The Journal of Bone and Joint Surgery

Image Quiz

A Complication of Modular Revision Total Knee Arthroplasty¹
(continued)

Dx: Disengagement of the locking screw from the femoral component. Note the subtle silhouette of the locking screw in the intercondylar area (arrow). Although subsidence of the femoral component was recognized, the loose locking screw had not been appreciated initially by the orthopaedic surgeon or by the radiologist.

Discussion

Metallic wear, fretting, and corrosion between the body of the prosthesis and the various modular components used in total knee arthroplasty require the surgeon to understand the potential long-term problems and to proceed with caution. In addition, loosening of screws used to secure blocks, wedges, or the intramedullary stems can be a problem. Dissociation of modular components after implantation is quite rare; however, surgeons must be aware of this potential complication and be prepared to correct it. Disengagement of the locking screw from the femoral component and the intramedullary stem may be caused by motion after implantation. Subsidence or micromotion of the stem may cause the Morse taper and the locking screw to become loose over time.

Alternatively, a mechanism of failure may be improper preassembly of the modular components before implantation. If the stem is not impacted into the Morse taper of the femoral housing with enough force at the time of assembly, the screw will not be fully seated when tightened. Any subsequent impaction of the stem may allow loosening of the screw. Thus, if during implantation the femoral component and stem assembly is impacted into the femur with enough force to seat the intramedullary stem farther within the Morse taper, the locking screw may begin to loosen. The construct then may be predisposed to accelerated loosening as the result of cyclical loading and micromotion of the prosthesis. Eventually, the screw will back out, as seen in this patient.

Such disengagement of the screw may be prevented by a repeat verification that the screw is tightly and properly seated after implantation of the femoral component and stem assembly. This mechanism of failure may also be due to failure of proper assembly of the modular components by the operative team.

Reference

1. Westrich GH, Hidaka C, Windsor RE. Disengagement of a locking screw from a modular stem in revision total knee arthroplasty. A report of three cases. J Bone Joint Surg Am. 1997;79:254-8.