INTRODUCTION

In total knee replacement (TKR) design, a proper coverage of bone surface is essential for good clinical results and long-term survivorship of prostheses. Many studies using the finite element analysis have demonstrated that a good match between prostheses and bone surface can even minimize the wear of the bearing surfaces (^{Liau et al., 2002}), improve stress distribution (^{Villa et al., 2004}), and prolong the service time of prosthesis (^{Figgie et al., 1989}). On the contrary, any mismatch between prosthesis and bone surface or malposition in the form of overhang or under sizing of the prosthesis may result in a number of severe complications. For the tibia prosthesis, overhang may result in the residual pain (^{Bonnin et al., 2016}). For the distal femur, the case of under sizing may cause prosthesis sinking, potentially leading to revision (^{Stulberg et al., 1995}).

Many studies have suggested that Asian patients may benefit from a smaller knee prosthesis than the Caucasians patients (^{Vaidya et al., 2000}; ^{Mahfouz et al., 2012}). A number of studies have also proven that the prostheses designed for European Caucasians are not appropriate for Asian ethnicity (^{Chaichankul et al., 2011}; ^{Sun et al., 2014}). The main reason has been identified by the knee morphological differences between Chinese population and Western population (^{Cheng et al., 2009}). To our knowledge, the available knee prostheses designed for Caucasians may not match the Chinese patients, especially in Mongolian Chinese. To our knowledge, many studies have focused on Han Chinese while few studies have focused on the natural knee anatomy in the Mongolian population. The aim of this study is to investigate the effects of nationality and sex on the size and shape of natural knees between Han and Mongolian Chinese of China. The work conducted in this study may provide better guidance for the design and selection of knee prostheses.

MATERIAL AND METHOD

All subjects were recruited at The Second Affiliated Hospital of Inner Mongolian Medical University, after achieving informed consent from the subjects. A total of 37 healthy natural knees, including Han Chinese 22 cases (13 males, 9 females) and Mongolian Chinese 15 (8 males, 7 females), were selected in this study. The range of age was 23-45 years, the average age was 36.0±7.2 years. All knees were healthy without any injuries or osteoarthritis, in terms of clinical examination and examination of computer tomographic (CT) images. For all subjects, a multi-slice helical CT scanner (120 kV, 80mA; Light Speed 16; GE, America) was used for scanning. The subject was required in a supine position with his or her lower limbs in a relaxed and neutral rotation. The scanning procedure was performed to acquire 0.625 mm CT slices (resolution, 512 × 512 pixels) with a field of view of 200 x 200 mm around knee joint. The scanning images were exported and saved in DICOM format. All digital three-dimensional (3D) reconstruction of knee was performed using the Mimics medical imaging program (version 16.0; Materialise, Leuven, Belgium). Then threedimensional models were imported to Geomagic studio (version 12.0; Geomagic, USA) for surface reconstruction. At last, morphological measurements were performed in Geomagic studio (version 12.0; Geomagic, USA).

The distal femoral morphology and the proximal tibia morphology were measured with a method reported by ^{Kwak et al. (2007}) as shown in Figure 1. The femoral mediolateral length (fML) was referenced by the femoral epicondyle axis, defined as the most salient point between the medial and lateral attachment on the femoral condyle. The femoral medial condyle anteroposterior length (fMAP) was defined as the distance from the most anterior point on the femur medial condyle to the posterior condylar line. The femoral lateral condyle anteroposterior length (fLAP) was taken as the distance from the most anterior point on the femur lateral condyle. The tibial mediolateral length (tML) was the maximum length between medial and lateral tibia plateau, parallel to the axis of the femoral condyle. The tibial lateral anteroposterior length (tLAP) was the length from the anterior lateral tibia plateau to the posterior plateau. The tibial medial anteroposterior length (tMAP) was the length from the anterior medial tibia plateau to the posterior plateau. The femoral aspect ratio (fML/fAP) and tibial aspect ratio (tML/tAP) were considered in our study to interpreting the results of knee shape. All measurements were recorded in millimeters.

The statistical analysis was performed in SPSS software 22.0 (SPSS, Chicago, IL). All measured lengths were summarized as the mean and standard deviation. Comparative statistical analyses between genders and nationalities were made using the independent sample t test, in analyzing the morphometric data, scatter plots along with a best-fit line were represented.

