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Effect of rotation on the radiographic appearance of the femoral canal
The Journal
of Arthroplasty
Vol. 9 No. 4 1994
Effect of Rotation on the Radiographic Appearance of the Femoral Canal Stephen G. J. Eckrich, MD, Philip C. Noble, MS, and Hugh S. Tullos, MD
Abstract: Radiographic templating is a key element of preoperative planning for cementless total hip arthroplasty, and it aids in the selection of an appropriate implant. Frequently, the radiographic projection of the proximal femur does not correspond to that of the femoral prosthesis on its template due to variations in patient positioning. This discrepancy is a potential source of error when predicting which femoral component will best fit within the femoral canal. To evaluate the effect of femoral rotation on the size and shape of its radiographic image, anteroposterior and lateral radiographs of 12 femora were prepared over a range of positions. Several medullary canal dimensions were measured for each projection. The changes in these dimensions were compared using the image at neutral rotation as a reference. Each femur was then implanted with an appropriately sized cementless prosthesis to determine its actual rotational orientation in the canal. On the anteroposterior projection, statistically significant changes in the width of the proximal canal with femoral rotation were noted. There was no statistically significant change in distal canal dimensions with rotation. On the lateral projection, the dimensions of the proximal canal changed significantly with internal rotation; however, external rotation had no effect on canal dimensions. In general, the magnitude and direction of the canal dimensions were highly variable. The final rotational orientation of the femoral component in the canal was quite variable with respect to the plane of the femoral neck. Errors in implant selection may be due to excessive reliance on preoperative ternplating, which can be misleading because of femoral rotation. Key words: radiographic templating, femoral rotation, cementless arthroplasty, projection.
Accurate radiographs play a fundamental role in the preoperative planning of total hip arthroplasty, which requires standardized radiographic views of the hip joint and the selection of appropriate prosthetic components. ’ ,’ Selecting an appropriate prosthesis involves overlaying templates of each prosthesis on the preoperative radiographic image of the proximal femur. In cemented femoral components, some component/canal mismatch is tolerated since the cement is intended to fill the gap between the prosthesis and medullary canal. However, ceFrom the Department cine, Houston, Texas.
of Orthopedic
Surgery,
Baylor
College
mentless femoral components do not allow this latitude because stable fixation of these implants depends on an intimate fit between the prosthesis and endosteal surface of the femoral cana1.2-4 It is often difficult to predict which cementless prosthesis provides the best fit to the femur on the basis of standard clinical radiographs, principally, because the femur is normally projected on its template in some degree of rotation with respect to the plane of the femoral prosthesis. To provide proper orientation on the anteroposterior (AP) projection, the proximal femur must be positioned so that the long axis of the femoral neck is parallel to the radiographic cassette and perpendicular to the x-ray beam. Similarly, for a correct lateral projection, the long axis of the femoral neck should be perpendicular to the
ofMedi-
Reprint requests: Stephen G. J. Eckrich, MD, Department of Orthopedic Surgery, Baylor College of Medicine, 6550 Fannin, Suite 2625, Houston, TX 77030.
419
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The Journal
of Arthroplasty
Vol. 9 No. 4 August
and parallel to the x-ray beam. To obtain this projection, the femur must be internally rotated to compensate for the normal anteversion of the head and neck. Frequently, the femur is externally rotated, either because the radiographic technique did not adequately compensate for femoral torsion or many patients suffering from arthritis of the hip joint are unable to internally rotate the extremity. Additionally, most radiographic templates have been designed on the assumption that the widest mediolateral dimension of the proximal canal is visualized on this AP projection, and that the prosthesis will ultimately lit in the canal in this orientation. These assumptions may be incorrect for a number of reasons. First, the widest dimension of the canal is often not visible ou the true AP radiograph.5 Second, the orientation of any cementless prosthesis implanted within the femur will be determined by other complex factors related to overall canal shape. The purposes of this study are to determine both the effect of femoral rotation on the radiographic dimensions of the proximal femur and the variation in femoral rotation that can be tolerated before the accuracy of femoral templating is significantly impaired. cassette
Materials
and
Methods
Previously, Noble et a1.6 showed that the most accurate single index of the overall size of the medullary canal was its mediolateral width, 20 mm distal to the lesser trochanter. Consequently, this measurement was used to select femora for this study. Twelve femora were selected, whose canal widths ranged from 16 to 27 mm and were 20 mm distal to the lesser trochanter. Femora were selected if they had no evidence of previous trauma, malignancy, or severe osteoporosis. A pin was placed through the center of the femoral head and the midpoint of the femoral neck at its narrowest point for radiographic measurement. Anteroposterior, lateral, and axial radiographs confirmed proper pin position. A spheric radiographic marker was glued to the tip of the lesser trochanter to provide a standard vertical reference on each radiograph. Each femur was mounted in a jig that allowed it to be rotated about its anatomic axis, defined by a line passing through both the midpoint of the bicondylar axis distally and a point just medial to the greater trochanter, approximately at the piriformis fossas, proximally. Rotation of the femur in this apparatus could be determined to + 0.5”. A standard radiographic cassette was mounted vertically, parallel to both the pin within the femoral neck and the intramedullary axis of the femoral shaft. This orientation
