The unintentional effect of unicompartmental knee arthroplasty on extraarticular deformity and of high tibial osteotomy on intraarticular deformity for the treatment of anteromedial osteoarthritis

Introduction: Unicompartmental Knee Arthroplasty (UKA) and High Tibial Osteotomy (HTO) are two valid options in the treatment of Anteromedial Osteoarthritis (AMOA) of the knee with UKA being mainly performed in cases of Intraarticular deformity (IA) and HTO in cases of Extraarticular deformity (EA). The exact unintentional effect of UKA on EA deformity and HTO on IA deformity is still not well understood. The aim of this study was to assess this unintentional effect of UKA on EA and HTO on IA deformities respectively. Materials and methods: This a single-center retrospective study in which 50 patients who underwent UKA and 50 patients who underwent medial opening wedge HTO (MWOHTO) for the treatment of AMOA were included. Overall, 35 males and 15 females underwent HTO with a mean age of 44.3 +/- 11.2 years while the mean age of the UKA group was 71.8 +/- 7.9 years in 23 males and 27 females. The radiological effect of UKA and HTO on each of the following angles: Hip-Knee-Ankle angle (HKA), Medial Proximal Tibial Angle (MPTA), mechanical Lateral Distal Femoral Angle (mLDFA) and Joint Line Convergence Angle (JLCA) on long film radiographs both pre- and postoperatively was measured. The postoperative values were compared to the preoperative values to detect the expected and unintended effects of each technique on IA and EA deformities. Results: In the HTO group, the MPTA has changed significantly as expected from 83.1 +/- 4.5 preoperatively to 88.9 +/- 3.9 postoperatively (p value < 0.001) correcting the existing preoperative EA varus without overcorrection. Similarly in the UKA group, the JLCA has also changed significantly as expected to correct the IA varus from 3.8 +/- 1.7 preoperatively to 0.9 +/- 1 postoperatively (p value < 0.001). On the other hand, the JLCA was unintentionally changed in the HTO group from 2.6 +/- 2.1 preoperatively to 1.6 +/- 2.4 postoperatively (p value = 0.03) partly correcting the IA varus deformity. Similarly, the MPTA showed a significant change that occurred inadvertently in the UKA group from 84.8 +/- 2.1 to 86.3 +/- 1.6 postoperatively (p value < 0.001). This unintentional increase in the MPTA also partly corrected the preexisting EA varus deformity. The mLDFA did not show a significant change neither in the HTO group (p value = 0.96) nor in the UKA group (p value = 0.94). Conclusion: In addition to intraarticular varus correction, UKA can partly correct the extraarticular varus deformity in AMOA even when resurfacing is exclusively attempted. Additionally, intraarticular deformity can be also partially managed by HTO along with the extraarticular varus correction even without performing overcorrection.