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Original Research |
1 Division of Musculoskeletal Radiology, Department of Radiology, University of
California, San Diego School of Medicine, 3350 La Jolla Village Dr., Mail Code
114, San Diego, CA 92161.
2 Department of Radiology, Veterans Affairs San Diego Healthcare System, San
Diego, CA.
Received August 1, 2007;
accepted after revision October 3, 2007.
Address correspondence to H. Torshizy
(htorshiz{at}hotmail.com).
Abstract
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MATERIALS AND METHODS. CT of the metatarsal heads was performed in six human cadaveric feet, and 3D reformatted images were analyzed. Subsequently, five feet were sectioned in planes corresponding to those at imaging for anatomic correlation, and one foot was anatomically dissected for correlation. The normal anatomic osseous contours of the metatarsal heads were analyzed and described.
RESULTS. Normal osseous anatomy of the greater (first) and lesser (second through fifth) metatarsal heads was identified. The normal osseous landmarks that simulate erosive changes were identified. Variations in the normal anatomic osseous contours, including those related to the medial and lateral condyles, were present in all specimens. The lateral condyle of the first metatarsal head was found to be more prominent than the medial condyle. In addition, anatomic variations in the normal osseous concavities of the lateral and medial aspects of each metatarsal head were measured. An intersesamoidal ridge, present on the plantar surface of the first metatarsal head, was identified in all specimens.
CONCLUSION. The normal anatomic contours of the metatarsal head are a potential major source for diagnostic error when viewing sectional CT and MR images in patients with suspected erosive arthritis. These normal variations, although common and varied, produce characteristic findings that can be differentiated from bone erosions.
Keywords: anatomy • ankle • arthritis • foot • metatarsal head • musculoskeletal imaging
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With the advent of novel therapies for arthritis that halt, and possibly reverse, erosive changes (i.e., tumor necrosis factor inhibitors and methotrexate), there has been a need for earlier detection and diagnosis of such erosive changes. This has been accompanied by a shift in paradigm from merely clinical examination and conventional radiography to more cross-sectional imaging methods, principally sonography, CT, and MRI. Although the osseous anatomy of the metatarsal heads is reasonably well recognized on conventional radiographs, the fine orthogonal plane anatomy is exceedingly more complex and less well understood. Furthermore, dedicated cross-sectional imaging of the metatarsophalangeal joint is not performed routinely.
Because of the complex osseous anatomic detail of both the greater and lesser metatarsophalangeal joints, ascertaining the cause of pathologic changes can often prove challenging. This is especially evident when evaluating erosive changes in the metatarsal heads because normal anatomic landmarks and contours often mimic true bone erosions. Differentiating between the two requires an in-depth understanding of the normal osseous anatomy of the metatarsal heads.
The purpose of our study is to characterize the normal osseous landmarks and contours of the metatarsal heads that simulate bone erosions using sectional radiography, high-resolution radiography, and CT with 3D reformatting; and to further correlate findings with those derived from anatomic inspection of the specimens. Emphasis has been placed on cross-sectional anatomic characterization.
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All specimens were harvested from six unembalmed cadavers (three women, three men; age range at death, 50–78 years; mean age at death, 70 years). The specimens were derived from legs cut through the distal portions of the tibia and fibula that were immediately deep frozen at –40°C in an ultra low Forma BioFreezer (Model 8158, Forma Scientific). All speci mens were allowed to thaw for 24 hours at room temperature before routine radiography.
In selected specimens, additional imaging was performed using high-resolution radiography (Hewlett Packard Faxitron X-ray System 43805N; 25 kV, 3/4-minute exposure) and CT (GE Healthcare LightSpeed; 0.6 mm, 120 kV, 190 mA). Three-dimensional images were reconstructed on a GE Healthcare Advantage Workstation (v4.2) using the bone surface contour algorithm.
After imaging, all cadaveric specimens were immediately frozen at –40°C for at least 24 hours. Subsequently, five of six specimens were sectioned transversely into 3-mm-thick slices corresponding to CT planes. The sliced transverse sections were further imaged with high-resolution radiography. The remaining specimen was dissected by an experienced musculoskeletal radiologist. Digital photography of all specimens was also performed.
Normal osseous anatomy of the metatarsal heads and of osseous structures simulating pathologic change were identified and characterized. Findings were then correlated with gross anatomic inspection and digital photography. Measurements of the depths of the central osseous concavities of the medial and lateral aspects of each metatarsal head were performed on CT images by first drawing a line tangential to the osseous surface. This line was then used as a reference, and depths of the concavities were measured using a second line perpendicular to the first. All data were recorded in a systematic fashion for each surface of the metatarsal head—dorsal, articular, plantar, lateral, and medial.
