Papers Accuracy of intramedullary replacement M. A. Elloy*,
M.P. Manning’
alignment in total knee
and R. Johnson”
*Department of Mechanical Engineering, Liverpool University, Liverpool, UK; ‘Royal Liverpool Hospital, Liverpool, UK; and ‘Arrowe Park Hospital, Upton, Wirral, Merseyside L49 5PE, UK Received
February
1990, accepted
November
1991
ABSTRACT The accuracy of a system of intramedullazy alignment using 6mm roa!s was assessed in 700 patients undergoing total knee replacements. Post-operative, fill length weight-bearing X-rays were used; the mechanical axisfiom head was used as the reference axis. The method of calculating the errors produced by fl exion and rotation of the limb in relation to the X-ray beam is described, the mean deviationfiom the mechanical axis in 700 cases being 0.67” valgus with a standard deviation of 2.47”. The maximum error wm 6.68” valgus and 4.62” varus. The puzpose of this study is twofold, first to assess the accuracy of this system of intramedullazy alignment and, second, to develop a method of correcting for apparent radiological m~a~ignment using standard radiographic equipment. Keywords: Total knee replacement,
alignment.
intramedullary,
X-ray correction
factor
INTRODUCTION The successful outcome of total knee replacement depends on several factors, one of the most important being the correct alignment of the components’-“. Ideally the prosthesis should lie central to the mechanical axis and at right angles to it (Figure 7). Misalignment of only 4” moves the mechanical axis to the outer third of the prosthesis, and 10” moves the axis outside the prosthesis. Failure is probable in the first situation and inevitable in the second”. The accepted definition of the mechanical axis is a line drawn from the centre of the femoral head to the centre of the ankle, when viewed in a true A/P direction HKA (Figure I). A true A/P view is one that is at right angles to the flexion axis of the knee; this is so for both the natural and the prosthetic knee. Many authors have used the tibio-femoral angle as a measure of alignment FKH (Figure 7) but it varies from patient to patient, whereas the mechanical axis is a constant”. We therefore feel that measurement of the mechanical axis is a more reliable method of determining knee alignment; it is also less prone to errors’?. The greatest difficulty with both of these methods is ensuring that the X-ray is taken in a true A/P plane, i.e. the X-ray plate is parallel to the flexion axis. To avoid such errors either the X-rays have to be screened to ensure a true A/P view or a correction factor should be applied. The former has considerable practical difficulties in a busy radiology depart-
Correspondence and reprint requests to: Mr R. Johnson, Knee Research Unit, Arrowe Park Hospital, Upton, Wirral, LA9 FiPE, UK. Q 199’2 Butterworth-Heinemann 0141-5425/92/05363-08
Figure 1
Ideal alignment
ment; we therefore prefer to accept some degree of rotation and flexion of the knee, provided the hip and ankle are clearly visible, and then apply an appropriate correction factor when necessary. In this series only seven of the 100 X-rays were perfectly radiologically aligned. In order to determine the correction factor it is necessary to have a precise knowledge of the eometry of the prosthesis. T f e Johnson/Elloy Knee is a meniscal knee with
for BES J. Biomed.
Eng. 1992, Vol. 14, September
363
Accuracyof intramedulhy alignment in TKR: M.A. Elloy et al.
