33rd Annual Scientific Meeting proceedings

Introduction
The navicular bursa is a common site of injury and sepsis in the feet of horses with navicular bursoscopy being an effective method of treatment in many cases. The deep digital flexor tendon (DDFT) forms the palmar/plantar border of the navicular bursa while the dorsal margin is composed of the distal sesamoidean impar ligament, the palmar/plantar fibrocartilage of the navicular bone, the suspensory ligaments of the navicular bone, and the “T” ligament that sits between the suspensory ligaments. Bursoscopic approaches were first described in the 1980s but have been refined and are now commonly used to treat different pathologies.

Bursoscopic Approaches
Transthecal Approach
The transthecal approach was first described by McIlwraith (1984), and later in the literature by Smith et al. (2007), as a technique to access the navicular bursa via the digital flexor tendon sheath (DFTS) with subsequent transection of the T ligament.^1,2 For this approach, the horse can be positioned in dorsal or lateral recumbency. The distal limb is clipped and sterilely prepped for incisions to be made at the level of the pastern joint. It is useful to grind the hoof with a Dremel^® rotary tool prior to surgical preparation in order to have it as clean as possible due to the close proximity of the incisions. The hoof should then be wrapped with a sterile barrier such as sterile gloves and an iodine-impregnated adhesive drape (IobanÔ). A fenestrated drape is placed over the limb with attachment just distal to the carpus or tarsus to prevent leg stands placed at the fetlock joint from interfering with instrument and arthroscope manipulation. A tourniquet placed above the carpus or tarsus is also recommended in case excessive bleeding is encountered. The landmarks for insertion of the arthroscope should be palpated before distension of the DFTS including the palmar/plantar border of the neurovascular bundle, the palmar/plantar eminence of P2 and the dorsal surface of the DDFT. Careful distension of the DFTS through a needle inserted axial to the sesamoid between the proximal digital annular ligament and palmar/plantar annular ligament is performed to avoid extravasation and distortion of anatomical landmarks. The DFTS is distended and the arthroscope is inserted through a small stab incision made in the outpouching of the DFTS between the digital annular ligaments just dorsal to the DDFT. The arthroscope is inserted perpendicular to the limb and once inside the tendon sheath can be used to guide a contralateral instrument portal with transillumination and needle placement. If the lesion is medial or lateral, an instrument portal ipsilateral to the lesion should be used. If both sides of the DDFT or bursa are affected, the arthroscope and instrument portals can be exchanged as needed. With the arthroscope pointed distally, the T ligament can be visualized and transected using an arthroscopic knife, beaver blade, or arthroscopic scissors. The incision should be made just dorsal to the DDFT to avoid entry into the coffin joint. A motorized resector is very helpful to complete transection and debride remaining fibers, especially in horses with chronic navicular bursitis that have thick, fibrotic T ligaments. Transection of the T ligament should be as wide as possible to facilitate entry into and manipulation within the navicular bursa. Following entry into the navicular bursa, the dorsal surface of the DDFT and palmar/plantar fibrocartilaginous surface of the navicular bone can be assessed. The transthecal approach allows for visualization of the proximal 2/3 of the bursa in most horses, however, visualization of the distal bursa is difficult without a second instrument portal used to apply palmar/plantar pressure to the DDFT. Due to the increased likelihood of DDFT pathology occurring in the suprsesamoidean and sesamoidean regions of the tendon, the transthecal approach facilitates endoscopic debridement of DDFT tears in most horses. Smaller and finer motorized resector tips, as well as small arthroscopic instruments, can be very useful for working within the navicular bursa. Horses with infrasesamoidean DDFT tears may be better treated with a direct approach to the navicular bursa.

