Nine hundred eight TKAs were performed for 671 patients, with individual study mean followups between 5 and 18 years, and individual patient followups between 2 and 25.7 years. Cochran Q-test statistics were calculated to assess for study heterogeneity with respect to the most common variables reported: pain, Knee Society (KS) clinical score, and KS functional score. Although study heterogeneity limited the ability to perform meta-analysis of pooled data, postoperative KS clinical and functional scores were available for all studies for determination of mean postoperative scores and average improvement.
The mean postoperative KS clinical score from these 13 studies was 90.9 points (95% confidence interval [CI], 88.3-95 points), which improved by an average of 47 points (range, 31-56 points). The postoperative mean KS functional score was 81.6 points (95% CI, 70.2-89.8 points) with an average improvement of 37 points (range, 15-41). Four studies documented preoperative and postoperative ROM with mean improvements from 102° to 109° flexion [9, 11, 22, 35]. Radiographic assessments reported in the studies noted nonprogressive radiolucencies observed in between 3.8% and 30% of cases (mean, 14.5%) with the patella and tibia being the most common locations (Table 3). Progressive radiolucencies were not commonly reported.
Component survivorship was either directly reported or estimated by Kaplan-Meier analysis in 10 of the studies with survivorship between 90.6% and 99% during the initial 6 to 10 years and between 85% and 96.5% for studies that projected at or beyond 15 years. Revisions for patellar component failure, instability, or infection were the most commonly reported complications, with component revisions uncommon. Fifty revision procedures were documented among the 908 TKAs (5.5%), with a range of revisions reported in individual studies from 3.4% to 18.6% at 2 to 25.7 years (mean, 5-20 years).
Study heterogeneity and limited data reporting attenuate the literature’s impact on guidance for surgical technique and implant selection in younger patients. Cochran’s Q-testing indicated significant heterogeneity among the 13 studies, with p values of 0 calculated for pain, clinical score, incomplete, and functional score. Five studies reported results for 414 TKAs without differentiating between lower demand patients with inflammatory arthropathy and potentially higher-demand patients with osteoarthritis or posttraumatic arthritis [5, 10, 11, 16, 28]. The mean improvement in KS clinical score was 43.6 points in these studies. Two studies (91 TKAs) reporting exclusively on patients with inflammatory arthritis, [4, 34] noted a mean improvement in KS clinical score of 40.8 points (range, 32-48 points). Six studies (402 TKAs) reported specifically on patients with a diagnosis of either osteoarthritis or posttraumatic arthritis [7, 9, 22, 24, 32, 35], and noted a mean improvement in clinical scores for this group of 42.9 points (range, 37-56 points). Cementless fixation was used in only one study in this group (118 TKAs)  with a KS clinical score improvement of 47 points. Cemented fixation was used for all patients in three studies [7, 9, 32], with a mean improvement in KS or HSS clinical score of 41.4 points (range, 37-56 points). A posterior cruciate retaining design was used exclusively in three studies [9, 24, 35], with a mean improvement in KS clinical score of 48.3 points (range, 40-56 points). A posterior cruciate substituting design was used exclusively in only two studies [7, 32], both reporting a mean HSS score improvement of 37 points.
TKA is being performed with increasing frequency in younger and more active adults, yet the published data on the clinical scores and long-term durability for TKA for these patients are limited. The purposes of this study were to assess the performance and durability of the TKA when performed in young patients and to determine whether the literature provides specific guidance with respect to implant selection or fixation technique for TKA in young patients.