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Anatomy of Greater Metatarsal Head
The dorsal surface of the first metatarsal head is convex in general
contour and lacks osseous depressions or interruptions in convexity at the
distal-most portions abutting the articular surface and joint space (Figs.
1 and
2A,
2B). The proximal-most portion
of the dorsal surface is characterized by an elevated osseous crest and a
ridge that separates a shallow concavity of the proximal phalangeal base from
the shaft (Fig. 2A,
2B). Both of these structures
act as attachment sites for a redundant joint capsule and are best appreciated
on gross specimens and 3D reformatted images.
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The articular surface of the first metatarsal head consists of two smooth cartilaginous covered regions that are convex in contour and continuous with one another: a smaller phalangeal segment and a larger inferior sesamoidal segment. Both surfaces articulate with the corresponding articular surface of the phalanx, albeit the superior phalangeal more so than the inferior sesamoidal. The inferior sesamoidal surface is further bisected by the intersesamoidal ridge, an anteroposteriorly oriented osseous ridge or crista located at the junction of the inner two thirds and the outer one third of the articular surface. The dorsal surface of the first metatarsal head is uniform in osseous structure and lacks normal regions that might simulate erosive changes on imaging.
The plantar surface of the first metatarsal head is grooved medially and laterally to articulate with the medial and lateral sesamoid bones, respectively (Figs. 3A, 3B, 3C and 4). These two concavities are separated by an osseous convexity, or protuberance, that is a continuation of the aforementioned intersesamoidal ridge. Both concavities and the intersesamoidal ridge are best appreciated on gross and consecutive axial CT images (Fig. 3A, 3B, 3C). Similar to the dorsal aspect, the plantar surface of the first metatarsal head also lacks normal osseous findings that simulate erosive changes. However, normal anatomic variations in both the orientation and the prominence of the intersesamoidal ridge have been reported in the literature [11].
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The medial and lateral sides of the first metatarsal head show well-developed bone tubercles, or epicondyles, at the dorsal-most aspect (Figs. 3A, 3B, 3C and 4). These tubercles are seen on all imaging techniques and on gross examination. On consecutive axial sections, the tubercles appear as rounded osseous protuberances that provide a gradual, rather than sharp, progression from the dorsal surface to the medial and lateral sides. The tubercles can be either in direct contact with the articular surface or separated from it by a shallow, conspicuous groove that is properly visualized on gross examination and 3D reformatted images. The lateral or innermost tubercle of the first metatarsal head is consistently more prominent than the medial or outermost one.
Functionally, the medial and lateral tubercles provide attachment for two intracapsular structures: a more dorsal metatarsophalangeal collateral ligament and a more plantar fan-shaped metatarsosesamoid ligament (also known as the suspensory metatarsosesamoid or metatarsoglenosesamoid ligaments) (Fig. 5A, 5B). In their course, both ligaments pass through prominent central osseous concavities present on both the medial and lateral aspects of the first metatarsal head (Figs. 3A, 3B, 3C and 4). These centralized depressions span most of the vertical distance of the first metatarsal head. Their superior-most border is delineated from the medial and lateral tubercles by an intervening acute osseous indentation, whereas their inferior-most border abuts the plantar surface. This is best seen on consecutive axial CT images and 3D reformatted images. The average depth of the medial concavity of the first metatarsal head is 0.88 mm, whereas that of the lateral concavity is 1.36 mm (Table 1). Further deepening of these concavities beyond the reported average values can represent normal anatomic variations that may be mistaken for erosions.
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In contrast to the dorsal and plantar surfaces, the medial and lateral aspects of the first metatarsal head contain several osseous landmarks that simulate abnormal erosive change. One such structure is the aforementioned sharp intervening groove interposed between the dorsal tubercles and the centralized osseous concavities bilaterally (Figs. 3A, 3B, 3C and 4). On axial CT images especially, this indentation can also make the centralized concavities appear deceivingly biconcave at the level at which both sesamoid bones and the intersesamoidal ridge are visualized (Figs. 3A, 3B, 3C and 4).
Similar to the dorsal surface, the progression from the medial and lateral aspects to the plantar surface of the first metatarsal head is smooth and gradual. It is denoted by an osseous prominence bilaterally that resembles the dorsal tubercles (Figs. 3A, 3B, 3C and 4). The vertically oriented metatarsosesamoid ligaments and the tendon of the abductor hallucis approximate the plantar prominences.
Anatomy of Lesser Metatarsal Heads
Similar to the first metatarsal, the dorsal surfaces of the lesser
metatarsals are convex in general contour and lack osseous depressions or
interruptions in convexity distally (Fig.
1). Proximally, the heads consist of an elevated osseous crest and
an osseous ridge that separate the concave base from the shaft of the proximal
phalanx. Functionally, both structures act as attachment sites for a redundant
joint capsule and can be best identified on 3D reformatted images and gross
specimens.
