Equine lameness due to disorders of the distal limb (Proceedings)

Pathology in the distal limb is one of the most common etiologies of lameness and poor performance in the equine athlete. This is particularly true in the case of forelimb lameness, where problems in the distal limb or foot account for greater than 50% of the lameness cases. However, distal limb problems are also common in the rear leg, and should not be overlooked.

Disorders of the distal limb is defined as problems occurring from structures distal to the fetlock. This is a complicated anatomic area involving numerous soft tissue and bony structures that when damaged can each be a source of pain and lameness.

The basic lameness exam is employed as for other causes of lameness. The nature of the lameness is highly variable, due to the large variety of disorders that can occur. The typical lameness caused by problems in the distal forelimb is a weight bearing lameness, and is worse on the hard surface. This means that the maximal head excursion (head up) is when the lame leg is on the ground. This is particularly true of lameness caused by problems in the joint and navicular bone. Disorders of the soft tissues within the distal limb and foot may also present with a weight bearing lameness, however some will alternatively present as a swinging leg lameness that can mimic lameness from problems higher in the leg. It is important to evaluate these horses on different surfaces and also with a rider, as the lameness may be variable under these different circumstances.

Physical exam and palpation of the distal limb is extremely important to isolate areas of palpable pain and sensitivity, as well as any joint effusion or swelling. A thorough examination of the hoof and hoof conformation should be done on both the sound and lame limb. Horses that have had a chronic, low grade lameness will often have a smaller, more upright hoof. When this is present, it can be determined that the horse has been under-loading the hoof for a long time. Hoof testers should be applied and can be a sensitive way to detect subsolar bruising or a subsolar abscess. However, I find hoof testers to be insensitive for detecting other causes of lameness related to the distal limb. Distal limb flexion is almost always positive when problems are present in the distal limb, but is by no means a specific test.

Once the basic lameness examination is complete, I generally proceed with diagnostic anesthesia of the palmar or plantar digital nerves. This block will desensitize the heel area, entire sole, coffin joint, and a variable amount of the pastern joint, so is not sensitive for any one problem. I prefer to do this block first because it is relatively non-invasive and is usually well tolerated by the animal. With additional time and volume of the medication, significant proximal diffusion can occur which can alleviate pain arising from the fetlock joint or either higher. Therefore only a small volume of local anesthetic (2-3 mL/nerve) should be used and the horse evaluated at 5-7 minutes post blocking. Efficacy of the block can be assessed by checking skin sensitivity at the heel areas with the horses ipsilateral eye covered. Proximal diffusion can be assessed by checking skin sensitivity higher up the leg.

Lameness caused by disorders of the distal limb are commonly bilateral, with one limb being more affected than the other. Often times when the most affected leg is blocked, the animal will begin displaying a lameness in the contralateral limb. When this happens, you can be assured that you have identified the region of pain in the blocked limb. The other limb is then similarly blocked to ensure that the problem is confined to the distal limb as well. In cases where routine diagnostic imaging have failed to identify the cause of the lameness or treatment has been unsuccessful, I may proceed with intra-articular anesthesia once the peri-neural block has worn off. Intra-articular anesthesia of the distal interphalangeal joint is performed first, followed by intra-articular anesthesia of the proximal interphalangeal joint if there has been no improvement. Even intra-articular anesthesia is not entirely sensitive, as the palmar digital nerves will course adjacent to the palmar and plantar pouch of the distal interphalangeal joint, so that the local anesthetic will diffuse to this area and anesthetize structures distal to the joint (navicular bone and all soft tissues of the heel). Intra-thecal anesthesia of the digital flexor tendon sheath can be performed if there is a suspicion of a distal deep digital flexor tendonitis. Occasionally diagnostic anesthesia of the navicular bursa is performed, however this technique is difficult in the unsedated animal. Aseptic preparation of the area is absolutely essential for these intra-synovial blocks to minimize any potential for contamination, which may be life threatening.