1994
was defined as neutral rotation, and was used as the reference projection for comparison with subsequent AP and lateral views of the femoral canal. The tubeto-femur-to-film distance was adjusted to provide a radiographic magnification of 15%, a standard used on many ternplating systems. Following correct positioning of the femur and apparatus, AP radiographs were taken in neutral rotation and at the following positions: 10” and 20” of internal rotation and lo”, 20”, 30”, and 40” of external rotation. This procedure was repeated for the lateral projection, with radiographs taken in neutral rotation and at 5”, lo”, and 15” of both internal and external rotation. Key endosteal dimensions were measured on each radiograph (Fig. 1). To ensure accurate comparison of the image of the femur on different radiographs, standardized axes were constructed using the following anatomic landmarks: ( 1) the femoral neck axis, defined by a line passing from the center of the femoral head to the center of the femoral neck at its narrowest point, and (2) the proximal and distal axes of the femoral canal, defined by lines that bisected the medullary canal at four midpoints placed at 2 cm intervals proximally and distally from the isthmus. The angle resulting from the intersection of these two lines determined the varus/valgus bow and the anterior bow on the AP and lateral radiographs, respectively. (The anterior bow of the femur coupled with femoral rotation creates a varus or valgus bow of the femoral shaft on the AP radiograph.) The change in femoral dimensions due to rotation, from the reference dimension on the AP and lateral radiographs taken at neutral rotation, was measured in either millimeters or degrees. The mean change in dimensions, measured for each incremental change in rotation, was statistically analyzed for significance using the Newman-Kuells test for comparison of multiple means. To determine the rotational orientation of a prosthesis within each femur, a cementless femoral prosthesis (Precision Osteolock, Howmedica, Rutherford, NJ) was implanted in each specimen following the manufacturer’s recommended procedure. The rotational orientation of the prosthetic component with respect to the original femoral neck axis was determined from axial radiographs using a pin that passed from the most medial to the most lateral point on the stem. Results
(Tables
1, 2)
All of the anatomic dimensions studied changed following femoral rotation; however, the average changes in some of the dimensions were not statisti-
Radiographic
Appearance
of the Femoral Canal
Eckrich et al.
l
421
1. Key femoral dimensions on the anteroposterior and lateral radiographs.A, B, C, canalwidths of the proximal femur; D, canal width 60 mm below lesser trochanter; E, canal width at isthmus; p, varus/valgusbow angle; 4, anterior bow angle. Fig.
Table
1. Amount
of Change Change
Measured Dimension
in parentheses
Dimensions (Anteroposterior
Due to Internal Rotation
20
Varus/valgus bow angle (degrees) Width of canal 20 mm above lesser trochanter (mm I Width of canal at lesser trochanter (mm) Width of canal 20 mm below lesser trochanter (mm ) Width of canal 60 mm below les~r trochanter (mm) Width of canal at isthmus (mm) Numbers
in Femoral
Reference Value 0”
10”
~ l.S(O.4)
-0.8lO.4)
m-2.1(0.6)
- 0.9(0.4)
51.0
r
-0.8(0.3)
33.8
1.2(0.5)
From the Reference Projection)
Change 10”
0.1 ? 0.3
40” 3.9(0.5)
1.9
0.9(0.3)
O.S(O.5)
--0 l(O.6)
k
1.3
0.7(0.2)
1.2(0.3)
0.9(0.4)
1.2
0.6(0.2)
1 l(O.4)
1.7(0
5)
2 6(0.7)
O.O(O.3)
0.6(0
41
(1.9(0.5)
O.S(O.2)
0.9(0
3)
Z.l(O.5)
e
0.9(0.4)
0.5(0.3)
20.0
-+ 1.0
0.9(0.4)
0.6(0.4)
16.6
+ 0.8
changes
Rotation 30 2.9(0.5)
25.7
significant
to External
2.2(0.5)
O.l(O.3)
statistically
Due 20”
Rotation
1.2(0.4)
l.O(O.4)
indicate
Value Due to Femoral
(P < .05).
-0.1(0.2)
0.3(0.2)
- 19(1.0)
0 S(O.7)
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The Journal Table
~-
of Arthroplasty
Vol. 9 No. 4 August
2. Amount of Change in Femoral Dimensions From the Reference Value Due to Femoral Rotation (Lateral Projection) Change
Measured
1994
Dimension
Due
to Internal
15”
Reference Dimension
IO’
Antenor
O.S(O.2)
Canal
2.2(0.6)
bow (degrees) wtdth 20 mm above lesser trochanter (mm) Canal wdth at lesser trochanter (mm) Canal width 20 mm below lesser trochanter (mm) Canal width 60 mm below lesser trochanter (mm) Canal width at isthmu,
Rotation
-0.6(0.3) l.Y(U
5O
4)
Change
0’
39.1
+ 0.7
31.4
i
16.1
i
1.7(0.3)
U.9(0
1.7(0.5)
1.3(0.3)
0 6(0.3)
3)
IO External
5’
0.4(0.3) l.l(O 31
1.9(0.5)
7.9
Due
i
0.6
10
Rotation 15”
-O.i(O.l, 0.2(0
3)
0. I(0 2) 0 h(O.5,
~0.4lU.3) u 7(O.K)
0.7
U.5(0
3)
I) 010 5)
O.X((I 8)
0.7
U.U(O. I )
0 llU.4)
O.O(U.5)
-0.6(0.3)
-0.4(0.2)
-0.2(0.21
23.1
+ 0.7
U.UlO 2)
(1.3(I) II
0.3(0.3)
- 0.3(0.2)
-0
-01(O.l)
-72.5
-2 I 2
0 O(O.1)
0 ii0
U.X(O.2,
I(O.2)
1)
(mm) Numbers
in parentheses
indicate
statistically
significant
changes
tally significant, possibly due to the large variation in the magnitude of change between femora. Interand intrafemoral variability was pronounced in the dimensions of canal width, particularly in the proximal femur.
