This clinical review covers conditions of the hoof complex itself and will not consider diagnostic and surgical conditions of the synovial structures of the hoof, such as diagnostic arthroscopy of the distal interphalangeal joint or diagnostic bursoscopy of the navicular bursa, nor will it consider internal fixation of fractures of the distal phalanx or navicular bones.
Surgery of the equine hoof differs significantly from elsewhere in the body owing to the unique surgical anatomy. The equine foot comprises the hoof, the skin between the bulbs of the heels and all the structures contained within. The hoof complex consists of the hoof capsule, sole, frog, digital cushion ungular (collateral) cartilages and the deep digital flexor tendon (Parks, 2003).
Disease processes involving the hoof complex can be broadly categorised into septic and non-septic causes.
Septic diseases associated with the hoof
Septic diseases requiring surgical intervention in the horse generally result from a penetrating foot injury, with wounds most commonly occuring on the solar surface of the foot when the horse stands on a sharp object, such as a nail. Puncture wounds are classified by their location on the foot and the depth of penetration (Redding and O'Grady 2012). While superficial wounds penetrating only the cornified tissue are of little consequence, deep wounds penetrating the germinal epithelium of the hoof are more serious as they are more likely to involve the structures within the hoof capsule.
Deep wounds penetrating the sole may damage the distal phalanx (pedal bone). Deep wounds penetrating the frog and heel area may involve the deep digital flexor tendon, distal sesamoidean impar ligament, navicular bursa, distal interphalangeal joint, digital flexor tendon sheath and the digital cushion. Injuries penetrating the coronary band may cause septic osteitis of the pedal bone, infection of the collateral cartilages of the distal phalanx (quittor), or septic arthritis of the distal interphalangeal joint.
Successful treatment of deep penetrating wounds therefore requires accurate preoperative planning to ascertain which structures are involved. Where possible, radiographs should be obtained before removal of the foreign body. Radiography, synoviocentesis of the distal interphalangeal joint, navicular bursa and digital flexor tendon sheath, contrast radiography (Redding and O'Grady, 2012) and magnetic resonance imaging (MRI) (Urraca del Junco, 2012) are all routinely used preoperatively.
Septic synovial structures
Where a synovial structure is involved, the horse is best treated surgically under general anaesthesia, with copious endoscopic lavage of the navicular bursa, distal interphalangeal joint or digital flexor tendon sheath as appropriate. The penetrating tract is also surgically debrided. Regional perfusion of antibiotics and intra-articular antimicrobial are routinely used at the end of surgery.
Endoscopic lavage of the navicular bursa, first described by Wright et al (1999), results in better outcomes in terms of resolution of infection and return to previous levels of athletic function compared to the previously used and more invasive ‘streetnail’ procedure (McIlwraith and Robertson, 1998). In the original study by Wright et al (1999), 10 out of 16 horses (63%) became sound and returned to their pre-injury level of performance, although a more recent multi-centre retrospective study by Findley et al (2014) showed a poorer level of survival to discharge (53/95; 56%) and return to previous level of athletic function (29/80; 36% of horses recovered from anaesthesia).
Sepsis of the distal interphalangeal joint may also occur secondary to a deep puncture wound of the frog, or can also result from a wound to the coronary band. Multiple synovial structures may be implicated with a foot penetration, for example, the distal interphalangeal joint and navicular bursa may be septic and both require lavage, particularly when the penetration is in the central sulcus of the frog and extends to involve the impar ligament which separates the distal interphalangeal joint from the navicular bursa.
Septic pedal osteitis and sequestrum formation
Penetrating injuries to the sole can also introduce bacteria to the solar surface of the distal phalanx and produce septic pedal osteitis (Redding and O'Grady, 2012). A dissecting subsolar abscess, hoof-wall avulsion, a soft tissue injury around the foot, or chronic laminitis with recurrent abscessation can occur as a result in septic pedal osteitis (Celeste and Szoke, 2005). Sequestrum formation of the pedal bone can also occur as a result of septic osteitis.
Septic pedal osteitis can be identified on radiographs as an area of osteolysis at the margins of the distal phalanx, gas density shadows in contact with bone on two orthogonal views and signs of chronic bone inflammation, including decreased bone radiopacity, generalised roughening of the solar margin of the distal phalanx and widening of the vascular channels of the distal phalanx. Sequestra or separate mineralised fragments may also be identified on radiographs (Cauvin and Munroe, 1998).
