Measurement of rotational alignment of the distal femur in total knee arthroplasty

Abstract

Objectives:  To measure and compare the included angle between the surgical transepicondylar axis (STEA) and the posterior condylar line (PCL) and the included angle between the femoral anteroposterior line (APL) and PCL, and to discuss the value of STEA, APL, and PCL as rotational alignment landmarks of the distal femur in total knee arthroplasty (TKA).

Methods:  Seventy‐five normal femoral specimens from Chinese adult cadavers were randomly selected. An axial photograph of every femoral specimen was taken with a digital camera and put into a personal computer. Using Photoshop 7.0.1 software, the included angle between the perpendicular line of APL and the PCL, noted as APA, together with the posterior condylar angle (PCA) between STEA and PCL were measured and compared using a paired‐samples t‐test.

Results:  The value for PCA was 3.67°± 1.62° (range, 0.75°–5.90°) and for APA 3.50°± 1.40° (range, 1.34°–5.65°). There was no significant difference between these two angles (t= 0.949, P= 0.359). Considering their relatively small means, these two angles showed wide variations.

Conclusions:  The rotational alignment of the femoral component can not accurately be determined by using PCL as a landmark. In order to get a proper rotational alignment of the femoral component in most cases of TKA, APL and STEA should be used as a double check.

Introduction

In total knee arthroplasty (TKA) improper rotation of the femoral and tibial components may lead to various patellofemoral complications, such as subluxation, dislocation, and wear ¹ . Despite the current high success rate of TKA, patellofemoral complications remain the most common reason for revision surgeries. The position of the patellar groove and flexion gap stability is determined by the femoral component of rotation. As for the femoral component, aligning it to the epicondylar axis of the femur has been a widely used method. The anteroposterior line (APL), posterior condylar line (PCL), gap technique, tibial shaft, and computer navigation are also used to determine the rotational alignment of the femoral component ² , ³ , ⁴ , ⁵ . The surgical epicondylar axis is considered to be the most reliable landmark for rotational alignment of the femoral component, however, locating it accurately during surgery is not easily achieved ³ , ⁴ . The APL and PCL are easily exposed and located during TKA. Many authors abroad have studied the relationship between these reference axes, but few reports relating to this issue have been found in the Chinese literature. So the purpose of the current study was to investigate the bone anatomy of the Chinese in order to determine an accurate technique for rotational alignment of the femur in TKA.

Materials and methods

Seventy‐five adult Chinese cadaveric femoral specimens were obtained from the Department of Human Anatomy, Southern Medical University. Of these femoral specimens, 55 were formalin‐infiltrated and 20 were dry specimens for demonstration. The sex and age of the corpses from whom these femora were taken was not known. Each of the selected femoral specimens had almost an intact cartilage surface, no fracture and no visible abnormality. All cadaveric specimens were obtained in an ethical way.

For each femur, the lateral epicondylar prominence and the central sulcus of the medial epicondyle were first located. Using a self‐made targeting tool as a guide, the distal femur was transfixed from medial to lateral epicondyle through the lateral epicondylar prominence and the central sulcus of the medial epicondyle by a 2 mm Kirschner wire (Changzhou Huasen Medical Device Company, Jiangsu Province, China). Thus, this Kirschner wire represented the surgical transepicondylar axis (STEA) ⁶ (Fig. 1).

Figure 1.

Photo of the distal femur showing Kirschner wire. 
A Kirschner wire transfixes the distal femur across the lateral epicondylar prominence and the medial sulcus of the medial epicondyle. This Kirschner wire represents the surgical transepicondylar axis (STEA).

The femur was then placed on a flat table and its shaft adjusted to obtain a good axial view. The patellar groove was identified and the deepest point on the anterior trochlea and the apex of the intercondylar notch was marked. A straight line between these two points is the APL, also called Whiteside's line (Fig. 2).

Figure 2.

A schematic diagram illustrating the key landmarks for defining rotational alignment of the distal femur. 
APL, anteroposterior line; PCA, posterior condylar angle, which is the included angle between PCL and STEA; PCL, posterior condylar line; STEA, surgical transepicondylar axis.

A Canon digital camera (Oita, Japan, 2002) was fixed on a tripod 2 m away from the distal femoral articular surface, and the lens center aimed at the elongation line of the femoral shaft. Then an axial photograph of the distal femur was taken.

The photograph was transferred to a personal computer. Using the image processing software Photoshop 7.0.1, the PCL, the parallel line of the PCL, and the APL were drawn. Then the included angle between the perpendicular of the APL and the PCL (APA), as well as the included angle (PCA) between the Kirschner wire line and the parallel line of the PCL, were measured (Fig. 3).

Figure 3.

An axial photograph of the distal femur showing the measurement of PCA and APA. 
PCA was measured between the line of the Kirschner wire and the dashed line, which is parallel to the PCL. APA is the included angle between the perpendicular of the APL and the PCL. The angle between the APL and the PCL was measured first, and then converted to APA by subtracting the value of the angle from 90°.

All data were presented as mean ± 1SD and a paired‐samples t‐test was done for statistical analysis with use of SPSS 12.5. The level of significance was considered to be P < 0.05.

Results

The value for PCA was 3.67°± 1.62° (range, 0.75°–5.90°) and for APA 3.50°± 1.40° (range, 1.34°–5.65°). Considering their relatively small means, both angles showed wide variations. There was no significant difference between these two angles (t = 0.949, P = 0.359).