Extramedullary
Dimensions
On the AP view at neutral rotation, the varus/valgus bow of the femoral shaft was essentially straight (average, 0.1” varus). With internal rotation, there was a consistent trend toward apparent valgus angulation, where the apparent varus angulation of the canal increased by approximately 1’ for each 10” of rotation of the femur. This trend was statistically significant, even though the values were small. On the lateral view, the anterior bow of the femur did not change significantly with internal or external rotation. Rotations of 15” from neutral resulted in changes of lessthan 1” in the angulation of the proximal and distal canals.
lntramedullary
Dimensions
(Fig. 2)
Rotation had a highly variable effect on the radiographic dimensions of the femoral canal. This variability was greatest in the proximal femur and was less distally. At 20 mm proximal to the lesser trochanter, only 10” of internal rotation on the AP projection caused a reduction in canal width of 0.9 + 0.4 mm (P < .05). However, external rotation did not significantly affect this dimension until the femur was rotated 40”. On the lateral projection, internal rotation of 10” produced a decreasein apparent canal width of 1.9 ? 0.4 mm (P < .05), whereas external rotation of up to 15” had no effect on canal width. The effects of femoral rotation were similar at the
(P < .05)
level of the lesser trochanter. On the AP projection, canal width was decreasedby 1 mm with 10” of internal rotation, but was increased by 1.2 mm with 20” of external rotation. Conversely, on the lateral projection, 10” of internal rotation caused an apparent 1.7 mm increase in canal width, while external rotation to 15” had no significant effect on canal dimcnsions. At 20 mm below the lesser trochanter, internal rotation to 20” had no significant effect on apparent canal width on the AP radiographs, while external rotation produced an apparent increase in canal width of 1.1 mm at 20”, 1.7 mm at 30”, and 2.6 mm at 40”. On the lateral projection, canal width did not change significantly until 15” of internal rotation, where it had increased by an average of 1.7 mm. Again, external rotation had no significant effect on canal width. The width of the medullary canal at the isthmus did not change significantly over the range of internal and external rotation in this study. The mean canal width at the isthmus was greater on the lateral than on the AP projection (22.5 and 16.6 mm, respectively).
Component
Orientation
(Fig. 3)
All except one of the femoral components fit into the canal in some degree of external rotation with respect to the plane of the femoral neck, corresponding to increased anteversion of the prosthesis with respect to the original femur. External rotation averaged 7.5” ? 6.3” (range, O”-22”). No significant relationships existed between the orientation of the implant and the location of the greatest width of the medullary canal at the proximal level.
Radiographic Anteroposterior
Appearance
of the Femoral
Canal
Lateral
Projection 20 m m above
lesser
l
Eckrich
et al.
Projection
trochanter
J
EKtERNAL
INTERNAL ROTATION
ROTATION
Level
of lesser
EXlERNAL ROTATtON
. f
trochanter
s
1, -1s
-1s
1C
15'
t
-s -1
-.
-a
. .
I
-3
tNTERNAL ROTATlON
20 m m below
lesser
EXlERNAL ROTAWN
4
trochanter 4-3 ..
T -15-
-1w
-5’
.,
5’
1
I IS
-2 . .
INTERNAL ROTATtOR
-3 --
4 4
EXIERNAL
INTERNAL ROTATION
ROTATtON
Level
4 . .
DCIERNAL ROTATION
of isthmc
-2 -3
cl-tNTERNAL ROTATlON
J-.
EXTERNAL
ROTATtON
INTERNAL ROTATION
-4
EXTERNAL ROTATlON
Fig. 2. Effect of rotation on canal widths at various levels. Error bars indicate one standard error.
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The Journal of Arthroplasty
Fig. 3. Cross-section
Vol. 9 No. 4 August 1994
of a right
femur at the osteotomy level, depicting the averageorientation of an implant in the canal. Line A-A’ is.the plane of the original femoral neck. Line BB’ is the plane of the stem through its widest point. Line C-C’ representsthe plane of the posterior portion of the femoral condyles.The plane of the implant is externally rotated 8” with respect to the plane of the original femoral neck (anglea) and 23” with respectto the posteriorborder of the femoralcondyles(anglep).