Fig. 1 the measurement of knee distal femoral (A) and proximal tibia (B). 

RESULTS AND DISCUSSION

The measurement results for femur are presented in Table I. It was observed that all measurements (fML, fMAP and fLAP) of femur had larger size in male than in female both in Mongolian Chinese and Han Chinese (P<0.05). Mongolian Chinese males had a larger femur (fML, fMAP and fLAP) than Han Chinese males (p<0.05).

Table I Average values of the femur morphology measurement (mm). 

It was observed that all measurements (tML, tMAP and tLAP) of tibia had larger size in male than those in female no matter in Mongolian Chinese or in Han Chinese (P<0.05) (Table II). Mongolian Chinese males had a larger tibia (tML, tMAP and tLAP) than Han Chinese males (P<0.05). Though Mongolian Chinese females had a larger tibia (tML, tMAP and tLAP) than those in Han Chinese females (P>0.05), there was no statistical difference. The statistical differences in tibia shape between Han Chinese male and Mongolian Chinese male was significant (P<0.05). Figure 2 shows the difference from nationality and gender of distal femur between the Han Chinese and Mongolian Chinese. The fML was significantly and positively correlated with the fMAP (Fig. 2A) while the fML/fMAP was significantly and negatively correlated with the fMAP (Fig. 2B). For the same fMAP values along the regression curves (Fig. 2A), both Mongolian Chinese males and Han Chinese males had a larger fML than those in the females and showed significant differences. Mongolian Chinese females and Han Chinese females showed no significant differences in fML, fMAP and fLAP although the values in Mongolian Chinese females were larger than those in Han Chinese females. On the other hand, both Mongolian Chinese and Han Chinese males had larger aspect ratio (fML/fAP) than those in females for the same fMAP values; sex showed significant differences in aspect ratio (fML/fAP) for both Han and Mongolian Chinese. Nationality showed significant differences in fML/fAP ratios in males.

Table II Average values of the tibia morphology measurement (mm). 

Fig. 2 Difference from nationality and sex of distal femur between the Han Chinese and Mongolian Chinese. 

Figure 3 shows the difference from nationality and sex of proximal tibia between the Han Chinese and Mongolian Chinese. The tML was significantly and positively correlated with the tMAP (Fig. 3A) while the aspect ratio (tML/tMAP) was significantly and negatively correlated with the Tmap (Fig. 3B). Both Mongolian Chinese males and Han Chinese males had larger aspect ratios (tML/tAP) than those in the females for the same tMAP values (Fig. 3B). Gender showed significant differences in aspect ratio (tML/tMAP) no matter in Han Chinese or Mongolian Chinese. Nationality showed significantly differences in aspect ratio (tML/tMAP) only in males.

Fig. 3 Difference from nationality and sex of proximal tibia between the Han Chinese and Mongolian Chinese and sex effects on Han Chinese males and females, Mongolian Chinese males and females. 

The tMAP was significantly and positively correlated with the fML, the tMAP was increased along with an increasing in the fML (Fig. 4A). The aspect ratio of tibia was significantly and negatively correlated with the fML, and the tMAP was increased along with an increasing in the fML (Fig. 4B).

Fig. 4 The morphological relationship between the tibia and femur in Han Chinese and Mongolian Chinese. 

The morphological character of the distal femur and proximal tibia is different among varius races, and was proved in many studies. But the difference of the distal femur and proximal tibia between divers nationalities still unknown in the same race for us, and to know the difference of the distal femur and proximal tibia is also the requirement of medical precision and individuation.

