Abstract
Introduction
There is no criteria to select cruciate retaining (CR) or posterior substitute (PS) component in total knee arthroplasty (TKA). In this study, extension and flexion gaps were measured intra-operatively with posterior cruciate ligament (PCL) remained to reveal characteristics of the gaps. Component type selection, CR or PS, was decided intra-operatively according to the gaps in each knee.
Materials and methods
One hundred and sixty knees with osteoarthritis were investigated. Extension gap (EG) was made by resection of 8 mm distal femur and 10 mm proximal tibia. After measurement of femoral AP size, about 4 mm bigger 4-in-1 femoral cutting guide than measured size was used for pre-cut of femoral posterior condyle[Figure 1]. With this technique, flexion gap (FG) was made 4 mm smaller than usual measured resection. The gaps were measured by a tension device with 30 pounds tension and FG was corrected by the amount of pre-cut. According to the EG and corrected FG, component type was decided. Too small FG usually needed PCL resection or (and) smaller size of femoral component to make enough final FG. On the other hand, large FG needed careful consideration to sacrifice PCL for adequate final FG. In these cases, CR component was selected usually. If necessary, soft tissue was released for good ligament balance. As the final step of the surgical procedure, the size of femoral component was decided for adequate final FG. It was changeable up to 4 mm larger than measured size[Figure 2].
Results
After pre-cut of femoral posterior condyle and correction of FG by the amount of pre-cut, the range of the gaps were 10∼31 mm (average 20.6±3.7) in flexion and 8∼29 mm (average 17.5±3.4) in extension. There were wide variations in both gaps. The range of the difference between corrected FG and EG was –4∼12 mm (average 3.1±3.3) and corrected FG was significantly larger than EG (P<0.001). Since PCL resection makes FG wider than EG, selection of PS implant would result in much larger final FG in many cases. Of course, larger size of femoral component was available to make FG smaller, but there was limitation. These cases were not suitable for PS component. On the other hand, there were some cases with smaller FG than EG. To select CR component in these cases, it was necessary to use smaller size of femoral component for enough final FG. It led to smaller posterior condylar offset and posterior flexion space. These cases were not suitable for CR component. Considering adequate size of femoral component, CR was used in 122 knees and PS in only 38 knees from the gaps.
Conclusion
Because of wide variations in EG and FG, it is difficult to use only one component, CR or PS, in every cases. Larger femoral component with PS or smaller component with CR than measured size is possible to use, but there is limitation. Considering adequate size of femoral component and the final gaps, selection of component type should be decided by intra-operative gap measurement in each knee.
