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Technique to detect migration of femoral components of total hip arthroplasties on conventional radiographs
Technique to Detect Migration of Femoral Components of Total Hip Arthroplasties on Conventional Radiographs
Richard D. Mulroy, Jr., MD,* Ronald C. Sedlacek, BS,* Daniel O. O'Connor, AS,* Daniel M. Estok II, MD,* and William H. Harris, MD t
Abstract: Critical to the evaluation of patients with painful total hip arthroplastics and to the assessment of the efficacy of fixation of new prosthetic designs and/or new insertion techniques is the ability to detect migration of the femoral components. Tile authors present a simple and effective technique using plain radiography that can detect migration of 1 mm with a high degree of accuracy. Key words: migration, subsidence, total hip arthroplasty, femoral component, Herbert screw, conventional radiograph
Radiographic evidence of migration of the femoral c o m p o n e n t is the most important diagnostic sign of femoral implant loosening in total hip arthroplasty (THA). In both cemented and cementless femoral components, migration of the stem in relation to the bone is definite evidence of femoral loosening. The critical problem in the assessment of femoral component migration and also in the accurate evaluation of radiolucent zones on conventional radiographs is the accurate repositioning of the femur (3). If tlle femur is not in the same rotation and flexion on each radiograph, accurate comparison is confounded. Because of the marked difficulty in accurately repositioning the femur for subsequent radiograph examinations, the m u c h more complex technique of roentgenographic stereophotogrammetry has been advocated to evaluate subsidence. While this latter method is highly accurate, it is expensive, requires From the *Orthopaedic Biomechanics Laboratory, and *the ttip and hnplant Unit, Department of Orthopaedic Surgery, Massachusetts General Hospital and *ttarvard Medical School, Boston, Massachusetts.
Supported by the William H. Harris Foundation, Boston, Massachusetts. Reprint requests: William H. Harris, MD, Department of Orthopaedic Surgery, Massachusetts General Hospital, Boston, MA 02114.
implantation of multiple metal beads in the femur as the fixed reference point, and is demanding and operator-sensitive (3, 5). Most investigators have found the technique to be too time-consuming and difficult for general use. For conventional radiographic techniques, Amstutz et al. (1) recommended superimposing a grid on the radiograph, centering the x-ray beam o n the femoral head, and standardizing rotation of the patient's leg in a jig as means of improving follow-up films so that changes in femoral position could be reduced and resolution could be increased. D e n h a m recommended using a metal slug placed in the femur as a fixed reference point (6). Although these techniques have improved the accuracy of the evaluation of subsidence to some degree, they are still less than satisfactory. It is important to be able to detect migration of the femoral component with greater accuracy, ease, and reproducibility than is n o w possible. We present a technique to reposition the femur reproducibly in taking conventional radiographs to enhance the accuracy of assessing migration of either cemented or cementless femoral implants on serial radiograph examinations.
$2
The Journal of Arthroplasty Vol. 6 Supplement October 1991
Materials and Methods Central to this technique is the use of a femoral c o m p o n e n t that contains an extraction hole that has a cylindrical contour and passes transversely through the stem in the anteroposterior (AP) direction. Under fluoroscopic guidance, the operated leg is rotated and/or flexed or extended until the extraction hole is seen as a perfect circle (Fig. 1), similar to the technique c o m m o n l y used during the placement of distal screws for interlocking intramedullary rods (4). In order to have a fixed reference point in the femur, after insertion of the femoral prosthesis, a Herbert screw (Zimmer) is placed in the greater trochanter in the plane of the femoral prosthesis. Placem e n t of the screw in a standardized and reproducible fashion is facilitated by use of a modified Huene alignment guide. The modification is a jig, fabricated
It
.
Fig. 1. AP radiograph ofa THA containing a femoral component with a transverse hole in the proximal portion. By aligning the hole so that it forms a perfect circle, the rotation and flexion-extension of the femoral component becomes accurately defined in a way that is highly reproducible. The Herbert screw in the greater trochanter is a permanent reference marker and cannot back out because of its unique thread configuration.
