Abstract
Background: Osteolysis of the femur has been a serious problem associated with some designs of total hip-replacement implants; it frequently leads to failure of the femoral component. We evaluated the effect of a circumferential plasma-spray porous coating on the rate of osteolysis in a study that included two groups of hips, each of which received an implant with the same design except for the extent of the porous coating. Our goal was to determine the possible role of circumferential porous coating in protecting the bone-implant interface from osteolysis.Methods: A series of consecutive primary total hip replacements performed with insertion of the Mallory-Head implant without cement was divided into two study groups. The first 126 hips (Group 1) were treated with a femoral stem that had a noncircumferential plasma-spray porous titanium coating. The next ninety hips (Group 2) were treated with a circumferentially coated stem of the same design. The average duration of radiographic follow-up was 7.8 years in Group 1 and 7.5 years in Group 2.Results: The average rate of polyethylene wear was similar for the two groups (0.187 millimeter per year in Group 1 and 0.189 millimeter per year in Group 2). The prevalence of osteolysis in Group 1 (40 percent; fifty of 126 hips) was significantly higher than that in Group 2 (10 percent; nine of ninety hips) (p < 0.001). Osteolysis remote from the joint space (distal to zones 1 and 7) was found in 11 percent (fourteen) of the hips in Group 1 but in none of those in Group 2 (p = 0.0004). The average total area of osteolysis in Group 1 (5.0 square centimeters) was significantly larger than that in Group 2 (2.9 square centimeters) (p < 0.05).Conclusions: A circumferential plasma-spray titanium porous coating on the femoral component of a total hip-replacement prosthesis inserted without cement appears to provide an effective barrier preventing wear debris from gaining access to the endosteal surface of the femur and the greater trochanter. This finding supports the hypothesis of the so-called effective joint space, which predicts that wear debris from the joint bearing can migrate, driven by intracapsular pressures, to all areas to which joint fluid has access and thus can result in osteolysis. The reduction of the prevalence of osteolysis and the elimination of osteolysis from the zones remote from the joint space by the use of a circumferential plasma-spray porous coating indicates that the femur was effectively sealed off from the joint space. We believe that the durability and longevity of the femoral component should be enhanced by the use of such a coating.
Since the introduction of the total hip replacement in 19597, various challenges have arisen in the pursuit of a more durable prosthesis. One of the most important clinical problems today is osteolysis resulting in periprosthetic bone loss and leading to aseptic loosening. Osteolysis is mediated by macrophages activated by wear debris (predominantly polyethylene) arising from the joint bearing surface1,6,12,21,27. This wear debris not only produces bone loss adjacent to the bearing surface but also can result in bone loss at sites distant from the joint space. Anthony et al.2 described localized osteolysis adjacent to defects in the cement mantles of femoral stems. They proposed that polyethylene debris in the joint fluid migrates along nonbonded implant-cement interfaces and reaches the endosteal cortex by way of these defects in the cement mantle. Schmalzried et al.22 also described areas of localized bone loss containing intracellular polyethylene debris remote from the actual joint space; this finding led to the concept of the so-called effective joint space. According to this hypothesis, debris can migrate, following the path of least resistance and driven by intracapsular pressure, to all areas that are accessible to joint fluid. Schmalzried et al. postulated that a barrier will block or delay this flow of debris, thereby protecting the bone-implant interface.
Several studies have suggested that fixation of the femoral stem with cement is associated with less osteolysis2,11,22 than is fixation of the femoral stem without cement10,19,25. Goetz et al.11 found that 0 percent of forty-one stems that had been precoated with polymethylmethacrylate and inserted with cement were associated with femoral osteolysis beyond the joint space compared with 29 percent (twelve) of forty-one stems that had a proximal porous coating (titanium wire-mesh pads with smooth, noncoated areas between the pads); both designs of stems had been matched with the same design of acetabular component. The authors concluded that cement was an effective barrier to wear debris and that it protected against osteolysis. Bobyn et al.4 showed that polyethylene debris can migrate along smooth, nonporous surfaces but not along porous surfaces with bone ingrowth. This finding suggests that the observation of increased osteolysis in association with some bone-ingrowth designs may have a more complex explanation than simply fixation of the stem without cement. It follows that a circumferential porous coating may be a more effective seal (or barrier) than a noncircumferential coating and the type of porous coating may have a role in preventing access to the endosteal surface of the femur.
