Physical examination

The horse should be examined for signs of hypovolemic shock, which include rapid, thready pulse, pale mucous membranes, delayed capillary refill time, and skin tenting. The superficial location of the median artery in the forelimb, the dorsal metatarsal artery in the hindlimb, and the digital arteries in the distal aspect of the limb make them prone to transection. Hemorrhage can typically be controlled, at least temporarily, by the application of a pressure bandage to the wound until the wound can be fully assessed. Immobilizing the injured limb by using a Kimzey Leg Saver Splint (Kimzey, Inc.) (Figure 17.5) or a piece of PVC pipe incorporated into a heavy bandage and extending down to the ground (Figure 17.6) prevents a partially lacerated tendon from disrupting completely and protects neurovascular structures from additional trauma. Splints are placed either dorsally or on the palmar surface of the forelimb and on the plantar surface of the hindlimb (the reader is referred to Chapter 7 for more information on splints). When the horse is able to balance on the stabilized limb, it is more likely to remain calm, enabling the exam to be completed.

The injured horse may need to be sedated or otherwise restrained to complete the examination, always while keeping in mind the systemic state of the horse. Severe blood loss and associated hypovolemic shock dictate selection and dosage of a sedative or tranquilizer. It is important that the veterinarian, owner, and handlers be protected from harm and that the horse not suffer more injury to the damaged tendon. A severely lame horse, in pain, can be a challenge to handle, and care must be taken to thoroughly assess the situation.

Physical examination should begin with a basic assessment of the horse’s temperature, pulse, respiratory rate, color of mucous membranes, and capillary refill time. The horse’s history, including information as to the duration of the injury, the horse’s vaccination status, and the use of any medications, should be recorded. The “big‐picture” information, such as hemorrhage or evidence of prior hemorrhage, weight‐bearing status, and pain level of the horse, as well as the horse’s overall demeanor, can be useful in formulating an initial plan. It is critical that the entire horse be examined so as not to miss additional injuries that may affect prognosis.

Careful observation of how the horse is willing to use the limb provides key information. Is the horse able to weight bear when stationary and to lock the stay apparatus? Does the foot stay flat on the ground, or does the fetlock or hock drop when the horse bears weight on the affected limb? Is there an abnormal pattern of flexion and extension that suggests that the reciprocal apparatus is no longer intact? Is there a stable bony column to support the horse? Is there any evidence of fluid drainage suggesting that a synovial structure is open? An understanding of normal anatomy is important when examining the distal aspect of the limb specifically for laceration of a digital flexor tendon.

Observation as the horse bears weight or takes a careful step or two on the injured limb is one of the easiest ways to assess the integrity of the primary support structures, including the digital extensor and flexor tendons, suspensory ligament, and their associated structures (e.g., tendon sheath, digital annular ligament, neurovascular structures). Transection of the common digital extensor tendon of the forelimb, or both the lateral and long digital extensor tendons (or below where the two join) of the hindlimb, results in knuckling of the fetlock when the horse attempts to walk, because the horse has lost its ability to extend the toe as the limb is advanced (Figure 17.7). When the limb is manually placed in a normal weight‐bearing position, a horse with a severed digital extensor tendon is able to bear weight on the limb without hyperextending the joints. Knuckling of the limb causes most horses to exhibit a substantial degree of anxiety, making restraint and stabilization with a splint an important aspect of treatment.

Transection of the tendons and ligaments on the palmar/plantar surface of the distal limb results in characteristic changes in joint position (Figure 17.8). Complete transection of the superficial digital flexor tendon (SDFT) at the metacarpal/metatarsal region or fetlock results in hyperextension of the metacarpo/metatarsophalangeal joint, characterized by “dropping” of the palmar/plantar aspect of the fetlock towards the ground (Figure 17.9). The toe remains on the ground. The pastern may subluxate dorsally after the SDFT is transected. A structure deep to the SDFT [e.g., the deep digital flexor tendon (DDFT) or suspensory ligament] may be injured in the absence of injury to a more superficial structure, such as the SDFT, but most lacerations involve the structures immediately underlying the skin before they involve deeper ones. The distinctive clinical sign indicative of complete transection of the DDFT is elevation of the toe off the ground (Figure 17.10), resulting from loss of the tension band produced by insertion of the DDFT on the palmar/plantar surface of the distal phalanx. Transection of both the SDFT and the DDFT leads to hyperextension of the metacarpo/metatarsophalangeal and distal interphalangeal joints, as well as elevation of the toe off the ground (Figure 17.11). When the suspensory ligament, in addition to both of the digital flexor tendons, is transected, support to the palmar/plantar aspect of the limb is completely lost, and the palmar/plantar surface of the fetlock rests on the ground with the toe pointing up (Figure 17.12). Because the DDFT and its associated sheath are the only support structures present on the palmar/plantar surface of the pastern, injuries in this area typically affect only the DDFT. The clinical signs associated with partial transection of a digital flexor tendon vary depending on the percentage of remaining intact tendon and the location of the injury.

