Diagnostic analgesia of the equine digit (Proceedings)

Until recently, there was little scientific basis for interpreting response to various techniques of diagnostic analgesia of the digit. As a result, some diseases, such as navicular disease, were often over-diagnosed, and others over-looked. Recently, some clinicians began to question long-held beliefs concerning analgesic techniques of the foot, especially that analgesia of the distal interphalangeal (DIP) joint localized pain to that structure or that a palmar digital nerve block (PDNB) localized pain to the palmar half of the foot. Within recent years, many studies have examined the response to diagnostic analgesia of horses lame because of pain in a particular structure or region of the foot. Clinical observations, anatomical studies, magnetic resonance images of feet of horses with lameness caused by foot pain, and results of clinical trials that created pain in certain structures of the foot have helped to clarify interpretation of the results of regional, intra-articular, and intra-bursal analgesia of the foot of horses. These studies have also highlighted the limitations of diagnostic analgesia of the horse's digit. In this paper, we present a summary of current knowledge of the use of analgesia to localize sites of pain within the digit of lame horses.

Assessment of lameness before application of local anesthetic techniques

Before performing regional anesthesia, the horse should be consistently and sufficiently lame so that any improvement in gait can be detected. Lunging or riding the horse may exacerbate a subtle lameness. The lameness of some horses improves or resolves during exercise, and so, for these horses, a false positive response to regional anesthesia may result if the horse has not been sufficiently exercised before it is examined for lameness. If a horse is subtly lame, independent observation and grading of its gait before and after regional anesthesia by two or more clinicians skilled at lameness examination may increase the accuracy of interpretation. Video-recordings of gait before and after diagnostic analgesia enable blinded assessment of response to local anesthetic techniques and remove bias from the subjective interpretation of results, especially when lameness is mild. Similarly, the use of computerized methods to detect and quantify gait asymmetry before and after the use of diagnostic analgesia can also result in more objective interpretation of the effect of regional analgesia.

Preparation of skin for injection of local anesthetic solution

Cleaning the site of injection with 70% isopropyl alcohol usually is sufficient for perineural administration of local anesthetic solution. When the distal portion of the limb is particularly dirty, however, the injection site should be scrubbed with antiseptic soap. As long as local anesthetic solution is deposited subcutaneously, complications from poor aseptic technique are unlikely. The clinician should be aware, however, of the potential for disastrous results when non-sterile technique is used, and a needle is misdirected into an adjacent synovial structure, such as the digital flexor tendon sheath (DFTS) or the proximal interphalangeal (PIP) joint. The injection site of a synovial structure should be aseptically prepared, but one study showed that clipping of hair over the injection site is not necessary for aseptic arthrocentesis, as long the site is scrubbed for at least seven minutes. Drawing the drug for intra-synovial deposition from an unused bottle may decrease the likelihood of introducing bacterial contaminates into the synovial structure.

Some clinicians consider the addition of an antibiotic to the local anesthetic solution to be contra-indicated because of the risk of intrasynovial precipitation of the antibiotic. Amikacin is a good choice of antibiotic, however, for clinicians that prefer to administer an antibiotic along with local anesthetic solution because 500 mg of amikacin administered into a normal joint provides a concentration above the reported MIC for most equine pathogenic bacteria for 72 hours with minimal inflammatory effects.

Choice of local anesthetic agent

The first recorded use of local anesthesia for diagnosis of lameness in the horse was in 1885 when a Pennsylvania veterinarian used cocaine for regional nerve blocks of the limbs. Cocaine was eventually replaced by procaine (Novocaine®) (developed in 1905) as the most commonly used local anesthetic agent in veterinary practice. Procaine, however, has no topical effect and so, is of no value when administered intra-articularly for lameness evaluation. By the late 1940s, lidocaine was the most commonly used local anesthetic agent for regional and intra-articular anesthesia in the horse. Mepivacaine HCl was available for human use in the early part of the 1960s and was approved for use in horses in the United States in 1977.

