• View in gallery

    Schematic transverse image of the canine manus with divisions representing the interdigital web spaces and regions of the digits (dorsal subcutaneous region not depicted). a = Axial region. b = Abaxial region. C = Central interdigital space. d = Dorsal region. L = Lateral interdigital space. M = Medial interdigital space. p = Palmar region.

  • View in gallery

    Transverse CT images of the manus of the cadaver of a healthy dog following injection of contrast medium into the medial interdigital web space, after which the medium extended dorsoproximally into the distal aspect of the dorsal subcutaneous space. A—Contrast medium is evident dorsal to the dorsal sesamoid bone of metacarpal bone III. B—Contrast medium is evident dorsal to the distal portion of metacarpal bone III, at the level of the proximal margin of the proximal sesamoid bones. The linear filling defect between the contrast medium and the dorsal cortex of metacarpal bone III represents the extensor tendons.

  • View in gallery

    Transverse CT images (A–C; from the distal aspect of the manus to the proximal aspect) and transverse anatomic section (D) of the manus of the cadaver of a healthy dog following injection of contrast medium into the lateral interdigital web space. Notice the marked distention of the injected interdigital web relative to the medial web space in panel A. In the CT images, the contrast medium tracks around the digital flexor tendons (arrows; A, B, and C) and digital extensor tendons (arrowhead; B). In panel D, the transverse section of the manus was obtained at approximately the same level as the CT image in panel B. Here, the blue contrast medium surrounds the proximal phalanx of digit V, and the extensor and flexor tendons are not infiltrated by the contrast medium.

  • View in gallery

    Transverse CT images (A–C; from the distal aspect of the manus to the proximal aspect) and transverse anatomic section (D) of the manus of the cadaver of a healthy dog following injection of contrast medium into the central interdigital web space. Notice the marked distention of the injected interdigital web space in panel A. The contrast medium extends around the digital flexor tendons (arrow) in panel B. In panel C, the proximal extent of the contrast medium (arrowhead) is evident just distal to the metacarpophalangeal joints; the proximal extent is more distal than was observed after injection of the medial and lateral web spaces. In panel D, the transverse section of the manus was obtained at approximately the same level as the image in panel B was obtained. The blue contrast medium can be seen extending into the adjacent web space on the left side of the image.

  • View in gallery

    Schematic transverse images of the typical extent of contrast medium (gray-shaded regions) after injection of the medial or lateral interdigital web space of a canine manus, advancing from distal to proximal (A–F). Accumulation of contrast medium tapered abruptly in the transverse planes immediately proximal to the plane depicted in panel F.

  • View in gallery

    Schematic transverse images of the typical extent of contrast medium (gray-shaded regions) after injection of the central interdigital web space of a canine manus, advancing from distal to proximal (A–F). Accumulation of contrast medium tapered abruptly at the level of the metacarpophalangeal joints (E), and usually only a small tubular structure representing a lymphatic vessel could be seen proximal to this point (F). When there was extension of the contrast medium to an adjacent web space, this extension was invariably evident at the levels depicted in panels B and C.

  • View in gallery

    Transverse CT image of the manus from the cadaver of a healthy dog following injection of contrast medium into the central interdigital web space. The contrast medium can be seen extending into the medial interdigital web space (arrow).

  • View in gallery

    Transverse CT image (A) and sagittal reformatted CT image (B) of a manus from a healthy dog following injection of contrast medium into the lateral interdigital web space. A lymphatic vessel is visible in cross section dorsally (arrow) in panel A. In panel B, the site of injection is represented by the asterisk. The linear structure filled with contrast medium (arrowheads) represents a lymphatic vessel. This vessel is distended with contrast medium immediately proximal to the interdigital web space but is poorly filled in the proximal metacarpus. M = Metacarpal bone.

