This Article Abstract Full Text (PDF) Free Letters to the Editor: Submit a response Alert me when this article is cited Alert me when Letters to the Editor are posted Alert me if a correction is posted Services E-mail this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Add to My File Cabinet Download to citation manager Rights and Permissions Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by TURNER, T. M. Articles by GALANTE, J. O. Search for Related Content PubMed PubMed Citation Articles by TURNER, T. M. Articles by GALANTE, J. O. Social Bookmarking                   What's this?

Maintenance of Proximal Cortical Bone with Use of a Less Stiff Femoral Component in Hemiarthroplasty of the Hip without Cement. An Investigation in a Canine Model at Six Months and Two Years*

T. M. TURNER, D.V.M., D. R. SUMNER, PH.D., R. M. URBAN, , R. IGLORIA, M.S. and J. O. GALANTE, M.D., CHICAGO, ILLINOIS

Investigation performed at Rush Institute of Arthritis and Orthopedics, Department of Orthopedic Surgery, Rush-Presbyterian-St. Luke's Medical Center, Chicago

	Abstract

Top Abstract Introduction Materials and Methods Results Discussion References

 
A canine model of hemiarthroplasty of the hip was used to determine if the use of a less stiff femoral stem can reduce the amount of bone loss induced by stress-shielding. Two types of stem were used: the stiffer stems were made of a titanium alloy, and the less stiff stems were composed of a cobalt-chromium-alloy core with an outer polymer layer. The stems were identical in shape, and both types were circumferentially coated along their entire length (except for the distal five millimeters) with commercially pure titanium fiber metal. Ten dogs with each type of stem were followed for six months, and twelve dogs with each type of stem were followed for two years. Loss of cortical bone from the proximal part of the femur was associated with both types of stem, but typically 50 per cent less bone was lost with the less stiff implants. Most of the cortical loss occurred at the subperiosteal surface. The amount of medullary bone adjacent to the proximal and distal aspects of both types of stem increased; the less stiff stems were associated with a greater increase in the proximal region, and the stiffer stems were associated with a greater increase in the distal region. Similarly, there were peaks in the amount of bone growth into the proximal and distal portions of both types of stem, with a greater peak in proximal bone growth into the less stiff stems and a greater peak in distal bone growth into the stiffer stems. CLINICAL RELEVANCE: The data indicate that an effective means of promoting proximal transfer of load from the implant to the host femur is to reduce the stiffness of the stem. This is a design-related mechanism to decrease stress-shielding, thereby suppressing the loss of bone in the proximal part of the femur following hemiarthroplasty of the hip performed without cement. Such bone loss can eventually lead to loosening of the component and make subsequent reconstruction difficult.

	Introduction

Top Abstract Introduction Materials and Methods Results Discussion References

 
With many of the total hip replacement prostheses that are currently available for use without cement, the femoral component becomes attached to the host by growth of bone into a porous surface¹⁸. Such stems tend to be larger than those designed to be inserted with cement so that the contact with the host bone and the initial stability of the implant are sufficient to permit biological fixation¹⁰. Thus, the proportion of the medullary cavity that is filled with high-modulus metal is greater than that after a procedure in which polymethylmethacrylate, which has a relatively low modulus, has been placed between the metallic implant and the host bone. A consequence is that a stem inserted without cement generally causes more stress-shielding (a reduction in the mechanical stress on the adjacent bone) than does a cemented stem. This factor often is noted to explain why there is more atrophy of the proximal part of the femur following insertion of a stem without cement than following insertion of a stem with cement¹⁰. This reduction in the bone mass is of concern because it may ultimately cause a loss of fixation and future reconstruction may be difficult.

Factors related to the design of the implant (the size, the stiffness, and the extent of the porous coating), related to the patient (age and initial bone mass), or related to both (the relative stiffness of the implant and host bone) have been found to affect the amount of bone loss following total hip arthroplasty^5,6. We previously reported, on the basis of research with a canine model, that the loss of bone from the proximal part of the femur cannot be reduced significantly by altering the type or location of the porous coating or even by eliminating the porous coating altogether^22-24.

One proposed approach to modulate the bone loss induced by stress-shielding is to use a less stiff stem¹⁵. With stems of this design, load transfer should be shifted proximally and, consequently, bone loss in this region would be expected to decrease. The efficacy of this design concept for preserving the cortical bone in the proximal part of the femur was investigated previously in two animal models of total hip arthroplasty without cement^1,14. In each study, less cortical bone was lost when a low-modulus stem was used; the loss was reduced at the subperiosteal and endocortical surfaces in one study¹ and at the haversian surface in the other¹⁴.