Another local anesthetic agent that has been used for regional and intra-articular analgesia in the horse is 1% hexylcaine HCL (Cyclaine), which has a shorter duration of action and elicits a milder inflammatory reaction than does lidocaine. Two percent prilocaine (Citanest) is also used for perineural anesthesia and intra-articular analgesia because it causes very little tissue reaction. Two percent lidoacaine HCl and 2% mepivacaine HCl are the local anesthetic agents most commonly used to induce regional anesthesia during lameness examination. Mepivacaine HCl is preferred for lameness examination by most clinicians because it has a more rapid onset of action and causes less tissue reaction than does lidocaine HCl. Although we have never observed lameness resulting from intra-articular administration of lidocaine in the elbow or shoulder joint of horses, we have observed temporary, severe lameness, presumably from inflammation, associated with its use in joints of the distal portion of the limb.

Experimentally, perineural administration of two or three percent ketamine HCl in isotonic saline solution or bicarbonate solution at the base of the sesamoid bones resulted in 15 to 25 minutes of analgesia, and 0.02 mg/kg detomidine HCl mixed with isotonic saline solution ameliorated lameness when administered into the coffin joint of horses made lame from set-screw pressure on the sole.

The choice of anesthetic agent may depend on its duration of action. Mepivacaine provides resolution of lameness for about 55 minutes when administered into the middle carpal joint of horses lame from pain in that joint. When mepivacaine is administered perineurally, its anesthetic effect lasts 90 to 120 minutes, and in some cases, up to three hours, which makes this local anesthetic agent valuable for examining a horse with lameness in multiple limbs or when multiple sites of pain on a limb are suspected. Lidocaine, which has an anesthetic effect of only 30 to 45 minutes when administered perineurally, might be the preferred local anesthetic agent when comparison of different techniques of diagnostic analgesia is likely to be used during the lameness examination.

Restraint of the horse for local anesthesia

Regional anesthesia of the distal portion of the limb of most horses can be accomplished using minimal restraint, but for fractious horses or for horses that have previously been subjected to regional anesthesia, using a lip twitch or lip chain is prudent. Not all horses respond in the same way to different methods of restraint, but for most horses, application of a lip twitch provides adequate restraint. The twitch works best when applied immediately prior to placing the needle.

If application of a lip twitch or lip chain does not provide sufficient restraint, xylazine (0.2 mg/kg) or detomidine (10 µg/kg) can be administered intravenously to most horses without decreasing the degree of lameness. Xylazine may be more useful for restraint than detomidine because of its shorter duration of action. A low dose, such as 5 mg, of intravenously administered acepromazine can also be useful for restraining a fractious horse during diagnostic analgesia without interfering with interpretation of the lameness examination. The degree to which sedation or tranquilization may interfere with assessment of gait, however, may depend on the severity of lameness and the skill of the clinician performing the examination. Because of the uncertainty of the effect of sedation or tranquilization on gait, a sedative or tranquilizer is best avoided, if possible. When the horse must be sedated to administer local anesthetic solution, antagonizing the sedative effects of an α2-agonist, such as xylazine, with yohimbine (0.12 ±0.02 mg/kg) or tolazoline (7.5 ± 1.1 mg/lb) may make interpretation of diagnostic analgesia easier.

Regional anesthesia of the distal portion of the limb usually requires multiple injections, and these injections are most safely administered with the limb held off the ground. When a nerve block is performed with the horse's limb on the ground, the contra-lateral limb can be lifted off the ground to enhance the safety of the procedure for the clinician. Some horses with a limb lifted off the ground, however, have a tendency to drop to their knees when the needle is inserted.