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Modeling of the spread of infection in the interdigital spaces of the manus in limbs from clinically normal dogs

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  • 1 Department of Small Animal Clinical Sciences, Virginia-Maryland Regional College of Veterinary Medicine, Virginia Tech and University of Maryland, Blacksburg, VA 24061.
  • | 2 Department of Small Animal Clinical Sciences, Virginia-Maryland Regional College of Veterinary Medicine, Virginia Tech and University of Maryland, Blacksburg, VA 24061.
  • | 3 Department of Small Animal Clinical Sciences, Virginia-Maryland Regional College of Veterinary Medicine, Virginia Tech and University of Maryland, Blacksburg, VA 24061.
  • | 4 Department of Small Animal Clinical Sciences, Virginia-Maryland Regional College of Veterinary Medicine, Virginia Tech and University of Maryland, Blacksburg, VA 24061.
  • | 5 Department of Biomedical Sciences and Pathobiology, Virginia-Maryland Regional College of Veterinary Medicine, Virginia Tech and University of Maryland, Blacksburg, VA 24061.

Abstract

Objective—To determine whether the pattern of extension of modeled infection from the interdigital web spaces in dogs is predictable and whether the distribution differs among initial injury sites.

Sample Population—Thawed frozen forelimbs from 23 cadavers of previously healthy adult medium- to large-breed dogs.

Procedures—The manus of each forelimb was evaluated by use of computed tomography (CT) before and after injection of radiopaque blue-staining contrast medium into the interdigital web spaces. Two veterinary radiologists reviewed the CT images and recorded the extent of contrast medium from each site. Each manus was dissected or sectioned transversely after imaging, and the extent of contrast medium accumulation was recorded and compared with locations of CT contrast enhancement. The Fisher exact test was performed to determine whether the pattern of contrast medium extension differed by injection site.

Results—Injections made in the interdigital web spaces of the canine manus led to unique and predictable patterns of extension into the surrounding soft tissues. That pattern of extension primarily involved the soft tissues of the digits.

Conclusions and Clinical Relevance—In humans, knowledge of common extension patterns from infected soft tissue spaces is used to predict the spread of disease within the hand and develop surgical plans that will minimize patient illness. Identification of the common sites of disease spread from the interdigital web spaces in dogs may help improve surgical planning and treatment for infection in the manus.

Abstract

Objective—To determine whether the pattern of extension of modeled infection from the interdigital web spaces in dogs is predictable and whether the distribution differs among initial injury sites.

Sample Population—Thawed frozen forelimbs from 23 cadavers of previously healthy adult medium- to large-breed dogs.

Procedures—The manus of each forelimb was evaluated by use of computed tomography (CT) before and after injection of radiopaque blue-staining contrast medium into the interdigital web spaces. Two veterinary radiologists reviewed the CT images and recorded the extent of contrast medium from each site. Each manus was dissected or sectioned transversely after imaging, and the extent of contrast medium accumulation was recorded and compared with locations of CT contrast enhancement. The Fisher exact test was performed to determine whether the pattern of contrast medium extension differed by injection site.

Results—Injections made in the interdigital web spaces of the canine manus led to unique and predictable patterns of extension into the surrounding soft tissues. That pattern of extension primarily involved the soft tissues of the digits.

Conclusions and Clinical Relevance—In humans, knowledge of common extension patterns from infected soft tissue spaces is used to predict the spread of disease within the hand and develop surgical plans that will minimize patient illness. Identification of the common sites of disease spread from the interdigital web spaces in dogs may help improve surgical planning and treatment for infection in the manus.

Abrasions, lacerations, and foreign body penetration are common in the manus of dogs, particularly in working dogs. Reports1,2 suggest that 35% to 52% of search-and-rescue and police dogs have had cuts, abrasions, or punctures during deployment, and 50% to 70% of those injuries were on the distal aspect of the limbs. In those reports, there were > 9 events/1,000 dog deployment hours, which represents the highest system-specific disease incidence rate among deployed dogs.

Even when manus wounds are treated, chronic infection can develop, and the prolonged illness can cause loss of function, requiring rehabilitation. Exploratory surgery and debridement of affected tissues are commonly required in situations of chronic manus infection, but it is often difficult to achieve a cure with a single procedure, and multiple surgeries may be required.3 Surgical exploration generally must be extensive because infection typically extends beyond the site of initial injury. The requirement of a large surgical field and the common need for multiple procedures often result in prolonged patient illness. A method for predicting the likely path of extension of a pathological process from a given injury site could help surgeons more efficiently plan their approaches and therefore minimize damage to unaffected anatomic structures.

