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Fractures of Proximal Portion of Fifth Metatarsal Bone: Anatomic and Imaging Evidence of a Pathogenesis of Avulsion of the Plantar Aponeurosis and the Short Peroneal Muscle Tendon¹

¹ From the Department of Radiology, School of Medicine, University of California, San Diego Medical Center, Calif (D.J.T., S.J.T., Y.K., D.R.); Department of Radiology, Veterans Administration San Diego Medical Center, Calif (D.J.T., S.J.T., Y.K., D.R.); and Department of Orthopedic Surgery, Scripps Clinic Medical Group, San Diego, Calif (M.J.B.). From the 2001 RSNA scientific assembly. Received March 18, 2002; revision requested April 30; revision received June 17; accepted July 1. Supported by Veterans Administration grant SA-360 and A. S. Onassis Public Benefit Foundation educational grant U-033. Address correspondence to D.J.T., 13 Papadopoulos St, Ioannina 45444, Greece (e-mail: rjtheodorou@hotmail.com).

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
PURPOSE: To evaluate the normal anatomy of the structures supporting the proximal portion of the fifth metatarsal bone and investigate the pathogenesis of fractures in this region.

MATERIALS AND METHODS: In two cadaveric feet, the region of the lateral component of the plantar aponeurosis (PAL), short peroneal muscle (SPM) tendon, and third peroneal muscle (TPM) tendon was dissected. These two foot specimens and four nondissected foot specimens were studied at magnetic resonance (MR) imaging. Two of the six specimens were studied at computed tomography (CT). Sectioning the nondissected foot specimens enabled anatomic correlation. In two additional specimens, simulation of the presumed mechanism of fifth metatarsal bone fracture was attempted. The radiographic, CT, and MR images obtained in 13 patients with fractures of the proximal portion of the fifth metatarsal bone were evaluated.

RESULTS: Anatomic, CT, and MR imaging studies revealed broad insertion of the PAL into the plantar aspect of the proximal portion of the fifth metatarsal bone in all specimens. The SPM tendon was consistently attached more distally and to the lateral side of the tuberosity, blending with the PAL fibers. The TPM tendon was inconsistently identified inserting anteriorly to the SPM tendon. No fracture was created in the specimens subjected to attempted injury. Frequent attachment of the PAL and the SPM tendon to the avulsed fragment was confirmed in clinical cases.

CONCLUSION: The pathogenesis of fractures of the proximal portion of the fifth metatarsal bone appears to be related to avulsion injury of PAL and SPM tendon fibers.

© RSNA, 2003

Index terms: Foot, anatomy, 465.92 • Foot, CT, 465.12111, 465.12115 • Foot, MR, 465.121411 • Foot, fractures, 465.41

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Fractures of the proximal portion of the fifth metatarsal bone are common foot injuries that cause dorsolateral proximal forefoot pain (1–3). Three fracture types in the proximal portion of the fifth metatarsal bone have been described: Jones fracture, tuberosity avulsion fracture, and diaphyseal stress fracture. Diaphyseal stress fractures can be further classified into three subtypes (3–6) (Fig 1). Because of the inherent poor blood supply of the proximal metadiaphyseal region of the fifth metatarsal bone (7,8), healing complications related to Jones fractures, such as delayed union or nonunion, can occur (3,8–11). These fractures are associated with increased disability and can be difficult to treat (6,9,11,12).

View larger version (25K): [in this window] [in a new window] [Download PPT slide]   Figure 1a. (a) Drawing shows the major structures attaching to the proximal portion of the fifth metatarsal bone (MT5). CU = cuboid bone, PAL = lateral component of the plantar aponeurosis, PB = peroneus brevis tendon (ie, short peroneal muscle [SPM] tendon), PT = peroneus tertius tendon (ie, third peroneal muscle [TPM] tendon). (b) Drawing shows the fracture zones at the proximal portion of the fifth metatarsal bone. 1 = zone of tuberosity avulsion fracture, 2 = zone of Jones fracture, 3 = zone of diaphyseal stress fracture. (Drawings courtesy of S.J.T.)

View larger version (22K): [in this window] [in a new window] [Download PPT slide]   Figure 1b. (a) Drawing shows the major structures attaching to the proximal portion of the fifth metatarsal bone (MT5). CU = cuboid bone, PAL = lateral component of the plantar aponeurosis, PB = peroneus brevis tendon (ie, short peroneal muscle [SPM] tendon), PT = peroneus tertius tendon (ie, third peroneal muscle [TPM] tendon). (b) Drawing shows the fracture zones at the proximal portion of the fifth metatarsal bone. 1 = zone of tuberosity avulsion fracture, 2 = zone of Jones fracture, 3 = zone of diaphyseal stress fracture. (Drawings courtesy of S.J.T.)

