Osteogenic protein 1 device stimulates bone healing to hydroxyapaptite-coated and titanium implants

This study evaluated the effects of osteogenic protein 1 (OP-1) placed in a gap around uncoated and hydroxyapatite (HA)-coated implants. Unloaded cylindrical titanium alloy implants were inserted in the femoral condyles of 16 skeletally mature dogs surrounded by a 3-mm gap. The gap around the implants was filled with 325 microg OP-1 in 130 mg bovine collagen type I as carrier (OP-1 device) or collagen carrier alone or left empty. All groups were tested with both HA-coated and uncoated implants, and the animals were sacrificed after 6 weeks. Implant fixation was determined by push-out test. Bone ongrowth and bone formation were evaluated by quantitative histomorphometry. OP-1 device enhanced mechanical fixation of uncoated and HA-coated implants, resulting in a higher shear strength than implants with collagen matrix and control implants. Bone ingrowth and bone formation in the gap were also stimulated 3-fold for OP-1 groups when compared with empty controls, but no difference was found between OP-1 groups and collagen matrix groups. This study suggests that the OP-1 device placed in a gap around uncoated and HA-coated implants is capable of enhancing mechanical fixation and periimplant bone formation. The collagen carrier alone also enhanced bone ongrowth and fixation significantly.     

Local alendronate increases fixation of implants inserted with bone compaction: 12-week canine study.

Bone compaction has been shown to increase initial implant fixation. Furthermore, bone compaction creates a peri-implant zone of autograft that exerts osteoconductive properties. We have previously shown that locally applied bisphosphonate (alendronate) at 4-week observation can preserve the autograft generated by bone compaction. We now investigate whether the increased amount of autograft, seen at 4 weeks, can increase implant osseointegration and biomechanical fixation. Porous-coated titanium implants were bilaterally inserted with bone compaction into the proximal part of tibia of 10 dogs. On the right side, local bisphosphonate was injected into the bone cavity prior to bone compaction immediately prior to implant insertion. On the left side, saline was used as control. Observation period was 12 weeks. Locally applied bisphosphonate significantly increased biomechanical implant fixation (approximately twofold), bone-to-implant contact (1.2-fold), and peri-implant bone volume fraction (2.3-fold). This study indicates that local alendronate treatment can increase early implant osseointegration and biomechanical fixation of implants inserted by use of bone compaction. Long term effects remain unknown.     

Hydroxyapatite coating enhances fixation of porous coated implants: A comparison in dogs between press fit and noninterference fit

Intimate contact at the bone-porous surface interface is not always achievable in noncemented prosthetic implantation. We investigated the effect of hydroxyapatite (HA) coating on skeletal attachment in non-interference fit 4 weeks after implantation in 6 mature dogs. The push-out test of HA-coated implants surrounded by a 1-mm gap showed a twofold increased shear strength and fivefold increased shear stiffness compared with titanium alloy (Ti) coated implants. The fixation of Ti implants was reduced by two thirds when inserted in a gap as compared with press fit, whereas HA-coated implants in gap showed anchorage close to implants in press fit. Only minor differences were found between HA and Ti implants in press fit. Histomorphometric analysis showed a significant increase in bone in direct contact to HA-coated implant as compared with Ti implants inserted both in gap and press fit. The study indicates that tightness of surgical fit is an important factor for sufficient fixation of the implant. However, our results demonstrate that hydroxyapatite coating almost eliminates the negative influence of noninterference fit between bone and unloaded implant.     

