Injuries and abnormalities involving the soft tissues and bones in the tarsal region of horses have been well described.1–7 Many of these conditions manifest primarily as lameness, but some may also result in joint effusion or diffuse swelling. Owing to the complex anatomy of this region, diagnosing abnormalities of the tarsal region in horses is challenging and can require a combination of imaging modalities, including radiography, ultrasonography, nuclear scintigraphy, CT, and MRI. Factors that must be considered when choosing the best diagnostic modality include expense, availability, potential risk factors (eg, the need for general anesthesia), and veterinarian expertise. To facilitate the diagnosis of problems involving the tarsal region in horses, previous studies4,8–23 have described the gross, ultrasonographic, CT, and MRI anatomy of this region. Surprisingly, however, especially given the widespread use of radiography by general and specialist equine practitioners, no studies to our knowledge have described the radiographically identifiable locations of the soft tissue attachments of the tarsal region in horses or the radiographic projections that can be used to visualize those locations.
Previous studies24–26 have found little association between the appearance of radiographic changes involving the distal tarsal joints in horses and the severity of lameness, suggesting that in some horses with pain localized to the tarsal region but no radiographically apparent bony changes, unrecognized soft tissue injuries may be present. Traditionally, information relating to soft tissue injuries that can be gained from radiographs has been limited to the localization of swellings, which are often grossly visible, and the presence of dystrophic mineralization or enthesophytes associated with chronic injuries.
The specific locations of soft tissue attachments have been described previously for the stifle and proximal interphalangeal joints in horses27–29 but not for the tarsal joint. Information on radiographic localization of the soft tissue attachments of the tarsal region could potentially aid practitioners in the identification of soft tissue injuries. In addition, recognition of enthesopathies alone or in conjunction with bony changes may indicate the need for additional imaging, such as nonstandard radiographic views, ultrasonography, CT, or MRI, to provide a diagnosis.
The objectives of the study reported here were to identify the radiographic locations of the soft tissue attachments in the tarsal region of horses and to describe any variability in the gross anatomy of those soft tissue attachments. We hypothesized that individual soft tissue attachments would be in distinct radiographically identifiable locations and would be best visualized on certain radiographic views. We also hypothesized that tarsal region soft tissue attachments would have minimal to no variability in gross anatomy between specimens.
Materials and Methods
Specimens and structures examined
Fifteen cadaveric tarsi harvested from 8 adult horses were used in the study. All horses had been euthanized for reasons unrelated to lameness and had no known tarsal region abnormalities. Owners of all horses provided informed consent for their inclusion in the study, and the study protocol was approved by the Michigan State University Institutional Animal Care and Use Committee.
For euthanasia, horses were sedated with xylazine (0.8 to 1.0 mg/kg, IV) and then given pentobarbital (90 to 95 mg/kg, IV) via a jugular vein catheter. The limbs were trimmed at the mid-tibia and stored at 4.4°C until dissected; all dissections were performed within 48 hours after euthanasia.
Eight limbs were used for dissection and radiography of soft tissue structures. The remaining 7 limbs were used only to evaluate anatomic variations in the insertion of the tendon of the fibularis tertius muscle. Soft tissue attachment sites were identified grossly for the following structures: long and short (superficial, middle, and deep parts) medial collateral ligaments; long and short (superficial, middle, and deep parts) lateral collateral ligaments; tendons of the gastrocnemius muscle, superficial digital flexor muscle, cranial tibial muscle, and fibularis tertius muscle; long plantar ligament; dorsal tarsal ligament; and origin of the suspensory ligament.