The purpose of our study was to evaluate the normal anatomy of the supporting structures of the proximal portion of the fifth metatarsal bone and investigate the pathogenesis of fractures in this region of the forefoot, with emphasis on the PAL and the SPM tendon.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Cadaveric Study
Six fresh feet were harvested from nonembalmed human cadavers (four men, two women; age range at time of death, 71–93 years; mean age, 82 years). The cadaveric foot specimens were obtained according to the policies and procedures of the University of California, San Diego Medical Center and the Veterans Administration San Diego Medical Center. The entire foot along with the region up to the middle portion of the calf was amputated, and the specimens were immediately deep frozen at -40°C (Bio-Freezer; Forma Scientific, Marietta, Ohio). The foot specimens were allowed to thaw for 24 hours at room temperature prior to imaging. All specimens were initially evaluated at radiography (in frontal, lateral, and oblique projections) so that those feet with injuries in the lateral aspect and/or moderate or severe arthritic changes could be excluded.

Magnetic resonance (MR) images of the six cadaveric feet were acquired with a 1.5-T superconducting MR imaging unit (Signa; GE Medical Systems, Milwaukee, Wis) and a 5-inch-diameter (ie, 12.7-cm) surface coil (Flex Coil; Medical Advances, Milwaukee, Wis). The cadaveric feet were placed in the neutral weight-bearing position inside the coil and immobilized with foam pads. The MR imaging protocol consisted of transverse, coronal, and sagittal T1-weighted spin-echo (repetition time msec/echo time msec, 600/22) sequences. For better delineation of the PAL, a special sagittal lateral-oblique imaging plane (determined from transverse images) parallel to the course of the SPM tendon or the abductor muscle of the little toe (19) was used.

The imaging parameters were as follows: 1-mm section thickness with no intersection gap, field of view of 10 x 10 cm in all three standard imaging planes (ie, transverse, coronal, and sagittal) and of 13 x 13 cm in the sagittal lateral-oblique plane, 512 x 256 matrix, acquisition time of 6 minutes 50 seconds, and two signals acquired. The MR images were evaluated by two authors (D.J.T., S.J.T.), who reached a consensus regarding the depiction of the following ligamentous and tendinous structures attaching to the fifth metatarsal bone: PAL, SPM tendon, TPM tendon, long plantar (ie, calcaneocuboid) ligament (LPL), tendon of the abductor muscle of the little toe, and origin of the flexor muscle of the little toe.

Computed tomographic (CT) images were successively obtained in two of the six cadaveric feet, including one foot that was later dissected. Transverse (ie, sections acquired parallel to the plantar surface of the foot) and coronal CT images (120 mAs, 150 kVp, 2-mm section thickness, 1-mm incremental table movement, and field of view of 9 x 9 cm for transverse images and 8 x 8 cm for coronal images) and sagittal CT reconstructions of the proximal portion of the fifth metatarsal bone were obtained with a spiral scanner (PQ-5000; Picker International, Cleveland, Ohio). The authors (D.J.T., S.J.T.) analyzed the CT images, paying particular attention to the structures attaching to the proximal portion of fifth metatarsal bone.

After MR imaging, an orthopedic surgeon (M.J.B.) who specializes in foot and ankle procedures dissected the supporting structures of the proximal portion of the fifth metatarsal bone in two of the six foot specimens to identify the anatomic course and metatarsal insertions of the tendinous and ligamentous structures. A longitudinal incision parallel to the plantar aspect of the foot was made at the lateral aspect of each foot by using a standard number 15 scalpel. The incision enabled access to the structures in the proximal lateral portion of the forefoot, and the orthopedic surgeon paid careful attention to preserve all of the major ligamentous and tendinous attachments adjacent to the base of the fifth metatarsal bone. The subcutaneous tissue and skin were then closed sequentially in layers. We took color photographs at all phases of the dissection to record the anatomic sites and relationships of the structures inserting into the proximal portion of the fifth metatarsal bone. The four nondissected specimens were refrozen for more than 72 hours and subsequently sectioned with a band saw in planes that corresponded closely to the MR imaging planes. Two authors (D.J.T., S.J.T.) compared the imaging and anatomic section findings and reached a consensus.

To simulate the presumed mechanism of fracture of the base of the fifth metatarsal bone, one author (M.J.B.) manually applied forceful plantar flexion of 85° and 45° of inversion to two additional thawed foot specimens. Because the specimens had been amputated at the level of the middle portion of the calf, we sutured the lateral peroneal tendons together with 3.0 vicryl and then stapled them onto the fibula to simulate active tendon eversion and plantar flexion during the application of stress to the foot. A heel clamp was used to stabilize the hindfoot in a neutral simulated weight-bearing position. In these two cadaveric feet, in which we attempted to simulate fracture of the proximal portion of the fifth metatarsal bone, radiographic evaluation was performed before and after the attempted simulated injuries, and CT (two foot specimens) and MR imaging (one foot specimen) examinations were performed after the attempted simulated injuries. The technical parameters used to acquire the cross-sectional images in these two foot specimens were the same as those used to acquire images in the other six cadaveric feet.

