[Analysis of the tibial baseplate coverage and symmetrical tibial prosthesis in primary total knee arthroplasty]

Objective: To investigate the tibial baseplate coverage with symmnetrical tibial prosthesis in primary total knee arthroplasty (TKA). Methods: From August 2014 to February 2015, 39 patients (39 knees) with varus knee osteoarthritis were retrospective reviewed in Department of Orthopaedics, Tianjin Xiqing Hospital. Nine males and 30 females; aged 50 to 78 years (67±6 year). 3D tibia models of the knee were built with Mimics 10.01 software, simulated TKA surgeries were performed. The morphology of the proximal tibia was measured, including the mediolateral (ML), anteroposterior (AP) and medial and lateral tibial plateau AP dimensions. 3D models of PFC Sigma tibial prosthesis were imported and the tibial coverage characters were analyzed. Results: The AP and ML dimensions of the tibial resection surface were (45.7± 3.4) mm, (71.7± 4.2) mm respectively, the plateau aspect ration ratio (AP/ML) was 63.8%±3.0%. The medial tibial plateau AP[(46.9± 3.3) mm]was significantly greater than the lateral side[(41.8±3.3) mm, P=0.000], the asymmetric ratio averaged 112.4%±6.9%. The medial tibial plateau, anteromedial, posteromedial, posterolateral unsatisfied coverage ratio were 33.3%, 76.9%, 20.5%, 33.3% respectively. There are 5 cases underhang and 3 cases overhang in the posteromedial side, whereas 1 case underhang and 12 cases overhang posterolateral, the overhang ratio was 30.8%. Posteromedial, posterolateral prosthesis coverage had linear positive correlation to the ipsilateral tibial plateau AP diameter. Conclusions: The medial AP diameter of tibial plateau is significantly lager than the lateral side in varus knee osteoarthritis patients. With symmetrical tibial baseplate placement in TKA, the prosthesis usually lead to medial tibial plateau anterior and posterior underhang and posterolateral overhang.