0198-021 1/0198-0211/91/1105-0254$03.00/0 FOOT& ANKLE Copyright 0 1991 by the American Orthopaedic Foot and Ankle Society, Inc
Three Dimensional Analysis of Calcaneal Fractures Steven M. Allon, M.D. and Dana C. Mears, M.D., Ph.D. Meadowbrook, Pennsylvania and Pittsburgh, Pennsylvania
3D CT techniques have been applied to other anatomic are as*^^-'^.'^ and two isolated reports each involving a single calcaneal fracture have been publ i ~ h e d . ' ~However, ,'~ the resolution of the calcaneal images has not been optimal and a reproducible protocol for procuring high quality 3D images of calcaneal fractures has been lacking. Through the development of a special protocol this project has extended the practical application of 3D CT scanning to include accurate assessment of 0s calcis fractures along with other complex fracture dislocations of the foot and ankle (Fig. 1C).
ABSTRACT Thirty calcaneal fractures in 22 patients were evaluated by conventional radiographs, two dimensional computed tomography scans (2D CT) and three dimensional computed tomography reformations (3D CT). A special protocol for the reproducible procurement of high quality 3 D CT scans was developed and is presented for the first time. Computer-generatedthree dimensional plastic models of fractured calcanei and videotaped presentations of fractured calcanei were made in selected cases. This study is the first to reproducibly demonstrate the usefulness of 3 D CT scans in visualizing a calcaneal fracture. This information highlights the rotationaldisplacementof major fracture fragments (multiplanar malrotation) and can facilitate the preparation of an appropriate surgical approach and reconstruction.
MATERIAL AND METHODS
Twenty-two patients who sustained 30 calcaneal fractures between July, 1986, and December, 1987, were evaluated with conventional two dimensional radiographs, 2D CT scans, and three dimensional reformations (3D CT scans). The patients were evaluated at hospitals affiliated with the University of Pittsburgh Orthopaedic Surgery Residency Program. Seven females and 15 males between the ages of 14 and 82 years were studied. Three females and five males sustained bilateral fractures. Five patients were involved in motor vehicle accidents while the remaining patients sustained falls. One elderly pedestrian was struck by a truck. She sustained massive traumatic injuries to her right lower extremity, including a comminuted calcaneal fracture. Subsequently, an above knee amputation was performed and the calcaneal fracture became available for more detailed study. 2D CT scans were performed with the patient in a supine position and the foot and leg in a bulky dressing or splint. The hip and knee were each flexed to between approximately 35" and 55" and supported with pillows or a triangular foam pad. Supplementary tape facilitated the immobilization of the limb (Fig. 2). This placement of the extremity permits between 30" and 45" of plantarflexion of the foot in relation to the tibia, and was the position used in nine patients. The foot was positioned between neutral and 30" in two patients, and in neutral flexion in one patient. The inclination of the remaining
This original study describes the application of three dimensional reformations in analyzing 0s calcis fractures. Such 3D CT reformations are generated from contiguous 2D CT images through the use of computer programs that make structures closer to the viewer appear whiter, and those further away appear darker (Fig. 1A). Intricate irregular curvatures and rotational displacement of surfaces are discerned by a scrutiny of these images (Fig. 1B). The treatment of calcaneus fractures has been fraught with difficulty due to many unsolved problems, including: 1. incomplete visualization of the three dimensional geometry of the bone, 2. inadequate representation of the rotational displacement of fracture fragments, 3. shortcomings in technical reduction of fractures in part because of the above issues, and, 4. limitations secondary to the surgical approach to these fractures. ~
Read in part at the American Orthopaedic Foot and Ankle Society 18th Annual Meeting in Atlanta, Georgia, February 7, 1988. Address correspondence and request for reprints to Steven M. Allon, M.D., Holy Redeemer Hospital and Medical Center, 1648 Huntingdon Pike, Meadowbrook, PA 19046.
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Fig. 1. A , 3D CT scan of a comminuted calcaneal fracture viewed from inferiomedially Objects closer to viewer appear brighter than those that are further away Note large sustentacular fragment B. viewed from inferioposterially, this 3D CT scan reveals a large tuberosity fracture fragment malrotated in the sagittal plane C, 3D CT scan seen from posteriomedial viewpoint demonstrates pilon fracture of distal tibia in addition to calcaneal fracture
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Fig. 2. Artist's rendering of extremity and foot position within CT gantry. Dotted lines represent sequential xray beams emanating from CT gantry in a plane perpendicular to examination table. Note that vertical x-ray beams cross the subtalar and calcaneocuboid joints using this protocol, thereby maximizing the articular surface area to be scanned at both joints. This greatly enhances the quality of the 3D images. See text for other important aspects of scanning protocol.
feet was not recorded inasmuch as the crucial role of positioning of the extremity was not appreciated early in the study. With the supine alignment of the lower extremity, the foot is positioned between 45" and 90" with respect to the gantry. The gantry is coaxial with the x-ray beam intentionally so that the latter is perpendicular to the examination table. The x-ray beam is thereby oriented perpendicular or with substantial obliquity with respect to the subtalar and calcaneocuboid articular and fracture surfaces (Fig. 2). This maximizes the articular suiface area which is to be scanned at both joints, and thereby significantly improves the quality of the three dimensional images. For the two dimensional scans, the cuts were contiguous, varying in thickness between 3 and 10 mm. A standard soft tissue algorithm was used for this study. In all but two patients the x-ray beam was aligned perpendicular to the table. In the other two cases the x-ray beam was tilted approximately 20' with respect to the table. For the three dimensional reformations of the images either a GE (General Electric, Milwaukee, WI) 9800 unit or a Cemax 1500 model (Cemax, Inc., Fremont, CA) was employed. Three dimensional reformations of the 0s calcis were obtained in increments of 45' around all axes of the bone. RESULTS Conventional Radiographs
Scrutiny of conventional radiographs provides an underestimate of the degree of comminution and the magnitude of rotational displacement of the fracture surfaces as well as the extent of subtalar facetal depression. The resolution of the fracture lines is further obscured by the presence of a splint and the frequent
Fig. 3. Plain radiograph of a comminuted calcaneal fracture. Such x-rays do not convey a complete 3D appreciation of the fracture pattern.
