Influence of extension stem length and diameter on clinical and radiographic outcomes of revision total knee arthroplasty

Abstract

Background: Extension stems provide stability to revision total knee arthroplasty (RTKA). Little is known regarding the relationship between stem characteristics and RTKA stability. We aimed to identify the relationship between canal filling ratio (CFR) and aseptic loosening following RTKA.

Methods: We retrospectively reviewed demographics, radiographic parameters, and outcomes associated with RTKA performed between 2008 and 2013 in a tertiary hospital. The inclusion criteria were: revision for aseptic loosening, hybrid fixation, minor bone defect, Zimmer® LCCK prosthesis, and follow-up > 24 months. Using the modified Knee Society radiographic scoring system, radiographic prosthesis loosening was defined as a radiolucent line (RLL) score ≥ 9 on the femoral side or ≥ 10 on the tibial side. We utilized receiver operating characteristic (ROC) curve analysis to evaluate the cutoff value for stem length and diameter in terms of prosthesis loosening or not. Furthermore, CFR-related parameters were analyzed with logistic regression to clarify their relationships with prosthesis loosening.

Results: Prosthesis loosening was detected in 17 of 65 patients included. On logistic regression analysis, male sex and severity of the tibial bone defect were associated with loosening. On multivariate analysis, male sex and bone defect severity were associated with loosening on the femoral side, while malalignment was associated with loosening on the tibial side. Protective factors included femoral CFR > 0.85, CFR > 0.7 for > 2 cm, and CFR > 0.7 for > 4 cm, as well as tibial CFR > 0.85.

Conclusions: To minimize loosening post-RTKA, femoral CFR > 0.7 for > 2 cm and tibial CFR > 0.85 are recommended. Risk factors may include male sex, bone defect severity, and malalignment.

Keywords: Aseptic loosening; Canal filling ratio; Extension stem; Revision total knee arthroplasty.

Fig. 1

Inclusion criteria. Patients receiving RTKA with Zimmer® LCCK during 2008–2013 in our hospital were included. Revisions for periprosthetic joint infection, severe bone defects, full cementation technique, and follow-up < 2 years were excluded f/u: follow-up period. Aseptic: revision TKA due to aseptic causes.

Fig. 2

Receiver operating curve analysis for predicting prosthesis loosening. Top, femoral component. Optimal results for CFR_max was 0.72 (area under the curve [AUC] = 0.77), and for CFR_0.7 length was 2 cm (AUC = 0.83). Bottom, tibial component. Optimal results for CFR_max was 0.83 (AUC = 0.66), and for CFR_0.7 length was 4.2 cm (AUC = 0.74). CFR (canal filling ratio) was calculated as the ratio between the stem width and the medullary canal width. CFR_x length indicates the stem length for which CFR is larger than x. CFR_max indicates the maximum CFR over the entire length of the stem

Fig. 3

Example of CFR parameter evaluation. Left, the stem diameter is 1.04 cm, whereas the narrowest part of medullary canal which is occupied by the stem is 1.42 cm. CFR_max = 1.04/1.42 = 0.73. CFR > 0.85: negative. Right, for the calculation of CFR_0.7 length, divide stem diameter (1.04 cm) by 0.7 equals to 1.49 cm. Measure the length from stem tip to the level where canal diameter is 1.49 cm. In this case, it is 1.15 cm. As a result, CFR_0.7 > 2 cm: negative, CFR_0.7 > 4 cm: negative, CFR_0.85length: 0 cm, CFR_0.7 length: 1.49 cm

Fig. 4

Kaplan-Meier survival curves describing survival free from prosthesis loosening. Left, femoral component. CFR_0.7 > 2 cm, CFR_0.7 > 4 cm, and CFR_0.85 were all associated with less loosening. Right, tibial component. Only CFR_0.85 was associated with less loosening CFR (canal filling ratio) was calculated as the ratio between the stem width and the medullary canal width. CFR_x > y indicates stem length > y for which CFR is larger than x.