When anesthetizing the foot of a fore limb, most clinicians prefer to hold the limb while facing in the opposite direction as the horse, whereas some prefer to face in the same direction as the horse. When facing in the same direction as the horse, the foot can be held between the clinician's knees to free both hands for the procedure, but the clinician is at risk of injury if the horse swings its limb caudally. When the clinician faces in the opposite direction as the horse, the procedure is performed using one hand because the other hand must hold the limb. Nerve blocks performed below the fetlock of the pelvic limb are most safely performed with the pelvic limb lifted, stretched caudally and the fetlock held on the thigh of the clinician performing the block.

Anesthesia of the palmar digital nerves

Clinicians have long believed that a positive response to anesthesia of the palmar digital nerves of lame horses localizes pain to the palmar third or half of the foot, including the palmar aspect of the DIP joint. Easter et al. (2000) found, however, that anesthesia of the palmar digital nerves just proximal to the bulbs of the heel alleviated lameness caused by endotoxin-induced pain in the DIP joint, indicating that the palmar digital nerves innervate the entire DIP joint. The study by Easter et al. corroborated an anatomical study that demonstrated that the dorsal branches of the palmar digital nerves do not innervate the DIP joint. I have observed that anesthesia of the palmar digital nerves does not resolve lameness in horses with osteoarthritis of the DIP joint if the subchondral bone of the middle phalanx is diseased. This is because subchondral bone is innervated by nerve branches that enter a bone through its nutrient foramen; these branches likely lie proximal to the site of the nerve block.

Some clinicians describe the proper site for anesthesia of the palmar digital nerves to be anywhere from the proximal margin of the lateral cartilage to the mid pastern region, but others believe that the nerves should be anesthetized near or distal to the proximal margin of the lateral cartilage. One theoretical advantage of anesthetizing the palmar digital nerves as far distally as possible is that anesthesia of the dorsal branches of the palmar digital nerve is more likely to be avoided. According to results of the study by Sack (1975) and that of Easter et. al. (2000), however, the dorsal branches are unlikely to contribute much more than sensory innervation to the dorsal aspect of the coronary band and dorsal laminae of the foot.

A more important reason for depositing local anesthetic solution as far distally in the pastern as possible when performing a PDNB is that more proximal deposition of local anesthetic solution increases the likelihood of causing analgesia of the PIP joint. When a 0.5- x 16-mm (25-ga, 5/8-in) needle is inserted over the palmar digital nerve one centimeter proximal to the proximal edge of the cartilage of the foot and directed distally, the needle tip is placed at the level of the proximal edge of the lateral cartilage for deposition of local anesthetic solution. Insertion of a needle at this site causes local anesthetic solution to be deposited at or slightly distal to the level of the palmar border of the PIP joint, because the height of the lateral cartilage in relation to the level of the palmar border of the PIP joint is probably similar for most horses. As another precaution to avoid inadvertent analgesia of the PIP joint, we also recommend that no more than 1.5 mL of local anesthetic solution be deposited over each palmar digital nerve and that the lameness be re-evaluated within 10 minutes of administering the nerve block.

Ring block

A ring block, performed at the level of a PDNB is unlikely to result in a positive response after a negative response to a PDNB, because the dorsal branches of the palmar digital nerves contribute little to sensation within the foot. The PDNB will already have anesthetized the entire foot, with exception of the dorsal portion of the coronary band and the dorsal laminae of the foot. The use of a pastern ring block performed at mid pastern level, however, should be considered as a better alternative to the ASNB, because the ASNB may inadvertently partially or entirely desensitize the fetlock joint in addition to the entire foot and pastern, which can lead to an erroneous localization of the pain in the foot or pastern region in horses with fetlock pain.

