Abstract
Background
Although the wear of conventional polyethylene liner becomes a serious problem in a long term follow up after total knee arthroplasty, there are few reports of measuring the polyethylene wear.
Questions/purposes
Is it possible to measure the linear wear rates in the non-cross-linked polyethylene liner used in the Press Fit Condylar (PFC) Sigma total knee system? Does the polyethylene wear influence on the clinical results?
Patients and Methods
The senior author and his colleagues performed total knee arthroplasties using the PFC knee system between 1997 and 2009. A follow-up was performed at minimum of 6 years after surgery, and 113 knee replacements (Female: 59 including 93 knee replacements, Male: 16 including 20 knee replacements) were clinically assessed and examined using X-ray imaging. An X-ray table was useful to make the X-ray accurately enter the tibial component at a parallel angle for anteroposterior image of radiograph when measuring the wear on knee prosthesis (Fig.1). The average follow-up period was 11.5 ± 3.4 years (range: 6.0–17.0) postoperatively.
Results
There was no loosening found in the knee prostheses. However, revision surgery was performed for a case with consistent varus instability. The wear rate in the polyethylene liner was an average of 0.08 ± 0.07 (range, 0.01–0.26) mm/year on the medial joint and 0.04 ± 0.03 (range, 0.0–0.11) mm/year on the lateral joint with significant difference. The X-ray imaging was useful to measure the wear accurately.
Conclusion
The medium-to-long term clinical results of the PFC Sigma Design total knee system with minor changes added have been good thus far for almost 20 years. The wear rate in the non-cross-linked polyethylene liner was very low. In the future, the extra long-term results will need to be researched further.
<Figure Legend>
Fig. 1 Measurement of polyethylene wear of the prosthetic knee joint. All measurements on the radiographs of the two-dimensional position of the polyethylene liner were performed by a single researcher with use of a computerized measurement system which had a computerized digitizer with an accuracy of ± 0.01 mm. Only the anteroposterior radiographs of the knee joint were used for the measurements. The thickness of polyethylene liner was measured in both the lateral (a) and medial (b) parts of the liner in each radiograph with correction of magnification by known tibial component medial-lateral diameter (c). The polyethylene wear rate was determined by comparing the thickness in the follow-up radiograph with the thickness in the initial three-week postoperative radiograph at the same locations. The thickness of the polyethylene was measured at its shortest distance in the same flexion angle of the knee joints.
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