Aims

We wanted to evaluate the effects of a bone anabolic agent (bone morphogenetic protein 2 (BMP-2)) on an anti-catabolic background (systemic or local zoledronate) on fixation of allografted revision implants.

Introduction

Hip and knee arthroplasty present surgeons with difficult bone loss. In these cases the use of morselized allograft is a well established way of optimizing early implant fixation. In revisions, the surgical field is potentially infected. The use of allograft bone creates a “dead space” in which the immune system has impaired access, and even a small amount of bacteria may therefore theoretically increase the risk of infection.

In vivo studies have shown that allograft bone is suitable as a vehicle of local antibiotic delivery.

We hypothesized that the allograft bone could be used as a local antibiotic delivery vehicle without impairing the implant fixation, tested by mechanical push-out.

Introduction: Revision hip implants have poorer clinical outcome than primary implants. The fixation of the implants is often compromised by the formation of an endosteal sclerotic bone rim during the process of aseptic loosening. The cracking procedure is a bone sparing, low energy surgical technique which produces a controlled local perforation of the sclerotic bone rim. In previous studies, we showed that fixation of revision implants significantly improved by the cracking technique for both titanium (Ti) and hydroxyapatite (HA) coated implants (1). In this study we compared the cracking technique with the common technique of reaming, which completely removes the sclerotic bone rim.

Methods: A paired animal study (n=10), in which revision cavities was created by 20 micromotion implant systems inserted in both knees. Micromotion was 0.5mm per gait cycle. After 8 weeks revision surgery was performed.

Crack revision: The splined crack tool was introduced over the implant piston with firm axial hammer blows. This producing controlled cracking and local perforation of the sclerotic endosteal rim. The tool is a 6.0 mm cylinder fitted with axially spaced 1.1 mm pointed splines (8.2 mm outer diameter).

Reaming revision: A flat bottomed reamer was inserted over the implant piston using one rotation per second. The outer diameter was 8.2 mm in order to remove the sclerotic bone rim. Stable revision Ti implants was inserted. Observation period was 4 weeks. Mechanical push-out tests were performed. Students’ paired t-test was used. Data presented as mean and SEM.

Results: Shear strength was markedly higher with the cracking procedure 1.33± 0.3MPa vs. 0.34 ± 0.2 MPa (p< 0.05). Similar results was seen for Stiffness 6.7± 2.0 vs. 1.6 ± 0.9 (p< 0.05) in favor of the cracking procedure. A non-significant increase was seen in energy absorption 170± 47 vs. 50± 29 (p=0.07).

Discussion: The cracking procedure improves the mechanical fixation of Ti revision implants compared to a reaming procedure. Shear strength and stiffness was consistently higher for all implant pairs. We have previously shown that the sclerotic bone rim is a barrier for bone ingrowth and that implants inserted with an intact sclerotic bone rim will have a poor biomechanical fixation. Additionally, revision implants inserted with the cracking technique obtained a mechanical fixation comparable to primary implants. Reaming procedures are often used in hip revision surgery. However, as loss of bone stock is a common feature of revision cavities, the reaming procedure may not always be an optimal preparation method of the bone. Excessive removal of bone by reaming may compromise the long term implant stability or increase the risk of peri-implant fractures. The cracking technique may be an alternative or supplemental procedure to reaming.

Introduction: Bone grafts should be biocompatible, mechanically stable and be replaced with new bone over time. BMP’s are known to increase bone formation around allografted implants, but have also been associated with increased graft resorption and implant instability. Bone resorption can be inhibited by bisphosphonates.

We hypothesized that topical bisphosphonate (Pamidronate, Mayne Pharma) in combination with rhBMP2 (InductOs, Wyeth) would give increased mechanical implant fixation and increased new bone formation without excessive allograft resorption. We looked at both porous-coated Ti implants and HA-coated implants.

Methods: Four 2.5 mm gap implants were inserted into the proximal humeri of each of 16 dogs. The gap around each implant was filled with fresh frozen impacted allograft with or without intervention treatment. Half the dogs received Ti-implants, the other half HA-implants. The 4 treatment groups were:

allograft alone (control)

The observation time was 4 weeks.

Results: For both the Ti and HA subgroup, the control-group had significantly better mechanical fixation than all other groups by push-out test. The fixation was twofold higher in the control group than the rhBMP2-group and more than 20-fold higher than the pamidronate group and combined group. The HA implants were twice as well fixed as the Ti implants with corresponding treatment.

The HA implants had less fibrous tissue and more new bone compared to the Ti implants. The fractions of allograft were the same.

The rhBMP2 group had more new bone and much less fibrous tissue than the mechanically superior control group. However, there was almost no allograft left in the rhBMP2 group due to extreme resorption.

The addition of pamidronate seemed to freeze bone metabolism around the implants. Neither in the pamidronate group nor in the combination group was there anything but minor new bone growth. The allograft was preserved. In the pamidronate group there was a dense, thick fibrous capsule around the implants. This was not the case in the combined rhBMP2-pamidronate group, and is most likely a positive effect of the rhBMP2.

Discussion: Topical pamidronate and rhBMP2 in combination and alone greatly weakened the mechanical fixation of the implants. The experiment confirms previous reports of mechanical instability of implants when BMPs are added to periimplanteric defects. Pamidronate alone had catastrophic effects on bone metabolism and implant fixation in this experiment.

The negative results with rhBMP2 may be due to over dosage, which warrants further preclinical testing. Despite the limitations of this animal study with non-loaded implants, the results encourage extreme caution in adjuvant therapies of arthroplastic surgery.