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inability to obtain the properly angled radiographs (Fig. 3). 2D CT
Two dimensional CT scans are not significantly affected by incidental motion of the patient, the angle of the gantry, or the field size (field of view). The use of bony algorithm does enhance 2D CT visualization of bone and fracture surfaces; however, the use of a soft tissue algorithm in obtaining the 2D CT provides better data for 3D CT images. Alignment of the foot in a perpendicular to substantially oblique angle with respect to the vertical orientation of the x-ray beam is optimal for visualization of the extent of subtalar disruption. When the x-ray beam is perpendicular or oblique to the subtalar fracture surfaces (Fig. 2), the fracture is well visualized in the 2D CT coronal image (Fig. 4). When the x-ray beam is aligned parallel to the subtalar joint irrespective of the position of the foot (Fig. 5), the resolution of the 2D and 3D scans is markedly compromised particularly with thicker 2 0 cuts. In such cases the magnitude and orientation of joint depression is not subject to an accurate determination by scrutiny of the 2D CT. This is because it has been shown that x-rays provide more detailed information when the angle of the x-ray beam is oriented perpendicular to the area of interest. In calcaneus fractures the areas of greatest interest are the overall alignment of the subtalar and calcaneocuboid joints as well as the orientation and displacement of fracture fragments of the calcaneus itself. Because the subtalar and calcaneo-
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cuboid joints are nearly perpendicular to one another and because the subtalar joint itself exhibits an undulating surface with substantial variation of the plane of one facet in relation to the others, optimal visualization of all of the calcaneal facets and the calcaneocuboid joint is obtained with the orientation that has been described. This is especially important when we consider that a 3D CT can only be made from the data obtained from a single 2D CT scan. For this reason, it is not possible to use the information from two different CT scans made in perpendicular planes. When the thickness of the cut exceeds 5 mm, the resolution of the 2D and 3D views deteriorates markedly so that morphologic features including the magnitude of joint depression, the presence of fracture surfaces, and the degree of comminution are rendered obscure. The degree of comminution is well visualized in the 2D CT coronal views and the 3D scans obtained from these 2D CTs. Unlike the 3D scans in this study, the coronal 2D CTs can reveal the presence of intra-articular comminution that may not be evident on the 3D scan. Scrutiny of the conventional 2D CT scans, however, provides a gross overestimation of the magnitude of comminution, and an underestimation of the rotational malalignment in multiple planes, as well as the presence and extent of joint depression. Since the completion of this study, both authors have independently been able to significantly improve upon the quality of 3D imaging of intra-articular comminution (see Fig.
8).
Fig. 4. Coronal 2D CT evaluation of bilateral comminuted calcaneal fractures. Fracture pattern can be well visualized with this type of image. While this 2D CT was produced with a soft tissue algorithm, a bony algorithm is preferredfor clearer 2D CT visualization of fracture morphology. The soft tissue algorithm however, provides better data for 3D images than does the bony algorithm.
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Fig. 5. The x-ray beam in this scout film is oriented parallel to long axis of foot. The 2D and 3D CT scans that result will be less than optimal for visualization of important subtalar joint irregularities.
3D CT
In a comparison of the morphologic features observed by a scrutiny of conventional radiographs and 2D and 3D scans, the 3D CT scans performed with appropriate protocol provide for superior resolution and excellent visualization. The quality of the 3D CT image is markedly influenced by the thickness of the cuts, the angle of the gantry, the motion of the patient, the position of the foot relative to the angle of the gantry, the field size, and the sophistication of the software program. The most important factors are cut thickness, field size, patient motion, and gantry tilt (Fig. 6). In addition, using a soft tissue algorithm in the 2D CT seems to enhance the quality of the 3D image. An increase in the thickness of the cut beyond 5 mm noticeably accentuates the alias or linear steppage artifact in the 3D scans. When the cut thickness is less
than 4 mm, the quality of the scans improves significantly. Tilting the gantry provokes a substantial degree of distortion in the 3D CT. This is a clinical observation that became apparent as the study progressed. In one case, an alias artifact combined with faulty orientation of the x-ray beam culminated in an irregular and distorted image with an apparent lengthening and flattening of the bones of the foot (Fig. 6A). Motion of the patient provokes an obvious disruption in the natural contour of the bone which adversely affects the quality of the 3D scan (Fig. 6B). Limiting the field size of the 2D CT scan to the area immediately surrounding the foot significantly improves the resolution of the 3D scan. With such a decrease in the area of view, the computer possesses more raw data per unit area with which to generate a detailed image. In a 3D scan, bone of insufficient thickness or density is erased from the image. In some cases this osteopenic appearance aids in an estimation of the cortical thickness and corresponding strength of the bone. Typically the resolution of fractured surfaces in the 3D scans is about 2 mm, or the width of an equivalent fracture gap. Occasionally, an undisplaced fracture line was visualized which was indicative of a resolution of approximately 1 mm (Fig. 6B). This degree of resolution compares favorably with that achieved previously in 3D imaging studies of pelvic and acetabular fracture^.^ Two dimensional CT scans and conventional radiographs are hampered by their inability to fully convey information relative to the vectors of rotational deformity of the fracture fragments. Such techniques are only capable of representing fractures in a single plane with any given image. By contrast, the magnitude of comminution, the degree of rotation, and the extent of joint depression can be well visualized with the 3D scans, especially when the above described protocol is employed (Fig. 7). The optimal resolution of a 3D CT scan is achieved when the resultant image of the 0s calcis is aligned and rotated in a plane perpendicular to the linear axis of the x-ray beam. Although the rotational deformity of the fracture documented in other planes is not as clearly resolved, it is still substantially evident. A careful study of the various 3D CT images of the foot accrued in multiple planes of rotation provides an enormous help for the observer to create an accurate three dimensional mental image of the complex geometry of a comminuted and displaced calcaneal fracture (Figs. 7 and 8). While previous workers have attempted to achieve a comparable degree of information by resorting to 2 0 CT sagittal reconstructions, this method
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Fig. 6. Examples of artifact in 3D CT scans. A , tilt and alias (linear steppage) artifacts. A gantry tilt of 20' results in apparent lengthening and flattening of the bones of the foot. The choppy appearance is due to a thickness of 7 mm in the 2D CT cuts. The view is from lateral and anteriosuperior perspective. B,motion artifact. Posteriorand slightly inferiolateral view of a tuberosity fracture. Horizontalband across the bone results from motion of the patient during the 2 0 CT scan. Note visualization of the vertical nondisplaced tuberosity fracture line.
