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Effect of Flexibility of the Femoral Stem on Bone-Remodeling and Fixation of the Stem in a Canine Total Hip Arthroplasty Model without Cement*

E. J. HARVEY, M.D., J. D. BOBYN, PH.D., M. TANZER, M.D., G. J. STACKPOOL, M.D., J. J. KRYGIER, C.E.T. and S. A. HACKING, M.ENG., MONTREAL, QUEBEC, CANADA

Investigation performed at Montreal General Hospital, McGill University, Montreal

	Abstract

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The purpose of this study was to compare, with regard to fixation of the implant and femoral bone resorption, two fully porous-coated stems of different stiffnesses in a canine total hip arthroplasty model. A bilateral arthroplasty was carried out with insertion of a titanium-alloy stem (which had stiffness properties comparable with those of the canine femur) on one side and with insertion of a composite stem (which was three to fivefold more flexible than the canine femur) on the contralateral side. Eight femora were evaluated at six months and eight, at eighteen months after the operation, to determine the extent of bone ingrowth, periprosthetic cortical area, intracortical porosity, and bone-remodeling. Despite the markedly greater flexibility of the composite stems, no significant difference could be detected (with the numbers available), with regard to the overall degree of femoral stress-shielding, cortical area, or cortical porosity, between these stems and the stiffer, titanium-alloy stems at either time-period. However, the composite stems had less bone ingrowth and more formation of radiopaque lines than did the titanium-alloy stems. At eighteen months, the values for bone ingrowth were 9.7 ± 5.38 percent (mean and standard deviation) for the composite stems compared with 28.1 ± 5.31 percent for the titanium-alloy stems (p = 0.003). Furthermore, the histological sections from the femora containing a composite stem showed radiopaque lines indicative of fibrous ingrowth approximately threefold more often than did those from the femora containing a titanium-alloy stem (p = 0.02). CLINICAL RELEVANCE: These findings demonstrate that there seems to be an effective limit beyond which increasing the stiffness of the femoral stem relative to that of the femur cannot prevent femoral stress-shielding. Use of a composite stem with a flexibility that was above this limit resulted in a negligible alteration in femoral bone-remodeling and a significant decrease in the extent of bone ingrowth (p = 0.003). These findings seem to confirm the theory that excessive flexibility of the stem may jeopardize interface stability. This information may be helpful in the design of femoral prostheses for clinical use.

	Introduction

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Femoral bone resorption that occurs secondary to stress-shielding has been investigated extensively since the development of porous-coated prostheses for insertion without cement in total hip arthroplasty. In the early 1970s, studies of animals revealed the potential for marked bone loss adjacent to well fixed bone-ingrown femoral prostheses^4,8,9. Several decades later, there is greater understanding of the incidence, frequency, and extent of stress-related bone resorption in the femur^7,8,10,24,26, and factors that can potentiate this problem have been identified^3-5,8. The clinical success of arthroplasty without cement on the femoral side may depend largely on the ability to achieve good fit and fill of the implant within the host bone in order to maximize initial stability and the potential for bone ingrowth. This requirement typically results in the use of implants that are larger and stiffer than those that are used in total hip arthroplasty with cement; thus, the potential for greater stress-shielding is increased.

The principal factor that increases the extent of periprosthetic bone resorption has been shown, both experimentally and clinically, to be the relative stiffness of the stem and the femur^2,4,6,8. Stiffness of both the femur and the implant is a function of material products and geometry. Comparison of the stiffness characteristics of human femora with those of stems designed to be inserted without cement has shown that the distal portion of small stems (those with a diameter of not more than twelve or thirteen millimeters), made of either cobalt-chromium or titanium alloy, is substantially less stiff in bending than the average human femur. Because smaller-diameter stems cause considerably less resorption than larger implants do, it has been empirically deduced that this mechanical relationship is a desirable design objective. As the diameter of the stem increases, the stiffness parameters increasingly exceed those of the femur and the tendency for more marked resorption increases.

