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Bone Remodeling Around Porous Metal Cementless Acetabular Components
The Journal of Arthroplasty Vol. 25 No. 5 2010
Bone Remodeling Around Porous Metal Cementless Acetabular Components R. Michael Meneghini, MD,* Kerry S. Ford, MD,z Cynthia H. McCollough, PhD,y Arlen D. Hanssen, MD,z and David G. Lewallen, MDz
Abstract: Bone remodeling around cementless acetabular components after total hip arthroplasty has not been well characterized. A randomized, prospective study of total hip arthroplasty was performed comparing 2 cementless acetabular implants: a solid titanium and a more elastic porous tantalum design. Seventeen hips (9 porous tantalum, 8 titanium) underwent quantitative computed tomography at mean of 7.7 years, and adjacent bone mineral density (BMD) was calculated. The absolute and relative decrease in BMD from preoperative level was less in zones 9 to 15 mm adjacent to the porous tantalum compared to the titanium component (P ≤ .02) and predominated posterosuperiorly. The relative BMD increased in all regions adjacent to the porous tantalum component from 5% to 40% over the control. This data demonstrates stress-shielding likely occurs less around a highly porous metal implant of material with an elastic modulus similar to bone. Keywords: bone mineral density, total hip arthroplasty, cementless, quantitative computed tomography, porous tantalum, bone-remodeling. © 2010 Elsevier Inc. All rights reserved.
The location and amount of periprosthetic bone stock are critical factors determining success during revision acetabular reconstruction after total hip arthroplasty (THA). Although remodeling and stress-shielding of the proximal femur in response to implants has been well characterized [1-8], the response to cementless acetabular components has not been as extensively studied. Finite element modeling of cementless acetabular components has predicted a concentration of the loads and stresses at the component periphery into the cortical bone which produces shielding of the proximal and medial trabecular bone [9,10] and a theoretical attenuation of bone density behind the implant [10-12]. To date, most investigation of bone density around acetabular components have utilized dual energy x-ray absorptiometry (DEXA), which precludes accurate assessment of the important retroacetab-
From the *Department of Orthopaedic Surgery, New England Musculoskeletal Institute, University of Connecticut Health Center, Farmington, Connecticut; yDepartment of Radiology, Mayo Clinic, Rochester, Minnesota; and zDepartment of Orthopaedic Surgery, Mayo Clinic, Rochester, Minnesota. Submitted September 8, 2008; accepted April 18, 2009. While the authors did not receive any benefits for this part of the study, benefits were received by Implex for the original study from which this cohort was taken. Therefore, it should be noted that benefits were received in support of this study from Implex. Also it should be noted that one or more of the authors have received royalties and payments from Implex/Zimmer. Reprint requests: R. Michael Meneghini, MD, Department of Orthopaedic Surgery, University of Connecticut Health Center, Medical Arts and Research Building, 4th Floor, #4016, 263 Farmington Ave, Farmington, CT 06034-4037. © 2010 Elsevier Inc. All rights reserved. 0883-5403/2505-0011$36.00/0 doi:10.1016/j.arth.2009.04.025
ular region [13-18]. Furthermore, longer-term follow-up studies are lacking, and no studies have directly compared bone remodeling in response to cementless acetabular implants composed of different core materials. A randomized, prospective study was undertaken to investigate the clinical and radiographic performance of cementless, monobloc acetabular components made of either traditional titanium-alloy or a newly developed porous tantalum material (Fig. 1). This provided a unique opportunity to study the difference in bone mineral density (BMD) adjacent to acetabular implants composed of titanium-alloy and porous tantalum in vivo. The elasticity of porous tantalum is proposed to create a more physiologic transfer of stresses to the periacetabular bone, theoretically decreasing detrimental stress-shielding. Three-dimensional finite element analysis has predicted that a lowstiffness, porous tantalum, monobloc implant will produce a more physiologic stress pattern over a traditional rigid metal backed acetabular design [19]. The study hypothesis is that the patients implanted with the highly porous tantalum acetabular implants will demonstrate a lesser degree of BMD loss compared with patients implanted with solid titanium components.