the following precise Appendix 1):
geometric
characteristics
(see
1. The profile of both metal tibia1 and femoral components are symmetrical in the A/P view. 2. The femoral condyles are true cylinders in, the lateral view. 3. The femoral condyles are flat in the A/P view. 4. The tibia1 component is a precise cone with a rectangular stem. It is implanted over long intramedullary alignment rods 6mm in diameter inserted into the femur and tibia. The entry holes into the femoral and tibia1 medullary canals are large, ensuring the rods take their alignment from the medullary canals and not the entry holes. The rods are used to locate instruments for making accurate bone cuts of the femur and tibia. A 7” angle adaptor is used to make the distal cut on the femur, thereby ensuring that all prostheses are inserted at this angle irrespective of the natural tibiofemoral angle. The components are introduced over the alignment rods ensuring that the prosthesis is always correctly aligned with the long axis of the bones. The purpose of this study is twofold: first to assess the accuracy of this system of intramedullary alignment; second to develop a method of correcting for apparent radiological misalignment using standard radiographic equipment. Figure 2
MATERIAL
AN-b METHOD
Patient selection All patients undergoing Johnson/Elloy knee replacement have a full length weight-bearing X-ray before discharge from hospital. One hundred of these X-rays were randomly selected, based on the following criteria: 1. Both components were cemented in. 2. The X-rays were of sufficient quality to show the ankle and the hip irrespective of the degree of rotation or flexion of the prosthesis in relation to the X-ray. The quality of the films in terms of bone definition was not considered important. 3. All males were selected in alphabetical order and then all females, again in alphabetical order, up to 100 cases. 4. Cases were rejected because of oor X-rays at either the ankle or the hip and a Pso if the metal components overlapped, which could occur with a thin plastic component and a flexed knee, making measurement impossible. Radiological assessment Long leg X-rays were taken with a (105 X 35 cm) cassette holder with three (35 x 35 cm) films and screens. The patients stood on a wooden frame with arm supports and their back to the cassette. The legs were positioned so that the patella faced directly forwards. In difficult cases, e.g. severe rheumatoids, the position of the knee was ignored. The cassettes were positioned so that the edges of the films were in direct contact. After processing the X-rays the three
364
J. Biomed. Eng. 1992, Vol. 14, September
Apparent valgus: total error Em= Et+ Ef
films were taped together on an X-ray viewing box to ensure that no errors were introduced due to malpositioning of the bones. The mechanical axis was drawn on the X-ray (Figure 2). A line (HA) was drawn from the centre of the head of the femur to the centre of the ankle, the centre of the head of the femur being determined by the use of concentric rings. The angle of alignment was measured as follows: l
l
l
A line (HK) was drawn on the X-ray from centre of the head of the femur to the centre of prosthesis. A line (KA) was drawn on the X-ray from centre of the ankle, i.e. bisecting the talus, to centre of the prosthesis. The centre of the prosthesis was the intersection the tibia1 stem axis and a line joining the femoral condyles.
the the the the of flat
The angle of error is the summation of the errors at the ankle (Et) and at the hip (Ef) as measured on
Figure 2 The measured error Em= Et-k E$ In a perfectly
aligned knee this angle should be 0” is flexion or hyperextension combined with rotation of the knee in relation to the X-ra beam, an apparent varus or valgus deformity is pro dy uced (Figure 3). Internal rotation produces a valgus deformity and external rotation a varus deformity; this artefact is superimposed on any actual valgus or varus misalignment. Correcting for these apparent radiological deformities depends on measuring, on the long leg X-ra s, the angles of misalignment at the hip and an kile
(Figure 3). If there
Accuracy of intramedullq
Figure
3
alignment in ThX: MA. Elloy et al
Perfectly aligned X-ray: apparent valgus
(Ef and Et). The total measured alignment error on the X-ray (Em) = Ef+ Et. If the rotation and flexion of the prosthesis produced by radiological misalignment is subtracted from, or added to this, then the true misalignment can be calculated. Calculation of flexion and rotation errors 1. Femoral (a) Rotation: When the femoral component is rotated in relation to the X-ray beam, symmetry of the profile is lost and one condyle becomes more prominent (Figwe 4).
Figure
5
Flexion of components
(b) Flexion: Femoral flexion or hyperextension causes a change in the height of the posterior femoral condyle to the top of the prosthesis. With increasing flexion this distance increases (Figure 5).
--w flmm --.
Figure 6 (c/w)-(%)];
Correction factor for femur. Rotation: Rf= tan ‘[ 1.269 flexion: Ff=sin-* (A/w 1.182 cos (Rj))
J. Biomed. Eng. 1992, Vol. 14, September
365
Accuracy of intramedullq
alignment in TKR: MA. Elloy et al.
To calculate the femoral rotation and flexion three measurements are necessary (Figure 6).
h = height
from the top of the component to the top of the femoral condyle; c = width from the centre of the component to the extreme edge of the flat of the most prominent condyle; w = total width across the condyles.
Tibia1 rotation
Rt= arcsin[ 1.4255( I$)] Tibia1 flexion
Ft = arcsin
I
;cos
The calculated
2. Tibial
alignment
I
artefact (EC) is then
EC = (Ff+ Ft) sin R$
(a) Rotation: The central dip of the tibia becomes displaced medially or laterally when rotated. This is a result of the conical shape being cut off at the front and back. (b) Flexion: When the rotated tibia is flexed a hump appears on either side of the central dip. The height of the hump from the top edge of the cone changes as the component is flexed or extended. On
(53.7 - Rt)
the tibia1 side,
three
measurements
are made
If this is positive the knee is in valgus and if negative the knee is in varus. The measured radiological angle is Em and is made positive if the knee is in valgus and negative if varus. The true radiological valgus is found by subtracting the valgus artefact EC from the measured valgus Eq i.e. true alignment (TA) = Em - EC. If this is negative then the knee is in varus.
