In the past few decades, TKA has achieved great development, but there is a high rate of dissatisfaction, often due to patellofemoral pathologies. In addition to surgical techniques, inappropriate prosthetic design often results in mismatches of the patellofemoral joints[10, 28]. It has been reported in the literature that even a millimeter-level error in TKA can lead to serious consequence . In this study, we made comprehensive measurements of patellofemoral joints as a whole, obtained gender differences, and exploratively found some morphological characteristics, which provided a reference for prosthetic design. Meanwhile, we used LASSO regression model to predict the normal values of TT-TG distances so as to enable precise clinical diagnoses and predict medial transfer of the tibial tuberosity accurately.
Many studies have reported on the measurements of patella thickness. The comparisons of patellar thickness with those measured in other studies are shown in Table 3. We used student’s t test to determine the significant differences between present results and those reported in other literatures. The result indicated that the patella thickness of Chinese tended to be smaller than that of the Caucasians, while comparable to that of the Korean and greater than that of the Indian[4, 9, 26, 35, 40, 50]. This finding is consistent with the results from previous studies [31, 35, 50]. The re-establishment of original thickness and adequate residual bone thickness is considered as key surgery guidelines in TKA . However, due to the mismatch of the patellar implants, the surgeons had to choose between the re-establishment of original thickness and adequate residual bone thickness. By choosing the former, the low residual bone thickness likely causes fracture and instability; by choosing the latter, the increased thickness of patella causes overstuffing of the patellofemoral joint and leads to anterior knee pain [1, 41]. Although several studies have shown that adverse clinical outcomes were not likely to occur if the overall and residual bone thickness of patella was maintained in a reasonable range (postoperative thickness within 3 mm of the original thickness of the patella, and residual thickness between 10 and 15 mm), the changes of the patella might affect the patellofemoral contact pressures, thus leading to complications of the patellofemoral joint . Therefore, the patellar prostheses with more available choices should be designed according to patellar characteristics.
We explained many gender differences from this study. The result showed that the dimensional indexes of the males were generally larger than those of females, which was consistent with previous studies [33, 40]. In terms of the shape, the patella and the femur of the females were thicker than those of males in the anterior-posterior direction for the same medial-lateral dimensions, which was consistent with the relatively small femoral aspect ratio in females. Therefore, the shape of the distal femur of the males was more "flat" compared to females, while females had a narrower distal femur than males. These results were comparable to those reported in previous studies[5, 19, 33, 51]. We found that the Wiberg index and the shape of the trochlear exhibited no dimorphisms between gender. Gillespie et al reported that no significant difference between gender was found in the medial and lateral flange, which was similar to our results. Based on these features, gender-specific prostheses should be designed in consideration of gender characteristics. However, more and more studies focused on not only gender differences but individual differences[3, 5]. Taking this issue into account, we exploratory analyzed the correlation coefficient between all parameters.
This study found that AFO, SA and the Wiberg index all varied greatly among individuals. These three indexes are the primary description of the patellofemoral shape and thickness. To avoid overstuffing and notching of patellofemoral compartment, the AFO should be treated appropriately. Matz et al reported that the probabilities of changes in AFO after TKA were 40% compared with those before TKA. Although some previous studies showed no significant differences between restoration of AFO and clinical outcomes, there was a trend toward improved outcomes[34, 42]. Other studies showed that if the AFO increased after TKA and the there was a risk of overstuffing due to the mismatch of the prosthesis, the pressure of the patellofemoral joint would increase, and then there would be complications such as anterior knee pain and decreased knee motion. If there was a risk of notching of the patellofemoral joint, it leaded to cause instability in knee flexion. Taking these issues into account, more and more studies analyzed the shape and variance of the distal femur. Lonner and Gillespie et al indicated that the overall variability of the anatomy of the distal femur should be taken into account but not gender difference[19, 32]. According to the individual differences, Everhart et al proposed a binary classification system to describe the shape of the distal femur and five binary categories were selected based on the aspect ratio, trochlear width, trochlear tilt, the ratio of medial and lateral trochlear width, and trochlear groove angle. Besides, Varadarajan et al reported that the laterally oriented proximal part and medially oriented distal part formed the intact trochlear groove, and there was a turning point to distinguish these two part. Moreover, Chen et al proposed a quaternary system based on the position of the turning point. Due to the greatly individual variance of the distal femur, more studies on different shapes of the femoral components should be focused, and prosthetic implants with greater varieties in sizes and shapes of anterior femoral condyle needed to be designed.