Careful palpation of the injury can be useful in confirming the information obtained by observing the stance assumed by the horse when it bears weight on the injured limb and in determining the extent of a partial laceration. Hair surrounding the wound should be clipped and the wound cleaned. Sterile gloves should be worn to prevent further contaminating wound, and for one’s own protection against zoonotic infection. Many horses must be sedated and/or the region of the wound desensitized by regional or local anesthesia to ensure that the wound is examined thoroughly and safely. If the wound appears to be severe and clearly requires surgical repair, it can be assessed more thoroughly with the horse anesthetized. The potential risks associated with recovery from general anesthesia should be taken into account when deciding whether to anesthetize an injured horse for surgical treatment. External coaptation in the form a cast may be required to prevent more damage to injured structures while the horse recovers from general anesthesia. The systemic status of the horse must also be taken into consideration before general anesthesia is selected as the method of restraint; if the injury appears to be severe, however, the horse should be anesthetized so that the wound can be assessed thoroughly, irrigated, debrided, and repaired.

In addition to visually assessing the injury and palpating the wound, the limb, while being palpated, should be flexed and extended, to better assess the injury. The ends of a completely severed tendon may retract a substantial distance proximally and distally to the wound, making the severed ends difficult to see or palpate. Ultrasonographic examination may be useful in assessing the extent of soft‐tissue injury; air trapped within the tissue planes of an open wound, however, creates artifacts that interfere with accurate sonographic examination. Nevertheless, use of ultrasound to assess vascular integrity in severe wounds remains an important adjunct to the rest of the examination. A horse with a completely disrupted or compromised blood supply has a grave prognosis for survival. Using radiography to rule out concurrent fracture is also warranted because the presence of bone involvement can impact the prognosis substantially.

General treatment considerations

Regardless of the structures involved, treatment of wounds involving a lacerated tendon is comprised of standard supportive care, including treatment of the horse for pain, dehydration, and/or hypovolemic shock, and of the wound for inflammation and contamination. Intravenous administration of fluids is indicated for horses suffering from hypovolemic shock resulting from hemorrhage or from dehydration resulting from decreased water consumption because of immobility and/or pain. In most cases, isotonic fluids, such as lactated Ringer’s solution, are adequate and should be administered at a rate appropriate to the horse’s needs. Administering plasma to treat for hypoproteinemia, or hypertonic saline solution or hetastarch to treat for hypovolemic shock, may be indicated in extreme circumstances. Additional nursing care specific to the horse’s systemic status and individual needs should be considered. This might be as simple as treating other less severe wounds, or more intensive, such as providing sole and frog support and cold therapy to prevent support limb laminitis. When bandaging or casting limits the mobility of the horse, care must be taken to provide bedding that enables easy movement within the stall.

For many horses, conventional non‐steroidal anti‐inflammatory drug (NSAID) therapy may suffice to keep them comfortable. In these cases, phenylbutazone (2.2–4.4 mg/kg) or flunixin meglumine (0.5–1.0 mg/kg) given twice daily provide excellent anti‐inflammatory and analgesic effects. Because most horses with a moderate or severe injury have an intravenous catheter in place for antibiotic therapy, the intravenous route is preferred for ease of administration and rapid, reliable effects.

Many horses with severe injuries may require analgesia beyond that provided by a NSAID, making it necessary to explore additional options to provide adequate analgesia. Administering an analgesic drug through a caudal epidural catheter is an excellent method of relieving pain in a hindlimb.^18–22 Continuous peripheral nerve blocks are another option to consider in cases requiring adjunct analgesia.^23,24 The goal of analgesic therapy is to make the horse comfortable enough to occasionally and modestly shift weight on to the injured limb just enough to prevent laminitis from developing in the contralateral limb. Other options for adjunctive analgesia include continuous intravenous infusion of lidocaine, ketamine, or butorphanol, fentanyl patches, morphine administered intramuscularly, and gabapentin;^25–27 these drugs, however, are generally only required if the horse displays extreme signs of pain or during the first 24–48 hours after injury. Because supporting limb laminitis is a major potential complication after tendon laceration, vigilance with respect to supportive care and close monitoring for signs of laminitis are crucial elements of nursing care.