0 A
A C’
C
B’
Discussion With the advent of cementless hip arthroplasty, accurate radiographic ternplating has become an important element in preoperative planning. However, two main sources of error arise from using plain radiographs for the selection of cementless prostheses and the assessment of their fit within the femoral canal. The first is radiographic magnification and the second is femoral rotation. Attempts have been made in the past to use radiographic techniques that minimize, or at least standardize, the amount of magnification produced when obtaining radiographs of the W3,7,8 Manufacturers of total hip systems have addressed this problem by providing templates of femoral components at various niagnifications, typically 15, 18, or 20%, in direct AP and lateral projections. Attempts to standardize femoral position have been made through the use of special jigs and chairs designed to hold the femur in a true AP or lateral position; however, these cumbersome and inconvenient methods are not popular.‘,” Additionally, because of variations in femoral torsion, it is difficult for an external apparatus to accurately place the femur in a truly AP or lateral orientation. Femoral anteversion averages 13” in normal hips,’ ’ and is increased 5”-6” in osteoarthritic hips.‘”
Unless the radiographic
technique
compensates for
anteversion, it will project the canal with 13”-20” of
external rotation. If the femur is held in additional external rotation due to hip disease, the deviation of the radiographic plane from that of the femoral neck will be even greater. The radiographic positioning technique commonly used to compensate for anteversion has the patient point their toes inward, with
the intention
of internally
rotating
the femur. The
amount of internal rotation actually achieved is un-
known and may be insignificant if the in-turning of the toes is primarily achieved through ankle and knee joint motion. Further, it is difficult to produce reproducible radiographs on a consistent basis using this technique due to variations in the amount of intoeing achieved at sequential examinations. The effect of femoral rotation on the shape of the femoral canal may be assessed through consideration of the normal cross-sectional shape of the medullary canal at various levels. Proximally, the canal is somewhat pear shaped, with its mediolateral width being greater than its anteroposterior width when viewed on the AP and lateral projections. The greatest crosssectional width of the proximal canal is not in the same plane as that of the femoral neck, but rather is variable, being externally rotated by 20” to over 40” from the plane of the femoral neck (Fig. 2). Proceeding distally, the canal gradually becomes elliptic, and the axis of the greatest width shifts until it lies approximately within the sag&al plane at the level of
the isthmus. Therefore, the dimensional changes due to femoral rotation vary with position along the longitudinal axis of the femur. For the “average” femur examined in this study, external rotation caused the width of the proximal canal to increase on the AP radiograph, while the width of the distal canal did not change significantly. However, a varus angulation of the canal was introduced (Fig. 4). Dimensional changes due to rotation vary, even within the same level of the femoral canal. The curvature of the medial canal wall is more acute than that of the lateral wail; thus, when projected in two dimensions, femoral rotation will cause more apparent change to the medial than to the lateral wall of the canal. The effect of all of these factors is that
femoral rotation does not simply magnify or shrink the dimensions of the canal uniformly, but changes
Radiographic
Appearance
of the Femoral Canal
-
I
Lesser Trochantc
l
Eckrich
et al.
425
10” Internal Rotation Neutral Rotation 10” External Rotation
Lateral Antenor
60mm
I f-
A Fig. 4. Change
Isthmus
in canal shape due to femoral rotation on the (A) anteroposterior and (B) lateral projections.
the entire shape due to the variation in the effects of rotation at different levels. The change in the overall shape of the canal may have as much influence on the accuracy of radiographic ternplating as the actual magnitude of the dimensional changes themselves. Radiographic templating assumes that the AP radiograph of the femur will orient the proximal canal at its widest point and, therefore, coincide with that of the prosthesis on its template. Templating also assumes that the implant will fit into the canal in that same orientation. This study suggests that for the average femur, the position of the widest dimension changes, even within the 40 mm segment of the canal, from 20 mm superior to 20 mm inferior to the lesser trochanter. This portion of the canal is of
greatest importance to implant stability and load transfer. In summary, femoral rotation has a significant effect on the dimensions of the femoral canal projected on AP and lateral radiographs. Although the magnitude and direction of these dimensional changes varies significantly among femora, their influence on the overall shape of the medullary canal are sufficient to significantly affect the selection of prosthetic components on the basis of preoperative ternplating. In light of the variability in the rotational orientation of the femoral prosthesis, it is difficult to recommend a specific positioning technique for preoperative radiographs. Regardless of the radiographic technique, we have demonstrated that variations in proximal femo-
426
The Journal
of Arthroplasty
Vol. 9 No. 4 August
ral anatomy are significant to prevent the appropriate femoral component from being selected from preoperative radiographs on a reliable basis. It is recommended that preoperative ternplating be used only as a general guide to implant selection, and that the final selection of prosthetic components be based on the use of instruments that accurately gauge the size of the canal during surgery. Routinely obtaining intraoperative radiographs should be considered.
1994
5.
6.
7.
8.
References 1. Cape110 WN: Preoperative planning of total hip arthroplasty. p. 249. In Anderson LD (ed): Institutional course lectures: The American Academy of Orthopedic Surgeons. CV Mosby, St. Louis, 1986 2. Engh CA, Bobyn JD: Biological Fixation in Total Hip Arthroplasty. Slack, Thorofare, NJ, 1985 3. Goodman S, Ruebenstein J, Schatzken J et al: Apparent changes in the alignment of the femoral component in hip arthroplasties associated with limb positioning. Clin Orthop 22 I :242, 1987 4. Hungerford DS, Border LS, Hedley AI< et al: Principles and techniques of cementless total hip arthroplasty. 1).
9.
10.
11. 12.