Hydroxyapatite coating enhances fixation of porous coated implants. A comparison in dogs between press fit and noninterference fit

Intimate contact at the bone-porous surface interface is not always achievable in noncemented prosthetic implantation. We investigated the effect of hydroxyapatite (HA) coating on skeletal attachment in noninterference fit 4 weeks after implantation in 6 mature dogs. The push-out test of HA-coated implants surrounded by a 1-mm gap showed a twofold increased shear strength and fivefold increased shear stiffness compared with titanium alloy (Ti) coated implants. The fixation of Ti implants was reduced by two thirds when inserted in a gap as compared with press fit, whereas HA-coated implants in gap showed anchorage close to implants in press fit. Only minor differences were found between HA and Ti implants in press fit. Histomorphometric analysis showed a significant increase in bone in direct contact to HA-coated implant as compared with Ti implants inserted both in gap and press fit. The study indicates that tightness of surgical fit is an important factor for sufficient fixation of the implant. However, our results demonstrate that hydroxyapatite coating almost eliminates the negative influence of noninterference fit between bone and unloaded implant.     

Transforming growth factor-6 stimulates bone ongrowth: Hydroxyapatite-coated implants studied in dogs

Unloaded cylindrical grit-blasted titanium (Ti-6A-4V) implants (6×10 mm) coated with hydroxyapatite ceramic were inserted into the proximal part of the humerus of 20 skeletally mature Labrador dogs. The implants were initially surrounded by a 2 mm gap. In 10 dogs, HA-coated implants without growth factor were inserted in one humerus and implants with 0.3 ug rh.TGF-61 adsorbed onto the HA coating were inserted in the contralateral humerus. In another group of 10 dogs, a dose of 3.0 mg rhTGF-β1 was tested in a similar design. All dogs were killed at 6 weeks after treatment. Results were evaluated by histomorphometry and mechanical push-out testing. Bone ongrowth was increased by one third, using the 0.3 mg rhTQF-β1 stimulation. Bone volume in the gap and mechanical testing showed no statistically significant differences between control and rhTGF-β1 stimulated implants. RhTGF-β1 only moderately enhanced bone ongrowth to hydroxyapatite-coated implants.     

The bovine bone protein lyophilisate Colloss improves fixation of allografted implants--an experimental study in dogs

BACKGROUND: Impacted morselized bone allograft is a well-established way of giving joint arthroplasties additional support in situations where there is insufficient bone stock. For long-term survival of the implant, early implant fixation is important. We hypothesized that Col-loss, a bone protein lyophilisate, might improve early implant fixation of allografted implants. METHOD: We inserted 4 porous-coated Ti implants in the distal femurs of 16 dogs. All implants were surrounded by a 2.5-mm gap, which was impacted with morselized allograft with or without Colloss. In each dog, the implants were treated with no Collos or low-, middle- or high-dose (0, 10, 20 and 40 mg) Colloss per cm3 allograft. The observation time was 4 weeks. RESULTS: Mechanical implant fixation was improved for all 3 groups with Colloss-treated implants (p < 0.05). The best anchorage was seen in the middle-dose group, where fixation was improved by 100%. We saw a dramatic reduction in fibrous tissue on the surface of the Colloss-treated implants (p < 0.001). The Colloss groups showed increased ongrowth of new bone (p < 0.01) and accelerated gap remodeling (p < 0.05). INTERPRETATION: Colloss can improve early osseointegration and fixation of allografted implants.     

Superior sealing effect of hydroxyapatite in porous-coated implants: experimental studies on the migration of polyethylene particles around stable and unstable implants in dogs

Background Migration of wear debris to the periprosthetic bone is a major cause of osteolysis and implant failure. Both closed-pore porous coatings and hydroxyapatite (HA) coatings have been claimed to prevent the migration of wear debris. We investigated whether HA could augment the sealing effect of a porous coating under both stable and unstable conditions.

Methods We inserted porous-surfaced knee implants, with and without HA coating, in 16 dogs, according to a paired, randomized study design. 8 dogs had 2 implants inserted into each knee using a stable implant device and 8 dogs received 1 implant in each knee using a micro-motion (500 μm) implant device. Implants had a periimplant gap of 0.75 mm. We then injected polyethylene (PE) particles or a control solution into the knee joints on a weekly basis.

Results After 16 weeks, the rating of particles around stable implants was reduced by the HA coating from a median value of 2 (1-4) to 1 (0-1) (p = 0.01) and during micromotion from 3 (2-4) to 1 (0-3) (p = 0.002). HA-coated implants had superior bone ongrowth during stable and unstable conditions. We found no difference in bone ongrowth between PE-exposed and vehicle-exposed implants.

