During total knee arthroplasty (TKA), the rotational alignment of the femoral component significantly affects not only femorotibial stability in flexion but also patellofemoral stability.1,3,6,7,21,23 Numerous studies have attempted to establish reproducible and reliable reference axes in the femoral condyles for setting the femoral component in appropriate alignment. Useful rotational reference axes proven reliable for this include the posterior condylar axis,14 the transepicondylar axis,8,20,27 and the midtrochlear line (Whiteside anteroposterior [AP] axis).4,30 Of these reference axes, the transepicondylar axis has been shown to be not only a useful anatomic reference axis, but also a functional flexion-extension axis.9,13,27 Furthermore, it has been reported that aligning the femoral component parallel to the transepicondylar axis resulted in optimum patellofemoral tracking and minimized femorotibial wear motion and instability.22 Therefore, the transepicondylar axis has been established as the mediolateral axis of the femur for TKA.
Compared with the femur, relatively little attention has been given to establishing reliable reference axes for correct rotational orientation of the tibial component. Various anatomic references have been proposed and used according to individual surgeons’ preferences and experiences.12,23 These include the orientation of the tibial tubercle,15,21,23 the posterior condylar line of the tibia,23 the transcondylar line of the tibia,11 the midsulcus of the tibial spine,10 the intermalleolar axis of the ankle, and the long axis of the foot (axis of the second metatarsus bone). It has been said that the tibial tray should be rotated externally to approximately the medial ⅓ of the tibial tubercle to maximize function.10,15 However, we are not aware of any published literature on the theoretical background of this technique, which seems to be established empirically. It has been reported that the medial ⅓ of the tibial tubercle technique might result in too much external rotation to the tibial component.10,28 If the medial border of the tubercle is used as an anterior landmark,16,21 surgeons need to know the other (posterior) anatomic landmark to recognize the AP orientation. The posterior condylar line of the tibia,23 the transcondylar line of the tibia,11,33 and the midsulcus of the tibial spine10 can be used to determine the rotational orientation of the tibial articular surface. However, osteophyte formation, deformity and bone loss of the articular surface of the tibia, and general anatomic variations can make it difficult to determine these reference axes in an operating field.23 Regarding the intermalleolar axis of the ankle and the long axis of the foot as it is held up in the neutral position of the ankle, significant anatomic variations among individuals have been shown by cadaver and computed tomography (CT) studies.11,31,33 Furthermore, those axes can be deformed in patients with rheumatoid or osteoarthritic changes in their ankles and feet. Therefore, it seems to be difficult to accurately determine the rotational orientation of the tibia using those reference axes.23 There is another method to rotationally align the tibial component, by which surgeons do not refer anatomy of the tibia.12,23 In this method, preparation of most of the joint surfaces is done, the trial of the femoral component is set, and then rotation of the tibial component with regard to the femoral component is indicated to the surgeon by the soft tissues that are tensed.12 The neutral position of the tibia is marked to parallel that of the femur in extension when longitudinal tension is applied across the joint. This method is dependent on correct femoral rotation and soft tissue balancing being done during the operation.
The objective of the current study is to identify new extraarticular anatomic landmarks indicating the AP orientation of the tibia using CT scans of the healthy knees, which would not be affected by destruction of the articular surface. We defined the AP axis of the tibia as a line perpendicular to the transepicondylar axis (the surgical epicondylar axis) of the femur and passing through the middle of the posterior cruciate ligament (PCL) at the tibial attachment in extended knees. Defining the AP axis of the tibia as mentioned previously makes it possible to investigate an anterior anatomic landmark for constructing the AP axis in the operation field. We investigated where the defined AP axis intersects the patellar tendon, because the patellar tendon is a solid extraarticular soft tissue and the medial and lateral edges of the tendon are clearly recognized on the CT scans and in an operating field. Finally, we investigated how much the tibial component was externally rotated when surgeons aligned it to the medial ⅓ of the patellar tendon, which conventionally is used as an anatomic landmark of rotational orientation of the tibial component.