Once I have determined the that the lameness is alleviated by local anesthesia of the palmar or plantar digital nerves or intra-articular anesthesia, I will proceed with diagnostic imaging. Radiology combined with ultrasonography is usually performed in almost all cases. Advanced diagnostic methodologies such as MRI has revolutionized the diagnosis of lameness in the distal limb, and is performed when possible. However, because of the increased cost and limited availability, MRI is usually only chosen with other diagnostic methods and treatment methods have failed to identify or alleviate the cause of the lameness.

Diagnostic radiology of the foot requires careful hoof preparation. The foot should be cleaned of all debris, particularly around the sulcus of the frog. If the hooves have not been trimmed recently, I prefer to lightly trim the frog and sole. In most cases the shoes are not removed for radiographs. However, if it appears that the shoes are blocking a possible lesion, they are removed and the foot is re-radiographed. In most situations, I will pack the sulcus of the frog with play-doh to eliminate the artifactual lines created by the frog. The standard radiographic projections of the foot include a lateral, dorso-palmar or plantar (DP), skyline of the navicular bone, 60 degree DP of the navicular bone, and 60 degree DP of the coffin bone. Additionally, while the foot is placed on the 60 degree block, I like to obtain lateral and medial 60 degree oblique radiographs. For evaluating foot balance, it is essential to use blocks where the central beam of the radiograph is centered on the solar surface of the coffin bone. If pathology is seen in the proximal interphalangeal joint or higher, additional radiographic projections are made.

Osteoarthritis of the distal interphalangeal joint (low ringbone) is commonly seen in older athletic horses with a chronic history of foot problems. Any evidence of osseous proliferation or remodeling around the distal interphalangeal joint is significant, as this is a high motion and very unforgiving joint in both the fore and hind limbs. Most often evidence of osteoarthritis is present on the lateral radiographic projection. However, more subtle changes in the joint may only be seen on the oblique 60 degree radiographic projections, as these views highlight the the dorsolateral and dorsomedial aspects of the distal interphalangeal joint. These areas should be scrutinized on properly positioned and exposed radiographs, as over-exposed projections can burn-out some of the less radiodense changes. The magnification and exposure manipulations available on digital radiographic units are particularly useful in evaluating this anatomic area.

Other osseous changes to the coffin bone are usually the result of a concurrent disease process. Distal translocation of the bone occurs with laminitis and severe flexural deformity cases. Pedal osteitis is a common finding and is characterized by variable resorption and osteolysis of the coffin bone. The coffin bone is a highly reactive bone and will respond by changing shape or density with disruptions of its normal circulation such as in cases of chronic subsolar bruising, chronic foot imbalance, laminitis, and infection. Fractures of the coffin bone are also common findings. In my practice, fractures of the coffin bone is the second most common cause of lameness in foals (behind septic arthritis). It is not uncommon for adult horses to fracture the coffin bone as well. Horses that kick the stall wall are particularly susceptible to this condition on the rear limb. Most coffin bone fractures that I see are non-articular, but one should pay particular attention to assess for joint integrity as this will dramatically affect the prognosis.

Significant radiographic changes to the navicular bone include shape changes (elongation) of the bone, enlarged synovial invaginations (lollipop lesions), increased number of synovial invaginations, remodeling or osseous proliferation at the proximal margin of the navicular bone, osseous fragments off of the distal border of the navicular bone, sclerosis or osteolysis of the navicular bone, and flexor surface lesions. The significance of some of these abnormalities is uncertain, as many sound horses can have varying degrees of these radiographic lesions. I have found that flexor surface lesions, particularly evidence of osteolysis and osseous remodeling are highly significant pathologic findings, as every horse that I have evaluated with this problem has been lame, and have been challenging cases to treat without neurectomy. This is most likely due to the fact that bony changes of this type will often be accompanied by deep digital flexor tendonitis adjacent to the abnormal bone as well as adhesions of the tendon to the bone. Evidence of osteolysis or a Òcyst-likeÓ lesion within the navicular bone also seems to be a highly significant finding. Increases in size or number of the synovial invaginations do not seem to be predictable for lameness. I have seen many cases in which I called these structures abnormally enlarged where the horses are sound and remain sound. In these cases these changes do not seem to worsen with time. I do however, think that these changes are an abnormality and should be monitored, but find it impossible to predict if there will be future problems or not. Remodeling along the proximal or distal border of the navicular bone including distal border fragments is evidence of a concurrent soft tissue problem as these are the sites of soft tissue attachments (the collateral sesamoidean ligament along the proximal border and the impar ligament along the distal border).

