The change of the posterior slope angle of proximal tibia following high tibial osteotomy

Objective High tibial osteotomy was an alternative in the management of knee osteoarthritis, it could release the knee pain, correct the mechanical axis of the affected limb, and delay the total knee replacement. We were aimed at measuring the change of posterior slope angle of tibial medial plateau after high tibial osteotomy and probe its clinical consequences. Methods From 1998 to 2001, there were fifty-eight knees of 38 patients with medial unicompartmental osteoarthritis treated with closing-wedge high tibial osteotomy. The present included 8 male and 30 female patients with the average age of 55.2 years ranging 41 to 65 years. The osteotomy line was 2 cm distal to articular surfaces, and the tibia was fixed with Giebel bladed plate and two oblique long screws. The posterior slope angles of tibia and Insall-Salvati index were measured both pre and postoperatively on lateral radiographs. The posterior slope angle of proximal tibia in this study was defined as the angle between vertical line to tibial axis and the surface parallel to subchondral bone. Insall-Salvati index was defined as the ratio of the length of patellar tendon to the largest vertical diameter of patella. The alteration of the above features was calculated and analyzed statistically with paired t test. Results The mean tibial posterior slope angle was 9.4°±3.0° preoperatively and 5.6°±2.6°postoperatively. The tibial posterior slope angle was averagely decreased about 3.8°±2.0°, Insall-Salvati index was 1.05±0.16 and 0.94±0.18 before and after high tibial osteotomy respectively. This index was averagely decreased about 0.15±0.10. Statistical analysis indicated that there was significant difference of posterior slope angle or Insall-Salvati index between pre and post operative(P 0.01). There were 7 cases of patella baja detected postoperatively, whereas none was found preoperatively. Conclusion The loss of tibial posterior slope angle after high tibial osteotomy may lead to such complications as patellar baja. Decreased tibial posterior slope angle itself and its related complications may exert negative effect on the results of total knee arthroplasty after high tibial osteotomy. Thorough understanding of the causes and consequences of decreased tibial posterior slope angle helps refining the techniques of high tibial osteotomy and preoperative planning of total knee arthroplasty if it is needed.