Abaxial sesamoid nerve block (ASNB)

Bassage and Ross (2003) distinguish between anesthesia of the palmar or plantar digital nerves performed at the base of the proximal sesamoid bones (i.e., a basisesamoid block) and anesthesia performed alongside the proximal sesamoid bones (i.e., an ASNB). They believe that the basisesamoid block is unlikely to anesthetize the dorsal branches of the palmar digital nerve, which is contrary to our experiences. Whether or not these dorsal branches are anesthetized probably has little bearing on the results of a lameness examination because the dorsal branches of the palmar digital nerves contribute little to sensation within the foot. Anesthesia of the digital nerves and their dorsal branches, at either level, alongside or at the base of the proximal sesamoid bones, desensitizes the foot, the PIP joint, middle phalanx and associated soft tissues, and the distal and palmar aspects of the proximal phalanx. Clinicians should be aware that an ASNB or basisesamoid nerve block may ameliorate or abolish pain within the MCP joint, especially if excessive time elapses before assessment to allow proximal migration of local anesthetic solution along the neurovascular bundle.

Performing the nerve block at the base of the proximal sesamoid bones (i.e., a basisesamoid block) decreases the likelihood of partially desensitizing the MCP joint. Using a small volume of local anesthetic solution (i.e., <2.5 mL) and directing the needle distally, rather than proximally, may also decrease the likelihood of partially anesthetizing the MCP joint. Because local anesthetic solution is likely to diffuse proximally after perineural injection, we recommend that the horse's gait be evaluated within 10 minutes of an ASNB.

Analgesia of the distal interphalangeal joint

Several approaches to the DIP joint have been described and include a lateral approach, a palmar approach, a dorsal perpendicular approach (i.e., perpendicular to the bearing surface), a dorsal parallel approach and a dorsal inclined approach. Administering local anesthetic solution into the DIP joint is most easily accomplished using a dorsal parallel or inclined approach, rather than the commonly used dorsal perpendicular approach.

Mepivacaine administered into the DIP joint desensitizes the DIP joint, the navicular bursa, the navicular bone, the toe region of the sole and the digital portion of the DDFT of most horses. When a large volume of mepivacaine HCl (e.g., 10 mL) is administered, the heel region of the sole may also be desensitized.

Possible explanations for analgesia of the navicular apparatus (i.e., the navicular bone and its associated ligaments) after local anesthetic solution is administered into the DIP joint include the desensitization of subsynovial nerves that supply sensory fibers to the navicular bone and its collateral sesamoidean ligaments and the desensitization of the palmar digital nerves where they lie in close proximity to the palmar pouch of the DIP joint. Additionally, Gough et al. (2002) found evidence for diffusion of local anesthetic solution from the DIP joint into the navicular bursa in cadaver limbs, and Keegan et al. (1996) demonstrated diffusion of mepivacaine from the DIP joint into the navicular bone in vivo. In both diffusion studies, the concentration of mepivacaine found in the navicular bursa or navicular bone was high enough to desensitize those structures.

A negative response to intra-articular analgesia of the DIP joint may not eliminate the navicular bone and its related structures as the source of lameness. In a study of 102 horses with chronic foot pain, Dyson (1995) found that 21% of horses failed to respond to intra-articular analgesia of the DIP joint but improved significantly after intrabursal analgesia of the navicular bursa. A later study showed that some lesions of the deep digital flexor tendon (DDFT) at the level of the tendon' s insertion to the distal phalanx may be more effectively desensitized by administration of local anesthetic solution into the navicular bursa than by analgesia of the DIP joint. Some of the lame horses in Dyson's study (1995) that responded to analgesia of the navicular bursa, but were unresponsive to analgesia of the DIP joint, may have had lesions of the insertion of the DDFT rather than disease of the navicular bone and its related structures.

Ten mL mepivacaine HCl administered into the DIP joint desensitizes the entire sole, but a smaller volume of mepivacaine (i.e., <6 mL) is unlikely to desensitize the heel portion of the sole. If lameness is improved by a PDNB, evaluation of the gait after intra-articular analgesia of the DIP joint with a low volume of mepivacaine (i.e., <6 mL) (after the effects of the PDNB have dissipated) may help to determine if pain in the soft tissues of the heel region is the cause of lameness. Pain is unlikely to originate from the sole of the heel if lameness is ameliorated by analgesia of the DIP joint using a low volume of mepivacaine. Calislar and St. Clair (1969) describe a diverticulum of the proximal palmar pouch of the DIP joint that lies proximal to the proximal palmar pouch. A large volume of analgesic solution may fill this diverticulum to anesthetize the palmar digital nerves above a point where a branch of the nerve ramifies to supply the heel region of the sole.

