Rotational Alignment of Femoral Component and Flexion Gap Balance in Patients With Distal Femoral Torsional Deformity Using Navigation-assisted TKA

Abstract

We evaluated the rotational alignment of the femoral component after total knee arthroplasty in 46 patients with distal femoral torsional deformity using a navigation-assisted gap technique. Preoperative distal femoral torsional angle and postoperative rotational deviation of the femoral component were measured using computed tomography. Flexion gap data were obtained from intraoperative navigation measurements. The mean rotational deviation of the femoral component was 4.1° (range, 2°-6°) internal rotation in reference to transepicondylar axis (TEA). The femoral component was not aligned within 3° in reference to TEA in 30 patients (65.2%). There was no significant difference of rotational deviation of the femoral component between unbalanced and balanced flexion gap groups (P=.65). There was a significant improvement of clinical outcomes after TKA. Navigation-assisted gap technique provided balanced flexion gaps in most patients, although there were wide rotational deviations of femoral components. Using the anatomic bony landmark method can result in excessive external rotation of the femoral component and unbalanced flexion gaps in patients with distal femoral torsional deformity.

Proper femoral component rotation in total knee arthroplasty (TKA) is important because inappropriate rotation of the femoral component may result in flexion imbalance and patellofemoral problems.^1-4 Rotational alignment of the femoral component can be determined by 2 methods: a measured resection technique and a balanced gap technique. There has been no consensus about which method results in a better outcome.^5-8 With measured resection technique, femoral component rotation is determined using fixed bony landmarks such as the transepicondylar axis (TEA), Whiteside’s line, and the posterior condylar axis (PCA) with 3° external rotation. This method is often satisfactory. However, rotational errors of the femoral component can occur in patients with distal femoral torsional deformity when only fixed bony landmarks are used. The purpose of our study was to evaluate rotational alignment of the femoral component in patients with distal femoral torsional deformity using a navigation- assisted gap technique. Our hypothesis was that the rotational alignment of the femoral component would not be well aligned with the TEA in patients with distal femoral torsional deformity.

Materials and Methods

After obtaining institutional ethical committee permission, we reviewed medical records to investigate rotational alignment of the femoral component in patients with distal femoral torsional deformity. The degree of external torsion of the distal femur was defined as the angle measured by computed tomography (CT) between a line drawn tangent to the most posterior part of the condyles and the epicondylar line drawn from the lateral epicondyle to the most prominent point of the medial epicondyle (Figure 1). Our inclusion criteria were (1) patients with distal femoral torsional deformity (>7° difference in angle between the TEA and the PCA), (2) patients who underwent cruciate-retaining TKA using navigation-assisted gap technique, and (3) patients who underwent preoperative and postoperative CT scan with informed consent. Forty-six patients who met the criteria were included in this retrospective study. There were 7 men and 39 women with a mean age of 68.2 years (range, 49-85 years). The mean follow-up period was 26.6 months (range, 24-51 months). Preoperatively, mechanical femorotibial angle (FTA) using lower extremity plain radiography, distal femoral torsional angle using CT scan, body mass index, knee range of motion (ROM), and Hospital for Special Surgery (HSS) score were collected.

Figure 1: The angle between posterior condylar axis and transepicondylar axis

Figure 1: The degree of external torsion of the distal femur is defined as the angle between posterior condylar axis (PCA) and transepicondylar axis (TEA).

All procedures were performed through a medial parapatellar approach, and an imageless navigation system was used (OrthoPilot version 4.0; B. Braun Aesculap, Tuttlingen, Germany). Kinematic and anatomic registration was made in the usual manner. Presurgical frontal and sagittal alignment, medial-lateral laxity in flexion and extension, and ROM were measured and recorded. If the soft tissue contracture was severe, we tried to release it before resecting the bone. The proximal tibia was first resected to align the mechanical axis to neutral. Soft tissue tension was then measured at full extension and at 90° flexion of knee using a spreader and a tensor with a slide ruler. Femoral planning helped determine the size and rotation of the femoral component for the balanced flexion and extension gap (Figure 2). Intraoperative navigation measurements of the medial and lateral gap were obtained and recorded. The balanced gap was defined as a gap difference <3 mm between the medial and lateral sides at full extension and 90° flexion of knee. The femoral cutting block was oriented according to femoral planning, and then femoral cuts were made. A cruciate-retaining mobile-bearing implant was used in all patients (E.motion; B. Braun Aesculap). Patella was not resurfaced.