Soft tissue spaces and myofascial compartments in the human hand4–7 and foot8–10 are important in confining disease processes such as inflammation, infection, and neoplasia.11–14 Because inflammation generally does not extend beyond the boundaries of such anatomic structures, surgical intervention can be directed at the affected space or compartment, and complete exploration of the hand or foot is usually unnecessary.12,14,15 We recently identified soft tissue spaces and myofascial compartments in the canine manus that are similar to those in humans.16 However, to our knowledge, distribution of infection from the interdigital spaces has not been described.

The purpose of the study reported here was to test the hypothesis that fluid injected into different interdigital spaces as a model of infection will extend into and be confined differently by the surrounding soft tissues. We anticipated that this model would yield a predictable path of disease based on the site of initial injury, with different sites of injury leading to spread of disease into different regions.

Materials and Methods

Sample collection—Forelimbs were removed from 23 medium- to large-breed dogs euthanized for reasons unrelated to forelimb disease and were frozen at −8°C immediately after removal. Limbs were to be excluded when there was evidence of orthopedic or soft tissue abnormalities; however, no such abnormalities were identified. Once frozen, the 46 limbs were separated into 5 batches of 9 or 10 limbs for imaging, and each batch was allowed to thaw in a walk-in cooler for approximately 16 hours before imaging. This thawing time allowed the limbs to be easily manipulated and injected during the imaging phase of the study.

Contrast medium preparation—Contrast medium components were mixed immediately prior to the first injections of each batch of limbs. The contrast medium mixture included 5 mL of iopromidea and 2 mL of indigo blue acrylic inkb diluted in 45 mL of 0.9% NaCl solution. The mixture was shaken approximately every 5 minutes to keep the blue pigment in suspension.

CT scanning and injection protocol—All limbs in each batch were imaged simultaneously with a 16-slice helical CT scanner.c Three forelimbs were positioned on the couch side-by-side, in an extended position, as for a patient positioned head-first in sternal recumbency. Cardboard platforms were used to support 2 additional layers of 3 or 4 limbs on top of the 3 forelimbs placed directly on the couch; the positioning of the limbs in the upper 2 rows was the same as that for the limbs in the lower row. Limb location was determined randomly. Before injection of contrast medium (precontrast), CT scans of the forelimbs were obtained from the distal aspect of the digits through the distal antebrachium with helical acquisitions of 0.5-mm slice thickness by use of a modified carpus protocol (0.5-second rotation, 300 mA, 120 kVp, 0.688 pitch factor, 250-mm field of view, and 512 × 512 matrix, with bone and soft tissue reconstruction algorithms).

Afterward, 1 of the 3 interdigital web spaces in each manus was injected with 3 mL of the contrast medium with a 22-gauge needle. The interdigital web space was defined as the skin and connective tissue between adjacent digits near the metacarpophalangeal joints; the 3 web spaces were referred to as medial (between digits II and III), central (between digits III and IV), and lateral (between digits IV and V). In preliminary tests, the amount of contrast medium administered yielded repeatable marked distention of the interdigital web space and extension of the contrast medium into the surrounding tissues in all specimens. The injection site for each limb was determined by random drawing. One limb in the first scan group (the group of 10 limbs) was not injected; thus, a total of 45 limbs were injected. After injection of the contrast medium, the limbs were returned to their previous positions on the CT couch and CT scans of the limbs (postcontrast) were repeated with the same protocol as was used for the precontrast scan.

CT data recording—Computed tomographic scans of the forelimbs were reviewed by 2 board-certified veterinary radiologists (CPO and JCJ) with the aid of a remote diagnostic workstation and commercially available DICOM viewing software.d Extension of the contrast medium at 8 predetermined CT slice locations was recorded through line drawings, and contrast medium dispersion was also described relative to recognizable anatomic structures such as individual phalanges.

Agreement between CT and anatomic findings—The forelimbs were refrozen after CT scanning. After freezing, 9 limbs were removed from the freezer for transverse sectioning with a band saw. The remaining specimens were later rethawed for conventional dissection. Manus selection for transverse sectioning and dissection was determined by random drawing, and all injection sites were represented by multiple specimens in each of the 2 sectioning methods. Extent of the blue-staining contrast medium was examined for correspondence with imaging findings.