Materials and Methods A randomized, prospective investigation of cementless primary total hip arthroplasty (THA) with cementless monobloc acetabular components composed of titaniumalloy and porous tantalum (Elliptical and Hedrocel Cups; Implex Corp, Allendale, NJ) was initiated in 1998 and completed in 1999. The monoblock porous tantalum acetabular component consists of a polyethylene liner
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742 The Journal of Arthroplasty Vol. 25 No. 5 August 2010
Fig. 1. (A) Image of the monobloc titanium and porous tantalum acetabular components. (Inset) Image of a sectioned monoblock porous tantalum component demonstrating the direct molding of the polyethylene into the highly porous tantalum material.
molded directly into the highly porous tantalum material (Fig. 1, inset) without the use of a rigid titanium inner core. The enrollment consisted of 100 patients with 50 THAs using a titanium acetabular component and 50 patients using a porous tantalum component. From this larger study cohort, 17 patients met inclusion criteria and underwent quantitative computed tomography (QCT) to evaluate the periacetabular bone density adjacent to the cementless acetabular component. Patients were excluded for age less than 50 years at the index surgery, the absence of a native contralateral hip, use of bone metabolism altering medication, and the inability to follow up for any reason. Twenty-two patients met the inclusion criteria and were eligible for enrollment. Five patients declined to participate due unwillingness to travel the long distance for follow-up. There were 10
Fig. 2. Schematic (left) and cross-section axial (right) view demonstrating the posterosuperior (A) and anterosuperior (B) periacetabular regions and 5 associated slices (or levels) involved in data collection.
Bone-Remodeling Cementless Acetabular Cups Meneghini et al
female and 7 male patients with an average age of 64.0 years at surgery (range, 46-76 years). Thirteen patients underwent THA for a diagnosis of osteoarthritis, 3 for osteonecrosis, and 1 for rheumatoid arthritis. Nine hips received a porous tantalum acetabular component and 8 received a monobloc titanium component. Six of the 9 patients with porous tantalum shells were female, and 4 of the 8 patients with titanium components were female. All acetabular components were inserted without screws. Institutional review board approval was obtained from the study institution for the original study, as well as the subsequent QCT analysis, and informed consent was obtained on each patient. All study patients underwent routine preoperative and postoperative clinical and radiographic evaluation that included a QCT scan at final follow-up. Bone mineral density was measured quantitatively with computed tomographic (CT) scans of the periacetabular bone around the implant, as well as the equivalent acetabular regions in the native contralateral hip. Data from the native contralateral hip served as a comparison to derive the interval change in bone density around the acetabular components. It has been reported that BMD in the native contralateral native hip of patients with THA is not statistically different from the preoperative level measured at an average of 1.3 years postoperatively [20]. The BMD was measured with QCT. The protocol was developed and is consistent in methodology to a similar study reported in the peer-reviewed literature [20] Each patient was positioned supine on a Sensation 64 CT scanner (Siemens Medical Solutions, Forchheim, Germany). A soft foam roll was positioned under the patient's lumbar spine to bring the pelvis into a flexed position. This flexed pelvic position, in combination with tilting of the CT scanner gantry, provided data acquisition from 2 distinct regions of the periacetabular bone. The 2 regions analyzed were anterosuperior and posterosuperior to the acetabular component (Fig. 2). Frontal scout images were obtained to ensure the pelvis was level and not tilted in the coronal plane. A lateral scout image was obtained to ensure a standardized pelvic tilt, with the inferior border of the obturator foramen being perpendicular to the image table within 5°. In addition, the lateral scout image was used to confirm a level pelvis in the coronal plane by observing superimposed obturator foramen. A calcium calibration phantom was positioned in the field of view posterior to the patient. The calibration phantom (Mindways Model 3) contained samples of know mineral density and the scanner calibration was confirmed daily. Once the patient and pelvis were positioned properly, the gantry was angled approximately 20° cephalad to the neutral vertical orientation and scanning was performed. To maintain a consistent and standardized periacetabular region of data collection, the gantry was then titled 20° in the caudal direction and a second exposure occurred. This provided data collection from 2 distinct regions of
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periacetabular bone: anterosuperior to the implant as well as posterosuperior to the implant (Fig. 2). The CT scan data was collected in each of the 2 regions by scanning the acetabular bone adjacent to the implant in five 3-mm-thick slices (or levels) that were 100 mm2 in cross-sectional area, providing 1.5 cm of data adjacent to the implant (Fig. 2). Bone mineral density was assessed within the 2 regions of interest, referenced to a cylinder of bone that extended peripherally away from the implant through the 5 crosssectional slices of data (Figs. 2A, B, 3A, B). Therefore, the most caudal cross-sectional data were directly adjacent to and within 3 mm of the acetabular implant. The CT numbers (Hounsfield units) corresponding to the calibration phantom samples were used to calculate a linear relationship between measured CT numbers and known calcium concentrations via linear regression. The resulting slope and intercept were applied to the CT numbers measured within the trabecular bone to
Fig. 3. Schematic (A) and cross-section axial view (B) demonstrating the calibration phantom and the 2 cylinders of bone where the BMD calculations were derived.