Figure 2: Femoral planning helps determine the size and rotational alignment of the femoral component for the balanced gaps.

Postoperatively, rotational alignment of femoral component was evaluated using CT scan. The rotational deviation of the femoral component from the anatomic landmark was determined by the angle between the line connecting the posterior condylar line of femoral implant and the transepicondylar line (Figure 3). The knee ROM, HSS score, and mechanical FTA at last follow-up were compared with preoperative data.

Figure 3: The rotational deviation of the femoral component from the transepicondylar axis (TEA) is defined as the angle between the line connecting the posterior condylar line of femoral component (FC) and the TEA.

Statistical analyses were performed using the SPSS (SPSS for Windows Release 15.0; SPSS Inc, Chicago, Illinois). The paired t test was used to determine the difference regarding ROM, HSS score, and mechanical FTA. P<.05 was considered statistically significant. Correlation between the rotational deviation of the femoral component and the distal femoral torsional angle was evaluated using Pearson’s correlation coefficient.

Results

The distribution of the distal femoral torsional angle and the rotational deviation of the femoral component from the TEA are listed in Table 1. The mean distal femoral torsional angle was 8.3° (range, 7°-12°). The mean rotational deviation of the femoral component was 4.1° (range, 2°-6°) internal rotation in reference to the TEA and 4.2° (range, 2°-6°) external rotation in reference to the PCA. The femoral component was not aligned within 3° in reference to the TEA in 30 patients (65.2%). There was less correlation between distal femoral torsional angle and rotational deviation of the femoral component from the TEA (r=-0.54). There were 6 cases with unbalanced gaps and 40 cases with balanced gaps. In the unbalanced group, the mean rotational deviation of the femoral component was 3.1° (range, 2°-6°) internal rotation with reference to the TEA. In the balanced group, the mean rotational deviation of the femoral component was 4.5° (range, 2°-6°) internal rotation with reference to the TEA. There was no significant difference of rotational deviation of the femoral component between the 2 groups (P=.65). The correlation coefficient between preoperative mechanical FTA and rotational deviation of the femoral component was 0.34. There were significant differences between preoperative and postoperative mechanical FTA, knee ROM, and HSS score (Table 2).

Table 1: Distribution of Distal Femoral Torsional Angle and Rotational Deviation of Femoral Component

Table 2: Comparison of Preoperative and Postoperative Clinical Outcomes

Discussion

The TEA, the anteroposterior line or Whiteside’s line, and the posterior condyles with 3° external rotation are used as references to rotationally position the femoral component in measured resection techniques.^5,9-12 The flexion gap can be either rectangular (balanced) or trapezoid (unbalanced). However, the philosophy of the classic balanced gap approach is that the knee must be balanced (ie, equal tension in medial and lateral soft tissue knee structures) in extension and flexion to achieve proper kinematics and stability of the knee.⁶ Each of the landmarks used in the measured resection techniques has its own pitfalls and can lead to femoral component malrotation and subsequent flexion instability.¹³ In our study, we investigated the rotational alignment of the femoral component in patients with distal femoral torsional deformity when implanted using the navigation-assisted gap technique. We observed that the femoral component was 4.5° (range, 2°-6°) internally rotated with reference to TEA in patients with balanced gaps. It is believed that using fixed bony landmarks is not a reliable method for rotational alignment of the femoral component in patients with distal femoral torsional deformity. Care should be taken to prevent misinterpretation while confirming whether the femoral component is aligned with the epicondylar axis or Whiteside’s line in patients with distal femoral torsional deformity.