Dissection
Standard textbooks and previous studies7–9,22,30 were used as references to guide dissection of the limbs. The skin was removed from the tarsal region of each limb, and the medial and lateral long collateral ligaments were identified. The long collateral ligaments were transected mid-body and reflected proximally and distally to their attachments. The components (superficial, middle, and deep) of the deeper, short collateral ligament were then identified. Most of the fascia overlying the dorsal aspect of the tarsal region was removed to expose the tendons of the fibularis tertius and cranial tibial muscles. These structures were individually dissected and reflected distally to their insertions. The dorsal portion of the joint capsule of the tarsal joint was then removed to gain intra-articular access to identify the origin of the dorsal tarsal ligament on the distomedial aspect of the talus. The dorsal tarsal ligament was then followed distally outside the joint to identify its insertion. On the plantar aspect, the tendons of the superficial digital flexor and gastrocnemius muscles and the long plantar ligament were identified and reflected to their attachment points. To identify the origin of the suspensory ligament, the tendons of the superficial and deep digital flexor muscles were transected at the mid-metatarsal region and reflected proximally to expose the proximoplantar aspect of the metatarsus. The tarsal canal was exposed by transecting the tarsal flexor retinaculum, which allowed further proximal retraction of the tendon of the deep digital flexor muscle to expose all aspects of the suspensory ligament origin, including the accessory ligament of the suspensory ligament. Primary attachment sites of the soft tissue structures and unique features of the attachments found during dissection were recorded.
Radiography
Radiographs were obtained as each structure was identified during dissection of the limbs, and the primary attachment sites (origin, insertion, or both) for each structure were individually localized on radiographs. To do this, 4-mm-diameter metallic beads were placed at each attachment site and held in position with radiolucent tape or glue. A series of standard radiographic views (DPl, LM, D35Pr45L-PlDiMO, and D45Pr55M-PlDiLO) were obtained for each structure, and additional nonstandard views (D10Pr15M-PlDiLO, D35Pr45L-PlDiMO [flexed 48°], and Pl70Pr-PlDiO [flexed]) were obtained as needed to avoid structure overlap and improve identification. For the D35Pr45L-PlDiMO (flexed 48°) view, the tarsal joint was placed in 48° of flexion while a radiograph was obtained as otherwise described for a standard D35Pr45L-PlDiMO view. The Pl70Pr-PlDiO (flexed) view included the distal aspect of the tibia and the talus, unlike the standard DPl (flexed) view, which generally only included the tuber calcanei with the x-ray beam perpendicular, or nearly perpendicular, to the receiver plate. By including the distal aspect of the tibia and all of the talus in the Pl70Pr-PlDiO (flexed) view, the origin and insertion of the short collateral ligaments were identified. All radiographs were acquired with a portable radiographic unit,a wireless plate,b and generator.c Settings were 70 to 71 kV and 1.6 to 2.0 mAs, depending on the size of the limb. To obtain the oblique views, radiolucent blocks were used to position the limb in the correct degree of obliquity as indicated with a laser goniometer.d
Radiographs were individually evaluated by each of the authors (a large animal surgery resident, a diplomate of the American College of Veterinary Surgeons [Large Animal], and a diplomate of the American College of Veterinary Surgeons [Large Animal] and the American College of Veterinary Sports Medicine and Rehabilitation [Equine]), who each identified the radiographic view on which each soft tissue attachment was best visualized. An intraclass correlation coefficient for the degree of agreement was determined with a 2-way random effects model and standard software.e The authors then discussed any disagreements related to preferred radiographic views, and a consensus list of preferred views that optimized evaluation of each soft tissue attachment was created. Attachment sites for structures that indirectly attached to bone through intervening fascia or that attached to other soft tissue structures rather than bone were excluded, with the exception of the branching sites of the tendons of the fibularis tertius and cranial tibial muscles on the dorsal aspect of the tarsal region. Attachment sites for each structure were then translated onto a composite radiographic image.
Results
Collateral ligaments
The origins and insertions of the long and short (superficial, middle, and deep parts) medial and lateral collateral ligaments were identified. The long lateral collateral ligament originated caudal to the groove for the tendon of the lateral digital extensor muscle on the lateral malleolus of the tibia, with fibers extending proximally on the lateral border of the distal aspect of the tibia. The main insertion of the long lateral collateral ligament was on the distolateral surface of the calcaneus with additional fibers extending to the plantarolateral aspect of the talus, lateral surface of the fourth tarsal bone, and lateral surfaces of the third and fourth metatarsal bones. Optimal radiographic views for evaluation of this structure included the standard DPl and D35Pr45L-PlDiMO views and the nonstandard D10Pr15M-PlDiLO view (Figures 1 and 2; Appendix). In addition, the D35Pr45L-PlDiMO (flexed 48°) view eliminated the superimposition of the calcaneus on the origin of the long lateral collateral ligament at the distolateral aspect of the tibia. The long lateral collateral ligament could also be visualized on the nonstandard Pl70Pr-PlDiO (flexed) view.