The design of implants with decreased stiffness, particularly those with larger diameters, should allow maximum bone retention by decreasing bone resorption. One of the more interesting approaches to the design of a flexible femoral implant involves the use of composite construction. One such type of composite design consists of a forged cobalt-chromium core coated by a layer of polyaryletherketone that is completely enveloped by titanium porous fiber metal³⁰. The cobalt-chromium core provides the bulk-strength requirements while the fiber metal provides a surface for bone ingrowth. The sandwiched layer of polyaryletherketone allows modulation of overall stiffness of the stem by increasing the layer's thickness with increases in the stem's size. The theoretical advantages of this composite stem are that it has a conventional material for taper-locking of the modular head; a conventional, relatively abrasion-resistant metal for bone contact and ingrowth; and a construction that can be adjusted for proximal and distal mechanical compatibility with the femur along the entire range of stem sizes.

With use of this composite technology, it is possible to create high-strength stems that are substantially less stiff than the femur. A femoral stem with low stiffness might allow for maximum bone ingrowth with minimum adverse bone-remodeling. However, given that excessive flexibility could result in bone-implant interfacial shear stresses that mitigate against bone ingrowth^12,13, it is necessary to balance the competing objectives of minimum bone-remodeling and maximum fixation of the implant. The purpose of this study was to compare, with regard to fixation of the implant and femoral bone resorption, two fully porous-coated stems of widely different stiffnesses in a total hip arthroplasty model in dogs.

	Materials and Methods

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Design of the Femoral Implants
A collarless femoral stem was chosen on the basis of a radiographic study of femora in dogs. The stem was straight with a cylindrical cross section distally and a generally tapered metaphyseal section that had a slight anteroposterior flare and a straight lateral aspect (Fig. 1). The length of the stem was ninety millimeters, and the diameter was ten millimeters. Commercially pure titanium fiber-metal was diffusion-bonded to the stem from the shoulder to the tip. The extent of this porous coating was chosen to maximize bone ingrowth and thus increase the potential for stress-related bone resorption. The neck was machined to a number-3 Morse taper to accommodate a modular sixteen-millimeter cobalt-chromium-alloy head with a neck length of zero or three millimeters.

View larger version (155K): [in this window] [in a new window]   FIG1: Fig. 1 Photograph showing the titanium-alloy stem (left), the composite stem (right), and the modular metal-backed acetabular cup that were used in the total hip arthroplasty.

Cross sections of the implants, obtained at one-centimeter intervals, were analyzed with regard to area and moments of inertia in order to compare the structural stiffness characteristics of the two types of stems. Calculations of bending stiffness were based on the product of the modulus of elasticity and the moment of inertia. The individual contributions to stiffness made by the solid-core material (titanium or cobalt-chromium alloy), intermediate polymer, and outer fiber metal (commercially pure titanium) were added to obtain the total stiffness (Table I). Cross sections of femora from six different dogs, in the size-range appropriate for implantation of a porous-coated prosthesis without cement, also were obtained at one-centimeter intervals for calculation of stiffness.

View larger version (27K): [in this window] [in a new window]   FIG2: Fig. 2 Graph showing bending stiffness in the coronal plane for the titanium-alloy stems and the composite stems as well as the range for six canine femora at one-centimeter intervals from the shoulder of the implant to the tip. The data were derived from stiffness calculations of cross sections on the basis of beam theory (modulus of elasticity x area moment of inertia).

Experimental Protocol
In order to allow for a direct comparison between the osseous response to the titanium-alloy femoral stems and the osseous response to the cobalt-chromium-polyaryletherketone-composite femoral stems, a bilateral total hip arthroplasty was performed, in stages three to four weeks apart, with insertion of one prosthesis of each type in each animal. The dogs were carefully screened radiographically to determine the appropriate size and shape of the femoral component. An effort was made to include only animals in which a press-fit or near press-fit of the distal aspect of the stem within the intramedullary canal was likely. The titanium-alloy stem and the composite stem were used alternately in the first operative procedure to eliminate a time-related bias in the results.