Surgical debridement and removal of any sequestrum is indicated. The approach will depend on the location of the lesion, with lesions on the distal surface or on the solar margin of the distal phalanx generally being approached through the ipsilateral aspect of the sole. An approach through the hoof wall may also be used where there is an associated abscess involving the dorsal, medial or lateral surface of the distal phalanx, although a defect is usually also required in the sole and extended in the distal hoof wall to improve exposure of the solar margin (Cauvin and Munroe, 1998). After exposing the pedal bone, necrotic bone can be readily identified as being discoloured, with a soft or crumbly consistency and a failure to bleed on curettage. Sequestra are identified as loose fragments of bone and removed. Medical management with systemic and local delivery of antibiotics is also used.
Infection of the collateral cartilages (quittor)
Quittor most commonly occurs secondary to wounds just above the coronary band and over the collateral cartilage; horses usually present with a chronic non-healing wound over the affected collateral cartilage with intermittent purulent discharge (Redding and O'Grady, 2012). Diagnosis is made on clinical appearance, radiography and ultrasonography (Smith, 2014), or less commonly with MRI (Meehan et al, 2016).
As with the other septic causes of hoof disorders, treatment of quittor consists of a combination of antimicrobial therapy (systemic and local delivery) and surgical debridement. Medical treatment alone is generally unsuccessful (Smith, 2014). This surgery is best performed under general anaesthesia in lateral recumbency with the affected cartilage uppermost. The distal interphalangeal joint is placed in extension to minimise inadvertent entry of the palmar/plantar joint pouch during dissection. An elliptical incision, with its base towards the coronary band, is made over the affected cartilage, with the incision starting just proximal to the coronary band itself. The flap is dissected distally to expose the diseased cartilage and discharging tracts, which are excised using sharp dissection and curettage. Where the more distal cartilage is affected, a window is made in the hoof wall to allow surgical access and postoperative drainage. The skin incision is closed to heal by primary intention (Honnas et al, 1988; Smith, 2014).
Non-septic diseases associated with the hoof
Keratoma
Keratomas are aberrant, hyperplastic, keratin masses and although their pathogenesis has not been conclusively determined, they are believed to develop secondary to inflammation induced by hoof wall trauma at the level of the coronary band or subsolar abscess (Katzman et al, 2019).
Clinically, keratomas may be recognised by variable levels of lameness, recurrent foot abscessation, deformation of the hoof capsule and displacement of the white line. Diagnosis is made based on these clinical signs, radiography (Lloyd et al, 1988) and more recently computed tomography (CT) and MRI (Getman et al, 2011). Keratomas are removed surgically after removal of the hoof wall over the mass, with further details considered in the sections to follow.
Pre-surgical planning
Identification of lesion location
Exact identification of the position of the lesion is critical when considering where to create access through the hoof wall. In some cases, such as in septic pedal osteitis where there is a draining tract, this may be relatively straightforward. A probe can be placed in the draining tract and a radiograph obtained to ascertain that the tract proceeds towards the area of pathology on the pedal bone. After ascertaining this, the tract is opened up, normally using a Dremel tool to allow access to the deeper structures which are curetted, debrided and lavaged.
Identification of the exact location of ideal hoof wall access where there is no draining tract and no abnormalities of the external hoof wall may not be so straightforward. In these cases, radiographs can be obtained with radiodense markers placed in an imaginary triangle over the lesion location. The location of the markers on radiographs can be used to accurately identify the correct site of surgical entry to the foot, with holes in the hoof wall made with a Dremel to mark the sites of the radiographic markers (Honnas and Moyer, 2000).
More recently, advanced imaging modalities such as MRI and CT have been shown to be extremely useful for accurately identifying and managing the correct site for surgical access for keratoma removal (Getman et al, 2011) and in management of quittor (infection of the collateral cartilages) (Meehan et al, 2016). With more accurate lesion identification, smaller hoof wall defects can be created while still ensuring adequate surgical access, which in turn leads to decreased instances of postoperative complications and a quicker return to athletic function (Boys Smith et al, 2006; Getman et al, 2011).
Standing vs general anaesthesia
The majority of hoof lesions can be accessed adequately in the standing horse and surgery of the distal limb lends itself well to being performed under standing sedation and regional analgesia of the foot, thus avoiding the associated risks and cost of general anaesthesia. In some cases, a horse's temperament or the site of the lesion may preclude standing surgery and general anaesthesia will be required.
