Zoledronic acid

The effect of zoledronic acid on bone ingrowth was examined in an animal model in which porous tantalum implants were placed bilaterally within the ulnae of seven dogs. Zoledronic acid in saline was administered via a single post-operative intravenous injection at a dose of 0.1 mg/kg. The ulnae were harvested six weeks after surgery. Undecalcified transverse histological sections of the implant-bone interfaces were imaged with backscattered scanning electron microscopy and the percentage of available pore space that was filled with new bone was calculated. The mean extent of bone ingrowth was 6.6% for the control implants and 12.2% for the zoledronic acid-treated implants, an absolute difference of 5.6% (95% confidence interval, 1.2 to 10.1) and a relative difference of 85% which was statistically significant. Individual islands of new bone formation within the implant pores were similar in number in both groups but were 69% larger in the zoledronic acid-treated group. The bisphosphonate zoledronic acid should be further investigated for use in accelerating or enhancing the biological fixation of implants to bone.

Porous materials have proven to be a very effective method for attaching prosthetic implants to the bony skeleton. The incidence of radiographie signs of bone ingrowth in total hip arthroplasty (THA) is reportedly 95% in uncomplicated primary hips, regardless of the type of porous coating or implant design.1-3 This is confirmed in studies of retrieved THAs.4,5 There remains a need to develop techniques which can enhance biological fixation. The more rapid the bone ingrowth, the faster the implant becomes protected against the disruptive forces of load bearing. This is particularly important in situations where bone healing is compromised or initial implant stability is more tenuous. Tissue ingrowth may protect the bone-implant interface against wear particle-induced osteolysis.6

Various methods have been investigated to improve bone ingrowth into porous implants, with varying degrees of success. These include the use of autograft and allograft,7,8 demineralised bone matrix,9,10 fibrin glue,11 calcium phosphate granules,12 collagen,13 periosteal activation agent,14 tricalcium phosphate coating,15,16 hydroxyapatite coating,17 prostaglandin F^sub 2^α,18 TGF-β^sub 2^,19,20 electrical stimulation,15,21 ultrasound stimulation22,23 and morphogenetic proteins.24-27

Bisphosphonates have been shown to increase the cellular response in both mature and healing bone.28,29 This has led to the use of oral bisphosphonate therapy to mitigate the osteolytic effects of wear debris.30,31 Bisphosphonates have been used to reduce periprosthetic bone loss through stress shielding mechanisms.32-34 In experimental studies, Little et al35 have shown that a single post-operative dose of pamidronate decreased the disuse osteopenia normally associated with bone lengthening and increased the amount and density of the regenerated bone. In a further study,36 they showed that one or two doses of the more potent zoledronic acid abolished osteopenia and increased the quality of the regenerate.

We hypothesised that bisphosphonates could have a positive effect on bone ingrowth into porous implants. The purpose of this study was therefore to investigate the potential of zoledronic acid to enhance bone ingrowth in a canine model using porous intramedullary implants.

Materials and Methods

Surgical implantation. Porous implants were inserted into the intramedullary canal of the ulna of skeletally mature, mongrel dogs weighing between 25 and 35 kg.22 The implants were cylindrical (50-mm long, 5-mm diameter) and fabricated from a porous tantalum biomaterial (Implex Corp., Allendale, New Jersey). The implants had a mean pore size of 430 μm (95% confidence interval (CI), 413 to 447) and a volume porosity of approximately 75% (Fig. 1). The porous tanatalum material has been studied in various animal models and used clinically.37,38

The surgical procedure was performed under general anaesthetic in sterile conditions. A 2-cm incision was made over the olecranon process and the triceps tendon was split by sharp dissection down to bone. Under fluoroscopic guidance, a 5-mm drill was orientated along the long axis of the ulna in line with the intramedullary canal for 5.5 cm. The porous implant was then tapped down the intramedullary canal with a punch (Fig. 2). The implant was countersunk to avoid irritation of the triceps tendon. The wound was irrigated and closed in a standard fashion. The procedure was repeated on the contralateral side. The positioning of the implants varied in depth of insertion within the canal and in orientation. This, together with differences in ulnar size, resulted in variability of the relationship of different parts of an implant to endosteal cortical bone.

