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TheEffectofAlendronate(Fosamax™)andImplantSurfaceon
BoneIntegrationandRemodelinginaCanineModel
SallyR.Frenkel,
1
WilliamL.Jaffe,
1
CraigDellaValle,
1
LaithJazrawi,
1
StephenMaurer,
1
AviBaitner,
1
KevinWright,
1
DebraSala,
1
MonicaHawkins,
2
PaulE.DiCesare
1
1
MusculoskeletalResearchCenterandDepartmentofOrthopaedicSurgery,NewYorkUniversityHospitalforJoint
Diseases,NewYork,NewYork
2
StrykerHowmedicaOsteonicsCorporation,Rutherford,NewJersey
Received13February2001;revised12June2001;accepted15June2001
Publishedonline00Month2001;DOI10.1002/jbm.0000
Abstract: Patientsathighriskforosteoporosisanditsassociatedmorbidity,including
postmenopausalwomen,arebeingpharmacologicallymanagedtostabilizeandimprovebone
mass.Alendronatesodium(Fosamax™)isacommonlyusedantiresorptiveagenteffectivein
osteopenicwomenforreducingboneresorption,increasingbonedensity,anddecreasing
fractureincidence.Withtheincreasedincidenceofalendronate-treatedwomenwhoare
undergoinghipreplacementorfracturerepairbyprosthesisplacement,dataareneededto
predicthowalendronateaffectshostboneintegrationwithuncementedsurfaces.Theaimof
thisstudywastodeterminetheeffectofalendronateonnewboneformationandattachment
toimplantsurfacesinanormalandsimulatedestrogen-deficient,calcium-deficientcanine
model,usinganimplantablebonegrowthchamber.Alendronatedidnotaffecthostbone
integrationtosurfacescommonlyusedinuncementedtotaljointarthroplasty,buttherewere
significantdifferencesdependentsolelyonthetypeofsurface.©2001JohnWiley&Sons,Inc.J
BiomedMaterRes(ApplBiomater)58:645–650,2001
Keywords: alendronate;implantinterface;bonegrowth;mechanicaltesting;caninemodel
INTRODUCTION
Uncementedtotalhiparthroplastyisoftenindicatedfor
youngerpatientswhoarelikelytoplaceahigherdemandon
theirprosthesesthanelderlyrecipients.Asthemeanlife
expectancyincreases,patientactivitylevelhasremainedhigh
intothesixthdecadeandbeyond.Asaresult,anincreasing
numberofuncementedtotalhiparthroplastiesarebeingper-
formedonpostmenopausalwomenwithdegenerativearthri-
tis.Withaheightenedawarenessofthemorbidityassociated
withosteoporosis,patientsathighrisk,includingpostmeno-
pausalwomen,arenowbeingpharmacologicallymanagedto
stabilizeandimprovebonemass.
1
Alendronatesodium
(Fosamax™,MerckInc.,Rahway,NJ)isacommonlyused
antiresorptivepharmacologicalagentthathasbeenshownto
beeffectiveinosteopenicwomenforreducingboneresorp-
tion,increasingbonedensity,anddecreasingfractureinci-
dence.
2
Thisagentisamemberofthenitrogen-containing
bisphosphonatefamilythatinhibitstheresorptiveactivityof
matureosteoclastsandcausesosteoclastapoptosis.
3
Unlike
otherbisphosphonates(e.g.,etidronateorpamidronate),alen-
dronatedoesnotimpairmineralization.
4
Long-termsurvivalofuncementedhipcomponentsisde-
pendentonbiologicalfixation.Toachievebiologicalfixation
ofuncementedorpress-fitimplants,closeappositionofbone
totheimplantsurfaceandinitialmechanicalstabilityare
required.
5
Osteoconductivecoatingsnotonlyenhanceboth
theintegrationofanimplantwithhostboneandthestrength
ofthebone–implantinterface,butalsoprolongtheimplant’s
usefullife.