744 The Journal of Arthroplasty Vol. 25 No. 5 August 2010 Table 1. Mean Bone Mineral Density Difference Calculations: Separate Regions BMD Difference (mg/mL) Porous Tantalum
Titanium
Posterosuperior Zone 1 Zone 2 Zone 3 Zone 4 Zone 5
−0.3 −14.1 −3.7 10.8 38.4
−21.2 −43.6 −42.9 −45.5 −28.6
Anterosuperior Zone 1 Zone 2 Zone 3 Zone 4 Zone 5
16.0 −9.5 −11.9 12.9 29.9
−19.2 −67.4 −62.9 −27.2 9.0
determine BMD. Bone mineral density was quantified in the five 3-mm cross-sectional slices in the 2 regions (anterosuperior and posterosuperior) of interest adjacent to the acetabular component. The BMD was compared to each of the 5 identical cross-sectional slices in the contralateral native hip to assess interval change. The difference between the periacetabular BMD of the contralateral native hip and the THA hip was calculated for each of the 5 periacetabular slices (levels). The relative difference was calculated as a percent of change from the contralateral native hip BMD. The means of the above measured changes in BMD were calculated and compared for differences between the 2 treatment groups. A 2-sample t test was used to assess differences between the 2 treatment groups and statistical significance was concluded for P ≤ .05.
Results All patients were ambulatory at the final follow-up. The mean follow-up was 7.7 years (range, 6.1-8.8 years) with a mean of 7.4 years in the tantalum group and 7.9 years in the titanium group. Harris Hip Scores improved from a mean 54 (range, 45-62) preoperatively to a mean of 92 (range, 71-100) postoperatively. There was no difference in the preoperative or postoperative Harris Hip Scores between the 2 patient groups (P = .3 and P = .8, respectively) with the numbers available. All radiographs and CT scans revealed well-fixed acetabular and femoral components. There were no radiolucent lines around the acetabular components and no evidence of implant migration. There were no osteolytic areas noted around any of the porous tantalum or titanium acetabular components at final radiographic review. Absolute BMD Difference The mean difference in BMD between the control hip and the periacetabular bone of the implanted hip was decreased in 9 of 10 acetabular levels (5 slices in each of the 2 regions, anterosuperior and posterosuperior) around the titanium cup and ranged from 19.2 to 67.4 mg/mL. The only level that demonstrated a mean
Percent BMD Change (%) P
Porous Tantalum
Titanium
P
.60 .37 .02 .006 .20
6.1 −8.8 −1.5 5.3 26.1
−12.2 −44.7 −31.1 −31.2 −11.0
.22 .12 .014 .017 .38
.38 .13 .16 .19 .45
31.7 12.2 28.3 41.0 30.1
6.5 −27.3 −37.4 −19.0 10.0
.41 .12 .10 .18 .47
increase in the BMD from the contralateral control hip was the anterosuperior level 5 (9.0 mg/mL), the most peripheral (or cephalad) level away from the titanium implant (Table 1). All 5 levels in the posterosuperior region adjacent to the titanium implant demonstrated a decrease in BMD. In contrast, 5 of the 10 acetabular levels in the porous tantalum group demonstrated in increase in the mean BMD difference over the control hip (range, 10.838.4 mg/mL), with 4 of the 5 located in 2 most cephalad levels (levels 4 and 5) in both anterosuperior and posterosuperior regions (Table 1). Greater BMD of the periacetabular bone around the porous tantalum over titanium implant was observed to be statistically significant in 2 posterosuperior levels (Fig. 4A), level 3 (P = .023; α = .05, β = 85.0%), and level 4 (P = .006; α = .05, β = 96.1%). These levels equate to a distance 9 to 15 mm from the implant surface. Percent Change in BMD (Relative Change) The mean percent change, or relative change in BMD from the control hip (considered the preoperative baseline), in BMD of the periacetabular bone was decreased in 8 of 10 acetabular levels around the titanium implant and ranged from 11.0% to 44.7% (Table 1). All 5 levels in the posterosuperior region adjacent to the titanium implant demonstrated a relative decrease in BMD. In contrast, 8 of 10 acetabular levels in the porous tantalum group demonstrated an increase in the mean percent change in BMD over the control hip and ranged from an increase of 5.3% to 41.0% in BMD (Table 1, Fig. 4B). The only 2 levels around the porous tantalum acetabular component that demonstrated a mean percentage decrease in BMD were in levels 2 and 3 of the posterosuperior region and were a nominal 8.8% and 1.5%, respectively. The mean percent change in periacetabular BMD around the porous tantalum implant was less than around the titanium implant in all 10 acetabular levels, and statistical significance was observed in level 3 (P = .014; α = .05, β = 90.9%) and level 4 (P = .017; α = .05, β = 88.5%) of the posterosuperior region (Table 1, Fig. 4B).