Two musculoskeletal radiologists (D.J.T., S.J.T.) reviewed the radiographic, CT, and MR images of these two cadaveric feet by consensus, with a focus on the presence or absence of fracture of the proximal portion of the fifth metatarsal bone. The orthopedic surgeon (M.J.B.) dissected these two cadaveric feet after they had been subjected to forceful plantar flexion to evaluate the integrity of the structures inserting into the proximal portion of the fifth metatarsal bone and to determine the presence or absence of fracture in this region of the foot.

Clinical Study
We conducted a prospective study involving 13 consecutive patients (nine men, four women; age range, 24–65 years; mean age, 48 years) with a clinical history of lateral forefoot injury and imaging evidence of acute fracture of the fifth metatarsal bone. For the purposes of this study, institutional review board approval from the Veterans Administration San Diego Medical Center and informed patient consent were obtained. The imaging evaluation of these patients included radiography of the ankle and foot in 13 patients and detailed MR imaging and CT examinations of the injured foot in seven and 12 patients, respectively. Some patients did not undergo both CT and MR imaging owing to reasons such as claustrophobia and pacemaker implant. The two musculoskeletal radiologists (D.J.T., S.J.T.) analyzed and interpreted the images and reached a consensus regarding findings. The same two radiologists evaluated the clinical history of the patients, paying particular attention to the reported mechanisms of injury.

All patients underwent cross-sectional imaging within 5 days after their injury. Radiographs were evaluated for the presence, location, orientation, fragmentation, and degree of displacement of the fracture. The conspicuity—that is, the visibility of the fracture line—and the radiographic evidence of associated osseous and soft-tissue injuries also were assessed. On cross-sectional images, the anatomic course and integrity of the structures attaching to the proximal portion of the fifth metatarsal bone—the relationship between these structures and the bone fragment, in particular—were evaluated. The presence or absence of marrow edema, joint effusion, and abnormal findings in adjacent bone structures and soft tissues also was evaluated. In most cases, these patient images were acquired with the same technical parameters that were used in the cadaveric studies. In two patients, however, T2-weighted spin-echo (2,000/80) and fast spin-echo (4,000/98) MR images with and without fat suppression were acquired in the transverse, coronal, and sagittal planes.

	RESULTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Cadaveric Study
Anatomic dissection.—The diaphyseal insertion of the TPM tendon into the fifth metatarsal bone was noted in one of the two foot specimens that were dissected. In both feet, the SPM tendon was identified as a strong broad-based structure attached to the dorsolateral surface of the tuberosity of the fifth metatarsal bone (Fig 2). In these two foot specimens, a sturdy, firm band of tissue, the PAL, extended from the calcaneus to the plantar surface of the tuberosity of the fifth metatarsal bone (Fig 2a). In both specimens, we observed fibers of the PAL blending imperceptibly with fibers of the SPM tendon and attaching to the tuberosity as a broad and strong structure (Fig 2b). In both specimens, we also observed a strong and broad slip from the LPL attaching to the plantar surface of the metatarsal base medially.

View larger version (111K): [in this window] [in a new window] [Download PPT slide]   Figure 2a. Pathologic specimens from same cadaver show normal anatomy of the lateral supporting structures of the proximal portion of the fifth metatarsal bone (MT5) from the lateral aspect of the foot. PB = peroneus brevis tendon (ie, SPM tendon). (a) Dissection of cadaveric specimen reveals the major supporting structures attaching to the base of the fifth metatarsal bone. Surgical scissors are inserted under the SPM tendon, close to the insertion of this tendon into the metatarsal bone. The entire course of the PAL and the broad site of attachment of this structure to the base of the fifth metatarsal bone are seen. (b) Gross cadaveric specimen demonstrates the bare proximal portion of the fifth metatarsal bone. Surgical clamps tighten the broad fibrous band (b) formed by fibers converging from the PAL and the SPM tendon. (c) Dissection of cadaveric specimen reveals the tendinous and ligamentous structures inserting into the proximal portion of the fifth metatarsal bone. The dark area marked by the arrow indicates tuberosity, and the thick black line around the area is the site of attachment of the PAL. The anterior frenular ligament (f) extending from the long peroneal muscle tendon (PL) to the base of the fifth metatarsal bone is seen between hemostats. The SPM tendon has been reflected over the long peroneal muscle tendon.

