M. J. Kwasny, Department of Preventive Medicine, Division of Biostatistics, Northwestern University, Feinberg School of Medicine, Chicago, IL, USA A. I. Edelstein, Department of Orthopaedic Surgery, Medical College of Wisconsin, Milwaukee, WI, USA D. W. Manning, Department of Orthopaedic Surgery, Northwestern University, Feinberg School of Medicine, Chicago, IL, USA.
Clin Orthop Relat Res. 2018 Dec;476(12):2418-2429. doi: 10.1097/CORR.0000000000000493.
Elevated body mass index (BMI) is considered a risk factor for complications after THA and TKA. Stakeholders have proposed BMI cutoffs for those seeking arthroplasty. The research that might substantiate BMI cutoffs is sensitive to the statistical methods used, but the impact of the statistical methods used to model BMI has not been defined.
QUESTIONS/PURPOSES: (1) How does the estimated postarthroplasty risk of minor and major complications vary as a function of the statistical method used to model BMI? (2) What is the prognostic value of BMI for predicting complications with each statistical method?
Using the American College of Surgeons National Surgical Quality Improvement Program from 2005 to 2012, we investigated the impact of BMI on major and minor complication risk for THA and TKA. Analyses were weighted with covariate-balancing propensity scores to account for the differential rate of comorbidities across the range of BMI. We specified BMI in two ways: (1) categorically by World Health Organization (WHO) BMI classes; and (2) as a smooth, continuous variable using splines. Models of risk for major complications (deep surgical site infection [SSI], pulmonary embolism, stroke, cardiac arrest, myocardial infarction, wound disruption, implant failure, unplanned intubation, > 48 hours on a ventilator, acute renal insufficiency, coma, sepsis, reoperation, or mortality) and minor complications (superficial SSI, pneumonia, urinary tract infection, deep vein thrombosis, or peripheral nerve injury) were constructed and were adjusted for confounding variables known to correlate with complications (eg, American Society of Anesthesiologists classification). Results were compared for different specifications of BMI. Receiver operating characteristic (ROC) curves were compared to determine the additive prognostic value of BMI.
The type of BMI parameterization leads to different assessments of risk of postarthroplasty complications for BMIs > 30 kg/m and < 20 kg/m with the spline specification showing better fit in all adjusted models (Akaike Information Criteria favors spline). Modeling BMI categorically using WHO classes indicates that BMI cut points of 40 kg/m for TKA or 35 kg/m for THA are associated with higher risks of major complications. Modeling BMI continuously as a spline suggests that risk of major complications is elevated at a cut point of 44 kg/m for TKA and 35 kg/m for THA. Additionally, in these models, risk does not uniformly increase with increasing BMI. Regardless of the method of modeling, BMI is a poor prognosticator for complications with area under the ROC curves between 0.51 and 0.56, false-positive rates of 96% to 97%, and false-negative rates of 2% to 3%.
The statistical assumptions made when modeling the effect of BMI on postarthroplasty complications dictate the results. Simple categorical handling of BMI creates arbitrary cutoff points that should not be used to inform larger policy decisions. Spline modeling of BMI avoids arbitrary cut points and provides a better model fit at extremes of BMI. Regardless of statistical management, BMI is an inadequate independent prognosticator of risk for individual patients considering total joint arthroplasty. Stakeholders should instead perform comprehensive risk assessment and avoid use of BMI as an isolated indicator of risk.
Level III, diagnostic study.
超重的身体质量指数（BMI）被认为是髋关节置换术和膝关节置换术（THA 和 TKA）后发生并发症的危险因素。利益相关者为寻求关节置换术的患者提出了 BMI 截止值。支持 BMI 截止值的研究对所使用的统计方法非常敏感，但用于建模 BMI 的统计方法的影响尚未确定。
问题/目的：（1） 用于建模 BMI 的统计方法的变化如何影响术后轻微和严重并发症的风险估计？（2） 对于每种统计方法，BMI 预测并发症的预后价值是什么？
使用美国外科医师学会全国手术质量改进计划（2005 年至 2012 年），我们研究了 BMI 对 THA 和 TKA 主要和次要并发症风险的影响。分析采用协变量平衡倾向评分加权，以说明 BMI 范围内各种共病的不同发生率。我们以两种方式指定 BMI：（1） 通过世界卫生组织（WHO）BMI 类别进行分类；（2） 使用样条作为连续的平滑变量。主要并发症（深部手术部位感染[SSI]、肺栓塞、中风、心脏骤停、心肌梗死、伤口破裂、植入物失败、需要插管>48 小时、急性肾功能不全、昏迷、败血症、再次手术或死亡）和轻微并发症（浅表 SSI、肺炎、尿路感染、深静脉血栓形成或周围神经损伤）的风险模型进行了构建，并根据与并发症相关的已知混杂因素进行了调整（例如，美国麻醉师协会分类）。比较了不同 BMI 规格的结果。比较了接收者操作特征（ROC）曲线，以确定 BMI 的附加预后价值。
BMI 参数化的类型导致术后关节置换并发症风险的不同评估，对于 BMI>30kg/m 和<20kg/m，样条规范的拟合度更好（Akaike 信息准则支持样条）。使用 WHO 类别对 BMI 进行分类建模表明，TKA 的 BMI 截止值为 40kg/m 或 THA 的 BMI 截止值为 35kg/m 与主要并发症的更高风险相关。以样条形式连续建模 BMI 表明，TKA 的主要并发症风险在 BMI 截止值为 44kg/m 和 THA 的 BMI 截止值为 35kg/m 时升高。此外，在这些模型中，风险不会随着 BMI 的增加而均匀增加。无论使用哪种建模方法，BMI 对并发症的预后价值都很差，ROC 曲线下面积在 0.51 到 0.56 之间，假阳性率为 96%至 97%，假阴性率为 2%至 3%。
建模 BMI 对术后并发症影响时所做的统计假设决定了结果。简单的 BMI 分类处理会创建任意的截止值，不应该用于告知更大的政策决策。BMI 的样条建模避免了任意的截止值，并在 BMI 的极端值处提供了更好的模型拟合。无论采用何种统计方法，BMI 都是评估考虑全关节置换术的个体患者风险的不足够的独立预后指标。利益相关者应进行全面的风险评估，并避免将 BMI 用作风险的单一指标。
III 级，诊断研究。