6
Titaniumandtitaniumalloyimplantscoatedwith
hydroxyapatite(HA)arewidelyusedtoencouragebiological
fixation,
7
duetothebiocompatibilityandmechanicalprop-
ertiesofthemetalsandthestabilityandconductivityofthe
HA.
8–10
Withtheincreasedincidenceofalendronate-treated
womenwhoareundergoinghipreplacementfordegenerative
arthritisorhipfracture,dataareneededtopredicthow
alendronateaffectshostboneintegrationtouncementedsur-
faces.
Thepresentstudywasconductedtodeterminetheeffectof
alendronateonboneattachmentandnewboneformationto
implantsurfacesinanormalandsimulatedestrogen-defi-
cient,calcium-deficientanimalmodel.Animplantablebone
growthchamber
11,12
wasusedtoevaluatetheeffectofalen-
dronateonhostboneintegrationtosurfacescommonlyused
inuncementedtotaljointarthroplasty.
Correspondenceto:SallyFrenkel,Ph.D.,HospitalforJointDiseases,301E.17St.,
NewYork,NY10003(E-mail:sally.frenkel@excite.com)
Contractgrantsponsor:MerckandCo.,Inc.
©2001JohnWiley&Sons,Inc.
645
METHODS
Sixteen skeletally mature tricolor hounds weighing 50–60
pounds were distributed into four treatment groups, and sam-
ples were analyzed at 24 weeks. Use of the implant chamber
produces six specimens for testing per limb, for a total Nof
192 specimens. To simulate estrogen deficiency accompanied
by low calcium intake, the experimental groups underwent
ovariectomy (OVX) and were fed a low-calcium (0.15%)
canine diet; control animals were fed a standard canine diet.
Eight weeks post-OVX, bone growth chambers were im-
planted into the animals.
13
The experimental groups consisted
of (1) intact animals treated with alendronate; (2) intact
untreated animals; (3) OVX animals treated with alendronate;
and (4) OVX untreated animals. Six specimens from each of
Groups 1–3 were processed for histological evaluation, and
the remaining 174 underwent mechanical testing. The study
protocol was approved by the Institutional Animal Care and
Use Committee.
Implant Chamber Design
Rectangular polyethylene implant chambers measured 8 mm
wide ⫻25 mm long ⫻10 mm deep (Figure 1). Twelve metal
test coupons with the experimental surfaces were placed
along slots cut into the top and bottom of the central space in
the implant, becoming the major surfaces lining the 10 ⫻1⫻
5-mm ingrowth channels created. The test coupons used in
the chambers were made from Ti6Al4V (ASTM F-136), with
three different surfaces: (1) arc-deposited commercially pure
titanium (CPTi; ASTM F-67) blasted with the use of a clean
blast medium (AD); (2) arc-deposited CPTi (ASTM F-67)
with a nominal 50-
m-thick plasma-sprayed hydroxyapatite
coating (AD/HA); and (3) chemically textured Ti6Al4V
(ASTM F-136) with a nominal 50-
m-thick plasma-sprayed
hydroxyapatite coating (ChemEtch/HA). Investigators were
blind as to both the type of coupon and its orientation within
the chamber.
A 2-mm lip built onto the implant sealed off the intramed-
ullary space, preventing ingrowth of periosteal new bone. The
row of ingrowth channels was wholly within the intramedul-
lary canal and not adjacent to any cortical bone surface. The
chambers were implanted in the lateral metaphysis of the
distal femur. After implantation, the ingrowth channel open-
ings faced the endocortical surface of the intact anterior and
posterior cortices. Upon sacrifice of the animal, each of the
six channels in each chamber, with its ingrown tissue and two
lining surfaces, became a sample that was mechanically
tested or processed for other forms of analysis.