is inadequate when compared to the application of an optimally performed 3D CT scan. The available 3 0 CT scans accrued from this preliminary investigation define morphological features of calcaneal fractures that are generally consistent with previous classification schemes with one exception. Whereas previous classification systems account for rotational deformity primarily in one and sometimes in two planes, the 3D CT scans in this study provide an accurate representation of the deformity in three planes. This improvement provides a clear mental image of the multiplanar malrotation of the fracture frag-
ments, and greatly facilitates an attempt to restore the anatomic configuration of the bone during an open reduction. Computer-assistedconstruction of three dimensional plastic models and videotaped presentations of the fractured calcaneus is another option which may aid the surgeon in the evaluation and treatment of a complex fracture. This can be accomplished with current 3 0 CT technology and was employed in selected cases in this study. In such models, the calcaneus can be disarticulated from the talus and cuboid to visualize intra-articular distortion.
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Fig. 7. Comminuted calcaneal fracture in a cadaveric specimen visualized in 3D CT scan performed with optimum protocol. Views are seen in multiple planes of rotation. A , superior view. Note oblique tuberosity fracture. B , lateral and, C, inferiolateralviews. Note subtalar joint subluxation and calcaneocuboid impaction and subluxation. D , inferiomedial view displays sustentacular and tuberosity fractures.
DISCUSSION Conventional Radiographs
The principal limitations associated with the use of conventional radiographs include the compromise of resolution documented in the presence of a splint or bulky dressing, the prolongation of the study needed to obtain the essential multiple views, and the intrinsic limitations in resolution of a three dimensional image that are consistent with the method. Even Broden’s views, often of value in a nonacute setting, are frequently difficult to obtain and uncomfortable for the patient in the acute and multiple trauma setting. 2D CT Scans
In the preparation of a 2D CT scan, substantial artifactual features arise in the subsequent 3D CT images unless the foot is positioned carefully with respect to the gantry. Also, the use of cuts thicker than 4 mm markedly compromises the quality of both the 2D and 3D CT images. Other factors that do not adversely affect the quality of the 2D CT images are the angle of the gantry and the field size. These factors, along with motion of the patient, do affect the quality
of the 3D CT scan. The type of algorithm used affects 2 0 and 3D images in opposite ways, with a soft tissue algorithm providing high quality 3D images. While a study of the 2D CT scans reveals the presence of intra-articular comminution, it does not provide an accurate indication of the magnitude of the injury, the degree of joint depression, or the extent of rotational malalignment (multiplanar malrotation). Typically, a study of 2D CT scans provides an overestimation of the degree of comminution while a comparable scrutiny of conventional radiographs provides an underestimation. Such 2D techniques can only illustrate images in a single plane parallel with the x-ray beam or gantry. The resultant plane of visualization, especially in the 2D CT image is essentially that of a single straight line through multiple undulating surfaces (the fractured subtalar facets, calcaneal body and neck, and the calcaneocuboid joint). Such images can give a false impression of the degree of joint depression and multiplanar malrotation, especially to the inexperienced viewer. This is commonly substantiated clinically when openly reducing and internally fixing a malrotated joint depression calcaneus fracture: one often finds that a
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large portion of the posterior facet has been sagittally fractured, impacted, and anteriorly rotated often up to 90". The appreciation of this rotation is often sorely missing from a standard 2D CT scan with coronal cuts and poorly visualized on standard sagittal 2D CT reconstructions. With regard to overestimation of intra-articular comminution in 2D CT scans, it is apparent that when a straight line transects a highly undulating surface such as the subtalar joint, it is not possible to determine if the points transected by that line remain contiguous on the undulating surface or if they are actually separated points (i.e., malrotated fracture fragments). This becomes easier to visualize if one draws a straight line through an undulating line. 3D CT Scans
For the preparation of a high resolution 3 0 CT scan, a special protocol was developed which pertains to the procurement of the corresponding 2D CT scan. The presence of a fiberglass or plaster splint does not detract from the quality of the 3D CT image inasmuch as the splint is routinely subtracted from the 3D image. Enlisting the cooperation of radiologists and x-ray technicians is important if the protocol to obtain high quality scans is to be followed properly. Likewise, high quality 3D hardware and software packages are essential for the production of high resolution images. At the time of this study, when the special protocol was followed, the corresponding 3D reformations were superior to the 2D CT scans in all but two parameters. The visualization of intra-articular comminution was not optimal. Likewise, the superimposition of the talus and the cuboid can obscure the articular surfaces of the 0s calcis. Currently these problems are being addressed by the subtraction of adjacent bony surfaces as part of the data manipulation by computer and software packages.' When this solution was employed for 3D CT studies of pelvic and acetabular fractures, it was shown to be extremely e f f e c t i ~ e .Since ~ . ~ the completion of this study, both authors have independently and successfully employed such techniques to visualize calcaneal fractures using such advanced software packages as those offered by the ISG system (ISG Technologies, Mississauga, Ontario, Canada) (Fig. 8). In all other respects, high quality 3D CTs provide information that greatly enhances information gained from conventional radiographs and 2 0 CT scans. This is the case particularly with reference to visualization of the sites and magnitude of joint depression, the degree and vector of rotational malalignment in multiple planes, and the degree of comminution. In contrast to other techniques, 3D CT images viewed in multiple