Bone-Remodeling Cementless Acetabular Cups Meneghini et al
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Fig. 4. Bar graph illustrating the mean absolute (A) and percent (B) change in BMD around the porous tantalum and titanium acetabular components in the posterosuperior region. Level 1 is closest to, and level 5, farthest from the implant.
Discussion Remodeling and stress-shielding of the proximal femur in response to implants has been well studied, yet the periacetabular bone response to cementless hemispherical cups is less well characterized. Finite element modeling of press-fit, uncemented acetabular components has predicted bone remodeling due to nonphysiologic transfer of loads to the peripheral cortical acetabular bone, which results in significant attenuation of bone density medial and inferior to the acetabular implant [10]. This stressshielding pattern has been corroborated in the short term via clinical DEXA studies of periacetabular bone after cementless total hip arthroplasty [13,15,18]. However, the use of DEXA methodology did not allow assessment of the retroacetabular bone density and remodeling located posterior to the implant. Furthermore, these studies failed to directly compare stress-shielding adjacent to acetabular components composed of materials with differing stiffness. The results in the current study support the development of decreased BMD around solid-metal osseointegrated cementless acetabular components. In a prospective analysis of bone density adjacent to cementless acetabular implants with quantitative CT, BMD values declined 20% to 33% from preoperative values at a minimum of 1-year follow-up [20]. The authors reported a mean interval reduction in BMD of 75 mg/mL between the preoperative and follow-up evaluations. The mean BMD at final 1-year follow up in this report of 26 THAs was 155 ± 60 μg/mL on the operated side and 205 ± 90 μg/mL on the untreated side at the second
level adjacent to the implant [20]. Furthermore, consistent with our data in this report, the authors demonstrated the magnitude of stress-shielding was greatest immediately adjacent to the implant. Although the authors reported less stress-shielding further from the implant, a reduction in BMD by 35 μg/mL was observed 10 mm from the implant [20]. Concerns exist that acetabular stress-shielding may facilitate or even exacerbate the osteolytic response to bearing surface particles over time. This decrease in bone density, with or without osteolysis, may compromise the subsequent success of revision acetabular reconstruction if required. It has been documented that improved success in revision acetabular surgery is obtained when a greater amount of viable bone stock is available for component fixation [21]. Therefore, it is logical that choosing an acetabular component that will minimize the decrease in BMD attenuation at the time of primary THA may ultimately provide a better outcome at the eventual revision surgery. Field et al [14] conducted a prospective study of bone density around a novel, flexible, horseshoe-shaped cementless acetabular component (Cambridge cup) comprising a 3-mm bearing surface of polyethylene molded to a 1.5-mm backing of carbon fiber-reinforced polybutyleneterephthalate. The authors demonstrated preservation of bone density and lack of stress-shielding in the superior and superomedial weight-bearing regions at 2 years after THA using the more flexible acetabular component [14]. The importance of preserving periprosthetic bone stock