Foot preparation
Before surgery, the foot to be operated on should be trimmed. It has also been advised that the other three feet also receive routine farriery attention, owing to the propensity for the other feet to be neglected after surgery, particularly if the horse is more painful postoperatively (Honnas and Moyer, 2000). Preoperative disinfection of the surgical site is more difficult when operating on the hoof as opposed to skin as a result of the environment in which the horse is kept and the direct of contact of the outer hoof wall, frog and sole with manure, bedding material or soil, as well as because of the crevices and lipid-rich periople/external layer of the hoof (Hennig et al, 2001). The routine technique of pre-surgical disinfection of the foot is rasping to remove the superficial hoof layer, followed by a povidone-iodine scrub, followed by a 24-hour submersion or bandaging of the hoof in povidone-iodine soaked cotton dressings (Hennig et al, 2001).
Intra-operative considerations
Access through the hoof wall
The main difficulty in equine foot surgery is the requirement in many cases to access the internal structures through the hoof wall. Various methods of doing this have been described, including use of complete or partial hoof wall resection (Boys Smith et al, 2006).
In a complete hoof wall resection, a cast saw is used to make parallel vertical cuts on either side of the hoof wall to be removed, with the cuts being connected distally along the white line and proximally, just distal to the coronary band. The hoof wall is grasped distally and levered upwards using a chisel (Boys Smith et al. 2006).
There are a number of methods of performing a partial hoof wall resection, making a window in the hoof wall over the abnormal tissue. These include using either a cast saw, a trephine (Boys Smith et al, 2006), a motorised handheld burr/Dremel (Honnas et al, 2003), or a cordless drill and Forstner bit (Honnas, 2011). As stated previously, partial hoof wall resections are generally preferred over complete hoof wall resections because of the reduced post-operative complications and the quicker return to athletic function.
After creating access through the outer hoof wall, the underlying sensitive tissues are identified. Unhealthy soft tissue is identified and removed by sharp dissection or with a curette, and any infected bone is usually readily identified as being discoloured and soft and is curetted to healthy margins (Figures 1a–1d) (Honnas et al, 2003).
Haemostasis
Application of an adequate torniquet, usually at the level of the proximal cannon/just below the carpus or tarsus, is critical to allow an unobscured surgical site following incision of the very sensitive vascular tissues of the foot. This is usually readily achieved either with an Esmarch tourniquet or simply tight application of Vetrap (or similar product). At the end of surgery and after debridement, the surgical site is firmly packed with gauze swabs and a foot bandage is applied (Honnas et al, 2003). This bandage is usually left in place for 24 hours, after which it can be changed with fresh swabs soaked in antimicrobial solutions placed in the defect.
KEY POINTS
- Surgery of the equine hoof provides challenges as compared to other anatomical sites.
- Developments in advanced imaging (magnetix resonance imaging and computed tomography) have improved preoperative planning.
- Septic diseases associated with the hoof, inc;luding septic synovial structures, septic pedal osteitis and quittor, generally do not respond to medical treatment with antibiotics alone and surgical intervention is key.
- To avoid post-operative complications it is preferable to perform partial hoof wall resections. As small a hoof wall defect as possible while still completely removing the abnormal tissue is the aim.
Postoperative considerations
Postoperatively, the horse will require restricted exercise or box rest, as well as bandaging of the foot until any defect made in the hoof granulates and cornification occurs. Horses may be in pain immediately after surgery, particularly where a large defect has been made in the hoof wall. These horses will require appropriate analgesia and stabilisation of the foot, such as placement of a bar shoe and/or a treatment plate where there is a solar defect (Redding and O'Grady, 2012). Some horses may require application of cast material around the hoof wall at the ground surface (Redding and O'Grady, 2012) or application of a half-limb cast for 10–14 days after surgery (Smith, 2014).
After a partial hoof wall resection, where the horse is comfortable filling of the defect is generally not required after cornification. However, where the horse is required to return to work, quickly filling of the defect with artificial hoof wall repair material may be requested by the owner (Honnas and Moyer, 2000). Another paper has also described the use of platelet-rich plasma combined with a sterile three-dimensional polylactic acid scaffold following hoof wall resection for treatment of keratoma in a limited number of horses (Leonardi et al, 2020).
Conclusions
Hoof surgery of the horse requires careful preoperative planning and accurate lesion diagnosis and location, which has been made possible by advances in imaging techniques such as MRI and CT. While surgical access to the internal structures of the hoof is not as straightforward as via a skin incision elsewhere in equine surgery, various methods have been described, with partial hoof wall resections being preferred over complete hoof wall resections to reduce postoperative pain and result in a quicker return to athletic function.