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Intramedullary alignment in total knee arthroplasty following proximal femoral instrumentation: an anatomical and radiographic study
The Knee 1994; 1:
83-85
lntramedullary alignment in total knee arthroplasty following proximal femoral instrumentation: an anatomical and radiographic study D J Roger, J V Bono, C A Paulsen Division of Orthopaedic
Surgery, Albany Medical College, Albany, NY, USA
Summary Correct component alignment is essential for the success of total knee arthroplasty. The use of intramedullary alignment in the femur has become the current standard for correct placement of the femoral component. The accuracy of this system is dependent on the degree to which the intramedullary guide rod approximates the anatomical axis of the femur. Cases in which the guide rod cannot be completely seated, such as after a femoral endoprosthesis, can result in increased differences between the anatomical axis and the guide rod axis. In these cases, additional referencing systems, such as extramedullary alignment or intra-operative radiography, may be useful to increase the accuracy of component position. IThe Knee 1994; i: 83-85) Key words: knee, atthroplasty, alignment, intramedullary
Introduction
Correct alignment of the prosthesis in total knee arthroplasty is of paramount importance to the function and survivorship of the implant. In the early designs of knee implants, the instrumentation that was used placed significant demands on the technical ability of the surgeon. Subsequent improvements in the design of the instrumentation have led to more predictable positioning of the implant. Specifically, the intramedullary alignment rod for orientation of the distal femoral resection has enabled the surgeon to perform reproducible cuts resulting in consistent component position (Figure 1). Cases in which there has been instrumentation of the proximal femur, e.g. a sliding hip screw or femoral endoprosthesis, can prevent complete seating of the rod. In these patients, the authors hypothesized that the incompletely seated rod might not approximate the true anatomical axis of the femur, and therefore could result in component malposition. The purpose of this
Accepted: March 1994 Correspondence and reprint requests to: D.J. Orthopaedic Surgery, A-280, Albany Medical Avenue, Albany, NY 12208, USA @ 1994 Butterworth-Heinemann 0968-0160/94/02083-03
Ltd
Roger MD, Division of College, New Scotland
study was to evaluate the effect of proximal femoral instrumentation on the position of the intramedullary alignment rod. Materials and methods
Seven cadaveric specimens with a known history of proximal femoral surgery were harvested by disarticulation of the femur at the hip joint and the knee joint. Four specimens had a compression screw and side plate, one had an Austin-Moore endoprosthesis, one had a bipolar endoprosthesis and one had three threaded pins in the femoral neck. Following disarticulation, the femora were evaluated anatomically and radiographically. Anatomically, the accuracy of the guide rod was assessed by sectioning the femur at the tip of the rod, and then measuring the distance from the middle of the guide rod to the endosteal cortex. Varusvalgus alignment was assessed by measuring the distance from the guide rod to the medial and lateral endosteal surfaces. Flexion-extension alignment was assessed by measuring the distance from the guide rod to the anterior and posterior endosteal surfaces. Mathematical triangulation was then used to calculate the difference between the anatomical axis and the guide rod axis. The accuracy of the guide rod was also assessed radiographically. Anteroposterior and lateral radiographs were obtained of all the disarticulated specimens
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The Knee 1994; 1: No 2
of valgus-2.6” of varus). In the lateral view, the guide rod was in 4.6” of flexion compared with the mechanical axis of the femur (range 3.1”-5.8’ of flexion). Collapse of the femoral neck was associated with a decrease in the difference between the mechanical axis and the guide rod axis in the anteroposterior view (Figure 2). Discussion Correct alignment of the lower extremity is essential for the success of total knee arthroplasty’-5. Studies have shown that the postoperative alignment of the lower extremity has a direct effect on the durability of the implant’. If the postoperative alignment has significant valgus or varus, loosening may occur at a greater rate compared with a correctly aligned lower extremity. Various instruments have been designed in order to assist the surgeon in constructing a correctly aligned lower extremity. The use of the intramedullary alignment guide for femoral cut orientation is common to most knee systems now in use. The guide rod is used as a referencing device which approximates the anatomical axis of the femur. From analysis of full-length standing pre-operative radiographs, the relationship between the anatomical and mechanical axis of the femur can be
Figure 1. Diagrammatic representation of the anatomical and mechanical axes of the femur and lower extremity.