Statistical analysis—Outcome was defined as extension of the contrast medium into any of 5 regions associated with each of the 4 digits (axial digit, abaxial digit, palmar digit, dorsal digit, or dorsal subcutaneous space of the metacarpus) or into any of the 3 web spaces (lateral, central, or medial; Figure 1). Thus, presence or absence of contrast medium within each of the 23 regions was considered for each injection. Subsequently, data regarding presence or absence of contrast medium were summarized in contingency tables of injection site (lateral, medial, or central) by digit region or web space. To test the null hypothesis that there was no association between site of injection and contrast medium migration to any of the digit regions or to any of the interdigital web spaces, the Fisher exact test was used. Statistical significance was set at a value of P ≤ 0.05. All analyses were performed by use of commercially available statistical software.e

Figure 1—
Figure 1—

Schematic transverse image of the canine manus with divisions representing the interdigital web spaces and regions of the digits (dorsal subcutaneous region not depicted). a = Axial region. b = Abaxial region. C = Central interdigital space. d = Dorsal region. L = Lateral interdigital space. M = Medial interdigital space. p = Palmar region.

Citation: American Journal of Veterinary Research 71, 3; 10.2460/ajvr.71.3.268

Results

CT scanning and injection protocol—Use of our CT scanning protocol allowed good visualization of all forelimbs. The contrast medium could be easily differentiated from the surrounding tissues, and the spatial resolution was sufficient to determine the extent of the contrast medium.

Sixteen injections were made into the medial interdigital web space, 15 into the central web space, and 14 into the lateral web space. Marked distention of the injected interdigital spaces resulting in displacement of the associated digits away from the web space was evident for all injections, and increasing resistance to injection was evident during each injection of contrast medium.

CT data recording—No difficulties in identifying the contrast medium were noticed during the recording of data. Skin contamination with contrast medium was identified on 2 limbs, and there was a focus of soft tissue mineralization that was not continuous with the contrast medium in 1 digit; however, neither of these situations hindered interpretation of contrast medium distribution. Gas bubbles were noticed as incidental findings at 2 injection sites.

Agreement between CT and anatomic findings—Computed tomographic findings and anatomic descriptions were nearly identical in all limbs. No discrepancies were evident between CT and anatomic descriptions of the transverse sections. However, in some situations of conventional manus dissection, it was difficult to determine whether the contrast medium extended into an adjacent interdigital web space or remained within the soft tissues of a digit. Evaluation of this contrast-agent distribution was much easier when a manus was viewed transversely (by means of CT or transverse sectioning).

Pattern of contrast medium extension—Repeatable and predictable patterns of contrast medium extension were observed with injection of the interdigital web spaces. In nearly all situations, injection of a web space caused filling of that web space and the adjacent margins of the associated digits (Table 1). For example, injection of the medial interdigital web space caused filling of that web space, the axial soft tissues of digit II, and the abaxial soft tissues of digit III. The contrast medium also commonly extended into the palmar and dorsal soft tissues of those digits. However, in all instances of dorsal extension, there was a filling defect immediately dorsal to the phalanges in the location of the digital extensor tendons, and in all instances of palmar extension, there was a filling defect immediately palmar to the phalanges in the location of the digital flexor tendons. When a soft tissue region was involved for a given digit, the region adjacent to the proximal phalanx was almost invariably affected (98% of instances), the region adjacent to the middle phalanx was relatively commonly affected (71%), and the contrast medium uncommonly extended to the level of the distal phalanx (17%).

Table 1—

Percentage of specimens with extension of contrast medium into the soft tissues of specific regions of the manus (n = 45) of the forelimbs of cadavers of healthy dogs, by injection site and as determined via CT.

Region of manusSite of injection
Medial web spaceCentral web spaceLateral web space
Digit II
   Abaxial19A0A0A
   Palmar75A0B0B
   Dorsal75A0B0B
   Dorsal subcutaneous space6A0A0A
   Axial100A0B0B
Medial web space100A53B0C
Digit III
   Abaxial100A20B0B
   Palmar94A87A0B
   Dorsal38A60A0B
   Dorsal subcutaneous space12A0A0A
   Axial6B87A0B
Central web space19B100A21B
Digit IV
   Axial0B100A7B
   Palmar0B87A93A
   Dorsal0B67A79A
   Dorsal subcutaneous space0B0B29A
   Abaxial0B7B100A
Lateral web space0C47B100A
Digit V
   Axial0B0B93A
   Palmar0B0B71A
   Dorsal0B0B93A
   Dorsal subcutaneous space0A0A0A
   Abaxial0A0A21A

Within each row, values with different superscript letters differ significantly (P ≤ 0.05).

