Healio
Healio
October 01, 2006
8 min read
Save

Rotational Alignment of Femoral Components in Total Knee Arthroplasty: Nonimage-based Navigation System Versus Conventional Technique

Abstract

A randomized prospective study was undertaken to compare the accuracy of rotational alignment of the femoral component in total knee arthroplasty using a nonimage-based navigation system and a conventional method. The authors randomly used two methods and implants in 55 patients and measured the angles between posterior condylar lines and transepicondylar axes on computed tomography images before and after surgery. Analysis showed no significant differences between the two techniques in mean errors (P>.05), but the navigation system showed higher accuracy than the 3° external rotation method by reducing outliers.

Proper rotational alignment of the femoral component in total knee arthroplasty (TKA) is an important factor for adequate postoperative patellofemoral and tibiofemoral kinematics and for balance of the flexion space at 90° of flexion. Moreover, proper rotational alignment may influence polyethylene insert wear and implant longevity.1,2 General consensus claims excessive internal rotation of the femoral component can lead to poor patellar tracking and imbalance of the flexion gap at 90° of flexion.3-5 However, controversy still exists in the literature regarding the amount of external rotation and the ideal anatomic axis for rotational alignment of the femoral component. Several reports suggest that the clinical transepicondylar axis (the line connecting the medial and lateral epicondylar prominences) most consistently recreates a balanced flexion space and normal patellofemoral tracking.6-8

Several conventional methods are used to obtain correct axial femoral alignment in TKA, including the transepicondylar axis method, Whiteside’s method, the tibial axis method, and the 3° external rotation-off posterior condyles method. Recently, computer-assisted navigation systems have been introduced to obtain more precise alignment, and the systems show excellent coronal alignment accuracy.9,10 The systems’ efficacies in terms of rotational alignment are not proven, however. In this study, the accuracy of a navigation system versus a conventional method in terms of the rotational alignment of femoral components by using computed tomography (CT) was compared in a prospective and randomized manner.

Materials and Methods

From July 2005 to December 2005, 55 patients who underwent TKA performed by a single surgeon were randomized prospectively into two groups (a conventional method group and a nonimage-based navigation method group). The participants in this study were patients with osteoarthritis who required primary TKA. Patients were excluded if they had a history of tibial or femoral fractures or pyogenic arthritis of the knee joints in a growth period or if they did not consent to the study. Randomization was done using a table of random numbers. All patients provided written consent, and the authors’ institutional review board approved the study protocol.

Two methods were used in this study. The 3° external rotation off posterior condyles method with The PFC Sigma Rotating Platform (Depuy Orthopaedics Inc, Warsaw, Ind) was used for the 28 patients in the conventional group. The OrthoPilot system (B. Braun Aesculap, Tuttlingen, Germany) was used for the 27 patients in the navigation group with e.motion (B. Braun Aesculap).

Using 8-channel multidetector CT (Lightspeed Ultra, GE Medical System, Milwaukee, Wis), axial CT images of the distal femur were obtained at a section thickness of 1 mm to 1.3 mm before and after implantation in all patients.

Surgical Technique

All TKA procedures were performed using a medial parapatellar arthrotomy with patellar eversion under pneumatic tourniquet control. In the conventional group, procedures were performed using standard instrumentation (an intramedullary guide for distal femoral resection and an extramedullary guide for proximal tibial resection). Femoral valgus angles for the intramedullary guide were determined on preoperative whole-length standing radiographs. After distal femoral bone resection, the posterior cruciate ligament was completely released. The proximal tibia was resected perpendicular to its mechanical axis with a posterior slope of 0° to 3°, using an extramedullary tibial guide. Extension gap balance was assessed using spacers and stress testing, and additional soft tissue release was performed when needed. For decision making concerning femoral component rotation, a rotational alignment axis, which was rotated 3° external to the posterior condylar axis, was established in the conventional group.

In the navigation group, the data-collecting process including identification of epicondyles was performed, and flexion gap balancing was assessed using laminar spreaders and the navigation system. The rotational alignment axis was defined according to the navigation system to achieve a symmetric flexion gap (Figure 1). After resections were performed, cutting planes were checked and documented.

figure 1

figure 2

Figure 1: Screen shot showing the amount of rotation required for a symmetric flexion gap using the navigation system. Figure 2: The posterior condylar angle is formed by the transepicondylar axis and a tangent line to posterior condyles. The AP axis of distal femur is usually perpendicular to the transepicondylar axis.

