The range of motion after TKA depends on many patient, surgical technique, and implant factors. Recently, high-flexion designs have been introduced as a means of ensuring or gaining flexion after TKA. Our purpose was therefore to investigate which factors influence postoperative range of motion and, specifically, if the polyethylene type within a single knee implant design influenced motion. We hypothesized postoperative flexion would correlate with preoperative flexion and that polyethylene/knee design would have little or no influence. However, if knee design is a major factor, we sought to explore whether patients with a certain preoperative flexion would benefit most from a certain polyethylene design.
We acknowledge several limitations. First, we included patients from four surgeons that may introduce some selection bias and/or subtle differences in surgical technique (although it could be considered an advantage because it resulted in a “better mix” of our sample and may strengthen the external validity). Second, there are disparate numbers of patients in each group (the vast majority being PS knees) representing differences in surgical practice and implant availability over the period of study. However, the large numbers in each group provided adequate statistical power. Third, we somewhat arbitrarily categorized the preoperative and postoperative flexion data (based on equal quartiles of the preoperative data and clinically relevant postoperative groupings), which may have influenced the results. However, we had relatively large numbers, prospectively collected data, and a single implant design (ie, Genesis II™). Finally, although not a randomized trial, the cohorts were essentially identical preoperatively (with respect to demographics and range of motion).
Postoperative flexion after TKA is undoubtedly related to several patient, surgical technique, and implant design factors. The results of our linear regression analysis confirmed our initial hypothesis that preoperative flexion was an important determinant of postoperative flexion. This was not unexpected because there is substantial previous work demonstrating the importance of preoperative flexion [9, 15, 19, 24, 33]. Also important to the model were the patient's gender (male gender associated with increased postoperative flexion) and BMI (increasing obesity associated with decreased postoperative flexion). The positive effect of male gender is difficult to explain clinically and, although statistically significant, the effect was relatively small (based on the variable's standardized beta coefficient within the model) and we are unaware of other literature reporting this finding. On the other hand, the negative effect of increasing BMI is much easier to understand in the clinical setting and is supported by several studies [10, 11, 16, 27, 28, 34, 38].
Contrary to our initial hypothesis, the linear regression model analysis demonstrated the polyethylene type/design did influence the postoperative flexion. In particular, independent of gender, BMI, and preoperative flexion, the use of a HF-PS and PS design implant resulted in a postoperative flexion increase of 8.2° and 5.6°, respectively, compared with a CR implant. Similarly, comparison of preoperative to postoperative flexion for each patient demonstrated those patients receiving a CR knee had, on average, no improvement in flexion (−0.4° ± 14.0°, mean ± SD) compared with an average increase of 7.4° ± 16.0° and 9.7° ± 20.4° for PS and HF-PS knees, respectively. The positive benefit to a PS knee design (over CR) was not completely expected because previous work remains controversial whether CR or PS knees are advantageous with respect to postoperative range of motion [6, 7, 9, 15, 18, 25].
Categorization of the preoperative and postoperative flexion into groups revealed the use of a HF-PS implant was associated with the greatest percentage of patients who gained flexion postoperatively in each of the preoperative flexion groups (except for the low flexion group in which implant type had no effect). The fact that all forms of analysis in this study demonstrated a potential advantage to the HF-PS design was somewhat surprising because a recently published randomized, controlled trial (comparing 50 PS with 50 HF-PS Genesis II™ knees) performed at our institution failed to show a difference in postoperative range of motion between groups . It is likely our randomized, controlled trial was underpowered to demonstrate the findings of this large database review series.
Other clinical results in the literature for high-flexion knee replacements have been quite variable. Several studies have demonstrated improved range of motion when comparing standard PS or CR with HF designs [5, 12, 17, 22, 37]. In contrast, other studies, including several randomized, controlled trials, have demonstrated little or no difference between high-flex and conventional knee designs [8, 20-22, 31, 32, 35, 39]. The potential downside of a high-flexion knee design is well illustrated in a paper from Han et al. , who reported on a 38% aseptic loosening and 21% revision rate in 72 high-flexion knees. Their paper demonstrated a clear association of loosening with those patients who achieved high flexion.
In summary, our data demonstrate postoperative range of motion after TKA is related to several factors, confirming the important role of the patient's preoperative range of motion. In addition, our review suggests knee design and, in particular, the use of a so-called “high-flexion” PS polyethylene design may be advantageous in maintaining or improving flexion postoperatively, especially in those patients with good preoperative range of motion.
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