MATERIALS AND METHODS
Twenty healthy male (mean age, 38.4 ± 6.7 years; range, 28 – 48 years) and female (mean age, 42.2 ± 6.8 years; range, 25–51 years) Japanese volunteers were used as subjects. They were informed of the risk of radiographic exposure during CT scans and written informed consent was obtained. The study protocol was approved according to the relevant regulations of our institution. Transverse CT scans of the subjects’ right knees were made at levels ranging from 40 mm proximal to the joint to 40 mm distal to the joint (X-vigor, Toshiba Medical Inc, Tokyo, Japan). During scanning, the right leg of the volunteer was set fixed in a wooden frame so that the knee was kept in an extended position (0° flexion) and placed parallel to the CT bed. The knee position was determined so that the subject felt a naturally extended—neither flexed nor hyperextended—knee position in the frame, without any feeling of internal or external rotation. Scans were made perpendicular to the tibial shaft axis at 3-mm intervals with a 2-mm-wide source beam. One woman with lateral subluxation of the patella observed on the CT scan was excluded from the investigation. The transverse CT scan images were processed using an image scanner (GT-8000, Seiko Epson Co, Tokyo, Japan) and entered into a PC (Macintosh PowerBook G3, Apple Computer, Inc, Cupertino, CA). The computer software packages used in the study were developed by us with the aid of a software developing language (REALbasic 2.1, REAL Software Inc, Austin, TX). These were used for overlaying CT scans spatially, for drawing lines or points on CT scans, projecting a line or a point on a scan to another scan, measuring relative widths, and for measuring the angles between two lines. The knee valgus angle (angle made by the femoral shaft axis and the tibial shaft axis) of each subject was measured using an AP scanogram obtained by CT. Using those CT scans on which the medial and lateral epicondyles could be recognized most clearly, the transepicondylar axis (surgical epicondylar axis) was determined connecting the sulcus of the medial epicondyle to the lateral epicondyle (Fig. 1A).2,8 In three women’s knees, the sulcus of the medial epicondyle could not be recognized on CT scans. In these subjects, the transepicondylar axis was assumed as a line rotating the clinical epicondylar axis (CEA, a line connecting the most prominent point of the medial epicondyle and the lateral epicondyle) internally by 3°.2,32 The transepicondylar axis was projected onto a scan of the tibial plateau, where the PCL was recognized in the posterior condylar notch, and the AP axis of the tibia was drawn as a line passing through the middle of the PCL and perpendicular to the projected transepicondylar axis (Fig. 1B). In this scan, the medial percentage width of the intersecting point of the patellar tendon and the AP axis was defined as m/l × 100, where the patellar tendon width was l and the width medial to the intersecting point was m. The transepicondylar axis then was projected to a scan in which the patellar tendon was seen attached to the tibial tubercle. In the same manner, the medial percentage width, m’/l’ x 100, of the intersecting point of the patellar tendon and the AP axis was measured at the patellar tendon attachment level (Fig. 2). Next, the angle between the AP axis and a line connecting the middle of the PCL and medial border of the patellar tendon at the tibial attachment was measured. The angle between the AP axis and a line connecting the middle of the PCL and the medial ⅓ of the patellar tendon at the level of the attachment also was measured (Fig. 3).
Measurements of the medial percentage width of the intersecting point of the patellar tendon and the AP axis were done by one observer (MA). Intraobsever variations of the measurements were assessed by repeating the measurements 10 times in three subjects. A maximum intraobserver difference of measurements was 4% and the largest standard deviation was 1.1%. Measurement of the angle between the AP axis and a line connecting the middle of the PCL and medial border of the patellar tendon was done by three independent observers (MA, MO, and TN). All angular measurements were repeated by the three observers and mean was regarded as a true value. A maximum interobserver difference was less than 2.0°
The mean knee valgus angle of the subjects in the current study was approximately the same as those of other studies on frontal knee alignment of healthy Japanese subjects, which was 4.2° ± 2.9° (range, −0.6°–+9.6°) for all subjects (n = 39). For the men (n = 20) this angle was 3.1° ± 2.8° (range, −0.6°–+8.8°) and for the women (n = 19) 5.3° ± 2.7° (range, 0.0°–+9.6°). There was a statistically significant difference between the mean valgus angle of the male and female subjects (unpaired t test, < 0.02).
At the level of the tibial plateau, the defined AP axis passed approximately 11% lateral to the medial edge of the patellar tendon. The mean medial percentage width of the intersecting point of the patellar tendon and the AP axis was 10.8% ± 9.8% (range, −9.3%–+30.0%) for all subjects. For the men, the mean value was 12.8% ± 9.0% (range, −4.5%–+26.9%) and for the women, it was 8.8% ± 10.4% (range, −9.3%–+30.0%) (Table 1).