We acknowledge substantial limitations of our review largely reflecting limitations in the existing body of literature. First, the majority of studies are small, retrospective, single-surgeon case series reporting on a heterogeneous group of patients with respect to diagnosis, activity levels, and surgical technique. Only six of the 13 articles that met our inclusion criteria focused specifically on patients with posttraumatic arthritis or osteoarthritis [7, 9, 22, 24, 32, 35]. Five studies included patients with a predominance of rheumatoid arthritis without distinguishing clinical outcomes on the basis of diagnosis or other potential considerations [5, 10, 11, 16, 28], and two studies exclusively reported on patients with rheumatoid arthritis [4, 34]. Second, although variations in surgical technique occurred in most studies, clinical scores were not reported specific to the techniques used, including posterior cruciate ligament management [4, 16, 32], fixation choice [22, 24], tibial component modularity [5, 10], and patellar management [11, 16]. Third, defining TKA performance may not be determined accurately from measurement tools traditionally reported in the literature. Although the KS clinical score consistently assesses knee status after TKA , its focus on pain and knee examination may not necessarily reflect physical function, return to desired level of activity, or patient satisfaction. Although the KS function score incorporates elements of activity, walking, and stair climbing, it was not assessed before and after surgery for the majority of available studies on TKAs in younger patients. Fourth, estimates for component survivorship (eg, Kaplan-Meier survivorship curves) often were reported, whereas actual long-term followup with patients enrolled in the studies did not occur for a majority of patients. Stern et al.  and Diduch et al.  presented the longest followup for studies focused on patients with osteoarthritis or posttraumatic arthritis, noting 87% survivorship at 18 years. However, the mean followup was 8 years (range, 3-18 years) and only 36 patients (31.5%) had documented followups beyond 10 years postoperatively. The authors calculated an annual implant failure rate up to the latest followup and made a linear determination of implant survivorship based on revisions that had occurred. Duffy et al., reporting predominantly on patients with inflammatory arthritis, noted 96% implant survival at 10 years but an increase in polyethylene wear-related failures at 15 years (85% survival), indicating the importance of followup through the end of the second decade . Because younger patients likely will live long enough to require revision surgery, more complete long-term followup is essential to provide an adequate assessment of durability. Continuing and reporting followup through the second decade are important to confirm whether implant survivorship follows a linear or exponential curve beyond the first decade. Finally, we excluded 37 articles published in languages other than English. As we were unable to perform an adequate review of these publications, we cannot exclude positive or negative impacts that these studies might have on the total body of literature. However, with the low percentage of articles that met our strict inclusion criteria with publication in English (13 of 206 studies), we anticipate similar weakness in reporting in the international literature.
Our review showed improvement in mean Knee Society clinical and functional scores that parallel the results reported in studies that have not separated patients on the basis of patient age [1, 2, 6, 8, 12, 19, 27, 30]. Although physician-generated scores suggest improvement, reporting biases that appear to overlook negative patient experiences were identified during this review, including minimizing the impact of revision procedures for patellar resurfacing, polyethylene wear, instability, and stiffness (Table 1). Bourne et al. recently reported patient satisfaction with TKA is achieved by only 81% of patients . Eight of the 13 studies in our review reported between 94% and 98% of patients having a good or excellent result as defined by the authors and generally attributed to a postoperative KS clinical score greater than 80. Notably, four of the six studies reporting on this patient group since 2000 noted good or excellent clinical scores for only 81% to 87% [4, 9, 16, 22], more consistent with the report by Bourne et al. . The inclusion of patient-derived assessment tools, (eg, SF-36, WOMAC, KOOS, satisfaction surveys) , assessment of activity level (eg, UCLA activity score) [25, 38], and functional performance testing (eg, stair climbing, timed ambulation) , could limit the potential for surgeon bias in reporting clinical and functional scores.
TKA component revisions were relatively uncommon in these series. A few studies noted failures that may have been related to design-specific considerations. Lonner et al. excluded 12 patients (27%) from their study group because an implant design was used that had a 75% rate of polyethylene wear and failure in these younger patients . Although component retention is a major factor in the complexity of revision surgery, implant-related failures may be underemphasized in the literature when only component survivorship is considered. Hofmann et al. noted five revisions (6.6%) performed for polyethylene wear and seven liner revisions (9.3%) for instability but did not present these as clinical failures .
