Our series of measurements allows to divide the morphology of the proximal tibia into three groups: type A, B and C, having type C the wider inner diaphyseal canal; in addition, significant sex differences were reported. This novel index and classification could be useful in predicting the risk of aseptic loosening, thus orienting the surgeon towards the choice of a rotating hinge implant. Like the Citak classification , there are other classifications, such as the Dorr classification , which describe anatomical norm variations of bony structures.
According to cut-off values for each sex, the ratio between the inner diameter of the tibial canal 16 cm distal to the knee joint, in relation to the inner diameter of the medullary canal 3 cm distal to the medial tibial eminence in AP knee radiographs, lead to the development of three anatomical classification groups: female patients: type A: <0.19, type B: 0.19-0.24, type C >0.24; male patients: type A: <0.19, type B: 0.19-0.23, type C: > 0.23, having type C the wider inner diaphyseal canal.
Excellent inter- (0.996 - 0.998) and intra-observer reliability (0.92 - 0.997) was demonstrated, which we were able to validate in our study (inter-/intra-observer reliability 0.997 - 0.999/0.998).
The femoral index resulting from the Citak classification was shown to be an independent predictor for aseptic femoral loosening of rotating hinge knee prosthesis . The measurements of the diameter of the internal femoral canal as the basis of the Citak classification were transferred to the tibia in this study. Also, we classified it analogue to the Citak classification of the femur in type A, B and C with quantitative cut-off values for each group and with a similar distribution for each type.
Citak et al. proposed a new classification for distal femur that may be helpful as pre-operative planning for rotating hinge prosthesis. Male and female patients were separate in different groups due to AP diameter found for each group and three types were described. Male type A with AP diameter under 19, type B between 19-24 and type C, 24 or above while female patients were classified as type A below 15, type B between 15 and 20, and type C, 20 or greater values. High inter- and intra-observer reliability indicates the classification to be a useful tool in making decisions on whether a hinged prosthesis is the best choice for a patient considering the risk of (AL). Citak´s classification for distal femur has an inter-observer reliability between 0.996 - 0.998 and an intra-observer reliability between 0.92 – 0.997 similar to the intra- and inter-observer reliability found on this study of 0.997 – 0.999 and 0.998 respectively, making it useful as a pre-operative tool for patient stratification. Also, we classified it analogue to the Citak classification of the femur in type A, B and C with quantitative cut-off values for each group and with a similar distribution for each type. In contrast to the novel classification system study, this study showed a moderate correlation between the AP diameter of proximal tibia and height. Although further investigation is needed, a type C with a wider inner diaphyseal diameter of the tibial canal may be helpful to predict AL of the tibial component in TKA tibially as has been shown femorally . It is suggested that its combined usage with the classification system for the distal femur facilitates decision-making processes regarding the implant and patient selection for rotating hinge implants.
Additionally, a novel index was developed; the distribution of the index ratio values demonstrated a wider range among the female study population, but similar cut-off values for both male and female patients. Ranging from 16%  to 65% , aseptic loosening has shown to be the most important cause of non-septic failures. Moreover, it is well known that an appropriate cementing technique is mandatory, but is becoming clear that patient related factors, such as bone stock quality and diaphyseal canal morphology, play an important role in prosthesis fixation and survivorship. The presented high failure rates could be justified by the fact that knee prosthesis with fixed hinge suffers from excessive abnormal stresses, such as tension, compression, and shearing forces. These stresses are transferred from the prosthesis to the bone, applying mostly to the interface . As suggested by Morgan-Jones et al. failure to gain adequate fixation in zone 2 (methaphysis) and 3 (diaphysis) can lead to early failure of a revision implant due to elevated shear stress; The concept of zonal fixation provides a working methodology applicable to both the tibia and the femur when planning revision knee replacement .
Taking this into account, the Citak classification system for distal femur offers a new possibility for patient stratification, evaluation and decision-making. Although no specific studies have been conducted focusing specifically on the tibial component alone, the use of both classification systems may be a better predictor for risk of AL and a tool to choose what is best for patients individually, preventing unwanted outcomes.
This study obtains the following limitations: Firstly, it has intrinsic limitations related to the retrospective study design. Secondly, ethnic differences among the study population were not considered, despite the existing proof for gender and race differences in knee morphology as described by Kim et al. who reported important differences among sex and races on the tibial medio-lateral and antero-posterior dimensions, while Mahfouz et al. found an evident larger femoral AP diameter, with a smaller aspect ratio comparing Caucasian and East Asian population and a larger aspect ratio of the tibia emerged when comparing Caucasian and Black population [15, 16]. Therefore, the generalizability of the study results is limited and needs further validation in other populations to increase external validity. Thirdly, analyses were carried out on randomly chosen radiographic images already stored in our database that were not adjusted-on-purpose or calibrated; however, all radiographs were performed according to a standardized methodology in our radiology department. The rationale behind that was to create a less controlled and more authentic setting suggesting higher external validity of the novel index. Lastly, the number of included patients was relatively low and requires further validation in larger multicenter studies to strengthen the level of evidence.
In conclusion, aseptic loosening represents the most frequent non-septic cause of rotating hinge knee prosthesis failure. It is well known that an appropriate fixation is mandatory, especially in a revision setting where it is difficult to have appropriate bone stock for increasing prosthesis’ survivorship.
Ultimately, the presented study project should be replicated to further test reliability of the classification, as a predictor for aseptic loosening in revision cases with different prosthesis designs, as well as its effects on implant survivorship. Lastly, future research should investigate the relationship between the novel femur and tibia classification.