Zone 4 femoral radiolucent lines in hybrid versus cemented total knee arthroplasties: are they clinically significant?

UNLABELLED: Aseptic loosening of the femoral component is an infrequent but important cause of failure in total knee arthroplasties. Inadequate support in the posterior femoral condylar region (radiographic Zone 4) has been associated with loosening. This zone also has been implicated as a site for ingress of wear debris particles leading to osteolysis. We determined the prevalence of Zone 4 radiolucent lines using fluoroscopy to obtain true lateral radiographs in a series of patients who had bilateral simultaneous total knee arthroplasties with a cemented femur on one side and a cementless porous ingrowth femur on the other. We hypothesized that cementless fixation would yield fewer Zone 4 radiolucent lines than cemented fixation. At an average 7.6 +/- 4.4 years followup, 11 of 16 patients (69%) with cemented femoral fixation had Zone 4 radiolucent lines, whereas none of the patients with cementless prostheses had radiolucent lines. Three of 11 radiolucent lines (27%) were progressive. Knee Society scores were similar for both groups. No femoral components in either group were clinically loose. When excellent initial stability was obtained, cementless femoral fixation yielded fewer Zone 4 radiolucent lines compared with cemented fixation. LEVEL OF EVIDENCE: Therapeutic study, Level III (retrospective comparative study). See the Guidelines for Authors for a complete description of levels of evidence.