planes essentially show the entire fracture as it exists in vivo (Figs. 7 and 8). The greater the number of 2D CT images made, the larger is the radiation exposure to the patient. There is no additional radiation exposure in obtaining a 3D CT beyond that in obtaining the 2D CT because the 3D images are generated from the same computer data used to form the 2D images. While attempts to employ 1- or 2-mm cuts are notable for the preparation of 3D scans of high quality, the application of 3-mm cuts is the optimal compromise in this study. This format yields high resolution, yet also provides for a lesser degree of radiation exposure and a briefer period of immobilization of the patient on the examination table than would be required for more narrow image slices. These cuts should be contiguous to maximize data retrieval and image quality. The advent of this series of 3D CT scans presents a new feature of calcaneal fractures not previously well defined in current classification systems: an accurate visual representation of the degree and vector of rotational malalignment in multiple planes, or multiplanar malrotation. CONCLUSIONS
A highly specific 3D imaging protocol has been devised which for the first time provides an excellent diagnostic tool in the assessment of a displaced, comminuted, and malrotated calcaneal fracture. The principal features of the protocol for the preparation of the corresponding 2D CT scan are: 1. the use of contiguous cuts with a thickness of 3mm, 2. the minimization of a motion artifact by an appropriate and comfortable form of immobilization, 3. the alignment of the foot and subtalar joint surfaces in a position oblique to perpendicular relative to the axis of the x-ray beam, 4. the orientation of the x-ray beam perpendicular to the examination table, 5. the use of a standard soft tissue algorithm, 6. the minimization of the field size, and 7. the use of current software programs and high quality computer packages.
The use of a conventional radiographic evaluation of a calcaneal fracture supplemented by a conventional 2D CT scan of optimal protocol can possess limitations referable to the definition of the degree and vector of rotational deformity and the magnitude of comminution and joint depression. When such studies are supplemented by high quality 3D CT images, the information gleaned is often significantly improved.
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Centers without such imaging capabilities will obviously continue to make maximum use of more standard modalities or send 2D CT computer tapes to centers with 3D imaging facilities if the patient’s clinical situation warrants and permits this. The less familiar and experienced one is with complex calcaneal fractures, the greater is the potential information gain from 3D imaging techniques. Even the experienced surgeon may find that he or she could gain increased appreciation of fracture anatomy with such techniques, as has been demonstrated in studies of the pelvis and other anatomic regions. In this study, 30 calcaneal fractures including one cadaveric specimen were evaluated by conventional plain radiographs as well as 2D and 3D CT scans. A special technique for the preparation of 3D CT images of uniformly high quality was devised. In a comparison of the 30 CT reformations with the previous radiographs and 2D CT images, the former views greatly enhanced the critical evaluation of the fractures especially with reference to the magnitude of joint depression, the extent of comminution and the degree and vector of rotational malalignment (multiplanar malrotation). This study demonstrates the usefulness of 3D CT scans in visualizing a calcaneal fracture. This information highlights the rotational displacement of major fracture fragments and facilitates the preparation of an appropriate surgical approach and reconstruction. ACKNOWLEDGMENTS
Dr. Allon gratefully acknowledges the support of his wife, Gloria A. Allon, R.N., and his parents, Meyer Michael and Marcella Allon, in the preparation of this study. REFERENCES 1. Adler, S.J., Vannier, M.W., Gilula, L.A., and Knapp, R.H.: Threedimensional computed tomography of the foot: optimizing the image. Comput. Med. Imaging Graph., 12(1):59-66, 1988. 2. Biondetti, P.R., Vannier, M.W., Gilula, L.A., and Knapp, R.H.: Three dimensional surface reconstruction of the carpal bones
3.
4. 5.
6. 7. 8.
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from CT scans: transaxial versus coronal technique. Comput. Med. Imaging Graph., 12(1):67-73, 1988. Burk, D.L., Jr., Mears, D.C., Cooperstein, L.A., Herman, G.T., and Udupa, J.K.: Acetabular fractures: three-dimensional computer tomographic imaging and interactive surgical planning. J. Comput Tomogr. 10(1):l-10, 1988. Burk, D.L., Jr., Mears, D.C., Kennedy, W.H., Cooperstein, L.A., and Herbert, D.L.: Three-dimensional computed tomography of acetabular fractures. Radiology, 155(1):183-186, 1985. Cutting, C., Bookstein, F.L., Grayson, B., Fellingham, L., and McCarthy, J.G.: Three-dimensional computed tomographic scanning and major surgical reconstructionof the head and neck. Mayo Clin. Proc., 61(7):546-555, 1986. DeMarino, D.P., Steiner, E., Poster, R.B., et al.: Three-dimensional computed tomography in maxillofacial trauma. Arch. Otolaryngol. Head Neck Surg., 112(2):146-150, 1986. Gillespie, J.E., Isherwood, I., Barker, G.R., and Quayle, A.A.: Three-dimensional reformations of computed tomography in the assessment of facial trauma. Clin Radiol. 38(5):523-526, 1987. Grodd, W., Dannenmaier, B., Petersen, D., and Gehrke, G.: Three-dimensional (3-D)image reconstruction of the facial skull and skull base in computerized tomography. Radiology,
27(11):502-510, 1987. 9. Koltai, P.J., and Wood, G.W.: Three dimensional CT reconstruction for the evaluation and surgical planning of facial fractures. Otolaryngol. Head Neck Surg., 95(1):10-15, 1986. 10. Lang, P., Genant H.K., Chafetz, N., Steiger, P., and Morris, J.M.: Three dimensional computed tomography and multiplanar reformations in the assessment of pseudarthrosis in posterior lumbar fusion patients. Spine, 13(1):69-75, 1988. 1 1 . Marentette, L.J., and Maisel, R.H.: Three-dimensional CT reconstruction in midfacial surgery. Otolaryngol. Head Neck Surg.