The purpose of the present study was to investigate, in a series of patients, the role of circumferential porous coating on the femoral component in protecting the bone-implant interface from osteolysis. We hypothesized that, compared with identical noncircumferentially plasma-spray porous-coated femoral stems, circumferentially plasma-spray porous-coated stems would be associated with fewer osteolytic lesions and would not be associated with osteolysis extending beyond the joint space. In addition, we surmised that the osteolysis associated with the noncircumferentially coated stems would be more prevalent adjacent to the non-porous-coated areas of the stem.
*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. No funds were received in support of this study.
†Texas Center for Joint Replacement, 6300 West Parker, Suite 220, Plano, Texas 75093.
‡2001 North MacArthur Boulevard, Irving, Texas 75061.
From February 1985 through December 1990, 241 consecutive primary total hip replacements were performed with insertion of the Mallory-Head plasma-spray proximally porous-coated titanium femoral stem (Biomet, Warsaw, Indiana) without cement in 210 patients. All operations were done by the senior ones of us (R. H. E., Jr., and W. C. H.). The patients were selected for total hip replacement without cement on the basis of radiographic evidence of good bone quality, defined as a tapering proximal part of the canal with thick diaphyseal cortices. No patient was excluded solely on the basis of age, gender, weight, level of activity, or pre-existing medical illness. Patients who were judged to have inadequate bone stock had fixation of the femoral component with cement.
The initial design of this component had a non-porous-coated lateral surface as well as a plasma-spray porous-coated medial surface, which wrapped partway anteriorly and posteriorly, on the upper one-third of the stem (Fig. 1-A). The porous coating covered 62.5 percent of the entire circumference. Subsequently, with the goal of promoting better fixation of the implant, the stem was modified with the application of a circumferential porous coating on the upper third, with maintenance of exactly the same geometry and other design features of the implant (Fig. 1-B). The stem is straight and is wedge-shaped in both the anteroposterior and the mediolateral plane. There are fins projecting anteriorly, posteriorly, and laterally. All of the operations were performed through an anterolateral approach with instrumentation provided by the manufacturer.
Twenty-two patients (twenty-five hips) had less than two years of radiographic follow-up: five patients (six hips) died less than two years after the operation, five patients (five hips) were lost to follow-up, and twelve patients (fourteen hips) failed to keep scheduled follow-up appointments but had been located and clinically assessed with a telephone interview at an average of 5.7 years postoperatively. Of these fourteen hips, one had removal of the prosthesis because of infection at eight years, one had revision of the acetabular component because of loosening at 1.5 years, and one had revision of the femoral component because of loosening at 1.4 years. All of these revisions were of partially porous-coated stems. The remaining nine patients (eleven hips) expressed satisfaction with the joint replacement but would not have a current radiograph made because of difficulty with travel and the absence of symptoms.
Therefore, the study included 216 hips in 188 patients, with an average radiographic follow-up interval of 7.5 years (range, 2.1 to 12.5 years) and an average clinical follow-up interval of 8.3 years (range, 2.5 to 12.5 years). Only fifteen patients (fifteen hips) had less than five years of clinical follow-up.
The first 126 hips were treated with the partially porous-coated design, and the remaining ninety hips were treated with the current design with the completely circumferential plasma-spray porous coating. The last partially coated stem was inserted on September 13, 1988. Eight hips were treated with a bipolar acetabular component; 205, with a hemispherical bone-ingrowth cup (a Hexloc component [Biomet, Warsaw, Indiana] in 189 and a Trispike cup [DePuy, Warsaw, Indiana] in sixteen); two, with a screw-in cup (T-tap; Biomet); and one, with a polyethylene cup inserted with cement. All of the femoral heads were titanium, with the exception of four chromium-cobalt heads on circumferentially coated stems. Five femoral heads measured twenty-two millimeters in diameter; 138, twenty-eight millimeters; and seventy-three, thirty-two millimeters.