Immediately after surgery seven test dogs with 14 ulnar implants were given a single intravenous dose of 0.1 mg/kg zoledronic acid (Novartis Pharma AG, Basel, Switzerland). Because of the systemic exposure of the test dogs to zoledronic acid it was necessary to use control data from an earlier experiment in which zoledronic acid was not used.22 Previous canine ingrowth studies have demonstrated that bone ingrowth into porous-coated implants is maximal at six to eight weeks.7,22,37 As the purpose of our study was to determine the effect of zoledronic acid on the early progression of bone ingrowth into intramedullary porous implants, the treatment period chosen was six weeks.

Histological examination. The bones were harvested, stripped of soft tissue, radiographed and processed for undecalcified hard-section histology. This involved dehydration in ascending solutions of ethanol, defatting in ether and acetone, and embedding in methylmethacrylate. Each implant was sectioned transversely into five sections at 1 cm intervals (Fig. 3). The sections were radiographed, polished, sputter-coated with gold-palladium and imaged with backscattered scanning electron microscopy. Bone that was observed around the implants, but not within the pores, was not quantified because of the uncertainty of the difference between new and pre-existing bone. For each section, computerised image analysis, based on gray level discrimination, was used to identify islands of bone within the implant pores and to generate quantitative information on the extent of bone ingrowth, defined as the percentage of the available porosity that was filled with new bone. Also tabulated was the area of each bone island within the implant pores in each histological section as well as the total number of bone islands. This enabled a calculation of the mean bone island size and number of bone islands. The quantitative analysis was performed by an independent observer who was blind to the implant groups. The computer-aided method for quantifying new bone formation was determined to have an intra-observer repeatability of ± 2%.

Statistical analysis. The quantitative histological data from the six control and 14 test implants were statistically analysed using a two-level hierarchical model. At the first level of the model, the set of results from the limbs of each dog was assumed to follow a normal distribution with dogspecific means and a global variance parameter. At the second level of the model, the means from each dog in each group from the first level followed a second normal distribution, with the mean representing the overall mean for the treatment or control groups and the variance representing the variability within the group.

A similar statistical model was also run where the results for each dog were allowed to vary with the distance along the limb. As these results were virtually identical to those from the model without this extra variable, only results from the simple hierarchical model are presented. The mean values for overall extent of bone ingrowth, number of bone islands within the implant pores and bone island size were analysed similarly with 95% CIs.

Results

A total of 28 histological sections from the external control implants and 67 sections from the zoledronic acid-treated implants (two control and three zoledronic acid-treated sections were lost due to preparation error) were examined. It was common to observe varying degrees of new bone formation within the intramedullary canal around the implants, probably resulting from the stimulus caused by reaming and implantation (Fig. 4). This new bone tended to be more dense where the implant was closer to endosteal bone and less dense in metaphyseal sections. New bone formation within the pores of the tantalum implants was observed in all sections to varying degrees (Fig. 4). There was a general tendency for more bone ingrowth at the implant periphery than in the centre. Small islands of bone were observed throughout many of the implant cross-sections.

The quantitative histological data are listed in Tables I and II. The mean extent of bone ingrowth for the six external control implants was 6.6% (95% CI, 3.2 to 10.0) while the mean extent of bone ingrowth for the 14 zoledronic acid-treated implants was 12.2% (95% CI, 9.2 to 15.2). The 5.6% difference of the means was significant (95% CI, 1.2 to 10.1). In relative terms, there was a mean of 85% more bone growth into the zoledronic acid-treated implants compared with controls.

The mean number of bone islands within the implant pores was 117 for the control (95% CI, 94 to 142) and 118 for the zoledronic acid-treated group (95% CI, 98 to 138). The size of the bone islands differed significantly between the groups, with a mean of 0.010 mm2 (95% CI, 0.009 to 0.012) for the control implants and a mean of 0.017 mm^sup 2^ (95% CI, 0.015 to 0.019) for the zoledronic acid-treated implants. The difference of the means was 0.007 mm^sup 2^, a relative difference of 69% that was statistically significant (95% CI, 0.004 to 0.010).

Discussion

A simple, controlled canine implant model was used to assess the effect of administering intravenous zoledronic acid on bone growth into porous intramedullary implants. The model had clinical relevance in that it resembled the period of early post-operative non-loading which is commonly recommended after uncemented arthroplasty. The dogs treated with a single post-operative dose of zoledronic acid showed almost twice as much net bone ingrowth six weeks after surgery compared with external controls. This is sufficiently encouraging to warrant further studies to determine the zoledronic acid dose response at different time periods and perhaps with different porous materials in both unloaded and loaded models.