(F&we 7):
Calculation of errors
h = height of the highest point on the tibia to the top
1. Measurement of the angles Et+ Ef on the long films is subject to errors of up to 0.7”. This comprises realistic errors in the measurement of the angles between lines, and construction errors in identifying the centres of the hip prosthesis and ankle. These we estimate to be + 2.5 mm, 1 mm and 1 mm respectively (Figure 8). 2. Realistic errors in the measurements h, c and w on the tibia1 and femoral components. These are shown in Figures 6 and 70 The total errors were a maximum of 0.3”. Thus the total measurement errors amount to 0.7 + 0.3 = 1”.
edge of the cone; c = width from the central narrowest side; ter= total width.
point of the dip to the
Using the following formulae (derived in Appendix 1) the rotation and flexion of the femoral and tibia1 components can be calculated. Internal
femoral rotation
Rf= arctan
1 269 L (:-+!I
if the knee is externally negative. Femoral
rotated the Rfmust
k2.5mm
be made
T
I-
H
.
flexion r
1.182 “OS W
Rf-21.322
+
f
i,
fl
mm
>
h
t f mm
t
Figure 7 Correction factor for tibia. Rotation: Rt = sin-‘[1.4255 (1 -2)(6/w)); flexion: Ff= sin-’ [(2A/w.cos (53.7 - Rt)]
366
J. Biomed. Eng. 1992, Vol. 14, September
Figure 8 Errors in identifying: the centre of the hip = H; the centre of the knee = K; the centre of the ankle = A
Accuracy Table 1
Percentage
of knees with various
degrees
of error
Degrees
Within Within Within Within Within Between
(No.1
Percentage
1 IL 3 4 5 5 and 7
37 19 17 15 10 2
100 90 80
3
70
E
60
:
50
(c 0
40
is '
30 20 10 n ”
M.P. Manning’
alignment in total knee
and R. Johnson”
*Department of Mechanical Engineering, Liverpool University, Liverpool, UK; ‘Royal Liverpool Hospital, Liverpool, UK; and ‘Arrowe Park Hospital, Upton, Wirral, Merseyside L49 5PE, UK Received
February
1990, accepted
November
1991
ABSTRACT The accuracy of a system of intramedullazy alignment using 6mm roa!s was assessed in 700 patients undergoing total knee replacements. Post-operative, fill length weight-bearing X-rays were used; the mechanical axisfiom head was used as the reference axis. The method of calculating the errors produced by fl exion and rotation of the limb in relation to the X-ray beam is described, the mean deviationfiom the mechanical axis in 700 cases being 0.67” valgus with a standard deviation of 2.47”. The maximum error wm 6.68” valgus and 4.62” varus. The puzpose of this study is twofold, first to assess the accuracy of this system of intramedullazy alignment and, second, to develop a method of correcting for apparent radiological m~a~ignment using standard radiographic equipment. Keywords: Total knee replacement,
alignment.
intramedullary,
X-ray correction
factor
INTRODUCTION The successful outcome of total knee replacement depends on several factors, one of the most important being the correct alignment of the components’-“. Ideally the prosthesis should lie central to the mechanical axis and at right angles to it (Figure 7). Misalignment of only 4” moves the mechanical axis to the outer third of the prosthesis, and 10” moves the axis outside the prosthesis. Failure is probable in the first situation and inevitable in the second”. The accepted definition of the mechanical axis is a line drawn from the centre of the femoral head to the centre of the ankle, when viewed in a true A/P direction HKA (Figure I). A true A/P view is one that is at right angles to the flexion axis of the knee; this is so for both the natural and the prosthetic knee. Many authors have used the tibio-femoral angle as a measure of alignment FKH (Figure 7) but it varies from patient to patient, whereas the mechanical axis is a constant”. We therefore feel that measurement of the mechanical axis is a more reliable method of determining knee alignment; it is also less prone to errors’?. The greatest difficulty with both of these methods is ensuring that the X-ray is taken in a true A/P plane, i.e. the X-ray plate is parallel to the flexion axis. To avoid such errors either the X-rays have to be screened to ensure a true A/P view or a correction factor should be applied. The former has considerable practical difficulties in a busy radiology depart-
Correspondence and reprint requests to: Mr R. Johnson, Knee Research Unit, Arrowe Park Hospital, Upton, Wirral, LA9 FiPE, UK. Q 199’2 Butterworth-Heinemann 0141-5425/92/05363-08
Figure 1
Ideal alignment
ment; we therefore prefer to accept some degree of rotation and flexion of the knee, provided the hip and ankle are clearly visible, and then apply an appropriate correction factor when necessary. In this series only seven of the 100 X-rays were perfectly radiologically aligned. In order to determine the correction factor it is necessary to have a precise knowledge of the eometry of the prosthesis. T f e Johnson/Elloy Knee is a meniscal knee with
for BES J. Biomed.