293. In Stillwell WY (cd): The art of total hip arthroplasty. Grune and Stratton, Orlando, 1987 Trotter M, Peterson RR: Transverse diameter of the femur: on roentgenograms and on bones. Clin Orthop 52:233, 1967 Noble PC, Alexander JW, Lindahl LJ et al: The anatomic basis of femoral component design. Clin Orthop 235: 148, 1988 Amstutz HC, Ouzounion T, Grower D et al: Grid radiograph: a simple technique for consistent high resolution visualization of the hip. J Bone Joint Surg 68A: 1052, 1986 Gorski JM, Schwartz L: A device to measure %-ray magnification in preoperative planning for cementless arthroplasty. Clin Orthop 202:302, 1986 Clarke IC, Gruen T, Mutos M, Amstutz HC: Improved methods for quantitative radiographic evaluation with particular reference to total hip arthroplasty. Clin Orthop 121:83, 1976 Kirkpatrick JS, Clarke IC, Amstutz HC, Jinnah RH: Radiographic techniques for consistent visualization of total hip arthroplasties. Clin Orthop 174: 158, 1983 Yoshioka Y, Cooke TDV: Femoral anteversion: asscssmcnt based on function axes. J Orthop Res 5:86, 1987 Reikeras 0, Bjerkreim I, Kolbenstvedt: Anteversion of the acetabulum and femoral neck in normals and in patients with osteoarthritis of the hip. Acta Orthop Stand 54:18, 1983
of Arthroplasty
Vol. 9 No. 4 1994
Effect of Rotation on the Radiographic Appearance of the Femoral Canal Stephen G. J. Eckrich, MD, Philip C. Noble, MS, and Hugh S. Tullos, MD
Abstract: Radiographic templating is a key element of preoperative planning for cementless total hip arthroplasty, and it aids in the selection of an appropriate implant. Frequently, the radiographic projection of the proximal femur does not correspond to that of the femoral prosthesis on its template due to variations in patient positioning. This discrepancy is a potential source of error when predicting which femoral component will best fit within the femoral canal. To evaluate the effect of femoral rotation on the size and shape of its radiographic image, anteroposterior and lateral radiographs of 12 femora were prepared over a range of positions. Several medullary canal dimensions were measured for each projection. The changes in these dimensions were compared using the image at neutral rotation as a reference. Each femur was then implanted with an appropriately sized cementless prosthesis to determine its actual rotational orientation in the canal. On the anteroposterior projection, statistically significant changes in the width of the proximal canal with femoral rotation were noted. There was no statistically significant change in distal canal dimensions with rotation. On the lateral projection, the dimensions of the proximal canal changed significantly with internal rotation; however, external rotation had no effect on canal dimensions. In general, the magnitude and direction of the canal dimensions were highly variable. The final rotational orientation of the femoral component in the canal was quite variable with respect to the plane of the femoral neck. Errors in implant selection may be due to excessive reliance on preoperative ternplating, which can be misleading because of femoral rotation. Key words: radiographic templating, femoral rotation, cementless arthroplasty, projection.
Accurate radiographs play a fundamental role in the preoperative planning of total hip arthroplasty, which requires standardized radiographic views of the hip joint and the selection of appropriate prosthetic components. ’ ,’ Selecting an appropriate prosthesis involves overlaying templates of each prosthesis on the preoperative radiographic image of the proximal femur. In cemented femoral components, some component/canal mismatch is tolerated since the cement is intended to fill the gap between the prosthesis and medullary canal. However, ceFrom the Department cine, Houston, Texas.
of Orthopedic
Surgery,
Baylor
College
mentless femoral components do not allow this latitude because stable fixation of these implants depends on an intimate fit between the prosthesis and endosteal surface of the femoral cana1.2-4 It is often difficult to predict which cementless prosthesis provides the best fit to the femur on the basis of standard clinical radiographs, principally, because the femur is normally projected on its template in some degree of rotation with respect to the plane of the femoral prosthesis. To provide proper orientation on the anteroposterior (AP) projection, the proximal femur must be positioned so that the long axis of the femoral neck is parallel to the radiographic cassette and perpendicular to the x-ray beam. Similarly, for a correct lateral projection, the long axis of the femoral neck should be perpendicular to the
ofMedi-
Reprint requests: Stephen G. J. Eckrich, MD, Department of Orthopedic Surgery, Baylor College of Medicine, 6550 Fannin, Suite 2625, Houston, TX 77030.
419
420
The Journal
of Arthroplasty
Vol. 9 No. 4 August
and parallel to the x-ray beam. To obtain this projection, the femur must be internally rotated to compensate for the normal anteversion of the head and neck. Frequently, the femur is externally rotated, either because the radiographic technique did not adequately compensate for femoral torsion or many patients suffering from arthritis of the hip joint are unable to internally rotate the extremity. Additionally, most radiographic templates have been designed on the assumption that the widest mediolateral dimension of the proximal canal is visualized on this AP projection, and that the prosthesis will ultimately lit in the canal in this orientation. These assumptions may be incorrect for a number of reasons. First, the widest dimension of the canal is often not visible ou the true AP radiograph.5 Second, the orientation of any cementless prosthesis implanted within the femur will be determined by other complex factors related to overall canal shape. The purposes of this study are to determine both the effect of femoral rotation on the radiographic dimensions of the proximal femur and the variation in femoral rotation that can be tolerated before the accuracy of femoral templating is significantly impaired. cassette
Materials
and
Methods
