V. M. Stetzelberger, A. M. Moosmann, M. Tannast, Department of Orthopaedic Surgery and Traumatology, Fribourg Cantonal Hospital, University of Fribourg, Fribourg, Switzerland.
V. M. Stetzelberger, S. D. Steppacher, M. Tannast, Department of Orthopaedic Surgery, Inselspital Bern, University of Bern, Bern, Switzerland.
Clin Orthop Relat Res. 2021 May 1;479(5):974-987. doi: 10.1097/CORR.0000000000001598.
Assessment of AP acetabular coverage is crucial for choosing the right surgery indication and for obtaining a good outcome after hip-preserving surgery. The quantification of anterior and posterior coverage is challenging and requires either other conventional projections, CT, MRI, or special measurement software, which is cumbersome, not widely available and implies additional radiation. We introduce the "rule of thirds" as a promising alternative to provide a more applicable and easy method to detect an excessive or deficient AP coverage. This method attributes the intersection point of the anterior (posterior) wall to thirds of the femoral head radius (diameter), the medial third suggesting deficient and the lateral third excessive coverage.
QUESTION/PURPOSE: What is the validity (area under the curve [AUC], sensitivity, specificity, positive/negative likelihood ratios [LR(+)/LR(-)], positive/negative predictive values [PPV, NPV]) for the rule of thirds to detect (1) excessive and (2) deficient anterior and posterior coverages compared with previously established radiographic values of under-/overcoverage using Hip2Norm as the gold standard?
We retrospectively evaluated all consecutive patients between 2003 and 2015 from our institutional database who were referred to our hospital for hip pain and were potentially eligible for joint-preserving hip surgery. We divided the study group into six specific subgroups based on the respective acetabular pathomorphology to cover the entire range of anterior and posterior femoral coverage (dysplasia, overcoverage, severe overcoverage, excessive acetabular anteversion, acetabular retroversion, total acetabular retroversion). From this patient cohort, 161 hips were randomly selected for analysis. Anterior and posterior coverage was determined with Hip2Norm, a validated computer software program for evaluating acetabular morphology. The anterior and posterior wall indices were measured on standardized AP pelvis radiographs, and the rule of thirds was applied by one observer.
The detection of excessive anterior and posterior acetabular wall using the rule of thirds revealed an AUC of 0.945 and 0.933, respectively. Also the detection of a deficient anterior and posterior acetabular wall by applying the rule of thirds revealed an AUC of 0.962 and 0.876, respectively. For both excessive and deficient anterior and posterior acetabular coverage, we found high specificities and PPVs but low sensitivities and NPVs.
We found a high probability for an excessive (deficient) acetabular wall when this intersection point lies in the lateral (medial) third, which would qualify for surgical correction. On the other hand, if this point is not in the lateral (medial) third, an excessive (deficient) acetabular wall cannot be categorically excluded. Thus, the rule of thirds is very specific but not as sensitive as we had expected.
Level II, diagnostic study.
评估髋臼前、后覆盖对于选择正确的手术适应证和获得髋关节保留手术后的良好效果至关重要。前、后覆盖的定量评估具有挑战性，需要使用其他常规投影、CT、MRI 或特殊的测量软件，这些方法繁琐、不普及，且会带来额外的辐射。我们引入了“三分法则”，作为一种很有前途的替代方法，可以提供一种更适用和简单的方法来检测髋臼前、后覆盖的过度或不足。该方法将前（后）壁的交点归因于股骨头半径（直径）的三分之一，内侧三分之一表示覆盖不足，外侧三分之一表示覆盖过度。
问题/目的：与以前建立的使用 Hip2Norm 作为金标准的髋臼下/上覆盖的放射学值相比，三分法则检测（1）过度和（2）髋臼前、后覆盖不足的有效性（曲线下面积 [AUC]、敏感度、特异度、阳性/阴性似然比 [LR(+)/LR(-)]、阳性/阴性预测值 [PPV、NPV]）如何？
我们回顾性评估了 2003 年至 2015 年期间我们机构数据库中所有因髋关节疼痛并可能适合髋关节保留手术的连续患者。我们根据各自的髋臼病理形态将研究组分为六个特定亚组，以覆盖整个前、后股骨覆盖范围（发育不良、覆盖过度、严重覆盖过度、髋臼前倾角过大、髋臼后倾、髋臼全面后倾）。从这个患者队列中，随机选择了 161 个髋关节进行分析。使用经过验证的髋臼形态评估计算机软件程序 Hip2Norm 确定髋臼前、后覆盖。在前、后骨盆 X 线片上测量前、后壁指数，并由一名观察者应用三分法则。
使用三分法则检测髋臼前、后壁过度覆盖的 AUC 分别为 0.945 和 0.933。同样，应用三分法则检测髋臼前、后壁不足覆盖的 AUC 分别为 0.962 和 0.876。对于髋臼前、后壁过度和不足覆盖，我们发现高特异性和 PPV，但低敏感度和 NPV。
当这个交点位于外侧（内侧）三分之一时，我们发现存在髋臼壁过度（不足）的可能性很大，这需要手术矫正。另一方面，如果这个点不在外侧（内侧）三分之一，则不能明确排除髋臼壁过度（不足）。因此，三分法则非常特异，但并不像我们预期的那样敏感。
二级，诊断研究。