Surgical Technique
The supracondylar region of the femur was approached by a
lateral skin incision extending distally along the lateral border of
the patellar tendon to the tibial tubercle. A drill template was
fixed to the lateral metaphysis with the use of Kirshner wires,
and a rectangle measuring 8 ⫻5 mm was marked by serial drill
holes. The template was positioned to allow the most distal
placement of the implant in the femoral metaphysis, equidistant
between the anterior and posterior cortices. The drill holes were
connected with an osteotome, and a lateral cortical bone window
was created. A 10-mm-deep rectangle of cancellous metaphy-
seal bone flush with the sides of the defect was removed to allow
a snug fit, and the chamber was inserted. Unicortical 2.7-mm
titanium bone screws were used to fix the implant, preventing
any motion. Closure was performed with the use of interrupted
resorbable sutures to repair the soft tissues, and interrupted 3-0
stainless-steel sutures for the skin. Animals were allowed full
postoperative weight bearing, and were given intramuscular
antibiotics (penicillin-G procaine) preoperatively and for the first
5 postoperative days.
Administration of Alendronate
Half the animals received subcutaneous injections of 2.5
g/kg alendronate 3 times a week from Day 7 postoperatively
until sacrifice; the remaining animals received vehicle control
injections. The dosing regimen was recommended by the
manufacturer specifically for canines in this application.
Sample Evaluation
All animals were sacrificed at 24 weeks postimplantion. At
sacrifice, femurs were harvested and a diamond wire saw was
used to isolate each implant from the surrounding bone. All
intact chambers underwent Faxitron (high-resolution) radiog-
raphy (30 kV) to allow qualitative assessment of bone pen-
etration into channels. Samples to be processed for histolog-
ical evaluation were placed in formalin, and plastic embed-
ded. Remaining chambers were carefully dismantled and the
samples removed for mechanical testing.
Mechanical Testing
Specimens were tested to failure in tension. The entire cou-
pon-tissue-coupon sandwich was placed into the custom
holding jig and mechanically tested with an Instron biaxial
servohydraulic testing system at a rate of 2.5% strain per
second. This mode of testing with this thin sandwich of tissue
induces a state of plane strain in the sample at a rate within
the range of physiological bone loading. After testing, the
failed specimens were removed from the test fixtures by
shearing the plates from the test fixture surfaces; this was
Figure 1. Schematic of implant chamber.
646 FRENKEL ET AL.
possible because of the relatively low shear strength of the
cyanoacrylate cement used to hold them in place in the testing
apparatus. All failure strengths are expressed in Newtons.
Following these tests, representative samples were fixed
for scanning electron microscopy (SEM).
Histomorphometry and Scanning Electron Microscopy
Specimens were fixed in 10% formalin and embedded in
methacrylate. Undecalcified sections were examined by light
microscopy following staining with Sanderson’s Rapid Bone
Stain. Histological evaluation was used to assess general
tissue response to the coupons. Following mechanical testing,
the failure surfaces were examined by SEM. Specimens were
deproteinated with sodium hypochlorite to expose the bone,
hydroxyapatite, and metal surfaces. To deproteinate, speci-
mens were placed in a solution of 50–50 Clorox™ sodium
hypochlorite and water for 30 min. Specimens were then
washed in five changes of tap water for 30 min each, followed
by graded ethanols (50–70–90–100%) for at least 2 h each.
This technique does not degrade the hydroxyapatite–bone
interface. The samples were then critical-point dried and
sputter coated with a 200-Å-thick layer of gold. The Faxitron
images of the bone and soft tissue taken parallel to the
tissue-implant surface were digitized; the amount of visible
bone penetration was calculated and expressed as a percent-
age of the total available surface area.
Statistical Analysis
Multivariate repeated-measures analysis of variance (ANOVA)
was used to analyze the mechanical testing and Faxitron data.
The assumption of sphericity was met for all analyses. The
Tukey test was used for all post hoc multiple comparisons.