Interpretation Compared to a pure plasma-sprayed porous coating, a layer of HA coating provides better bone ongrowth and protects the bone-implant interface against the migration of wear debris under both stable and unstable conditions.     

Superior sealing effect of hydroxyapatite in porous-coated implants

Background Migration of wear debris to the periprosthetic bone is a major cause of osteolysis and implant failure. Both closed-pore porous coatings and hydroxyapatite (HA) coatings have been claimed to prevent the migration of wear debris. We investigated whether HA could augment the sealing effect of a porous coating under both stable and unstable conditions.

Methods We inserted porous-surfaced knee implants, with and without HA coating, in 16 dogs, according to a paired, randomized study design. 8 dogs had 2 implants inserted into each knee using a stable implant device and 8 dogs received 1 implant in each knee using a micro-motion (500 μm) implant device. Implants had a periimplant gap of 0.75 mm. We then injected polyethylene (PE) particles or a control solution into the knee joints on a weekly basis.

Results After 16 weeks, the rating of particles around stable implants was reduced by the HA coating from a median value of 2 (1–4) to 1 (0–1) (p = 0.01) and during micromotion from 3 (2–4) to 1 (0–3) (p = 0.002). HA-coated implants had superior bone ongrowth during stable and unstable conditions. We found no difference in bone ongrowth between PE-exposed and vehicle-exposed implants.

Interpretation Compared to a pure plasma-sprayed porous coating, a layer of HA coating provides better bone ongrowth and protects the bone-implant interface against the migration of wear debris under both stable and unstable conditions.     

The bovine bone protein lyophilisate Colloss improves fixation of allografted implants—an experimental study in dogs

Background?Impacted morselized bone allograft is a well-established way of giving joint arthroplasties additional support in situations where there is insufficient bone stock. For long-term survival of the implant, early implant fixation is important. We hypothesized that Col-loss, a bone protein lyophilisate, might improve early implant fixation of allografted implants.

Method?We inserted 4 porous-coated Ti implants in the distal femurs of 16 dogs. All implants were surrounded by a 2.5-mm gap, which was impacted with morselized allograft with or without Colloss. In each dog, the implants were treated with no Collos or low-, middle- or high-dose (0, 10, 20 and 40 mg) Colloss per cm³ allograft. The observation time was 4 weeks.

Results?Mechanical implant fixation was improved for all 3 groups with Colloss-treated implants (p < 0.05). The best anchorage was seen in the middle-dose group, where fixation was improved by 100%. We saw a dramatic reduction in fibrous tissue on the surface of the Colloss-treated implants (p < 0.001). The Colloss groups showed increased ongrowth of new bone (p < 0.01) and accelerated gap remodeling (p < 0.05).

Interpretation?Colloss can improve early osseointegration and fixation of allografted implants.     

Osteogenic protein 1 device increases bone formation and bone graft resorption around cementless implants

In each femoral condyle of 8 Labrador dogs, a non weight-bearing hydroxyapatite-coated implant was inserted surrounded by a 3 mm gap. Each gap was filled with bone allograft or ProOsteon with or without OP-1 delivered in a bovine collagen type I carrier (OP-1 device). 300 microg OP-1 was used in the 0.75 cc gap surrounding the implant. After 3 weeks, the OP-1 device enhanced implant fixation by 800% (p <0.05) in the ProOsteon group, but OP-1 bad no significant effect on implant fixation in the allograft group. By adding the OP-1 device, the volume fraction of woven bone close to the implant increased from 12% to 20% (p < 0.05) in the bone allograft group and from 6% to 25% (p < 0.05) in the ProOsteon group. The volume fraction of bone allograft decreased from 29% to 9% (p < 0.05) in the OP-1 treated group versus 33% to 30% in the allograft group not treated with OP-1. No resorption of ProOsteen was found. In conclusion, OP-1 accelerates resorption of bone allograft and enhances new bone formation around cementless implants grafted with bone allograft or semisynthetic hydroxyapatite bone substitute. Our findings do not support the use of ProOsteon alone around cementless implants.     