The operation was performed with use of standard sterile techniques. The hip joint was exposed through a modified anterolateral approach. After a lateral skin incision had been made, the interval between the gluteus medius and the tensor fasciae latae was opened to expose the anterior aspect of the hip capsule. The gluteus medius and the vastus lateralis were elevated with a small flake of bone off the anterior portion of the greater trochanter.

After an anterior capsulotomy and sectioning of the ligamentum teres, the hip was partially dislocated and a provisional osteotomy of the femoral neck was performed. The acetabulum was exposed, excess soft tissue was resected, and the transverse acetabular ligament was partially excised. The acetabulum was progressively reamed under irrigation to a diameter of twenty-seven millimeters in order to create a one-millimeter press-fit. The metal backing of the component was impacted into position and was secured with one or two screws before insertion of the polyethylene liner.

Preparation of the femur started with insertion of a small tapered curet into the piriformis fossa in order to broach the femoral canal. The intramedullary canal was progressively enlarged in 0.5-millimeter increments to a diameter of 9.5 millimeters, creating a 0.5-millimeter press-fit for the stem. The femoral canal was rasped to prepare the metaphyseal region, and the stem was impacted forcefully into position. A modular femoral head was attached to the neck taper, and the hip was reduced. The trochanteric bone flake was reattached with two interrupted transosseous nonresorbable sutures. The incision was closed in a standard three-layer fashion with use of resorbable sutures.

Four dogs (eight hips) were studied for six months postoperatively, and four dogs (eight hips) were studied for eighteen months postoperatively. Previous studies of total hip arthroplasty in dogs have shown little change in the tissue response to a functioning implant, in terms of the extent of both tissue ingrowth and periprosthetic remodeling, after six months^2,28,29. Twelve to eighteen months is sufficient for evaluation of the end point of bone-remodeling^3,4,28,29. Anteroposterior and lateral radiographs of each hip were made before and within the first week after the operation. Subsequent radiographs were made at six-month intervals and at the end of the study period. The radiographs were inspected for canal fill (defined as the extent of filling measured at the isthmus), resorptive bone-remodeling, bone ingrowth, and regions of formation of radiopaque lines that would indicate fibrous encapsulation. Radiographic characteristics of the femur were documented by location according to the seven contiguous zones described by Gruen et al.¹¹. Radiographs of the acetabular cup were examined for the presence or absence of periprosthetic radiolucency.

The animals were killed with an intravenous dose of Nembutal (pentobarbital). The femora and acetabula were removed and stripped of soft tissue, and contact radiographs were made with high-resolution film and use of a Faxitron apparatus (Hewlett-Packard, San Diego, California). The specimens were fixed in 70 percent ethanol and were processed for histological analysis of undecalcified thin sections. This involved dehydration in ascending solutions of ethanol, defatting in ether and acetone, and embedding in methylmethacrylate. Small holes were drilled in the femora and acetabula in locations that were not critical in order to facilitate penetration of the various solutions.

The femora were sectioned transversely at one-centimeter intervals, starting at the shoulder of the implant. Sections approximately two millimeters thick were obtained at nine intervals. The intervals were subdivided to correspond with the seven zones of Gruen et al.¹¹ (Table II). The ninth interval was not included in the data analysis because it represented the nonporous tip of the stem. Additional thinning and polishing were performed before sputter-coating with gold-palladium to enable analysis with backscattered scanning electron microscopy. Sections from the third through seventh intervals were analyzed with regard to the volume fraction of bone ingrowth, defined as the percentage of the available porous space that was filled with new bone. Care was taken to account for the reduced thickness of the porous coating available for ingrowth in the composite implants, which was due to the partial infiltration with polyaryletherketone polymer. The paired bone-ingrowth data were compared between the two stem designs at each time-period. Statistical significance was assessed with use of analyses of variance and two-tailed paired Student t tests.