At the level of the patellar tendon attachment, the AP axis passed at the medial border of the tendon on average. The mean medial percentage width of the intersecting point of the patellar tendon and the AP axis was −0.2% ± 10.4% for all subjects (range, −23.6%–+23.0%). The distribution of results was monomodal with the peak lying between −5% and 0% (Fig. 4). The mean value for the men was 1.7% ± 10.6% (range, −16.9%–+23.0%) and for the women it was −2.3% ± 10.0% (range, −23.6%–+11.8%) (Table 1). The mean angle between the AP axis and the line connecting the middle of the PCL and the medial border of the patellar tendon attachment was 0.0° ± 2.8° for all subjects (range, −6.3°–+5.2°). The distribution was monomodal with the peak between 0° and 2.5° (Fig. 5). The mean angle for the men was 0.4° ± 2.5° (range, −3.1°–+5.2°), and that for the women was −0.4° ± 3.1° (range, −6.3°–+4.8°) (Table 2).
On average, the line connecting the middle of the PCL and the medial ⅓ of the patellar tendon at the attachment level rotated externally by 10° relative to the AP axis. The mean angle between the AP axis and the line connecting the middle of the PCL and the medial ⅓ of the patellar tendon at the attachment level was 10.0° ± 4.2° for all subjects (range, 1.6°–19.5°); for the men it was 9.1° ± 3.6° (range, 4.1°–15.9°), and for the women it was 10.8° ± 4.6° (range, 1.6°–19.5°) (Table 2).
The rotational relationship between the femoral and tibial components is an important factor affecting the overall function and durability of a TKA.7,19,22,23,29 If a knee prosthesis with low constraint in axial rotation is used, rotational mismatch of the components can result in subluxation of the femorotibial joint and premature wear or breakage of the polyethylene insert.19,30 By contrast, if a knee prosthesis with high constraint in axial rotation is used, the mismatch can result in impingement between the polyethylene and CoCr, leading to limitation in knee motion, rotational malalignment between the knee and ankle, and toe-in or toe-out gaits. Furthermore, the rotational relationship of components can affect patellar stability and function.1,7,21,23,25 Excessive internal rotation of the tibial component increases the risk of patellar subluxation and lateral excessive wear of the patella.7,21,25 Conversely, excessive external rotation also may result in abnormal tracking of the patella.24
On the femoral side, the transepicondylar axis has been established as the mediolateral axis of the femur and setting the femoral component parallel to the transepicondylar axis seems reasonable. Compared with the femoral side, reliable reference axes for correct rotational orientation of the tibial component has not been established. Although various anatomic references have been proposed to determine the tibial orientation, those references are used according to individual surgeons’ preferences and experiences. Therefore, it would be worth establishing a well-defined reference axis for determining tibial rotational orientation during surgery. We defined the AP axis of the tibia in an extended knee position as a line perpendicular to the transepicondylar axis and passing through the middle of the PCL. Based on this definition, the extraarticular anterior anatomic landmark was searched using CT scans. Reasons for the definition are the following. First, the femoral component is set parallel to the transepicondylar axis and rotational matching of the knee components is checked in the extension position during surgery. Second, the PCL seems to be a useful posterior landmark to determine the AP axis because the ligament, which attaches to the posterior condylar notch of the tibia, is in the center of the knee. Furthermore, it is present in more than 99% of knees at the time of primary TKA26 and the middle of the PCL at the tibial attachment is identified easily during surgery. Third, the PCL is clearly identified as an elliptical patch of soft tissue in the notch seen on CT scans of the proximal tibia.
The major limitations of the current study include that the number of subjects was limited (total number of subjects was 39, 20 males and 19 females), and the study population was limited to Japanese subjects. The data included in the current study may be typical for knees of Asian subjects and there may be anatomic differences in the Caucasian population. Therefore, the results of this study should be interpreted cautiously. Another limitation of the current study was potential errors in making the measurements. Assessment of the interobserver and intraobserver variation of the measurements showed that the computer-assisted measuring methods introduced in the current study had reasonable reliability. However, the AP axis of the tibia was defined based on the transepicondylar axis (the mediolateral axis of the femur), which was projected to the CT scans of the proximal tibia beyond the knee. Therefore, the variability of measurements included variability resulting from axial rotation between the femur and tibia, because the CT scans were obtained at 0° knee flexion, whereas the knee has a little freedom of longitudinal rotation. Adding to normal anatomic variations, this may be why the measurements varied in the current study, although the knee position was determined so that the subject felt a naturally extended knee position without any feeling of internal or external rotation.