Standard DPI (A and B) and LM (C and D) radiographic views and a nonstandard PI70Pr-PIDiO (flexed; E) radiographic view illustrating locations of soft tissue attachments in the tarsal region of horses, along with a color legend (F) of the structures evaluated. Illustrated locations represent composite findings based on dissection and radiographic imaging of 8 hind limbs from healthy adult horses.
Citation: American Journal of Veterinary Research 81, 5; 10.2460/ajvr.81.5.406
Standard D45Pr55M-PlDiLO (A and C) and D35Pr45L-PlDiMO (B and D) radiographic views and nonstandard DI0Prl5M-PlDiLO (E) and D35Pr45L-PlDiMO (flexed 48°; F) radiographic views illustrating locations of soft tissue attachments in the tarsal region of horses. Illustrated locations represent composite findings based on dissection and radiographic imaging of 8 hind limbs from healthy adult horses. The D10Pr15M-PlDiLO view was the preferred view for evaluating the origins and insertions of the collateral ligaments, except for the deep part of the short medial collateral ligament. The D35Pr45L-PlDiMO (flexed 48°) view was the preferred view for evaluating the origin of the long lateral collateral ligament and avoiding overlap with the calcaneus. See Figure 1 for color key.
Citation: American Journal of Veterinary Research 81, 5; 10.2460/ajvr.81.5.406
The 3 parts of the short lateral collateral ligament originated on the lateral malleolus cranial to the origin of the long lateral collateral ligament. The superficial part of the short lateral collateral ligament originated cranial to the malleolar groove and inserted on the lateral aspect of the talus and calcaneus, over the middle aspect of the talocalcaneal joint, as it ran in a plantarodistal direction just distal to the coracoid process of the calcaneus. The middle part of the short lateral collateral ligament originated deep to the superficial part of the short lateral collateral ligament and inserted dorsal to the superficial part on the lateral surface of the talus. The deep part of the short lateral collateral ligament originated deep to the middle part of the short lateral collateral ligament and inserted on the lateral surface of the talus, dorsal and proximal to the insertion of the middle part of the short lateral collateral ligament. The deep part of the short lateral collateral ligament was the flattest of the 3 short lateral collateral ligament parts, whereas the superficial and middle parts were about the same width as each other and were clearly defined. When the tarsus was in full extension, the 3 parts of the short lateral collateral ligament were almost perpendicular to the long lateral collateral ligament. Optimal radiographic views for evaluating the short lateral collateral ligament included the standard DPl and D45Pr55M-PlDiLO views and the nonstandard D10Pr15M-PlDiLO and Pl70Pr-PlDiO (flexed) views (Figures 1 and 2).
In all limbs, the long medial collateral ligament was less well defined, compared with the thicker long lateral collateral ligament. The long medial collateral ligament originated from the medial malleolus and distomedial aspect of the tibia. A pronounced fascial layer blended with the margins of the long medial collateral ligament making it less distinct across the medial aspect of the tibiotarsal joint. A more superficial portion of the long medial collateral ligament inserted on the fused first and second tarsal bones and the proximomedial aspects of the second and third metatarsal bones in conjunction with the fascial layers. A deeper, more defined portion inserted primarily on the distal tuberosity of the talus and adjacent areas distally, including the medial aspect of the central and third tarsal bones and the third metatarsal bone. Optimal radiographic views included the standard DPl view and nonstandard D10Pr15M-PlDiLO view (Figures 1 and 2).