The purposes of the present study were (1) to determine, with use of a canine model, if it is possible to reduce the amount of cortical bone lost following hemiarthroplasty of the hip by using a less stiff porous-coated femoral component inserted without cement; (2) to identify the surfaces at which the cortical bone mass changes; and (3) to determine if structural changes in the periprosthetic cancellous bone and the amount and distribution of bone ingrowth are influenced by the stiffness of the stem.

	Materials and Methods

Top Abstract Introduction Materials and Methods Results Discussion References

 
A less stiff or a stiffer fully porous-coated femoral component was implanted unilaterally without cement in two separate groups of dogs. One group was followed for six months and the other, for two years. At the final examination, the bone growth into the porous coating was measured and the cortical area, the cortical bone area fraction, and the medullary bone area fraction in the operatively treated femur were compared with those in the contralateral (control) femur. Forty-four dogs were studied: ten with each type of implant were evaluated at six months and twelve with each type, at two years. The experiment was performed in accordance with institutional and federal guidelines.

Implants
Eighty-eight-millimeter-long stems were used (Fig. 1). The implants were circumferentially porous-coated along the entire length of the stem, except for the distal five millimeters, with a 1.5-millimeter-thick fiber-metal surface made of commercially pure titanium. The substrate of the stiffer stem was a titanium alloy (Ti-6Al-4V), onto which the porous surface had been diffusion-bonded. The less stiff stems were composed of a cast cobalt-chromium-alloy core surrounded by a layer of polyaryletherketone. The porous coating was heat-staked into the polymer in the stems studied at six months, and it was attached to the polymer by an injection process in the stems studied at two years. There were seams of polymer on the medial and lateral faces of the proximal part of the less stiff stems; these seams separated the anterior and posterior fiber-metal pads, were approximately one millimeter wide, and were confluent with the outer aspect of the fiber-metal surface. The corresponding areas of the titanium-alloy stems had slight recesses reaching the substrate.

View larger version (80K): [in this window] [in a new window]   FIG1: Fig. 1 Photograph of the less stiff (left) and stiffer (right) femoral stems, which had the same shape and were both circumferentially porous-coated along the entire length, except for the distal five millimeters.

Operative Procedures
Conditioned large adult male mongrel dogs first were screened radiographically to ensure that the femoral canal was an appropriate size, that the congruity of the hip was normal, and that skeletal maturity had been reached. The details of the operative procedure have been described previously²³. Briefly, each implant was press-fit into a precisely prepared cavity in the proximal part of the femur. The medullary cavity was reamed to a size that was one millimeter smaller than the external dimensions of the implant. A hemiarthroplasty was done in each dog to avoid the generation of polyethylene wear debris. All of the femoral heads were made of cobalt-chromium alloy and were available in different diameters (in one-millimeter increments); the size was chosen to match that of the acetabulum. The head was affixed to the locking taper at the proximal part of the stem after the stem had been impacted.

Follow-up Procedures
The dogs were housed in individual runs, and their activity was not restricted. Cephalexin (one gram) was given intraoperatively and was administered once every eight hours for the first five postoperative days. The dogs were assessed daily and were evaluated clinically once a week. Radiographs were made preoperatively, immediately postoperatively, and at one, three, and six months; the dogs that were followed for two years also had radiographs made at twelve, eighteen, and twenty-four months. The dogs were killed with a lethal intravenous injection of a supersaturated solution of barbiturates.

Preparation and Analysis of the Samples
Comparable sections were obtained from the operatively treated femur and the contralateral (control) femur¹⁹. Each femur was sectioned perpendicular to the long axis at one-centimeter intervals, beginning one centimeter proximal to the lesser trochanter and extending two centimeters inferior to the distal tip of the stem (Fig. 2). Undecalcified specimens were processed for histological analysis and for the measurement of the cortical area (the area between the subperiosteal and endocortical surfaces)^19,21, the cortical bone area fraction, the medullary bone area fraction, and bone ingrowth^22,23. The absolute amounts of bone tissue gained or lost at the subperiosteal, endocortical, and haversian surfaces were calculated from the measurements of the cortical area and the cortical bone area fraction²².

View larger version (17K): [in this window] [in a new window]   FIG2: Fig. 2 Drawing showing the location of the sections. The cortical area, the cortical bone area fraction, and the medullary bone area fraction were determined in sections C, D, F, I, and K. Bone ingrowth was measured in sections C, D, F, and I. Intermediate sections (B, E, G, H, and J; not shown) were also made, and the histological analysis of the interface was carried out with use of all sections.

