Severe, Unrelenting Lameness

The severe, unrelenting form of MTP joint lameness is associated with intraarticular fractures or severe, end-stage OA. Horses are obviously lame at the walk, cannot be trotted, and may be non–weight bearing. Deformity of the MTP joint may be obvious, as in horses with comminuted fractures of the proximal phalanx. Effusion is obvious and diffuse; periarticular soft tissue swelling may be present. Horses with acute, displaced condylar fractures of the MtIII may have acute, progressive edema of the diaphyseal region, and those with comminuted fractures of the proximal phalanx often have severe soft tissue swelling. Palpation and flexion elicit severe pain and, in horses with comminuted or displaced fractures, crepitus. Radiographs are usually diagnostic. This category includes severe, complete, or comminuted fractures (e.g., spiral fractures of the MtIII); comminuted or complete fractures of the proximal phalanx; midbody fractures of the PSBs; complex fractures involving the MtIII, the proximal phalanx, and the PSBs; luxation or subluxation; and end-stage OA. Horses with acute tendonitis of the deep digital flexor tendon (DDFT) with tenosynovitis can be severely lame, and intrathecal analgesia of the digital flexor tendon sheath (DFTS) may only partially remove pain. Ultrasonographic examination is required. However, some sports horses with DDFT lesions show lameness that is challenging to diagnose because of its sporadic nature.

Intermittent, Severe Lameness

Horses with intermittent, severe lameness of the MTP joint may be able to train and perform at some level but develop severe lameness afterward. When exhibiting clinical signs, horses are lame at the walk, even toe-touching lame, and are obviously lame at the trot (grade 3 or 4 of 5), but after resting and receiving nonsteroidal antiinflammatory drugs (NSAIDs), horses are often able to gallop, jog, or train within 1 to 5 days. In some instances horses may be able to race, only to become lame once again. When walking in hand, horses often show marked lameness while turning, even if they are sound while walking in a straight line. Obvious signs of inflammation usually are not present, but horses generally respond positively to the lower limb flexion test and may show a painful response to deep palpation. Horses with incomplete midsagittal or dorsal (frontal) fractures of the proximal phalanx often manifest a painful response when firm digital pressure is placed on the proximal, dorsal aspect (see Figure 6-37). Effusion may be present, but it is often absent or minimal even in horses with incomplete fractures. Because clinical signs may be difficult to detect, diagnostic analgesia is often necessary. Radiographs are usually diagnostic, but if findings are equivocal or the radiographs are obtained before radiological changes develop, scintigraphic examination or follow-up radiographic examination in 10 to 14 days is required. Digital and computed radiography can be helpful. Conditions such as incomplete fractures of the MtIII, the proximal phalanx, and the PSBs and moderate OA are in this category.

Chronic, Low-Grade Lameness

Diagnosis of chronic, low-grade MTP joint lameness is difficult. Specific signs that localize lameness to the MTP joint are lacking, and lameness may be only subtle to mild (grade 1 of 5) at the trot in hand. Horses examined at the track and carrying a rider may show only mild lameness. In racehorses, effusion and the response to lower limb flexion varies, but it can be negative. There is often mild increase in temperature, a subtle but useful sign of subchondral bone injury. In nonracehorses there may be effusion of the MTP joint and often the DFTS, but effusion is often long-standing and may be easily overlooked if reported by the handler as preexistent. Palpation may reveal mildly suspicious areas (e.g., pain over the abaxial aspect of the PSBs) in horses with sesamoiditis, but in most horses no abnormalities are noted. Concomitant lameness of the MTP joint and stifle region, called intralimb compensatory lameness, occurs most often in racehorses but is recognized in nonracehorses, and clinical signs may not abate until both sites are blocked or treated. The association between the stifle and MTP joint is difficult to explain. In young horses with “loose stifles,” knuckling of the MTP joint occurs when horses, particularly STBs, are jogged or worked slowly. Perhaps stretching of MTP joint capsule attachments or early subchondral bone trauma occurs during knuckling. Horses with “loose stifles” are thought to have patellar ligament and muscular instability and laxity and may benefit from counterirritant injection and simultaneous management of the MTP joint problem.