Many different methods have been investigated for their potential to augment the rate and extent of bone ingrowth into porous implants. One of the more notable effects was reported by Tanzer et al22 using the identical model in which porous tantalum implants were subjected to daily 20-minute treatments of non-invasive low intensity ultrasound for six weeks. A mean increase in bone ingrowth of 119% was obtained with ultrasound treatment compared with controls. Sumner et al26 recently showed, in a gap model using porous-coated rods, that local delivery of recombinant human bone morphogenetic protein-2 (rhBMP-2) to the implant site enhanced bone ingrowth by a factor of 3.5 compared with controls four weeks after surgery. Substantial increases in bone ingrowth have also been reported by Bragdon et al24 and Barrack et al25 using recombinant morphogenetic proteins in canine acetabular gap models using porous-coated cups.

The increased potency of zoledronic acid compared with other bisphosphonates makes it a logical choice to enhance net bone ingrowth. The quantitative data on the bone islands that formed within the implant pores revealed that the mean number of sites of new bone formation within both implant groups was essentially the same but that the size of each site was greater with exposure to zoledronic acid. This finding is consistent with the documented suppression of osteoclastic remodelling with bisphosphonate therapy39 and the study of Smith et al40 suggesting that osteoblastic activity is not increased in the presence of bisphosphonates.

The marked enhancement of bone ingrowth afforded by this simple therapy could have very important benefits in any clinical application in which implants require mechanical attachment to the skeleton. The positive effects of bisphosphonates will have to be compared with those recently described for bone morphogenetic proteins.24-26 Bone morphogenetic proteins, which increase bone turnover, are unlikely to offer protection in the longer term against stress-shielding osteopenia or osteolysis, whereas potent bisphosphonate therapy could do so. In a recent study of impaction arthroplasty in a sheep model, Howie et al41 noted rapid resorption of bone graft and one stem subsidence after administration of OP-1 (bone morphogenetic proteins). Although the power of that study was insufficient to make strong conclusions, it highlights the theoretical risk of approaches which increase bone turnover in the presence of stress shielding. The relative merits of increasing or decreasing bone turnover in these situations remain unclear as both approaches can result in increases in bone mass.

The authors are grateful for the manufacture and donation of the implants by Implex Corp (Allendale, New Jersey), donation of zoledronic acid by Novartis Pharma AG (Basel, Switzerland), histological analysis by Dorota Karabasz, statistical analysis by Dr. Lawrence Joseph, and financial support from the Canadian Institutes for Health Research.

No other benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article.

References

1. Engh CA, Claus AM, Hopper RH, Engh CA. Long-term results using the anatomic medullary locking hip prosthesis. Clin Orthop2001;393:137-46.

2. Teloken MA, Bissett G, Hozack WJ, Sharkey PF, Rothman RH. Ten to fifteen-year follow-up after total hip arthroplasty with a tapered cobalt-chromium femoral component (tri-lock) inserted without cement. J Bone Joint Surg [Am] 2002;84-A: 2140-4.

3. D'Antonio JA, Capello WN, Manley MT, Geesink R. Hydroxyapatite femoral stems for total hip arthroplasty: 10- to 13-year followup. Clin Orthop 2001;393:101-11.

4. Pidhorz LE, Urban RM, Jacobs JJ, Sumner DR, Galante JO. A quantitative study of bone and soft tissues in cementless porous-coated acetabular components retrieved at autopsy. J Arthroplasty 1993;8:213-25.

5. Sychterz CJ, Claus AM, Engh CA. What we have learned about long-term cementless fixation from autopsy retrievals. Clin Orthop 2002;405:79-91.

6. Bobyn JD, Jacobs JJ, Tanzer et al. The susceptibility of smooth implant surfaces to peri-implant fibrosis and migration of polyethylene wear debris. Clin Orthop 1995; 311:21-39.

7. Kienapfel H, Sumner DR, Turner TM, Urban RM, Galante JO. Efficacy of autograft and freeze-dried allograft to enhance fixation of porous coated implants in the presence of interface gaps. J Orthop Res 1992;10:423-33.

8. McDonald DJ, Fitzgerald RH Jr, Chao EYS. The enchancement of fixation of a porous-coated femoral component by autograft and allograft in the dog. J Bone Joint Surg [Am] 1988;70-A:728-37.