88(1):48-52, 1988. 12. Millman, A.L., Lubkin, V., and Gersten, M.: Three-dimensional reconstructionof the orbit from CT scans and volumetric analysis of orbital fractures: its role in the evaluation of enophthalmos (editorial) Adv. Ophthalmic Plast. Reconstr. Surg., 6265-268,
1987. 13. Pozzi Mucelli, R. S., Muner, G., Pozzi Mucelli, R., Pozzi Mucelli, M., Marotti, F., and Dalla Palrna, L.: Three-dimensional computed tomography of the acetabulum. Eur. J. Radiol.
6(30):168-177, 1986. 14. Sartoris, D.J., and Resnick, D.: Diagnostic imaging approach to calcaneal fractures. J. Foot Surg., 28(6):524-529, 1987. 15. Sartoris, D.J., Resnick, D., Bielecki, D., Gershuni, D., and Meyers, M.: Computedtomography with multiplanar reformation and three-dimensional image reconstruction in the preoperative evaluation of adult hip disease. Int. Orthop., 12(1):1-8, 1988. 16. Sundberg, S.B., Clark, B., and Foster, B.K.: Three-dimensional reformation of skeletal abnormalities using computed tomography. J. Pediatr. Orthop., 6(4):416-420, 1986.
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Three Dimensional Analysis of Calcaneal Fractures Steven M. Allon, M.D. and Dana C. Mears, M.D., Ph.D. Meadowbrook, Pennsylvania and Pittsburgh, Pennsylvania
3D CT techniques have been applied to other anatomic are as*^^-'^.'^ and two isolated reports each involving a single calcaneal fracture have been publ i ~ h e d . ' ~However, ,'~ the resolution of the calcaneal images has not been optimal and a reproducible protocol for procuring high quality 3D images of calcaneal fractures has been lacking. Through the development of a special protocol this project has extended the practical application of 3D CT scanning to include accurate assessment of 0s calcis fractures along with other complex fracture dislocations of the foot and ankle (Fig. 1C).
ABSTRACT Thirty calcaneal fractures in 22 patients were evaluated by conventional radiographs, two dimensional computed tomography scans (2D CT) and three dimensional computed tomography reformations (3D CT). A special protocol for the reproducible procurement of high quality 3 D CT scans was developed and is presented for the first time. Computer-generatedthree dimensional plastic models of fractured calcanei and videotaped presentations of fractured calcanei were made in selected cases. This study is the first to reproducibly demonstrate the usefulness of 3 D CT scans in visualizing a calcaneal fracture. This information highlights the rotationaldisplacementof major fracture fragments (multiplanar malrotation) and can facilitate the preparation of an appropriate surgical approach and reconstruction.
MATERIAL AND METHODS
Twenty-two patients who sustained 30 calcaneal fractures between July, 1986, and December, 1987, were evaluated with conventional two dimensional radiographs, 2D CT scans, and three dimensional reformations (3D CT scans). The patients were evaluated at hospitals affiliated with the University of Pittsburgh Orthopaedic Surgery Residency Program. Seven females and 15 males between the ages of 14 and 82 years were studied. Three females and five males sustained bilateral fractures. Five patients were involved in motor vehicle accidents while the remaining patients sustained falls. One elderly pedestrian was struck by a truck. She sustained massive traumatic injuries to her right lower extremity, including a comminuted calcaneal fracture. Subsequently, an above knee amputation was performed and the calcaneal fracture became available for more detailed study. 2D CT scans were performed with the patient in a supine position and the foot and leg in a bulky dressing or splint. The hip and knee were each flexed to between approximately 35" and 55" and supported with pillows or a triangular foam pad. Supplementary tape facilitated the immobilization of the limb (Fig. 2). This placement of the extremity permits between 30" and 45" of plantarflexion of the foot in relation to the tibia, and was the position used in nine patients. The foot was positioned between neutral and 30" in two patients, and in neutral flexion in one patient. The inclination of the remaining
This original study describes the application of three dimensional reformations in analyzing 0s calcis fractures. Such 3D CT reformations are generated from contiguous 2D CT images through the use of computer programs that make structures closer to the viewer appear whiter, and those further away appear darker (Fig. 1A). Intricate irregular curvatures and rotational displacement of surfaces are discerned by a scrutiny of these images (Fig. 1B). The treatment of calcaneus fractures has been fraught with difficulty due to many unsolved problems, including: 1. incomplete visualization of the three dimensional geometry of the bone, 2. inadequate representation of the rotational displacement of fracture fragments, 3. shortcomings in technical reduction of fractures in part because of the above issues, and, 4. limitations secondary to the surgical approach to these fractures. ~
Read in part at the American Orthopaedic Foot and Ankle Society 18th Annual Meeting in Atlanta, Georgia, February 7, 1988. Address correspondence and request for reprints to Steven M. Allon, M.D., Holy Redeemer Hospital and Medical Center, 1648 Huntingdon Pike, Meadowbrook, PA 19046.
