Medial proximal tibial angle after medial opening wedge HTO: A retrospective diagnostic test study

Background: Medial proximal tibial angle (MPTA) is the commonly used angle, which is simply measured from the knee radiographs. It can determine the correction angle in medial opening wedge high tibial osteotomy (MOWHTO). The hypothesis of our study is that post-osteotomy MPTA can predict the change in correction angle, and we aimed to determine the optimal MPTA with which to prevent recurrent varus deformity after MOWHTO. Materials and Methods: Between January 2002 and April 2010, radiographs of 59 patients, who underwent 71 MOWHTOs using the locking-compression osteotomy plates without bone grafts, were evaluated for the change of the MPTA. The MPTA was measured preoperatively and one and twelve months postoperatively. The changes of MPTA between one and twelve months were classified into valgus, stable, and varus change. The predicting factors were analyzed using analysis of variance (ANOVA) and Bonferroni multiple comparisons. The receiver operating characteristic (ROC) curve was used to find out the cut off point for preventing the recurrent varus deformity. Results: The overall preoperative, and one and twelve month postoperative MPTA values were 84.4 +/- 2.4 degrees, 97.2 +/- 4.1 degrees, and 96.3 +/- 3.6 degrees, respectively. Between one and twelve months, 39 knees displayed reduced varus change (-2.8 +/- 2.1 degrees), 18 knees displayed no change, and 14 knees displayed a greater valgus change (+2.9 +/- 2.1 degrees). The best factor for predicting these changes was the one month MPTA value (P = 0.006). By using the ROC curve, a one month MPTA of 95 was analyzed as the cut off point for preventing the recurrent varus deformity. With MPTA >= 95 degrees, 92.3% of the osteotomies exhibited stable or varus change and 7.7% exhibited valgus change. However, with MPTA <95 degrees, 47.4% exhibited stable or varus change and 52.6% exhibited valgus change (P < 0.001, odds ratio = 13.3). Conclusion: The postoperative MPTA can be used to predict the change in correction angle and an MPTA of at least 95 degrees is the crucial angle with which to prevent recurrent varus deformity.