The combined effect of parathyroid hormone and bone graft on implant fixation

Abstract

Impaction allograft is an established method of securing initial stability of an implant in arthroplasty. Subsequent bone integration can be prolonged, and the volume of allograft may not be maintained. Intermittent administration of parathyroid hormone has an anabolic effect on bone and may therefore improve integration of an implant. Using a canine implant model we tested the hypothesis that administration of parathyroid hormone may improve osseointegration of implants surrounded by bone graft. In 20 dogs a cylindrical porous-coated titanium alloy implant was inserted into normal cancellous bone in the proximal humerus and surrounded by a circumferential gap of 2.5 mm. Morsellised allograft was impacted around the implant. Half of the animals were given daily injections of human parathyroid hormone (1-34) 5 μg/kg for four weeks and half received control injections. The two groups were compared by mechanical testing and histomorphometry. We observed a significant increase in new bone formation within the bone graft in the parathyroid hormone group. There were no significant differences in the volume of allograft, bone-implant contact or in the mechanical parameters. These findings suggest that parathyroid hormone improves new bone formation in impacted morsellised allograft around an implant and retains the graft volume without significant resorption. Fixation of the implant was neither improved nor compromised at the final follow-up of four weeks.

Fig. 1

The implant lies in cancellous bone in the proximal humerus. The radiograph was taken at the time of bone harvest after four weeks treatment with parathyroid hormone or a control. The implant measures 10 mm × 6 mm and is surrounded by a 2.5 mm gap impacted with morsellised allograft bone secured by an 11 mm washer.

Fig. 2

Drawing showing the technique of sectioning. Implant in-situ in humerus metaphyseal bone. An outer section of 3.5 mm is taken for mechanical testing. The inner section of 6.5 mm is for histomorphometry with four serial sections around the centre of the implant after random rotation around its axis.

Fig. 3

Histomorphometry – The region of interest (ROI) is defined on both sides of the implant. The ROIs are illustrated on one side of the implant above. Tissue ongrowth (surface fraction) at the surface of the implant, the tissue volume (volume fraction) in the grafted gap of 2.5 mm (divided into an outer region (1500 μm) and inner region reaching the surface of the implant) and tissue volume in region of intact non-implanted bone (1000 μm) are defined. The regions were defined from the end-washer margin as a fixed point with a 100 μm clearance at the gap-intact-bone-interface and 500 μm below the washer. ROI were defined at magnification × 1.25. Total assessment magnification × 30.3. For histomorphometric assessment, the corresponding magnifications were × 20 and × 402.

Fig. 4

Drawing showing mechanical testing. An axial push-out test is performed with the specimen placed on a metal platform with a central opening. The specimen thickness = 3.5 mm, the implant diameter = 6 mm, the support hole diameter = 7.4 mm and the preload = 2 N. Displacement velocity is 5 mm/min. The load (Pa) displacement (mm) curve enables calculation of the ultimate shear strength (MPa), apparent shear stiffness (MPa/mm) and the total energy absorption (J/m²).

Fig. 5

Photomicrographs of representative histological samples showing a-b) control and c-d) parathyroid hormone (PTH). Left images a-c) bar 2000 μm and right images b-d) bar 500 μm. Staining technique 0.4% basic fuchsin (red) and 2% light green (green = bone) and black = implant. PTH shows increase in bone in the morsellised impacted gap with numerous connective trabeculae of wo ven bone and elements of bone allograft. In the control group bone graft appears with sparse new bone.

Fig. 6

Mechanical testing - Box plots showing results from mechanical testing push-out-to-failure. Maximum shear stiffness, maximum shear strength, and total energy absorption. n(control) = 9, n(PTH) = 8. Mann-Whitney test, p < 0.05 PTH compared to control (PTH, parathyroid hormone).