Cementation of a polyethylene liner into a metal shell. Factors related to mechanical stability.
BACKGROUND
Various clinical situations may make polyethylene liner exchange desirable in the setting of a well-fixed metal shell. Options have included a simple polyethylene liner exchange or revision of the entire acetabular shell. A recently introduced technique involves cementation of a new liner into a well-fixed metal shell. The purposes of this study were to quantify the mechanical integrity of this method of liner fixation, to evaluate the factors that may influence immediate liner fixation, and to compare this construct with a standard locking mechanism.


METHODS
One modular acetabular cup design was evaluated. Variables that may contribute to the mechanical integrity of the construct were evaluated via lever-out and multiaxis dislocation testing methods. The variables included the size of the liner (undersized versus oversized), type of liner (all-polyethylene versus modular design), modification of the modular liner (unmodified versus grooved), and groove configuration (cruciform versus circumferential). Metal shells with and without screw-holes were also tested. The modular locking mechanism was used as the control.


RESULTS
None of the constructs failed at the cement-metal interface. All undersized liners required significantly higher loads to failure than either the controls (p < 0.001) or the oversized liners (p < 0.001). Oversized unmodified liners failed at significantly lower loads than the controls did (p < 0.01). The creation of circumferential grooves in the oversized liners significantly improved the strength of the constructs (p < 0.01), making them comparable with the controls. No significant differences were found among the four undersized groups (p > 0.3). The standard locking construct (control) and the oversized unmodified construct failed at a force of 2000 N in the multiaxis dislocation test. No other construct failed with use of this test mode.


CONCLUSIONS
Cementation of a polyethylene liner into a metal shell can be stronger than a conventional locking mechanism if the liner is undersized. Cementation of an oversized liner into a shell should be performed with caution. The long-term durability of this fixation remains unknown.
Bonner
Kevin F
KF
Department of Orthopaedic Surgery, Naval Medical Center Portsmouth, Virginia 23708, USA.
Delanois
Ronald E
RE
Harbach
Greg
G
Bushelow
Michael
M
Mont
Michael A
MA
D000077:Acetabulum / Q000293:injuries* / Q000601:surgery*; D001696:Biomechanical Phenomena; D002484:Cementation; D005592:Fracture Fixation; D050723:Fractures, Bone / Q000601:surgery*; D006801:Humans; D007593:Joint Instability / Q000601:surgery*; D007595:Joint Prosthesis; D008670:Metals / Q000627:therapeutic use*; D020959:Polyethylene / Q000627:therapeutic use*; D011474:Prosthesis Design; D015203:Reproducibility of Results
D008670:Metals; D020959:Polyethylene