Hydroxyapatite enhances long-term fixation of titanium implants

The aim of this study was to evaluate osseous integration of hydroxyapatite coated titanium implants over time as compared to pure titanium. In 20 rats the medullary cavity of both femoral bones was entered by an awl from the trochanteric area. With steel burrs it was successively reamed to a diameter of 2.0 mm. Nails with a diameter of 2.0 mm and with a length of 34 mm were inserted into the medullary cavity; a pure titanium nail on the left side and a titanium nail entirely plasma-sprayed with hydroxyapatite (HA) on the right side. The surface roughness of the pure titanium was characterized by Ra 2.6 microm and Rt 22 microm, and HA had a roughness of Ra 7.5 (arithmetical mean roughness) microm and Rt (maximum profile height) 52 microm. The rats were randomized to a follow-up of 6 and 12 months, respectively. At sacrifice the femoral bones were dissected free from soft tissues. The bones were radiographed and then immersed in fixative. A specimen-slice of about 5 mm thickness was prepared from the region under the trochanter minor with a water cooled band-saw. Sample preparation for undecalcified tissue followed the internal guidelines at the laboratories of Biomaterials/Handicap Research. At 6 months the median bone bonding contact of the implants was 40% (range 0-92) in the titanium group and 34% (0-86) in the HA group. At 12 months the median bone bonding contact was 51% (0-97) in the titanium group and 86% (72-98) in the HA group. In conclusion, we found a significant (p = 0.001) increase in bone bonding contact from 6 to 12 months of the HA coated nails and significantly (p = 0.043) enhanced bone bonding contact in HA coated nails at 12 months as compared to pure titanium nails.     

Hydroxyapatite and carbon coatings for fixation of unloaded titanium implants

The aim of this study was to investigate the interaction between bone and pure titanium, titanium coated with hydroxyapatite (HA), and titanium coated with carbon in a rat femur model. In 25 rats, the medullary cavity of both femurs was entered by an awl from the trochanteric area. With steel burrs it was successively reamed to a diameter of 2.0 mm. Nails with a diameter of 2.0 mm and with a length of 34 mm were inserted in a random manner; either a pure titanium nail, a titanium nail entirely plasma-sprayed with a 75-100-microm layer of HA or a titanium nail coated with 2-10-microm carbon. The surface roughness of the pure titanium was characterized by Ra 2.6 microm and Rt 22 microm. Ra of HA was 7.5 microm and Rt 52 microm, and of carbon Ra was 0.4 microm and Rt 4.0 microm. Twelve rats were randomized to a follow up of 8 weeks, and the remaining 13 rats were followed for 16 weeks. At sacrifice both femora were dissected free from soft tissues and then immersed in fixative. A specimen slice of about 5 mm thickness was prepared from the subtrochanteric region with a water-cooled band-saw. Sample preparation for un-decalcified tissue followed the internal guidelines at the laboratories of Biomaterials/Handicap Research. At 8 weeks the median bone bonding contact of the implants was 43% (range 0-74) in the titanium group, 39% (0-75) in the HA group, and 3% (0-59) in the carbon group. At 16 weeks the corresponding figures were 58% (0-78) in the titanium group, 51% (15-75) in the HA group, and 8% (0-79) in the carbon group. In conclusion, we found great variability in bone bonding contact. In general, carbon-coated nails had reduced bone bonding contact both at 8 and at 16 weeks as compared to pure titanium or titanium coated with hydroxyapatite.     

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.     

In vivo effects of coating loaded and unloaded Ti implants with collagen, chondroitin sulfate, and hydroxyapatite in the sheep tibia.