Radiologic Measurements

Computed tomographic images were acquired digitally using a picture archiving and communication system, and the measurements were subsequently carried out using 3-D imaging software (V-works 5.0, Cybermed, Seoul, Korea). The computerized measuring system allows surgeons to measure angles between lines in different planes.

To determine the precision of the navigation system and instruments in rotational alignment, the authors compared navigation data collected intraoperatively with those obtained from CT images. During TKA, planned angles of rotation of the femoral cutting guide and angles of rotation after bone cutting and implantation were documented and later compared with angles of rotation in postoperative CT images.

To assess preoperative and postoperative rotational alignments, posterior condylar angles (PCAs) formed by the clinical transepicondylar axis and a tangent line to posterior condyles were measured (Figure 2). Postoperative rotational alignment with a PCA value of more than ±3° was defined as an outlier.

Statistical Analysis

The Mann-Whitney U test and the 2 test were used to compare angles between the two groups and data between navigation and CT images. The analysis was performed using SPSS version 11.5 (SPSS Inc, Chicago, Ill).

table 1

table 2

Results

No patient in either group was excluded or lost from the study, and no significant differences were observed between the two groups in terms of sidedness, gender distribution, age at TKA, or other demographic data (Table 1).

Significant difference was not observed between the navigation system intraoperative data and postoperative CT data in terms of angles of rotation (Table 2). The mean values of PCA from the navigation system and CT images measured intraoperatively and preoperatively were 2.98°±3.74° and 5.87°±1.88°, respectively, and these values were different significantly (P=.00).

The mean values of preoperative and postoperative PCAs measured on CT images were 5.27°±1.71° and 2.17°±2.31° in the conventional group and 5.87° ± 1.88° and 2.05°±2.53° in the navigation group. No significant difference was found between the two groups (P=.39 and P=.12). The percentage of postoperative PCA outliers in the conventional group was more than the percentage in the navigation group (P=.04) (Figure 3).

figure 3

Figure 3: Graph showing the postoperative posterior condylar angles of both groups, which represent deviations from the transepicondylar axis. The painted area indicates rotational alignment outliers of femoral components.

Discussion

The authors sought to demonstrate whether the nonimage-based navigation system was more accurate than the conventional 3° external rotation off posterior condyles method for the rotational alignment of the femoral components in TKA. The nonimage-based navigation system and instruments reproduced precisely the planned rotation of the femoral component. The navigation system demonstrated original posterior condylar angles inaccurately, which might be attributable to an error in the landmark identification for the transepicondylar axis. Low reproducibility has been reported for intraoperative measurements of the transepicondylar axis during TKA.11

When using the classic method of bone resection, the 3° external rotation off posterior condyles method has been used to make a rectangular flexion gap at 90° of flexion, because the average tibial slope in the frontal plane was reported to be 3° of varus with reference to the mechanical axis of the tibia.12 A study performed in China, however, demonstrated a significantly larger medial inclination of the knee joint in patients and suggested performing a more external rotation of the femoral component to make a rectangular flexion gap at 90° in TKA.13 Moreover, the transepicondylar axis reportedly displays a parallel relationship with the perpendicular line of the tibial mechanical axis and recreates a balanced flexion gap.14,15 According to the results, the mean PCA values in both groups were over 5°. The finding concurs with a previous report that suggested the commonly reported 3° of external rotation might be insufficient to make a rectangular flexion gap at 90°. In addition, another study reported that the transepicondylar axis might be closer to the functional axis for patellofemoral articulation, because Whiteside’s anteroposterior axis is perpendicular to the functional axis.16 As noted above, the transepicondylar axis is proposed to be the most ideal rotational reference providing functional kinematics.7 Postoperative PCA values should be as close to 0° as possible. The mean value of PCA in the navigation system was similar to that measured using the conventional method. However, outliers of more than 3° were found less commonly in the navigation group.