Fig. 2. The alignment jig used to insert the Herbert screw in the greater trochanter. The jig places the screw in the midcoronal plane of the femoral component. of stainless steel (Fig. 2), which inserts into the extraction hole of the femoral prosthesis and attaches to the Huene guide. Use of this assembly assures the accurate placement of the screw using the standard Herbert screw instrumentation. A short ( 1 4 - m m ) screw is used to avoid contact b e t w e e n the screw and either the cement or prosthesis. The Herbert screw, with its special threaded features, has a n advantage over use of a bead or slug, which could migrate. In addition, the absence of a screw head is advantageous in preventing a trochanteric bursitis. Prior to discharge of the patient, an anteroposterior (AP) film is taken of the operated hip using the technique described above. The x-ray b e a m is centered over the hole in the prosthesis to prevent distortion, and the kV and m a are recorded. On successive follow-up examinations, films of the patient's hip are obtained with the same technique, such that the hole in the femoral prosthesis is once again a perfect circle. One can correct for any changes in radiograph magnifications by measuring femoral head diameter on serial films. To evaluate two serial films, a vertical line is d r a w n through the midportion of, the femoral shaft (Fig. 3). A line perpendicular to this midfemoral axis is d r a w n from the tip of the Herbert screw to the intersection with the midfemoral axis. A second line is d r a w n perpendicular to the midfemoral axis from a n y fixed point on the prosthesis, usually the top edge of the extraction hole. A vertical m e a s u r e m e n t is t h e n made b e t w e e n the intersects of these two lines, along the midfemoral axis. Any change in this vertical distance or in the position of the prosthesis relative to the screw indicates migration. To test for small
Radiographic Technique to Detect Femoral Component Migration 9 Mulroy et al. Femoral axis
I
eosure distance between line A1-A2ond B1-B 2
$3
fixed distance from the x-ray source such that the cylindrical extraction hole was perpendicular to the x-ray beam. A baseline radiograph was taken. The prosthesis f e m u r complex was then rotated or flexed in relation to the x-ray b e a m on its m o u n t i n g such that radiographs were taken with the extraction hole in the prosthesis at varying degrees of rotation and flexion. To assess the sensitivity of the method, k n o w n differences of 1 m m in subsidence are generated in cadaver femurs and in artificial femurs m a d e from c o m pressed sawdust. A cementless femoral implant w a s placed at a fixed distance from the calcar, m e a s u r e d from a pin fixed in the anterior femoral cortex and a pin placed in the hole in the prosthesis. The distance between the pins was varied by impacting the prosthesis into the femur, changing the initial distance in increments of 1 m m over a 6 - m m range. At each n e w location of the prosthesis in the f e m u r an AP film was taken with the b e a m centered at the hole. A blinded observer assessed t h e subsidence by measuring the films. Twenty such pairs of film were studied.
Results
Fig. 3..Drawing of an AP radiograph, illustrating how measurements are made to detect subsidence. changes in position of the f e m u r in both rotation and flexion, a cementless prosthesis with a cylindrical extraction hole was placed using standard technique in an artificial f e m u r m a d e of compressed sawdust. The prosthesis f e m u r complex was then m o u n t e d a
The cylindrical hole provides an accurate index of position of the femur in both rotation and flexion. Figure 4 (A, B) shows the distortion of the image of the hole in successive rotations of only 3 ~ of a f e m u r containing such a prosthesis. Figure 5 demonstrates the contour of the hole b e t w e e n 10 ~ of internal rotation and 10 ~ of external rotation and b e t w e e n 10 ~ of flexion and 10 ~ of extension. Although rotational displacement is c o m m o n and important, the m e a s u r e m e n t of small a m o u n t s of ro-
Fig. 4. AP radiographs of a femoral component inserted in a sawbones (anatomical model, Pacific Research Laboratories, Vashan Island, WA). (A) Position of the limb for the correct radiograph. Note that the upper hole appears as a perfect circle. Prior to taking the radiograph shown in (B), the femur was rotated only 3~ into external rotation. The upper hole is no longer a perfect circle. Three degrees of malrotation are readily detectable.
$4
The Journal of Arthroplasty Vol. 6 Supplement October 1991
EXTERNAL ROTATION
"11
//
HYPEREXTENSION
Fig. 5. Composite illustrations showing the deviation of the appearance of the hole at 5~ and 10~ of malposition compared to a perfect circle (center). This shows the sensitivity of the repositioning of the femoral component for these radiographic examinations. It also makes it far more accurate in determining changes in radiolucent zones, since malrotation is either sharply reduced or eliminated as a cause of changes in the appearance of the radiolucent zones.
for either cementless or cemented femoral c o m p o nents. The ability to measure small amounts of subsidence is critical to the evaluation of the fixation of all femoral components and especially critical in assessment of new designs, evolution of n e w fixation techniques, and study of custom components. This new technique is of particular benefit in assessing the stability of cementless components since the diagnosis of small amounts of migration of these components on plain films is extremely difficult. The placement of the Herbert screw in the greater trochanter serves as a fixed reference point in the femur. The use of this technique requires the prosthesis to have an extraction hole. Currently, some manufacturers do not place an extraction hole in their designs. They should strongly consider adding this feature in the future so that this technique can be utilized. Although stereophotogrammography (3, 5) has a higher resolution than this technique, its cost and complexity and the difficulty of maintaining the expensive and observer-dependent capability for large numbers of follow-up examinations over 1 0 - 2 0 years make its widespread use problematic. In contrast, this new approach is a simple m e t h o d that is widely available.