The articulating surface of the lesser metatarsal heads is condylar in shape, extending more on the plantar aspect than on the dorsal. Just as with the first metatarsal, the articular surface of the lesser metatarsal heads is continuous with both the dorsal and plantar surfaces. However, unlike that of the first metatarsal, the articular surface is not bisected by an intersesamoidal-like osseous ridge.
The plantar surface has two marginal articular proximal extensions, a medial and a lateral, separated by an intervening centralized concavity (Fig. 6A, 6B). In general, the lateral extension is more prominent than the medial. The intervening central concavity may be mistaken for an abnormal induced change. These structures can be best seen on 3D CT reconstructed images and on gross specimens (Fig. 6A, 6B).
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As with the first metatarsal head, the progression from the medial and lateral concavities to the plantar surface is smooth and gradual, denoted by osseous convexities that resemble the dorsal tubercles (Figs. 8A, 8B, 8C and 9). The vertically oriented metatarsosesamoid ligaments and the tendon of the abductor hallucis muscle approximate these plantar prominences (Fig. 7).
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The normal osseous anatomy of the metatarsal heads is complex. A variety of osseous protuberances and concavities, and subtle and distinct changes in contour, exist amid a broader spectrum of soft-tissue structures. Because of this intricate anatomy, differentiation between normal anatomic findings and abnormal changes can often prove to be difficult and challenging because normal osseous structures can simulate erosive changes on imaging. Although the radiographic anatomy of the osseous structure of the metatarsal heads is reasonably well recognized, the fine orthogonal plane anatomy is exceedingly more complex; therefore, it is less well understood. To better differentiate normal osseous anatomy from pathologically induced changes, an indepth understanding of the normal osseous contours of the metatarsal heads is necessary.
To our knowledge, ours is the first study to describe those anatomic features of the metatarsal heads that simulate erosive changes using CT with 3D reformatting, sectional radiography, high-resolution radiography, and dissection correlation in cadaveric specimens. Special emphasis has been placed on cross-sectional description and depiction of osseous anatomy pertaining to everyday clinical practice.
Anatomic Considerations
When evaluating osseous structures of both the greater and lesser
metatarsal heads that may mimic erosive changes, it useful to systematically
characterize findings according to location: dorsal surface, plantar surface,
transition from dorsal and plantar aspects to the medial and lateral aspects,
and medial and lateral aspects.
First Metatarsal Head
The dorsal surface of the first metatarsal head is continuously convex in
general contour. It lacks osseous landmarks that simulate erosive changes.
Therefore, inconsistencies and discontinuities in general shape should suggest
pathologically induced changes. The plantar surface of the first metatarsal
head consists of two osseous concavities separated by an intervening osseous
convexity that is an extension of the intersesamoidal ridge. Each osseous
concavity further articulates with a sesamoid bone. The plantar surface
exhibits a great deal of variation in osseous anatomy. Variations in
prominence and orientation of the intersesamoidal ridge, in either a more
medial or a more lateral direction with respect to the long axis of the first
metatarsal, have been reported
[11,
13]. Similar to the dorsal
surface, the osseous anatomy of the plantar surface lacks osseous landmarks
that can mimic erosive changes.
The transition from the dorsal to the medial and lateral aspects of the first metatarsal head is smooth and gradual, corresponding to the contour of the aforementioned bone tubercles or epicondyles. Functionally, these tubercles provide attachment for the metatarsophalangeal collateral ligaments and the metatarsosesamoid ligaments (also known as the suspensory metatarsosesamoid or metatarsoglenosesamoid ligaments). Note that stability of the first metatarsophalangeal joint is provided by the ligamentous and tendinous insertions of the sesamoid apparatus, the plantar metatarsophalangeal ligament, and the flanking collateral ligaments [14].
As mentioned previously, the lateral or innermost tubercle of the first metatarsal head is consistently more prominent than the medial or outermost tubercle. This may in part be a physiologic response to patterns of stress because bone responds to stress by remodeling its structure, including shape, trabe cular distribution, and density distribution, to withstand stress and strain (Wolff's law) [15, 16]. Studies have shown the trabeculae pattern of the lateral aspect of the first metatarsophalangeal to be consistently less well developed than the medial aspect [17]. This finding is consistent with the fact that the metatarsal head articulates only from its lateral aspect.
The transition from plantar to medial and lateral aspects is convex and gradual. The medial and lateral aspects of the first metatarsal head contain several osseous landmarks that may resemble abnormally induced changes. First, the prominent medial and lateral central osseous concavities, which house the metatarsophalangeal collateral ligament and metatarsosesamoid ligament, may themselves mimic erosive changes. Based on measurements from six cadaveric specimens, the average depth of the medial concavity of the first metatarsal is 1.0 mm (range, 0.5–1.6 mm) and that of the lateral aspect is 1.42 mm (range, 1.0–2.1 mm) (Table 1). Further deepening of the central medial and lateral concavities beyond the reported average values may represent normal anatomic variations that may be mistaken for erosive changes.