Diagnostic analgesia is required to localize pain causing lameness to the MTP joint, but in many horses lameness is difficult to accurately assess with the horse at a trot in hand. It is extremely important to recognize the need to employ perineural diagnostic analgesic techniques in addition to or in lieu of intraarticular analgesia in order to abolish all pain associated with the MTP joint. Radiological findings may be normal and horses are referred for scintigraphic examination, which often reveals stress-related bone injury. Special radiographic images may be needed to evaluate the distal aspect of the MtIII or for accurate identification of osteochondral fragments located in the dorsal and plantar aspects of the joint. In this category are conditions such as stress-related bone injury of the MtIII, early OA, osteochondral fragments that occur traumatically or as the result of osteochondrosis, and sesamoiditis.

Diagnostic Analgesia

Articular pain originating from the MTP joint can usually be abolished or at least partially alleviated by intraarticular analgesia. Because only one injection is required, intraarticular analgesia is easier and safer to perform than perineural techniques, but pain originating from subchondral bone may not abate or may be only partially alleviated by intraarticular analgesia. Therefore, it is extremely important to recognize that the low plantar perineural technique or a variation must be used, because it is more effective in alleviating pain from all sources (see Chapter 10).

A variation of the low plantar block, the lateral plantar metatarsal block, can be performed in horses with stress-related bone injury of the distal plantarolateral aspect of the MtIII (see Chapter 107).⁵ This block is particularly valuable in horses with bilateral lameness, because after one limb is blocked, the horse becomes obviously lame in the other. Other sources of pain located laterally in the MTP joint, such as sesamoiditis, small fractures of the PSBs, can be blocked by this technique, but the most common reason horses improve after lateral plantar metatarsal analgesia is alleviation of subchondral bone pain of the distal aspect of the MtIII.

In horses with chronic, low-grade lameness it may be necessary to perform analgesia and watch the horse train. Resolution of subtle signs, such as bearing in or out, not feeling right behind, performing dressage maneuvers, or, in STB racehorses, being on a shaft or a line, may be the only sign of a positive response to diagnostic analgesia. Although perineural analgesia may result in slight loss of proprioception, it is generally preferable, because intraarticular analgesia may result in a false-negative response. If pain is elicited by firm palpation of the dorsoproximal aspect of the proximal phalanx, suspect a midsagittal fracture. Nerve blocks are contraindicated because of the risk of creating a complete or comminuted fracture.

The clinician should be aware that it is possible to inadvertently block pain associated with subchondral bone of the MTP joint when performing plantar digital or plantar perineural analgesia. This occurs most frequently in racehorses with midsagittal fracture of the proximal phalanx but can occur in horses with OA or other conditions (see Chapters 10 and 41).

Radiography and Radiology

Examination should include dorsoplantar (DPl), lateromedial (LM), dorsolateral-plantaromedial oblique (DL-PlMO), and dorsomedial-plantarolateral oblique (DM-PlLO) images. A flexed LM image is useful to evaluate the sagittal ridge for the presence of osteochondrosis lesions and to see dorsal frontal fractures of the proximal phalanx. Vacuum phenomenon can occur because the MTP joint can be placed in extreme flexion. Sudden decompression of the joint during stress flexion is believed to cause what appears to be an air artifact in the distal plantar aspect of the MtIII.⁶

Horizontal oblique images are useful to evaluate the proximal-dorsal aspect of the proximal phalanx for the presence of osteochondral fragments, but overlap between the base of the PSBs and the proximal aspect of the proximal phalanx can hide lesions associated with the distal-plantar aspect of the joint. Overlap in the plantar aspect of this joint is more common than in the metacarpophalangeal joint and occurs when the distal hindlimb is not vertically positioned. The x-ray beam should be angled down 15 to 20 degrees to separate the PSBs and proximal phalanx (Figure 42-1).

Fig. 42-1 A, Dorsal 45° lateral-plantaromedial oblique digital radiographic image of a 3-year-old Standardbred filly with maladaptive subchondral bone remodeling obtained using a horizontal x-ray beam. Often, there is overlap between the base of the proximal sesamoid bones (PSBs) and proximal aspect of the proximal phalanx (white arrow), but in this well-positioned view no overlap exists. A medial, plantar osteochondral fragment can be seen (black arrow). B, Dorsal 20° proximal, 45° lateral-plantarodistal medial oblique digital radiographic image of the same filly showing a large radiolucent lesion in the distal aspect of the lateral condyle of the third metatarsal bone (MtIII) that was not seen in a conventional oblique image (A). A proximodistal (down-angled) radiographic image opens up the space between the PSBs and the proximal phalanx. C, A similar down-angled oblique image of the plantaromedial aspect of the joint allows complete visibility of the plantar osteochondral fragment (arrow).