with the guide rod in place. Each radiograph then had lines drawn on it representing the anatomical, mechanical and guide rod axis. Using trigonometric analogue tables, differences between the anatomical axis and mechanical axis were calculated on the anteroposterior views. In addition, the difference between the guide rod axis and the anatomical axis was calculated. On the lateral radiographs, the difference between the guide rod axis and the mechanical axis was determined.
Results On the basis of the anatomical measurements, the guide rod was in 0.9” of flexion compared with the anatomical axis of the femur (range 0.1” of extension1.4” of flexion). In the varus-valgus plane, the guide rod was in 0.6” of valgus (range O.l”-1.0” of valgus). Radiographic analysis of the specimens showed that the difference between the anatomical axis and the mechanical axis in the anteroposterior view was 5.7” (range 1.7”-9.4”). The difference between the mechanical axis and the guide rod axis in the anteroposterior view was 4.2” (range 2.0”8.2”). The guide rod was in an average of 1.5” of varus compared with the anatomical axis (0.3”
Figure 2. 1bteroposterior view of incompletely seated sliding hip compression screw (note guide rod fl-ollowing -.....i the shortening oft he femoral neck).
Roger et al.
Intramedullary
alignment
in total knee arthroplasty
85
ascertained. Once this has been determined, the prosthesis can be positioned so that the load-bearing surface is perpendicular to the mechanical axis of the lower extremity. Proximal femoral instrumentation prior to total knee arthroplasty will prevent complete seating of the intramedullary guide rod for the femoral component. On the basis of this study, this will place the distal femoral cut into an average of 4.6” of flexion compared with the mechanical axis of the femur (Figure 3). The anterior bow of ,the femur makes it difficult to define precisely the anatomical axis of the femur in the lateral view. The results of this study also show that an incompletely seated femoral guide rod will place the distal femoral cut in an average of 1.5” of varus compared with the anatomical axis of the femur. The maximum difference in this study between the guide rod axis and the anatomical axis was 2.6”. In the case of actual knee arthroplasty, knowledge of this information could allow the surgeon to compensate for this difference, and more accurately position the implant. The authors conclude that incomplete seating of the intramedullary guide rod in total knee arthroplasty can lead to discrepancies between the guide rod axis and
the anatomical axis of the femur. While these differences are small, they can contribute to inaccurate alignment of the femoral component. Additional reference systems, such as extramedullary alignment and/or intraoperative radiography, might be useful in maximizing the accuracy of femoral component position.
a
b
References Bargren JH, Blaha JD, Freeman MAR. Alignment in total knee arthroplasty: correlated biomechanical and clinical observations. Clin Orthop 1983; 173: 178-83 Chao EYS. Biomechanics of high tibia osteotomy. In: Symposium on Reconstructive Surgery of the Knee. St Louis: CV Mosby, 1978: 14340 Dorr LD, Conaty JP, Schreiber R, Mehne DK, Hull D. Technical factors that influence mechanical loosening of total knee arthroplasty. In: Dorr LD, ed. The Knee: Papers of the First Scientific Meeting of The Knee Society.
Baltimore: University Park Press, 1985: 121-35 Hungerford DS, Krackow KA, Kenna RV. Alignment in total knee arthroplasty. In: Dorr LD, ed. The Knee: Papers of the First Scientific Meeting of The Knee Society.
Baltimore: University Park Press, 1985: 9-21 Moreland JR. Mechanisms of failure in total knee arthroplasty. Clin Orthop 1988; 226: 49-64
Laskin RS. Alignment of total knee components. Orthopedics 1984; 7: 62-72
Figure 3. (a) Anteroposterior view of incompletely seated guide rod following cemented femoral endoprosthesis; (b) lateral view of incompletely seated guide rod following cemented femoral endoprosthesis (note the flexion of the guide rod compared with the mechanical axis of the femur).