Current management

Background and purpose Allografts are often used during revision hip replacement surgery for stabilization of the implant. Resorption of the allograft may exceed new bone formation, and instability of the prosthesis can develop. We investigated whether strontium could regulate the imbalance of fast resorption of allograft and slower formation of new bone, because it is both an anabolic and an anticatabolic agent.

Method Strontium was added to the implant interface environment by doping a hydroxyapatite bone graft extender. 10 dogs each received 2 experimental titanium implants. The implants were inserted within a 2.7-mm concentric gap in cancellous bone. The gap was filled with 50% (v/v) allograft mixed with 50% bone graft extender. The extender either had 5% strontium doping (SrHA) or was undoped (HA). After 4 weeks, osseointegration and mechanical fixation were evaluated by histomorphometry and by push-out test.

Results SrHA bone graft extender induced a 1.2-fold increase in volume of new bone, a 1.2-fold increase in allograft remaining in the gap, and a 1.4-fold increase in surface area of the bone graft extender material in contact with new bone compared to HA bone graft extender. All these increases were statistically significant.

SrHA bone graft extender did not significantly improve ongrowth of bone onto the implants or improve any of the mechanical push-out parameters compared to HA bone graft extender.

Interpretation Doping of the HA bone graft extender with 5% strontium increased gap healing, preserved more of the allograft in the gap, and increased the ongrowth of bone onto the bone graft extender material, but did not improve mechanical fixation.     

Platelet rich plasma and fresh frozen bone allograft as enhancement of implant fixation. An experimental study in dogs.

Platelet rich plasma (PRP) is an autologous source of growth factors. By application of PRP around cementless implants alone or in combination with bone allograft chips, early implant fixation and gap healing could be improved. We inserted two porous HA coated titanium implants extraarticularly in each proximal humerus of eight dogs. Each implant was surrounded by a 2.5 mm gap. Four treatments were block randomized to the four gaps in each dog: Treatment 1: empty gap, treatment 2: PRP, treatment 3: fresh frozen bone allograft, treatment 4: fresh frozen bone allograft+PRP. PRP was prepared from each dog prior to operation by isolating the buffycoat from centrifuged blood samples. Platelet count in PRP was increased 670% compared to baseline level. Calcium/thrombin was added to degranulate platelets and form a gel. Three weeks after surgery, push-out test and histomorphometri was performed. After three weeks, the non-allografted implants had poor mechanical properties. Bone grafting significantly increased implant fixation, bone formation in the gap and bone growth on the implant surface. We found no significant effect of PRP alone or mixed with bone allograft on implant fixation or bone formation. In conclusion, we showed the importance of bone allografting on early implant fixation and bone incorporation but we found no effect of PRP. More studies are needed to investigate the effect and possible clinical applications of platelet concentrates which are now being commercialised.     

Effect of local TGF-��1 and IGF-1 release on implant fixation: comparison with hydroxyapatite coating: A paired study in dogs

Background and purpose Hydroxyapatite (HA) coating stimulates the osseointegration of cementless orthopedic implants. Recently, locally released osteogenic growth factors have also been shown experimentally to stimulate osseointegration so that bone fills gaps around orthopedic implants. Here, we have compared the effect of local release of TGF-β 1 and IGF-1 with that of hydroxyapatite coating on implant fixation.Method Weight-bearing implants with a 0.75-mm surrounding gap were inserted bilaterally in the knees of 10 dogs. Growth factors were incorporated in a biodegradable poly(D,L-lactide) coating on porous coated titanium implants. Plasma-sprayed HA implants served as controls. The dogs were killed at 4 weeks and the implants were evaluated by mechanical push-out test and by histomorphometry.Results There was no difference in any of the mechanical parameters. Bone ongrowth was 3-fold higher for HA-coated implants (p < 0.001). For growth factor-coated implants, bone volume was 26% higher in the inner half of the gap and 28% higher in the outer half compared to HA (p < 0.03).Interpretation- The mechanical fixation of porous-coated titanium implants with local growth factor release is comparable to that of HA coating. While HA mainly stimulated bone ongrowth, local release of TGFβ 1 and IGF-1 stimulated gap healing.     