Diagnostic analgesia of the collateral ligaments of the DIP joint

Lameness caused by disease of the collateral ligaments of the DIP joint often fails to improve significantly after analgesia of the DIP joint itself. Only 24% of 30 horses with MRI evidence of collateral ligament disease showed a significant improvement in lameness after intra-articular analgesia of the DIP joint, and none of the horses improved after intra-bursal analgesia of the navicular bursa. A PDNB, however, improved lameness significantly in 72% of these horses, and all became sound after an ASNB. Consequently, when lameness is abolished by a PDNB but not by analgesia of the DIP joint or navicular bursa, and no radiographic abnormalities of the foot can be detected, collateral desmitis of the DIP joint must be considered as a possible cause of lameness, as long as the presence of solar heel pain can be eliminated with hoof testers. Clinical impressions suggest that the degree of improvement in lameness associated with collateral ligament injury after PDNB is determined by the extent of the injury and the level at which the palmar digital nerve is anesthetized. The further proximal the level of the injury within the collateral ligament, the less likely that lameness is affected by analgesia of the DIP joint or a PDNB. This observation suggests that the branches innervating the collateral ligaments arise from the palmar digital nerves, proximal to the site where the palmar digital nerves abut the palmar pouch of the DIP joint or from the dorsal branches of the palmar digital nerve. A pastern ring block or ASNB may be required to abolish lameness when osseous remodeling at the origin of the collateral ligament at the level of the middle phalanx is seen during radiographic examination,

Intra- articular and perineural analgesia of the proximal interphalangeal (PIP) joint (pastern joint)

A positive response to intra-articular analgesia of the PIP joint localizes pain causing lameness to that joint. Not all horse lame because of PIP joint pain, however, respond positively to intra-articular analgesia of the PIP joint. Intra-articular analgesia may relieve pain caused by disease of periosteal and capsular soft tissue, but when joint disease involves subchondral bone, lameness may not resolve. Because subchondral bone is innervated by nerves that enter the bone marrow via nutrient foremen, anesthesia of nerves proximal to branches that enter the nutrient foremen may be necessary, in some cases, to resolve joint pain and lameness. Because the pastern joint is innervated by branches of the palmar digital nerves and their dorsal branches either an ASNB or a basisesamoid nerve block resolves lameness caused by pain in the PIP joint. Perineural anesthesia of the palmar digital nerves and their dorsal branches at this location should also anesthetize nerves before they give rise to branches that enter the nutrient foremen of the proximal phalanx and resolve lameness caused by subchondral bone pain.

Several techniques for analgesia of the PIP joint have been described. I prefer the palmar approach described by Miller et. al. (1996) because using this approach joint fluid is usually recovered to indicate proper needle placement and because it seems to be less resented by the horse than a dorsal approach. The procedure is performed with the limb held. A 20-ga, 1-inch (0.90 x 25-mm) needle is inserted perpendicular to the long axis of the pastern, close to the palmar border of the first phalanx, just proximal to the easily palpable transverse bony prominence on the proximopalmar aspect of the middle phalanx. The needle is inserted through the skin with the joints of the distal portion of the limb extended. These joints are flexed, and the needle is advanced into the palmar pouch of the PIP joint.

Intra-articular analgesia of the fetlock joint

Although a positive response to intra-articular analgesia of the MCP localizes disease to the MCP joint in most horses, not all lameness associated with an injury of the MCP joint resolves with intra-articular analgesia. Whereas lameness caused by intra-articular fragmentation, synovitis, capsulitis, and osteoarthritis improves or resolves after intra-articular analgesia, lameness caused by injury of the subchondral bone or peri-articular ligaments improves only after a low, 4-point nerve block. As an exception to this rule, lameness caused by desmitis of the proximal portion of the straight or an oblique distal sesamoidean ligament is frequently abolished by intra-articular anesthesia of the fetlock joint.