9. McLaughlin RE, Reger SI, Bolander M, Eschenroeder HC. Enhancement of bone ingrowth by the use of bone matrix as a biologic cement. Clin Orthop 1984;183:255-61.

10. Cook SD, Salkeld SL, Patron LP, Barrack RL. The effect of demineralized bone matrix gel on bone ingrowth and fixation of porous implants. J Arthroplasty 2002;17: 402-8.

11. Kienapfel H, Sumner DR, Turner TM, et al. Efficacy of autograft, freeze dried allograft and fibrin glue to enhance fixation of porous-coated implants in the presence of interface gaps [abstract]. Trans Orthop Res Soc 1990;15:432.

12. Russotti GM, Okada Y, Fitzgerald RH Jr, Chao EYS, Gorski JP. Efficacy of using a bone graft substitute to enhance biological fixation of a porous metal femoral component. In: Brand RA, ed. The Hip. Proc of 15th open science meeting of Hip Society, St Louis: Mosby, 1987:120-54.

13. Longo JA, Weinstein AM, Medley AK. The effects of collagen on tissue growth into a porous polyethylene ingrowth model In: Christel R Meunier A, Lee AJC, eds. Biological and biomechanical performances of biomaterials. Amsterdam: Elsevier Science Publishers B.V., 1986:483-8.

14. Alberts LR. Effects of periosteal activation agent on bone repair and ingrowth. J Biomed Mater Res 1987;21:429-42.

15. Berry JL, Geiger JM, Moran JM, Skraba JS, Greenwald AS. Use of tricalcium phosphate or electrical stimulation to enhance the bone-porous implant interface. J Biomed Mater Res 1986;20:65-77.

16. Cook SD, Thomas KA, Kay JF, Jarcho M. Hydroxyapatite-coated porous titanium for use as an orthopedic biologic attachment system. Clin Orthop 1988;230:303-12.

17. Rivero DP, Fox J, Skipor AK, Urban RM, Galante JO. Calcium phosphate-coated titanium implants for enhanced skeletal fixation. J Biomed Mater Res 1988;22:191-201.

18. Trancik T, Vinson N. The effect of prostaglandin F^sub 2^ alpha on bone ingrowth into a porous-coated implant [abstract]. Trans Orthop Res Soc 1990;15:167.

19. Sumner DR, Turner TM, Purchio AF, et al. Enhancement of bone ingrowth by transforming growth factor beta. J Bone Joint Surg [Am] 1995;77-A:1135-47.

20. Sumner DR, Turner TM, Urban RM, et al. Locally delivered rh TGF-beta2 enhances bone ingrowth and bone regeneration at local and remote sites of skeletal injury. J Orthop Res 2001;19:85-94.

21. Rivero DP, Landon GC, Skipor AK, Urban RM, Galante JO. Effect of pulsing electromagnetic fields on bone ingrowth in a porous material [abstract]. Trans Orthop Res Soc 1986;11:492.

22. Tanzer M, Kantor S, Bobyn JD. Enhancement of bone growth into porous intramedullary implants using non-invasive low intensity ultrasound. J Orthop Res 2001;19: 195-9.

23. Tanzer M, Harvey E, Kay A, Morton P, Bobyn JD. Effect of noninvasive low intensity ultrasound on bone growth into porous-coated implants. J Orthop Res 1996;14: 901-6.

24. Bragdon CR, Doherty AM, Rubash HE, et al. Efficacy of MBP-2 to induce bone ingrowth in gap and non-gap regions of a THR model. Clin Orthop 2003;417:50-61.

25. Barrack RL, Cook SD, Patron LP, et al. Induction of bone ingrowth from an acetabular defect to a porous surface with osteogenic protein-1. Clin Orthop 2003;417:41-9.

26. Sumner DR, Turner TM, Urban RM, et al. Locally delivered rhBMP-2 enhances bone ingrowth and gap healing in a canine model. J Orthop Res 2004;22:58-65.

27. Cole BJ, Bostrom MPG, Pritchard TL, et al. Use of bone morphogenetic protein 2 on ectopic porous coated implants in the rat. Clin Orthop 1997;345:219-28.

28. Green JR, Müller K, Jaeggi KA. Preclinical pharmacology of CGP42'446, a new, potent, heterocyclic bisphosphonate compound. J Bone Miner Res 1994;9:745-51.