Local bisphosphonate treatment increases fixation of hydroxyapatite‐coated implants inserted with bone compaction

It has been shown that fixation of primary cementless joint replacement can independently be enhanced by either: (1) use of hydroxyapatite (HA) coated implants, (2) compaction of the peri‐implant bone, or (3) local application of bisphosphonate. We investigated whether the combined effect of HA coating and bone compaction can be further enhanced with the use of local bisphosphonate treatment. HA‐coated implants were bilaterally inserted into the proximal tibiae of 10 dogs. On one side local bisphosphonate was applied prior to bone compaction. Saline was used as control on the contralateral side. Implants were evaluated with histomorphometry and biomechanical push‐out test. We found that bisphosphonate increased the peri‐implant bone volume fraction (1.3‐fold), maximum shear strength (2.1‐fold), and maximum shear stiffness (2.7‐fold). No significant difference was found in bone‐to‐implant contact or total energy absorption. This study indicates that local alendronate treatment can further improve the fixation of porous‐coated implants that have also undergone HA‐surface coating and peri‐implant bone compaction. © 2008 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 27:189–194, 2009     

Bone compaction enhances fixation of weight-bearing hydroxyapatite-coated implants

The effect of bone compaction vs conventional drilling on the fixation of hydroxyapatite-coated implants was examined in a weight-bearing canine model. In each dog, one knee joint had the implant cavity prepared with drilling, the other with compaction. Eight dogs were euthanized after 2 weeks and 8 dogs after 4 weeks. Femoral condyles from additional 7 dogs represented time 0. Compacted specimens had significantly higher bone implant contact and energy absorption at time 0. Compaction significantly increased ultimate shear strength at 0 and 2 weeks. There was no significant difference in implant fixation after 4 weeks. The results of this study suggest that compaction may be beneficial in optimizing the crucial initial implant stability, even when hydroxyapatite-coated implants with osteoconductive properties are inserted in vivo.     

Bone compaction enhances fixation of weightbearing titanium implants

Implant stability is crucial for implant survival. A new surgical technique, compaction, has increased in vitro implant stability and in vivo fixation of nonweightbearing implants. However, the in vivo effects of compaction on weightbearing implants are unknown. As implants inserted clinically are weightbearing, the effects of compaction on weightbearing implants were examined. The hypothesis was that compaction would increase implant fixation compared with conventional drilling. Porous-coated titanium implants were inserted bilaterally into the weightbearing portion of the femoral condyles of dogs. In each dog, one knee had the implant cavity prepared with drilling, and the other knee was prepared with compaction. Eight dogs were euthanized after 2 weeks, and eight dogs were euthanized after 4 weeks. Femoral condyles from an additional eight dogs represented Time 0. Compacted specimens had higher bone-implant contact and periimplant bone density at 0 and 2 weeks, but not at 4 weeks. A biphasic response of compaction was found with a pushout test, as compaction increased ultimate shear strength and energy absorption at 0 and 4 weeks, but not at 2 weeks. This biphasic response indicates that compaction enhances implant fixation by mechanical and biological mechanisms. Therefore, compaction might have potential value in total joint replacement in the future.     