Several techniques are described for intra-articular analgesia of the MCP joint. I prefer the technique described by Misheff and Stover (1991), during which the limb is positioned in partial flexion by holding the foot with one hand, while the needle is introduced through the collateral sesamoidean ligament into the space between the articular surface of the lateral sesamoid bone and the palmar aspect of the lateral metacarpal condyle. A volume of 10 mL mepivacaine is administered, and the lameness is assessed after 10 minutes.

Analgesia of the navicular bursa

A study comparing various techniques for inserting a needle into the navicular bursa showed that a method described by Verschooten et. al. (1991) was the most accurate. Using this method, a 20-gauge, 8.9-cm (3.5-inch), disposable, spinal needle is inserted between the bulbs of the heel just proximal to the coronary band, and the needle is advanced along a sagittal plane aiming for a point 1 cm below the coronary band, midway between the dorsal and palmar limits of the coronary band. The spinal needle is advanced until the tip of the needle contacts bone. At this time, a digital lateromedial radiograph can be taken to check the position of the needle prior to injection 3 to 4 mL of mepivacaine. Another method of determining success of the procedure is to examine the foot radiographically immediately after injecting the bursa, provided that 0.5 to 1.0 mL of contrast medium was added to the local anesthetic solution. Radiographic identification of contrast medium within the bursa is evidence of a successful bursal injection.

Successful centesis of the bursa can also be determined ultrasonographically. For an ultrasonographic view of the navicular bone, the frog is trimmed to pliable tissue, and then soaked in warm water for 30 to 120 minutes; the time of soaking depends on the moisture content of the frog. The needle is advanced into the navicular bursa, using Verschooten's method of centesis, and then the position of the needle tip is determined using a 7.5 MHz linear probe placed on the frog in a sagittal orientation. The needle is properly placed when the tip of the needle contacts the flexor surface of the navicular bone.

As an alternative to radiographic verification, we believe that successful centesis of the bursa can be assumed if the first several milliliters of local anesthetic solution are easily administered and then, as pressure within the bursa increases as several more millilitres are administered, administration becomes more difficult, resulting in refilling of the syringe barrel with local anesthetic solution when pressure on the plunger is released. When the tip of the needle is inserted too far proximally, it passes proximal to the navicular bone to enter the palmar pouch of the DIP joint, and when this occurs, a much larger volume of fluid (e.g., > 7 mL) can be administered before administration becomes difficult. Synovial fluid can usually be aspirated from the palmar pouch of the DIP joint but not from the navicular bursa.

A positive response to administration of local anesthetic solution into the navicular bursa indicates disease of the navicular bursa, the navicular bone, and/or its supporting ligaments, solar toe pain, or disease of the distal portion of the DDFT. Even though analgesia of the DIP joint results in analgesia of the navicular bursa, analgesia of the navicular bursa does not result in analgesia of the DIP joint. Analgesia of the navicular bursa may help to differentiate pain associated with disease of the DIP joint from pain associated with disease of the navicular bone and associated structures. Pain arising from the DIP joint can likely be excluded as a cause of lameness when lameness is attenuated within 10 minutes by analgesia of the navicular bursa.

One possible explanation for the observation that analgesia of the DIP joint causes analgesia of the navicular bursa but analgesia of the navicular bursa does not cause analgesia of the DIP joint is that the site of direct contact between the palmar pouch of the DIP joint and the palmar digital nerves is located proximal to the origin of the deep branches that innervate the DIP joint and the navicular bursa, whereas the site of direct contact between the navicular bursa and the palmar digital nerves is located distal to these branches. Another explanation is that local anesthetic solution may diffuse more slowly from the navicular bursa to the DIP joint than from the DIP joint to the navicular bursa.