The superficial part of the short medial collateral ligament originated on the cranial surface of the medial malleolus and inserted on the proximal and distal aspects of the tuberosity of the talus. The middle part of the short medial collateral ligament originated just distal to the superficial part of the short medial collateral ligament and inserted on the distomedial aspect of the calcaneus. The deep part of the short medial collateral ligament originated deep to the middle part of the short medial collateral ligament and inserted just distal to the tuberosity of the talus. The superficial part of the short medial collateral ligament was the flattest and the middle part was the roundest and best defined of the 3 parts. The standard DPl and D35Pr45L-PlDiMO views and the nonstandard D10Pr15M-PlDiLO and Pl70Pr-PlDiO (flexed) views optimized evaluation of these structures (Figures 1 and 2). One notable exception was that the D10Pr15M-PlDiLO view did not improve identification of the attachments of the deep part of the short medial collateral ligament. The standard D45Pr55M-PlDiLO view offered better identification of the insertion of this structure with less overlap; however, evaluation of the origin was suboptimal with this view also.
Tendon of the gastrocnemius muscle
In all 8 limbs, the tendon of the gastrocnemius muscle inserted on the proximoplantar aspect of the tuber calcanei after rotating laterally around the tendon of the superficial digital flexor muscle. The standard LM, D35Pr45L-PlDiMO, and D45Pr55M-PlDiLO views and nonstandard Pl70Pr-PlDiO (flexed) view were the most useful for evaluating the insertion of the tendon of the gastrocnemius muscle (Figures 1 and 2).
Tendon of the superficial digital flexor muscle
The tendon of the superficial digital flexor muscle spiraled medially around the tendon of the gastrocnemius muscle and became flat as it coursed distally over the tuber calcanei. It was secured in this region by retinacular attachments on the lateral and medial aspects of the proximal portion of the tuber calcanei. The standard DPl and D35Pr45L-PlDiMO views and the nonstandard Pl70Pr-PlDiO (flexed) view were the most useful for evaluating the retinacular attachments of the tendon of the superficial digital flexor muscle (Figures 1 and 2).
Long plantar ligament
In all 8 limbs, the long plantar ligament originated on the plantar surface of the proximal to mid-body portion of the tuber calcanei and coursed distolaterally to insert on the base of the fourth metatarsal bone, with no direct attachment to the fourth tarsal bone. Indirect attachments of the long plantar ligament to the fourth tarsal bone were through its relationship with the intervening fascia of the long lateral collateral ligament. Fibers distal to the base of the fourth metatarsal bone merged with the deep fascia and periosteum in the region of the proximal portion of the fourth metatarsal bone. The preferred radiographic views for evaluating the attachments of this structure were the standard LM and D35Pr45L-PlDiMO views (Figures 1 and 2).
Tendon of the cranial tibial muscle
On the dorsoproximal aspect of the tarsal region, the tendon of the fibularis tertius muscle bifurcated to allow the tendon of the cranial tibial muscle to migrate more dorsally. The tendon of the cranial tibial muscle also bifurcated as it coursed dorsal to the tendon of the fibularis tertius muscle, forming dorsal and medial branches. The dorsal branch coursed distally across the tarsal region to insert on the dorsoproximal aspect of the third metatarsal bone. The medial branch, also known as the cunean tendon, coursed distomedially to insert on the fused first and second tarsal bones. A combination of the 4 standard radiographic views was necessary for complete evaluation of the insertion of the tendon of the cranial tibial muscle (Figures 1 and 2).