Calcium phosphate coatings for fixation of bone implants: Evaluated mechanically and histologically by stereological methods

The osteoconductive properties of HA coatings are well-documented. HA coating is able to enhance bone ingrowth and to reduce early migration of both hip and knee prostheses. Despite the clinical use of HA-coated prostheses several aspects relevant to HA coatings have not been elucidated. The optimum coating quality and surface texture is still a matter of debate. Moreover, the significance of coating resorption is controversial. It has been suggested that resorption disintegrates the coating and reduces the bonding strength between implant and bone and the strength of the coating-implant interface, which might lead to implant loosening, coating delamination and acceleration of third body wear processes. This thesis aimed to investigate the effects of Ca-P coating type, quality and surface texture on mechanical fixation, bone ingrowth and loss of coating in experimental models in dogs and man. Furthermore, the significance of systematic sampling in bone histomorphometry using the unbiased stereological vertical section method was analyzed. Results . The first group of studies showed that HA-coated implants with porous-coated surface demonstrated increased energy absorption compared with grit-blasted implants during both non weight-bearing and weight-bearing conditions with controlled micromotion of 500 &#119 m. In addition, the HA coating delaminated on gritblasted implants during mechanical testing in contrast to porous-coated implants. Histomorphometry showed increased bone ingrowth to grit-blasted implants demonstrating that surface topology influenced surface activity. The next series of studies focused on the effects of Ca-P coating type, HA versus FA, during stable weight-bearing and non weight-bearing conditions. In dogs, no difference in mechanical fixation and bone ingrowth was demonstrated. However, in humans, HA-coated implants had significantly greater bone ongrowth than FA-coated implants after one year. The third group of studies evaluated the effects of HA coating crystallinity during controlled micromotion of 250 &#119 m. After 16 weeks, low crystalline (50%) HA coating accelerated mechanical fixation and bone ingrowth compared with high crystalline HA (75%). High crystalline HA achieved significantly better anchorage from 16 to 32 weeks whereas mechanical fixation of low crystalline HA was unchanged. In all studies, loss of Ca-P coating was evaluated. It was demonstrated that the coatings were resorbed, partially, in vivo irrespective type and quality of the coating. HA coverage on porouscoated implants was significantly more reduced than on grit-blasted implants in dogs. No difference in overall resorption between HA and FA coatings was demonstrated. However, in humans, significantly less HA and FA coating was resorbed when bone was present on the coating surface compared with bone marrow or fibrous tissue. In addition, resorption of HA was greater than FA in the presence of bone marrow indicating that FA was more stable than HA. Low (50%) crystalline HA coating was significantly more reduced compared with high (75%) crystalline HA at both 16 and 32 weeks. However, no further coating loss was observed from 16 to 32 weeks suggesting two phases of coating resorption: Phase I (0-16 weeks) with rapid coating loss, and phase II (16-32 weeks) with slow loss. Another important finding was that continuous loading and micromovements of 150 &#119 m accelerated resorption in contrast to immobilization of the implant. In addition, unstable fibrous anchored implants had significantly more loss of HA coating as compared with bony anchored implants. In all studies, resorbed coating was partly replaced by bone in direct contact with the implant surface suggesting durable implant fixation. Sampling efficiency in the unbiased stereological vertical section method was analyzed in order to find an optimal sampling design for histomorphometric analyzes at different sampling levels (humans, sections, fields of view and number of counting items) with different sampling intensities. The analysis showed that only minor changes in variances were observed when the initial scheme of 14 sections from each implant was reduced to include only one of the two possible implant sides, every third field of view and half the probe density, reducing the total workload at the microscope to less than 10% on all sections. In addition, the number of sections for analysis could be reduced to every fourth section per implant (3-4 sections for evaluation) without significantly increase in variance. The study demon strated that biological variation contributed to the majority of the total observed variance. Conclusion . The present series of investigations demonstrated that Ca-P coating type and quality and the underlying surface texture had significant influence on either mechanical fixation, bone in/ongrowth and loss of coating in dogs and man. In addition, the sampling design for histomorphometry could be optimized without reducing the quality of the data.     