The contrast medium generally remained confined to the soft tissues of the digits, without extension proximal to the level of the metacarpophalangeal joints. Extension of the contrast medium proximally into the most distal portion of the dorsal subcutaneous space, associated with the distal metacarpal bones and dorsal sesamoid bones, occurred in only 7 regions following 6 injections (13% of limbs; Figure 2). The contrast medium within the web space tapered to a point proximally at approximately the level of the third and fourth metacarpophalangeal joints, although the precise proximodistal location varied with each manus (Figures 3–6).

Figure 2—
Figure 2—

Transverse CT images of the manus of the cadaver of a healthy dog following injection of contrast medium into the medial interdigital web space, after which the medium extended dorsoproximally into the distal aspect of the dorsal subcutaneous space. A—Contrast medium is evident dorsal to the dorsal sesamoid bone of metacarpal bone III. B—Contrast medium is evident dorsal to the distal portion of metacarpal bone III, at the level of the proximal margin of the proximal sesamoid bones. The linear filling defect between the contrast medium and the dorsal cortex of metacarpal bone III represents the extensor tendons.

Citation: American Journal of Veterinary Research 71, 3; 10.2460/ajvr.71.3.268

Figure 3—
Figure 3—

Transverse CT images (A–C; from the distal aspect of the manus to the proximal aspect) and transverse anatomic section (D) of the manus of the cadaver of a healthy dog following injection of contrast medium into the lateral interdigital web space. Notice the marked distention of the injected interdigital web relative to the medial web space in panel A. In the CT images, the contrast medium tracks around the digital flexor tendons (arrows; A, B, and C) and digital extensor tendons (arrowhead; B). In panel D, the transverse section of the manus was obtained at approximately the same level as the CT image in panel B. Here, the blue contrast medium surrounds the proximal phalanx of digit V, and the extensor and flexor tendons are not infiltrated by the contrast medium.

Citation: American Journal of Veterinary Research 71, 3; 10.2460/ajvr.71.3.268

Figure 4—
Figure 4—

Transverse CT images (A–C; from the distal aspect of the manus to the proximal aspect) and transverse anatomic section (D) of the manus of the cadaver of a healthy dog following injection of contrast medium into the central interdigital web space. Notice the marked distention of the injected interdigital web space in panel A. The contrast medium extends around the digital flexor tendons (arrow) in panel B. In panel C, the proximal extent of the contrast medium (arrowhead) is evident just distal to the metacarpophalangeal joints; the proximal extent is more distal than was observed after injection of the medial and lateral web spaces. In panel D, the transverse section of the manus was obtained at approximately the same level as the image in panel B was obtained. The blue contrast medium can be seen extending into the adjacent web space on the left side of the image.

Citation: American Journal of Veterinary Research 71, 3; 10.2460/ajvr.71.3.268

Figure 5—
Figure 5—

Schematic transverse images of the typical extent of contrast medium (gray-shaded regions) after injection of the medial or lateral interdigital web space of a canine manus, advancing from distal to proximal (A–F). Accumulation of contrast medium tapered abruptly in the transverse planes immediately proximal to the plane depicted in panel F.

Citation: American Journal of Veterinary Research 71, 3; 10.2460/ajvr.71.3.268

Figure 6—
Figure 6—

Schematic transverse images of the typical extent of contrast medium (gray-shaded regions) after injection of the central interdigital web space of a canine manus, advancing from distal to proximal (A–F). Accumulation of contrast medium tapered abruptly at the level of the metacarpophalangeal joints (E), and usually only a small tubular structure representing a lymphatic vessel could be seen proximal to this point (F). When there was extension of the contrast medium to an adjacent web space, this extension was invariably evident at the levels depicted in panels B and C.