Direct Approach
The direct approach is achieved with the horse positioned and prepared in a similar fashion, but the arthroscope is inserted through a skin incision made just proximal to the collateral cartilage and palmar/plantar to the neurovascular bundle. The arthroscope cannula is advanced distally along the dorsal border of the DDFT to enter the bursa.^3,4 The direct approach can be more challenging, especially for inexperienced surgeons, therefore, injection of contrast material into the navicular bursa followed by imaging-guided arthroscope placement can be considered. The direct approach is very useful for treating horses with penetrating injuries and sepsis of the navicular bursa as it avoids penetration of other synovial structures. As mentioned previously, it is also useful for treating distal (infrasesamoidean) lesions in the navicular bursa, keeping in mind that the proximal bursal recess is not easily accessible.

Pathologic Conditions of the Navicular Bursa
Deep Digital Flexor Tendinopathies
Advances in diagnostic imaging, especially magnetic resonance imaging (MRI), have shed light on the prevalence of DDFT injuries in the front feet of horses. One study found that 80% of horses having an MRI due to foot pain had DDFT pathology.⁵ Dorsal fibrillation and dorsal border splits and tears occur commonly in the suprasesamoidean DDFT with tendon fibers often recoiling into this region. Inflammation and granulation tissue associated with extruded tendon fibers can lead to chronic bursitis and in some horses, adhesions of the tissue to the T ligament, collateral sesamoidean ligaments, and navicular bone. Sagittal tears of the DDFT are most common, however, dorsal plane tears can also occur.⁶ Sagittal tears tend to narrow as they extend distally. Bursoscopy allows for debridement of the torn tendon fibers, fibrin and adhesions which is thought to limit further inflammation and facilitate healing of the DDFT. Debridement of DDFT tears is best achieved using motorized resectors with smaller diameter resector tips being most useful. Small arthroscopic instruments and suction punch rongeurs can also be helpful for debriding tendon fibers with the goal of creating smooth edges.

Although MRI has greatly advanced our detection of DDFT lesions in this region, bursoscopy appears to be the most sensitive tool for diagnosing lesions of the DDFT with Smith & Wright (2012) visualizing lesions in 103 of 105 (98%) bursae evaluated bursoscopically. In the same study, 48 of 55 (87%) of limbs that had an MRI and 7 of 8 (88%) limbs that had a CT performed were diagnosed with a DDFT lesion. Not unexpectedly, in the same group of horses, 71 of 105 (68%) limbs had no significant radiologic abnormalities.

Horses with DDF tendinopathies that are treated with bursoscopy and subsequent rehabilitation, have been found to have a fair to good prognosis for returning to work. A large retrospective study found that 61% of horses returned to work with 42% of horses returning to their previous level of performance.⁶ Other studies have shown similar results with approximately 60% of horses sound 12 months after surgery.⁷ Horses with extensive tears, concurrent navicular bone flexor cortical erosions, and more chronic navicular bursitis are less likely to return to work.⁶ The prognosis for horses with similar pathologies that are treated conservatively has been reported to be much lower with 28% of horses with DDF tendinopathies and 5% of horses with combined navicular bone and DDFT lesions returning to full work.⁸

Navicular Bursitis
Navicular bursitis can be classified as effusive or proliferative.⁹ Horses with effusive navicular bursitis have an acute accumulation of fluid in the bursa while horses with proliferative bursitis are generally more chronic in nature with thickening and fibrosis of the bursal wall, synovial tissue, and T ligament, with or without adhesion formation. Effusive bursitis appears to be effectively treated with intrathecal injections with corticosteroids, hyaluronic acid, and orthobiologics in many horses. Since proliferative navicular bursitis is usually found in conjunction with other pathology,¹⁰ such as DDF tendinopathies, navicular bursoscopy can be an effective tool as it allows debridement of proliferative tissue, adhesions and torn tendon fibers if they are present.