Tendon of the fibularis tertius muscle
The location of the dorsal and dorsomedial attachments of the tendon of the fibularis tertius muscle varied in breadth, as did the number of branches to the central and third tarsal bones. In 4 of the 8 limbs that were dissected and radiographed, the dorsal attachment extended in a continuous, fan-like fashion from the dorsomedial aspect of the central and third tarsal bones and continuing to the dorsolateral aspect of the third metatarsal bone (Figure 3) In the other 4 limbs, the tendon had a finger-like configuration forming dorsomedial, dorsal, dorsolateral, and 2 lateral (superficial and deep) branches. Of the additional 7 limbs that underwent dissection alone, 2 had a tendon of the fibularis tertius muscle with simpler dorsal and lateral insertions, 1 had the finger-like configuration, and 4 had the fan-like configuration. All limbs had a small branch from the lateral margin of the tendon of the fibularis tertius muscle that was continuous with the dorsal branch of the tendon of the cranial tibial muscle near the bifurcation of the tendon of the fibularis tertius muscle on the dorsoproximal aspect of the tarsal region. In 3 limbs, a small branch of the tendon of the fibularis tertius muscle also inserted directly on the distodorsal aspect of the third tarsal bone and dorsoproximal aspect of the third metatarsal bone, independent of the primary attachments. The 2 lateral (superficial and deep) branches of the tendon of the fibularis tertius muscle attached on the distolateral aspect of the calcaneus and proximal aspect of the fourth tarsal bone in all limbs. The middle tarsal retinaculum of the tendon of the long digital extensor muscle merged with the superficial lateral branch of the tendon of the fibularis tertius muscle in all limbs. The tendon of the fibularis tertius muscle, over the dorsal aspect of the tarsal region, was always in close contact with the tarsal joint capsule beneath it and with the branches of the tendon of the cranial tibial muscle dorsal to it. Careful dissection was required to distinguish between these structures. A combination of the 4 standard radiographic views was necessary for complete evaluation of the insertion of the tendon of the fibularis tertius muscle (Figures 1 and 2). The radiographic metallic markers were noted to be in line and closer to each other on the limbs with the fan-like configuration and more separate in the limbs with the finger-like configuration.
Photographs of anatomic specimens illustrating the insertion of the tendon of the fibularis tertius muscle in the tarsal region of 15 cadaveric equine limbs. A—In 8 limbs, the tendinous insertion had a fan-like configuration. The tendon of the long digital extensor muscle (star) has been reflected. B—In 5 limbs, the tendinous insertion had a finger-like configuration. The medial branch of the tendon of the cranial tibial muscle (cunean tendon; red arrow) and the dorsal branch of the tendon of the cranial tibial muscle (black arrow) have been reflected distally. C—In 2 limbs, the tendinous insertion consisted of lateral and dorsal branches. The medial branch of the tendon of the cranial tibial muscle (red arrow) can be seen; the dorsal branch of the tendon of the cranial tibial muscle has been removed.
Citation: American Journal of Veterinary Research 81, 5; 10.2460/ajvr.81.5.406
Dorsal tarsal ligament
The dorsal tarsal ligament consistently originated on the dorsal surface of the distal tuberosity of the talus and extended distally to insert in a fan-like fashion on the dorsal aspect of the central and third tarsal bones and dorsoproximal aspect of the third metatarsal bone (Figure 4) Osteophytosis and ankylosis consistent with severe osteoarthritic changes were noted at the centrodistal tarsal joint in 4 of the 8 limbs that underwent dissection and radiography. In the specimens with osteoarthritis, the dorsal tarsal ligament was tightly adhered across the dorsomedial aspect of the centrodistal tarsal joint, making it impossible to differentiate an obvious joint capsule and joint space. In the limbs without gross or radiographic changes consistent with osteoarthritis, the dorsal tarsal ligament and joint capsule of the centrodistal tarsal joint could be easily dissected and elevated to reveal a normal joint space. A combination of the 4 standard radiographic views was necessary for complete evaluation of the dorsal tarsal ligament attachments (Figures 1 and 2).
Photograph of an anatomic specimen (A) and a DPl radiographic view (B) illustrating the location of the dorsal tarsal ligament (dashed lines) in the tarsal region of horses. Notice that radiographically, there is narrowing, sclerosis, and lysis of the centrodistal tarsal joint (stars), consistent with osteoarthritis. Compare the gross appearance of the centrodistal tarsal joint (ovals) and overlying dorsal tarsal ligament in limbs without (C) and with (D) osteoarthritis.