Previously, Noble et a1.6 showed that the most accurate single index of the overall size of the medullary canal was its mediolateral width, 20 mm distal to the lesser trochanter. Consequently, this measurement was used to select femora for this study. Twelve femora were selected, whose canal widths ranged from 16 to 27 mm and were 20 mm distal to the lesser trochanter. Femora were selected if they had no evidence of previous trauma, malignancy, or severe osteoporosis. A pin was placed through the center of the femoral head and the midpoint of the femoral neck at its narrowest point for radiographic measurement. Anteroposterior, lateral, and axial radiographs confirmed proper pin position. A spheric radiographic marker was glued to the tip of the lesser trochanter to provide a standard vertical reference on each radiograph. Each femur was mounted in a jig that allowed it to be rotated about its anatomic axis, defined by a line passing through both the midpoint of the bicondylar axis distally and a point just medial to the greater trochanter, approximately at the piriformis fossas, proximally. Rotation of the femur in this apparatus could be determined to + 0.5”. A standard radiographic cassette was mounted vertically, parallel to both the pin within the femoral neck and the intramedullary axis of the femoral shaft. This orientation
1994
was defined as neutral rotation, and was used as the reference projection for comparison with subsequent AP and lateral views of the femoral canal. The tubeto-femur-to-film distance was adjusted to provide a radiographic magnification of 15%, a standard used on many ternplating systems. Following correct positioning of the femur and apparatus, AP radiographs were taken in neutral rotation and at the following positions: 10” and 20” of internal rotation and lo”, 20”, 30”, and 40” of external rotation. This procedure was repeated for the lateral projection, with radiographs taken in neutral rotation and at 5”, lo”, and 15” of both internal and external rotation. Key endosteal dimensions were measured on each radiograph (Fig. 1). To ensure accurate comparison of the image of the femur on different radiographs, standardized axes were constructed using the following anatomic landmarks: ( 1) the femoral neck axis, defined by a line passing from the center of the femoral head to the center of the femoral neck at its narrowest point, and (2) the proximal and distal axes of the femoral canal, defined by lines that bisected the medullary canal at four midpoints placed at 2 cm intervals proximally and distally from the isthmus. The angle resulting from the intersection of these two lines determined the varus/valgus bow and the anterior bow on the AP and lateral radiographs, respectively. (The anterior bow of the femur coupled with femoral rotation creates a varus or valgus bow of the femoral shaft on the AP radiograph.) The change in femoral dimensions due to rotation, from the reference dimension on the AP and lateral radiographs taken at neutral rotation, was measured in either millimeters or degrees. The mean change in dimensions, measured for each incremental change in rotation, was statistically analyzed for significance using the Newman-Kuells test for comparison of multiple means. To determine the rotational orientation of a prosthesis within each femur, a cementless femoral prosthesis (Precision Osteolock, Howmedica, Rutherford, NJ) was implanted in each specimen following the manufacturer’s recommended procedure. The rotational orientation of the prosthetic component with respect to the original femoral neck axis was determined from axial radiographs using a pin that passed from the most medial to the most lateral point on the stem. Results
(Tables
1, 2)
All of the anatomic dimensions studied changed following femoral rotation; however, the average changes in some of the dimensions were not statisti-
Radiographic
Appearance
of the Femoral Canal
Eckrich et al.
l
421
1. Key femoral dimensions on the anteroposterior and lateral radiographs.A, B, C, canalwidths of the proximal femur; D, canal width 60 mm below lesser trochanter; E, canal width at isthmus; p, varus/valgusbow angle; 4, anterior bow angle. Fig.
Table
1. Amount
of Change Change
Measured Dimension
in parentheses
Dimensions (Anteroposterior
Due to Internal Rotation
20
Varus/valgus bow angle (degrees) Width of canal 20 mm above lesser trochanter (mm I Width of canal at lesser trochanter (mm) Width of canal 20 mm below lesser trochanter (mm ) Width of canal 60 mm below les~r trochanter (mm) Width of canal at isthmus (mm) Numbers
in Femoral
Reference Value 0”
10”
~ l.S(O.4)
-0.8lO.4)
m-2.1(0.6)
- 0.9(0.4)
51.0
r
-0.8(0.3)
33.8
1.2(0.5)
From the Reference Projection)
Change 10”
0.1 ? 0.3
40” 3.9(0.5)
1.9
0.9(0.3)
O.S(O.5)
--0 l(O.6)
k
1.3
0.7(0.2)
1.2(0.3)
0.9(0.4)
1.2
0.6(0.2)
1 l(O.4)
1.7(0
5)
2 6(0.7)
O.O(O.3)
0.6(0
41
(1.9(0.5)
O.S(O.2)
0.9(0
3)
Z.l(O.5)
e
0.9(0.4)
0.5(0.3)
20.0
-+ 1.0
0.9(0.4)
0.6(0.4)
16.6
+ 0.8
changes
Rotation 30 2.9(0.5)
25.7
significant
to External
2.2(0.5)
O.l(O.3)
statistically
Due 20”
Rotation
1.2(0.4)
l.O(O.4)
indicate
Value Due to Femoral
(P < .05).
-0.1(0.2)
0.3(0.2)
- 19(1.0)
0 S(O.7)
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The Journal Table
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of Arthroplasty
Vol. 9 No. 4 August
2. Amount of Change in Femoral Dimensions From the Reference Value Due to Femoral Rotation (Lateral Projection) Change
Measured
1994
Dimension
Due
to Internal
15”
Reference Dimension
IO’
Antenor
O.S(O.2)
Canal
2.2(0.6)
bow (degrees) wtdth 20 mm above lesser trochanter (mm) Canal wdth at lesser trochanter (mm) Canal width 20 mm below lesser trochanter (mm) Canal width 60 mm below lesser trochanter (mm) Canal width at isthmu,
Rotation
-0.6(0.3) l.Y(U
5O
4)
Change
0’
39.1
+ 0.7
31.4
i
16.1
i
1.7(0.3)
U.9(0
1.7(0.5)
1.3(0.3)
0 6(0.3)
3)
IO External
5’
0.4(0.3) l.l(O 31
1.9(0.5)
7.9
Due
i
0.6
10
Rotation 15”
-O.i(O.l, 0.2(0
3)
0. I(0 2) 0 h(O.5,
~0.4lU.3) u 7(O.K)
0.7
U.5(0
3)
I) 010 5)
O.X((I 8)
0.7
U.U(O. I )
0 llU.4)
O.O(U.5)
-0.6(0.3)
-0.4(0.2)
-0.2(0.21
23.1
+ 0.7
U.UlO 2)
(1.3(I) II
0.3(0.3)
- 0.3(0.2)
-0
-01(O.l)
-72.5
-2 I 2
0 O(O.1)
0 ii0
U.X(O.2,
I(O.2)
1)
(mm) Numbers
in parentheses
indicate
statistically
significant
changes
tally significant, possibly due to the large variation in the magnitude of change between femora. Interand intrafemoral variability was pronounced in the dimensions of canal width, particularly in the proximal femur.