Diagnostic ultrasonography is a useful adjunctive diagnostic procedure to evaluate the soft tissue structures of the distal limb. Routine examination can be performed of the flexor tendons, digital flexor tendon sheath, distal sesamoidean ligaments, collateral sesamoidean ligament, collateral ligaments of the distal interphalangeal (DIP) joint, impar ligament, navicular bursa, and distal interphalangeal joint. The technique is limited to soft tissue structures outside the limits of the cornified hoof tissue. With careful preparation of the frog, this can be used as an acoustic window to visualize the axial part of the deep digital flexor tendon, navicular bursa, flexor surface of the navicular bone, and impar ligament.

The most common soft tissue disorders of the distal limb that can be detected with diagnostic ultrasonography include deep digital flexor (DDF) tendonitis and desmitis of the collateral ligaments of the DIP joint. The deep digital flexor tendon in the distal limb is a bi-lobed structure, and can be evaluated in weight bearing and non-weight bearing, both in transverse and longitudinal fashion. Both lobes of the DDF should be evaluated carefully. Most lesions of the tendon will cause some degree of asymmetry between the lobes, and this can be the first indication of a tendonitis. In addition, if the lesion extends proximally, some degree of tenosynovitis of the digital flexor tendon sheath can be seen via ultrasound. Linear margin tears of the DDF tendon are also common and are difficult to see on ultrasound. Most cases will however have some degree of asymmetry between lobes of the tendon and tenosynovitis in the digital flexor tendon sheath. It is important to remember that the DDF tendon transitions from a fibrous structure to a fibrocartilagenous structure as it crosses the flexor surface of the navicular bone. While the fibrous part of the tendon will have the characteristic echogenic appearance, the fibrocartilagenous part will appear hypoechoic. Approximately 40-60% of the collateral ligaments of the DIP joint can be visualized via ultrasound, depending on the conformation of the hoof. The remaining part of the ligament lies beneath the cornified hoof wall. Symmetry between the lateral and medial collateral ligaments is very important as the injured ligament will be larger due to swelling and scar tissue. Occasionally core lesions can be seen within the ligament or even small avulsion fractures from the origin on the distal second phalanx. The distal sesamoidean ligaments should also be evaluated for symmetry or any obvious ultrasonographic abnormalities. In my practice, the oblique distal sesamoidean ligaments are the most common of the distal sesamoidean ligaments to be injured, in both the fore and hind limbs.

Nuclear scintigraphy is occasionally utilized to aid in the diagnosis of pathology in the distal limb. Typical cases where scintigraphy is used are those with severe lameness that can be alleviated by diagnostic anesthesia of the palmar or plantar digital nerves, but there is no obvious pathology identified by the standard diagnostic imaging methods. The most common finding in cases such as these are increased radiopharmaceutical uptake in the distal phalanx due to a radiographically occult fracture. Additionally, increased radiopharmaceutical uptake in the navicular bone can be seen in horses that are radiographically normal. This is probably evidence of early navicular disease where there is active bone remodeling but no apparent radiographic abnormalities yet.

Magnetic resonance imaging (MRI) is the most sensitive method of detecting pathology with the soft tissues and bones of the distal limb, and has totally revolutionized the diagnosis of distal limb problems. I will utilize this technology when a diagnosis is not completely clear by the standard imaging techniques, or in cases where the response to therapy is not typical. MRI cannot be used as a screening test, so the source of the lameness must be localized using diagnostic anesthesia. MRI evident lesions are often seen in bony and soft tissue structures that appear radiographically and ultrasonographically normal. Additionally, the metabolic activity of these lesions (acute vs. chronic) can be ascertained. Multiple sources of injury are common MRI findings. The most common MRI abnormalities of the distal limb in my practice include bone edema in the navicular bone, collateral ligament desmitis of the DIP joint, DDF tendonitis, and oblique distal sesamoidean ligament desmitis. Even in cases where pathology is evident on radiographs or ultrasound, MRI can help further taylor therapy and give a more accurate prognosis.