Eng. 1992, Vol. 14, September
363
Accuracyof intramedulhy alignment in TKR: M.A. Elloy et al.
the following precise Appendix 1):
geometric
characteristics
(see
1. The profile of both metal tibia1 and femoral components are symmetrical in the A/P view. 2. The femoral condyles are true cylinders in, the lateral view. 3. The femoral condyles are flat in the A/P view. 4. The tibia1 component is a precise cone with a rectangular stem. It is implanted over long intramedullary alignment rods 6mm in diameter inserted into the femur and tibia. The entry holes into the femoral and tibia1 medullary canals are large, ensuring the rods take their alignment from the medullary canals and not the entry holes. The rods are used to locate instruments for making accurate bone cuts of the femur and tibia. A 7” angle adaptor is used to make the distal cut on the femur, thereby ensuring that all prostheses are inserted at this angle irrespective of the natural tibiofemoral angle. The components are introduced over the alignment rods ensuring that the prosthesis is always correctly aligned with the long axis of the bones. The purpose of this study is twofold: first to assess the accuracy of this system of intramedullary alignment; second to develop a method of correcting for apparent radiological misalignment using standard radiographic equipment. Figure 2
MATERIAL
AN-b METHOD
Patient selection All patients undergoing Johnson/Elloy knee replacement have a full length weight-bearing X-ray before discharge from hospital. One hundred of these X-rays were randomly selected, based on the following criteria: 1. Both components were cemented in. 2. The X-rays were of sufficient quality to show the ankle and the hip irrespective of the degree of rotation or flexion of the prosthesis in relation to the X-ray. The quality of the films in terms of bone definition was not considered important. 3. All males were selected in alphabetical order and then all females, again in alphabetical order, up to 100 cases. 4. Cases were rejected because of oor X-rays at either the ankle or the hip and a Pso if the metal components overlapped, which could occur with a thin plastic component and a flexed knee, making measurement impossible. Radiological assessment Long leg X-rays were taken with a (105 X 35 cm) cassette holder with three (35 x 35 cm) films and screens. The patients stood on a wooden frame with arm supports and their back to the cassette. The legs were positioned so that the patella faced directly forwards. In difficult cases, e.g. severe rheumatoids, the position of the knee was ignored. The cassettes were positioned so that the edges of the films were in direct contact. After processing the X-rays the three
364
J. Biomed. Eng. 1992, Vol. 14, September
Apparent valgus: total error Em= Et+ Ef
films were taped together on an X-ray viewing box to ensure that no errors were introduced due to malpositioning of the bones. The mechanical axis was drawn on the X-ray (Figure 2). A line (HA) was drawn from the centre of the head of the femur to the centre of the ankle, the centre of the head of the femur being determined by the use of concentric rings. The angle of alignment was measured as follows: l
l
l
A line (HK) was drawn on the X-ray from centre of the head of the femur to the centre of prosthesis. A line (KA) was drawn on the X-ray from centre of the ankle, i.e. bisecting the talus, to centre of the prosthesis. The centre of the prosthesis was the intersection the tibia1 stem axis and a line joining the femoral condyles.
