Efficacy of Patient-Specific Instruments in Total Knee Arthroplasty: A Systematic Review and Meta-Analysis

Thienpont, Emmanuel MD, MBA, PhD; Schwab, Pierre-Emmanuel MD; Fennema, Peter DSc

Journal of Bone & Joint Surgery - American Volume: 15 March 2017 - Volume 99 - Issue 6 - p 521–530
doi: 10.2106/JBJS.16.00496
Evidence-Based Orthopaedics

Background: Patient-specific instrumentation (PSI) was introduced with the aim of making the procedure of total knee arthroplasty more accurate and efficient. The purpose of this study was to compare PSI and standard instrumentation in total knee arthroplasty with regard to radiographic and clinical outcomes as well as operative time and blood loss.

Methods: A meta-analysis was conducted in accordance with the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) statement. PubMed and Embase were searched from 2011 through 2015. We included randomized controlled trials and cohort studies that reported the effect of PSI on the aforementioned outcomes. The primary end point was deviation from the mechanical axis by >3°. Random and fixed-effect models were used for analysis.

Results: A total of 44 studies, which included 2,866 knees that underwent surgery with PSI and 2,956 knees that underwent surgery with standard instrumentation, were evaluated. The risk of mechanical axis malalignment was significantly lower for PSI, with a pooled relative risk of 0.79 (p = 0.013). The risk of tibial sagittal-plane malalignment was higher for PSI than for standard instrumentation (relative risk = 1.32, p = 0.001), whereas the risk of femoral coronal-plane malalignment was significantly lower (relative risk = 0.74, p = 0.043). The risk of tibial coronal-plane malalignment was significantly higher for PSI only when employing fixed-effect meta-analysis (relative risk = 1.33, p = 0.042). Minor reductions in total operative time (−4.4 minutes, p = 0.002) and blood loss (−37.9 mL, p = 0.015) were noted for PSI.

Conclusions: PSI improves the accuracy of femoral component alignment and global mechanical alignment, but at the cost of an increased risk of outliers for the tibial component alignment. The impact of the increased probability of tibial component malalignment on implant longevity remains to be determined. Meta-analyses indicated significant differences with regard to operative time and blood loss in favor of PSI. However, these differences were minimal and, by themselves, not a substantial justification for routine use of the technology.

Level of Evidence: Therapeutic Level II. See Instructions for Authors for a complete description of levels of evidence.

1University Hospital Saint Luc, Brussels, Belgium

2AMR Advanced Medical Research, Männedorf, Switzerland

E-mail address for E. Thienpont: emmanuel.thienpont@uclouvain.be

Article Outline

Up to 25% of patients are not satisfied with the postoperative outcome of total knee arthroplasty, on account of pain and functional limitations1. The socioeconomic impact of the high prevalence of residual symptoms is great. Hence, there is a need to improve clinical outcomes for patients. Unsatisfactory outcomes of total knee arthroplasty are the result of the complex interaction of various factors. Results can be improved by optimizing implant design, surgical techniques, patient selection, and management of patient expectations2-4.

Implant malalignment is generally believed to be a common cause of failure5. Various technologies have been proposed to improve total knee arthroplasty alignment. One of the most recently introduced technologies is customization of the instrumentation to be used during total knee arthroplasty―i.e., patient-specific instrumentation (PSI). The aims of PSI are to improve the accuracy of implantation, reduce operative time, and facilitate the workflow in the operating room6. PSI is based on computed tomography (CT) or magnetic resonance imaging (MRI), sometimes combined with radiographs of the lower extremity. The imaging is used by implant manufacturers to develop 3-dimensional models of the patient’s anatomy7. Implant component size and positioning can be planned utilizing the osseous landmarks used during conventional total knee arthroplasty. These models are then used to produce disposable pinning or cutting blocks to help the surgeon reproduce the surgical plan during the surgical intervention.

PSI is potentially beneficial if it leads to greater surgical accuracy during arthroplasty or if it increases the efficiency of the surgical process8. A worldwide increase in the number of PSI-assisted cases indicates that this technology has a substantial impact on the current knee arthroplasty market, in spite of the substantial economic costs that come with the use of the product8,9.