At the completion of the study, the tibial bone-mineral content in the operatively treated and contralateral (control) limbs was measured in vitro with use of photon absorptiometry (DPA or SP2; Lunar, Madison, Wisconsin)⁸ or dual energy x-ray absorptiometry (DPXL; Lunar). For a given pair, the same machine was used.

Analysis of the Data
The bone-remodeling response to the presence of the implant was calculated as the difference between the values for the operatively treated femur and those for the control femur. For all but the medullary bone area fraction, the response was normalized to the control value—([treated - control]/control) x 100 per cent—so that the relative magnitude of the effect could be appreciated. Some of the control values for the medullary bone area fraction near the middle of the femoral shaft were zero (which yields an undefined effect). Accordingly, we simply subtracted the control value for this variable from the value for the operatively treated femur to estimate the remodeling response. For all of the remodeling variables, we interpreted the response as representing a change from the baseline (the preoperative value). Strictly speaking, this should be considered as an inferred change because we did not know the actual baseline values for the operatively treated limb. Thus, we inferred that the difference between the values for the treated limb and those for the control limb represented a change on the basis of the fact that bone mass and architecture are equivalent in the hindlimbs of dogs^8,19 and the assumption that the changes in the control limb over the course of the experiment were slight. This assumption is justified, given that a recent study showed that there were only slight changes in the bone mass of the tibia of the control limb after a hemiarthroplasty in dogs¹⁶. In addition, the data were used to determine the contribution of the subperiosteal, endocortical, and haversian surfaces to changes in the amount of bone within the cortex of the treated femur.

Statistical analyses included a separate multivariate analysis of variance for repeated measures for the tibial bone-mineral content, the cortical area, the cortical bone area fraction, the medullary bone area fraction, and the bone ingrowth. The between-subjects factors were stiffness and time, and the within-subjects factors were section (for all except the tibial bone-mineral content) and side (for all except bone ingrowth). When differences were found, t tests for independent or paired samples were used to determine which specific differences were significant. In addition, using data obtained at two years we examined the circumferential distribution of bone ingrowth by including the anatomical quadrant as an additional within-subjects factor. We considered p < 0.05 to be significant for all of the analyses, but because of the potential multiplicity problem caused by having performed numerous tests we report the exact p values for the analyses of variance so that the reader can apply a more cautious criterion if desired.

These parametric tests make certain assumptions about the structure of the data—namely, normality and equality of variance. For each type of stem at each time-period, skewness and kurtosis were checked for each variable to determine if the distribution deviated significantly from normality¹⁷. In addition, the equality of the variances was assessed with the Levene test. A log transformation improved the structure of the data for bone ingrowth, and the log-transformed data for this variable were used in the statistical analyses. We found no need to transform any of the other variables. Because the data on bone ingrowth were subjected to transformations before the statistical tests, we report the 95 per cent confidence intervals rather than the standard deviations; the confidence limits are asymmetrical about the mean¹⁷.

	Results

Top Abstract Introduction Materials and Methods Results Discussion References

 
All of the animals resumed weight-bearing within three days and exhibited normal function within seven to ten days after the operation. Radiographic evidence of thinning of the anterior and anteromedial cortex and of decreased density of the medial cortex adjacent to the proximal one-half of both types of stem was first seen at three months. At six months, there appeared to be complete resorption or severe thinning of the anteromedial cortex in six of the ten femora with a stiffer stem and in three of the ten with a less stiff stem, but the cortical thinning encompassed a greater area in the femora with a stiffer stem. At twenty-four months, the anteromedial cortex appeared to be completely resorbed or severely thinned in seven of the twelve femora with a stiffer stem and in none of the twelve with a less stiff stem. In addition, there was no radiographic evidence of subsidence of any stem and there were no acetabular changes at either six or twenty-four months.

Inferred Change in the Tibial Bone-Mineral Content
Although we could not detect a significant association, with the numbers available, between the asymmetry of the tibial bone-mineral content (the value for the tibia in the treated limb compared with that for the tibia in the contralateral [control] limb) and the type of stem, at either time-period, the asymmetry associated with the less stiff stems was significantly greater (p < 0.0005) at two years than at six months (Table III). The change from six to twenty-four months associated with the stiffer stems was not found to be significant.