Citation: American Journal of Veterinary Research 81, 5; 10.2460/ajvr.81.5.406
Origin of the suspensory ligament
On gross examination, the proximolateral aspect of the origin of the suspensory ligament, at the level of the proximal aspect of the plantar cortex of the third metatarsal bone, appeared continuous with the dorsal wall of the tarsal canal in all 8 limbs evaluated. A ligamentous structure covered by a thin fascia was identified on the dorsal border of the tarsal canal when the flexor tendons were removed. When this fascia was removed, a proximal continuation of the suspensory ligament was exposed (Figure 5) This ligamentous proximal extension had bony attachments at the plantar aspect of the fourth tarsal bone and plantarodistal aspect of the calcaneus, as previously described.31 A combination of the 4 standard radiographic views was necessary for complete evaluation of the origin of the suspensory ligament and its proximal extension (Figures 1 and 2).
Photograph of an anatomic specimen (A) and DPI (B) and LM (C) radiographic views illustrating the origin (red oval) and proximal extension of the origin (yellow oval) of the suspensory ligament in the tarsal region of horses. In the photograph, needles have been inserted to illustrate the location of the second and fourth metacarpal bones, and the flexor tendons have been reflected medially. In the radiographic images, metallic markers were inserted to highlight the origin of the suspensory ligament on the proximoplantar aspect of the third metatarsal bone and the proximal extension of the origin of the suspensory ligament on the plantar aspect of the fourth tarsal bone and calcaneus.
Citation: American Journal of Veterinary Research 81, 5; 10.2460/ajvr.81.5.406
Agreement on preferred radiographic views
The mean intraclass correlation coefficient for absolute agreement on the preferred radiographic views for visualization of the soft tissue attachments was 0.98, indicating excellent agreement. Any structures for which the authors did not agree were discussed, and a consensus on the optimal set of preferred radiographic views was obtained (Appendix).
Discussion
Results of the study reported here supported our hypotheses that attachments of the proposed soft tissue structures would be in distinct radiographically identifiable locations and that visualization of individual soft tissue attachments would be optimized with certain radiographic views, including some nonstandard views. We also found minimal gross anatomic variability in the location of the soft tissue attachments of the tarsal region between specimens, with the exception of the tendon of the fibularis tertius muscle. Additionally, we found that the dorsal tarsal ligament was adhered to the dorsomedial aspect of the centrodistal tarsal joint in limbs with radiographic osteophyte formation and ankylosis consistent with severe osteoarthritis. Information provided by our study should assist in radiographic examination of the soft tissues of the tarsal region in horse, which may guide practitioner recommendations for advanced imaging or treatment if lesions are seen on radiographs.
Nonstandard radiographic views can be helpful to characterize areas of interest that are superimposed on standard views. In previous imaging studies1,3,4,11–14,17,18,20 describing the anatomy of the collateral ligaments of the tarsal joint, only 1 or 2 components were easily identified. Other authors1,4,18,20 have referred to the short collateral ligaments as 1 structure with a single origin and insertion, rather than as 3 individual structures, as previously described grossly.9 Traditionally, the DPl (flexed) radiographic view, commonly known as the calcaneal skyline view, has been used to evaluate the tuber calcanei, sustentaculum tali, and part of the talocalcaneal joint.32 Our findings indicated that this view can also be helpful in evaluating the origins and insertions of the medial and lateral short collateral ligaments when the angle of the x-ray beam is modified to include the distal portion of the tibia and all of the talus. We found that these attachments were best visualized when the x-ray beam was positioned at a 70° angle to the receiving plate as opposed to a 90° angle with the traditional DPl (flexed) view. Advanced imaging modalities have been unable to distinguish among the 3 components of the medial and lateral short collateral ligaments, possibly because of the oblique orientation of these structures. The Pl70Pr-PlDiO (flexed) radiographic view allowed a unique perspective of this region that cannot be reproduced with other advanced imaging modalities.
Two other nonstandard views were useful for assessing the collateral ligament attachments in the present study. The D35Pr45L-PlDiMO (flexed 48°) view eliminated superimposition of the calcaneus over the distoplantarolateral aspect of the tibia, where the lateral long collateral ligament originated. The D10Pr15M-PlDiLO view improved characterization of the lateral collateral ligaments on the dorsolateral aspect and the medial collateral ligaments on the plantaromedial aspect of the tarsal region. Previous radiographic studies27–29 of the soft tissue attachments of the stifle and proximal interphalangeal joints in horses have proven clinically useful as guides for equine practitioners, and a recent study33 demonstrated the usefulness of a novel radiographic view of the stifle joint in the diagnosis of cranial cruciate lesions in horses. Similarly, we believe that findings of the present study, particularly those findings related to these nonstandard views, will prove helpful when characterizing lesions of the collateral ligaments of the tarsal joint.