References tational migration on a frog lateral or true lateral radiograph is less satisfactory. While this technique is most valuable in the coronal plane, some information about rotational micromotion is provided by two observations. First, if magnification is corrected, any change in the length of tile screw in a radiograph in which the cylindrical hole is a perfect circle indicates rotational migration. Second, if the outline of the femur differs from the appearance on the prior film but the outline of the prosthesis is exact in the two films, rotational micromotion has occurred. The reading error for this new technique to evaluate femoral stem subsidence was 0.24 m m (range, 0.01-0.68). Six additional series of films were reviewed by three blinded observers. T w o - w a y analysis of variance found n o significant difference among the three examiners.
Discussion Radiographic demonstration of subsidence of the femoral c o m p o n e n t is definite evidence of migration
1. Amstutz HC, Ouzounian T, Grauer D et al: The grid radiograph: a simple technique for consistent high-resolution visualization of the hip. J Bone Joint Surg 68A:1052, 1986 2. Bell AL, Brand RA: Roentgenographic changes in proximal femoral dimensions due to hip rotation. Clin Orthop 240:194, 1989 3. Chafetz N, Baumrind S, Murray WR, Genant HK: Femoral prosthesis subsidence in asymptomatic patients: a stereophotogrammetric assessment. " Invest Radiol 19:235, 1984 4. Medoff RJ: Insertion of tile distal screw in interlocking nail fixation of femoral shaft fractures. J Bone Joint Surg 68A:1275, 1986 5. Mjoberg B, Selvik G, Hansson LI et al: Mechanical loosening of total hip prostheses: a radiographic and roentgen stereophotogrammetric study. J Bone Joint Surg 68B:770, 1986 6. Paterson M, Fulford P, Denham R: Loosening of the femoral component after total hip replacement: the thin black line and the sinking'hip. J Bone Joint Surg 68B:392, 1986
Richard D. Mulroy, Jr., MD,* Ronald C. Sedlacek, BS,* Daniel O. O'Connor, AS,* Daniel M. Estok II, MD,* and William H. Harris, MD t
Abstract: Critical to the evaluation of patients with painful total hip arthroplastics and to the assessment of the efficacy of fixation of new prosthetic designs and/or new insertion techniques is the ability to detect migration of the femoral components. Tile authors present a simple and effective technique using plain radiography that can detect migration of 1 mm with a high degree of accuracy. Key words: migration, subsidence, total hip arthroplasty, femoral component, Herbert screw, conventional radiograph
Radiographic evidence of migration of the femoral c o m p o n e n t is the most important diagnostic sign of femoral implant loosening in total hip arthroplasty (THA). In both cemented and cementless femoral components, migration of the stem in relation to the bone is definite evidence of femoral loosening. The critical problem in the assessment of femoral component migration and also in the accurate evaluation of radiolucent zones on conventional radiographs is the accurate repositioning of the femur (3). If tlle femur is not in the same rotation and flexion on each radiograph, accurate comparison is confounded. Because of the marked difficulty in accurately repositioning the femur for subsequent radiograph examinations, the m u c h more complex technique of roentgenographic stereophotogrammetry has been advocated to evaluate subsidence. While this latter method is highly accurate, it is expensive, requires From the *Orthopaedic Biomechanics Laboratory, and *the ttip and hnplant Unit, Department of Orthopaedic Surgery, Massachusetts General Hospital and *ttarvard Medical School, Boston, Massachusetts.
Supported by the William H. Harris Foundation, Boston, Massachusetts. Reprint requests: William H. Harris, MD, Department of Orthopaedic Surgery, Massachusetts General Hospital, Boston, MA 02114.
implantation of multiple metal beads in the femur as the fixed reference point, and is demanding and operator-sensitive (3, 5). Most investigators have found the technique to be too time-consuming and difficult for general use. For conventional radiographic techniques, Amstutz et al. (1) recommended superimposing a grid on the radiograph, centering the x-ray beam o n the femoral head, and standardizing rotation of the patient's leg in a jig as means of improving follow-up films so that changes in femoral position could be reduced and resolution could be increased. D e n h a m recommended using a metal slug placed in the femur as a fixed reference point (6). Although these techniques have improved the accuracy of the evaluation of subsidence to some degree, they are still less than satisfactory. It is important to be able to detect migration of the femoral component with greater accuracy, ease, and reproducibility than is n o w possible. We present a technique to reposition the femur reproducibly in taking conventional radiographs to enhance the accuracy of assessing migration of either cemented or cementless femoral implants on serial radiograph examinations.
