Lysosomal Enzyme Production at the Interface Surrounding Loose and Well-fixed Cemented Tibial Hemiarthroplasties in the Rabbit Knee

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Abstract
Fourteen mature New Zealand white female rabbits had a right, cemented, tibial hemiarthroplasty using a stemmed, fluted, titanium alloy, condylar-type prosthesis. In one group (seven rabbits), polymethyl methacrylate (PMMA) was used to cement the prosthesis firmly. In a second group (seven rabbits), the prosthesis was treated with cement ex vivo; the prosthesis and cured cement were then implanted, and rotated once within the bone to ensure that the prosthesis was loose fitting. Roentgenograms performed postoperatively and at 3 months were graded for new (i.e., not present on the immediate postoperative radiograph) radiolucent lines. At 3 months, the tissue adjacent to the implant was harvested sterilely and cultured over a 3-day period; the tissues and culture supernatants were then assayed for total protein, DNA content, and lysosomal enzyme activity (N-acetyl-β-D-glucosaminidase and β-glucuronidase). The mean cumulative grading of new lucent lines was 0.4 ± 0.3 (mean ± standard error) for the well-fixed prosthetic group and 2.0 ± 0.6 for the loose prosthetic group. The tissue surrounding loose prostheses contained more DNA and total protein, and produced greater amounts of lysosomal enzymes compared to well-fixed prostheses. The control left sides were not statistically different for any parameter analyzed. The increased DNA content demonstrates an increase in cellularity of the tissue surrounding loose prostheses. Normalization of the relative amount of enzyme released as a function of cellularity (DNA) suggests that the influx of cells into the area surrounding loose prostheses may be more important to the overall increase in lysosomal enzyme release than increased production of lysosomal enzymes by individual cells.