When evaluating whether further deepening is a manifestation of a disease process versus a normal anatomic variant, it is helpful to consider such deepening in the context of the provided clinical history and in a broader context of other osseous findings that suggest a pathologic cause. Most notably, these findings include erosions or disruption of cortex integrity throughout the head; loss of convexity of the dorsal surface; loss of the convexity and gradual transitioning from dorsal surface to medial and lateral aspects, or from plantar surface to medial and lateral aspects; evidence of osteophytes or other abnormal bone outgrowths; and other osseous abnormalities of the shaft or base of the metatarsals. In addition, it is important to evaluate the contour of the centralized concavities, making certain that a smooth and gradual concavity is maintained. When these findings are found in conjunction with deepening of the centralized concavities, an abnormal cause is more suggestive.
A second osseous structure that may be mistaken for an erosive change on the lateral and medial aspects is the sharp intervening osseous groove interposed between the centralized medial and lateral osseous concavities and the medial and lateral tubercles bilaterally. On consecutive axial sections, it makes the medial and lateral aspects of the first metatarsal head appear biconcave. This is especially evident with respect to the medial aspect of the first metatarsal head, where the groove is most prominent.
Lesser Metatarsal Heads
Similar to the first metatarsal head, the dorsal surface of the lesser
metatarsal heads is convex in general contour and lacks osseous structures
that simulate erosive patterns of change. Again, a discrepancy from this
general shape should suggest an abnormally induced change.
Unlike the first metatarsal head, however, the plantar aspect of the lesser metatarsal head contains a concavity that may be mistaken for a sequela of disease. This concavity, which is best appreciated on axial CT and 3D reformatted images, is centrally located on the plantar aspect between the medial and lateral proximal articular extensions of the plantar surface. The lateral plantar prominence is more prominent than the medial aspect. To our knowledge, this finding has not been explained in the literature, and studies similar to those of the first metatarsal head have not been performed to characterize differences in trabecular pattern between the medial and lateral prominences. Although biomechanical forces may be a contributing factor, great controversy exists in the literature as to the distribution of peak plantar pressure among the metatarsal heads [18–22].
As in the first metatarsal head, the transition from dorsal to medial and lateral aspect is convex and gradual as denoted by the medial and lateral tubercles. These tubercles serve as an attachment site for the metatarsophalangeal ligament and the metatarsoglenoid or suspensory metatarsoglenoid ligament. Unlike the first metatarsal head, no appreciable difference in size was noted between the medial and lateral tubercles.
Just as for the first metatarsal head, the medial and lateral aspects of the lesser metatarsal heads contain the same inherent osseous structures that simulate bone erosions. These include the centralized osseous concavities and the sharp intervening osseous groove interposed between the centralized medial and lateral osseous concavities and the medial and lateral tubercles. Further deepening of the central medial and lateral concavities beyond these reported average values may represent normal anatomic variations that may be mistaken for erosive changes of the medial and lateral aspects. When evaluating whether further deepening of the concavities of the lesser metatarsal heads is a manifestation of disease processes as opposed to normal anatomic variation, the same criteria were taken into consideration as used in evaluating the first metatarsal head. In addition to simulating erosive changes, the intervening osseous groove interposed between the centralized concavities and tubercles makes the medial and lateral aspects appear biconcave, although this is not as pronounced in the lesser heads as in the medial aspect of the first metatarsal head.
Study Limitations
We acknowledge several limitations to this study. Our sample size of
examined cadavers (n = 6) was small. However, findings were
correlated across a spectrum of imaging techniques, including CT with 3D
reformatting, high-resolution radiography, and con ventional radiography.
Furthermore, all findings were ultimately correlated with the gold standard of
gross specimens. In addition, no pathologic sample consisting of cadavers with
osseous manifestations of osteoarthritis, such as osteophytes, was avail able
for comparison.
Conclusion
The osseous anatomy of the metatarsal heads is complex. Differentiating
actual erosive changes from normal osseous structures can be difficult because
normal osseous structures simulate erosive change. To our knowledge, our study
is the first to characterize those anatomic features of the metatarsal heads
that simulate erosive changes. In doing so, findings were consistently
correlated across a diverse spectrum of imaging techniques and with gross
specimens, all while maintaining emphasis on cross-sectional description and
depiction of osseous anatomy as it would apply to clinical practice. Because
MRI is being ordered more routinely to diagnose erosions in the metatarsal
heads early in the course of arthritis, knowledge of the pseudoerosions gained
from CT can be helpful when interpreting MR images and radiographs.
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