Inferred Change in the Cortical Area
The loss of cortical area (range, 8.2 to 37.7 per cent) was significant in the proximal three sections of femora with both types of implant at both time-periods (p < 0.05), and the loss was greater in the more proximal sections (Figs. 3 and 4). The loss associated with the stiffer implants typically was 50 to 100 per cent greater than that associated with the less stiff stems. For example, the mean loss in section D at two years was 16.6 per cent in association with the less stiff stems and 32.3 per cent in association with the stiffer stems. At both time-periods, there tended to be an increase in the cortical area at the tip (section I) of the less stiff stem and distal to the tip (section K) for both types of stem; however, with the numbers available, we could not detect a significant difference between the two types of stem with regard to the cortical areas in these distal sections. Thus, in the analysis of variance, the cortical area varied as a function of the stiffness, side, and section (p = 0.001 for stiffness-by-side-by-section interaction term; Table IV), but the length of time that the stem had been in place had no significant effect.

View larger version (110K): [in this window] [in a new window]   FIG3: Fig. 3 Contact radiographs of two-millimeter-thick cross sections taken from the region of the lesser trochanter (section C) at six months. The upper panels show radiographs of a section from a femur with a stiffer stem and a comparable section from the contralateral (control) femur. The lower panels show radiographs of a section from a femur with a less stiff stem and a comparable section from the contralateral femur. There is evidence of more cortical thinning adjacent to the stiffer stem than adjacent to the less stiff stem.

View larger version (15K): [in this window] [in a new window]   FIG4: Fig. 4 Graphs showing the mean change in the cortical area associated with the less stiff (circles) and stiffer (squares) stems, according to the section, at both time-periods. The I-bars indicate the 95 per cent confidence intervals. An asterisk indicates p < 0.05 and a double asterisk, p < 0.01, for the difference between the values for the two types of stem.

Inferred Change in the Cortical Bone Area Fraction
The cortical bone area fraction associated with both types of stem was less in the operatively treated femora than in the contralateral (control) femora. This effect varied as a function of the section and time (p = 0.004 for the time-by-section interaction term) but did not vary as a function of the stiffness of the implant (Table IV). Specifically, there was a significant loss of cortical bone area fraction associated with both types of stem at two years (p < 0.0005 for side; Table IV). The greatest decrease in the cortical bone area fraction occurred proximally (section C), reaching a maximum of 8.2 per cent, but the decrease ranged from 1.4 to 4.9 per cent in the more distal sections (Fig. 5). The effect of time was most apparent in section D: the change in the cortical bone area fraction associated with both types of stem was no more than 0.5 per cent at six months but was 3.0 per cent for the less stiff stems and 4.0 per cent for the stiffer stems at two years.

View larger version (14K): [in this window] [in a new window]   FIG5: Fig. 5 Graphs showing the mean change in the cortical bone area fraction associated with the less stiff (circles) and stiffer (squares) stems, according to the section, at both time-periods. The I-bars indicate the 95 per cent confidence intervals. An asterisk indicates p < 0.05 and a plus sign, p < 0.1, for the difference between the values for the two types of stem.

Inferred Changes at the Three Cortical Bone Surfaces
Most of the bone loss occurred at the subperiosteal surface in association with both types of stem at both time-periods (Fig. 6). The loss of bone at the haversian surface (a decrease in the cortical bone area fraction) accounted for only a minor portion of the net change in the amount of cortical bone and was much more similar in magnitude in the various sections than was the loss at the subperiosteal surface. The change at the endocortical surface varied somewhat more than that at the haversian surface, but the loss of bone at that surface in the proximal part of the femur, relative to the magnitude of the loss at the subperiosteal surface, was substantial only in association with the stiffer stems at two years. There tended to be a gain of bone at the subperiosteal surface and a loss of bone at the endocortical surface adjacent to the tip (section I) of both types of stem at two years.

View larger version (31K): [in this window] [in a new window]   FIG6: Fig. 6 Graphs showing the mean amount of bone loss at the subperiosteal, endocortical, and haversian surfaces along the lengths of the less stiff and stiffer stems. The net change in the cortical bone mass is also shown. Only the means are shown for the sake of clarity.

Inferred Change in the Medullary Bone Area Fraction
The medullary bone area fraction associated with both types of stem increased significantly (p < 0.05) in certain sections at both time-periods, and no significant decreases were found in any section (Fig. 7). The pattern of change varied according to the stiffness of the stem (p < 0.0005 for stiffness-by-side-by-section interaction term; Table IV). In particular, the less stiff stems were associated with greater increases (p < 0.01) in the medullary bone area fraction in the proximal sections (section C at two years and section D at six months; Fig. 7). There were significant increases in the medullary bone area fraction adjacent to the tip (section I) of both types of stem (p < 0.05), with a tendency for a greater increase in association with the stiffer stems. The medullary bone area fraction increased significantly (p < 0.05) distal (section K) to both types of stem at two years, with the increase being greater (p < 0.01) for the stiffer stems.

