Backside wear of polyethylene tibial inserts: mechanism and magnitude of material loss.
BACKGROUND
Wear of the underside of modular tibial inserts (backside wear) in total knee replacements has been reported by several authors. Although, for some implant designs, this phenomenon seems to contribute to osteolysis, the actual volume of material lost through wear of the backside surface has not been quantified. This study describes the results of computerized measurements of tibial inserts of one design known to be associated with a high prevalence of backside wear in situ.


METHODS
A series of retrieved total knee components of one design were examined. The duration of implantation of the retrieved components ranged from thirty-six to 146 months. Laser surface profilometry and computer-aided design software were used to develop individual three-dimensional models of each worn, retrieved tibial insert to compare with scanned unused inserts. Volumetric subtraction of both models revealed the material lost because of backside wear.


RESULTS
Worn and unworn areas on the backside surface were easily identified by stereomicroscopy and laser profilometry. The computer reconstructions showed that, in all retrievals, all unworn surfaces on the nonarticulating surface lay in one plane. The average volume (and standard deviation) of the material lost because of backside wear was 925 +/- 637 mm(3) (range, 197 to 2720 mm(3)). On the basis of the time in situ for each implant, the average volumetric wear rate was 138 +/- 95 mm(3)/yr.


CONCLUSIONS
The predicted volume of material removed because of backside wear is substantial and may be sufficient to induce osteolysis. Our results suggest that peg-like protrusions are not generated by the extrusion of polyethylene into screw-holes within the base-plate but by abrasion of the underside of the bearing insert, leaving the protruding pegs as the only remnants of the original surface.
Conditt
Michael A
MA
Institute of Orthopedic Research and Education, 6550 Fannin, Suite 2512, Houston, TX 77030, USA. mconditt@bcm.tmc.edu
Thompson
Matthew T
MT
Usrey
Molly M
MM
Ismaily
Sabir K
SK
Noble
Philip C
PC
D000368:Aged; D019645:Arthroplasty, Replacement, Knee; D015992:Body Mass Index; D001835:Body Weight; D019544:Equipment Failure Analysis; D005260:Female; D006801:Humans; D007091:Image Processing, Computer-Assisted; D007720:Knee Prosthesis; D008297:Male; D008875:Middle Aged; D020959:Polyethylene; D011474:Prosthesis Design; D011475:Prosthesis Failure; D012307:Risk Factors; D013499:Surface Properties
D020959:Polyethylene