Alendronate does not inhibit early bone apposition to hydroxyapatite-coated total joint implants: a preliminary study

BACKGROUND: Alendronate is a pyrophosphate analogue of bisphosphonate that has been shown to inhibit osteoclastic bone resorption. Bone formation and remodeling are necessary to establish initial fixation of uncemented implants, especially those coated with a bioactive surface such as hydroxyapatite. Because the process of bone-remodeling that culminates in new-bone formation is thought to be initiated by osteoclastic bone resorption, it is appropriate to test the influence of osteoclast-inhibiting medications on bone apposition to hydroxyapatite-coated implants. METHODS: Twelve dogs underwent staged bilateral total hip arthroplasty, with twenty weeks between the first and second operations, with use of a titanium-alloy femoral stem that had a proximal macrotextured surface and a plasma-sprayed hydroxyapatite coating. Six of the dogs received oral alendronate therapy from the time of the surgery until they were killed; the other six dogs were untreated controls. The animals were killed four weeks after the second operation. Sections from matched implant sites (proximal, middle, and distal) were histologically analyzed. The linear extent of bone apposition, the linear extent and the thickness of the hydroxyapatite coating, and the total amount of cortical and trabecular bone were measured with the use of an interactive image analysis system. RESULTS: There were no significant differences in radiographic or histologic findings between the two groups at either four or twenty-four weeks. Although the extent of the hydroxyapatite coating decreased significantly with time in both groups (p < 0.01), we identified no significant influence of alendronate on the extent of bone apposition, the extent or thickness of the hydroxyapatite coating, or the cortical or trabecular bone area around the implants. CONCLUSIONS: Many patients who are receiving alendronate for osteoporosis or other disorders may also be candidates for cementless total joint arthroplasty. Although bone formation is generally thought to be initiated by and coupled with bone resorption, our results suggest that alendronate has no discernible effect on the initial fixation of or the short-term bone-remodeling around hydroxyapatite-coated femoral total joint implants.     

Alendronate treatment in the revision setting, with and without controlled implant motion: an experimental study in dogs

Introduction Bisphosphonates have been proposed to delay or prevent loosening of joint replacement implants by reducing bone resorption. It is known, however, that implant motion prevents the bone anchorage necessary to maintain secure implant fixation.

Methods We used our experimental implant model with controlled motion to evaluate the relative effects of implant motion and bisphosphonate. We implemented our established 8-week experimental revision protocol to obtain a bony and soft tissue setting of revision joint replacement in 16 dogs. At 8 weeks, we had stabilized half of the implants. The other half of the implants continued pistoning. Half of the dogs were exposed to alendronate (oral).

Results Stabilization of the revision implant was more effective at improving fixation (higher shear strength) than administering alendronate. As expected, the fibrous membrane remained under unstable conditions, even with alendronate. With alendronate and stabilized implants, increased bone was observed near the sclerotic shell of the revision cavity, but it was reduced with alendronate when the implant was unstable.

Interpretation Our findings suggest that it may be difficult for alendronate administration alone to rescue implants that are already loose. In implants that have not progressed to loosening, alendronate may increase bone density at the border with the sclerotic shell, but the effect of this bone in delaying eventual loosening is not known.     

No effect of autologous growth factors (AGF) around ungrafted loaded implants in dogs

Autologous growth factors (AGF) is a growth-factor-rich concentrate of platelets, white blood cells and fibrinogen. Application of AGF was presumed to improve implant fixation and gap healing of non-grafted, loaded implants. We inserted one loaded titanium implant intra-articularly in each medial femoral condyle of eight dogs. Each implant was surrounded by a 0.75 mm gap. One implant in each dog was coated with AGF prior to implantation whereas the contralateral implant served as a control. AGF was prepared by isolating the buffy-coat from blood and further concentrated using an Interpore Cross UltraConcentrator. Platelet counts were increased from a median baseline of 168×10³/μl to 1003×10³/μl in AGF. However, AGF had no significant effect on implant fixation or bone formation. Even though AGF increased ultimate shear strength and energy absorption by approximately 50%, these differences had a p-value less than 0.05. The sample size in this study was small and any negative conclusions should be taken with caution due to low statistical power. We have previously demonstrated that AGF significantly improves fixation and incorporation of grafted implants. AGF might require mixing with an osteoconductive grafting material in order to provide a scaffold on which to foster bone growth and to keep the growth factors on location for a prolonged period.