The in vivo effects of coating titanium implants with organic extracellular matrix molecules were examined in the sheep tibia. Titanium screws (5.0 mm) were coated with type I collagen (Ti/Coll) or type I collagen and chondroitin sulfate (Ti/Coll/CS) by biomimetic fibrillogenesis. Uncoated screws (Ti) and screws coated with hydroxyapatite (Ti/HA) served as control. Six adult female sheep received one screw of each type to stabilize a midshaft tibial fracture with external fixation. Four cylindrical implants of 4-mm outer diameter and 3.3-mm inner diameter with the same coatings were inserted into the tibial head. No pin track infections were seen at the time of implant retrieval 6 weeks after implantation. Extraction torque was greater for Ti/HA (1181 Nmm) and Ti/Coll/CS (1088 Nmm) compared to Ti/Coll (900 Nmm) and Ti (904 Nmm) [N.S.]. Newly formed bone was noted around all coated screws within the medullary cavity. Macrophage and osteoclast activity was significantly reduced around Ti/Coll/CS in both types of implants compared to uncoated controls (p < 0.05). Osteoblast activity was significantly increased around loaded Ti/Coll and Ti/Coll/CS screws compared to uncoated Ti screws (p < 0.05). Microtomographic evaluation (SRmicroCT) revealed no significant differences in new bone formation around the unloaded tibial head implants.Coating of external fixation devices with of type I collagen and chondroitin sulfate appears to have similar effects with respect to stability and bone healing as HA but with less osteoclast activity. These findings were more pronounced under loaded than unloaded conditions in the sheeptibia.     

The unreamed femoral and tibial nail-design and insertion behavior

Summary Background: The insertion of locking nails into the medullary cavity without previous reaming of the latter has come to meet most prerequisites desirable for biological osteosynthesis. The present experimental study, which was performed on cadaver bones is based on the measurement of the intramedullary pressure exercised by solid titanium nails of different profiles and diameters inserted in the medullary cavity of femora and tibia. Methods: In all, 132 pressure measurements were carried out in 4 cadaver femora after insertion of 5 different solid nails of 9 mm diameter, and 6 solid nails of 10 mm diameter. Our examinations were carried out in 4 cadaver tibia, using nails of 8 to 10 mm diameter and 84 pressure measurements were done. All of these nails were mad of titanium alloy but differed in profile and surface conditions. Results: Femur: With implants of 9 mm diameter, maximum insertion forces of 234 to 327 N and maximum pressure values between 5 and 11 bar were measured, whereas with nails of 8 to 10 implants of 10 mm diameter the respective values were 191 to 285 N and 3 to 12 bar. Tibia: With implants of 8 to 10 mm diameter, maximum insertion forces of 103 to 340 N and pressure values of 0.43 to 8.05 bar were measured, whereas with nails of 9 and 10 mm diameter, insertion forces amounted to 226 to 325 N, and pressure values to 0.77 to 12.27 bar. Conclusions: Our experimental model confirmed the influence of the shape of the nail on its insertion behavior. Proper selection of the nail diameter is thus decisive for unproblematic implantation. The pressure developed during insertion is on average higher with nails of smaller diameters, as rapid insertion will increase pressure and the marrow substitute cannot flow off fast enough on account of its viscosity. Thus it is not only the diameter of the nail that is decisive for pressure development in the medullary cavity but also the shape of its cross-section.      

Intramedullary pressure and bone marrow fat extravasation in reamed and unreamed femoral nailing.

This study was designed to investigate whether intramedullary pressure and embolization of bone marrow fat are different in unreamed compared with conventional reamed femoral nailing in vivo. In a baboon model, the femoral shaft was stabilized with interlocking nailing after a midshaft osteotomy. Intramedullary pressure was measured in the distal femoral shaft fragment at the supracondylar region. Extravasation of bone marrow fat was determined by the modified Gurd test (range: 0-5) with blood samples from the vena cava inferior. Data were monitored in eight unreamed and eight reamed intramedullary femoral nailing procedures. Intramedullary pressure increased in the unreamed group to 76 +/- 25 mm Hg (10.1 +/- 3.3 kPa) during insertion of 7-mm nails and in the reamed group to 879 +/- 44 mm Hg (117.2 +/- 5.9 kPa) during reaming of the medullary cavity. Insertion of 9-mm nails after the medullary cavity had been reamed to 10 mm produced an intramedullary pressure of 254 +/- 94 mm Hg (33.9 +/- 12.5 kPa) (p < 0.05). Fat extravasation in the unreamed group was recorded with a score of 2.9 +/- 0.4 for the Gurd test during nailing with 7-mm nails, whereas in the reamed group significantly more fat extravasation was noticed during the reaming procedures, with a score of 4.6 +/- 0.1. Liberation of fat during insertion of 9-mm nails after reaming was recorded with a score of 3.5 +/- 0.4. In both groups, a positive correlation of fat extravasation with the rise in intramedullary pressure was found (reamed group: r(s) = 0.868; unreamed group: r(s) = 0.698), resulting in significantly less liberation of bone marrow fat in the unreamed stabilized group than in the reamed control group (p < 0.05). The data indicate that fat embolization during nailing procedures after femoral osteotomy increases with increasing intramedullary pressure and occurs in a lesser degree in unreamed than in reamed intramedullary femoral shaft stabilization.     