Discussion

Rotational alignment of the femoral component affects patellar tracking, patellofemoral contact points and pressures, and rotational alignment of the knee. Several methods have been proposed to establish correct rotational alignment of the femoral component. Insall's gap technique consists of rotating the femoral cutting block to create a rectangular flexion gap ⁶ . In most ordinary TKA, because the normal upper tibia has a 3° slope, perpendicular resection of the upper tibial surface can remove more bone from the lateral surface of the tibial plateau than from the medial surface. To obtain a rectangular flexion gap, a larger amount of bone should be resected from the medial posterior condyle than from the lateral posterior condyle of the femur. On account of this, a 3° external‐rotating angle relating to the posterior condylar axis is usually used when resecting the posterior condyle. However, this is contentious. Griffin et al. have stated that there is significant variability in the magnitude of the PCA, and that TKA systems which determine femoral component rotation based on automatic rotation relative to the posterior condyles would result in numerous malrotated femoral components ⁷ . The posterior condylar axis is not a feasible landmark both for this reason, and also because there can be bone loss in either the medial posterior or lateral posterior condyle, usually as a result of a severely osteoarthritis knee, making this method even more inaccurate. In this study, the authors confirmed the opinion of Griffin et al. The femora of the Chinese have similar characteristics, for the standard deviations were large relative to the small means of APA and PCA. It was concluded that, because the posterior condylar axis has a wide variation in Chinese femora, using it as a landmark for determining rotational alignment may be inaccurate.

Stiehl et al. have proposed a technique for establishing femoral rotational alignment using the tibial shaft axis ⁵ . They considered that posterior condyle resection using the tibial shaft axis can restore the anatomy of patellofemoral relationships, and minimize patellofemoral complications. Yoshioka et al. found that the mean transepicondylar line is at a right angle to the mechanical axis of the femur in the frontal plane, and that with the knee flexed to 90°, the transepicondylar line also makes a right angle to the long axis of the tibia ⁸ . In their study, the posterior extent of the condyles in relation to the transepicondylar line showed wide variations, the posterior extent of the lateral condyle was smaller than that of the medial condyle. The transcondylar valgus angle, or the distal extent of the condyles to transepicondylar line was also variable, the distal extent of the lateral condyle was usually smaller than that of the medial condyle. A similar opinion has been reported by Stiehl et al. ⁵ . They reported that, using computed tomography, the posterior condylar axis method gave a PCA of 5° and 4° compared with the transepicondylar axis, whereas the tibial shaft axis technique measured 0° and 1°. Thus, patellar fracture and the need for lateral retinacular release were diminished when this technique was compared with the posterior condylar axis method in similar implants.

Insall et al. thought that identifying the epicondylar axis required some additional soft‐tissue dissection in order to define the anatomic positions of the medial and lateral epicondyles ⁶ . As described in their study, the center of the medial epicondyle is located in the sulcus, which lies between the proximal origin of the superficial medial collateral ligament (MCL) and the distal origin of the deep MCL. The medial epicondylar ridge at the origin of the superficial MCL can be identified by isolating the condylar vessels which lie proximal and anterior to the medial epicondylar ridge. From these vessels the epicondylar ridge can readily be outlined; the center of this outline is the sulcus, which can typically be palpated. The lateral epicondyle can be identified easily because it is the most prominent point on the lateral aspect of the distal femur. However, Griffin et al. have proposed that the center of the medial epicondyle is hard to identify, for the sulcus of the medial epicondyle is a hoof type shallow fossa with a diameter of more than 1 cm, but a depth of only 0.2–2.2 mm ⁷ . Incorrect location tends to occur commonly during surgery when the sulcus is covered by the MCL.

Arima et al. defined rotational alignment of the femoral component in the valgus knee using the anteroposterior axis of the distal femur ⁴ , ⁹ . They stated that a line perpendicular to the anteroposterior axis was consistently at approximately 4° external rotation relative to the posterior condylar surfaces (3.89°± 1.77°), while the epicondylar axis was more difficult to define, and showed greater variation (4.43°± 2.81°). In this study, the value for PCA was 3.67°± 1.62° (range, 0.75°–5.90°) and that for APA 3.50°± 1.40° (range, 1.34°–5.65°). There was no significant difference between these two angles. So the anteroposterior line had similar characteristics to the transepicondylar axis in regard to determining rotational alignment of the femoral component. Taking account of their relatively small means, both the PCA and APA showed wide variations. So in many TKA the rotational alignment of the femoral component will not accurately be determined by using PCL as a landmark.

The perpendicularity of the transepicondylar line to the mechanical axis of the femur and the tibia with the knee flexed to 90° makes it a sound landmark for rotational alignment of the femoral component when resecting the posterior condyles. This landmark correlates well with the APL described by Arima et al. ⁴ , ⁹ . The advantages of the epicondylar line as compared with the APL are that it is less dependent on patellofemoral dysplasia or arthritis, that it is less variable, and that it is never significantly internally rotated relative to the posterior condylar line. It can be used in both primary TKA and revision knee replacement. Double checking rotation by drawing the APL and a parallel line to the epicondylar axis should ensure proper femoral rotational alignment in most TKA.