View larger version (16K): [in this window] [in a new window]   FIG7: Fig. 7 Graphs showing the mean change in the medullary bone area fraction associated with the less stiff (circles) and stiffer (squares) stems, according to the section, at both time-periods. The I-bars indicate the 95 per cent confidence intervals. A double asterisk indicates p < 0.01 and a plus sign, p < 0.1, for the difference between the values for the two types of stem.

Volume Fraction of Bone Ingrowth
There were peaks in the amount of bone ingrowth proximally and distally, with less bone growth into the middle portion of both types of stem; there was significantly more bone growth into the proximal portion of the less stiff stems and into the distal portion of the stiffer stems (Fig. 8). We could not detect a significant change in the total amount of bone growth into the less stiff stems over time; the amount of bone ingrowth was 30.2 per cent (95 per cent confidence interval, 26.6 to 34.3 per cent) at six months and 26.1 per cent (95 per cent confidence interval, 21.5 to 31.6 per cent) at two years. However, the amount of bone growth into the stiffer stems decreased: it was 27.8 per cent (95 per cent confidence interval, 25.2 to 30.7 per cent) at six months and 21.4 per cent (95 per cent confidence interval, 17.4 to 26.2 per cent) at two years (p = 0.025). The change with time was dependent not only on the stiffness of the implant but also on the section (p = 0.013 for stiffness-by-time-by-section interaction term; Table IV). There was a trend toward decreased bone growth into the proximal sections (C and D) of the stiffer stems as a function of time, but this was not true for the less stiff stems. Thus, at two years there was nearly 50 per cent less (p = 0.002) bone growth in section C of the stiffer stems (19.0 compared with 36.8 per cent in the less stiff stems) and approximately 25 per cent less (p = 0.026) bone growth in section D of the stiffer stems (20.0 compared with 27.8 per cent) (Fig. 8). There was a decrease (p = 0.011) in the bone ingrowth from six months to two years in the middle portion of the less stiff stems (section F) but not in that section of the stiffer stems. Distally (section I), there was a decrease in bone growth from six months to two years into both types of stem, with the difference being significant (p = 0.033) for the stiffer stems.

View larger version (14K): [in this window] [in a new window]   FIG8: Fig. 8 Graphs showing the mean volume fraction of bone ingrowth into the less stiff (circles) and stiffer (squares) stems, according to the section, at both time-periods. The I-bars indicate the 95 per cent confidence intervals. An asterisk indicates p < 0.05 and a double asterisk, p < 0.01, for the difference between the values for the two types of stem.

Histological Analysis of the Interface
The histological findings at the interface between the porous coating and the surrounding tissue at two years were similar to those observed at six months. Bone ingrowth was present at all levels of all stems. The ingrown bone was found in the superficial one-half to two-thirds of the porous coating of both types of stem and often reached the substrate. In areas of bone ingrowth, the interface was characterized by bridging of trabeculae and continuity of marrow. Most of the bone was lamellar, although woven bone also was present at the proximal portion of the stem. Fibrocartilage was not observed at the interface of any of the stems.

Fibrous tissue usually was present within the porous coating and at the interface in areas where there was no bone ingrowth. The fibrous tissue at the interface was seen primarily at the middle portion of the stem, especially sections F and G, where there was less bone ingrowth than there was at the proximal and distal sections. At six months, there was fibrous tissue over less than one-half of the interface in sections F and G of nine of the ten less stiff stems and six of the ten stiffer stems. At two years, there was fibrous tissue at the interface at the middle of nine of the twelve stiffer stems, and the extent of this tissue was similar to that found at six months. In contrast, there was fibrous tissue at the interface in sections F and G of all twelve of the less stiff stems at two years; it was present over less than one-third of the interface of six stems and over more than one-half of the interface of the other six. In addition, fibrous tissue was more prevalent in the distal sections (H and I) of the less stiff stems at two years than it was in either type of stem at six months or in the stiffer stems at two years.

The seams of the polymer on the lateral and medial surfaces of all of the less stiff stems at both time-periods were associated with the formation of a fibrous membrane that protruded into the adjacent marrow spaces. The thickness of the membrane increased from six months to two years but was otherwise unchanged in character (parallel collagen fibers and fibrocytes without histiocytic infiltration).

