Alignment changes after open-wedge high tibial osteotomy result in offloading in the patellofemoral joint: a SPECT/CT analysis

Purpose The patellofemoral (PF) joint may be adversely affected by medial open-wedge high tibial osteotomy (OWHTO). This study aimed to evaluate the PF compartmental changes using combined single-photon emission computed tomography (SPECT) and conventional computed tomography (CT) after OWHTO to provide clinical guidance regarding the PF joint pressure and force. Methods Patients with medial osteoarthritis and varus malalignment > 5 degrees were treated using OWHTO. Patients with a minimum 2-year follow-up were included in the study. The patellar positions were evaluated based on the radiographic parameters. The changes in chondral lesions during second-look arthroscopic examination were evaluated, and the PF joint arthritis grade was recorded on patellar Merchant radiographs using Kellgren-Lawrence classification. The PF compartmental changes according to SPECT/CT analysis after OWHTO were evaluated in all patients. The scintigraphic uptake was graded on four scales. Patients were divided into improved and unimproved groups according to the PF compartmental grade using the SPECT/CT uptake grading system. Results At a mean follow-up period of 47.0 months (range 25-74 months), the mean mechanical femorotibial angle changed significantly from varus 6.3 degrees (range 5-12 degrees) to valgus 2.6 degrees (range 0-8 degrees);p < 0.001) postoperatively. The radiological parameters presenting patellar positions, including the tibial slope, patellar convergence angle, and lateral tilt angle, did not change significantly between the preoperative values and the 2-year follow-up values. The mean patellar height significantly decreased (0.07 +/- 0.14,p = 0.001 according to the Blackburn-Peel index and 0.32 +/- 0.23,p < 0.001 using the modified Insall-Salvati ratio). The average tibial tubercle to trochlear groove (TT-TG) distance significantly decreased from 14.1 to 12.2 mm (p < 0.001). The Q angle also significantly decreased from 9.8(o)to 7.7(o)(p = 0.008). Chondral lesions of the patella and trochlear groove revealed significant deterioration; at 2 years after OWHTO, the arthritic grades of the PF joints worsened significantly, as determined by radiography (p = 0.007). Scintigraphic uptake in the PF joint was significantly lower (from 2 to 1) at 2 years postoperatively compared to that immediately after the index operation (p < 0.001). Only 4 of 56 (7.1%) patients showed increased uptake. Comparison between the improved and unimproved groups according to scintigraphic uptake changes revealed that the changes in the cartilage status on the patellar undersurface and TT-TG distance were the most significant predictive factors of increased scintigraphic uptake in the PF joint after OWHTO. Conclusion Alignment correction by OWHTO result in PF compartment offloading and should be considered when identifying the surgical indications for OWHTO.