Variables that were analyzed in this study included the design of the femoral stem, the follow-up interval, the diameter of the femoral head, the affected femoral zones13, the area of the osteolysis, the amount and rate of polyethylene wear, the polyethylene thickness, the size of the acetabular component, and the abduction angle of the acetabular cup. Any geographic punched out area of decreased bone density that had not been present on the initial postoperative radiograph was interpreted as osteolysis. These lytic lesions are easily distinguishable from stress-shielding, which is more diffuse in appearance. Radiolucent lines were defined as lucent lines with a sclerotic border, and they were considered to be different from osteolysis, which has no surrounding sclerosis. The area of osteolysis on the most recent follow-up anteroposterior radiograph was measured with a radiographic digitizer (Orthographics, Salt Lake City, Utah), by tracing the outline of the lesion. Linear polyethylene wear was determined with use of a computer radiographic digitizer according to the method of Livermore et al.18 and comparing the measurement on the initial postoperative radiograph with that on the most recent follow-up radiograph. The known diameter of the prosthetic femoral head was used to correct for magnification. Intraobserver variability was determined to be ±0.4 millimeter. The angle of abduction of the cup was the angle between a line drawn along the face of the cup and a line drawn through the inferior margins of the ischial tuberosities.
Statistical evaluation was performed with the Fisher exact test and the Student t test where applicable at the 95 percent confidence level. Associations were determined with standard linear regression analysis. Survivorship analysis was done with the life-table method.
The 126 partially porous-coated stems (Group 1) were followed radiographically for an average of 7.8 years (range, 2.1 to 12.5 years) and clinically for an average of 8.6 years (range, 2.5 to 12.5 years). The ninety circumferentially porous-coated stems (Group 2) were followed radiographically for an average of 7.5 years (range, 2.1 to 12.5 years) and clinically for an average of 8.2 years (range, 2.5 to 12.5 years). These differences were not significant (p = 0.18 for radiographic follow-up, and p = 0.11 for clinical follow-up). The two groups were also similar in terms of diagnosis, gender, and age. The diagnosis was osteoarthritis in eighty-two hips in Group 1 and sixty-five hips in Group 2, avascular necrosis in nineteen hips in Group 1 and eleven hips in Group 2, congenital dislocation in six hips in Group 1 and nine hips in Group 2, rheumatoid arthritis in two hips in Group 1 and three hips in Group 2, Legg-Calvé-Perthes disease in two hips in Group 1 and one hip in Group 2, and another diagnosis in fifteen hips in Group 1 and one hip in Group 2. In Group 1, sixty-three of the hips were in male patients and sixty-three were in female patients. In Group 2, thirty-four of the hips were in male patients and fifty-six were in female patients. The average age was fifty-five years in Group 1 and fifty-four years in Group 2.
At the latest follow-up examination, 157 femoral stems (seventy-six in Group 1 and eighty-one in Group 2) were not associated with osteolysis. The average duration of radiographic follow-up was 7.3 years for the Group-1 stems that were not associated with osteolysis and 7.1 years for the Group-2 stems that were not associated with osteolysis; this difference was not significant (p = 0.25). Five of the 157 stems were revised. Two were revised because of infection at 4.6 and 9.8 years; one, because of instability at nine years; one, because of early loosening at three years; and one, because of chronic pain at 5.3 years. None of these stems were associated with osteolysis at the time of the revision, and the acetabular component was also revised in all five hips. All of the revised stems were partially coated (Group 1).
Fifty-nine femoral stems were associated with osteolysis at the latest follow-up examination. Fifty (40 percent) of the 126 stems in Group 1 and nine (10 percent) of the ninety stems in Group 2 were associated with osteolysis, which was a significant difference (p < 0.001). These areas of osteolysis were found primarily at the joint line (zones 1 and 7 of Gruen et al.13) (Fig. 2). In contrast, the radiolucent zones (Fig. 3) were distributed primarily around the non-porous-coated distal aspects of both stem designs. The average duration of follow-up of the stems that were associated with osteolysis was 8.6 years in Group 1 and 8.1 years in Group 2; this difference was not significant (p = 0.18).
Of the fifty-nine stems that were associated with osteolysis, four (all in Group 1) were revised, all with accompanying revision of the acetabular component. Two stems were revised because of loosening and subsidence; one of them was associated with osteolysis in zones 1, 5, and 6 (Fig. 4), and the other was associated with osteolysis in zones 3, 5, and 7. Another of the revised stems was associated with osteolysis in zones 1 and 2; it showed no subsidence and there were no progressive radiolucent lines in zones elsewhere around this stem, but it was revised because of the osteolysis and a fear of future loosening. However, this component was very difficult to remove because of bone growth into the porous area of the stem. The fourth stem was revised at another institution; there was an osteolytic lesion in zone 1, but there were no other radiolucent lines and the stem was not clinically loose and was difficult to remove according to the operating surgeon (Fig. 5). The remaining stems that were associated with osteolysis were functioning well at the time of the latest follow-up.