The serial thin sections from both femora were paired at each level, and radiographs were made. Computer-aided image analysis was used to digitize and quantify the cortical area defined by the periosteal and endosteal envelopes of each section. Paired comparisons of the cortical area were made between the titanium-alloy stems and the composite stems at each one-centimeter interval and were averaged for each time-period. This provided a macroscopic measurement of periprosthetic bone-remodeling. The depth of bone ingrowth was not calculated. Differences were analyzed for significance with use of analyses of variance and two-tailed paired Student t tests. The sections from the third, fifth, and seventh intervals were analyzed to determine intracortical porosity, a histological parameter of resorptive bone-remodeling. The mean intracortical porosity at each interval was compared between the two stem designs with use of two-tailed Student t tests. A Bonferroni adjustment was made to compare the overall values. For all statistical analyses, the level of significance was p < 0.05.

The acetabular cup was cut coronally to produce three one-millimeter-thick sections for contact radiography and scanning electron microscopy. The implants were categorized as those with a stable interface, denoted by multiple sites of bone ingrowth, or those with uncertain functional stability, denoted by continuous fibrous-tissue encapsulation.

	Results

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Clinical Function and Fixation of the Acetabular Cup
After the operation, the dogs were allowed unrestricted activity in their cages. Limited walking usually began within two or three days, and full activity was typical within two to three weeks. The animals were allowed one hour of daily unrestricted activity, including running and jumping, in a large run.

There were three early postoperative dislocations. All necessitated open reduction, which was successful without additional complications.

Careful attention was paid to the function of the limbs and signs of the animal favoring one limb or limping. Six animals had a normal gait pattern throughout the study period. Two dogs—one that was evaluated at six months (Dog 4) and one that was evaluated at eighteen months (Dog 8)—limped on the side with the composite stem (Table III). The animal in the six-month group had a slight limp, which was noticeable during the middle two months of the study period. The animal in the eighteen-month group had periodic episodes of more obvious limping between nine and eighteen months after the operation. In both animals, the acetabular cup on the side with the composite stem was surrounded by a continuous radiolucent line and was encapsulated by fibrous tissue, a finding consistent with the gait abnormality. The other fourteen acetabular implants appeared stable radiographically (Table III) and were found to be fixed by bone ingrowth on histological analysis.

Radiographic and Histological Findings

Bone Ingrowth
Radiographically, all implants had characteristic signs of bone ingrowth in various regions along their length. There were no apparent differences in the amount or distribution of bone ingrowth between the six and eighteen-month evaluations. Densification of bone immediately adjacent to the porous fiber metal, suggestive of bone ingrowth and load transfer, was observed locally at both time-periods (Figs. 3 and 4). Marked new endosteal bone formed about the tip of both the titanium-alloy and the cobalt-chromium-composite stems in three animals (Figs. 4 and 5-A).

View larger version (88K): [in this window] [in a new window]   FIG3: Fig. 3 Anteroposterior radiograph of the femora of Dog 2, made at six months, showing a close intramedullary fit of both implants. The small cortical defects seen medially are drill-holes that were used to facilitate penetration of fixatives. There is little apparent difference in bone-remodeling between the two stem designs, with the exception of a slightly greater loss of proximal medial density on the side with the composite stem (right).

View larger version (97K): [in this window] [in a new window]   FIG4: Fig. 4 Anteroposterior radiograph of the femora of Dog 7, made at eighteen months, showing two-millimeter undersizing of both implants. There is more marked proximal medial bone resorption in association with the titanium-alloy stem (left). The thickness of the diaphyseal bone is comparable between the two sides. Proximal and distal drill-holes for histological processing are evident. There is marked bone densification near the tip of both stems and some endosteal bone densification near the tip of both stems and in the middle region of the titanium-alloy stem.

View larger version (87K): [in this window] [in a new window]   FIG5-A: Fig. 5-A Anteroposterior radiograph of the femora of Dog 6, made at eighteen months, showing one-millimeter undersizing of both stems. There is more proximal medial resorption in association with the titanium-alloy stem (left).

View larger version (90K): [in this window] [in a new window]   FIG5-B: Fig. 5-B Paired serial transverse cross sections from Dog 6, showing more marked proximal resorption in association with the titanium-alloy stem (left) at the third and fourth intervals (two middle pairs). Additional bone densification is seen adjacent to the titanium-alloy stem in the most distal pair.