Extramedullary
Dimensions
On the AP view at neutral rotation, the varus/valgus bow of the femoral shaft was essentially straight (average, 0.1” varus). With internal rotation, there was a consistent trend toward apparent valgus angulation, where the apparent varus angulation of the canal increased by approximately 1’ for each 10” of rotation of the femur. This trend was statistically significant, even though the values were small. On the lateral view, the anterior bow of the femur did not change significantly with internal or external rotation. Rotations of 15” from neutral resulted in changes of lessthan 1” in the angulation of the proximal and distal canals.
lntramedullary
Dimensions
(Fig. 2)
Rotation had a highly variable effect on the radiographic dimensions of the femoral canal. This variability was greatest in the proximal femur and was less distally. At 20 mm proximal to the lesser trochanter, only 10” of internal rotation on the AP projection caused a reduction in canal width of 0.9 + 0.4 mm (P < .05). However, external rotation did not significantly affect this dimension until the femur was rotated 40”. On the lateral projection, internal rotation of 10” produced a decreasein apparent canal width of 1.9 ? 0.4 mm (P < .05), whereas external rotation of up to 15” had no effect on canal width. The effects of femoral rotation were similar at the
(P < .05)
level of the lesser trochanter. On the AP projection, canal width was decreasedby 1 mm with 10” of internal rotation, but was increased by 1.2 mm with 20” of external rotation. Conversely, on the lateral projection, 10” of internal rotation caused an apparent 1.7 mm increase in canal width, while external rotation to 15” had no significant effect on canal dimcnsions. At 20 mm below the lesser trochanter, internal rotation to 20” had no significant effect on apparent canal width on the AP radiographs, while external rotation produced an apparent increase in canal width of 1.1 mm at 20”, 1.7 mm at 30”, and 2.6 mm at 40”. On the lateral projection, canal width did not change significantly until 15” of internal rotation, where it had increased by an average of 1.7 mm. Again, external rotation had no significant effect on canal width. The width of the medullary canal at the isthmus did not change significantly over the range of internal and external rotation in this study. The mean canal width at the isthmus was greater on the lateral than on the AP projection (22.5 and 16.6 mm, respectively).
Component
Orientation
(Fig. 3)
All except one of the femoral components fit into the canal in some degree of external rotation with respect to the plane of the femoral neck, corresponding to increased anteversion of the prosthesis with respect to the original femur. External rotation averaged 7.5” ? 6.3” (range, O”-22”). No significant relationships existed between the orientation of the implant and the location of the greatest width of the medullary canal at the proximal level.
Radiographic Anteroposterior
Appearance
of the Femoral
Canal
Lateral
Projection 20 m m above
lesser
l
Eckrich
et al.
Projection
trochanter
J
EKtERNAL
INTERNAL ROTATION
ROTATION
Level
of lesser
EXlERNAL ROTATtON
. f
trochanter
s
1, -1s
-1s
1C
15'
t
-s -1
-.
-a
. .
I
-3
tNTERNAL ROTATlON
20 m m below
lesser
EXlERNAL ROTAWN
4
trochanter 4-3 ..
T -15-
-1w
-5’
.,
5’
1
I IS
-2 . .
INTERNAL ROTATtOR
-3 --
4 4
EXIERNAL
INTERNAL ROTATION
ROTATtON
Level
4 . .
DCIERNAL ROTATION
of isthmc
-2 -3
cl-tNTERNAL ROTATlON
J-.
EXTERNAL
ROTATtON
INTERNAL ROTATION
-4
EXTERNAL ROTATlON
Fig. 2. Effect of rotation on canal widths at various levels. Error bars indicate one standard error.
423
424
The Journal of Arthroplasty
Fig. 3. Cross-section
Vol. 9 No. 4 August 1994
of a right
femur at the osteotomy level, depicting the averageorientation of an implant in the canal. Line A-A’ is.the plane of the original femoral neck. Line BB’ is the plane of the stem through its widest point. Line C-C’ representsthe plane of the posterior portion of the femoral condyles.The plane of the implant is externally rotated 8” with respect to the plane of the original femoral neck (anglea) and 23” with respectto the posteriorborder of the femoralcondyles(anglep).