$2
The Journal of Arthroplasty Vol. 6 Supplement October 1991
Materials and Methods Central to this technique is the use of a femoral c o m p o n e n t that contains an extraction hole that has a cylindrical contour and passes transversely through the stem in the anteroposterior (AP) direction. Under fluoroscopic guidance, the operated leg is rotated and/or flexed or extended until the extraction hole is seen as a perfect circle (Fig. 1), similar to the technique c o m m o n l y used during the placement of distal screws for interlocking intramedullary rods (4). In order to have a fixed reference point in the femur, after insertion of the femoral prosthesis, a Herbert screw (Zimmer) is placed in the greater trochanter in the plane of the femoral prosthesis. Placem e n t of the screw in a standardized and reproducible fashion is facilitated by use of a modified Huene alignment guide. The modification is a jig, fabricated
It
.
Fig. 1. AP radiograph ofa THA containing a femoral component with a transverse hole in the proximal portion. By aligning the hole so that it forms a perfect circle, the rotation and flexion-extension of the femoral component becomes accurately defined in a way that is highly reproducible. The Herbert screw in the greater trochanter is a permanent reference marker and cannot back out because of its unique thread configuration.
Fig. 2. The alignment jig used to insert the Herbert screw in the greater trochanter. The jig places the screw in the midcoronal plane of the femoral component. of stainless steel (Fig. 2), which inserts into the extraction hole of the femoral prosthesis and attaches to the Huene guide. Use of this assembly assures the accurate placement of the screw using the standard Herbert screw instrumentation. A short ( 1 4 - m m ) screw is used to avoid contact b e t w e e n the screw and either the cement or prosthesis. The Herbert screw, with its special threaded features, has a n advantage over use of a bead or slug, which could migrate. In addition, the absence of a screw head is advantageous in preventing a trochanteric bursitis. Prior to discharge of the patient, an anteroposterior (AP) film is taken of the operated hip using the technique described above. The x-ray b e a m is centered over the hole in the prosthesis to prevent distortion, and the kV and m a are recorded. On successive follow-up examinations, films of the patient's hip are obtained with the same technique, such that the hole in the femoral prosthesis is once again a perfect circle. One can correct for any changes in radiograph magnifications by measuring femoral head diameter on serial films. To evaluate two serial films, a vertical line is d r a w n through the midportion of, the femoral shaft (Fig. 3). A line perpendicular to this midfemoral axis is d r a w n from the tip of the Herbert screw to the intersection with the midfemoral axis. A second line is d r a w n perpendicular to the midfemoral axis from a n y fixed point on the prosthesis, usually the top edge of the extraction hole. A vertical m e a s u r e m e n t is t h e n made b e t w e e n the intersects of these two lines, along the midfemoral axis. Any change in this vertical distance or in the position of the prosthesis relative to the screw indicates migration. To test for small
Radiographic Technique to Detect Femoral Component Migration 9 Mulroy et al. Femoral axis
I
eosure distance between line A1-A2ond B1-B 2
$3
fixed distance from the x-ray source such that the cylindrical extraction hole was perpendicular to the x-ray beam. A baseline radiograph was taken. The prosthesis f e m u r complex was then rotated or flexed in relation to the x-ray b e a m on its m o u n t i n g such that radiographs were taken with the extraction hole in the prosthesis at varying degrees of rotation and flexion. To assess the sensitivity of the method, k n o w n differences of 1 m m in subsidence are generated in cadaver femurs and in artificial femurs m a d e from c o m pressed sawdust. A cementless femoral implant w a s placed at a fixed distance from the calcar, m e a s u r e d from a pin fixed in the anterior femoral cortex and a pin placed in the hole in the prosthesis. The distance between the pins was varied by impacting the prosthesis into the femur, changing the initial distance in increments of 1 m m over a 6 - m m range. At each n e w location of the prosthesis in the f e m u r an AP film was taken with the b e a m centered at the hole. A blinded observer assessed t h e subsidence by measuring the films. Twenty such pairs of film were studied.