The effects of hydroxyapatite coating and bone allograft on fixation of loaded experimental primary and revision implants

We used our established experimental model of revision joint replacement to examine the roles of hydroxyapatite coating and bone graft in improving the fixation of revision implants. The revision protocol uses the S?balle micromotion device in a preliminary 8-week period of implant instability for the presence of particulate polyethylene. During this procedure, a sclerotic endosteal bone rim forms, and a dense fibrous membrane is engendered, having macrophages with ingested polyethylene and high levels of inflammatory cytokines. At the time of revision after 8 weeks, the cavity is revised with either a titanium alloy (Ti) or a hydroxyapatite (HA) 6.0 mm plasma-sprayed implant, in the presence or absence of allograft packed into the initial 0.75 mm peri-implant gap. The contralateral limb is subjected to primary surgery with the same implant configuration, and serves as control. 8 implants were included in each of the 8 treatment groups (total 64 implants in 32 dogs). The observation period was 4 weeks after revision. Outcome measures are based on histomorphometry and mechanical pushout properties. The revision setting was always inferior to its primary counterpart. Bone graft improved the revision fixation in all treatment groups, as also did the HA coating. The sole exception was revision-grafted HA implants, which reached the same fixation as primary Ti and HA grafted implants. The revision, which was less active in general, seems to need the dual stimulation of bone graft and HA implant surface, to obtain the same level of fixation associated with primary implants. Our findings suggest that the combination of HA implant and bone graft may be of benefit in the clinical revision implant setting.     

The effects of hydroxyapatite coating and bone allograft on fixation of loaded experimental primary and revision implants

We used our established experimental model of revision joint replacement to examine the roles of hydroxyapatite coating and bone graft in improving the fixation of revision implants. The revision protocol uses the Søballe micromotion device in a preliminary 8-week period of implant instability for the presence of particulate polyethylene. During this procedure, a sclerotic endosteal bone rim forms, and a dense fibrous membrane is engendered, having macrophages with ingested polyethylene and high levels of inflammatory cytokines. At the time of revision after 8 weeks, the cavity is revised with either a titanium alloy (Ti) or a hydroxyapatite (HA) 6.0 mm plasma-sprayed implant, in the presence or absence of allograft packed into the initial 0.75 mm peri-implant gap. The contralateral limb is subjected to primary surgery with the same implant configuration, and serves as control. 8 implants were included in each of the 8 treatment groups (total 64 implants in 32 dogs). The observation period was 4 weeks after revision. Outcome measures are based on histomorphometry and mechanical pushout properties. The revision setting was always inferior to its primary counterpart. Bone graft improved the revision fixation in all treatment groups, as also did the HA coating. The sole exception was revision-grafted HA implants, which reached the same fixation as primary Ti and HA grafted implants. The revision, which was less active in general, seems to need the dual stimulation of bone graft and HA implant surface, to obtain the same level of fixation associated with primary implants. Our findings suggest that the combination of HA implant and bone graft may be of benefit in the clinical revision implant setting.     

Hydroxyapatite coating modifies implant membrane formation. Controlled micromotion studied in dogs.