Several published meta-analyses have been based on early comparative studies and focused primarily on radiographic outcomes7,10-14. However, several studies that were not included in those analyses have been published recently, and a number of those studies evaluated intraoperative and clinical outcomes. The aim of the present study was to provide an update to previously published meta-analyses by including the most current literature as well as other efficacy end points such as operative time, blood loss, and clinical outcome.

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Materials and Methods

Protocol and Registration

This systematic review and meta-analysis was performed using a predetermined protocol in accordance with the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) statement15. The study protocol was registered with PROSPERO (CRD42016036175).

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Search Strategy and Study Selection

An electronic search of PubMed and Embase from January 1, 2011, to December 31, 2015, was conducted. The electronic search was complemented by hand-searching the references of the retrieved articles. The full search strategy is available in the Appendix. We searched for studies that compared primary total knee arthroplasty performed with PSI and with standard instrumentation, utilizing a combined text and MeSH (medical subheading) search with the following terms: total knee arthroplasty, patient-specific instrumentation, custom-fit instrumentation, and custom-made guides. Both randomized and nonrandomized comparative studies were included.

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Eligibility Criteria

The selection of studies was made using the titles and abstracts, and 2 reviewers (P.-E.S., P.F.) conducted the screening. Potential studies had the full text retrieved and were screened against the eligibility criteria. Studies were eligible if (1) PSI and standard instrumentation were compared; (2) the study population comprised patients who underwent primary total knee arthroplasty; (3) at least 1 of the study outcomes of interest was reported; (4) the studies were published in English, French, German, or Dutch; and (5) clinical outcome data were reported at a mean follow-up of at least 6 months.

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Data Extraction

Two reviewers (P.-E.S., P.F.) used a standardized form to extract data. A third reviewer (E.T.) was used to resolve disagreements in eligibility, data extraction, or quality assessment.

Extracted data included radiographic alignment of the mechanical axis on standing full-extremity radiographs in full extension and malalignment of the femoral and tibial components in the coronal, sagittal, and axial planes. Malalignment was defined as a measurement that was >3° from the intended value7. Mechanical alignment was defined as the medial angle between the femoral mechanical axis, which runs from the center of the femoral head to the center of the distal portion of the femur, and the tibial mechanical axis, which runs from the center of the proximal portion of the tibia to the center of the ankle joint. Femoral coronal alignment was defined as the angle between the articular surface of the femoral component and the mechanical axis of the femur, with an ideal perpendicular angle of 90°. Femoral sagittal alignment was defined as the angle between the distal femoral cut line and the femoral anterior cortex. Femoral axial alignment was defined as the alignment of the femoral component in relation to the transepicondylar axis in the axial plane as determined by CT or MRI. Tibial component coronal alignment was defined as the angle between the inferior surface of the tibial component and the tibial mechanical axis. Tibial sagittal slope varied by implant and surgeon preference; for the present study, the deviation from the intended slope was used16. Intraoperatively, tourniquet time and total operative time were extracted, along with the proportion of patients receiving blood transfusion. Postoperatively, Knee Society Score17 data were extracted.

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Quality Assessment

The methodological quality of the included studies was assessed by the same 2 reviewers (P.-E.S., P.F.). A modified version of the Detsky Quality Assessment Scale was used as described previously, as not all items in the original instrument were applicable for nonrandomized studies7,18.

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Statistical Analysis

Meta-analysis was performed to compare the PSI and standard instrumentation groups with regard to radiographic, surgical, and clinical end points. Data were summarized as the ratio of relative risk or the difference between means. For studies that did not report standard deviations (SDs), it was calculated from p values, confidence intervals, or standard errors. If such information was not available, the reviewers contacted the corresponding authors for additional information.

Random-effect models19 were used throughout because of the anticipated heterogeneity7. However, analyses that included <10 studies were performed with use of fixed-effect models according to Mantel and Haenszel20.

Meta-analysis of proportions was performed after a Freeman-Tukey (double arcsine) transformation of the data21. The Miller equation was used for the corresponding back-transformation22.

Study heterogeneity was quantified using the I2 statistic23. To explore the source of heterogeneity, meta-regression analysis was performed, using the log-transformed odds ratio for binary variables or the difference in means for continuous outcome data as dependent variables. For this analysis, the manufacturer, baseline risk of malalignment, study design (randomized versus nonrandomized), region where the study was conducted (Europe versus other), and Detsky Quality Assessment Scale were selected as covariates in the regression model. The primary assessment used the restricted maximum likelihood with random-effect weighting and the Hartung-Knapp modification24. Meta-regression analysis was only considered when at least 20 studies were available for analysis. Methods of analysis and covariates in the meta-regression were specified before study commencement.