746 The Journal of Arthroplasty Vol. 25 No. 5 August 2010 cannot be overstated in a time where patients are receiving hip arthroplasty at younger ages and life expectancy continues to increase, escalating the likelihood of an eventual revision operation. Porous tantalum has emerged as a biologically and mechanically viable biomaterial. In addition to rapid bone ingrowth and increased interface strength [22], porous tantalum provides increased material elasticity and a surface frictional coefficient greater than other implant surfaces. The modulus of elasticity of porous-tantalum is between cortical and cancellous bone, significantly less than titanium and chromium cobalt materials [23]. This elasticity of porous tantalum likely creates a morephysiologic transfer of stresses to the periacetabular bone, theoretically decreasing detrimental stress-shielding. In a finite element analysis of periacetabular stresses, Brown et al [19] demonstrated that acetabular monobloc implants composed of porous tantalum produced a bone stress pattern closer to the predicted stress pattern induced by cemented all-polyethylene acetabular shells, rather then cementless titanium shells. The data obtained in our current study, obtained with QCT at a mean of 7.7 years, supports a more physiologic stress transfer to the underlying subchondral bone via the more elastic porous tantalum monobloc acetabular components. This study resulted from a unique opportunity to study a group of patients from a larger prospective, randomized cohort study comparing acetabular implants differing in core material of either porous tantalum or titanium at intermediate-term follow up. The results uniquely demonstrate that stress shielding continues to occur beyond the short term in the acetabular bone adjacent to well-fixed titanium components. There was a statistically significant difference between the change in measured BMD between the 2 patient groups of porous tantalum and titanium implants in multiple zones adjacent to the acetabular implant in the posterosuperior region (P b .02) as detailed in Table 1 (Fig. 4A and B). These in vivo results at intermediate follow-up corroborate the finite element analysis predictions by Brown et al [19] and support consideration of the elastic properties of the implant materials when designing cementless, pressfit acetabular components. However, it is plausible that the improved stress transfer to the periacetabular bone is a result of the improved ingrowth achieved with the optimized frictional coefficient and biologically conducive surface properties of porous tantalum, rather than the elastic characteristics alone. Finally, this study provided bone density measurements from the retroacetabular region posterior to the implant using QCT methodology. Retroacetabular bone density is technically difficult to obtain with DEXA, and our data demonstrates this region of interest is susceptible to significant stress-shielding, compared to the anterosuperior bone density (Table 1). This study does have limitations. Primarily, the lack of preoperative bone density measurements in the subjects necessitated the use of the contralateral native hip as a
control for an estimation of preoperative BMD. Although certainly not optimal, the data from the native contralateral hip serves as a valid comparison to derive the interval change in the bone remodeling around the acetabular components. Despite these limitations, the authors believe the scientific merit of this data is supported by the derivation of patients from a prospective, randomized study; the quality and anatomic reach of the QCT and BMD calculations; and the intermediate follow-up of 7.7 years. In summary, these data demonstrates stress-shielding continues beyond the short-term around standard solid titanium cementless acetabular components. The data also supports that stress-shielding likely occurs less around a highly porous metal implant of material with an elastic modulus similar to bone.
Acknowledgments The authors would like to thank Jon Camp and his team in the Biomedical Imaging Resource Lab at Mayi Clinic for converting the QCT data to calibrated bone density in the regions of interest. The authors would also like to thank Cynthia McCollough’s team in the CT Clinical Innovation Center at Mayo Clinic for their efforts and contributions in protocol development, as well as data acquisition.
