To explore the source of heterogeneity, meta-regression analysis was performed with the log of the odds ratio of mechanical malalignment as the dependent variable. The meta-regression did not yield any significant association between malalignment and any of the independent variables (Table III). All variables together explained 54.4% of the variance among studies.
For the analysis of operative time, data from 26 studies with a total of 3,480 knees were pooled6,30,33,35-43,47,49,53-55,57,59-66. The difference in mean total operative time favored PSI (−4.4 minutes [95% CI, −7.2 to −1.7 minutes]; p = 0.002) (Fig. 3). There was substantial heterogeneity among studies (I2 = 93.5%; p < 0.001). Meta-regression analysis performed with the difference in mean operative time as the dependent variable did not yield any significant association between operative time and any of the independent variables (Table III). All variables together explained 4.9% of the variance among studies.
Six studies could be included in the meta-analysis of the postoperative Knee Society Score, with the longest follow-up time being 16.7 ± 8.2 months (range, 6 to 24 months)33,52,57,58,66,67. For the knee score, a mean difference of 1.5 (95% CI, −0.26 to 3.3; p = 0.093) was found. For the function score, a mean difference of 4.3 (95% CI, 1.5 to 7.2; p = 0.003) in favor of PSI was found. The pooled mean values for the knee and function scores are presented in Table IV.
We created funnel plots by plotting the treatment effect against the reciprocal of its standard error. For mechanical axis malalignment, graphical assessment of the funnel plot (see Appendix) alongside the borderline p value of the Harbord modified test27 (Table V) indicated that publication bias may have been present for mechanical axis malalignment. However, the relative risk did not change after trim-and-fill analysis, and the result was still significant in favor of PSI (p = 0.035). No publication bias was observed for any of the other radiographic outcomes, either by graphical assessment of the funnel plots (not shown) or by the Harbord modified test (Table V). Publication bias could not be ruled out for blood loss, as indicated by the funnel plot (see Appendix) and by the Egger test25,26 (Table V). No publication bias was observed for operative time, as indicated by the Egger test (Table V).
To verify the robustness of our findings, meta-analyses were repeated with application of fixed-effect models to the data. The pooled relative risk of tibial coronal-plane malalignment was 1.33 (95% CI, 1.00 to 1.75; p = 0.042). Sensitivity analysis with the exclusion of imputed data did not materially alter the findings of the study (not shown).
With malalignment defined as a deviation of >3°, PSI slightly decreased the risk of malalignment of the mechanical axis, and it decreased the risk of malalignment of the femoral component in the coronal plane. Except for coronal-plane alignment of the femoral component, PSI did not decrease the risk of malalignment of the individual components. Fixed-effect meta-analysis yielded no differences with regard to the alignment achieved by PSI in the axial plane; however, only a small number of studies that reported this outcome (despite the large number of total knee arthroplasties that have been implanted with PSI globally), which did not allow robust conclusions to be drawn.
In terms of surgical accuracy, the effectiveness of PSI as found in the present study was less than the effectiveness of surgical navigation, as found in previous reports in the literature. A systematic review of published meta-analyses comparing surgical navigation and standard instrumentation found significant reductions in the risk of malalignment in the coronal and sagittal planes when surgical navigation was used, with relative risks generally <0.569. In the present meta-analysis, a slight reduction (21%) in the risk of mechanical axis malalignment was found, without any consistent advantages with regard to implant component alignment.
Although an adverse impact of inadequate restoration of leg alignment on postoperative outcome has been a long-held tenet, the authors of recent studies have concluded that the impact of mechanical coronal-plane malalignment may be smaller than originally believed, which may cast doubt on the premise of PSI70.
Notably, the risk of malalignment of the tibial component was approximately 30% greater for PSI than for standard instrumentation in both the coronal and sagittal planes, although the significance of the relative risk of coronal-plane malalignment depended on the modeling assumptions. Recent literature has revealed that changes in alignment of the tibial component, in both varus and valgus directions, produce greater increases in contact stress and pressure71 and in the failure rate than malalignment of the femoral component does72. Moreover, compensation for a varus or valgus orientation of the tibial component by alteration of the femoral component alignment led to a significant increase in the failure rate from 3.2% to 7.8%72. In the future, data from large case series or registries need to determine whether the increased probability of tibial component malalignment affects long-term implant survival.