Several limitations to the present study should be noted. First, patients with a history of tibial or femoral fractures were excluded. For patients with deformed femurs or tibias, conventional methods are limited in terms of the use of standard instrumentation and the attainment of accurate alignment, and thus, navigation may be superior. Therefore, for ethical reasons, the authors used the navigation system for patients with deformities. The second limitation involves the selective nature of the patient population. All patients in the study had osteoarthritis and neutral or varus alignment. Thus, the findings of the study are not applicable for patients with valgus knees, which are typically known to have a small lateral condyle.5 A limitation in the use of CT should also be noted. Differences in the loss of posterior femoral articulating cartilage could not be assessed using CT; CT reduces PCA measurements. The authors considered that CT does not significantly influence results, because this limitation applies equally to both groups.

Conclusion

A nonimage-based navigation system reproduced precisely planned femoral rotation but could not demonstrate actual PCAs, which may be due to errors in the identification of femoral epicondyles. Although the navigation system showed little difference in terms of mean measurements, the system significantly improved the accuracy of rotational alignment of femoral components in TKA against the 3° external rotation off posterior condyles method by reducing outliers.

References

  1. Akagi M, Matsusue Y, Mata T, et al. Effect of rotational alignment on patellar tracking in total knee arthroplasty. Clin Orthop Relat Res. 1999; 366:155-163.
  2. Insall JN, Scuderi GR, Komistek RD, et al. Correlation between condylar lift-off and femoral component alignment. Clin Orthop Relat Res. 2002; 403:143-152.
  3. Barrack RL, Schrader T, Bertot AJ, et al. Component rotation and anterior knee pain after total knee arthroplasty. Clin Orthop Relat Res. 2001; 392:46-55.
  4. Berger RA, Crossett LS, Jacobs JJ, Rubash HE. Malrotation causing patellofemoral complications after total knee arthroplasty. Clin Orthop Relat Res. 1998; 356:144-153.
  5. Kuster MS, Stachowiak GW. Factors affecting polyethylene wear in total knee arthroplasty. Orthopedics. 2002; 25:235-242.
  6. Berger RA, Rubash HE, Seel MJ. Determining the rotational alignment of the femoral component in total knee arthroplasty using the epicondylar axis. Clin Orthop Relat Res. 1993; 286:40-47.
  7. Churchill DL, Incavo SJ, Johnson CC, Beynnon BD. The transcondylar axis approximates the optimal flexion axis of the knee. Clin Orthop Relat Res. 1998; 356:111-118.
  8. Hollister AM, Jatana S, Singh AK, et al. The axes of rotation of the knee. Clin Orthop Relat Res. 1993; 290:259-268.
  9. Krackow KA, Phillips MJ, Bayers-Thering M, et al. Computer-assisted total knee arthroplasty: navigation in TKA. Orthopedics. 2003; 26:1017-1023.
  10. Sparmann M, Wolke B, Czupalla H, et al. Positioning of total knee arthroplasty with and without navigation support: a prospective, randomized study. J Bone Joint Surg Br. 2003; 85:830-835.
  11. Jenny J, Boeri C. Low reproducibility of the intra-operative measurement of the transepicondylar axis during total knee replacement. Acta Orthop Scand. 2004; 75:74-77.
  12. Moreland JR, Bassett LW, Hanker GJ. Radiographic analysis of the axial alignment of the lower extremity. J Bone Joint Surg Am. 1987; 69:745-749.
  13. Tang WM, Zhu YH, Chiu KY. Axial alignment of the lower extremity in Chinese adults. J Bone Joint Surg Am. 2000; 82:1603-1608.
  14. Matsuda S, Miura H, Nagamine R, et al. A comparison of rotational landmarks in the distal femur and the tibial shaft. Clin Orthop Relat Res. 2003; 414:183-188.
  15. Poilvache PL, Insall JN, Scuderi GR, Font-Rodriguez DE. Rotational landmarks and sizing of the distal femur in total knee arthroplasty. Clin Orthop Relat Res. 1996; 331:35-46.
  16. Akagi M, Yamashita E, Nakagawa T, et al. Relationship between frontal knee alignment and reference axes in the distal femur. Clin Orthop Relat Res. 2001; 388:147-156.

Authors

Drs Han, Seong, Lee, and Chul Lee are from the Department of Orthopedic Surgery, Seoul National University College of Medicine, Seoul, Korea.