In the present study, radiographic evaluation of the tendinous insertion of the gastrocnemius muscle and the retinacular attachments of the tendon of the superficial digital flexor muscle on the tuber calcanei showed that these structures are in close approximation to each other but can be evaluated individually on the Pl70Pr-PlDiO (flexed) view. Injury sites of the tendons of the superficial digital flexor and gastrocnemius muscles have been described.10,34–36 An interesting observation was that intrathecal tearing of the calcaneal insertion of the tendon of the superficial digital flexor muscle was accompanied by disruption of the fibrocartilage.35,37 Longer-term follow-up with Pl70Pr-PlDiO (flexed) radiographic views in these horses with damage to the fibrocartilage would be warranted to determine whether bony changes occur.
Branching of the tendon of the fibularis tertius muscle was found in the present study to be more extensive than what has been previously described. Historically, the tendon of the fibularis tertius muscle is described as having simple dorsal and lateral insertions.22,23,30 In the present study, most of the limbs had a fan-like or finger-like configuration of the insertion, with only 2 specimens having the historically described appearance. Radiographic characterization of the insertion of the tendon of the fibularis tertius muscle in the present study confirmed its complex nature. The fibularis tertius muscle plays an important role as part of the reciprocal apparatus to coordinate flexion and extension of the tarsal and stifle joints and in stabilizing the tarsal joint in the stance phase by limiting extension.38,39 Whether this variability in tendon insertion is purely genetic or has environmental or conformational components is currently unknown. Therefore, future studies investigating environmental and conformational abnormalities in relation to the configuration of the tendinous insertion of the fibularis tertius muscle are warranted.
The tendon of the fibularis tertius muscle has historically been described as a flat structure that is split or perforated by the tendon of the cranial tibial muscle.22,23,30 In the present study, however, we found a more complex, tunnel-type bifurcation of the tendon of the fibularis tertius muscle that enclosed the tendon of the cranial tibial muscle for a short distance and was intimately associated with the tarsal joint capsule. Thus, the layering we identified of the dorsal structures over the tibiotarsal joint, from deep to superficial, was tarsal joint capsule, tendon of the fibularis tertius muscle and intervening fascia, tendons of the cranial tibial and long digital extensor muscles laterally with corresponding tarsal retinacula, deep and superficial fascial layers, and skin. Although the tunnel-type bifurcation of the tendon of the fibularis tertius muscle has been described previously,9 the clinical relevance was not appreciated for its potential implications when evaluating this region ultrasonographically and arthroscopically. Ultrasonographically, this complex bifurcating and layering and the close proximity of structures requires careful interpretation when lesions are suspected in this region. Arthroscopically, this layering may limit the ability to identify the tendons of the long digital extensor and cranial tibial muscles from inside the joint, unless substantial damage of the tarsal joint capsule and tendon of the fibularis tertius muscle is present. Further cadaveric studies comparing the arthroscopic and gross appearance could help clarify the clinical relevance.
Published information about the dorsal tarsal ligament is limited to small excerpts in older anatomy textbooks.23,30 In our study, 4 limbs with radiographic changes consistent with osteoarthritis (severe osteophytosis and ankylosis) on the dorsomedial aspect of the centrodistal tarsal joint had tight adherence of the dorsal tarsal ligament in this region. We were unable to ascertain whether these changes were a result of primary osteoarthritis within the centrodistal tarsal joint, enthesophytes involving the dorsal tarsal ligament attachments on the central and third tarsal bones, lesions of the interosseous tarsal ligaments, or some combination of these. However, on the basis of our findings, it could be proposed that the dorsal tarsal ligament may play an important role in the development of osteoarthritis of the centrodistal tarsal joint, especially given that the severity of pain localized to the tarsal region does not always correspond to the severity of bony changes on radiographs.5,16,24,25 Horses with pain causing lameness localized to the distal tarsal joints without radiographic changes could potentially have an injury of the dorsal tarsal ligament or the previously identified interosseous ligament of the tarsus.26 To further elucidate any relationship between osteoarthritis of the centrodistal tarsal joint and the dorsal tarsal ligament, further clinical, histologic, and advanced imaging studies involving a greater number of specimens would be required.