Using a horizontal x-ray beam in a DPl image, the PSBs are superimposed over the distal aspect of the MtIII and the MTP joint. To better evaluate these areas, dorsal 15° proximal-plantarodistal oblique (D15° Pr-PlDiO), flexed DPl, and standing 125-degree DPl images should be used. Proximolateral (medial)-distolateral (medial) and proximoplantar-distoplantar tangential images of the PSBs are occasionally used to evaluate the abaxial and plantar surfaces of the PSBs, respectively.

Approximately 5% to 10% of normal horses have a small unilateral or bilateral radiolucent notch (<1 mm in length) in the sagittal grove of the proximal phalanx that is seen in a DPl image and should not be confused with a midsagittal fracture. I have seen this “notch” most commonly in the DPl image of the STB racehorse. If clinical signs are consistent with midsagittal fracture, scintigraphic examination is recommended. Flattening of the plantar distal aspect of the MtIII condyles is not as common as in the metacarpophalangeal joint, but increased radiopacity of the plantar aspect of the MtIII can be seen in horses with stress-related bone injury and early OA if well-exposed LM and flexed LM images are obtained. I question the clinical significance of flattening of the condyles of the MtIII in most horses. Digital and computed radiographs are useful in the evaluation of horses with stress-related bone injury and incomplete fractures. Computed tomography (CT) and magnetic resonance imaging (MRI; see later) are also useful, but availability is limited.

Scintigraphic Examination

Scintigraphic examination is the best way to establish a diagnosis in many horses with intermittent, severe, or chronic low-grade lameness of the MTP joint. The most common scintigraphic findings in the MTP joint of STB and TB racehorses are focal areas of increased radiopharmaceutical uptake (IRU) that involve the distal plantarolateral aspect of the MtIII (see Figure 19-20, A; Figures 42-2 and 42-3).⁷ This IRU is a form of stress-related bone injury and early OA that is found in many racehorses with chronic, low-grade lameness, bilateral hindlimb lameness, and poor performance. Special radiographic images may then reveal increased radiopacity or radiolucent defects (see Figures 42-1 and 42-3). Similar scintigraphic findings are seen in horses with lateral condylar fractures of the MtIII. Focal areas of IRU are seen in horses with midsagittal or dorsal frontal fractures of the proximal phalanx (see Figure 19-20, C), sesamoiditis (Figure 42-4), and osteochondrosis of the plantar process of the proximal phalanx (see Figure 23-1; Figure 42-5). Often areas of IRU are found in unusual sites, such as those associated with the medial PSB (osteochondral fragments of the abaxial border and sesamoiditis), intersesamoidean ligament injury with radiolucent defects in one or both PSBs, incomplete fractures of the PSBs, and medial condylar fractures of the MtIII. Any focal area of IRU located medially in the MTP joint should be investigated, because incidental findings are unusual in this location.

Fig. 42-2 A, Plantar (lateral to the left) and flexed lateral (on the right) left hindlimb and B, plantar (lateral to the right) and lateral (on the right) right hindlimb delayed (bone) phase scintigraphic images of the metatarsophalangeal (MTP) joint in two Standardbred racehorses. All images show focal increased radiopharmaceutical uptake of the distal, plantarolateral aspect of the third metatarsal bone, which is the most common scintigraphic finding in the MTP joint of racehorses.

Fig. 42-3 A, Lateral (on the left) and plantar (lateral is to the left) bone phase scintigraphic images of a metatarsophalangeal joint. There is increased radiopharmaceutical uptake in the plantar aspect of the lateral condyle of the third metatarsal bone. B, Dorsolateral proximal-plantaromedial distal oblique xeroradiographic image showing a radiolucent defect (arrow) of the same area. This defect can easily be missed on routinely positioned images, but the increased radiopharmaceutical uptake seen in this area scintigraphically (A) prompted further investigation.

Fig. 42-4 Flexed lateral delayed (bone) phase scintigraphic image of a 2-year-old Standardbred racehorse with lateral sesamoiditis. There is focal moderate increased radiopharmaceutical uptake in the proximal sesamoid bones (arrow). When these findings were combined with those of the plantar image, the increased radiopharmaceutical uptake was localized to the lateral proximal sesamoid bone.