View larger version (111K): [in this window] [in a new window] [Download PPT slide]   Figure 2b. Pathologic specimens from same cadaver show normal anatomy of the lateral supporting structures of the proximal portion of the fifth metatarsal bone (MT5) from the lateral aspect of the foot. PB = peroneus brevis tendon (ie, SPM tendon). (a) Dissection of cadaveric specimen reveals the major supporting structures attaching to the base of the fifth metatarsal bone. Surgical scissors are inserted under the SPM tendon, close to the insertion of this tendon into the metatarsal bone. The entire course of the PAL and the broad site of attachment of this structure to the base of the fifth metatarsal bone are seen. (b) Gross cadaveric specimen demonstrates the bare proximal portion of the fifth metatarsal bone. Surgical clamps tighten the broad fibrous band (b) formed by fibers converging from the PAL and the SPM tendon. (c) Dissection of cadaveric specimen reveals the tendinous and ligamentous structures inserting into the proximal portion of the fifth metatarsal bone. The dark area marked by the arrow indicates tuberosity, and the thick black line around the area is the site of attachment of the PAL. The anterior frenular ligament (f) extending from the long peroneal muscle tendon (PL) to the base of the fifth metatarsal bone is seen between hemostats. The SPM tendon has been reflected over the long peroneal muscle tendon.

View larger version (100K): [in this window] [in a new window] [Download PPT slide]   Figure 2c. Pathologic specimens from same cadaver show normal anatomy of the lateral supporting structures of the proximal portion of the fifth metatarsal bone (MT5) from the lateral aspect of the foot. PB = peroneus brevis tendon (ie, SPM tendon). (a) Dissection of cadaveric specimen reveals the major supporting structures attaching to the base of the fifth metatarsal bone. Surgical scissors are inserted under the SPM tendon, close to the insertion of this tendon into the metatarsal bone. The entire course of the PAL and the broad site of attachment of this structure to the base of the fifth metatarsal bone are seen. (b) Gross cadaveric specimen demonstrates the bare proximal portion of the fifth metatarsal bone. Surgical clamps tighten the broad fibrous band (b) formed by fibers converging from the PAL and the SPM tendon. (c) Dissection of cadaveric specimen reveals the tendinous and ligamentous structures inserting into the proximal portion of the fifth metatarsal bone. The dark area marked by the arrow indicates tuberosity, and the thick black line around the area is the site of attachment of the PAL. The anterior frenular ligament (f) extending from the long peroneal muscle tendon (PL) to the base of the fifth metatarsal bone is seen between hemostats. The SPM tendon has been reflected over the long peroneal muscle tendon.

Anatomic sectioning.—At macroscopic inspection of the coronal, transverse, sagittal, and sagittal lateral-oblique anatomic sections, the course of the major tendinous and ligamentous structures to the fifth metatarsal attachment sites was visualized and compared with the findings on the corresponding MR images. The TPM tendon was identified in two of the four foot specimens as an oblique structure inserting into the superior surface of the fifth metatarsal bone, anterior to the SPM tendon. In all four specimens, the SPM tendon was identified as a thick band of tissue coursing obliquely and becoming broader just before its insertion into the styloid apophysis of the fifth metatarsal bone (Figs 3, 4). In these four feet, the PAL was a firm, flat band of tissue with broad attachment to the inferior surface of the proximal portion of the fifth metatarsal bone as far posteriorly as the tip of the metatarsal base (Figs 3, 4).

View larger version (130K): [in this window] [in a new window] [Download PPT slide]   Figure 3a. Supporting structures of the proximal portion of the fifth metatarsal bone (MT5) in a cadaveric foot, with MR imaging-anatomic correlation. (a) Coronal T1-weighted MR image (600/22) obtained at the level of the base of the fifth metatarsal bone shows the SPM tendon (curved arrow) inserting into the dorsal surface of the base of the fifth metatarsal bone and the PAL (open arrow) inserting into the plantar surface of the bone. (b) Coronal anatomic slice approximately 3 mm posterior to the imaging section in a shows the SPM tendon (curved arrow) and the PAL (open arrow) inserting into the proximal portion of the fifth metatarsal bone.

View larger version (158K): [in this window] [in a new window] [Download PPT slide]   Figure 3b. Supporting structures of the proximal portion of the fifth metatarsal bone (MT5) in a cadaveric foot, with MR imaging-anatomic correlation. (a) Coronal T1-weighted MR image (600/22) obtained at the level of the base of the fifth metatarsal bone shows the SPM tendon (curved arrow) inserting into the dorsal surface of the base of the fifth metatarsal bone and the PAL (open arrow) inserting into the plantar surface of the bone. (b) Coronal anatomic slice approximately 3 mm posterior to the imaging section in a shows the SPM tendon (curved arrow) and the PAL (open arrow) inserting into the proximal portion of the fifth metatarsal bone.