View larger version (52K): [in this window] [in a new window]   FIG6: Fig. 6 Paired serial transverse cross sections from Dog 2, made at six months, showing a stable interface on both sides. Faint partial radiopaque lines have formed around both stems in some sections. Overall, there is little difference in cortical area between the femora.

View larger version (57K): [in this window] [in a new window]   FIG7: Fig. 7 Paired serial transverse cross sections from Dog 3, made at six months, showing a stable interface and slightly more cortical thinning in association with the titanium-alloy stem (left) at the most proximal three intervals. A slight medial periosteal reaction is present distally on the side with the composite stem.

View larger version (96K): [in this window] [in a new window]   FIG8-A: Fig. 8-A Scanning electron micrographs of a section from a titanium-alloy stem (Fig. 8-A) and a section from a composite stem (Fig. 8-B), from a dog evaluated at six months, showing bone ingrowth arising from new endosteal-bone formation. There is shallower ingrowth in the composite stem because the inner half of the fiber-metal porous coating has been infiltrated with polyaryletherketone (not visible with backscattered scanning electron microscopy) to create a bonded interface.

View larger version (91K): [in this window] [in a new window]   FIG8-B: Fig. 8-B Scanning electron micrographs of a section from a titanium-alloy stem (Fig. 8-A) and a section from a composite stem (Fig. 8-B), from a dog evaluated at six months, showing bone ingrowth arising from new endosteal-bone formation. There is shallower ingrowth in the composite stem because the inner half of the fiber-metal porous coating has been infiltrated with polyaryletherketone (not visible with backscattered scanning electron microscopy) to create a bonded interface.

Despite signs of gross stability of the implant and bone ingrowth, the plain radiographs of some femora revealed isolated regions in which a distinct, thin, generally noncontinuous radiopaque line was present. Analysis of the titanium-alloy stems revealed a radiopaque line in one zone of Gruen et al.¹¹ in two femora and in two zones in two femora. Analysis of the composite stems demonstrated a radiopaque line in six of the eight femora, four of which had a line in at least two zones.

Examination of the cross sections for the presence of radiopaque lines more clearly revealed different reactions to the two stem designs (Table III). Five of the eight femora containing a titanium-alloy stem had a partial radiopaque line in at least one cross section. Six (21 percent) of the twenty-eight sections had a radiopaque line at six months compared with four (14 percent) at eighteen months. In none of these sections was the radiopaque line continuous around the entire circumference of the implant. In contrast, all eight femora containing a composite stem had a radiopaque line in at least one cross section (Figs. 9, 10-A, 10-B through 10-C). Thirteen (46 percent) of the twenty-eight sections had a radiopaque line at six months; with the numbers available, this rate was not found to be significantly higher than that seen in association with the titanium-alloy stems. Fifteen sections (54 percent) had a radiopaque line at eighteen months; this rate was significantly higher than that associated with the titanium-alloy stems (p = 0.02). In six sections, the radiopaque line was continuous around the implant. There tended to be more radiopaque lines on the side containing the composite stem in the dogs that had less canal fill. All dogs that did not have filling of the canal had three or four more sections with a radiopaque line on the side with the composite stem compared with the side with the titanium-alloy stem (Fig. 9).

View larger version (89K): [in this window] [in a new window]   FIG9: Fig. 9 Paired serial transverse cross sections from Dog 7, in which both stems were undersized by two millimeters, showing the difference in the tissue response between the stems. Bone-bridging and bone ingrowth are evident primarily on the side with the titanium-alloy stem (left), whereas prominent radiopaque lines developed adjacent to the composite stem (right).

View larger version (88K): [in this window] [in a new window]   FIG10-A: Figs. 10-A and 10-B: Backscattered scanning electron micrographs of Dog 3, made at the third interval at six months. Fig. 10-A: There is bone-bridging and bone ingrowth on the side with the titanium-alloy stem.

View larger version (80K): [in this window] [in a new window]   FIG10-B: Fig. 10-B There is a radiopaque line on the side with the composite stem (see Fig. 7).

