Arthrosonography: Joints of the head, neck and front limb (Proceedings)

Diagnostic ultrasonography has, more recently, been applied to the assessment of other less traditional musculoskeletal problems such as evaluation of bone, joints, muscle and nerves. The soft tissue structures of the head, neck, joints of the forelimb and foot can be evaluated ultrasonographically; yielding important diagnostic information that cannot be obtained radiographically.

Normal Sonographic Anatomy of the Joints of the Forelimb

The Head

The temporomandibular joint contains an echogenic fibrocartilagenous disc that separates the larger dorsal compartment from the smaller ventral compartment. No free fluid is visible in the normal joint. The disc appears as a homogeneously echogenic triangle with its base laterally. The disc is attached to the thin echogenic joint capsule and measures 2 cm in thickness caudally and caudolaterally, thinning to 0.5 cm rostrolaterally. The hypoechoic layer of articular cartilage adjacent to the disc measured up to 3 mm in foals but was difficult to measure in adults.

The Neck

Ultrasonographic examination of the neck is best performed following radiographic and scintigraphic examination of the neck to help localize the areas of potential pathology. Ultrasonographic evaluation of the normal atlanto-occipital articulation and the cervical region has been described in horses. Ultrasound guided injections of the atlanto-occipital joint have been performed for myelograms with the injection of the contrast material into the subarachnoid space. The detection of the "chair" sign is an excellent guide for performing ultrasound guided injections of the cervical facet joints.

Shoulder

The muscles of the shoulder girdle are the stabilizers the shoulder joint and thus, all the musculature on the craniolateral aspect of the shoulder girdle should be evaluated. The biceps brachii tendon originates on the supraglenoid tubercle of the scapula and occupies the intertuberal groove of the humerus. The biceps tendon is imaged coursing over the cranial aspect of the shoulder. A bursa lies below and around the tendon in the groove. There is normally a small amount of fluid visualized in the bicipital bursa. A tendinous band extends from the greater to the lesser tubercle, anchoring the biceps tendon. It is also possible to image the insertion of the infraspinatus muscle in the caudal eminence of the greater tubercle from the lateral aspect of the shoulder. The thin joint capsule is best imaged from a lateral and caudolateral view of the shoulder joint. Synovial fluid is not normally imaged in the shoulder joint. The articular margins of the lateral and caudal portions of the humeral head are the only visible articular cartilage and are best imaged from the lateral and caudolateral views.

Elbow

The collateral ligaments of the elbow joint are easily imaged sonographically as homogeneous echoic structures with parallel fiber alignment from their origins on the medial and lateral epicondyle of the humerus to their insertions on the medial and lateral tuberosity of the radius. There is a long superficial part of the medial collateral ligament that inserts on the medial border of the radius immediately distal to the interosseous space.

Carpus

The extensor tendons are normally imaged over the dorsal aspect of the carpal joints. The extensor carpi radialis tendon is located on the dorsal midline of the carpus and inserts on the proximal aspect of MC3. It is echoic with a parallel fiber pattern and is surrounded by a synovial sheath containing a very small amount of anechoic fluid. The common (long) and the smaller lateral digital extensor tendons are located on the craniolateral aspect of the radius and carpus. The long digital extensor tendon courses medially to a position along the dorsal midline distal to the carpus. The lateral extensor tendon joins the long digital extensor tendon just beneath the carpus. At the carpal level both tendons have synovial sheaths. The long digital extensor sheath may be visible with a small rim of anechoic fluid within, while the sheath associated with the lateral digital extensor tendon is not usually visible. The extensor carpi obliquus tendon is a short tendon crossing the dorsal aspect of the carpus from lateral to medial. It is surrounded by a sheath and has a bursa lying under its insertion near the medial collateral ligament of the carpus. The carpal sheath extends from 8 -10 cm proximal to the accessory carpal bone to a similar distance distal to the accessory carpal bone, surrounding the SDFT and DDFT in the carpal canal. The carpal sheath is imaged as a thin circular echogenic structure encircling the SDFT and DDFT, with fluid usually visible in the carpal sheath between the DDFT and ICL in the proximal to mid metacarpal region along the medial aspect of the limb. The wall of the carpal sheath is normally a thin echogenic structure that is difficult to separate from the tendons, except where fluid is contained within the sheath. When distended, the carpal sheath will bulge primarily along the proximal, middle and medial two thirds of the metacarpus. The carpal sheath may also distend along the distolateral aspect of the carpus, cranial to the ulnaris lateralis.