The TT-TG distance had significant positive correlation with tubercle sulcus angle (TSA) and Q-angle, and was considered to be objective and reliable in the quantification of extensor mechanism malalignment and patellar instability[21, 23]. In the previous studies, the measurement of the TT-TG distance was mainly used in image overlapping technology based on CT and MRI. However, several literatures have reported the inaccuracy of the current measurement[2, 21], and we found that mild adduction or abduction of the lower extremity resulted in a greater change in this value. In this study, we took this issue into account, and used the online reslice of the mimics software to standardize the selection of images, so that the collected transverse picture was as perpendicular to the mechanical axis of the lower limb as possible, which greatly ensured the accuracy of measurement.
This study reported the average CT-based TT-TG distance to be 13.62 ± 1.76 mm. The average TT-TG distance from the research of Hernigou et al. was 13 mm, which was measured based on CT data and was similar to our results. Tse et al. showed by MRI that the average TT-TG distance was 10.1 mm in healthy Chinese. In a research conducted in New Zealand, Pandit et al. reported the average MRI-based values to be 9.91 mm for males and 10.04 mm for females. Hinckel et al reported that the MRI-based TT-TG distance was 3.1–3.6 mm smaller than CT-based TT-TG distance, which explained the inconsistency of the above results. At present, more and more literatures recognized the limitation of the absolute threshold of TT-TG distance. Although 20 mm was the main diagnosis threshold for surgical application, there are some disputes about the value. Franciozi et al reported that tibial tubercle osteotomy combined with medial patellofemoral ligament reconstruction (MPFLR) resulted in better outcomes compared with MPFLR alone in the treatment of recurrent patellar instabilities in patients with a TT-TG distance of 17 to 20 mm. Graf et al reported the inaccuracy of surgical intervention, and demonstrated the need for combining the TSA and the TT-TG distance to avoid overcorrection during medial tibial tubercle osteotomy. Our result reported that the TT-TG distance had a positive correlation with height and the knee size, which was comparable to other literatures[11, 16, 23]. Moreover, several literatures described that application of the TT-TG indices (ratio of the TT-TG distance to the tibial maximal mediolateral axis) obtained more reliable and standardized results[7, 16]. Hernigou et al used fML and tML to establish normal values of TT-TG distances in Greek. However, they also raised doubts about whether the two parameters were applied as the best predictors. Taking these questions into account, the present study applied LASSO regression model to analyze the best predictors of the normal TT-TG distances. LASSO regression is a machine learning method that can shrink the coefficients of variables that do not contribute information to the model to zero and is well suited to feature selection for high-dimensional data. Using this method, we obtained four parameters to predict normal TT-TG distance, namely height, fML, tML and tAP, to achieve the best accuracy and convenience. The prediction formula obtained by us provided a more accurate reference for clinical determination of patellar instability, rather than the absolute values or TT-TG indices. Meanwhile, the formula can play a guiding role in the more accurate localization of the tibial tubercle during the tibial tubercle osteotomy.
Insufficiency of the study
Limitations of the present study include the relatively small sample size. With all measurements derived from knees with extension, there was no data from the knees with 30° of flexion. Thus, the patellar tilt, patellar height, bisect offset, and other factors could not be obtained. We are continuing to study additional subjects and to acquire data from knees with a flexion angle of 30°. Another limitation of the present study was that the formula for predicting TT-TG distance has not been clinically verified, and more studies on the clinical effectiveness of the formula need to be performed.