Citation: American Journal of Veterinary Research 71, 3; 10.2460/ajvr.71.3.268

Contrast medium extended to the next adjacent interdigital web space (ie, from a medial or lateral web space injection to the central web space or from a central web space injection to the medial, lateral, or both web spaces) in several limbs (Figure 7; Table 1). However, in these situations, the contrast medium never extended further to the next adjacent digit. Small tubular structures in the dorsal soft tissues of the manus extended from the most proximal aspect of the injected interdigital web space in all specimens (Figure 8), and these structures were identified as lymphatic vessels after dissection. In 7 of the 45 (16%) limbs, the contrast medium was also evident in dorsal lymphatic vessels extending from a web space adjacent to the injected space. Palmar lymphatic vessels containing the contrast medium were identified extending from the injected space in 2 (4%) limbs and from a noninjected space in 2 other limbs.

Figure 7—
Figure 7—

Transverse CT image of the manus from the cadaver of a healthy dog following injection of contrast medium into the central interdigital web space. The contrast medium can be seen extending into the medial interdigital web space (arrow).

Citation: American Journal of Veterinary Research 71, 3; 10.2460/ajvr.71.3.268

Figure 8—
Figure 8—

Transverse CT image (A) and sagittal reformatted CT image (B) of a manus from a healthy dog following injection of contrast medium into the lateral interdigital web space. A lymphatic vessel is visible in cross section dorsally (arrow) in panel A. In panel B, the site of injection is represented by the asterisk. The linear structure filled with contrast medium (arrowheads) represents a lymphatic vessel. This vessel is distended with contrast medium immediately proximal to the interdigital web space but is poorly filled in the proximal metacarpus. M = Metacarpal bone.

Citation: American Journal of Veterinary Research 71, 3; 10.2460/ajvr.71.3.268

Statistical analysis—The distribution of contrast medium differed significantly among injection sites (Table 1). Contrast medium injected into a given web space was significantly (P < 0.003) more likely to be found within that web space and the adjacent margins of the associated digits than with injections at the other 2 interdigital web spaces. Whereas the contrast medium also commonly extended into the dorsal and palmar tissues of the adjacent digits as well, there were no significant (P > 0.28) differences in frequency of extension to these areas when comparing injections made in the 2 web spaces associated with these digits (eg, extension from the medial and central web spaces to the palmar or dorsal aspect of digit III).

Discussion

Results of the study reported here indicated that injection of contrast medium into the interdigital web spaces of the manus in forelimbs obtained from cadavers of healthy dogs resulted in a predictable pattern of extension of the material into the surrounding soft tissues, as was hypothesized. This is important because the interdigital web spaces are among the most common sites for retention of foreign bodies in dogs17; consequently, knowledge of the likely paths of foreign body migration or spread of infection would be beneficial for surgical planning. Although use of imaging techniques would be beneficial for identification of the extent of inflammation or the presence of a foreign body,18,19 this is not always possible, and exploratory surgery based on probable extent of disease may be required.

The findings in the present study suggested that surgical exploration should begin in the affected interdigital web space because the web spaces are able to contain a large amount of material. The proximal extent of the interdigital web space is approximately at the level of the metacarpophalangeal joints. In our model, spread of contrast medium to the immediately adjacent soft tissues of the related digits occurred almost invariably, and these tissues should be considered to be involved as well. Involvement of the dorsal and palmar soft tissues of the adjacent digits was also common, so exploration of these areas may be required in most situations. Whereas there was no infiltration of contrast medium into the digital extensor and flexor tendons, these structures could be secondarily affected by dorsal or palmar inflammation through adhesion formation.

Just as pertinent for surgical planning are those sites that were involved less commonly in our study because failure to recognize spread of disease to such areas would result in continued illness and the requirement for additional surgical intervention. In the present study, there were 21 instances of contrast medium extension to the next interdigital web space in 18 of the 45 (40%) limbs (3 injections of the central web space resulted in extension to both the medial and lateral web spaces). Because the amount of material in many of these instances was small, it could easily be overlooked. Likewise, involvement of the dorsal subcutaneous space adjacent to 7 metacarpal bones occurred after 6 injections (in 13% of limbs). This finding suggested that injury to the distal aspect of a manus could lead to infection in the dorsal aspect of the metacarpus, which is a site that may not commonly be surgically explored in dogs with a more palmar injury.