Many studies have reported on the size differences of distal femur resulted from the gender for Han Chinese population. Cheng et al. reported the average fML dimensions based on CT to be 74.4±2.9 mm and 66.8±3.1 mm for males and females in northeastregion of China, respectively. However, the different Chinese nationalities have been rarely compared. In this study, the sizes of Han and Mongolian Chinese knees were measured and compared. The measured fML, fLAP and fMAP of Han Chinese were larger than the previous study (^{Wang et al., 1992}; Cheng et al.), maybe as a result of different definition of measured length of femur and tibia or measured methods. For example, for Cheng et al., the tibial mediolateral (tML) dimension was taken as the longest mediolateral length of the proximal tibial cut surface 6 mm below the medial aspect of the tibial plateau with a 7° posterior slope. For Wang et al., the measuements based on the x-rays, so it was not very accurately. And the sizes of femur and tibia of Mongolian Chinese were larger than those of Han Chinese. The present study showed that the shape of Han Chinese knees was generally wider than Mongolian Chinese knees in the same sex.

In our study, females also had a narrower distal femur than males both in Han Chineseand Mongolian Chinese according to the statistical analysis, and these results were consistent with the previous studies (^{Mensch & Amstutz, 1975}; Chin et al., 2002; ^{Hitt et al., 2003}). Compared with western population, Mongolian Chinese females had a significantly wider average aspect ratio (fML/fAP) than Caucasian females [1.27±0.04 vs. 1.19 (^{Lonner et al., 2008}) or 0. 99 (^{Thienpont et al., 2016})]. This suggested that not only the TKA prosthesis design should consider the sex factor, but also the nationality differences. The morphological differences of femoral between Han Chinese and Mongolian Chinese females (P<0.05) cannot be accounted by the differences in size alone, but also the shape variation between these two nationalities. The components designed based on Han Chinese knee data may lead to a mediolateral component overhang in Mongolian Chinese for the same gender. An overhanging prosthesis is more likely to cause soft tissue imbalance, and unfavorable patellofemoral stress distribution. It is often needed to downsize the femoral components during TKA operation to avoid this clinical overcome. However, this may also result in an undesirable complication. For example, notching of the anterior cortex can predispose to periprosthetic fractures and over-resection of the posterior femoral condyles resulting in an imbalance between the flexion and extension gaps.

The geometry of tibial plateau is considered as an important factor in TKA design and has a direct impact on the biomechanics of tibiofemoral joint (^{Hashemi et al., 2008}). The tibial plateau parameters were measured and compared in this study. The tMAP had larger values than tLAP, which is similar to the previous reports (Hitt et al.; Kwaket al.). However, the size of proximal tibia is larger than the reported results (^{Wevers et al., 1994}; Hitt et al.; Kwak et al.) of the Asia-Pacific population including Chinese population. In addition, the mean aspect ratio (tML/ tAP) showed the significant difference between male and female in our study (p?0.05), and the shape of tibia in female are narrower and smaller than that in male in the same nationality, which is consistent with previous findings (Kwak et al.; Cheng et al.; ^{Lim et al., 2013}; ^{Piriou et al., 2014}). Moreover, most current prostheses used in China are designed for Caucasian population with an unchanged aspect ratio. A significant aspect ratio difference between Han Chinese and Mongolian Chinese was found in this study. The measurement results implied that it is necessary to adjust the aspect ratio during components design for achieving a better clinical outcome.

The morphology of femur and tibia should be considered as a whole in the process of knee prosthesis design. The morphological relationship between the tibia and femur of Han Chinese and Mongolian Chinese was investigated in this study. We found that the tML was positively correlated with the fML and fAP, the trend of which was similar to that reported by Cheng et al. Therefore it is important to consider the morphological factors of tibia and femur as a whole for the prosthesis design.

Some limitations of this study should be discussed. First, the sample number of natural knees in this study was limited, and more samples should be recruited in next step. Second, a direct dimensional comparison with the current available knee prostheses was not performed. Moreover, the three-dimensional knee models were made from CT images, where the effects from the cartilage thickness of the femur and tibia were not considered. Despite these, the present study provides useful and essential knowledge about the femur and tibia of Mongolian Chinese and Han Chinese.

CONCLUSION

Our study showed that the sizes of femur and tibia of the males were larger than those of the female for both Han and Mongolian Chinese. Furthermore, the sizes of femur and tibia of Mongolian Chinese were larger than those of Han Chinese. The aspect ratios of femur and tibia of Mongolian Chinese were smaller than that of Han Chinese. The effects of nationality and sex on the size and shape of proximal tibia and distal femur were significant (p<0.05). This study implied that engineer design prothesis should taken into account the differences of nationalities.