RESULTS
Tissue Penetration and Bone-to-Surface Contact:
Light Microscopy
No significant difference in tissue penetration or bone-to-surface
contact was observed between alendronate-treated and untreated
specimens, or between OVX and intact specimens. A low-
magnification view of an entire chamber is shown in Figure 2.
Tissue on uncoated surfaces was almost entirely fibrous. When
bone was seen within an uncoated channel, it was typically not
in direct contact with the metal surface, but was rather a pointed,
conical projection of bone into the channel’s center (Figure 3).
The percentage of bone-to-surface contact observed in uncoated
specimens was 24.7 ⫾1.3%.
The tissue-penetration patterns seen on both types of HA-
coated surfaces were similar (Figure 4). There was virtually no
visible fibrous tissue interposed between bone and the contact
surface in these channels, and tissue characteristics were those of
mature bone that was primarily lamellar in nature. The mean
percentage of bone-to-surface contact observed in HA-coated
specimens was 84.3 ⫾3.7%. This was significantly greater than
Figure 2. Light photomicrograph of longitudinal section through in-
tact chamber. From the top down, Channels 2 and 5 were lined with
AD; remaining channels were lined with HA-coated surfaces. Note
extensive bone ingrowth in Channels 3 and 4, and complete pene-
tration in Channel 6. Bone is in intimate contact with surfaces in these
channels. In Channels 3 and 4 note marrow elements in center.
Channels 2 and 5 contain fibrous tissue almost exclusively; Channel
1 shows bone at either end, but the center is fibrous. Sanderson’s
Rapid Bone Stain, ⫻25.
Figure 3. Light photomicrograph of AD surface. Note fibrous tissue
(F) in contact with metal (black) and thin trabecula of bone (B) within
the channel. Sanderson’s Rapid Bone Stain, ⫻67.
647ALENDRONATE AND HYDROXYAPATITE-COATED SURFACES
the percentage for AD specimens (p⬍.05). Bone can be seen
attached to and within the HA, and it appears to be well inte-
grated into the coating (Figure 4). Active osteoid seams are
present, and bridging bone can be seen in the channel centers
(Figure 4). As can be seen in Figure 1 (Channels 4 and 6), a
section through an HA-coated channel is often similar in ap-
pearance to a longitudinal section through a mature long bone.
Tissue Penetration into Channels; Faxitron Morphometry
The percentage of bone penetration into the channels is
shown in Table I. A paucity of bone penetration into channels
lined with AD coupons was observed. The channels contain-
ing HA-coated coupons of both types showed much greater
and in some cases nearly complete penetration of bone,
across both the length and diameter of the channels, than did
AD coupons. Percent penetration was highest in AD/HA
channels, but this percentage was not significantly different
from that observed in ChemEtch/HA-coated channels. For
both types of HA-coated specimens, the percentage was sig-
nificantly higher than that observed in AD channels (Table I).
SEM Analysis of Mechanically Tested Samples
The AD coupons exhibited little if any adherent tissue, and
virtually no bone was seen on the metal in the examined spec-
imens. This finding is in keeping with the mechanical testing
results, in which AD surfaces demonstrated a significantly lower
tensile strength. Both types of HA-coated specimens showed
bone interdigitating with the coating (Figure 5).
In both types of HA-coated specimens, failure occurred
variably along the length of the channel. Within a given
channel, failure occurred through the bone remote from the
coating, through the interface of the coating and bone, and in
some small regions by delamination of coating from the
metal. In areas where delamination occurred, fragments of
HA were often seen embedded in the metal. The bone in these
regions was so strongly integrated with the coating that it was
capable of pulling the HA off of the metal, leaving the
observed fragments of HA. Where failure occurred through
bone, some bone could be seen on the surface of the HA
coating, again indicating the strength of the bond between
coatings and new bone. In addition to this strong bond, the
tensile strength observed is probably also the result of the
lack of delamination of HA seen on either coated surface.