This meta-analysis revealed that PSI yielded slight advantages in terms of blood loss and operative time, and it reduced the risk of allogeneic transfusion. Lower blood loss may be attributable to the fact that violation of the femoral medullary canal was avoided when using PSI, as well as to the decrease in operative time. However, the effect of blood loss reduction may only be relevant in surgical procedures performed without a tourniquet64,65, which was not established in our study. Nevertheless, the difference in blood loss was small and probably not of clinical relevance. In addition, no significant advantage was found with regard to tourniquet time. The reason that a slight but significant difference was found for the operative time but not for the tourniquet time may be that different studies were included in each analysis. Different techniques of anesthesia (e.g., whether or not local infiltration of analgesic agents is used) and closure can influence the total operative time. The present study was unable to establish whether the minor improvements in terms of operative time offset the additional nonsurgical resource hours required of the surgeon9 plus the incremental increase in time for preoperative imaging. Hence, the economic impact of the technology remains unknown. However, a pooled difference in mean operative time of only 4 minutes is, by itself, not a justification for routine use of the technology. No differences were found with regard to the knee score, but a significantly higher function score was found. As the number of studies that could be evaluated was small, we consider these findings to be preliminary.
Substantial heterogeneity was found among the studies. We were unable to demonstrate significance for any of the covariates during the meta-regression. Additionally, the meta-regression was unable to show that randomized clinical trials or studies exhibiting a higher risk of bias reported a stronger association of instrumentation with the risk of malalignment of the mechanical axis or with operative time. The meta-regression analysis did not yield significant differences among the manufacturers of the systems.
The present study provides substantially more information on the efficacy of PSI than a previous meta-analysis7. Although point estimates for the relative risk of mechanical axis malalignment in the current meta-analysis are similar to our previous estimates7, the present study has greater precision.
This analysis has several strengths. First, as a systematic review and meta-analysis of all available studies on the efficacy of PSI, it has greater power than the included studies. Many of the included studies have small sample sizes and, as a consequence, low statistical power. The present study has been able to partially overcome this issue. Second, it contains both efficiency end points and radiographic and clinical efficacy end points, allowing a comprehensive appraisal of the technology.
This analysis has several limitations. First, although we attempted to acquire unpublished and missing data for eligible cohorts by contacting authors, our conclusions may be influenced by publication bias. However, although publication bias could not be ruled out for all outcomes, the most important study conclusions appear to be robust with regard to the presence of publication bias. Second, the sample size for several outcomes was limited and did not allow assessment of heterogeneity and publication bias. Third, we encountered substantial heterogeneity among studies, and the meta-analysis was unable to identify any of the covariates of interest as sources of the heterogeneity. Because of the joint impact of these limitations, inferences should be drawn from this study with caution.
In the search for improvements in functional outcomes for patients and increased survival of prostheses, there is an ongoing need to introduce new technologies. New device technologies and the associated clinical benefits need to be carefully balanced against the possible incremental costs and associated risks. The present meta-analysis revealed an increased risk of malalignment of the tibial component for PSI, and its impact on implant longevity remains to be seen. This study showed that PSI only has a minor impact on the risk of malalignment of the mechanical axis. Moreover, there is still a lack of evidence of clinical effect associated with the use of PSI. We believe that this technology therefore cannot be recommended for routine use in standard primary total knee arthroplasty cases.
The full search strategy and figures showing the forest plots for the outcomes and funnel plots for publication bias are available with the online version of this article as a data supplement at jbjs.org (http://links.lww.com/JBJS/A150).
NOTE: The authors thank their colleague researchers for responding to their queries to provide additional information. The authors also thank Ingrid Schaefer Sprague (AMR Advanced Medical Research) for her editorial support.
Investigation performed at the University Hospital Saint Luc, Brussels, Belgium, and AMR Advanced Medical Research, Männedorf, Switzerland
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