0 A
A C’
C
B’
Discussion With the advent of cementless hip arthroplasty, accurate radiographic ternplating has become an important element in preoperative planning. However, two main sources of error arise from using plain radiographs for the selection of cementless prostheses and the assessment of their fit within the femoral canal. The first is radiographic magnification and the second is femoral rotation. Attempts have been made in the past to use radiographic techniques that minimize, or at least standardize, the amount of magnification produced when obtaining radiographs of the W3,7,8 Manufacturers of total hip systems have addressed this problem by providing templates of femoral components at various niagnifications, typically 15, 18, or 20%, in direct AP and lateral projections. Attempts to standardize femoral position have been made through the use of special jigs and chairs designed to hold the femur in a true AP or lateral position; however, these cumbersome and inconvenient methods are not popular.‘,” Additionally, because of variations in femoral torsion, it is difficult for an external apparatus to accurately place the femur in a truly AP or lateral orientation. Femoral anteversion averages 13” in normal hips,’ ’ and is increased 5”-6” in osteoarthritic hips.‘”
Unless the radiographic
technique
compensates for
anteversion, it will project the canal with 13”-20” of
external rotation. If the femur is held in additional external rotation due to hip disease, the deviation of the radiographic plane from that of the femoral neck will be even greater. The radiographic positioning technique commonly used to compensate for anteversion has the patient point their toes inward, with
the intention
of internally
rotating
the femur. The
amount of internal rotation actually achieved is un-
known and may be insignificant if the in-turning of the toes is primarily achieved through ankle and knee joint motion. Further, it is difficult to produce reproducible radiographs on a consistent basis using this technique due to variations in the amount of intoeing achieved at sequential examinations. The effect of femoral rotation on the shape of the femoral canal may be assessed through consideration of the normal cross-sectional shape of the medullary canal at various levels. Proximally, the canal is somewhat pear shaped, with its mediolateral width being greater than its anteroposterior width when viewed on the AP and lateral projections. The greatest crosssectional width of the proximal canal is not in the same plane as that of the femoral neck, but rather is variable, being externally rotated by 20” to over 40” from the plane of the femoral neck (Fig. 2). Proceeding distally, the canal gradually becomes elliptic, and the axis of the greatest width shifts until it lies approximately within the sag&al plane at the level of
the isthmus. Therefore, the dimensional changes due to femoral rotation vary with position along the longitudinal axis of the femur. For the “average” femur examined in this study, external rotation caused the width of the proximal canal to increase on the AP radiograph, while the width of the distal canal did not change significantly. However, a varus angulation of the canal was introduced (Fig. 4). Dimensional changes due to rotation vary, even within the same level of the femoral canal. The curvature of the medial canal wall is more acute than that of the lateral wail; thus, when projected in two dimensions, femoral rotation will cause more apparent change to the medial than to the lateral wall of the canal. The effect of all of these factors is that
femoral rotation does not simply magnify or shrink the dimensions of the canal uniformly, but changes
Radiographic
Appearance
of the Femoral Canal
-
I
Lesser Trochantc
l
Eckrich
et al.
425
10” Internal Rotation Neutral Rotation 10” External Rotation
Lateral Antenor
60mm
I f-
A Fig. 4. Change
Isthmus
in canal shape due to femoral rotation on the (A) anteroposterior and (B) lateral projections.
the entire shape due to the variation in the effects of rotation at different levels. The change in the overall shape of the canal may have as much influence on the accuracy of radiographic ternplating as the actual magnitude of the dimensional changes themselves. Radiographic templating assumes that the AP radiograph of the femur will orient the proximal canal at its widest point and, therefore, coincide with that of the prosthesis on its template. Templating also assumes that the implant will fit into the canal in that same orientation. This study suggests that for the average femur, the position of the widest dimension changes, even within the 40 mm segment of the canal, from 20 mm superior to 20 mm inferior to the lesser trochanter. This portion of the canal is of
greatest importance to implant stability and load transfer. In summary, femoral rotation has a significant effect on the dimensions of the femoral canal projected on AP and lateral radiographs. Although the magnitude and direction of these dimensional changes varies significantly among femora, their influence on the overall shape of the medullary canal are sufficient to significantly affect the selection of prosthetic components on the basis of preoperative ternplating. In light of the variability in the rotational orientation of the femoral prosthesis, it is difficult to recommend a specific positioning technique for preoperative radiographs. Regardless of the radiographic technique, we have demonstrated that variations in proximal femo-
426
The Journal
of Arthroplasty
Vol. 9 No. 4 August
ral anatomy are significant to prevent the appropriate femoral component from being selected from preoperative radiographs on a reliable basis. It is recommended that preoperative ternplating be used only as a general guide to implant selection, and that the final selection of prosthetic components be based on the use of instruments that accurately gauge the size of the canal during surgery. Routinely obtaining intraoperative radiographs should be considered.
1994
5.
6.
7.
8.
References 1. Cape110 WN: Preoperative planning of total hip arthroplasty. p. 249. In Anderson LD (ed): Institutional course lectures: The American Academy of Orthopedic Surgeons. CV Mosby, St. Louis, 1986 2. Engh CA, Bobyn JD: Biological Fixation in Total Hip Arthroplasty. Slack, Thorofare, NJ, 1985 3. Goodman S, Ruebenstein J, Schatzken J et al: Apparent changes in the alignment of the femoral component in hip arthroplasties associated with limb positioning. Clin Orthop 22 I :242, 1987 4. Hungerford DS, Border LS, Hedley AI< et al: Principles and techniques of cementless total hip arthroplasty. 1).
9.
10.
11. 12.
293. In Stillwell WY (cd): The art of total hip arthroplasty. Grune and Stratton, Orlando, 1987 Trotter M, Peterson RR: Transverse diameter of the femur: on roentgenograms and on bones. Clin Orthop 52:233, 1967 Noble PC, Alexander JW, Lindahl LJ et al: The anatomic basis of femoral component design. Clin Orthop 235: 148, 1988 Amstutz HC, Ouzounion T, Grower D et al: Grid radiograph: a simple technique for consistent high resolution visualization of the hip. J Bone Joint Surg 68A: 1052, 1986 Gorski JM, Schwartz L: A device to measure %-ray magnification in preoperative planning for cementless arthroplasty. Clin Orthop 202:302, 1986 Clarke IC, Gruen T, Mutos M, Amstutz HC: Improved methods for quantitative radiographic evaluation with particular reference to total hip arthroplasty. Clin Orthop 121:83, 1976 Kirkpatrick JS, Clarke IC, Amstutz HC, Jinnah RH: Radiographic techniques for consistent visualization of total hip arthroplasties. Clin Orthop 174: 158, 1983 Yoshioka Y, Cooke TDV: Femoral anteversion: asscssmcnt based on function axes. J Orthop Res 5:86, 1987 Reikeras 0, Bjerkreim I, Kolbenstvedt: Anteversion of the acetabulum and femoral neck in normals and in patients with osteoarthritis of the hip. Acta Orthop Stand 54:18, 1983