We studied the influence of controlled micromovements between bone and porous titanium alloy implants with and without hydroxyapatite coating. A dynamically loaded unstable device producing approximately 150-microns axial translation of knee implants during each gait cycle was developed. Stable implants served as controls. Matched stable and unstable implants with either porous titanium (Ti) or hydroxyapatite (HA) coating surrounded by a gap of 0.75 mm were inserted into the weight-bearing regions of the medial femoral condyles in 14 mature dogs. Histologic analysis after 4 weeks showed a fibrous membrane surrounding both types of implants subjected to micromovements, whereas various amounts of bone ingrowth was obtained in the stable implants. The membrane around unstable HA implants was thinner than that around unstable Ti implants. Islands of fibrocartilaginous tissue characterized the membrane around unstable HA implants, whereas fibrous connective tissue surrounded unstable Ti implants. The collagen concentration of the fibrous membranes was higher around unstable HA implants compared with Ti implants. Instability reduced the shear strength of the implants. However, the shear strength of unstable HA implants exceeded that of the Ti implants, both unstable and stable. The greatest shear strength was obtained by stable HA implants, i.e., tenfold greater than that of stable Ti implants. The gap-healing capacity around stable HA implants increased toward the HA surface, and was greater than that around Ti implants. Our study demonstrates that micromovements between bone and implant inhibit bone ingrowth and lead to the development of a fibrous membrane. The superior fixation of unstable HA implants compared with unstable Ti implants may be ascribed to the presence of fibrocartilage, a higher collagen concentration, and radiating orientation of collagen fibers in the membrane. The strongest mechanical anchorage and the greatest amount of bone ingrowth was obtained by stable implants coated with hydroxyapatite.     

Unusual Complications during Removal of Intramedullary Nails after Lower Leg Fractures Tibial Shaft Fissure and Inability to Remove the Nail

Patients and Methods: Between January 1995 and December 1999, 279 fractures of the lower leg were stabilized by crural intramedullary nailing (ACE^® interlocking nails, DePuy). Only under certain circumstances was the medullary cavity reamed. This paper reports on ten patients with whom unusual complications arose during nail removal. Results: In two cases, the nail jammed in the fracture callus during its removal, so that the nail had to be left in place. In eight further cases, the nail could only be removed with extreme difficulty and the use of special instruments; in six of the cases, however, a long fissure in the dorsal tibial shaft was visible under intraoperative X-ray fluoroscopy. Postoperative patient mobilization was achieved using either a brace (allowing full weight bearing on the leg) or crutches (partial weight bearing on the leg). The tibial medullary canal was reamed in nine of the ten patients, and a nail relatively wide in diameter (11-14 mm) was inserted. Conclusions: The ACE^® interlocking nail employed is bent 5° ventrally over the final 50 mm to the tip. This does not only facilitate nail insertion, but the nail also adapts to the physiological anteversion of the distal tibia. As a result of its bent shape and large diameter and in the case of considerable endosteal callus formation with associated narrowing of the medullary cavity the rigid titanium nail can no longer pass through. This may lead to a fissure when removing the nail, or even result in the total inability to remove the nail. Another reason is the dorsal groove in nails S 11 mm in diameter, into which bone grows at the site of callus formation. When removing the nail, the distal end of the nail (with no groove) has to slide over this dorsal bony ridge, thus narrowing the available lumen of the medullary canal by about 20% in the anteroposterior direction. The manufacturer has announced a change in the nail design.     

Effects of graded reaming on fracture healing: Blood flow and healing studied in rat femurs

In 30 rats, closed bilateral fractures of the femur were produced. On the left side intramedullary reaming was performed to 1.6 mm, and the fracture fixed with a steel pin with a diameter of 1.6 mm. On the right side the femoral canal was reamed to 2.0 mm and a hollow steel tube with a diameter of 2.0 mm was used for fixation. An additional 8 rats were used to obtain mechanical, dimensional and flow data on intact femurs, and another 10 rats were used to study the acute flow changes caused by fracture and different degrees of reaming and fracture.