View larger version (83K): [in this window] [in a new window]   FIG10-C: Fig. 10-C Backscattered scanning electron micrograph of Dog 8, made at eighteen months, showing a prominent radiopaque line around a composite stem. The more extensive intracortical porosity is probably due to disuse atrophy caused by an uneven gait.

Bone-Remodeling
The immediate postoperative anteroposterior radiographs indicated that, in general, the diaphyseal portion of the femoral canal was well filled by the implant (Table III). In ten of the sixteen femora, the stem either completely filled the canal or was within one millimeter of the endosteal cortex (Figs. 3 and 5-A). In the remaining six femora, the stem was undersized by 1.5 to two millimeters (Fig. 4).

On the basis of plain radiographs, the patterns of bone-remodeling were generally similar at six and eighteen months. Within three to six months after the operation, there was clear radiographic evidence of a localized loss of cortical density or thinning in eight femora. This was evident in the proximal medial cortex (zone 7) and, to a lesser degree, in the proximal anterior cortex. Bone loss in zone 7 was minor or was similar in the two femora in two animals at six months and in two animals at eighteen months. In one animal that was evaluated at six months bone loss was slightly more marked in association with the composite stem, and in one animal that was evaluated at eighteen months it was much more evident in association with the titanium-alloy stem (Figs. 5-A and 5-B).

The radiographs of the cross sections revealed some variability in the bone-remodeling response between animals but demonstrated no general differences between time-periods. Six of the twelve paired femora had only minor resorptive changes along the entire length of the stem (Fig. 6). The other six femora had more noticeable resorption (Figs. 5-B and 7). Proximal anterior and medial resorption (in zone 7) was typically more marked than the resorption in the other zones. The thin sections clearly revealed marked localized cortical thinning, leading to exposure of the anteromedial edge of the titanium-alloy implant in two animals and the anteromedial edge of the composite implant in another animal (Figs. 5-B and 7). Other than these three instances of local resorption, there was no full-thickness cortical resorption in any zone of any femur. The two femora in which the composite stem was paired with a fibrous-encapsulated acetabular cup showed a generalized loss of cortical thickness around the stem in zones 2, 3, 5, and 6. In the dog that was evaluated at eighteen months, cortical thinning was also present distal to the level of the tip of the stem, a finding that is consistent with disuse atrophy from unequal limb-loading.

The intracortical porosity of the cortical bone surrounding the implants was highest proximally (range, 6.6 to 7.1 percent) and lowest distally (range, 1.6 to 1.9 percent); however, with the numbers available, the difference between the femora containing a titanium-alloy stem and those containing a composite stem was not found to be significant at any of the three intervals that were analyzed (Fig. 11).

View larger version (18K): [in this window] [in a new window]   FIG11: Fig. 11 Bar graph showing the percentage of intracortical porosity as a function of the type of stem and the interval at which the porosity was evaluated.

	Discussion

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Two types of fully porous-coated femoral stems that had identical dimensions but different structural stiffnesses were compared in the current study. The titanium-alloy stem had a stiffness comparable with that of the canine femur, whereas the cobalt-chromium-polyaryletherketone-composite stem was threefold less stiff than the canine femur proximally and four to fivefold less stiff distally. Use of the composite stem resulted in no clear decrease in stress-shielding compared with that produced by the titanium-alloy stem. An increase in the flexibility of the stem beyond that provided by the titanium-alloy stem resulted in a negligible alteration in femoral bone-remodeling and a decrease in the extent of bone ingrowth. Finite-element analysis of stress-shielding relationships between femoral prostheses and surrounding bone has indicated that the extent of periprosthetic stress-shielding, and hence the expected level of bone resorption, can be increasingly attenuated with increasing flexibility of the stem^12,13,31. However, in the same studies it was predicted that the stresses between the implant and the bone that are due to interfacial shear motion also will increase with increasing flexibility of the stem. Excessive motion at the interface may jeopardize the stability of the implant. Therefore, a balance must be struck between the competing objectives of stress-shielding and interface stability.