the the the the of flat
The angle of error is the summation of the errors at the ankle (Et) and at the hip (Ef) as measured on
Figure 2 The measured error Em= Et-k E$ In a perfectly
aligned knee this angle should be 0” is flexion or hyperextension combined with rotation of the knee in relation to the X-ra beam, an apparent varus or valgus deformity is pro dy uced (Figure 3). Internal rotation produces a valgus deformity and external rotation a varus deformity; this artefact is superimposed on any actual valgus or varus misalignment. Correcting for these apparent radiological deformities depends on measuring, on the long leg X-ra s, the angles of misalignment at the hip and an kile
(Figure 3). If there
Accuracy of intramedullq
Figure
3
alignment in ThX: MA. Elloy et al
Perfectly aligned X-ray: apparent valgus
(Ef and Et). The total measured alignment error on the X-ray (Em) = Ef+ Et. If the rotation and flexion of the prosthesis produced by radiological misalignment is subtracted from, or added to this, then the true misalignment can be calculated. Calculation of flexion and rotation errors 1. Femoral (a) Rotation: When the femoral component is rotated in relation to the X-ray beam, symmetry of the profile is lost and one condyle becomes more prominent (Figwe 4).
Figure
5
Flexion of components
(b) Flexion: Femoral flexion or hyperextension causes a change in the height of the posterior femoral condyle to the top of the prosthesis. With increasing flexion this distance increases (Figure 5).
--w flmm --.
Figure 6 (c/w)-(%)];
Correction factor for femur. Rotation: Rf= tan ‘[ 1.269 flexion: Ff=sin-* (A/w 1.182 cos (Rj))
J. Biomed. Eng. 1992, Vol. 14, September
365
Accuracy of intramedullq
alignment in TKR: MA. Elloy et al.
To calculate the femoral rotation and flexion three measurements are necessary (Figure 6).
h = height
from the top of the component to the top of the femoral condyle; c = width from the centre of the component to the extreme edge of the flat of the most prominent condyle; w = total width across the condyles.
Tibia1 rotation
Rt= arcsin[ 1.4255( I$)] Tibia1 flexion
Ft = arcsin
I
;cos
The calculated
2. Tibial
alignment
I
artefact (EC) is then
EC = (Ff+ Ft) sin R$
(a) Rotation: The central dip of the tibia becomes displaced medially or laterally when rotated. This is a result of the conical shape being cut off at the front and back. (b) Flexion: When the rotated tibia is flexed a hump appears on either side of the central dip. The height of the hump from the top edge of the cone changes as the component is flexed or extended. On
(53.7 - Rt)
the tibia1 side,
three
measurements
are made
If this is positive the knee is in valgus and if negative the knee is in varus. The measured radiological angle is Em and is made positive if the knee is in valgus and negative if varus. The true radiological valgus is found by subtracting the valgus artefact EC from the measured valgus Eq i.e. true alignment (TA) = Em - EC. If this is negative then the knee is in varus.
(F&we 7):
Calculation of errors
h = height of the highest point on the tibia to the top
1. Measurement of the angles Et+ Ef on the long films is subject to errors of up to 0.7”. This comprises realistic errors in the measurement of the angles between lines, and construction errors in identifying the centres of the hip prosthesis and ankle. These we estimate to be + 2.5 mm, 1 mm and 1 mm respectively (Figure 8). 2. Realistic errors in the measurements h, c and w on the tibia1 and femoral components. These are shown in Figures 6 and 70 The total errors were a maximum of 0.3”. Thus the total measurement errors amount to 0.7 + 0.3 = 1”.
edge of the cone; c = width from the central narrowest side; ter= total width.
point of the dip to the
Using the following formulae (derived in Appendix 1) the rotation and flexion of the femoral and tibia1 components can be calculated. Internal
femoral rotation
Rf= arctan
1 269 L (:-+!I
if the knee is externally negative. Femoral
rotated the Rfmust
k2.5mm
be made
T
I-
H
.
flexion r
1.182 “OS W
Rf-21.322
+
f
i,
fl
mm
>
h
t f mm
t
Figure 7 Correction factor for tibia. Rotation: Rt = sin-‘[1.4255 (1 -2)(6/w)); flexion: Ff= sin-’ [(2A/w.cos (53.7 - Rt)]
366
J. Biomed. Eng. 1992, Vol. 14, September
Figure 8 Errors in identifying: the centre of the hip = H; the centre of the knee = K; the centre of the ankle = A
Accuracy Table 1
Percentage
of knees with various
degrees
of error
Degrees
Within Within Within Within Within Between
(No.1
Percentage
1 IL 3 4 5 5 and 7
37 19 17 15 10 2
100 90 80
3
70
E
60
:
50
(c 0
40
is '
30 20 10 n ”