The joint capsule and collateral ligaments of the carpus are common to all 3 carpal joints. Both ligaments originate on the styloid process of the radius. The lateral collateral ligament extends from the lateral styloid process distally to the fourth and third metacarpal bones. The long, superficial lateral collateral ligament is present as well as the short, deep lateral collateral ligament. The lateral digital extensor tendon passes through the long superficial and short deep lateral collateral ligaments. The lateral collateral ligament is 27-29 mm wide by 5 mm thick and narrows over the ulnar carpal bone to 19-21 mm where it is slightly thicker. The lateral collateral ligament becomes thicker and wider over the fourth carpal bone (8-10 mm) by (25-29 mm) and becomes crescent shaped as it inserts. The medial collateral ligament is similar but is stronger and wider distally where it attaches to the second and third metacarpal bones. The flexor retinaculum and the palmar border of the medial collateral ligament blend together. The collateral ligaments should be homogeneously echoic with a parallel fiber pattern. Heterogeneous fat areas are present in the antebrachiocarpal joint that appear hypoechoic relative to the echoic joint capsule. Fluid is not normally imaged in the dorsal aspect of the antebrachiocarpal and intercarpal joints.

Fetlock

The long and short medial and lateral collateral ligaments have a homogeneous echogenicity and parallel fiber pattern. The short collateral ligament is oriented obliquely and is more difficult to follow sonographically. It originates from the medial and lateral epicondylar fossa and courses distopalmarly to insert on the abaxial surface of the medial and lateral proximal sesamoid bone and palmar or plantar surface of the proximal phalanx. The long, superficial collateral ligaments originate from the distal lateral and medial epicondyles of the metacarpus or metatarsus and insert onto the proximal lateral or medial eminences of P1. The dorsal aspect of the metacarpophalangeal or metatarsophalangeal joint capsule is thick. The proximal synovial fold of the metacarpo-metatarsophalangeal joint should not exceed 2 mm in thickness. A small amount of anechoic joint fluid is normally imaged in the metacarpophalangeal or metatarsophalangeal joint in the dorsal synovial recess and in the proximopalmar or proximoplantar recess. Numerous synovial villi are visible in the proximopalmar or proximoplantar recess. The dorsal aspect of the articular cartilage of the distal metacarpus or metatarsus is imageable in the standing horse, while the distal aspect can only be imaged in the flexed fetlock joint. The normal articular cartilage in the dorsal aspect of the metacarpal or metatarsal condyle is approximately 1 mm in thickness and thins distally to approximately 0.5 mm.

Pastern

The collateral ligaments of the proximal interphalangeal (PIP) joint also appear as homogeneously echoic structures with a parallel fiber pattern. They extend from distal P1 to proximal to mid P2 along the lateral and medial aspects of the limb. They originate from the lateral and medial eminences and adjacent distal depressions of the distal end of the proximal phalanx and insert on the lateral and medial eminences of the proximal end of the middle phalanx. Fluid is not normally imaged in the proximal interphalangeal joint. There are also strong palmar/plantar ligaments of the PIP joint that also appear homogeneously echoic with a parallel fiber pattern. There is a central pair that originates on the palmar/plantar triangular rough area in the mid portion of the proximal phalanx with a second pair located more proximally and abaxially. Both sets of palmar ligaments of the PIP joint insert on the proximopalmar/proximoplantar aspect of the second phalanx and its complementary fibrocartilage. The insertions of the central ligaments blend with the insertions of the superficial digital flexor tendon and the straight distal sesamoidean ligament.

The collateral ligaments of the distal interphalangeal (DIP) joint originate in the depressions on the lateral and medial side of the distal part of the middle phalanx. These ligaments widen distally to insert on the abaxial depressions on the extensor process of the third phalanx and the dorsal aspects of the collateral cartilages and cannot be viewed in their entirety. The distal interphalangeal joint can be imaged from the palmar or plantar surface of the pastern where it appears as a small anechoic space immediately proximal to the navicular bone and dorsal to the collateral sesamoidean ligaments of the navicular bone. The dorsal part of the distal interphalangeal joint can also be imaged just above the coronary band on the dorsal midline.

Foot

The collateral ligaments of the navicular bone, the coffin joint and the podotrochlear (navicular) bursa can be imaged from the palmar or plantar aspect of the foot angling distally between the bulbs of the heel with a high frequency transducer containing a curved footprint. After soaking the foot in wet bandages overnight and trimming the frog, the transcuneal approach can be used to image the podotrochlear apparatus. The application of emu oil to the frog can help improve contact between the transducer and the frog. These ultrasound scans are performed with a 7.5 sector scanner transducer, a 6.0 MHz microconvex or a 7.5 MHz linear array transducer. The impar ligament is a short wide ligament that runs between the distal sesamoid bone and the third phalanx and it can be imaged with this technique. The impar ligament measures 2-3 mm in thickness and is imaged in contact with the navicular bursa on its palmar surface. The origin of the impar ligament from the distal sesamoid bone and its insertion on the distal phalanx can be imaged. The deep digital flexor tendon is echoic if the ultrasound beam can be oriented perpendicular to the long axis of its fibers. The heterogeneous hypoechoic tissue in the superficial portion of the scan represents the digital cushion.