83-85
lntramedullary alignment in total knee arthroplasty following proximal femoral instrumentation: an anatomical and radiographic study D J Roger, J V Bono, C A Paulsen Division of Orthopaedic
Surgery, Albany Medical College, Albany, NY, USA
Summary Correct component alignment is essential for the success of total knee arthroplasty. The use of intramedullary alignment in the femur has become the current standard for correct placement of the femoral component. The accuracy of this system is dependent on the degree to which the intramedullary guide rod approximates the anatomical axis of the femur. Cases in which the guide rod cannot be completely seated, such as after a femoral endoprosthesis, can result in increased differences between the anatomical axis and the guide rod axis. In these cases, additional referencing systems, such as extramedullary alignment or intra-operative radiography, may be useful to increase the accuracy of component position. IThe Knee 1994; i: 83-85) Key words: knee, atthroplasty, alignment, intramedullary
Introduction
Correct alignment of the prosthesis in total knee arthroplasty is of paramount importance to the function and survivorship of the implant. In the early designs of knee implants, the instrumentation that was used placed significant demands on the technical ability of the surgeon. Subsequent improvements in the design of the instrumentation have led to more predictable positioning of the implant. Specifically, the intramedullary alignment rod for orientation of the distal femoral resection has enabled the surgeon to perform reproducible cuts resulting in consistent component position (Figure 1). Cases in which there has been instrumentation of the proximal femur, e.g. a sliding hip screw or femoral endoprosthesis, can prevent complete seating of the rod. In these patients, the authors hypothesized that the incompletely seated rod might not approximate the true anatomical axis of the femur, and therefore could result in component malposition. The purpose of this
Accepted: March 1994 Correspondence and reprint requests to: D.J. Orthopaedic Surgery, A-280, Albany Medical Avenue, Albany, NY 12208, USA @ 1994 Butterworth-Heinemann 0968-0160/94/02083-03
Ltd
Roger MD, Division of College, New Scotland
study was to evaluate the effect of proximal femoral instrumentation on the position of the intramedullary alignment rod. Materials and methods
Seven cadaveric specimens with a known history of proximal femoral surgery were harvested by disarticulation of the femur at the hip joint and the knee joint. Four specimens had a compression screw and side plate, one had an Austin-Moore endoprosthesis, one had a bipolar endoprosthesis and one had three threaded pins in the femoral neck. Following disarticulation, the femora were evaluated anatomically and radiographically. Anatomically, the accuracy of the guide rod was assessed by sectioning the femur at the tip of the rod, and then measuring the distance from the middle of the guide rod to the endosteal cortex. Varusvalgus alignment was assessed by measuring the distance from the guide rod to the medial and lateral endosteal surfaces. Flexion-extension alignment was assessed by measuring the distance from the guide rod to the anterior and posterior endosteal surfaces. Mathematical triangulation was then used to calculate the difference between the anatomical axis and the guide rod axis. The accuracy of the guide rod was also assessed radiographically. Anteroposterior and lateral radiographs were obtained of all the disarticulated specimens
84
The Knee 1994; 1: No 2
of valgus-2.6” of varus). In the lateral view, the guide rod was in 4.6” of flexion compared with the mechanical axis of the femur (range 3.1”-5.8’ of flexion). Collapse of the femoral neck was associated with a decrease in the difference between the mechanical axis and the guide rod axis in the anteroposterior view (Figure 2). Discussion Correct alignment of the lower extremity is essential for the success of total knee arthroplasty’-5. Studies have shown that the postoperative alignment of the lower extremity has a direct effect on the durability of the implant’. If the postoperative alignment has significant valgus or varus, loosening may occur at a greater rate compared with a correctly aligned lower extremity. Various instruments have been designed in order to assist the surgeon in constructing a correctly aligned lower extremity. The use of the intramedullary alignment guide for femoral cut orientation is common to most knee systems now in use. The guide rod is used as a referencing device which approximates the anatomical axis of the femur. From analysis of full-length standing pre-operative radiographs, the relationship between the anatomical and mechanical axis of the femur can be
Figure 1. Diagrammatic representation of the anatomical and mechanical axes of the femur and lower extremity.
with the guide rod in place. Each radiograph then had lines drawn on it representing the anatomical, mechanical and guide rod axis. Using trigonometric analogue tables, differences between the anatomical axis and mechanical axis were calculated on the anteroposterior views. In addition, the difference between the guide rod axis and the anatomical axis was calculated. On the lateral radiographs, the difference between the guide rod axis and the mechanical axis was determined.
