Our study was the first investigation that ascertained the risk factors of PFF in the early postoperative period within 3 months. In univariate analysis, our significant factors, including age, RA, and neurologic disease, were similar to previous studies[3,5,6]. Although several pre-existing neurological diseases, including epilepsy, Parkinson's disease, and poliomyelitis were proposed as the risk factors[9], our study only had Parkinson patients. These diseases were related to an increase in falls. Regarding the operated site, Zainul-Abidin et al.[1] reported left-sided surgery was a significant risk factor. However, the opposite side was reported in our study. The relevance of this factor was still unexplained.
In terms of prosthetic design, our study revealed no CR design used in PFF group. Besides, we also demonstrated that PS design was the risk factor for early PFF, which this factor had never been reported. Alden et al.[7] reviewed 49 intraoperative femoral fractures from 17,389 primary TKA. These fractures could occur during exposure, bone preparation, and trialing of the component. They found that the PS design had a higher risk of intraoperative femoral fracture than CR design. The relative risk was 4.74. From their conclusions, we hypothesized that the intercondylar box cut of PS design might cause stress riser or intraoperative occult fracture. It might lead to early PFF in some patients.
The relationship between prosthetic and distal femoral bone sizes was another concern that we investigate. In univariate analysis, smaller distal femoral bone and larger prosthesis compared to bone or PDFW ratio were the risk factors of PFF. However, these factors were not significant when fitting to the regression model. For femoral component positioning, the correlation between malalignment and PFF was not well documented. Although LDFA in PFF group was significantly higher than the control group, the amount of difference was not clinically important.
The most significant finding of our study existed that age and dyslipidemia were independent risk factors of early PFF. The cut-off point of age was > = 75 years, while the odds ratio of dyslipidemia was 6.63 (95%CI, 1.11 to 39.8). Several pieces of evidence about the effects of dyslipidemia on bone health had been proposed. Experimental studies showed that oxidized low-density lipoprotein (LDL) suppressed the differentiation of bone marrow stromal cells into osteoblasts and also promoted adipocytes. Consequently, it might result in fatty marrow and bone mass reduction. Hypercholesterolemia could reduce bone mass via the inactivation of the LRP5/6 receptor and the involvement of Wnt signaling[10,11]. Pirie et al.[12] found that hyperlipidemia could induce secondary hyperparathyroidism, imparted bone regeneration, and mechanical strength in the animal model. In the cohort study, Poiana et al.[13] reported the significant correlation between bone mass density (BMD) and lipid profile in osteoporotic patients aged > 51 years. Furthermore, Bauer et al.[14] conducted a large meta-analysis of 151,500 patients and suggested that the use of statin medications for hyperlipidemia was associated with reduced fracture risk in an older woman. The association between dyslipidemia and PFF was not previously determined. As we have known, our study was a fundamental study that discovered this risk factor. However, our study had a small sample size, which made the wide CI and did not collect the data about the statin user and level of lipid profile. Therefore, a larger well-controlled study was required to address this issue.
The most common mechanism for a supracondylar fracture was a low-velocity fall. Although AFN more than 3 mm with a sharp corner at the proximal end of a femoral component provided the highest stress concentration in a biomechanical study[15], a recent prospective clinical trial could not show the correlation between AFN and supracondylar fracture[16]. Our study also could not determine AFN as a risk factor. For the condylar fracture, Vestermark et al.[3]. found that seven patients sustained a condylar fracture in the acute postoperative setting. Five patients had preoperative valgus deformity and sustained fracture of unloaded medial condyle. The other two patients had preoperative varus deformity and sustained fracture of unloaded lateral condyle. The authors called this type of fracture as "early femoral condyle insufficiency fracture". Comparable to our study, all lateral condylar fractures had preoperative varus deformity. While 54.5% of medial condylar fractures had preoperative valgus deformity (Figure 3), we believed that insufficiency fracture might explain this phenomenon. Nevertheless, the remaining 45.5% of medial condylar fracture was still associated with preoperative varus deformity. Due to a fracture of the loaded medial condyle, it should be caused by a technical error during surgery.
In clinical application, appropriate surgical exposure, avoiding excessive bow cut if PS design was used, gentle trial reduction, and prosthesis insertion were essential for minimizing this complication. Because early PFF was not found in the CR design used, we recommended that the use of this design might be beneficial for high-risk patients. For surgeons who preferred PS design, intraoperative surveillance for occult fracture, and preparation of the backup femoral stem should be performed. Likewise, we thought that the prophylactic femoral stem insertion was another strategy to prevent PFF. A finite element study revealed that periprosthetic stress was reduced through the use of a femoral stem. It might help mitigate PFF risk[17]. However, big high-quality data was necessitated for stratifying or scoring the risk factors and identifying the appropriate patients.
There were several limitations to our study. First, our research was a retrospective design; retrieving some of the information that we need might be troublesome. Osteoporosis was one of the most critical factors that contribute to PFF risk. Bernatz et al.[18] reported that one-quarter of total joint arthroplasty patients met the criteria to receive osteoporosis medications. This lack of preoperative osteoporosis screening and treatment has also happened in our study. Second, our study's small sample size might decrease statistical power to detect the other significant risk factors. Third, most patients in this study were female that had a higher risk. Thus, our results could not be applied to male patients. Fourth, all radiographic outcomes were measured from short radiographs because we had not sent the full-length radiographs routinely in the early postoperative period. However, Alzahrani et al.[19] illustrated the good to the excellent correlation of short and full-length radiographs. They also suggested that short radiographs could be an appropriate substitute for full-length radiographs for evaluating postoperative coronal alignments. Lastly, although we tried to detect the consequent intraoperative PFF from immediate postoperative radiographs, it was challenging to distinguish the early PFF from occult intraoperative PFF.
In conclusion, we found that age and dyslipidemia were independent risk factors for early PFF. The cut-off point of age was > = 75 years, with a sensitivity of 75.0% and specificity of 78.1%. The odds ratio of dyslipidemia was 6.63 (95% CI, 1.11 to 39.8). The further well-controlled studies with large sample size were needed to elucidate this research question. It would support us in doing strategic planning for preventing this complication.