Collateral Ligament Desmitis and Joint Injuries in the Head, Neck and Forelimb

The Head

Irregular bone along the articular margins has been imaged in horses with degenerative joint disease of the temporomandibular joint, as has narrowing of the joint space, thickening of the fibrous joint capsule and disintegration of the disc material with replacement of the disc by a fibrous mass of tissue.

The Neck

Pathology in the ligamentum nuchae has been detected ultrasonographically, with fractures detected at the base of the skull at the origin of the ligamentum nuchae. Sonographic evaluation of the cervical facet joints has revealed bony proliferative change associated with degenerative joint disease. Ultrasound guided intra-articular injection of the cervical facet joints is in common use in equine practice for horses with cervical pain and stiffness. Identification of the "chair" sign facilitates the ultrasound guided placement of the needle into the cervical facet joint.

Shoulder

The muscles of the shoulder girdle are the stabilizers the shoulder joint and injuries to these muscles can be detected ultrasonographically. Biceps tendinitis has been most frequently reported and often occurs with an associated bicipital bursitis. Injuries to the infraspinatus muscle and tendon of insertion are much less common. Muscle fiber disruption with secondary fibrosis is a common cause of a shortened anterior phase of the stride in horses with shoulder lameness.

Elbow

Collateral ligament injuries of the cubital joint are uncommon and are most frequently associated with trauma. Injuries to the lateral collateral ligament and avulsion fractures of the origin of the lateral collateral ligament from the humeral epicondyle have been reported. Injuries to the lateral collateral ligament may also be associated with fractures of the lateral tuberosity of the radius and penetration of the cubital joint. The injured collateral ligament is usually enlarged, hypoechoic and lacking the normal fiber pattern.

Carpus

Injuries to the lateral and medial collateral ligaments are visible sonographically while those of the intercarpal ligaments are not usually visible. Avulsion fractures and disruption of the medial collateral ligaments of the carpus have been described most frequently. Again trauma is usually the cause of these injuries, which result in enlargement of the collateral ligament, a decrease in its echogenicity and loss of fiber alignment.

Fetlock

Collateral ligament injuries are more common in the fetlock joint than previously recognized and may occur more frequently in the lateral collateral ligament of the metatarsophalangeal joint. The affected collateral ligaments are enlarged, hypoechoic and usually demonstrate a diffuse area of fiber damage. Avulsion fractures of the origin or insertion of these collateral ligaments have been detected. Periarticular soft tissue swelling is usually present in these horses. Collateral ligament desmopathy was identified ultrasonographically in 21% of horses with lameness localized to the fetlock joint region in one study. Villonodular synovitis is also frequently diagnosed ultrasonographically by measuring the thickness of the dorsal synovial plica. The proximal synovial fold of the metacarpo-metatarsophalangeal joint should not exceed 4 mm in thickness. Intersesamoidean desmitis has also been documented ultrasonographically with the detection of hypoechoic to anechoic areas of fiber disruption within the intersesamoidean ligament. Lysis along the attachment of the intersesamoidean ligament to the axial surfaces of the sesamoid bones is common in horses with intersesamoidean desmitis. Proximal palmar annular desmitis usually occurs in conjunction with a digital sheath tenosynovitis. Palmar digital annular ligament constriction occurs with thickening of the palmar digital annular ligament. Combined thicknesses of the skin and palmar digital annular ligament of 4-5 mm have been reported in horses with palmar annular desmitis. Four different types of annular ligament constriction have been reported.

Pastern

Collateral ligament desmitis and desmitis of the palmar/plantar ligaments of the proximal interphalangeal joint have been demonstrated ultrasonographically. The collateral ligaments or palmar/plantar ligaments are enlarged, hypoechoic and lack the normal parallel fiber pattern. Avulsion fractures associated with the origins or insertions of these ligaments have been detected, but most frequently involve the insertions of the palmar/plantar ligaments of the proximal interphalangeal joint as the palmar/plantar eminences of P1 are usually affected. Desmitis of the palmar/plantar ligaments of the proximal interphalangeal joints have been reported and have been reported alone and in association with injuries to the superficial digital flexor tendon and distal sesamoidean ligaments.

Foot

Irregularities of the flexor surface of the navicular bone have been detected ultrasonographically using the transcuneal approach. Protrusions of the hyperechoic bony surface, lysis of the bone and fractures have all been identified. Hyperechoic areas within the deep digital flexor tendon and impar ligament consistent with mineralization have been imaged ultrasonographically in horses with foot pain.