The stiffness relationship between the implant and the femur can be adjusted by changing either the material or the geometry. Titanium-alloy implants, for example, have approximately one-half the stiffness of cobalt-chromium-alloy implants because of a twofold difference in the elastic modulus. Both radiographic and quantitative evaluations of bone-mineral density with use of dual-energy x-ray absorptiometry have revealed that titanium-alloy stems fixed without cement are associated with less periprosthetic bone resorption than are cobalt-chromium stems fixed without cement^17,18,24. Inevitably, even with use of titanium alloy, larger stems are quite stiff relative to the femur and cause more resorption. Geometric features such as flutes, grooves, or slots help to reduce stiffness by decreasing the cross-sectional area or moment of inertia. However, there are practical limitations to the extent that the geometry of all-metal femoral stems can be modified. Because of strength limitations of surgical alloys, particularly after the heat treatments required for bonding of porous coatings, stiffness-reduction characteristics have limited applicability. The metaphyseal portion of the stem cannot tolerate meaningful geometric modification (such as the creation of grooves) without fatigue strength being jeopardized. Similarly, flutes or slots, which can reduce the geometric contribution to stiffness by as much as 25 percent in larger stems, can be incorporated only into the distal few centimeters of the stem because of the need to preserve mechanical properties in the middle third.

For many years, attempts to reconcile the divergent goals of flexibility of the stem and strength have focused on composite-materials technology^22,25. It must be recognized, however, that increased flexibility of the implant may increase the shear stresses at the bone-implant interface during loading, and this in turn could create differential motion between the implant and the bone and cause loosening or mechanical abrasion of the implant. Polymer-coated prostheses originated with the isoelastic design of Mathys, which consists of a stem with a stainless-steel core and a thick casing of polyacetyl resin²⁰. Although the results of use of this design have been favorable in some short-term follow-up studies^1,20, rates of aseptic mechanical loosening as high as 32 percent (eleven of thirty-four) within a mean of forty-two months after the operation also have been reported¹⁵. Instability of the stem and the formation of radiopaque lines were attributed to excessive flexibility of the implant, even when there had been an initial press-fit within the intramedullary canal¹⁵. Clinical trials with use of a similar design, a titanium stem coated with a relatively thin layer of porous polysulfone (a high-strength thermoplastic material with well documented bone-ingrowth characteristics), generally were not successful²¹, and high rates of revision, evidence of mechanical failure of the polymer coating, and associated osteolysis led to the abandonment of this design²¹.

Carbon-carbon and carbon-polymer-composite materials have been studied extensively as candidates for a new generation of implants^14,19,22,25. Their potential advantage is their combination of high fatigue strength and elastic moduli that are closer to those of bone than to those of metals. These materials can be fabricated in nonhomogeneous and nonisotropic combinations so that structural stiffness can be varied within different regions of the implant and more closely matched to that of bone. However, recent clinical trials with use of a carbon-fiber-polysulfone-composite femoral component revealed problems with stability²¹. Roentgen stereophotogrammetric analysis of the first eleven implants indicated progressive subsidence and rotation of all of the implants for as long as three years after the operation²¹.

Overall, the tissue response to the composite stems in the present study resulted in less bone ingrowth and more radiopaque lines than did the tissue response to the titanium-alloy stems. Only at the more distal intervals, where the stem was closely apposed to (and thus supported by) endosteal bone, was the bone-ingrowth response similar between the two stem designs. It has been well documented that excessive motion at the bone-implant interface of porous-coated femoral stems results in fibrous-tissue ingrowth rather than osseointegration^16,23. Radiographically, this is manifested by a thin radiopaque line that is adjacent to but separate from the ingrowth surface. It is interesting to note that none of the composite stems were completely encased in fibrous tissue or surrounded by a radiopaque line. Instead, the radiopaque lines had a tendency to form in diaphyseal regions, where the differences in stiffness between the stem and the femur were maximum. Transverse sections showing partial bone ingrowth and incomplete radiopaque lines suggested that deformation of the composite stem can vary enough locally to result in a differential tissue response.