Several investigators found a significant difference between the extent of diffusion of various drugs from the DIP joint to the navicular bursa and the extent of diffusion of these drugs from the bursa to the DIP joint. In one study, 4 times more of a mixture of luxol-fast blue dye and mepivacaine diffused from the DIP joint into the navicular bursa (65%) than vice versa (12.5%). In a cadaver study, significantly more mepivacaine was found in the navicular bursa after injection of the DIP joint with mepivacaine than the converse.

In addition to experimental findings concerning the effect of analgesia of the navicular bursa, clinical observations indicate that a positive response to intra-articular analgesia of the DIP joint and a negative response to intra-bursal analgesia of the navicular bursa incriminate pain within the DIP joint as the cause of lameness. This clinical observation is valid if solar pain can be eliminated with hoof testers as a cause of lameness.

The effect of time on interpretation of analgesia of the DIP joint or navicular bursa

Some clinicians have assumed that improvement in lameness observed within 10 minutes after injection of the DIP joint with local anesthetic solution indicates that lameness is caused by pain in the DIP joint alone and that improvement observed more than 10 minutes after injection is caused by diffusion of local anesthetic solution into the navicular bursa or around the nerves providing sensory innervation to the navicular bone and its associated structures. This assumption appears to be invalid because a positive response to intra-articular analgesia of the DIP joint has been observed to occur within 5 to 8 minutes of injection in a majority of horses with navicular disease or experimentally-induced navicular bursal pain.

Results of several trials indicate that the effect of intra-articular analgesia of the DIP joint or of intrabursal analgesia of the navicular bursa on lameness should be assessed soon after injection (i.e., within 5 to 10 min) because after this time, the structures that become desensitized by diffusion of the anesthetic solution become uncertain.

Analgesia of the digital flexor tendon sheath

Synoviocentesis of the digital flexor tendon sheath can be performed by placing a 20- to 22-gauge needle into one of the sheath's several pouches. Access to these pouches is not difficult when the sheath is distended with synovial fluid but is often difficult when it is not. An approach through the palmar/plantar annular ligament of the fetlock was found to be reliable for consistent synoviocentesis of the DFTS. Using this approach, the fetlock joint is flexed to a dorsal angle of 225o, the needle is placed through the skin at the level of the midbody of the lateral proximal sesamoid bone, and then through the palmar annular ligament, 3 mm axial to the palpable palmar border of the lateral proximal sesamoid bone, immediately palmar to the palmar digital neurovascular bundle. The needle is inserted in a transverse plane and advanced at an angle of 45 o to the sagittal plane, aiming toward the central intersesamoidean region, to a depth of 1.5 to 2.0 cm. This technique results in reduced time and fewer attempts required for successful entry into the sheath than the proximal lateral approach. The synovium in this location is less villous, cellular, and mobile than in the proximal pouch, and therefore, synoviocentesis is less likely to result in synovial hemorrhage. Ten mL of local anesthetic solution provides adequate desensitization of the DFTS.A recent study demonstrated that pain induced in the toe and heel regions of the sole, pain associated with synovitis of the DIP joint, and pain associated with synovitis of the navicular bursa were not significantly attenuated by intrathecal analgesia of the DFTS. It is logical, therefore, to assume that analgesia of the DFTS desensitizes only structures that are contained within or border on the sheath itself (i.e., the superficial and deep digital flexor tendons, the straight and oblique distal sesamoidean ligaments, and the annular ligaments of the fetlock and pastern).

Diagnostic analgesia of the digital portion of the DDFT

An ASNB abolished or improved lameness localized to the foot in all of 46 horses found, using magnetic resonance imaging, to have a significant abnormality in the digital portion of the DDFT. A PDNB, analgesia of the DIP joint, or analgesia of the navicular bursa, however, ameliorated lameness of only about two-thirds of these horses.