Although guidelines may be extrapolated from research that did not meet the criteria of this systematic review, substantiation of specific techniques for TKA in young patients has not been documented in the literature. The two most recent studies most clearly stated their inclusion criteria, and used a single implant and standardized surgical technique, suggesting improvement in reporting quality [9, 35]. However, the authors did not define whether their studies included all young patients who underwent surgery or simply those treated with the specified protocol.
Because our systematic review was performed to consider TKA performance and durability, we excluded some studies [13, 15, 29, 36] that nonetheless provide valuable information in our efforts to understand the results of TKA performed for young patients. National, academic institutional, and community registries have reported on revision surgery rates among young patients. Himanen et al. reported lower 10-year survival rates among patients younger than 60 years, male gender, and a diagnosis of osteoarthritis . W-Dahl et al. noted increased cumulative rates of revision for patients younger than 55 years compared with older patients, but lower revision rates for TKA than for other surgical procedures performed in this patient group, including unicompartmental arthroplasty and proximal tibial osteotomy . Rand et al. reported survivorship analysis for 11,606 TKAs performed in one institution during a 22-year period and noted a lower estimated component survivorship at 10 years for patients younger than 55 years and for patients with a primary diagnosis of osteoarthritis . Gioe et al. reported on implant survival from a community registry for more than 1000 TKAs in patients younger than 55 years and noted higher failure rates for male patients and for patients undergoing either unicompartmental knee arthroplasty or cementless TKA . Although the registry data are valuable in identifying potential implant- or technique-related implant survivorship data, important clinical questions including durability of implants under sustained high activity levels, fixation techniques, implant design selection, surgical techniques, and the influence of perioperative protocols and rehabilitation on functional recovery would be answered with greater certainty from retrospective or prospective studies conducted at a higher level of evidence. The existing body of literature regarding TKA in patients younger than 55 years consists of studies with Level IV evidence. Although the levels of evidence for a specific study indicate the potential for bias and not the validity of results, the strength of conclusions may be less certain when a collective body of evidence contains studies with only the lowest levels of evidence . Although prospective data collection might be inferred from the majority of the articles, only one study specified prospective entry or analysis . Although Diduch et al.  likely included patients initially enrolled in the study reported by Stern et al.  from the same institution, they did not specify whether the additional patients added to the study were added in a retrospective or prospective fashion.
General reporting quality weaknesses were noted across the studies when using the STROBE criteria (Table 4). Although all studies gave appropriate introduction to the reasons for assessing this patient group, only two studies specifically identified a study purpose or hypothesis [22, 35]. None of the studies discussed or explained missing data or defined how they accounted for patients lost to followup. Although clinical outcomes were reported in a similar fashion by all of the studies, survivorship data rarely were reported with confidence intervals [11, 35], and only the most recently published study  specifically acknowledged both statistical methods for evaluating data and the presence of either bias or limitations. These improvements in the most recently published studies may reflect a change in research quality reporting or an elevation in standards for publication.
The use of TKA is increasing among patients younger than 55 years. This collection of data provides important and more generalized information regarding the results of TKA in young patients and may be useful as baseline or comparative data for future studies with this patient population. The existing literature suggests surgeon-reported improvements in pain occur in a similar fashion as in other patients undergoing TKA. However, many of these younger patients are likely to outlive the period of reporting in the current literature. With a relatively low volume of young patients in most individual surgeon practices, multicenter collaboration and improving the details of data submitted to registries will be important to provide more reliable information to guide surgical decisions. Future research should be directed to answer questions regarding TKA implant design considerations, fixation methods, and the effect of activity levels on the durability of TKA among the most young and active patients, who may become a large subpopulation undergoing TKAs during the next 20 years. Improvements in data reporting, emphasized since release of the STROBE statement, will be beneficial in improving the quality of systematic reviews and meta-analyses to help guide clinical decisions.
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