We investigated the relationships between the patellar tendon and the AP axis defined above. The tibial tubercle has been used intraoperatively to determine the rotational alignment of the tibial component as an extraarticular bony landmark. However, surgeons subjectively locate medial and lateral borders of the tubercle and it is difficult to define clearly those borders on CT scans. On the contrary, the patellar tendon is clearly identifiable as a flat area of soft tissue surrounded by low-density fat tissue on CT scans. The medial and lateral borders of the tendon can be distinguished from the thin medial and lateral patellar retinacula. The tendon also is clearly identified in the operating field and is not affected by arthritis or deformity because it is an extraarticular soft tissue.
At the level of the tibial plateau, the AP axis on average passed at 10.8% ± 9.8% of the width of the patellar tendon, lateral to the medial border. At the level of the patellar tendon attachment to the tibial tubercle, the AP axis passed on average at 0.2% ± 10.4% of the width of the patellar tendon, medial to the medial border of the patellar tendon (Table 1). These results show that the relationship between the AP axis and the patellar tendon varies depending on cutting levels of the CT scans. At the patellar tendon attachment level, the intersecting point of the AP axis relative to the patellar tendon distributed monomodally and the peak of the distribution was located at approximately zero (Fig. 4). This observation suggested that the line connecting the middle of the PCL to the medial border of the patellar tendon attachment is perpendicular to the transepicondylar axis and prompted us to measure the angle between the AP axis and the line. The mean and modal angles between the AP axis and the line were 0.0° ± 2.8° (Table 2; Fig 5). Therefore, a line connecting the middle of the PCL to the medial border of the patellar tendon at the attachment level can provide a reproducible and reliable line parallel to the AP axis of the tibia. Therefore, if the femoral component is set parallel to the transepicondylar axis and the tibial component is set perpendicular to the line, the components align correctly for rotation in an extended knee position.
The mean angle between the AP axis and the line connecting the middle of the PCL and the medial ⅓ of the patellar tendon at the level of the tubercle attachment was 10.0° ± 4.2°. If the femoral component is set parallel to the transepicondylar axis and the tibial component is set perpendicular to the line, the tibial component would have approximately 10° excessive external rotation relative to the femoral component in an extended knee position. However, this external rotation of the tibial component may cause the tibia to rotate internally by the femorotibial conformity of the prosthesis, resulting in medial placement of the tibial tubercle and reduction of the Q angle. The normal medial pivoting motion of the knee, which is observed in the in vivo normal knee motion under weightbearing conditions,5,9,18 seems to be beneficial for the patellofemoral joint to reduce lateral excessive pressure because the motion reduces the Q angle in flexion. In a joint with a total knee prosthesis, the medial pivoting motion of the femorotibial joint seldom can be reproduced and is small if it exists.17 Setting the tibial component to align to the medial ⅓ of the tibial tubercle therefore may be beneficial to the patellofemoral joint, although the femorotibial joint may be at risk.
In many instrumentation systems for TKA, cutting guides for the proximal tibia have a posterior slope to minimize bone loss. Cutting the proximal tibia with a posterior slope, it is important for surgeons to know exactly where the anterior aspect of the tibia really is, because when the cutting guide is externally rotated, posteromedial (varus) slope will be produced and when the guide is internally rotated, a posterolateral (valgus) slope will result.16 If direction of the posterior slope is set parallel to the line connecting the middle of the PCL and the medial border of the patellar tendon attachment, the proximal tibia would be cut neutrally in an AP view and the posterior slope intended should be produced in a lateral view. Therefore, the AP orientation proposed in the current study would be useful to obtain a correct perpendicular cut of the proximal tibia.
In the current study, the AP axis of the tibia was defined as a line passing through the middle of the PCL and perpendicular to the transepicondylar axis in an extended position of the knee. The medial border of the patellar tendon attachment was identified as an anterior anatomic landmark to determine the AP axis in the operation field. We now are using the line connecting the middle of the PCL and the medial border of the patellar tendon attachment as the AP axis of the tibia during TKAs. The line is reliable and reproducible. Furthermore, when planning placement of the components preoperatively using computer simulation techniques, the AP axis of the tibia is useful to determine the rotational alignment of the tibial component, because the line can be drawn on the cut surface of the proximal tibia independently from the transepicondylar axis of the femur.
We thank Yamada K, Tatsumi Y, and Miyata Y (Kinki University Hospital, Osaka-Sayama City, Osaka, Japan) for technical assistance and advice in CT scanning.
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