Foreign-body granulomas were not present in any of the animals. Occasionally, foreign-body giant cells were found on the surfaces of the polyaryletherketone or the titanium fibers. A very small amount of particulate debris from the prosthetic devices was observed in the periprosthetic tissues. Most of the particles consisted of opaque, apparently carbon fibers that measured approximately ten micrometers in diameter and thirty to 200 micrometers in length. The carbon fibers were commonly found in the interface tissues around all of the stiffer stems and, to a much lesser degree, around the less stiff stems. These particles were found free within the marrow or fibrous tissue immediately adjacent to the porous coating, with no cellular response. The most likely source of these fibers was the manufacturing fixtures used during sintering of the porous surface. There also were minute opaque shards and granules within apparent lymphatic channels in several specimens. Whether these finer particles represented carbon or titanium debris could not be determined with light microscopy.

	Discussion

Top Abstract Introduction Materials and Methods Results Discussion References

 
The data in the present study of dogs showed that the use of a less stiff stem can reduce the amount of cortical bone that is lost following hemiarthroplasty with implantation of a fully porous-coated femoral stem without cement. For clinical situations in which there has already been a substantial loss of bone—such as in patients who have relatively thin cortices and large medullary cavities, older patients who have osteopenia, and patients who need a revision arthroplasty—fully porous-coated less stiff stems may be beneficial.

A number of the architectural changes that were observed in the present study are consistent with the findings in mechanical models that have indicated that the use of a less stiff stem can enhance proximal stress transfer^2,11,25. First, the preservation of cortical bone associated with a less stiff stem was consistent with the findings in these analytical models and in previous experimental investigations^1,14. Furthermore, the presence of more bone ingrowth proximally and a greater medullary bone area fraction associated with the less stiff stems support the analytical predictions that a reduction of the stiffness of the stem enhances the proximal load transfer from the implant to the femur. These findings provide strong support for the hypothesis that stress-shielding is an important cause of bone loss following hip arthroplasty and that reduction of the stiffness of the stem offers a promising means of promoting proximal stress transfer and avoiding atrophy of cortical bone in the proximal part of the femur.

The beam model⁹ used to characterize the stiffness of the implants was meant to provide a first-order approximation of the degree of stress-shielding. This simple model gives a reasonable estimation of the effect of the implants on the change in stresses in the host bone⁴. Calculations indicate that the human femur has less axial stiffness than implants made of either cobalt-chromium alloy or titanium alloy⁴, which is in agreement with our findings for the canine model. However, although the bending stiffness of the implants never exceeded that of the canine femora in our study, the bending stiffness of an implant—especially if it is large (eighteen millimeters or more in diameter) or made of cobalt-chromium alloy—can exceed the stiffness of the human femur⁴. Thus, it is not surprising that there have been reports of periprosthetic bone loss in patients that appears to exceed the amount of bone loss that was observed in the present study^6,13.

Loss of bone following total hip arthroplasty may be caused by a decreased use of the limb as well as by the local stress-shielding effects of the prosthesis. The slightly lower bone-mineral content in the tibiae of the operatively treated limbs compared with that in the contralateral (control) limbs is consistent with our previous observations^8,22. The magnitude of this change was small and may indicate subclinical disuse, possibly related to the so-called end-of-stem pain associated with this canal-filling design, increased bone-remodeling in the involved limb (the regional acceleratory phenomenon⁷), gain of bone in the contralateral limb, or a combination of these factors. In any case, the response was equivalent for the two types of stem, and it is unlikely that differences in function account for the differences between the periprosthetic bone-remodeling and the bone ingrowth associated with the two types of implant. Nevertheless, a small portion of the periprosthetic bone loss may reflect this global phenomenon rather than the local stress-shielding effects of the implant.

The relative importance of haversian remodeling (a change in the cortical bone area fraction), as opposed to geometric alterations of the cortex (formation and resorption of bone at the subperiosteal and endocortical surfaces), to cortical bone loss associated with an intramedullary implant is a matter of controversy. Some investigators have noted that the stiffness of the stem influences the cortical porosity¹⁴, but two studies in which the relative importance of these bone envelopes was examined directly showed that the geometric changes in the cortex were far more important to the net change in the cortical bone mass than were decreases in the cortical bone area fraction (that is, increased cortical porosity). One of these studies involved an experimental total hip arthroplasty model²² and the other, an experimental intramedullary rod model³. Our results indicate that most of the changes in the cortical bone mass occur at the subperiosteal and endocortical surfaces. Furthermore, the relative invariance of cortical bone area fraction in relation to the stiffness of the implant in the present study suggests that a change in the cortical bone area fraction (porosity) may represent a non-specific response to the operative procedure and the presence of an implant.