Fracture and reaming reduced total bone and cortical bone blood flows to about one third of normal flow, with no differences between the 1.6-mm and the 2.0-mm reamed bones. At 4 weeks, total bone flow was about double and cortical bone flow about 4 times increased in the 1.6-mm group. In the 2.0-mm reamed bones increases of approximately 5 times in total bone flow and of about 7 times in cortical flow were found. Callus flow was about twice the size of the respective cortical flow in both groups. Both total and cortical flows gradually subsided, without differences between the 2 groups. At 12 weeks, the callus area in the 2.0-mm group was greater than in the 1.6-mm group, while bone dimensions were greater in the 2.0-mm group at 4 and 12 weeks. Bending moment and rigidity were greater in the 1.6-mm group than in the 2.0-mm one at every time interval; no differences were found in fracture energy.

We conclude that, in terms of healing, modest reaming is preferable to extensive reaming. The adverse effect of extensive reaming is not due to excessive flow derangement at the acute stage or to impaired vascularity at the phase of remodeling.     

Unusual Complications during Removal of Intramedullary Nails after Lower Leg Fractures Tibial Shaft Fissure and Inability to Remove the Nail

Patients and Methods: Between January 1995 and December 1999, 279 fractures of the lower leg were stabilized by crural intramedullary nailing (ACE^® interlocking nails, DePuy). Only under certain circumstances was the medullary cavity reamed. This paper reports on ten patients with whom unusual complications arose during nail removal. Results: In two cases, the nail jammed in the fracture callus during its removal, so that the nail had to be left in place. In eight further cases, the nail could only be removed with extreme difficulty and the use of special instruments; in six of the cases, however, a long fissure in the dorsal tibial shaft was visible under intraoperative X-ray fluoroscopy. Postoperative patient mobilization was achieved using either a brace (allowing full weight bearing on the leg) or crutches (partial weight bearing on the leg). The tibial medullary canal was reamed in nine of the ten patients, and a nail relatively wide in diameter (11–14 mm) was inserted. Conclusions: The ACE^® interlocking nail employed is bent 5° ventrally over the final 50 mm to the tip. This does not only facilitate nail insertion, but the nail also adapts to the physiological anteversion of the distal tibia. As a result of its bent shape and large diameter and in the case of considerable endosteal callus formation with associated narrowing of the medullary cavity the rigid titanium nail can no longer pass through. This may lead to a fissure when removing the nail, or even result in the total inability to remove the nail. Another reason is the dorsal groove in nails ≥ 11 mm in diameter, into which bone grows at the site of callus formation. When removing the nail, the distal end of the nail (with no groove) has to slide over this dorsal bony ridge, thus narrowing the available lumen of the medullary canal by about 20% in the anteroposterior direction. The manufacturer has announced a change in the nail design.     

Temperature rise during reamed tibial nailing

The current study determined the temperature rise during reamed tibial intramedullary nailing in vivo. Eighteen adult patients were studied. The tibial medullary canal diameter ranged from 8 to 11 mm and was reamed to at least 1.5 mm above the required nail diameter with AO reamers. Reaming of the medullary cavity ranged from 9 to 12 mm before nail insertion. Intraoperative monitoring of the heat produced during reaming of the medullary cavity was done by inserting two platinum resistance thermometer probes into the cortical bone at the short isthmic segment of the tibial shaft. The probes were connected to a data logger, and temperature readings were taken every 5 seconds during each reaming procedure. The mean tibial temperature before initiation of reaming was 35.6 degrees C (standard deviation, +/- 0.5 degrees), and peak temperatures recorded were from 36.1 degrees C to 51.6 degrees C. A direct correlation was observed between temperature elevation and amount of reaming. With reaming above 10 mm, tibias with a canal diameter of 8 mm showed a statistically higher temperature rise compared with tibias with a canal diameter of 9, 10, or 11 mm. No patients had intraoperative or postoperative complications related to skin or bone thermal necrosis, and bony healing progressed uneventfully. The small amount of reaming required to insert a nail into a normal 9-, 10-, or 11- mm tibial canal does not seem to produce a clinical problem. Reaming smaller canals (8 mm) to a larger size may induce a significant heating effect.     