The total area of the osteolysis averaged 4.6 square centimeters: 5.0 square centimeters in Group 1 and 2.9 square centimeters in Group 2, which was a significant difference (p < 0.05). No osteolysis was seen distal to zones 1 and 7 (adjacent to the joint space) in Group 2. In contrast, in Group 1, there was osteolysis extending along the lateral aspect (zones 2 and 3) of ten stems and osteolysis extending around the tip (zones 4, 5, and 6) of four additional stems. Therefore, fourteen hips in Group 1 had osteolysis remote from the joint space (that is, distal to zones 1 and 7) compared with none in Group 2; this difference was significant (p = 0.0004). Most of the distal osteolytic areas were found adjacent to the lateral, noncoated zones of the partially coated stems (Fig. 2). Only one stem, which was clinically loose, was associated with an osteolytic lesion in zone 6, the medial porous area common to both stem designs.
Although four of the Group-1 stems that were associated with osteolysis were revised, only two were actual clinical failures, and the other forty-six partially coated and all nine of the circumferentially coated stems that were associated with osteolysis were functioning well without evidence of loosening (Fig. 6).
The survival rate in the entire series (the original 241 stems), with failure defined as revision of the stem for any reason, was 93.7 ± 3.9 percent at ten years. The survival rate with failure defined as aseptic loosening was 95.3 ± 2.9 percent at ten years. A total of eleven stems, all of the partially coated design (Group 1), were revised. Three were revised because of infection; one, because of instability; and seven, because of pain or loosening. Two of these seven stems were revised because of osteolysis and pain, but they were not clinically loose, as described. With failure defined as aseptic loosening or pain (that is, excluding the three stems that were revised because of infection, the one that was revised because of instability, and the two revised stems that were shown to be stable), the rate of survival of the Group-1 stems was 93.9 ± 6.9 percent and that of the Group-2 stems was 100 percent at ten years.
In both groups, the femoral osteolytic lesions were most common in the greater trochanter (in forty-eight hips in Group 1 and in six in Group 2; Fig. 2). The average sizes of the lesions were similar (4.5 square centimeters in Group 1 and 4.3 square centimeters in Group 2, a difference that was not significant). However, the trochanteric cysts were as large as 15.4 square centimeters in this study. Of the fifty-four femora in which there was a trochanteric osteolytic lesion, two (one with a 6.2-square-centimeter cyst and one with a 7.4-square-centimeter cyst) had a pathological fracture and chronic trochanteric nonunion.
Of the fifty-nine hips with osteolysis about the stem, twenty-four had revision of the acetabular cup because of loosening, osteolysis, or polyethylene wear. Four of these failed acetabular cups were matched with a Group-1 femoral stem that was revised. In the remaining twenty hips that had a revision of the cup and osteolysis about the stem, the lesions about the stem were treated with bone graft at the operation and the stem was preserved.
The average amount of linear wear in this series was 1.37 millimeters, and the average rate of wear was 0.19 millimeter per year. Group 1 had an average of 1.36 millimeters of wear, with an average rate of wear of 0.187 millimeter per year, and Group 2 had an average of 1.30 millimeters of wear, with an average rate of wear of 0.189 millimeter per year. No significant difference could be detected between the two groups with regard to either the amount of linear wear (p = 0.46) or the rate of wear (p = 0.44).
On comparison of the two treatment groups in the subset of stems associated with osteolysis, we were unable to detect a significant difference with regard to the average amount of linear wear (1.91 millimeters in Group 1 and 1.14 millimeters in Group 2; p = 0.08) or the average rate of wear (0.250 millimeter per year in Group 1 and 0.139 millimeter per year in Group 2; p > 0.05). In the subset of stems that were not associated with osteolysis, there was a slightly significant difference between the groups with regard to linear wear (1.03 millimeters in Group 1 and 1.32 millimeters in Group 2; p = 0.02) and the rate of wear (0.14 millimeter per year in Group 1 and 0.19 millimeter per year in Group 2; p = 0.018). The clinical importance of these significant differences is only marginal.