The development of radiopaque lines was more marked in femora that had incomplete filling of the canal. This suggests that stems with low stiffness characteristics are more susceptible to excessive motion and failure of bone ingrowth in association with suboptimum fit and fill of the canal; that is, they rely more on structural support from the femur to stabilize the bone-implant interface. Although the goal of all procedures in which the implant is inserted without cement is to fill the canal and maximize initial stability, this is not always possible, despite careful preoperative planning and operative technique. Thus, stems with a distal stiffness that is four to fivefold lower than that of the femur appear to be at higher risk for fibrous as opposed to osseous integration. It is possible that suboptimum fixation of a low-stiffness stem, resulting from extensive fibrous ingrowth in the diaphysis, could cause favoring of one limb and bone resorption due to disuse atrophy.

Another important finding was that the adverse effect of the composite stem on the bone-implant interface was not associated with a meaningful gain in periprosthetic bone retention. Only at six months was there a significant difference (p = 0.03) between the stem designs with regard to the cortical area over a portion of the bone-implant construct (at the fourth through eighth intervals), and there was more bone in the femora with the titanium-alloy stems. Other differences at six and eighteen months were relatively small and were not found to be significant, with the numbers available. Intracortical porosity was similar between the two types of stems. This is an important finding because it indicates that there is a limit to the beneficial effect of increased flexibility of the stem on resorptive bone-remodeling. It would be counterproductive to design implants in order to achieve minor gains in bone retention at the risk of interface stability.

In a previous study, implantation of fully porous-coated cobalt-chromium stems that were three to fourfold stiffer than the canine femur resulted in marked periprosthetic cortical thinning and extensive regions of full-thickness diaphyseal cortical resorption^2,4. In the present study, with the exception of localized proximal medial and anterior cortical thinning in some femora, the amount of bone resorption surrounding the fiber-metal-coated titanium-alloy stems was insubstantial. None of the femora had full-thickness cortical resorption. These findings are comparable with earlier findings in a canine study in which porous-beaded hollow titanium stems were used⁴. The titanium-alloy implants in the present study and the hollow implants in the earlier study had stiffness characteristics that were generally the same as or slightly lower than those of the canine femur. The results of the current study indicate that the additional flexibility of the composite stems was not particularly helpful in decreasing resorptive bone-remodeling.

The findings with regard to bone-remodeling in the present study are not in complete agreement with those of a previous comparison of the same two stem designs in a unilateral hemiarthroplasty model in dogs; in that study, there was 30 to 50 percent less cortical resorption in the proximal regions around the composite stem³⁰. It is difficult to know if this difference was due to differences between dogs that could result from use of a unilateral as opposed to a bilateral model. No details of the extent of filling of the canal by the stems among the different animals were provided for the unilateral model. In addition, there were no comparisons of the extent of bone ingrowth between the two stem designs and no details about the formation of radiopaque lines; both of these factors could substantially alter patterns of periprosthetic bone-remodeling.

Because of the strength limitations of metal alloys, only composite technology provides the latitude necessary to overcome the mismatches in stiffness between the stem and the femur that occur in association with larger implants. Unlike metal stems, which have substantially greater stiffness proximally compared with distally, composite technology enables both distal and proximal stiffness to be maintained at low levels relative to the femur. It is this unique property that has led to the use of composite technology in the clinical setting, in the hope that the benefits of hip arthroplasty without cement can be maximized while the risk of stress-shielding is minimized. However, this study indicates that there is a limit to which flexibility of the stem can be increased if the optimum balance between bone-remodeling and bone ingrowth is to be achieved.

NOTE: The authors thank K. Smith and U. Kanaan, B.Eng., for their invaluable technical assistance.

View larger version (20K): [in this window] [in a new window]   FIG12: Fig. 12 Graph showing the percentage difference in the cortical area of the cross sections of the composite stems relative to that of the titanium-alloy stems at six and eighteen months. The data are presented according to one-centimeter intervals 2 through 8 as measured from the shoulder of the stem. The data point for each interval at each time-period was derived from the mean cortical area of three histological cross sections (I-bars = standard deviations).

	Footnotes

	References

Top Abstract Introduction Materials and Methods Results Discussion References

 

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