Antibiotic therapy is an important component of wound therapy and is especially critical for wounds involving tendons and their associated structures. Antibiotics are often administered systemically and regionally. Broad‐spectrum antibiotics, such as penicillin (22–44 000 IU/kg IV q 6 hours) or ceftiofur (2.2 mg/kg IV q 12 hours) and gentamicin (6.6 mg/kg IV q 24 hours) are indicated for most horses that have suffered a tendon laceration. Trimethoprim sulfa (30 mg/kg PO q 12 hours) may provide adequate coverage against most common contaminants, and its use may be sufficient if the wound does not involve a synovial structure. Whenever possible, a sample of (synovial) fluid or tissue from the wound for cytologic examination and/or culture and sensitivity testing should be collected to guide the selection of antibiotics. Based on lack of response to therapy, cytologic findings, or more appropriately, sensitivity testing, administering an antibiotic with a broad anaerobic spectrum, such as metronidazole, may be necessary. Duration of therapy is based subjectively on prior experience of the clinician, the severity, degree of contamination and duration of the wound, structures involved, and response to treatment. Antibiotics are administered intravenously as early as possible after tendon injury has been diagnosed; this is most likely to occur in consultation with a referral center with the aim to administer the initial dose prior to transport of the horse. Interim antibiotic therapy is an empiric drug selection, instituted while awaiting results of bacterial culture and antibiotic sensitivity testing. For all severely infected wounds, rational antibiotic therapy must be guided by culture and sensitivity results. Consequently, the referring veterinarian should collect a sample for culture and sensitivity testing prior to the initiation of interim antibiotic therapy and either submit it or send it along with the horse for testing at the referral hospital. The reader is referred to Chapter 16 for more information on the diagnosis and management of wounds involving synovial structures. Table 19.1 lists common bacterial isolates from various wounds in horses, and provides recommendations for interim systemic antibiotic therapy.

The large amount of tissue trauma and contamination in wounds involving a tendon result in a prolonged phase of debridement, which in turn, delays the formation of a healthy bed of granulation tissue. Antibiotic therapy should be administered for at least 3–5 days. Only for minimally contaminated wounds treated early (less than 3–6 hours after injury) would antibiotic therapy be maintained for as little as 3–5 days. Total duration of antibiotic therapy usually ranges from 7–28 days. A lengthy period of administration is most often associated with wounds involving a synovial structure and those with substantial contamination, a slow response to therapy, and/or persistent infection. For more information, refer to Table 19.5, which provides guidelines for duration of antibiotic therapy in specific types of wounds.

The increased use of regional antibiotic therapy in recent years has greatly improved the prognosis for horses with extensive tissue damage to a limb, with or without damage to a synovial structure. The antibiotic should be administered regionally and systemically for optimal results. Regional antibiotic delivery has the distinct advantage of achieving a much higher concentration of the antibiotic at the wound than can be achieved by systemic administration where high doses may be unacceptably expensive or cause detrimental side‐effects. Additionally, a wider range of antibiotic options that can be delivered regionally enables the selection of an antibiotic with a susceptibility pattern or spectrum of activity different to that being administered systemically. A complete review of regional antibiotic delivery is beyond the scope of this chapter, but many excellent resources on the subject are available^28–30 and more information can be found in Chapter 19 of this book.

The choice of administering an antibiotic intravenously, intraosseously, intrasynovially, topically, or by sustained release from beads implanted at the wound is made on a case‐by‐case basis and depends on many factors, including the nature of the specific injury and the availability of a peripheral vein for delivery. Intravenous regional antibiotic delivery is a common choice because of general ease of administration and distribution to synovial structures and the surrounding soft tissues. Frequency of regional delivery depends on severity of contamination or infection, response to initial treatment, ease of delivery, and to some extent, prior experiences of the clinician. Although the literature contains various reports on the pharmacokinetics of regionally delivered antibiotics, the majority concern normal horses, and how these data may translate to horses with inflammation and possibly increased blood flow to the injured area is unknown. In many cases, therapy is administered by this route every other day, but frequency of administration can sometimes be reduced to every third day if the horse responds well to therapy.

In recent years, the use of regenerative therapies has gained popularity in veterinary medicine, especially in equine practice. Mesenchymal stem cells (MSC) are commonly delivered to the site of tendon and ligament injury caused by overuse, in the hope of stimulating a stronger healing response and reducing the incidence of re‐injury. Sources of cells include bone marrow and adipose tissue. Additional biologic therapies, such as platelet rich plasma (PRP), concentrated bone marrow aspirate, autologous conditioned serum (irap® plus, Dechra Veterinary Products), and ECM (ACell Vet™, ACell, Inc.), have also been described for treating tendinitis and desmitis. Application of regenerative therapies in general wound healing is covered in Chapter 22 of this book. The use of regenerative therapies has not yet been described for adjunctive treatment of tendon and ligament lacerations, but theoretically, their beneficial effects would apply. Use of PRP gel (with or without MSC) or ECM at the site of tendon injury may provide a structural scaffold and bioactive proteins capable of contributing to a regenerative response. Judicious use of products shown to be safe for use in open wounds, especially those that can be produced for point‐of‐care application, may be beneficial.

Extensor tendon laceration