View larger version (127K): [in this window] [in a new window] [Download PPT slide]   Figure 4a. Supporting structures of the proximal portion of the fifth metatarsal bone (MT5) in a cadaveric foot, with MR imaging-anatomic correlation. LP = LPL, PB = peroneus brevis tendon (ie, SPM tendon). (a) Sagittal T1-weighted MR image (600/22) of the lateral aspect of the foot shows the sites of insertion of the SPM tendon, PAL, and LPL into the proximal portion of the fifth metatarsal bone. (b) Sagittal anatomic slice shows the SPM tendon, PAL, and LPL inserting into the proximal portion of the fifth metatarsal bone. The tendon of the abductor muscle of the little toe (arrow) is seen coursing distally between the SPM tendon and the PAL.

View larger version (120K): [in this window] [in a new window] [Download PPT slide]   Figure 4b. Supporting structures of the proximal portion of the fifth metatarsal bone (MT5) in a cadaveric foot, with MR imaging-anatomic correlation. LP = LPL, PB = peroneus brevis tendon (ie, SPM tendon). (a) Sagittal T1-weighted MR image (600/22) of the lateral aspect of the foot shows the sites of insertion of the SPM tendon, PAL, and LPL into the proximal portion of the fifth metatarsal bone. (b) Sagittal anatomic slice shows the SPM tendon, PAL, and LPL inserting into the proximal portion of the fifth metatarsal bone. The tendon of the abductor muscle of the little toe (arrow) is seen coursing distally between the SPM tendon and the PAL.

MR Imaging
The normal anatomy and anatomic variations of most of the stabilizing structures of the proximal portion of the fifth metatarsal bone were well delineated in the coronal, sagittal, sagittal lateral-oblique, and transverse planes on the available MR images of seven cadaveric feet, including one in which we attempted to create a fracture (Figs 3a, 4a). All tendinous and ligamentous structures were visualized as homogeneous bands of low signal intensity surrounded by areas of high signal intensity, which correlated with fatty tissue on the macroscopic sections.

The TPM tendon was visualized in two of the seven foot specimens at MR imaging. The SPM tendon and PAL were identified in all seven specimens and had an MR imaging appearance that corresponded to their appearance at anatomic sectioning and dissection. In all specimens, the broad insertion of the SPM tendon was delineated in the sagittal, sagittal lateral-oblique, and transverse planes on MR images (Fig 5a). Owing to an oblique orientation, the PAL was best visualized in the sagittal lateral-oblique imaging plane in all specimens. In the sagittal lateral-oblique and transverse planes, the PAL was seen inserting into the base of the fifth metatarsal bone more posteriorly than the SPM tendon, at the typical site of the avulsion fracture of the tip of the tuberosity (Fig 5b). In these MR imaging planes, the PAL and the SPM tendon were converged and attached together at the proximal portion of the fifth metatarsal bone.

View larger version (109K): [in this window] [in a new window] [Download PPT slide]   Figure 5a. Transverse T1-weighted MR images (600/22) of a fifth metatarsal bone specimen show the sites of attachment of (a) the SPM tendon (arrow) and (b) the PAL (arrow). The image in a was obtained at a level 6 mm superior to b.

View larger version (100K): [in this window] [in a new window] [Download PPT slide]   Figure 5b. Transverse T1-weighted MR images (600/22) of a fifth metatarsal bone specimen show the sites of attachment of (a) the SPM tendon (arrow) and (b) the PAL (arrow). The image in a was obtained at a level 6 mm superior to b.

Attempted Simulation of Fracture of the Proximal Portion of the Fifth Metatarsal Bone: Imaging-Anatomic Correlation
In the two cadaveric feet in which simulation of injury to the proximal portion of the fifth metatarsal bone was attempted, we were not able to reproduce a fracture at the base of the fifth metatarsal bone. In one of these two feet, a spiral fracture at the distal third of the diaphysis of the fifth metatarsal bone was created. No fracture of the proximal portion of the fifth metatarsal bone was visualized on radiographs of the foot. In addition, neither MR imaging nor CT depicted any fracture line or gap between the proximal portion of the fifth metatarsal bone and the ligamentous and tendinous structures attached to it. The absence of fracture of the proximal portion of the fifth metatarsal bone was confirmed at dissection of the specimens performed after the attempted fracture.

Clinical Study
Concomitant fractures of the lateral aspect of the cuboid bone (Fig 6a) were seen at imaging in two (15%) of the 13 patients, and one (8%) of the 13 patients also had an oblique fracture of the distal portion of the fibula. One (50%) of the two patients with a cuboid bone fracture developed an additional spiral fracture of the distal diaphysis of the fifth metatarsal bone (Fig 6a). Pain to the lateral part of the forefoot and soft-tissue edema were encountered in all cases, and 10 (77%) of the 13 patients developed ecchymosis. The signal intensity characteristics of bone marrow edema (ie, high signal intensity on T2-weighted images and low signal intensity on T1-weighted images) at the proximal portion of the fifth metatarsal bone were seen in two (15%) of the 13 patients for whom T2-weighted MR images were available.

