ROTATIONAL ALIGNMENT AND ENTRY POINTS IN FEMORAL IMPLANTATION IN CONVENTIONAL TOTAL KNEE ARTHROPLASTY – A BONE MODEL STUDY

Abstract

Introduction: Correct alignment in both coronal and sagittal planes has been shown to be associated with longevity of total knee arthroplasty. The majority of procedures are performed using an intramedullary rod with a femoral cutting jig, with a 5°–7° offset depending upon the anatomical and mechanical axes. The cutting jig rotates around the rod and therefore the rotational alignment of the jig will also affect the cut and final component position (in addition to the rod entry point). It is interesting that rotational alignment of the femoral component is often assessed after the distal resection has been made. The distal resection plane determines the final position of the femoral component, influences patellar tracking and medial/lateral, flexion/extension balancing. This study measures the resultant effect on the distal femoral resection when entry point and jig rotation are varied.

Materials and Methods: The distal femoral resection was carried out in sawbones with three different entry points (central, inferior and superolateral) in neutral alignment and rotations of 10° (internal and external) about the transepicondylar axis. The resulting plane of the cut was assessed by a graphical method measuring the changes in orientation of the alignment rod in space before and after the distal cut. A computer navigation system was used to measure the varus/valgus and flexion/extension angles of the distal cut. This experiment was done thrice, in a total of 27 sawbones and the average values were recorded.

Results: The results varied considerably in the sagittal plane with central and inferior entry points. Internal rotation of the jig around a central entry point produced hyperextension (mean 3.3°) and external rotation caused flexion (mean 1.8°). Using an inferior entry point, flexion of the distal plane improved from an average 3° in neutral rotation to 1.6° on internal rotation; external rotation worsened flexion to an average of 4.3°. The angles digressed in both sagittal and coronal planes with a superolateral entry point; rotations of the distal cutting jig caused hyperextension (maximum of 7.5°). Coronal alignment ranged from 4.5° of varus to 5° of valgus in neutral alignment and rotations around a superolateral entry point.

Conclusion: The study demonstrates that there is a possibility of a compound error from misplaced entry point and that malrotation prior to distal resection is real. This error would invariably be extrapolated in the subsequent steps of conventional knee arthroplasty. Computer assisted arthroplasty may have a role in avoiding this surgical error.