Statistical differences in the contrast medium extension based on site of injection also provided possible evidence of necessary sites for surgical exploration in an injured manus. For example, if the abaxial tissues of digit III are known to be infected, involvement of the medial interdigital web space is also highly likely. However, contrast medium rarely extended from the central interdigital web space to the abaxial aspect of digit III, so exploration of the central web space in such a situation may not be as critical. Additionally, the differences in contrast medium extension from the 3 interdigital web spaces and the marked distensibility of these spaces indicated that these 3 structures can likely be considered discrete soft tissue spaces, similar to those identified elsewhere.16 Retention of contrast medium within the web spaces was incomplete because the volume of contrast medium injected was deliberately chosen to simulate disease spread.

The anatomy of the interdigital web spaces in the manus of dogs appears to be analogous to the anatomy of the interdigital web space in human hands. In humans, the interdigital webs are recognized as distinct subcutaneous fascial spaces in which pus may collect.12,20,21 Indeed, it has been reported that the interdigital web spaces are one of the most common sites of pus collection, second only to the pulp spaces of the finger tips.21 Human web spaces are not continuous with each other and are separated by the joint capsule and collateral ligaments of the intervening metacarpophalangeal joints as well as by the digital extensor tendons.22 However, similar to the findings in our model in dogs, web space infections in humans may spread distally into the digits, medially or laterally into an adjacent web space, or proximally to the dorsal aspect of the metacarpus.21 A fourth direction of spread from the interdigital web spaces described for the human hand is proximally, along a lumbrical canal into the midpalmar space.21 We did not recognize this last path of spread in our study, possibly because of the differing anatomy between dogs and humans, including the vestigial nature of the lumbrical muscles in dogs and the association of canine lumbrical muscles with the flexor tendons rather than with the midpalmar space as in humans.23

Although our study provided a model for disease spread in the canine manus, we recognize that additional research involving live dogs and actual infection would be required to better assess patterns of disease extension. Soft tissue structures may tend to confine pathological processes in humans, but this is not always the situation. Depending on the chronicity and severity of infection and the anatomy of the initially affected soft tissue space, the inflammatory process may extend to a neighboring space.24,25 It is understood that cadaver models cannot replace clinical experiments and experience, as the conditions of infection and increased pressure in soft tissue spaces and myofascial compartments cannot be faithfully replicated in cadavers.6,9 Because of the actions of enzymes released during infection, it is not possible to fully evaluate the resistance of barriers to the spread of disease in a cadaver model; structures recognized as discrete compartments anatomically may not confine disease, particularly when an infection is severe or chronic.24,25 Additionally, although injection studies5,10 of the distal aspect of extremities in humans have involved specimens that were frozen and thawed, the effects of freezing could alter the strength or elasticity of the tissues. For example, ex vivo studies26,27 of human tendons have revealed that tendons that were frozen and thawed had increased stiffness and decreased load-bearing before failure, compared with fresh tendons. Thus, it is possible that some fascial planes in our specimens failed prematurely because of the effects of freezing, and in vivo spread of infection would not be as dramatic in fresh tissues as that identified in this study. Nonetheless, findings of studies20,28,29 concerning disease spread in hands from human cadavers appear to correspond well with clinical experience, and we believe our results likewise represent a meaningful depiction of disease spread from the interdigital web spaces in dogs.

ABBREVIATIONS

CT

Computed tomography

DICOM

Digital Imaging and Communications in Medicine

a.

Ultravist, Berlex, Montville, NJ.

b.

Super Pigmented Acrylic Ink, Speedball Art Products Co, Statesville, NC.

c.

Aquilion TSX-101A Multislice CT Scanner, Toshiba America Medical Systems, Tustin, Calif.

d.

eFilm Workstation, version 2.1, Merge Healthcare, Milwaukee, Wis.

e.

SAS, version 9.2, SAS Institute Inc, Cary, NC.

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Contributor Notes

Dr. Ober's present address is Veterinary Clinical Sciences Department, College of Veterinary Medicine, University of Minnesota, Saint Paul, MN 55108.

Supported by the American Kennel Club Canine Health Foundation.

This manuscript represents a portion of a dissertation submitted by Dr. Ober to Virginia Polytechnic Institute and State University as partial fulfillment of the requirements for a Doctor of Philosophy degree.

Address correspondence to Dr. Ober (cpober@umn.edu).