Fig. 42-5 A, Plantar (left, lateral is to the right) and flexed lateral delayed (bone) phase scintigraphic images of a 3-year-old Standardbred trotter. There is focal mild increased radiopharmaceutical uptake (IRU) involving the proximal aspect of the proximal phalanx (arrows), evidence of mild bone modeling associated with lateral and medial axial articular osteochondral fragments of the plantar process of the proximal phalanx. IRU of the lateral aspect of the distal phalanx in the hindlimb is a common incidental scintigraphic finding. B, Dorsal 20° proximal lateral-plantarodistal medial oblique digital radiographic image of the same horse showing the presence of lateral (white arrow) and medial (black arrow) articular osteochondral fragments. Osteochondrosis lesions occur in this region more commonly on the medial aspect of the proximal phalanx but can be biaxial. C, Intraoperative arthroscopic photograph (plantar is to the left and proximal is uppermost) showing the medial fragment interposed between the base of the medial proximal sesamoid bone (PSB) and the proximal phalanx. The medial condyle of the third metatarsal bone (MtIII) is immediately dorsal to the fragment. The arthroscope was positioned in the lateral plantar pouch, and separate instrument incisions were used to remove both the medial (shown) and lateral fragments.

Scintigraphic changes are usually less pronounced in nonracehorses, and negative or equivocal results often occur. The most common scintigraphic findings in jumpers and dressage horses with MTP joint lameness are diffuse mild areas of IRU in all bones or focal mild or moderate IRU in the dorsal or central aspects of the joint that involve the distal aspect of the MtIII and proximal aspect of the proximal phalanx and are associated with radiological evidence of marginal osteophytes and enthesophytes. There is a distinct difference in the scintigraphic and radiological appearance of the MTP joint of nonracehorses and racehorses, because in racehorses IRU is generally focal in nature and IRU and radiological changes involve the plantar aspect of the joint. In a 2004 study of clinically sound horses, generalized, even radiopharmaceutical uptake across the distal aspect of the MtIII and the proximal aspect of the proximal phalanx was found.⁸ Radiopharmaceutical uptake was significantly higher in the condyles of the MtIII of hindlimbs than in forelimbs, there was no effect of age, and there was a trend for region of interest ratios to be higher in the right hind MTP joint when compared with the left.⁸ In some nonracehorses arthroscopic examination has revealed full-thickness cartilage damage that primarily involves the distal dorsomedial aspect of the MtIII (medial condyle) and is consistent with OA. Focal IRU in the proximal aspect of the plantar pouch is seen in horses with severe OA (an ominous finding) (see Figure 19-20, B). I believe this finding is caused by accumulation of subchondral bone fragments in this location or modeling of the distal plantar aspect of the MtIII.