Publication bias was investigated for outcomes with at least 10 included studies25. Graphically assessed funnel plots were evaluated for the presence of publication bias. We used the Egger regression test25,26 for continuous outcome data and the Harbord modified test27 for binary outcome variables to test for small-study effects. If publication bias was thought to be present, a trim-and-fill method was used to adjust for publication bias28.

Sensitivity analyses were conducted to assess the potential impact of modeling assumptions (fixed versus random effect) and the impact of imputation of missing data on the study findings.

The level of significance was set at p < 0.05. All statistical analyses were carried out using R software (version 3.2.3; R Foundation for Statistical Computing) with use of the Metafor package29.

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Results

The search strategy identified 531 studies, of which 487 were excluded after screening (Fig. 1). The data set for the meta-analysis was based on the remaining 44 studies, which evaluated 2,866 knees that underwent surgery using PSI and 2,956 knees that underwent surgery using standard instrumentation.

Study baseline characteristics are summarized in Table I. Twenty randomized studies and 24 observational studies were included. The risk of bias, as determined by the modified Detsky score, is presented in Table I. The mean methodological quality was higher in randomized clinical trials (mean score [and standard deviation], 14.2 ± 3.8) than in cohort studies (mean, 10.8 ± 3.3) (p = 0.003).

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Risk of Malalignment

The proportion of knees with mechanical axis malalignment of >3° was reported in 29 studies with 3,479 knees30-58. The pooled risk of malalignment was higher for standard instrumentation (25.7%; 95% confidence interval [CI], 21.6% to 30.0%) than for PSI (20.2% [95% CI, 15.6% to 25.1%]). The pooled relative risk of malalignment was 0.79 (95% CI, 0.65 to 0.95; p = 0.013) (Fig. 2). There was substantial heterogeneity among studies (I2 = 51.0%; p < 0.001). A significantly higher probability of malalignment with use of PSI was found for the tibial component in the sagittal plane (pooled relative risk, 1.32 [95% CI, 1.12 to 1.56]; p = 0.001). PSI yielded a lower probability of femoral component malalignment in the coronal plane (pooled relative risk, 0.74 [95% CI, 0.55 to 0.99]; p = 0.043). No significant difference was found for the probability of tibial malalignment in the coronal plane. Results for individual component alignments are summarized in Table II, and the forest plots are presented in the Appendix.

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.

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Intraoperative Variables

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.

For tourniquet time, 9 studies with 1,313 knees were pooled6,35,45,47,55,60,62,67,68. No significant difference was found (difference in mean, −0.4 minutes [95% CI, −1.7 to 1.0 minutes]; p = 0.597).

For blood loss, 12 studies were pooled36,38-40,45,49,60,63-65,67,68. PSI was associated with a slight reduction in blood loss (difference between means, −37.9 mL [95% CI, −68.4 to −7.4 mL]; p = 0.015). There was substantial heterogeneity among studies (I2 = 91.2%; p < 0.001). PSI was also associated with a lower relative risk of transfusion (0.61 [95% CI, 0.43 to 0.86]; p = 0.004). The pooled mean values and pooled mean differences for the perioperative outcomes are summarized in Table IV.

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Clinical Outcome

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.

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Publication Bias

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).

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Sensitivity Analysis

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).

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Discussion

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.

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Appendix Cited Here...

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

Disclosure: This study was funded by Zimmer Biomet (Winterthur, Switzerland). The sponsor had no involvement in the writing of the report or in the decision to submit the results for publication. On the Disclosure of Potential Conflicts of Interest forms, which are provided with the online version of the article, one or more of the authors checked “yes” to indicate that the author had a relevant financial relationship in the biomedical arena outside the submitted work; “yes” to indicate that the author had a patent and/or copyright, planned, pending, or issued, broadly relevant to this work; and “yes” to indicate that the author had other relationships or activities that could be perceived to influence, or have the potential to influence, what was written in this work (http://links.lww.com/JBJS/A149).

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