Because lameness caused by disease of the DDFT within the foot may fail to improve significantly after analgesia of the palmar digital nerves, the DIP joint, or the navicular bursa, we believe that a portion of the DDFT within the foot and distal to the DFTS receives its sensory supply from more proximal deep branches of the medial and lateral palmar digital nerves that enter the DFTS. Improvement of lameness in horses with similar lesions of the DDFT after intrathecal analgesia of the digital synovial sheath has been described.

Performing intrathecal analgesia of the DFTS on horses with lameness that is unchanged after anesthesia of the palmar digital nerves but resolves after an ASNB, may be useful. Resolution of lameness after intrathecal analgesia of the DFTS justifies suspicion of a lesion within the digital portion of the DDFT.

Timing of intra-articular analgesia and intra-articular treatments

Although we can find no references for the recommendation that intra-articular administration of medication, such as corticosteroids or hyaluronic acid, be delayed for several days after intra-articular administration of local anesthetic solution, such a delay seems to be a common practice of many veterinarians. The reason for this delay seems to be the belief that concurrent or sequential intra-articular administration of medication substantially increases the risk of joint infection or that inflammation caused by the local anesthetic solution may dampen the therapeutic response to intra-articular medication.

Two recent studies investigated the effect of concurrent or sequential intra-articular administration of local anesthetic solution and medication in equine joints. Results of one of those studies indicated that same-day administration of local anesthetic solution and hyaluronan and/or cortisone does not pose a substantial risk of joint sepsis, provided that adherence to aseptic technique is strict. A study that examined the effect of concurrent intra-articular administration of mepivacaine and triamcinolone acetonide found that mepivacaine had no effect on the potency or duration of action of triamcinolone.

Anesthesia of the hind foot compared to the forefoot

Innervation of the distal portion of the pelvic limb differs slightly from that of the thoracic limb because the medial and lateral dorsal metatarsal nerves, which are branches of the deep peroneal nerve, provide additional innervation to the digit and third metatarsal bone. Because of this additional nerve supply to the distal portion of the pelvic limb, some clinicians recommend that the dorsal metatarsal nerves be anesthetized when performing either a PDNB or an ASNB, by administering local anesthetic solution subcutaneously medial and lateral to the long digital extensor tendon. Other clinicians, however, believe that supplementing a PDNB or ASNB with anesthesia of the dorsal metatarsal nerves is unnecessary. The dorsal metatarsal nerves should be anesthetized with a subcutaneous ring block when performing regional anesthesia to localize lameness to the fetlock of a pelvic limb because these nerves supply innervation of the dorsal portion of the fetlock. Carter and Hogan (1996) claim, however, that most lameness involving the distal portion of the pelvic limb can be evaluated adequately without blocking the dorsal metatarsal nerves.

False positive and false negative results of diagnostic analgesia of the foot

Clinicians should be aware that techniques of diagnostic analgesia of the horse's foot might provide misleading information concerning the site of pain causing lameness. Possible causes of false positive results have been discussed previously in this article but are summarized below:

• Failure to appreciate that a horse has "warmed out" of lameness. The clinician should be convinced that lameness will not diminish with further exercise before proceeding with diagnostic analgesia.

• Failure to re-evaluate lameness within an appropriate time, thus allowing proximal diffusion of anesthetic solution.

• Clinician bias due to the expectation that the block will ameliorate lameness, especially if the lameness is subtle.

Possible causes of false negative results are:

• Local anesthetic solution could unknowingly be injected into a blood vessel.

• Misdirection of a needle causing deposition of local anesthetic solution into an underlying synovial structure, rather than perineurally.

• Misdirection of a needle causing deposition of local anesthetic solution outside the fascia surrounding the neurovascular bundle.

• Failure of intra-articular anesthesia to alleviate subchondral bone pain or pain associated with the joint capsule or collateral ligaments.

• Clinician bias due to the expectation that the block will not ameliorate lameness, especially if the lameness is subtle.

References supplied upon request