The average size of the acetabular cup in this series was fifty millimeters in both groups, and the average abduction angle of the cup was 44 degrees in both groups. The average polyethylene thickness was 4.6 millimeters in the dome and 3.3 millimeters in the side wall in Group 1 and 5.4 millimeters in the dome and 4.0 millimeters in the side wall in Group 2. Using standard linear regression analysis, with the numbers available, we were unable to detect a significant relationship between osteolysis and the abduction angle of the cup, the diameter of the femoral head, or the polyethylene thickness in either group.
Willert and Semlitsch27, in 1976, were among the first to describe the mechanism of accumulation of wear debris and the development of an inflammatory response leading to bone resorption and loosening of the implant. Over the next two decades, this response was further described in detail1,6,12,21. Recent data suggest that macrophages, fibroblasts, and endothelial cells are activated by wear debris and release interleukin-1 (bone-resorbing cytokine) and tumor necrosis factor, which initiate the osteolytic cascade. Macrophages may also resorb bone by the release of oxide radicals and hydrogen peroxide3,20,21,26. This wear debris can include metal, polyethylene, and polymethylmethacrylate; however, Schmalzried et al.22 demonstrated that osteolysis occurs with polyethylene debris alone, in the absence of identifiable cement or metallic debris.
The average rate of wear, 0.19 millimeter per year, found in the current study is in the middle of the range reported in the literature (0.005 to 0.6 millimeter per year8,9,16). A retrieval study of seventy-two Charnley total hip prostheses in which an all-polyethylene acetabular component was matched with a twenty-two-millimeter-diameter femoral head showed a rate of wear of 0.15 millimeter or less per year for 75 percent (fifty-four) and more than 0.15 millimeter per year for the remaining 25 percent (eighteen)9. In a series of ninety-eight hips in which a twenty-eight-millimeter-diameter femoral head had been used, Livermore et al.18 found an average rate of linear wear of 0.08 ± 0.07 millimeter per year.
The two stem designs that we studied performed very differently from one another even though they were associated with similar amounts of wear debris. The partially coated stems failed more often and were more frequently associated with osteolysis than the circumferentially coated stems. We did not detect a significant difference between the two stem designs with regard to the average amount or rate of linear wear, either in the series as a whole or in the subset of stems associated with osteolysis. All of the stems that failed were of the partially coated design. Excluding the three revisions because of infection and the one failure because of instability, seven partially coated stems were revised. Two of these revisions were of stable stems. In retrospect, the osteolysis associated with these two stable stems could have been treated with bone-grafting and the stems could have been preserved. Thus, five partially coated stems actually failed because of aseptic loosening or pain. Four of these revisions were done in the first five years of the study period, and the remaining revision was performed at 10.4 years, although the stem had been painful for many years. In contrast, no circumferentially coated stems were revised in this series, and none appeared to be at risk for failure due to osteolysis or mechanical causes as of the latest follow-up evaluation.
The better performance of the circumferentially coated stems appears to be related to both the presumably superior initial fixation that results from more porous coating as well as the evident protection of the bone-implant interface resulting from the seal produced by the circumferential application of the porous coating. There may have been a small so-called learning curve effect, as the Group-2 stems were used later in the series, but better operative technique would not have provided the protection of the endosteum from osteolysis that was seen in Group 2.
The average durations of follow-up of these two stem designs were almost the same, but the prevalence of osteolysis in Group 1 (40 percent; fifty of 126) was significantly higher than that in Group 2 (10 percent; nine of ninety) (p < 0.001). The average total area of the osteolysis in Group 1 (5.0 square centimeters) was also significantly larger that that in Group 2 (2.9 square centimeters) (p < 0.05). As we had hypothesized, osteolytic lesions were more common in the femoral zones13 with no porous coating, and osteolysis was seen at the tip of the stem only in Group 1 (the partially coated stems); the debris gained access to the tip down the smooth lateral side of the implant. No lesions were seen remote from the joint line in any hip with a circumferentially coated stem. This observation is in contrast to the findings in a study of a different implant design by Tanzer et al.25, who reported that most of the osteolytic lesions were distal to the porous pads, in zones 2, 3, 5, and 6 of Gruen et al.13. These findings clearly demonstrate that a circumferential plasma-spray coating can create a seal that protects the distal aspect of the stem from debris generated at the bearing surface.