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Fig. 1. A , 3D CT scan of a comminuted calcaneal fracture viewed from inferiomedially Objects closer to viewer appear brighter than those that are further away Note large sustentacular fragment B. viewed from inferioposterially, this 3D CT scan reveals a large tuberosity fracture fragment malrotated in the sagittal plane C, 3D CT scan seen from posteriomedial viewpoint demonstrates pilon fracture of distal tibia in addition to calcaneal fracture
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Fig. 2. Artist's rendering of extremity and foot position within CT gantry. Dotted lines represent sequential xray beams emanating from CT gantry in a plane perpendicular to examination table. Note that vertical x-ray beams cross the subtalar and calcaneocuboid joints using this protocol, thereby maximizing the articular surface area to be scanned at both joints. This greatly enhances the quality of the 3D images. See text for other important aspects of scanning protocol.
feet was not recorded inasmuch as the crucial role of positioning of the extremity was not appreciated early in the study. With the supine alignment of the lower extremity, the foot is positioned between 45" and 90" with respect to the gantry. The gantry is coaxial with the x-ray beam intentionally so that the latter is perpendicular to the examination table. The x-ray beam is thereby oriented perpendicular or with substantial obliquity with respect to the subtalar and calcaneocuboid articular and fracture surfaces (Fig. 2). This maximizes the articular suiface area which is to be scanned at both joints, and thereby significantly improves the quality of the three dimensional images. For the two dimensional scans, the cuts were contiguous, varying in thickness between 3 and 10 mm. A standard soft tissue algorithm was used for this study. In all but two patients the x-ray beam was aligned perpendicular to the table. In the other two cases the x-ray beam was tilted approximately 20' with respect to the table. For the three dimensional reformations of the images either a GE (General Electric, Milwaukee, WI) 9800 unit or a Cemax 1500 model (Cemax, Inc., Fremont, CA) was employed. Three dimensional reformations of the 0s calcis were obtained in increments of 45' around all axes of the bone. RESULTS Conventional Radiographs
Scrutiny of conventional radiographs provides an underestimate of the degree of comminution and the magnitude of rotational displacement of the fracture surfaces as well as the extent of subtalar facetal depression. The resolution of the fracture lines is further obscured by the presence of a splint and the frequent
Fig. 3. Plain radiograph of a comminuted calcaneal fracture. Such x-rays do not convey a complete 3D appreciation of the fracture pattern.
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inability to obtain the properly angled radiographs (Fig. 3). 2D CT
Two dimensional CT scans are not significantly affected by incidental motion of the patient, the angle of the gantry, or the field size (field of view). The use of bony algorithm does enhance 2D CT visualization of bone and fracture surfaces; however, the use of a soft tissue algorithm in obtaining the 2D CT provides better data for 3D CT images. Alignment of the foot in a perpendicular to substantially oblique angle with respect to the vertical orientation of the x-ray beam is optimal for visualization of the extent of subtalar disruption. When the x-ray beam is perpendicular or oblique to the subtalar fracture surfaces (Fig. 2), the fracture is well visualized in the 2D CT coronal image (Fig. 4). When the x-ray beam is aligned parallel to the subtalar joint irrespective of the position of the foot (Fig. 5), the resolution of the 2D and 3D scans is markedly compromised particularly with thicker 2 0 cuts. In such cases the magnitude and orientation of joint depression is not subject to an accurate determination by scrutiny of the 2D CT. This is because it has been shown that x-rays provide more detailed information when the angle of the x-ray beam is oriented perpendicular to the area of interest. In calcaneus fractures the areas of greatest interest are the overall alignment of the subtalar and calcaneocuboid joints as well as the orientation and displacement of fracture fragments of the calcaneus itself. Because the subtalar and calcaneo-
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cuboid joints are nearly perpendicular to one another and because the subtalar joint itself exhibits an undulating surface with substantial variation of the plane of one facet in relation to the others, optimal visualization of all of the calcaneal facets and the calcaneocuboid joint is obtained with the orientation that has been described. This is especially important when we consider that a 3D CT can only be made from the data obtained from a single 2D CT scan. For this reason, it is not possible to use the information from two different CT scans made in perpendicular planes. When the thickness of the cut exceeds 5 mm, the resolution of the 2D and 3D views deteriorates markedly so that morphologic features including the magnitude of joint depression, the presence of fracture surfaces, and the degree of comminution are rendered obscure. The degree of comminution is well visualized in the 2D CT coronal views and the 3D scans obtained from these 2D CTs. Unlike the 3D scans in this study, the coronal 2D CTs can reveal the presence of intra-articular comminution that may not be evident on the 3D scan. Scrutiny of the conventional 2D CT scans, however, provides a gross overestimation of the magnitude of comminution, and an underestimation of the rotational malalignment in multiple planes, as well as the presence and extent of joint depression. Since the completion of this study, both authors have independently been able to significantly improve upon the quality of 3D imaging of intra-articular comminution (see Fig.
8).
Fig. 4. Coronal 2D CT evaluation of bilateral comminuted calcaneal fractures. Fracture pattern can be well visualized with this type of image. While this 2D CT was produced with a soft tissue algorithm, a bony algorithm is preferredfor clearer 2D CT visualization of fracture morphology. The soft tissue algorithm however, provides better data for 3D images than does the bony algorithm.
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Fig. 5. The x-ray beam in this scout film is oriented parallel to long axis of foot. The 2D and 3D CT scans that result will be less than optimal for visualization of important subtalar joint irregularities.