View larger version (55K): [in this window] [in a new window] [Download PPT slide]   Figure 6a. Images obtained in a 54-year-old man with an acute fracture of the tip of the tuberosity (ie, most proximal portion of the fifth metatarsal bone). (a) Anteroposterior radiograph of the foot shows a bone chip (thin straight arrow) at the site of attachment of the PAL. Note the concomitant fracture (curved arrow) of the lateral aspect of the cuboid bone, as well as the diaphyseal spiral fracture (thick straight arrow) of the distal portion of the fifth metatarsal bone. (b) Transverse CT image of the foot shows the tiny bone fragment (small arrow) at the tip of the tuberosity and the fibers of the PAL (large arrow) attached to the tip of the tuberosity.

View larger version (68K): [in this window] [in a new window] [Download PPT slide]   Figure 6b. Images obtained in a 54-year-old man with an acute fracture of the tip of the tuberosity (ie, most proximal portion of the fifth metatarsal bone). (a) Anteroposterior radiograph of the foot shows a bone chip (thin straight arrow) at the site of attachment of the PAL. Note the concomitant fracture (curved arrow) of the lateral aspect of the cuboid bone, as well as the diaphyseal spiral fracture (thick straight arrow) of the distal portion of the fifth metatarsal bone. (b) Transverse CT image of the foot shows the tiny bone fragment (small arrow) at the tip of the tuberosity and the fibers of the PAL (large arrow) attached to the tip of the tuberosity.

View larger version (158K): [in this window] [in a new window] [Download PPT slide]   Figure 7a. Images obtained in a 42-year-old man with an acute fracture of the tuberosity of the fifth metatarsal bone. (a) Lateral radiograph of the foot shows the fracture (arrow) traversing the proximal portion of the fifth metatarsal bone. (b) Transverse T1-weighted spin-echo MR image (366/14) shows the fracture (large arrow) and the PAL (small arrows) attached to the avulsed fragment. (c) Transverse T1-weighted spin-echo MR image (366/14) obtained at a level approximately 16 mm superior to b shows the fracture (large arrow) and the SPM tendon (small arrows) inserting into the lateral aspect of the tuberosity.

View larger version (157K): [in this window] [in a new window] [Download PPT slide]   Figure 7b. Images obtained in a 42-year-old man with an acute fracture of the tuberosity of the fifth metatarsal bone. (a) Lateral radiograph of the foot shows the fracture (arrow) traversing the proximal portion of the fifth metatarsal bone. (b) Transverse T1-weighted spin-echo MR image (366/14) shows the fracture (large arrow) and the PAL (small arrows) attached to the avulsed fragment. (c) Transverse T1-weighted spin-echo MR image (366/14) obtained at a level approximately 16 mm superior to b shows the fracture (large arrow) and the SPM tendon (small arrows) inserting into the lateral aspect of the tuberosity.

View larger version (146K): [in this window] [in a new window] [Download PPT slide]   Figure 7c. Images obtained in a 42-year-old man with an acute fracture of the tuberosity of the fifth metatarsal bone. (a) Lateral radiograph of the foot shows the fracture (arrow) traversing the proximal portion of the fifth metatarsal bone. (b) Transverse T1-weighted spin-echo MR image (366/14) shows the fracture (large arrow) and the PAL (small arrows) attached to the avulsed fragment. (c) Transverse T1-weighted spin-echo MR image (366/14) obtained at a level approximately 16 mm superior to b shows the fracture (large arrow) and the SPM tendon (small arrows) inserting into the lateral aspect of the tuberosity.

View larger version (64K): [in this window] [in a new window] [Download PPT slide]   Figure 8a. Images obtained in a 57-year-old man with an acute Jones fracture. (a) Anteroposterior radiograph of the foot shows a fracture (arrow) anterior to the tuberosity of the fifth metatarsal bone. (b) Transverse CT image of the plantar aspect of the foot shows the fracture (large arrow) of the proximal diaphysis of the fifth metatarsal bone. The PAL (small arrow) is seen inserting into the proximal portion of the fifth metatarsal bone. (c) Transverse CT image of the foot obtained approximately 2 mm superior to b shows the fracture (large arrow) and the SPM tendon (small arrow) attached to the lateral side of the fragment.

View larger version (77K): [in this window] [in a new window] [Download PPT slide]   Figure 8b. Images obtained in a 57-year-old man with an acute Jones fracture. (a) Anteroposterior radiograph of the foot shows a fracture (arrow) anterior to the tuberosity of the fifth metatarsal bone. (b) Transverse CT image of the plantar aspect of the foot shows the fracture (large arrow) of the proximal diaphysis of the fifth metatarsal bone. The PAL (small arrow) is seen inserting into the proximal portion of the fifth metatarsal bone. (c) Transverse CT image of the foot obtained approximately 2 mm superior to b shows the fracture (large arrow) and the SPM tendon (small arrow) attached to the lateral side of the fragment.

