Does tobramycin impregnation of allograft bone affect implant fixation? - an experimental study in 12 dogs

Abstract

Clinical trials have used antibiotic impregnated impacted bone allograft in revisions of infected arthroplasties. By this method high local antibiotic concentration and good control of infection was achieved. Toxicity studies, however, suggest that high local antibiotic concentration can impair osteoblast replication. We therefore asked whether impregnating morselized allograft bone with different quantities of tobramycin before impaction would impair implant fixation. We implanted three cylindrical (10 mm × 6 mm) porous-coated titanium implants into the distal femurs of 12 dogs. The implants were surrounded by a circumferential gap of 2.5 mm into which a standardized volume of morselized allograft bone, with or without tobramycin, was impacted. In each animal, the bone graft was impregnated with either 0 mg (control), 50 mg (low dose), or 200 mg (high dose) of tobramycin per 1 mL of bone graft. At the end of the 4 weeks experimental period, the implants with surrounding bone were evaluated by histomorphometric analysis and mechanical push-out test. We found no difference between the treatment groups regarding new bone formation, bone graft resorption, or implant fixation. There was, however, a tendency toward a decrease in implant fixation with higher tobramycin dose. The present study is unable to provide evidence on whether the use of topical tobramycin with allograft is safe or whether it indeed can impair implant fixation. The tendency toward an impaired implant fixation warrants further preclinical studies. Its current clinical use should be weighed against its possible positive effects on preventing infection in complicated revisions.

Keywords: implant fixation; impregnation; morselized allograft bone; osseointegration; tobramycin.

FIGURE 1.

(A) The location of the gap implants in the distal femur is shown in a post-euthanasia radiograph. (B) The experimental porouscoated, titanium implant with the dimensions is shown. The last end-screw is mounted after impaction grafting of the gap. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

FIGURE 2.

(A) The implant is inserted and the peri-implant gap of 2.5 mm is empty. (B) The gap is impacted with morselized allograft bone with or without tobramycin. (C) The gap is closed with a top screw.

FIGURE 3.

On the left, a specimen is placed on a metal platform with central opening. The specimen thickness is 3.5 mm, the implant diameter is 6 mm, and the support hole is 7.4 mm (preload 2N, displacement velocity is 5 mm/minute). On the right, the load (N) displacement (mm) curve enables the calculation of ultimate shear strength (MPa), apparent shear stiffness (MPa/mm), and total energy absorption (KJ/m²).

FIGURE 4.

Histology: Representative histologic sections of the control group (A: Allograft) and intervention groups (B: Allograft + low dose tobramycin; C: Allograft + high dose tobramycin) form the same animal. Control: A-1: Overview of implant and bone graft; A-2: Thin fibrous tissue membrane in contact with implant; A-3: Allograft bone chip with ongrowth of new bone. Low dose: B-1: Overview of implant and bone graft; B-2: Allograft bone chip with ongrowth of new bone in contact with implant; B-3: Fibrous tissue membrane in contact with implant. High dose: C-1: Overview of implant and bone graft; C-2: Thin fibrous tissue membrane in contact with implant; C-3: Allograft bone chip with ongrowth of new bone. Ipl = Implant, Nb = New bone, Ab = Allograft bone, Ms = Marrow space, Fib = Fibrous tissue. (Stain, toluidine blue; magnification, ×28 (left), ×230 (right). [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]