Before one considers treatment options, an accurate diagnosis of the condition must be ascertained. Any underlying foot imbalance issues must be addressed to maximize the success of treatment of conditions of the distal limb. Accurately placed lateral and dorso-palmar/plantar radiographs are particularly useful to determine balance. It is also imperative to incorporate a farrier as part of a team approach to the problem. Additionally, a post-treatment rehabilitation program is equally as important, and often times overlooked. Periodic re-examinations are important to determine response to treatment.

Most problems within the foot are accompanied by some degree of inflammation and effusion within the DIP joint. Most of these horses will be positive to distal limb flexion. Therefore, intra-articular injection of the DIP joint with anti-inflammatory medications makes therapeutic sense in the first attempt at treatment, as it is relatively non-invasive compared to other therapeutic options. My typical coffin joint injection includes hyaluronate and triamcinolone. The duration or rest following injection depends on the diagnosis. This treatment method will return many horses with only mild pathology to soundness relatively quickly. However, the therapy may need to be repeated at variable intervals. Horses with pathology of the navicular bone and bursa may fail to improve after treatment of the DIP joints. For these horses, I prefer to inject the navicular bursa with anti-inflammatories (typically the same medications used in the coffin joint) This procedure is technically more difficult and invasive than coffin joint injections, as the needle must be placed through the deep digital flexor tendon which may lead to future problems. Some form of diagnostic imaging should be used when injecting the navicular bursa. I prefer to use fluoroscopy to guide my needle with the leg in a non-weight bearing position, as real time guidance of the needle allows for minor manipulations to ensure precise needle placement. Alternatively, radiology can be utilized.

Treatment of soft tissue pathology within the distal limb again requires a precise diagnosis. Typically a combination therapy is utilized, usually with extracorporeal shock wave therapy and local infiltration of interleukin-1 receptor antagonist protein (IRAP). Soft tissue pathology that occurs at the insertion sites to bone typically respond to shock wave therapy alone. Shock wave and IRAP injections are typically repeated approximately every 2 weeks for 2-4 treatments, depending on the case and clinical response. IRAP can also be injected intra-articularly within the DIP joint to assist in the treatment of joint or soft tissue problems. The most common indication for intra-articular IRAP in my practice is cases of collateral ligament of the DIP joint desmitis, where it is used in combination with shock wave therapy of the affected collateral ligament. When fiber disruption or core lesions exist, some sort of intra-lesional therapy such as stem cell therapy alone or in combination with platelet rich plasma may be utilized. Ultrasound guided injections ensures precise placement of the medication.

Horses with radiographically evident osteolysis are candidates for treatment with systemic tiludronate. This drug can help alleviate the pain associated with bone remodeling, and assist with improving bone density. Treatment with this drug alone usually will not return a horse to complete soundness, but can dramatically improve many horses, and can be used in combination with other therapies. For horses with end stage navicular disease, where other treatments have failed or have provided only short term pain relief, surgical neurectomy of the palmar digital nerves may be the only treatment option. I use this surgery only as a last ditch effort when retirement is not an option. Case selection is important, as horses with severe erosions along the flexor surface of the navicular bone are not good candidates for surgery as they may rupture their flexor tendons once the pain has been removed. Additionally, in horses with secondary pathology, particularly osteoarthritis of the DIP joint, the outcome following neurectomy is usually not satisfactory.

Successful treatment of lameness caused by pathology within the distal limb requires a precise diagnosis. This can only be achieved by systematic lameness evaluation, diagnostic anesthesia, and diagnostic imaging. Due to the complex anatomy of the distal limb and hoof, advanced imaging modalities such as magnetic resonance imaging can be extremely useful in determining an accurate diagnosis, and in turn an accurate prognosis. Various treatment options exist for pathology within the bone and soft tissues of the distal limb, but must be tailored to the individual and problem. An appropriate post-treatment rehabilitation program may be equally important in ensuring success.