3D CT
In a comparison of the morphologic features observed by a scrutiny of conventional radiographs and 2D and 3D scans, the 3D CT scans performed with appropriate protocol provide for superior resolution and excellent visualization. The quality of the 3D CT image is markedly influenced by the thickness of the cuts, the angle of the gantry, the motion of the patient, the position of the foot relative to the angle of the gantry, the field size, and the sophistication of the software program. The most important factors are cut thickness, field size, patient motion, and gantry tilt (Fig. 6). In addition, using a soft tissue algorithm in the 2D CT seems to enhance the quality of the 3D image. An increase in the thickness of the cut beyond 5 mm noticeably accentuates the alias or linear steppage artifact in the 3D scans. When the cut thickness is less
than 4 mm, the quality of the scans improves significantly. Tilting the gantry provokes a substantial degree of distortion in the 3D CT. This is a clinical observation that became apparent as the study progressed. In one case, an alias artifact combined with faulty orientation of the x-ray beam culminated in an irregular and distorted image with an apparent lengthening and flattening of the bones of the foot (Fig. 6A). Motion of the patient provokes an obvious disruption in the natural contour of the bone which adversely affects the quality of the 3D scan (Fig. 6B). Limiting the field size of the 2D CT scan to the area immediately surrounding the foot significantly improves the resolution of the 3D scan. With such a decrease in the area of view, the computer possesses more raw data per unit area with which to generate a detailed image. In a 3D scan, bone of insufficient thickness or density is erased from the image. In some cases this osteopenic appearance aids in an estimation of the cortical thickness and corresponding strength of the bone. Typically the resolution of fractured surfaces in the 3D scans is about 2 mm, or the width of an equivalent fracture gap. Occasionally, an undisplaced fracture line was visualized which was indicative of a resolution of approximately 1 mm (Fig. 6B). This degree of resolution compares favorably with that achieved previously in 3D imaging studies of pelvic and acetabular fracture^.^ Two dimensional CT scans and conventional radiographs are hampered by their inability to fully convey information relative to the vectors of rotational deformity of the fracture fragments. Such techniques are only capable of representing fractures in a single plane with any given image. By contrast, the magnitude of comminution, the degree of rotation, and the extent of joint depression can be well visualized with the 3D scans, especially when the above described protocol is employed (Fig. 7). The optimal resolution of a 3D CT scan is achieved when the resultant image of the 0s calcis is aligned and rotated in a plane perpendicular to the linear axis of the x-ray beam. Although the rotational deformity of the fracture documented in other planes is not as clearly resolved, it is still substantially evident. A careful study of the various 3D CT images of the foot accrued in multiple planes of rotation provides an enormous help for the observer to create an accurate three dimensional mental image of the complex geometry of a comminuted and displaced calcaneal fracture (Figs. 7 and 8). While previous workers have attempted to achieve a comparable degree of information by resorting to 2 0 CT sagittal reconstructions, this method
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Fig. 6. Examples of artifact in 3D CT scans. A , tilt and alias (linear steppage) artifacts. A gantry tilt of 20' results in apparent lengthening and flattening of the bones of the foot. The choppy appearance is due to a thickness of 7 mm in the 2D CT cuts. The view is from lateral and anteriosuperior perspective. B,motion artifact. Posteriorand slightly inferiolateral view of a tuberosity fracture. Horizontalband across the bone results from motion of the patient during the 2 0 CT scan. Note visualization of the vertical nondisplaced tuberosity fracture line.
is inadequate when compared to the application of an optimally performed 3D CT scan. The available 3 0 CT scans accrued from this preliminary investigation define morphological features of calcaneal fractures that are generally consistent with previous classification schemes with one exception. Whereas previous classification systems account for rotational deformity primarily in one and sometimes in two planes, the 3D CT scans in this study provide an accurate representation of the deformity in three planes. This improvement provides a clear mental image of the multiplanar malrotation of the fracture frag-
ments, and greatly facilitates an attempt to restore the anatomic configuration of the bone during an open reduction. Computer-assistedconstruction of three dimensional plastic models and videotaped presentations of the fractured calcaneus is another option which may aid the surgeon in the evaluation and treatment of a complex fracture. This can be accomplished with current 3 0 CT technology and was employed in selected cases in this study. In such models, the calcaneus can be disarticulated from the talus and cuboid to visualize intra-articular distortion.
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Fig. 7. Comminuted calcaneal fracture in a cadaveric specimen visualized in 3D CT scan performed with optimum protocol. Views are seen in multiple planes of rotation. A , superior view. Note oblique tuberosity fracture. B , lateral and, C, inferiolateralviews. Note subtalar joint subluxation and calcaneocuboid impaction and subluxation. D , inferiomedial view displays sustentacular and tuberosity fractures.
DISCUSSION Conventional Radiographs
The principal limitations associated with the use of conventional radiographs include the compromise of resolution documented in the presence of a splint or bulky dressing, the prolongation of the study needed to obtain the essential multiple views, and the intrinsic limitations in resolution of a three dimensional image that are consistent with the method. Even Broden’s views, often of value in a nonacute setting, are frequently difficult to obtain and uncomfortable for the patient in the acute and multiple trauma setting. 2D CT Scans
In the preparation of a 2D CT scan, substantial artifactual features arise in the subsequent 3D CT images unless the foot is positioned carefully with respect to the gantry. Also, the use of cuts thicker than 4 mm markedly compromises the quality of both the 2D and 3D CT images. Other factors that do not adversely affect the quality of the 2D CT images are the angle of the gantry and the field size. These factors, along with motion of the patient, do affect the quality
of the 3D CT scan. The type of algorithm used affects 2 0 and 3D images in opposite ways, with a soft tissue algorithm providing high quality 3D images. While a study of the 2D CT scans reveals the presence of intra-articular comminution, it does not provide an accurate indication of the magnitude of the injury, the degree of joint depression, or the extent of rotational malalignment (multiplanar malrotation). Typically, a study of 2D CT scans provides an overestimation of the degree of comminution while a comparable scrutiny of conventional radiographs provides an underestimation. Such 2D techniques can only illustrate images in a single plane parallel with the x-ray beam or gantry. The resultant plane of visualization, especially in the 2D CT image is essentially that of a single straight line through multiple undulating surfaces (the fractured subtalar facets, calcaneal body and neck, and the calcaneocuboid joint). Such images can give a false impression of the degree of joint depression and multiplanar malrotation, especially to the inexperienced viewer. This is commonly substantiated clinically when openly reducing and internally fixing a malrotated joint depression calcaneus fracture: one often finds that a
