDFT branches ranges from 0.3 to 0.4 cm2 in the distal portion of zone P1A where the branch begins, increases to 0.4 to 0.6 cm2 in zone P1B, and further enlarges to 0.6 to 0.8 cm2 near the insertion.
The DDFT has an oval to bilobed appearance in the pastern and is imaged on the palmar midline of the pastern until the DDFT is lost from view distally.1-6,12-15 The two lobes are symmetrical. The fibers of the DDFT extend obliquely from a deeper to a more superficial position in the more distal portion of the pastern and are separated from the SSL by an anechogenic space. The dorsopalmar thickness and lateral to medial width of the DDFT decrease in the mid pastern and increase again in the distal aspect of the pastern. The DDFT measures 5 to 10 mm (palmar to dorsal) in the proximal aspect of the pastern, slightly less in the mid pastern, and 7 to 12 mm in the distal aspect of the pastern. The width of the DDFT in a lateral-to-medial direction ranges from 18 to 33 mm in the proximal aspect of the pastern, decreasing to 15 to 23 mm in the mid pastern, and increasing in the distal aspect of the pastern to 23 to 32 mm.3 Along the dorsal aspect of the DDFT in the mid pastern region is a synovial fold of the DFTS that is imaged readily, surrounded by a small amount of anechogenic synovial fluid. In the distal pastern region, the palmar aspect of the DDFT adheres to the synovial membrane of the DFTS.
Injury to the OSLs is often missed ultrasonographically because the origin and proximal to midportion of each OSL (where the majority of the injuries are) are not imaged from the palmaromedial and palmarolateral aspects of the limb. The origin of the medial or lateral OSL is best found by placing the ultrasound transducer over the base of the medial or lateral PSB and scanning distally over the bone to its base, angling the transducer proximally to image the origin of the respective OSL.1-6,12,13 Alternatively, the origin of an OSL can be found by following the SL branches distally over the respective PSBs to the base. Immediately distal to the base of the PSB is the origin of an OSL, best located initially in its transverse section as a large, round to oval structure. The OSLs merge in the distal part of zone P1A into a broad, rectangular band dorsal to the DDFT. The OSLs then insert on the palmar or plantar aspect of the proximal phalanx in zone P1B. An OSL is the most difficult tendon or ligament to follow longitudinally to its insertion because the ligament extends diagonally from its origin to its insertion in two different planes. Following the medial or lateral OSL from its origin to the main body of OSL requires a transducer angle of about 45 degrees from the base of the PSBs to the palmar midline of the proximal phalanx. Properly aligning the transducer and eliminating off-normal incidence artifact is difficult. The OSLs may appear less echogenic because of an oblique orientation. The OSLs are thickest in the medial to lateral direction proximally. Each OSL measures 12 to 20 mm (lateral to medial) in the proximal aspect of P1A, decreasing to 9 to 17 mm just before their convergence, and 0 to 9 mm (one side only) at their insertion. The palmar to dorsal thickness of the OSLs is 5 to 12 mm in P1A, decreasing to 2 to 6 mm just proximal to the convergence, and decreasing again to 0 to 3 mm at the insertion. The mean CSA of an OSL determined with MRI is reported to be 0.86 cm2 in the proximal third, 0.56 cm2 in the middle third, and 0.40 cm2 in the distal third.19 However, there may be size differences between the medial and lateral OSLs.20
The origin of the SSL is found by angling the transducer in a proximal and dorsal direction from the proximal-most aspect of the pastern, just underneath the ergot, to image the ligament and the base of the PSBs. The SSL becomes a more oval-shaped structure and is palmar to the OSLs in zone P1B. The SSL remains dorsal to the DDFT as it inserts on the scutum medium.
The dorsal-to-palmar thickness of the SSL gradually increases as the medial to lateral width decreases. The SSL measures 5 to 9 mm (palmar to dorsal) proximally, increasing slightly over the distal aspect of the proximal phalanx to 6 to 12 mm, and increasing again to 8 to 14 mm at the scutum medium. The medial-to-lateral thickness of the SSL ranges from 17 to 30 mm in zone P1A, decreases to 10 to 15 mm, and then widens over the scutum medium to 45 to 65 mm.3 The SSL is echogenic with normal parallel fiber alignment throughout, except at the insertion. A central symmetric hypoechoic area is commonly imaged in clinically normal horses at the insertion of the SSL.16 Care must be taken to be sure that lesions are not created at the insertion of the SSL because of the difficulty in aligning the transducer perpendicular to the ligamentous fibers owing to the horse’s heel. Comparison of this area with the contralateral normal limb should aid in determining whether a suspected lesion is real.
The collateral ligaments of the proximal interphalangeal joint are easiest to examine by imaging in both longitudinal and transverse planes. The collateral ligaments of the proximal interphalangeal joint are homogeneously echogenic structures, with a parallel fiber pattern at the origin on the distal aspect of the proximal phalanx to the insertion on the proximal half of the middle phalanx.1,2
The proximal interphalangeal joint is easiest to image initially in the longitudinal plane by identifying the joint space, and then a transverse evaluation of the joint can be made.7 Fluid normally is not imaged in the proximal interphalangeal joint.
The palmar ligaments of the proximal interphalangeal joint can be imaged from the origin on the middle of the proximal phalanx to the insertion on the proximal palmar aspect of the middle phalanx. These ligaments are paired on the medial and lateral aspects of the pastern and are located by placing the transducer dorsal to the branch of the SDFT. The more abaxial branch originates first and is easier to follow than the more axial branch. Each branch has a round to oval shape, is homogeneously echogenic with a parallel fiber pattern, and must be followed individually from origin to insertion.