If the only consideration in the design of stems were loss of cortical bone, very flexible implants would be designed. Of course, there are other considerations, such as the durability of the implant, which preclude this type of design with currently available materials. Even if sufficiently strong material were available to allow for the manufacture of very flexible stems, the potential for initial micromotion and late excessive stress at the interface might preclude the utility of such stems^2,11,25. We can conclude that the initial micromotion was not excessive in our canine model, as the bone growth into the less stiff stems was the same as or more than that into the stiffer stems; this finding is consistent with that of a previous short-term study of bone growth into porous-coated low-modulus implants¹². Although there was more fibrous tissue at the interface of the less stiff stems, particularly at two years, we did not observe evidence of trabecular failure at the interface, as might be expected with less stiff implants if the interface stresses were too high.

	Footnotes

 
*One or more of the authors has received or will receive benefits for personal or professional use from a commercial party related directly or indirectly to the subject of this article. In addition, benefits have been or will be directed to a research fund, foundation, educational institution, or other non-profit organization with which one or more of the authors is associated. Funds were received in total or partial support of the research or clinical study presented in this article. The funding sources were National Institutes of Health Grant AR16485 and Zimmer USA, Warsaw, Indiana.

Department of Orthopedic Surgery, Rush-Presbyterian-St. Luke's Medical Center, 1653 West Congress Parkway, Chicago, Illinois 60612.

Department of Anatomy, Rush Medical College, Rush-Presbyterian-St. Luke's Medical Center, 600 South Paulina Street, Chicago, Illinois 60612. E-mail address: rsumner@rush.edu.

	References

Top Abstract Introduction Materials and Methods Results Discussion References

 