Results
Fig. 3..Drawing of an AP radiograph, illustrating how measurements are made to detect subsidence. changes in position of the f e m u r in both rotation and flexion, a cementless prosthesis with a cylindrical extraction hole was placed using standard technique in an artificial f e m u r m a d e of compressed sawdust. The prosthesis f e m u r complex was then m o u n t e d a
The cylindrical hole provides an accurate index of position of the femur in both rotation and flexion. Figure 4 (A, B) shows the distortion of the image of the hole in successive rotations of only 3 ~ of a f e m u r containing such a prosthesis. Figure 5 demonstrates the contour of the hole b e t w e e n 10 ~ of internal rotation and 10 ~ of external rotation and b e t w e e n 10 ~ of flexion and 10 ~ of extension. Although rotational displacement is c o m m o n and important, the m e a s u r e m e n t of small a m o u n t s of ro-
Fig. 4. AP radiographs of a femoral component inserted in a sawbones (anatomical model, Pacific Research Laboratories, Vashan Island, WA). (A) Position of the limb for the correct radiograph. Note that the upper hole appears as a perfect circle. Prior to taking the radiograph shown in (B), the femur was rotated only 3~ into external rotation. The upper hole is no longer a perfect circle. Three degrees of malrotation are readily detectable.
$4
The Journal of Arthroplasty Vol. 6 Supplement October 1991
EXTERNAL ROTATION
"11
//
HYPEREXTENSION
Fig. 5. Composite illustrations showing the deviation of the appearance of the hole at 5~ and 10~ of malposition compared to a perfect circle (center). This shows the sensitivity of the repositioning of the femoral component for these radiographic examinations. It also makes it far more accurate in determining changes in radiolucent zones, since malrotation is either sharply reduced or eliminated as a cause of changes in the appearance of the radiolucent zones.
for either cementless or cemented femoral c o m p o nents. The ability to measure small amounts of subsidence is critical to the evaluation of the fixation of all femoral components and especially critical in assessment of new designs, evolution of n e w fixation techniques, and study of custom components. This new technique is of particular benefit in assessing the stability of cementless components since the diagnosis of small amounts of migration of these components on plain films is extremely difficult. The placement of the Herbert screw in the greater trochanter serves as a fixed reference point in the femur. The use of this technique requires the prosthesis to have an extraction hole. Currently, some manufacturers do not place an extraction hole in their designs. They should strongly consider adding this feature in the future so that this technique can be utilized. Although stereophotogrammography (3, 5) has a higher resolution than this technique, its cost and complexity and the difficulty of maintaining the expensive and observer-dependent capability for large numbers of follow-up examinations over 1 0 - 2 0 years make its widespread use problematic. In contrast, this new approach is a simple m e t h o d that is widely available.
References tational migration on a frog lateral or true lateral radiograph is less satisfactory. While this technique is most valuable in the coronal plane, some information about rotational micromotion is provided by two observations. First, if magnification is corrected, any change in the length of tile screw in a radiograph in which the cylindrical hole is a perfect circle indicates rotational migration. Second, if the outline of the femur differs from the appearance on the prior film but the outline of the prosthesis is exact in the two films, rotational micromotion has occurred. The reading error for this new technique to evaluate femoral stem subsidence was 0.24 m m (range, 0.01-0.68). Six additional series of films were reviewed by three blinded observers. T w o - w a y analysis of variance found n o significant difference among the three examiners.
Discussion Radiographic demonstration of subsidence of the femoral c o m p o n e n t is definite evidence of migration
1. Amstutz HC, Ouzounian T, Grauer D et al: The grid radiograph: a simple technique for consistent high-resolution visualization of the hip. J Bone Joint Surg 68A:1052, 1986 2. Bell AL, Brand RA: Roentgenographic changes in proximal femoral dimensions due to hip rotation. Clin Orthop 240:194, 1989 3. Chafetz N, Baumrind S, Murray WR, Genant HK: Femoral prosthesis subsidence in asymptomatic patients: a stereophotogrammetric assessment. " Invest Radiol 19:235, 1984 4. Medoff RJ: Insertion of tile distal screw in interlocking nail fixation of femoral shaft fractures. J Bone Joint Surg 68A:1275, 1986 5. Mjoberg B, Selvik G, Hansson LI et al: Mechanical loosening of total hip prostheses: a radiographic and roentgen stereophotogrammetric study. J Bone Joint Surg 68B:770, 1986 6. Paterson M, Fulford P, Denham R: Loosening of the femoral component after total hip replacement: the thin black line and the sinking'hip. J Bone Joint Surg 68B:392, 1986