29. Pataki A, Müller K, Green JR, et al. Effects of short-term treatment with the bisphosphonates zoledronate and pamidronate on rat bone: a comparative histomorphometric study on the cancellous bone formed before, during, and after treatment. Anat Rec 1997;249:458-68.

30. Shanbhag AS, Hasselman CT, Rubash HE. Inhibition of wear debris mediated osteolysis in a canine total hip arthroplasty model. Clin Orthop 1997;344:33-43.

31. Shanbhag AS, May D, Cha C, Kovach C, Hasselman CT, Rubash HE. Enhancing net bone formation in canine total hip components with bisphosphonates [abstract]. Trans Orthop Hes Soc 1999;24:255.

32. Soininvaara TA, Jurvelin JS, Miettinen HJA, et al. Effect of alendronate on periprosthetic bone loss after total knee arthroplasty: a one-year, randomized, controlled trial of 19 patients. Calcif Tissue Int 2002;71:472-7.

33. Venesmaa PK, Kroger JP, Miettinen HJ, et al. Alendronate reduces periprosthetic bone loss after uncemented primary total hip arthroplasty: a prospective randomized study. J Bone Miner Res 2001;16:2126-31.

34. Wilkinson JM, Stockley I, Peel NF et al. Effect of pamidronate in preventing local bone loss after total hip arthroplasty: a randomized, double-blind, controlled trial. J Bone Miner Res 2001;16:556-64.

35. Little DG, Cornell MS, Briody J, et al. Intravenous pamidronate reduces osteoporosis and improves formation of the regenerate during distraction osteogenesis: a study in immature rabbits. J Bone Joint Surg [Br] 2001;83-B:1069-74.

36. Little DG, Smith NC, Williams P, et al. Zoledronic acid prevents osteopenia and increases bone strength in a rabbit model of distraction osteogenesis. J Bone Miner Res 2003;18:1300-7.

37. Bobyn JD, Stackpool GJ, Hacking SA, Tanzer M, Krygier JJ. Bone ingrowth characteristics and interface mechanics of a new porous tantalum biomaterial. J Bone Joint Surg [Br] 1999:81-B:907-14.

38. Bobyn JD, Tok K-K, Hacking SA, Tanzer M, Krygier JJ. The tissue response to porous tantalum acetabular cups: a canine model. J Arthroplasty 1999;14:347-54.

39. Day J, Ding M, Bednarz P, et al. Bisphosphonates affect the apparent modulus of trabecular bone through architecture and not mineralization [abstract]. Trans Orthop Res Soc 2002;27:85.

40. Smith EJ, Bugler J, Peat RA, et al. Zoledronic acid modulates bone repair through transiently delayed remodelling [abstract]. Trans Orthop Res Soc 2003;28:351.

41. Howie DW, McGee MA, Findlay DM, et al. Use of OP-1 in femoral impaction grafting in a sheep hemiarthroplasty model [abstract]. Trans Orthop Res Soc 2003;28: 32.

J. D. Bobyn,

S. A. Hacking,

J. J. Krygier,

E. J. Harvey,

D. G. Little,

M. Tanzer

From McGill University, Montreal, Canada and The Children's Hospital at Westmead, Westmead, Sydney, Australia

* J. D. Bobyn, PhD, Associate Professor Departments of Surgery and Biomedical Engineering

* E. J. Harvey, MD, Assistant Professor

Division of Orthopaedics, Department of Surgery

* M. Tanzer, MD, Associate Professor

* S. A. Hacking, MEng, Graduate Student Department of Biomedical Engineering

McGill University, Montreal, Quebec, Canada.

* J. J. Krygier, CET, Technical Director

Jo Miller Orthopaedic Research Lab, Montreal General Hospital, Montreal, Quebec, Canada.

* D. G. Little, MD Orthopaedic Research Unit, The Children's Hospital at Westmead, Westmead, New South Wales, Australia.

Correspondence should be sent to Dr J. D. Bobyn at Montreal General Hospital Research Institute, Room LS1-409, 1650 Cedar Avenue, Montreal, Quebec H3G 1A4, Canada; e-mail: jdbobyn@hotmail.com

©2005 British Editorial Society of Bone and Joint Surgery

doi:10.1302/0301-620X.87B3. 14665 $2.00

J Bone Joint Surg [Br] 2005;87-B:416-20.

Received 2 June 2003; Accepted after revision 26 May 2004

Copyright British Editorial Society of Bone & Joint Surgery Mar 2005
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