Navicular Bone Lesions
Few reports of bursoscopic treatment of navicular bone lesions exist. Fibrocartilage and flexor cortical lesions are not uncommonly identified on MRI and curettage of the palmar/plantar flexor cortical lesions can be performed, however, the efficacy of this procedure remains unknown.¹¹ Regardless, the prognosis for horses with flexor cortical erosions appears to be significantly worse than DDFT tears alone.⁶ A recent study described the use of bursoscopically-guided core osteostixis of osseous cyst-like lesions of the navicular bone in 7 horses.¹² In this study, all 7 horses had one limb randomly assigned to treatment with osteostixis while the other limb was assigned to navicular bursoscopy only. Reduction in lameness compared to baseline was significantly better for limbs treated with osteostixis thus providing preliminary evidence that this may be a useful treatment of osseous cyst-like lesions.

Penetrating Injuries and Sepsis
Navicular bursoscopy via the direct approach can be used to treat penetrating injuries and sepsis of the navicular bursa. This commonly occurs when a horse steps on a foreign object, such as a nail, that enters the foot in the palmar/plantar ½ of the sole. The object must penetrate the DDFT in order to enter the navicular bursa and can also cause injury to the palmar/plantar surface of the navicular bone. It should be noted that foreign objects can also penetrate the impar ligament leading to contamination of the coffin joint and/or the T ligament leading to contamination of the DFTS. Navicular bursoscopy allows for identification and debridement of foreign material and for debridement of the damaged DDFT and navicular bone. The penetrating tract through the sole can often be used as an instrument portal. Motorized resectors are particularly useful for debridement of the penetrating tract. Although penetrating injuries and sepsis can be treated via bursostomy performed through a solar approach, bursoscopic approaches require less tissue resection to access the bursa and may be associated with a better prognosis. Wright (1999) reported 63% of horses returning to work following bursoscopic debridement,³ while previous bursotomy reports found only ~32% of horses returned to work.¹³ However, it should be noted that Suarez-Fuentes et al. (2018) recently found that 84% of horses were able to return to work following bursotomy.¹⁴

Complications
Complications of navicular bursoscopy include minor iatrogenic damage to the fibrocartilaginous surface of the navicular bone during arthroscope and instrument manipulation, damage to the digital vessels, damage to the palmar/plantar nerve leading to the neuritis, and surgical site infection. Overall, complications appear to be rare.

Postoperative Management and Rehabilitiation
Some surgeons have advocated for treatment of the navicular bursa with tissue plasminogen activator (tPA) following bursoscopy, either via the tendon sheath following a transthecal approach or via direct injection into the navicular bursa following a direct approach, in attempts to decrease postoperative adhesion formation. This author uses a dose of 500mg of tPA once daily for 3 days, however, the efficacy of this treatment remains unknown. Serial injection of the navicular bursa with orthobiologics after surgery can be performed with some surgeons advocating for the use of mesenchymal stem cells or autologous blood-based products, while other surgeons avoid bursal injections due to concerns of increasing the risk of adhesion formation. Therapeutic shoeing including a 2-4^o wedge, rolled toe, and adequate heel support is recommended immediately after surgery. In cases of penetrating solar injuries, a hospital plate is a helpful addition to keep the solar wound clean, free of debris and accessible if needed. Horses likely benefit from entering a progressive walking program beginning the day following surgery to limit adhesion formation. Walking programs often include 6 to 12 weeks of hand walking, followed by progressive tack walking for 6-12 weeks and then slow introduction of trot. It is recommended that horses work on even, firm footing and avoid circles until trotting soundly for a period of time. In horses with DDFT injuries, serial ultrasonography of the pastern and proximal navicular bursa can be used to examine the DDFT for any propagating lesions. Depending on the pathology, horses generally require 6-12 months before they are back in full work.

Conclusions
Although navicular bursoscopy can be technically challenging, especially for inexperienced surgeons, it is an effective treatment method for pathologies of the navicular bursa including DDFT lesions, navicular bursitis, navicular bone lesions, and sepsis. The prognosis is better for horses with more acute injuries and in horses that do not have concurrent lesions of the flexor cortex of the navicular bone.

References

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