In a study comparing stems fixed with cement with porous ingrowth femoral stems fixed without cement, Goetz et al.11 considered only lesions that caused scalloping of the endosteal cortical surface, away from the joint line (that is, distal to zones 1 and 7). They defined osteolysis as extensive if the lesions occupied at least six zones or had an area of more than ten square centimeters; as intermediate if the lesions occupied three, four, or five zones or had an area of 2.5 to ten square centimeters; and as mild if the lesions occupied one or two zones or had an area of less than 2.5 square centimeters. They excluded from the analysis all focal lesions at the joint space and made no mention of lesions in the greater trochanter. If their definitions were applied to the data in the present study, only fourteen femora, rather than fifty-nine, would have been considered to have had osteolysis; the osteolysis would have been graded as intermediate in three of these femora and as mild in eleven. The overall prevalence of osteolysis would therefore have been 11 percent (fourteen of 126) in Group 1 and none (of ninety) in Group 2 compared with a prevalence of 29 percent (twelve of forty-one) for the porous stems in the study by Goetz et al., in which the average duration of follow-up was only five years and in which six of the twelve osteolytic lesions were around the tip of the femoral stem. Goetz at al. noted "no evidence of accelerated wear of the polyethylene" in any hip in their series.
The greater trochanter (zone 1) was the most common location of osteolysis in both groups in the present study. Forty-eight hips in Group 1 and six in Group 2 had a lesion of the greater trochanter, which was a significant difference (p < 0.001). While there were many fewer trochanteric lesions in Group 2, the average area of these lesions was almost the same as the average area in Group 1, a finding that is consistent with almost the identical wear rates and follow-up intervals in the two groups. Nonetheless, the circumferential coating afforded remarkable protection of the greater trochanter, suggesting that debris gains access to the cancellous trochanteric bone through the implant-bone interface. Several large lesions of the greater trochanter were seen in this study, and two trochanteric fractures occurred through cysts measuring more than five square centimeters, suggesting that lesions of this size are associated with an increased risk for fracture. Seventeen of the fifty-four trochanteric lesions were at least five square centimeters. Therefore, the rate of fracture through these larger cysts was two of seventeen. Treatment of these lesions with bone-grafting is definitely desirable before a fracture has occurred because reconstruction is difficult if not impossible after the trochanter has fragmented.
This study supports the theory of the so-called effective joint space that was proposed by Schmalzried et al.22. This theory postulates that debris can be driven to all areas accessible to joint fluid, even areas remote from the immediate joint space such as the tip of the femoral stem, but will not go where joint fluid is effectively restricted. This migration is caused by high intracapsular pressures, ranging from 200 to 700 millimeters of mercury (26.66 to 93.31 kilopascals)15,22,23, which can occur during normal activities of daily living. In the present study, osteolytic lesions were found more commonly in areas with no barrier to debris transport—that is, the lateral, noncoated part of the stems in Group 1. Lesions did not develop in remote areas that were sealed off from the joint space, as demonstrated in Group 2. This study also supports the findings of Bobyn et al.4, who showed, in an animal model, that polyethylene debris preferentially tracks along a smooth bone-implant interface rather than a porous interface.
Not all circumferential porous coatings have provided an effective barrier to wear debris. Kim and Kim17 found osteolysis in association with 33 percent (thirty-eight) of 116 PCA (Porous-Coated Anatomic) stems (Howmedica, Rutherford, New Jersey). Woolson and Maloney28 observed osteolysis adjacent to 22 percent (fifteen) of sixty-nine HGP (Harris-Galante porous-coated) stems (Zimmer, Warsaw, Indiana), which are partially coated, after only three and one-half years of follow-up. The porous coatings used in these studies were circumferential sintered beads and noncircumferential titanium wire-mesh pads. Common to these porous coatings are interconnecting porous channels, which theoretically can permit access of joint fluid despite bone ingrowth. Plasma-spray porous surfaces have no continuous channels through the porous layer. Also, titanium has a known affinity for bone ongrowth5,14,24, which may enhance the effectiveness of the barrier against debris.
In summary, our study has shown that a circumferential plasma-spray titanium porous coating is very effective at protecting the endosteal femoral bone-implant interface as well as the greater trochanter compared with a noncircumferential porous coating on an identical stem. The circumferential plasma-spray coating completely protected the bone-implant interface distal to the level of the joint space from osteolysis in this series. Therefore, the circumferential plasma-spray coating appears to provide an effective seal of the femoral canal, preventing debris-containing joint fluid from gaining access to the endosteal bone surface. The importance of this observation is that prevention of femoral osteolysis can be expected to prolong the longevity of these circumferentially coated stems.
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