gms | German Medical Science

Objectives: Improper implant fit has been found to increase the prevalence of clinically significant knee pain and implant loosening in off-the-shelf (OTS) total knees. Recently, a customized, individually made (CIM) TKA has been introduced that is designed to exactly fit the femoral and tibial components of the patient's native geometry in three dimensions. The purpose of this study was to compare the tibial fit of a CIM TKA to that of OTS TKAs intra-operatively, in the same patient.

Methods: Fourty-four (44) patients undergoing TKA with a CIM system were compared to assess the fit of the tibial tray intra-operatively. After tibial preparation, a series of tibial trials from 3 different OTS-TKA designs were fit to the operative knee. Each trial was optimally sized and positioned based on the surgeon's judgment, while maintaining proper rotational alignment. Implant fit data (overhang and underhang) for the best matched tibial trial of each OTS knee was recorded in four tibial zones (antero-medial, antero-lateral, postero-medial, and postero-lateral) (Figure 1). Once all measurements were complete, the CIM tibial tray was implanted, and all measurements were repeated.

Results: Analysis reveals that surgeons prefer to undersize the tibia to prevent overhang of the cortical bone. In spite of this preference, significant overhang of >3mm of the tibial component in any one zone was seen in 16% for OTS1, and 18% each for OTS 2 and OTS 3 TKAs. None (0%) of the CIM TKA, experienced tibial tray overhang of >1mm. For the four zones analyzed, underhang of the tibial component >3mm was seen in 18% of CIM TKAs, and an average 40% in the 3 OTS groups (39%, 39% and 43% for OTS-1, 2 and 3 respectively). Additionally, there were individual cases among the OTS groups where significant overhang and underhang was seen for the same tibial trial or under-hang was evident in more than one zone. There were no such cases with the CIM tibial trays. In the 3 OTS groups, underhang was most frequently seen in the postero-medial zone, while the antero-lateral zone was the most frequent zone experiencing overhang.

Discussion: Results show that CIM TKA can significantly improve tibial fit in all regions of the resected tibial plateau. This could play an important role in reducing knee pain and patient dissatisfaction, resulting from overhanging components, soft-tissue impingement and implant loosening due to poor tibial bone support and resultant subsidence. We also noted that increased tibial coverage could be attained by upsizing the OTS implants, however this led to a significant internal rotation of components in order to avoid unacceptable overhang, which is a known source of postoperative pain. By providing better coverage on the tibial plateau, a CIM TKA can reduce the instances of the surgeon making compromises on sizing the tibial component in order to achieve correct rotational alignment.