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large portion of the posterior facet has been sagittally fractured, impacted, and anteriorly rotated often up to 90". The appreciation of this rotation is often sorely missing from a standard 2D CT scan with coronal cuts and poorly visualized on standard sagittal 2D CT reconstructions. With regard to overestimation of intra-articular comminution in 2D CT scans, it is apparent that when a straight line transects a highly undulating surface such as the subtalar joint, it is not possible to determine if the points transected by that line remain contiguous on the undulating surface or if they are actually separated points (i.e., malrotated fracture fragments). This becomes easier to visualize if one draws a straight line through an undulating line. 3D CT Scans
For the preparation of a high resolution 3 0 CT scan, a special protocol was developed which pertains to the procurement of the corresponding 2D CT scan. The presence of a fiberglass or plaster splint does not detract from the quality of the 3D CT image inasmuch as the splint is routinely subtracted from the 3D image. Enlisting the cooperation of radiologists and x-ray technicians is important if the protocol to obtain high quality scans is to be followed properly. Likewise, high quality 3D hardware and software packages are essential for the production of high resolution images. At the time of this study, when the special protocol was followed, the corresponding 3D reformations were superior to the 2D CT scans in all but two parameters. The visualization of intra-articular comminution was not optimal. Likewise, the superimposition of the talus and the cuboid can obscure the articular surfaces of the 0s calcis. Currently these problems are being addressed by the subtraction of adjacent bony surfaces as part of the data manipulation by computer and software packages.' When this solution was employed for 3D CT studies of pelvic and acetabular fractures, it was shown to be extremely e f f e c t i ~ e .Since ~ . ~ the completion of this study, both authors have independently and successfully employed such techniques to visualize calcaneal fractures using such advanced software packages as those offered by the ISG system (ISG Technologies, Mississauga, Ontario, Canada) (Fig. 8). In all other respects, high quality 3D CTs provide information that greatly enhances information gained from conventional radiographs and 2 0 CT scans. This is the case particularly with reference to visualization of the sites and magnitude of joint depression, the degree and vector of rotational malalignment in multiple planes, and the degree of comminution. In contrast to other techniques, 3D CT images viewed in multiple
planes essentially show the entire fracture as it exists in vivo (Figs. 7 and 8). The greater the number of 2D CT images made, the larger is the radiation exposure to the patient. There is no additional radiation exposure in obtaining a 3D CT beyond that in obtaining the 2D CT because the 3D images are generated from the same computer data used to form the 2D images. While attempts to employ 1- or 2-mm cuts are notable for the preparation of 3D scans of high quality, the application of 3-mm cuts is the optimal compromise in this study. This format yields high resolution, yet also provides for a lesser degree of radiation exposure and a briefer period of immobilization of the patient on the examination table than would be required for more narrow image slices. These cuts should be contiguous to maximize data retrieval and image quality. The advent of this series of 3D CT scans presents a new feature of calcaneal fractures not previously well defined in current classification systems: an accurate visual representation of the degree and vector of rotational malalignment in multiple planes, or multiplanar malrotation. CONCLUSIONS
A highly specific 3D imaging protocol has been devised which for the first time provides an excellent diagnostic tool in the assessment of a displaced, comminuted, and malrotated calcaneal fracture. The principal features of the protocol for the preparation of the corresponding 2D CT scan are: 1. the use of contiguous cuts with a thickness of 3mm, 2. the minimization of a motion artifact by an appropriate and comfortable form of immobilization, 3. the alignment of the foot and subtalar joint surfaces in a position oblique to perpendicular relative to the axis of the x-ray beam, 4. the orientation of the x-ray beam perpendicular to the examination table, 5. the use of a standard soft tissue algorithm, 6. the minimization of the field size, and 7. the use of current software programs and high quality computer packages.
The use of a conventional radiographic evaluation of a calcaneal fracture supplemented by a conventional 2D CT scan of optimal protocol can possess limitations referable to the definition of the degree and vector of rotational deformity and the magnitude of comminution and joint depression. When such studies are supplemented by high quality 3D CT images, the information gleaned is often significantly improved.
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Centers without such imaging capabilities will obviously continue to make maximum use of more standard modalities or send 2D CT computer tapes to centers with 3D imaging facilities if the patient’s clinical situation warrants and permits this. The less familiar and experienced one is with complex calcaneal fractures, the greater is the potential information gain from 3D imaging techniques. Even the experienced surgeon may find that he or she could gain increased appreciation of fracture anatomy with such techniques, as has been demonstrated in studies of the pelvis and other anatomic regions. In this study, 30 calcaneal fractures including one cadaveric specimen were evaluated by conventional plain radiographs as well as 2D and 3D CT scans. A special technique for the preparation of 3D CT images of uniformly high quality was devised. In a comparison of the 30 CT reformations with the previous radiographs and 2D CT images, the former views greatly enhanced the critical evaluation of the fractures especially with reference to the magnitude of joint depression, the extent of comminution and the degree and vector of rotational malalignment (multiplanar malrotation). This study demonstrates the usefulness of 3D CT scans in visualizing a calcaneal fracture. This information highlights the rotational displacement of major fracture fragments and facilitates the preparation of an appropriate surgical approach and reconstruction. ACKNOWLEDGMENTS
Dr. Allon gratefully acknowledges the support of his wife, Gloria A. Allon, R.N., and his parents, Meyer Michael and Marcella Allon, in the preparation of this study. REFERENCES 1. Adler, S.J., Vannier, M.W., Gilula, L.A., and Knapp, R.H.: Threedimensional computed tomography of the foot: optimizing the image. Comput. Med. Imaging Graph., 12(1):59-66, 1988. 2. Biondetti, P.R., Vannier, M.W., Gilula, L.A., and Knapp, R.H.: Three dimensional surface reconstruction of the carpal bones
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