References 1. Engh CA, et al. Porous-coated total hip replacement. Clin Orthop Relat Res 1994;89. 2. Engh CA, et al. A quantitative evaluation of periprosthetic bone-remodeling after cementless total hip arthroplasty. J Bone Joint Surg Am 1992;74:1009. 3. Engh CA, et al. Quantification of implant micromotion, strain shielding, and bone resorption with porous-coated anatomic medullary locking femoral prostheses. Clin Orthop Relat Res 1992;1:13. 4. Engh Jr CA, et al. Clinical consequences of stress shielding after porous-coated total hip arthroplasty. Clin Orthop Relat Res 2003;157. 5. Jacobs JJ, Sumner DR, Galante JO. Mechanisms of bone loss associated with total hip replacement. Orthop Clin North Am 1993;24:583. 6. Kiratli BJ, et al. Determination of bone mineral density by dual x-ray absorptiometry in patients with uncemented total hip arthroplasty. J Orthop Res 1992;10:836. 7. Ritter MA, Fechtman RW. Distal cortical hypertrophy following total hip arthroplasty. J Arthroplasty 1988;3:117. 8. Trevisan C, et al. Dual x-ray absorptiometry for the evaluation of bone density from the proximal femur after total hip arthroplasty: analysis protocols and reproducibility. Calcif Tissue Int 1993;53:158. 9. Huiskes R. Finite element analysis of acetabular reconstruction. Noncemented threaded cups. Acta Orthop Scand 1987;58:620. 10. Levenston ME, et al. Computer simulations of stress-related bone remodeling around noncemented acetabular components. J Arthroplasty 1993;8:595. 11. Beaupre GS, Orr TE, Carter DR. An approach for timedependent bone modeling and remodeling—theoretical development. J Orthop Res 1990;8:651.
Bone-Remodeling Cementless Acetabular Cups Meneghini et al 12. Carter DR, Fyhrie DP, Whalen RT. Trabecular bone density and loading history: regulation of connective tissue biology by mechanical energy. J Biomech 1987;20:785. 13. Digas G, Karrholm J, Thanner J. Different loss of BMD using uncemented press-fit and whole polyethylene cups fixed with cement: repeated DXA studies in 96 hips randomized to 3 types of fixation. Acta Orthop 2006;77:218. 14. Field RE, et al. Bone remodeling around the Cambridge cup: a DEXA study of 50 hips over 2 years. Acta Orthop 2006;77:726. 15. Kim YH, Yoon SH, Kim JS. Changes in the bone mineral density in the acetabulum and proximal femur after cementless total hip replacement: alumina-on-alumina versus alumina-on-polyethylene articulation. J Bone Joint Surg Br 2007;89:174. 16. Korovessis P, Piperos G, Michael A. Periprosthetic bone mineral density after Mueller and Zweymueller total hip arthroplasties. Clin Orthop Relat Res 1994;214. 17. Laursen MB, Nielsen PT, Soballe K. Bone remodelling around HA-coated acetabular cups: a DEXA study with a 3year follow-up in a randomised trial. Int Orthop 2007;31:199.
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18. Sabo D, et al. Periprosthetic mineralization around cementless total hip endoprosthesis: longitudinal study and crosssectional study on titanium threaded acetabular cup and cementless Spotorno stem with DEXA. Calcif Tissue Int 1998;62:177. 19. Brown TD, Poggie RA, Pederson DR. Finite element analysis of peri-acetabular stress of cemented, metal-backed, and porous tantalum backed acetabular components. Trans Orthop Res Soc 1999;24:747. 20. Wright JM, et al. Bone density adjacent to press-fit acetabular components. A prospective analysis with quantitative computed tomography. J Bone Joint Surg Am 2001; 83-A:529. 21. Lewallen DG. Acetabular revision: technique and results. In: Morrey BF, editor. Joint Replacement Arthroplasty. 3rd ed. Philadelphia: Churchill Livingstone; 2003 2003: p. 824. 22. Bobyn JD, et al. Characteristics of bone ingrowth and interface mechanics of a new porous tantalum biomaterial. J Bone Joint Surg Br 1999;81:907. 23. Cohen R. A porous tantalum trabecular metal: basic science. Am J Orthop 2002;31:216.
