The consequences of anterior femoral notching in total knee arthroplasty. A biomechanical study.

BACKGROUND

Notching of the anterior femoral cortex during total knee arthroplasty has been implicated as a cause of subsequent periprosthetic supracondylar femoral fracture. However, other than observational clinical data, no reliable association between these events has been established, to our knowledge. The purpose of the present study was to investigate the biomechanical effects of notching of the anterior femoral cortex.

METHODS

The femoral component of a total knee replacement was implanted in twelve matched pairs of human cadaveric femora; one specimen in each pair had preservation of the anterior femoral cortex, and the other had a full-thickness cortical defect created just proximal to the anterior flange of the femoral component. The pairs were then subjected to either bending or torsional loading to failure. Both the fracture pattern and the quantitative load to failure were analyzed. Two matched pairs were excluded from the analysis because of inadvertent fracture during placement of the component.

RESULTS

Following the application of a bending load, femora with notching of the anterior femoral cortex sustained a short oblique fracture that originated at the cortical defect proximal to the femoral component and femora without notching had a midshaft fracture. In contrast, notching of the anterior femoral cortex had no effect on the fracture pattern that was observed after the application of a torsional load. The mean load to failure was significantly reduced by notching in both testing modes. Notching decreased bending strength from 11,813 to 9690 newtons (18 percent; p = 0.0034), and it decreased torsional strength from 134.7 to 81.8 newton-meters (39.2 percent; p = 0.01).

CONCLUSIONS

Biomechanical testing demonstrated that notching of the anterior femoral cortex significantly lessens the load to failure following total knee arthroplasty and influences the subsequent fracture pattern. These effects are manifested in different ways under the two loading conditions: the fracture pattern is altered under bending load, and there is a greater quantitative decrease in load to failure with torsional loading.

CLINICAL RELEVANCE

Weakening of the femur by notching of the anterior cortex after total knee arthroplasty may warrant alteration in the customary postoperative regimen for these patients. Manipulation of a total knee replacement with a notched anterior femoral cortex should probably be avoided.