Clinical significance of distal femur morphology in a healthy Mongolian youth population – Scientific Reports

With improvements in quality of life, changes in lifestyle habits, and increases in average patient age, the incidence and detection rate of knee disorders are increasing annually and the number of Chinese people receiving TKA has increased significantly. TKA is a major advancement in the development of orthopedic science in the last half century with a wide range of indications, and it has become an important way to improve knee function and improve the quality of life for patients with osteoarthritis, rheumatoid arthritis, severe knee trauma, and other serious knee diseases. Its postoperative results are good and the long-term follow-up results are satisfactory. However, 20% of patients with TKA are still unsatisfied after surgery¹², and postoperative complications are particularly worrying, which has attracted widespread attention from orthopedic surgeons. The postoperative results, long-term outcomes, and incidence of postoperative complications after TKA are closely related to knee prosthesis design, intraoperative osteotomy volume, osteotomy angle, soft tissue balance, and prosthesis placement level. Importantly, the data used in prosthesis design are the most influential in achieving a match in prosthesis geometry and dimensions and the osteotomy surface of the patient undergoing surgery, i.e., it is crucial that the intraoperatively applied prosthesis fits the patient’s knee geometry. It is well known that the placed knee prosthesis can lead to accurate knee joint alignment and significant changes in the biomechanical and anatomical structure of the knee joint^13,14,15. Therefore, the matching of the artificial joint prosthesis and the osteotomy surface of the femoral condyle is an important influential factor in determining TKA surgery success and long-term prosthesis patency.

With regard to the above issues, relevant studies have demonstrated that Chinese body size is shorter than and the anteroposterior and left–right diameters of the proximal tibia are smaller than those of Caucasians¹⁶; similarly, the same problem exists with the femoral condyle prosthesis. Due to this inter-ethnic difference, it is not uncommon for imported artificial joint prostheses to be poorly matched when applied to Chinese patients. This mismatch between the prosthesis and the osteotomy surface can lead to knee pain, limited popping, and motion after TKA, ultimately affecting the knee’s normal function¹⁷. Such morphological data of the knee joint vary across ethnic groups worldwide¹⁸. Therefore, it is necessary to study the geometry of the femoral condyles of the knee joint in normal Mongolians and assess the differences between them and other Western ethnic and racial groups.

The analysis of this group of data revealed inter-sex differences in the geometry of the femoral condyle in Mongolians. All parameters of the femoral condyle, including the ML/LAP, were significantly greater in men than in women^11,19. This indicates that the sagittal diameter of the knee condyle was larger in women than in men when the transverse diameter of the femoral condyle was the same; in the case of the same sagittal diameter, the transverse diameter of the femoral condyle was smaller in women than in men, which can lead to a tendency of the osteotomy surface to hang past the femoral condyle when a knee prosthesis is placed in the female knee if the sagittal match is used as the criterion for prosthesis selection, that is overhang^5,6,9. This can lead to postoperative pressure and irritation of the soft tissues around the knee, which can cause painful symptoms and, in severe cases, affect soft tissue balance^6,20,21. If the prosthesis is selected by matching of the transverse diameter, excessive osteotomy of the posterior condyle will occur, which will easily lead to unequal internal extension and flexion gaps similar to the findings of Western scholars³.

It was once believed that the morphological differences were small between sexes of the same race despite the parameters of the knee condyle varying. Thus, as long as the morphological design of the prosthesis was reasonable, different models could be used for different individuals to ensure proper matching. This view is deficient. The current commercially available knee prosthesis is designed and fabricated based on the geometric parameters of the knee joint of Caucasian men, resulting in a high degree of their adaptation to men, and when used by women, the problem of mismatch is likely to occur. Therefore, to achieve better knee function and prolong prosthesis life, prosthesis design and application must be refined and sex-specific. Plaster et al.²². suggested that when women use a knee prosthesis designed based on the geometric parameters of the female knee, early function was superior to that of other non-female parameter-based prostheses. In Kim’s²³ article, is noted that there was significant difference in the incidence of overhang of femoral component between the gender-specific implant and the traditional implant, but in terms of range of motion, Hospital for Special Surgery knee score, radiographic result, patella tilt angle and displacement, no significant difference was observed between two groups. But in other article⁵ point that femoral component overhang of > 3 mm in at least one zone was associated with an almost twofold increased risk of knee pain more severe than occasional or mild at 2 years after surgery (odds ratio, 1.9; 95% confidence interval, 1.1 to 3.3), however, femoral component overhang was not a significant predictor of postoperative flexion. The majority of knee prostheses currently used in China are imported or copied from imported prostheses, which also suffer from the above problems.

Although the use of domestic joint prostheses has surpassed that of imported joint prostheses in recent years, most of the domestic joints are mainly imitations and the geometric parameters are still derived from the measurement data of Caucasian men in Europe and the United States. This has led to frequent mismatches in the intraoperative use of prostheses in the national population. According to Zhang’s findings²⁴ ML/LAP ratio of the domestic mainstream imported prosthesis system was 1.04–1.16, much smaller than the measurement data of this group. According to the Tables 3, 4, it can be seen that the mainstream knee prosthesis is quite different from the normal Mongolian morphology, because the SD value is not provided in the reference, so statistical comparison cannot be made, but comparing the ML/LAP value can find that the difference is huge. The comparison of the measurement data of this group with those of European and American and several Asian ethnic groups also revealed that most of the parameters of this group were smaller than the European and American data, while some were similar to those of Japanese (Tables 5 and 6). The differences in the measurement data of femoral condyles between normal Mongolians and different ethnic groups were significant, which would probably cause difficulty choosing the intraoperative prosthesis type and affect the long-term results of the knee joint as well as the survival time of the prosthesis. As seen in the above table, the differences in distal femoral geometry between different ethnic groups and races were significant, while the differences in ML/LAP ratios between different ethnic groups were statistically significant by the abstract independent samples t-test. Because of the poor fit between this prosthesis and the national knee osteotomy surface as well as its insufficient coverage, the gap between the actual service life of the prosthesis and the patient’s expected life span is large and repair operations are bound to increase, which undoubtedly increases the economic burden of the national population and the pain of the secondary operation.

In conclusion, there is an urgent need to design and manufacture an artificial knee prosthesis system that is suitable for national use. If combined with adequate preoperative surgical design and preparation, China’s artificial knee arthroplasty technology will achieve further needed improvements.