View larger version (79K): [in this window] [in a new window] [Download PPT slide]   Figure 8c. Images obtained in a 57-year-old man with an acute Jones fracture. (a) Anteroposterior radiograph of the foot shows a fracture (arrow) anterior to the tuberosity of the fifth metatarsal bone. (b) Transverse CT image of the plantar aspect of the foot shows the fracture (large arrow) of the proximal diaphysis of the fifth metatarsal bone. The PAL (small arrow) is seen inserting into the proximal portion of the fifth metatarsal bone. (c) Transverse CT image of the foot obtained approximately 2 mm superior to b shows the fracture (large arrow) and the SPM tendon (small arrow) attached to the lateral side of the fragment.

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
The fifth metatarsal bone consists of a base, tuberosity (ie, styloid process), diaphysis, neck, and head. The base articulates proximally with the cuboid bone and medially with the fourth metatarsal bone. Dorsal and plantar cuboideometatarsal, intermetatarsal, and capsular ligaments; the SPM tendon; and the PAL provide stability to the lateral Lisfranc complex (ie, tarsometatarsal joint) (4,20,21). Despite anatomic variations in its size and shape, the tuberosity of the fifth metatarsal bone protrudes downward and laterally (3,14,21). At the dorsolateral aspect of the tuberosity, the sturdy SPM tendon inserts over a relatively large area, the longest dimension of which is almost four times the diameter of the tendon itself (3,14,21). Distal to the tuberosity, the TPM tendon has a weaker attachment to the superior surface of the metatarsal diaphysis (14,20,21).

The strong PAL, which may contain muscle fibers, connects the tuberosity to the lateral margin of the medial calcaneal tubercle (19–22). The broad slip from the LPL attaches to the plantar surface of the fifth metatarsal bone base (21). Along its course from the calcaneus to the base of the proximal phalanx of the fifth toe, the abductor muscle of the little toe passes under and around, and usually attaches to, the base of the fifth metatarsal bone (14,20). The flexor muscle of the little toe originates from the plantar surface of the base of the fifth metatarsal bone, and the dorsal and plantar interosseous muscles arise from the diaphysis of this bone (14,20,21,23).

Fractures of the proximal portion of the fifth metatarsal bone are common foot injuries, and they may have diverse prognoses and treatments depending on the location and acuteness of the injury (3–6,14). Three types of fracture of the proximal portion of the fifth metatarsal bone have been described: Jones fracture, tuberosity avulsion fracture, and diaphyseal stress fracture (3–5,24–26). The clinical diagnosis of these fractures can be difficult when they are in the acute stage because of pain, muscle spasm, and soft-tissue edema (2,13,17,27). Routine radiographs of the foot, however, may not depict avulsion fractures of the base of the fifth metatarsal bone (17). Furthermore, the fracture needs to be distinguished from apophyseal distraction fractures in the immature skeleton, a normal or injured sesamoid or vesalian bone, or injuries to the tarsometatarsal complex (3,14). Thus, it is important to recognize a fracture and distinguish among the different fracture types to avoid the potential clinical complications associated with delayed union, nonunion, or repeat fracture after union if the appropriate treatment is delayed or not rendered (1,11,14,28,29).

The most common fracture of the proximal portion of the fifth metatarsal bone is fracture of the tuberosity, which is usually extraarticular (4,30). Tuberosity avulsion fractures were dominant in the patients examined in the present study: 11 (85%) of 13 patients had them. We observed intraarticular extension of the fracture in one (8%) patient, who underwent surgery.

The Jones fracture is an acute forefoot injury that is believed to be related to the application of a large adduction force to the forefoot with the ankle in plantar flexion (1,4). Stewart (31) defined a true Jones fracture as a transverse fracture at the junction of the diaphysis and the metaphysis, without extension distally to the fourth and fifth intermetatarsal articulations. This fracture occurs at the area between the insertion of the TPM and SPM tendons (14), and medial comminution of the fracture is a common finding (31). In the patient with a Jones fracture in this study, there was medial comminution of the fracture with no intraarticular involvement.

The least common fractures of the proximal portion of the fifth metatarsal bone are diaphyseal stress fractures (3,4), which occur within the proximal 1.5 cm of the fifth metatarsal diaphysis. Diaphyseal stress fractures in this area of the foot are considered to be distraction fractures (4), which begin as microfractures and then progress to complete fractures (10). The patient with this type of fracture in our series had metatarsus adductus, a structural foot deformity that has been associated with stress fractures of the lateral metatarsal bones (32) and sustained injury to the foot.