Osteoconductivity of thermal-sprayed silver-containing hydroxyapatite coating in the rat tibia

A silver-containing hydroxyapatite (Ag-HA) coating has been developed using thermal spraying technology. We evaluated the osteoconductivity of this coating on titanium (Ti) implants in rat tibiae in relation to bacterial infection in joint replacement. At 12 weeks, the mean affinity indices of bone formation of a Ti, an HA, a 3%Ag-HA and a 50%Ag-HA coating were 97.3%, 84.9%, 81.0% and 40.5%, respectively. The mean affinity indices of bone contact of these four coatings were 18.8%, 83.7%, 77.2% and 40.5%, respectively. The indices of bone formation and bone contact around the implant of the 3%Ag-HA coating were similar to those of the HA coating, and no significant differences were found between them (bone formation, p = 0.99; bone contact, p = 0.957). However, inhibition of bone formation was observed with the 50%Ag-HA coating. These results indicate that the 3%Ag-HA coating has low toxicity and good osteoconductivity, and that the effect of silver toxicity on osteoconductivity depends on the dose.     

Growth of new bone guided by implants in a murine calvarial model

New methods to increase vertical bone growth are needed to permit dental implant placement in patients with low alveolar ridge height after extended periods of tooth loss. While ectopic rodent models are typically used to evaluate new osteogenic implant surface coatings, a more relevant intramembraneous rodent model was needed to address the particular clinical need to grow a new layer of bone above an existing layer of bone. In this study we report on a novel murine calvaria model in which successful vertical bone growth around miniaturized dental implants was achieved when using non-glycosylated bone morphogenetic protein-2 (ng/rhBMP-2). Twenty CD-1 mice received two Ti implants each consisting of a Ti ring implant stabilized by a Ti screw into the occipital calvarial bone. Four groups were evaluated: control Ti, Ti+20 mug ng/rhBMP-2, hydroxyapatite (HA)-coated Ti, and HA+20 mug ng/rhBMP-2. The mice were sacrificed 21 days following implant placement. MicroCT analysis showed no new bone formation around the untreated Ti or the HA-coated implants, but demonstrated new bone growth in every dimension around and above the Ti+ng/rhBMP-2 and the HA+ng/rhBMP-2 treated implants. Histopathologic analysis showed that a thin fibrous capsule covered the untreated Ti implants. Limited bone-to-implant contact (BIC) was observed for the HA-coated implants, while in contrast both ng/rhBMP-2 treated groups exhibited extensive new supracalvarial woven bone that covered the implant and merged with the calvarial plate. Histomorphometrically, supracalvarial bone heights and bone widths and BIC were not statistically different from one another for the two ng/rhBMP-2 treated groups. However, the total supracalvarial bone surface area was significantly greater (p<0.05) for the Ti+ng/rhBMP-2 implants (7.2 mm(2)) than the HA+ng/rhBMP-2 (4.0 mm(2)) treated implants. The bone density within 1 mm around the implant was also significantly greater (p<0.05) for the Ti+ng/rhBMP-2 implants (9.9%) than the HA+ng/rhBMP-2 (4.0%) implants, indicating that HA coatings may not be required for sustained release when non-glycosylated BMP-2 is used. This new murine model is capable of discriminating between various bone augmentation strategies and may represent a clinically more relevant model for alveolar bone augmentation than the commonly used ectopic muscle pouch or long bone models.     

Bone graft incorporation around titanium-alloy- and hydroxyapatite-coated implants in dogs.