Bone Remodeling Around Porous Metal Cementless Acetabular Components R. Michael Meneghini, MD,* Kerry S. Ford, MD,z Cynthia H. McCollough, PhD,y Arlen D. Hanssen, MD,z and David G. Lewallen, MDz
Abstract: Bone remodeling around cementless acetabular components after total hip arthroplasty has not been well characterized. A randomized, prospective study of total hip arthroplasty was performed comparing 2 cementless acetabular implants: a solid titanium and a more elastic porous tantalum design. Seventeen hips (9 porous tantalum, 8 titanium) underwent quantitative computed tomography at mean of 7.7 years, and adjacent bone mineral density (BMD) was calculated. The absolute and relative decrease in BMD from preoperative level was less in zones 9 to 15 mm adjacent to the porous tantalum compared to the titanium component (P ≤ .02) and predominated posterosuperiorly. The relative BMD increased in all regions adjacent to the porous tantalum component from 5% to 40% over the control. This data demonstrates stress-shielding likely occurs less around a highly porous metal implant of material with an elastic modulus similar to bone. Keywords: bone mineral density, total hip arthroplasty, cementless, quantitative computed tomography, porous tantalum, bone-remodeling. © 2010 Elsevier Inc. All rights reserved.
The location and amount of periprosthetic bone stock are critical factors determining success during revision acetabular reconstruction after total hip arthroplasty (THA). Although remodeling and stress-shielding of the proximal femur in response to implants has been well characterized [1-8], the response to cementless acetabular components has not been as extensively studied. Finite element modeling of cementless acetabular components has predicted a concentration of the loads and stresses at the component periphery into the cortical bone which produces shielding of the proximal and medial trabecular bone [9,10] and a theoretical attenuation of bone density behind the implant [10-12]. To date, most investigation of bone density around acetabular components have utilized dual energy x-ray absorptiometry (DEXA), which precludes accurate assessment of the important retroacetab-
From the *Department of Orthopaedic Surgery, New England Musculoskeletal Institute, University of Connecticut Health Center, Farmington, Connecticut; yDepartment of Radiology, Mayo Clinic, Rochester, Minnesota; and zDepartment of Orthopaedic Surgery, Mayo Clinic, Rochester, Minnesota. Submitted September 8, 2008; accepted April 18, 2009. While the authors did not receive any benefits for this part of the study, benefits were received by Implex for the original study from which this cohort was taken. Therefore, it should be noted that benefits were received in support of this study from Implex. Also it should be noted that one or more of the authors have received royalties and payments from Implex/Zimmer. Reprint requests: R. Michael Meneghini, MD, Department of Orthopaedic Surgery, University of Connecticut Health Center, Medical Arts and Research Building, 4th Floor, #4016, 263 Farmington Ave, Farmington, CT 06034-4037. © 2010 Elsevier Inc. All rights reserved. 0883-5403/2505-0011$36.00/0 doi:10.1016/j.arth.2009.04.025
ular region [13-18]. Furthermore, longer-term follow-up studies are lacking, and no studies have directly compared bone remodeling in response to cementless acetabular implants composed of different core materials. A randomized, prospective study was undertaken to investigate the clinical and radiographic performance of cementless, monobloc acetabular components made of either traditional titanium-alloy or a newly developed porous tantalum material (Fig. 1). This provided a unique opportunity to study the difference in bone mineral density (BMD) adjacent to acetabular implants composed of titanium-alloy and porous tantalum in vivo. The elasticity of porous tantalum is proposed to create a more physiologic transfer of stresses to the periacetabular bone, theoretically decreasing detrimental stress-shielding. Three-dimensional finite element analysis has predicted that a lowstiffness, porous tantalum, monobloc implant will produce a more physiologic stress pattern over a traditional rigid metal backed acetabular design [19]. The study hypothesis is that the patients implanted with the highly porous tantalum acetabular implants will demonstrate a lesser degree of BMD loss compared with patients implanted with solid titanium components.