Results On the basis of the anatomical measurements, the guide rod was in 0.9” of flexion compared with the anatomical axis of the femur (range 0.1” of extension1.4” of flexion). In the varus-valgus plane, the guide rod was in 0.6” of valgus (range O.l”-1.0” of valgus). Radiographic analysis of the specimens showed that the difference between the anatomical axis and the mechanical axis in the anteroposterior view was 5.7” (range 1.7”-9.4”). The difference between the mechanical axis and the guide rod axis in the anteroposterior view was 4.2” (range 2.0”8.2”). The guide rod was in an average of 1.5” of varus compared with the anatomical axis (0.3”
Figure 2. 1bteroposterior view of incompletely seated sliding hip compression screw (note guide rod fl-ollowing -.....i the shortening oft he femoral neck).
Roger et al.
Intramedullary
alignment
in total knee arthroplasty
85
ascertained. Once this has been determined, the prosthesis can be positioned so that the load-bearing surface is perpendicular to the mechanical axis of the lower extremity. Proximal femoral instrumentation prior to total knee arthroplasty will prevent complete seating of the intramedullary guide rod for the femoral component. On the basis of this study, this will place the distal femoral cut into an average of 4.6” of flexion compared with the mechanical axis of the femur (Figure 3). The anterior bow of ,the femur makes it difficult to define precisely the anatomical axis of the femur in the lateral view. The results of this study also show that an incompletely seated femoral guide rod will place the distal femoral cut in an average of 1.5” of varus compared with the anatomical axis of the femur. The maximum difference in this study between the guide rod axis and the anatomical axis was 2.6”. In the case of actual knee arthroplasty, knowledge of this information could allow the surgeon to compensate for this difference, and more accurately position the implant. The authors conclude that incomplete seating of the intramedullary guide rod in total knee arthroplasty can lead to discrepancies between the guide rod axis and
the anatomical axis of the femur. While these differences are small, they can contribute to inaccurate alignment of the femoral component. Additional reference systems, such as extramedullary alignment and/or intraoperative radiography, might be useful in maximizing the accuracy of femoral component position.
a
b
References Bargren JH, Blaha JD, Freeman MAR. Alignment in total knee arthroplasty: correlated biomechanical and clinical observations. Clin Orthop 1983; 173: 178-83 Chao EYS. Biomechanics of high tibia osteotomy. In: Symposium on Reconstructive Surgery of the Knee. St Louis: CV Mosby, 1978: 14340 Dorr LD, Conaty JP, Schreiber R, Mehne DK, Hull D. Technical factors that influence mechanical loosening of total knee arthroplasty. In: Dorr LD, ed. The Knee: Papers of the First Scientific Meeting of The Knee Society.
Baltimore: University Park Press, 1985: 121-35 Hungerford DS, Krackow KA, Kenna RV. Alignment in total knee arthroplasty. In: Dorr LD, ed. The Knee: Papers of the First Scientific Meeting of The Knee Society.
Baltimore: University Park Press, 1985: 9-21 Moreland JR. Mechanisms of failure in total knee arthroplasty. Clin Orthop 1988; 226: 49-64
Laskin RS. Alignment of total knee components. Orthopedics 1984; 7: 62-72
Figure 3. (a) Anteroposterior view of incompletely seated guide rod following cemented femoral endoprosthesis; (b) lateral view of incompletely seated guide rod following cemented femoral endoprosthesis (note the flexion of the guide rod compared with the mechanical axis of the femur).