To evaluate cancellous allogenic bone graft incorporation into porous-coated implants, the fixation of titanium alloy-(Ti) and hydroxyapatite-(HA) coated implants with and without bone graft was compared. An unloaded model with unilateral carragheenin-induced osteopenia of the knee was used in 12 mature dogs. Ti- and HA-coated cylinders were implanted in the distal femoral condyles and centralized in 2-mm overreamed drill holes. Allogenic, fresh-frozen (-80 degrees) cancellous bone graft was packed around the implants in six dogs. In a matched group of six other dogs, the implants were left in overreamed canals without bone graft. After six weeks the interface shear strength of grafted Ti-coated implants had significantly increased compared to the nongrafted Ti implants. However, HA coating used without bone graft was capable of enhancing the bone-implant interface shear strength to nearly the same degree. The fixation of grafted Ti- and HA-coated implants was equal. No significant difference in implant fixation was found between osteopenic and control bone. Histomorphometric evaluation of mineralizing surfaces in direct contact with the implant confirmed the results from the push-out test. Bone-implant fixation when using allogeneic fresh-frozen cancellous bone graft in osteopenic and control bone was enhanced by hydroxyapatite coating but the HA coating alone appeared to offer almost the same improvement in anchorage in 2-mm defects. Loss of bone stock around loose prosthetic implants often requires bone grafting. However, because of anatomic constraints in joint prosthetic surgery, a complete filling of defects with bone graft is difficult, and areas of gaps between bone and implant will remain. Provided mechanical stability of the prosthesis, the results reported here suggest that these areas will probably be filled early with new mineralizing bone if the prosthesis is coated with a thin layer of hydroxyapatite.     

The effect on cortical bone of reaming and filling of canine tibial diaphysis with inert bone wax.

In canine studies the effect of intramedullary reaming on tubular bone was investigated in 4 dogs. Intramedullary reaming was further compared with reaming and intramedullary filling with bone wax in 6 dogs. Bone blood perfusion was measured by a microsphere technique and bone remodelling activity by 99mTc-MDP uptake. From histological sections bone necrosis and remodelling activity were estimated. The biological response increased with the surgical trauma. If the medullary cavity was only reamed, endosteal apposition was the predominant reaction. Obturation of the medullary cavity resulted in more vigorous subperiosteal and cortical reaction. It is concluded that the remodelling processes differ significantly between reamed bone and bone where the medullary cavity is reamed and blocked. Thus when testing bone cement the studies should include a control operation with obturation of the medullary canal in a way simulating bone cement.     

Crack Revision Improves Fixation of Uncemented HA-coated Implants Compared with Reaming: An Experiment in Dogs

The crack procedure is a surgical technique for preparing the implant cavity at revision of loose joint replacement components. It disrupts the neocortical bone shell that typically forms around the cavity. Using an animal model, we compared the crack technique with reaming. Twenty micromotion implants were inserted bilaterally into the knees of 10 dogs according to our revision protocol, allowing formation of a standardized revision cavity (loose implant, fibrous tissue, and sclerotic bone rim). Eight weeks later we performed revision surgery. On the control side, in which the neocortex was removed, the cavity was reamed. On the intervention side, in which the neocortex was perforated but left in situ, the cavity was cracked. For revision we used non-motioning hydroxyapatite (HA)-coated, plasma-sprayed titanium implants. Observation after revision was 4 weeks. The implants revised by the crack technique had better mechanical fixation in all mechanical parameters by the push-out test. The crack revisions also provided more new bone formation around the implants compared with the reamed revisions but had no effect on new bone ongrowth. The data suggest using this bone-sparing technique may be superior to reaming in terms of achieving improved early implant fixation of uncemented HA-coated revision implants.     

Bone particles disturb new bone formation on the interface of the titanium implant after reaming of the marrow cavity

Histomorphometric studies were conducted in rats to determine whether bone particles would disturb new bone formation on the interface of titanium implants inserted after reaming of the marrow cavity. In eighty 10-week-old female Wistar rats, smooth-surfaced titanium alloy implants were inserted bilaterally into the marrow cavity after reaming in the distal femur. There were three experimental groups: in the irrigated femora, sterile saline was flushed through the medullary canal; in the particle femora, autologous bone particles were inserted into the intramedullary cavity; and in the reamed femora, the implant was inserted without procedures after reaming. The rats were sacrificed at one, two, four or eight weeks postoperatively, and Villanueva bone staining was applied for histomorphometric studies. The bone volume of new bone on the interface of the implant in the irrigated femora was greater than that in the particle or the reamed femora throughout the study period. The results suggest that clearance of bone particles by irrigation after reaming of the marrow cavity significantly facilitates new bone formation on the interface of implants by one week. The findings also suggest the potential clinical application of total canal irrigation prior to insertion of cementless femoral components as well as cemented prosthesis.