Flexion gap balancing is important for a stable knee in flexion and better ROM.^7,14 In gap balancing methodology, the femoral component is positioned parallel to the resected proximal tibia in 90° flexion with each collateral ligament equally tensioned. It is likely that the measured resection technique results in excessive external rotation of the femoral component in patients with distal femoral torsional deformity. This leads to unbalanced flexion gaps and flexion instability. Fehring⁶ compared flexion gap balance in 100 TKAs using gap balancing versus a measured resection technique in which bony landmarks were the primary determinant of femoral component rotation. Compared with knees in which gap balancing was used, rotational errors of at least 3° occurred in 45% of patients when rotation was determined from fixed bony landmarks. In our series, femoral component was not aligned within 3° in reference to TEA in 30 patients (65.2%). This result supports soft tissue balancing using a navigation-guided gap technique as a reliable method for achieving symmetrical flexion gaps, especially in patients with distal femoral torsional deformity.¹⁵

Our study showed that there were wide interindividual variations of the distal femoral torsional angle, which is consistent with previous results.¹⁶ However, it is difficult to identify distal femoral torsional deformity intraoperatively. The surgeon’s ability to identify accurately this bony landmark during surgery is poor. Kinzel et al¹⁷ reported on a series of 74 TKAs in which the femoral epicondyles were marked with pins intraoperatively, and postoperative CT scans were performed to assess the accuracy of epicondylar identification. The researchers observed that the epicondyles were correctly identified to within ±3° in only 75% of the cases, with a wide range of error from 6° of external rotation to 11° of internal rotation. Yau et al¹⁸ performed an additional analysis and similarly found a wide range of error in intraoperative surgeon identification of the femoral epicondyles (28° error range; 11° external rotation to 17° of internal rotation). Galaud et al¹⁶ reported that computed navigation does not provide a reliable or reproducible evaluation of the epiphyseal torsion. Because of inaccurate identification of femoral epicondyles, the related navigated measurement is not considered to be reliable or reproducible. It is believed that preoperative CT scan is a reliable method to identify distal femoral torsional deformity.

In our study, 6 patients had unbalanced flexion gap even though navigation provided real-time information. However, there was statistically significant improvement in ROM, FTA, and HSS in all patients at the last follow-up. This finding could be attributed to the fact that even the knees in the unbalanced group had a relatively small amount of asymmetry in soft tissue balance. Long-term follow-up is needed.

There are several limitations of our study. First, cartilage thickness could not be accurately assessed when we defined the PCA line. Cartilage denudation in the medial femoral posterior condyle is common and could affect interobserver variations, thus affecting results. Second, patients included in this study had >7° of distal femoral torsional angle. Patients (excluded in this study) with a smaller degree of distal femoral torsional deformity could have introduced selection bias that could affect our results. Furthermore, the amount of force on the tensor may have affected the amount of femoral component rotation applied manually and was not standardized.

Conclusions

This study showed that navigation-assisted gap technique provided balanced flexion gaps in most patients with distal femoral torsional deformity. The femoral component was not aligned with bony landmarks such as transepicondylar axis or Whiteside’s line in patients with distal femoral torsional angle >7°. The femoral component was a mean of 4.5° (range, 2°-6°) internally rotated with reference to TEA when balanced flexion gaps were achieved in this study. The determination of femoral component rotation using anatomic bony landmarks method alone can result in excessive external rotation of the femoral component and unbalanced flexion gaps in patients with distal femoral torsional deformity. The soft tissue balancing using a navigation-guided gap technique can be a reliable method for achieving symmetrical flexion gaps, especially in patients with distal femoral torsional deformity.

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Authors

Drs Lim, Bae, Neogi, Seok, and Kim are from the Department of Orthopedic Surgery, Korea University College of Medicine, Guro Hospital, and Dr Wang is from the Department of Orthopedic Surgery, Korea University College of Medicine, Ansan Hospital, Seoul, Korea.

Drs Lim and Bae are consultants for B. Braun Aesculap. Drs Neogi, Wang, Seok, and Kim have no relevant financial relationships to disclose.

Correspondence should be addressed to: Ji Hoon Bae, MD, Department of Orthopedic Surgery, Korea University College of Medicine, Guro Hospital, 80 Guro-dong, Guro-gu, Seoul 152-703, Korea.

doi: 10.3928/01477447-20090915-61