Limitations of the present study were those inherent with the use of cadavers and include the mixed population, limited history, and absence of premortem clinical and imaging examinations.
In conclusion, soft tissue attachments of the tarsal region in horses were identified in distinct radiographic locations, and certain radiographic views were found to be useful in evaluating specific structures, including standard and nonstandard radiographic views. Generally, the soft tissue attachments in this region had minimal anatomic variability, with the exception of the tendon of the fibularis tertius muscle. Findings of the present study provide guidelines for clinicians and practitioners to use when performing radiography of the tarsal region in horses that could alert them to possible soft tissue injuries and indicate the need for further diagnostic testing.
Acknowledgments
This study was funded through Michigan State University's Freeman Endowed Research Funds. The funding source did not have any involvement in the study design, data analysis and interpretation, or writing and publication of the manuscript.
The authors declare that there were no conflicts of interest.
Presented in oral abstract form at the American College of Veterinary Surgeons Summit, Phoenix, October 2018.
ABBREVIATIONS
| D10Pr15M-PlDiLO | Dorsoproximomedial-plantarodistolateral oblique view made at 10° proximal to the supporting surface and 15° medial to the dorsoplantar line |
| D35Pr45L-PlDiMO | Dorsoproximolateral-plantarodistomedial oblique view made at 35° proximal to the supporting surface and 45° lateral to the dorsoplantar line |
| D45Pr55M-PlDiLO | Dorsoproximomedial-plantarodistolateral oblique view made at 45° proximal to the supporting surface and 55° medial to the dorsoplantar line |
| DPI | Dorsoplantar view |
| DPI (flexed) | Dorsoplantar view of the calcaneus made with the tarsal joint flexed |
| LM | Lateromedial view |
| PI70Pr-PlDiO (flexed) | Plantaroproximal-plantarodistal oblique view made at 70° proximal to the receiving plate with the tarsal joint flexed |
Footnotes
Sound Eklin, Carlsbad, Calif.
Canon Inc, Farmington Hills, Mich.
MinXRay Inc, Northbrook, Ill.
Halo Medical Devices, Subiaco, Australia.
ICC calculator, Mangold International GmbH, Arnstorf, Germany.
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Appendix
Preferred radiographic views for visualization of the attachment sites of soft tissue structures of the tarsal region in 8 cadaveric equine limbs.
| Standard radiographic views | Nonstandard radiographic views | ||||||
|---|---|---|---|---|---|---|---|
| Soft tissue structure | DPI | LM | D35Pr45L-PlDiMO | D45Pr55M-PlDiLO | D10Pr15M-PlDiLO | Pl70Pr-PlDiO (flexed) | D35Pr45L-PlDiMO (flexed 48°) |
| Long LCL | X | X | X | X | |||
| Short LCL: superficial part | X | X | X | X | |||
| Short LCL: middle part | X | X | X | X | |||
| Short LCL: deep part | X | X | X | X | |||
| Long MCL | X | X | |||||
| Short MCL: superficial part | X | X | X | X | |||
| Short MCL: middle part | X | X | X | X | |||
| Short MCL: deep part | X | X | X | ||||
| Gastrocnemius tendon | X | X | X | X | |||
| Superficial digital flexor tendon | X | X | X | ||||
| Long plantar ligament | X | X | |||||
| Cranial tibial tendon | X | X | X | X | |||
| Fibularis tertius tendon | X | X | X | X | |||
| Dorsal tarsal ligament | X | X | X | X | |||
| Suspensory ligament origin | X | X | X | X | |||
LCL = Lateral collateral ligament. MCL = Medial collateral ligament.