The exact pathogenesis of fractures of the proximal portion of the fifth metatarsal bone has been debated, in part because of the intricate anatomy of the lateral aspect of the Lisfranc joint (3,4,21). Imprecise definitions of fracture types and locations, however, have added to some of the confusion (3,4,16). The most frequent error has been to label tuberosity avulsion fractures and proximal diaphyseal fractures as Jones fractures. The identification and precise localization of this fracture with CT and MR imaging enabled easy differentiation among the fracture types in our series. Jones (13) believed that the lateral Lisfranc complex was so reinforced by strong ligamentous attachments that injury to it would result in fracture rather than dislocation of the fifth metatarsal bone. In his original article (13), Jones did not indicate that the fracture he described was an avulsion injury.

In a dynamic force analysis study, Kavanaugh et al (1) demonstrated that the Jones fracture results from the application of vertical and mediolateral forces on the fifth metatarsal bone, not from inversion of the foot. In a cadaveric study of 20 specimens, Dameron (14) observed that only the SPM tendon appeared to be strong enough to cause an avulsion fracture of the proximal portion of the fifth metatarsal bone with inversion of the plantar flexed foot. In 1984, Richli and Rosenthal (16) suggested that the PAL is the structure that produces tuberosity avulsion fracture, with a mechanism of injury that includes inversion and plantar flexion of the forefoot. Pao et al (17) reported six avulsion fractures of the tip of the tuberosity and noted that the avulsed fragment occurred at the insertion site of the PAL.

Certain observations seem to favor the plantar aponeurosis as the contributing structure in avulsion fractures: First, the PAL attaches to the plantar surface of the tip of the tuberosity, which is the most common site of avulsion fractures, whereas the SPM tendon lies superiorly and has a more distal, broad, and oblique attachment to the tuberosity of the fifth metatarsal bone (3,4,21). Second, displacement of an avulsion fracture is uncommon; it is most likely to occur with muscle contraction of the SPM and pull of the SPM tendon rather than pull of the flat plantar aponeurosis on the bone fragment (4,30). Third, experience with the surgical treatment of these fractures indicates that the avulsed fragment can be excised without functional compromise at the site of insertion of the SPM tendon (33).

In our study, imaging-anatomic correlation enabled us to visualize the broad and firm insertion of the PAL directly at the tip of the tuberosity, the oblique-lateral insertion of the SPM tendon (extending more distally at the tuberosity), and the inconsistent and more distant insertion of the TPM tendon. We found interdigitating fibers between the PAL and the SPM tendon; these fibers formed a strong structure and were attached to the proximal portion of the fifth metatarsal bone at the typical site of the tuberosity fracture.

Findings at analysis of the clinical (ie, patient) cases confirmed the attachment of the PAL and the SPM tendon to the avulsed fragment. Although we were not able to create a fracture in our cadaveric specimens, we speculate that the pathogenesis of fractures proximal to the tuberosity is related to a violent pull of the strong and extensive structure formed by the converging fibers of the PAL and the SPM tendon. We were not able to simulate the mechanism of fracture, presumably because the cadaveric specimens could not be subjected to vertical loading, which in the clinical setting is facilitated by weight-bearing conditions. During weight bearing, the plantar aponeurosis tightens and tension at the site of insertion of this structure increases (19,21,34).

Our study had some limitations: First, simulation of fracture of the proximal portion of the fifth metatarsal bone was not possible, because the mechanism of injury was difficult to recreate in cadaveric specimens. Second, the limited numbers of cadavers and clinical (ie, patient) cases may have resulted in an underestimation of normal anatomic variations. Third, selection bias is introduced when only patients who have injuries to the lateral part of the forefoot are examined, as was the case in our study. Fourth, the visualization of small structures such as the anterior frenular ligament on MR images was not possible. Finally, surgical correlation was possible in only one case.

Our study results suggest that the PAL and the interdigitating fibers of the SPM tendon have an important role in the pathogenesis of fractures of the proximal portion of the fifth metatarsal bone. Further biomechanical studies that include simulation of the mechanism of injury under weight-bearing conditions may elucidate the roles of the PAL, the SPM tendon, or both, in determining the pathomechanical origin of injuries involving the base of the fifth metatarsal bone.

	FOOTNOTES

 
Abbreviations: LPL = long plantar ligament, PAL = lateral component of the plantar aponeurosis, SPM = short peroneal muscle, TPM = third peroneal muscle

Author contributions: Guarantors of integrity of entire study, D.J.T., S.J.T., Y.K., D.R.; study concepts and design, D.J.T., S.J.T., D.R.; literature research, D.J.T., S.J.T., Y.K.; clinical studies, D.J.T., S.J.T., M.J.B.; experimental studies, D.J.T., S.J.T., M.J.B., Y.K.; data acquisition, D.J.T., S.J.T., Y.K.; data analysis/interpretation, D.J.T., S.J.T., Y.K., M.J.B.; manuscript preparation, D.J.T., S.J.T.; manuscript definition of intellectual content, editing, and revision/review, D.J.T., S.J.T., D.R.; manuscript final version approval, D.R., D.J.T.

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