1. Bobyn, J. D.; Mortimer, E. S.; Glassman, A. H.; Engh, C. A.; Miller, J. E.; and Brooks, C. E.: Producing and avoiding stress shielding. Laboratory and clinical observations of noncemented total hip arthroplasty. Clin. Orthop., 274: 79-96, 1992.
2. Cheal, E. J.; Spector, M.; and Hayes, W. C.: Role of loads and prosthesis material properties on the mechanics of the proximal femur after total hip arthroplasty. J. Orthop. Res., 10: 405-422, 1992.[Medline]
3. Dallant, P.; Meunier, A.; Guillemin, G.; Christel, P.; Crolet, J.-M.; and Sedel, L.: Cortical bone response to orthopaedic implant rigidity: an experimental study. In Biological and Biomechanical Performance of Biomaterials, pp. 441-446. Edited by P. Christel, A. Meunier, and A. J. C. Lee. Amsterdam, Elsevier, 1986.
4. Dujovne, A. R.; Bobyn, J. D.; Krygier, J. J.; Miller, J. E.; and Brooks, C. E.: Mechanical compatibility of noncemented hip prostheses with the human femur. J. Arthroplasty, 8: 7-22, 1993.[Medline]
5. Engh, C. A., and Bobyn, J. D.: The influence of stem size and extent of porous coating on femoral bone resorption after primary cementless hip arthroplasty. Clin. Orthop., 231: 7-28, 1988.
6. Engh, C. A.; McGovern, T. F.; Bobyn, J. D.; and Harris, W. H.: A quantitative evaluation of periprosthetic bone-remodeling after cementless total hip arthroplasty. J. Bone and Joint Surg., 74-A: 1009-1020, Aug. 1992.[Abstract/Free Full Text]
7. Frost, H. M.: The regional acceleratory phenomenon. A review. Henry Ford Hosp. Med. J., 31: 3-9, 1983.[Medline]
8. Goethgen, C. B.; Sumner, D. R.; Platz, C.; Turner, T. M.; and Galante, J. O.: Changes in tibial bone mass after primary cementless and revision cementless total hip arthroplasty in canine models. J. Orthop. Res., 9: 820-827, 1991.[Medline]
9. Huiskes, R.: On the modelling of long bones in structural analyses. J. Biomech., 15: 65-69, 1982.[Medline]
10. Huiskes, R.: The various stress patterns of press-fit, ingrown, and cemented femoral stems. Clin. Orthop., 261: 27-38, 1990.
11. Huiskes, R.; Weinans, H.; and Van Rietbergen, B.: The relationship between stress shielding and bone resorption around total hip stems and the effects of flexible materials. Clin. Orthop., 27: 124-134, 1992.
12. Jasty, M.; Bragdon, C. R.; Maloney, W. J.; Mulroy, R., Jr.; Haire, T.; Crowninshield, R. D.; and Harris, W. H.: Bone ingrowth into a low-modulus composite plastic porous-coated canine femoral component. J. Arthroplasty, 7: 253-259, 1992.[Medline]
13. Kilgus, D. J.; Shimaoka, E. E.; Tipton, J. S.; and Eberle, R. W.: Dual-energy x-ray absorptiometry measurement of bone mineral density around porous-coated cementless femoral implants. Methods and preliminary results. J. Bone and Joint Surg., 75-B(2): 279-287, 1993.
14. Maistrelli, G. L.; Fornasier, R. V.; Binnington, A.; McKenzie, K.; Sessa, V.; and Harrington, I.: Effect of stem modulus in a total hip arthroplasty model. J. Bone and Joint Surg., 73-B(1): 43-46, 1991.
15. Morscher, E. W., and Dick, W.: Cementless fixation of "isoelastic" hip endoprostheses manufactured from plastic materials. Clin. Orthop., 176: 77-87, 1983.
16. Smith, A. M.; Turner, T. M.;; and Sumner, D. R.: Unilateral hip replacement causes bilateral changes in tibial bone mineral content in a canine model. J. Bone and Min. Res., 11: 693-696, 1996.[Medline]
17. Sokal, R. R., and Rohlf, F. J.: Biometry. The Principles and Practice of Statistics in Biological Research. Ed. 2. San Francisco, W. H. Freeman, 1981.
18. Sumner, D. R., Jr., and Galante, J. O.: Bone ingrowth. In Surgery of the Musculoskeletal System, edited by C. McC. Evarts. Ed. 2, vol. 1, pp. 151-176. New York, Churchill Livingstone, 1990.
19. Sumner, D. R.; Turner, T. M.; and Galante, J. O.: Symmetry of the canine femur: implications for experimental sample size requirements. J. Orthop. Res., 6: 758-765, 1988.[Medline]
20. Sumner, D. R.; Bryan, J. M.; Urban, R. M.; and Kuszak, J. R.: Measuring the volume fraction of bone ingrowth: a comparison of three techniques. J. Orthop. Res., 8: 448-452, 1990.[Medline]
21. Sumner, D. R.; Devlin, T. C.; Winkelman, D.; and Turner, T. M.: The geometry of the adult canine proximal femur. J. Orthop. Res., 8: 671-677, 1990.[Medline]
22. Sumner, D. R.; Turner, T. M.; Urban, R. M.; and Galante, J. O.: Remodeling and ingrowth of bone at two years in a canine cementless total hip-arthroplasty model. J. Bone and Joint Surg., 74-A: 239-250, Feb. 1992.[Abstract/Free Full Text]
23. Turner, T. M.; Sumner, D. R.; Urban, R. M.; Rivero, D. P.; and Galante, J. O.: A comparative study of porous coatings in a weight-bearing total hip-arthroplasty model. J. Bone and Joint Surg., 68-A: 1396-1409, Dec. 1986.[Abstract/Free Full Text]
24. Van Rietbergen, B.; Huiskes, R.; Weinans, H.; Sumner, D. R.; Turner, T. M.; and Galante, J. O.: The mechanism of bone remodeling and resorption around press-fitted THA stems. J. Biomech., 26: 369-382, 1993.[Medline]
25. Weinans, H.; Huiskes, R.; and Grootenboer, H. J.: Effects of material properties of femoral hip components on bone remodeling. J. Orthop. Res., 10: 845-853, 1992.[Medline]

CiteULike

Connotea

Del.icio.us

Facebook

Technorati

Twitter

This article has been cited by other articles:

				E. J. HARVEY, J. D. BOBYN, M. TANZER, G. J. STACKPOOL, J. J. KRYGIER, and S. A. HACKING Effect of Flexibility of the Femoral Stem on Bone-Remodeling and Fixation of the Stem in a Canine Total Hip Arthroplasty Model without Cement J. Bone Joint Surg. Am., January 1, 1999; 81(1): 93 - 107. [Abstract] [Full Text]

This Article Abstract Full Text (PDF) Free Letters to the Editor: Submit a response Alert me when this article is cited Alert me when Letters to the Editor are posted Alert me if a correction is posted Services E-mail this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Add to My File Cabinet Download to citation manager Rights and Permissions Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by TURNER, T. M. Articles by GALANTE, J. O. Search for Related Content PubMed PubMed Citation Articles by TURNER, T. M. Articles by GALANTE, J. O. Social Bookmarking                   What's this?

Stanford University Libraries' HighWire Press (TM) assists in the publication of JBJS Online.
Copyright © 1997 by The Journal of Bone and Joint Surgery, Inc. Online ISSN: 1535-1386.
The Journal of Bone and Joint Surgery®, JBJS®, JB&JS®, and eJBJS® are registered in the U.S. Patent and Trademark Office.