Abstract
Introduction
Although, the total knee arthroplasty (TKA) procedure is performed to make the same extension gap (EG) and flexion gap (FG) of the knee, it is not clear how the gaps can be created equally. According to earlier reports, the gaps after bone resection (bone gaps) differ from the gaps after the trial component of the femur is set (component gaps), because of the thickness of the posterior condyle of the femoral component and the tension of the posterior capsule. The surgeon can only check the component gaps after completing the bone resection and setting the trial component and it difficult to adjust the gaps even when the acquired component gaps are inadequate. To resolve this problem, we developed a “pre-cut trial component” for use in a pre-cut technique for the femoral posterior condyle (Fig. 1). This specially made trial component allows us to check the component gaps before the final bone resection of the femur.
Materials and methods
The pre-cut trial component is composed of an 8-mm-thick usual distal part and a 4-mm-thick posterior part of the femoral component, and lacks an anterior part of the femoral component. With this pre-cut trail component, 152 knees were investigated. The EG was made by standard resection of distal femur and proximal tibia. The FG was made by a 4 mm pre-cut from the posterior condylar line of the femoral posterior condyle (Fig. 2). The rotation of the pre-cut line is initially decided by anatomical landmarks. Once all of the osteophytes are removed and the bone gaps are checked, the pre-cut trial component is attached to the femur and the component gaps are estimated with the patella reduction (Fig. 3). In our experiments, these gaps were the same as the component gaps after the usual trial component was set via the measured resection technique. Finally, the femur is completely resected according to the measurements of the component gaps with the pre-cut trial component.
Results
The bone gaps were 18.4±2.4 (mean ± standard deviation) mm in extension and 16.5±2.7 mm in flexion. From these results, the expected component gaps were 10.8±2.7 (bone gap −8) mm in extension and 12.5±2.7 (bone gap −4) mm in flexion. After the pre-cut trial component was set, the measured component gaps were 9.4±2.8 mm in extension and 12.5±2.8 mm in flexion. The EG became 1.5±1.0 mm smaller than expected, and the change of FG was 0.2±0.5 mm. While no large decrease of EG was noted, the variation was not insubstantial (0∼5 mm).
Conclusion
The difference between the bone gap and component gap is very important for an adequate EG and FG in the TKA procedure. Yet with the conventional technique, the component gap is impossible to estimate before the final bone resection. If unacceptable results are discovered after the component gaps are estimated, the gaps are difficult to correct. With the technique we present here, the component gaps can be checked before final bone resection and truly precise gap control can be attained.