Figure 4. Light photomicrograph of HA-coated surface. HA is gran-
ular material (C) on black metal surface. Note bone (B) along and
within the HA coating (C) , osteoid seam lined with osteoblasts (O),
bridging bone (BB) forming between trabeculae, with blood vessel
just above bridge. Also note absence of fibrous layer between bone
and coating. Sanderson’s Rapid Bone Stain, ⫻67.
TABLE I. Effect of Alendronate Treatment on Bone Penetration: Percentage of Bone Penetration (ⴞS.E.M.) into Experimental
Channels as Measured with Image Analysis of Digitized Faxitron High-Resolution x Rays of the Chambers
Surface Type
Intact Dogs OVX Dogs
Alendronate No Drug Alendronate No Drug
AD 50.7 ⫾3.05 48.1 ⫾5.25 35.8 ⫾2.41 58.2 ⫾13.28
AD/HA 84.0 ⫾1.05* 75.9 ⫾5.27* 92.0 ⫾3.40* 69.5 ⫾9.01
ChemEtch/HA 78.7 ⫾3.41* 81.9 ⫾2.08* 77.7 ⫾3.62* 67.7 ⫾5.70
Note. AD ⫽arc-deposited commercially pure titanium (CPTi). AD/HA ⫽arc-deposited CPTi with a plasma-sprayed hydroxyapatite coating. ChemEtch/HA ⫽chemically
textured Ti6Al4V with a plasma-sprayed hydroxyapatite coating.
* Significantly greater than AD, p⬍.05.
Figure 5. Backscattered electron image of AD/HA surface: B, bone;
HA, hydroxyapatite. Note integration of bone with coating. Intimate
interdigitation of bone with coating can be seen. Original magnifica-
tion: ⫻100.
648 FRENKEL ET AL.
Mechanical Testing
As indicated in Table II, alendronate had no significant effect
on the strength of bone attachment at the different implant
surfaces. Similarly, alendronate treatment caused no signifi-
cant differences in mechanical testing results between OVX
and intact animals.
There were, however, significant differences in quantity
and strength of bone that depended only upon the type of
surface (Table II). AD/HA surfaces formed the mechanically
strongest bonds with the bone. The ChemEtch/HA-coated
surfaces were somewhat weaker (p⬎.05), with the AD
surfaces significantly weaker than both HA-coated surface
types (p⬍.05). In some AD specimens, there was insuffi-
cient bone ingrowth to provide any tensile strength at the
tissue interface. These specimens collapsed upon removal
from the chamber and could not be tested. They were re-
garded as having zero measurable tensile strength, and were
included in the failure strength analysis.
DISCUSSION
The canine implantable chamber model used in this study has
been employed previously to evaluate the biological response
of intramedullary bone to various implant materials and sur-
faces.
11,12,14,15
The system allows sensitive comparison of the
bone ingrowth response to several different sample surfaces
at the same surgical site under controlled conditions. It has a
significant advantage over plug push-out tests used to test
bone bonding to materials: It permits direct measurement of
adhesion using tensile testing instead of measuring shear
strength, which is often complicated by the surface roughness
of the test material.
11,16
In this study, the model allowed
simultaneous screening of the effect of alendronate on the
bone–implant interface on a variety of simulated implant
surfaces. Implantation was performed 8 weeks post-ovariec-
tomy, based on reports that histological features of the bone
are consistent with those of human osteoporotic bone at this
time.
13
The single endpoint of 24 weeks was based on our
previous findings that maximal penetration by bone, with
some early remodeling, is achieved at this time.
14,15
The typical clinical alendronate dose, administered orally,
is 10 mg per day. The gastrointestinal absorption of alendro-
nate is 0.7%, making the effective absorbed dose approxi-
mately 1.2
g/kg/d based on a 60-kg patient.
17
The canine
dose of 2.5
g/kg dose used in this study was thus twice the
human dose. Subcutaneous injection assured more uniform
delivery of the administered dose.