Magnetic Resonance Imaging

MRI is useful to evaluate bone and soft tissue abnormalities in the MTP joint and nearby structures (see Chapters 21, 107, and 108). In fact, in some horse-dense locations such as Newmarket, England, examination of the metacarpophalangeal and MTP joints using standing low-field MRI has become as commonplace as scintigraphic examination and is often preferentially requested by TB trainers.⁹ In TB and STB racehorses with maladaptive or nonadaptive bone remodeling of the distal aspect of the MtIII, MRI examination reveals extensive subchondral bone damage characterized predominately by low signal intensity of the plantar aspect of the lateral condyle of MtIII on T1- and T2-weighted images, a finding supporting the existence of chronic, sclerotic subchondral bone (Figure 42-6, A and B). Within the areas of low signal intensity is often a small area of high signal intensity near the articular cartilage (see Figure 42-6, A and B). In fat-suppressed short tau inversion recovery (STIR) image sequences, small, focal areas of high signal intensity within sclerotic bone are often found, indicating the presence of necrotic, and perhaps ischemic, bone, but widespread areas of high signal intensity consistent with bone edema from acute trauma are not often seen (Figure 42-6, C). Although numerous authors or speakers have characterized these lesions as bone bruises, bone edema (fluid accumulation) characteristic of bone bruises found in subchondral bone in people is not a hallmark of this common lesion in horses. Large areas of increased signal intensity likely indicated the presence of a fracture of the MtIII. Areas of increased signal intensity within sclerotic subchondral bone in horses with repetitive stress injuries could represent proteinaceous fluid, but likely represent regions of necrotic bone or granulation tissue.⁹ Areas of necrotic bone seen on magnetic resonance (MR) images correspond to radiolucent defects. Areas of bone loss, necrotic subchondral bone, or areas of intense resorption within sclerotic bone may warrant consideration when trying to manage horses with this lesion (see later). In horses with acute-onset clinical signs consistent with acute subchondral bone injury or fracture, bone edema can be more prominent. In 13 horses, most of which were nonracehorses, with lameness of metacarpophalangeal or MTP joints and without radiological abnormalities, the most common finding was decreased signal intensity in T1-weighted images, indicating the presence of sclerotic subchondral bone.¹⁰ In nine horses, decreased signal intensity in T-2*-weighted images was consistent with sclerosis, but five had increased signal intensity in fat-suppressed STIR images consistent with what was described as fluid accumulation within the sclerotic regions.¹⁰ Importantly, MRI abnormalities and subchondral bone lesions were found not only in racehorses but in horses used for show jumping and general purpose riding.¹⁰ Lesions similar to those described for racehorses were seen in the MTP joint of sports horses.¹¹ Focal areas of IRU seen scintigraphically appeared in MR images as areas of low signal intensity on T1- and T2-weighted images with or without small areas of increased signal intensity in STIR images, indicating the presence of chronic, repetitive stress injuries of the distal aspect of the MtIII in sports horses in addition to acute, traumatic injuries.¹¹ In an earlier study of 11 horses (eight racehorses), subchondral bone damage was identified using MRI, although there were no or equivocal radiological abnormalities.¹² Four horses had lesions in the metacarpophalangeal or MTP joints. Horses with acute-onset lameness had subchondral bone injury characterized by increased signal intensity in STIR and T2-weighted images.¹² In two horses, decreased signal intensity in proton density images (similar to T1-weighted images) indicated the presence of sclerotic subchondral bone of the distal aspect of the MtIII, but dense bone was surrounded by areas of increased signal intensity in STIR sequences.¹² Only two horses had bone scintigraphy performed, and areas of IRU in the damaged subchondral bone were seen.¹² Horses with acute subchondral bone injury should improve with rest, whereas those with chronic, sclerotic, and osteoarthritic subchondral bone may improve temporarily with rest but long-term prognosis is guarded. MRI was useful in the evaluation of horses with oblique and straight distal sesamoidean desmitis in 27 horses with lameness localized to the metacarpophalangeal or MTP joint region, of which 17 had lameness and injury in the hindlimb.¹³ Careful examination of soft tissue structures associated with the MTP joint, including use of diagnostic ultrasonographic examination, is necessary, because in some horses the results of diagnostic analgesia will be ineffective at differentiating authentic sources of pain in the region.

Fig. 42-6 Sagittal, low-field (0.25T) magnetic resonance images, obtained with the horse under general anesthesia, of a 4-year-old Thoroughbred racehorse with lameness localized to the lateral aspect of the right metatarsophalangeal joint by using lateral plantar metatarsal analgesia. A, T1-weighted image showing decreased signal intensity (arrows) surrounding an area of increased signal intensity (arrowheads). B, T2-weighted image showing the same signal distribution, indicating there is dense, sclerotic subchondral bone surrounding an area of necrotic, ischemic subchondral bone or fluid accumulation. C, Short tau inversion recovery (STIR) image showing increased signal intensity (arrow), confirming the presence of an area of necrotic subchondral bone within dense, sclerotic bone typical of a horse with stress-related subchondral bone damage.

Ultrasonographic Examination

Ultrasonographic examination of the MTP joint region is indicated for suspensory branch desmitis (see Chapter 72). The abaxial aspect of the PSBs should be examined carefully to identify tearing and small avulsion fractures. Proliferative synovitis (villonodular synovitis) is unusual in the MTP joint. The DDFT should be evaluated carefully if tenosynovitis of the DFTS is present¹⁴ (see Chapter 74). Ultrasonographic examination is useful in horses with intersesamoidean and collateral ligament injuries or those with wounds and draining tracts to look for foreign material or communication with the articular surface. The distal sesamoidean ligaments should also be examined carefully, because intraarticular analgesia of the MTP joint has the potential to resolve lameness associated with injury to these ligaments, which can be present with no localizing clinical signs.

Computed Tomography

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