The degree of osseous ingrowth at the implant–host inter-
face is critical for the long-term survival of the arthroplasty.
Application of a thin HA coating to implant surfaces is
widely recognized as a means of enhancing bone attachment
to the surface.
18–21
A biochemical bond is believed to form
between bone and HA coatings, resulting in improved im-
plant stability.
22
Several investigators have reported that HA
coatings increase mechanical shear strength and bone contact
at the implant interface
23,24
and reduce micromotion of the
implant.
25
AD/HA-lined and ChemEtch/HA channels formed
stronger bonds with the bone than AD-lined channels. AD/
HA-lined and ChemEtch/HA channels did not differ. This is
consistent with the work cited above,
23,24
demonstrating that
HA-coated implant surfaces promote bone ongrowth and
mechanically stronger interfaces. Alendronate treatment did
not influence the strength of bone attachment to any surface.
Bisphosphonates inhibit bone resorption by being incor-
porated selectively into osteoclasts and interfering with the
resorptive activity of osteoclasts.
26–28
Because alendronate
has an estimated half-life in human bone of 8–10 years, Peter
et al.
29
performed a long-term study of its safety in a canine
model. They found that after 3 years of alendronate treatment,
there were no spontaneous fractures and no changes in the
structural properties of femoral or vertebral bone. Another
study of fracture repair during alendronate administration
reported that although there was slower remodeling of the
fracture callus, bone formation and mineralization were not
inhibited, and there were no adverse effects on mechanical
properties.
30
Concurring with these safety results is a recent
study of canine hip arthroplasty followed by 6 months’ alen-
dronate therapy, which found no effect of alendronate on a
wide range of bone biomechanical properties.
31
Approved for use in the treatment of osteoporosis, alen-
dronate may also prove useful in the treatment of wear-
debris–induced osteolysis associated with hip and knee im-
plants. A recent canine study demonstrated that alendronate
inhibited such osteolysis over a 6-month period, despite the
presence of wear particles.
32
A clinical study found that bone
loss due to stress shielding was reduced in patients treated
with alendronate following total hip replacement.
33
In a study
TABLE II. Effect of Alendronate Treatment on Strength of Bone Attachment: Failure Strengths (in Newtons, ⴞS.E.M.) of Samples
Mechanically Tested in Tension
Surface Type
Intact Dogs OVX Dogs
Alendronate No Drug Alendronate No Drug
AD 89.8 ⫾19.7 51.3 ⫾12.9 36.2 ⫾6.0 105.6 ⫾14.8
AD/HA 178.4 ⫾14.4* 153.7 ⫾11.8* 218.2 ⫾10.2* 163.4 ⫾26.5*
ChemEtch/HA 124.5 ⫾17.8 95.5 ⫾11.3 92.9 ⫾29.1 125.2 ⫾17.4
Note. AD ⫽arc-deposited commercially pure titanium (CPTi). AD/HA ⫽arc-deposited CPTi with a plasma-sprayed hydroxyapatite coating. ChemEtch/HA ⫽chemically
textured Ti6Al4V with a plasma-sprayed hydroxyapatite coating.
* Significantly greater than AD, p⬍.05.
649ALENDRONATE AND HYDROXYAPATITE-COATED SURFACES
of therapeutic management measures including calcium and
vitamin D supplementation, estrogen, bisphosphonates, intra-
nasal calcitonin, raloxifene, and fluoride salts, the strongest
evidence for antifracture capability was observed with alen-
dronate.
34
Alendronate would appear to have clinical appli-
cability in several situations related to the problem of limited
survival of uncemented prostheses.
Early stabilization of implants is critical to their long-term
survival. Our results indicate that administration of 2.5
g/kg
of alendronate caused no detectable effect on bone growth or
strength of attachment at the interface during the first 6
months after implant placement. This suggests that recipients
of joint prostheses may undergo alendronate therapy without
affecting the short-term host tissue response to the implant.
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