Materials and Methods A randomized, prospective investigation of cementless primary total hip arthroplasty (THA) with cementless monobloc acetabular components composed of titaniumalloy and porous tantalum (Elliptical and Hedrocel Cups; Implex Corp, Allendale, NJ) was initiated in 1998 and completed in 1999. The monoblock porous tantalum acetabular component consists of a polyethylene liner
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742 The Journal of Arthroplasty Vol. 25 No. 5 August 2010
Fig. 1. (A) Image of the monobloc titanium and porous tantalum acetabular components. (Inset) Image of a sectioned monoblock porous tantalum component demonstrating the direct molding of the polyethylene into the highly porous tantalum material.
molded directly into the highly porous tantalum material (Fig. 1, inset) without the use of a rigid titanium inner core. The enrollment consisted of 100 patients with 50 THAs using a titanium acetabular component and 50 patients using a porous tantalum component. From this larger study cohort, 17 patients met inclusion criteria and underwent quantitative computed tomography (QCT) to evaluate the periacetabular bone density adjacent to the cementless acetabular component. Patients were excluded for age less than 50 years at the index surgery, the absence of a native contralateral hip, use of bone metabolism altering medication, and the inability to follow up for any reason. Twenty-two patients met the inclusion criteria and were eligible for enrollment. Five patients declined to participate due unwillingness to travel the long distance for follow-up. There were 10
Fig. 2. Schematic (left) and cross-section axial (right) view demonstrating the posterosuperior (A) and anterosuperior (B) periacetabular regions and 5 associated slices (or levels) involved in data collection.
Bone-Remodeling Cementless Acetabular Cups Meneghini et al
female and 7 male patients with an average age of 64.0 years at surgery (range, 46-76 years). Thirteen patients underwent THA for a diagnosis of osteoarthritis, 3 for osteonecrosis, and 1 for rheumatoid arthritis. Nine hips received a porous tantalum acetabular component and 8 received a monobloc titanium component. Six of the 9 patients with porous tantalum shells were female, and 4 of the 8 patients with titanium components were female. All acetabular components were inserted without screws. Institutional review board approval was obtained from the study institution for the original study, as well as the subsequent QCT analysis, and informed consent was obtained on each patient. All study patients underwent routine preoperative and postoperative clinical and radiographic evaluation that included a QCT scan at final follow-up. Bone mineral density was measured quantitatively with computed tomographic (CT) scans of the periacetabular bone around the implant, as well as the equivalent acetabular regions in the native contralateral hip. Data from the native contralateral hip served as a comparison to derive the interval change in bone density around the acetabular components. It has been reported that BMD in the native contralateral native hip of patients with THA is not statistically different from the preoperative level measured at an average of 1.3 years postoperatively [20]. The BMD was measured with QCT. The protocol was developed and is consistent in methodology to a similar study reported in the peer-reviewed literature [20] Each patient was positioned supine on a Sensation 64 CT scanner (Siemens Medical Solutions, Forchheim, Germany). A soft foam roll was positioned under the patient's lumbar spine to bring the pelvis into a flexed position. This flexed pelvic position, in combination with tilting of the CT scanner gantry, provided data acquisition from 2 distinct regions of the periacetabular bone. The 2 regions analyzed were anterosuperior and posterosuperior to the acetabular component (Fig. 2). Frontal scout images were obtained to ensure the pelvis was level and not tilted in the coronal plane. A lateral scout image was obtained to ensure a standardized pelvic tilt, with the inferior border of the obturator foramen being perpendicular to the image table within 5°. In addition, the lateral scout image was used to confirm a level pelvis in the coronal plane by observing superimposed obturator foramen. A calcium calibration phantom was positioned in the field of view posterior to the patient. The calibration phantom (Mindways Model 3) contained samples of know mineral density and the scanner calibration was confirmed daily. Once the patient and pelvis were positioned properly, the gantry was angled approximately 20° cephalad to the neutral vertical orientation and scanning was performed. To maintain a consistent and standardized periacetabular region of data collection, the gantry was then titled 20° in the caudal direction and a second exposure occurred. This provided data collection from 2 distinct regions of
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periacetabular bone: anterosuperior to the implant as well as posterosuperior to the implant (Fig. 2). The CT scan data was collected in each of the 2 regions by scanning the acetabular bone adjacent to the implant in five 3-mm-thick slices (or levels) that were 100 mm2 in cross-sectional area, providing 1.5 cm of data adjacent to the implant (Fig. 2). Bone mineral density was assessed within the 2 regions of interest, referenced to a cylinder of bone that extended peripherally away from the implant through the 5 crosssectional slices of data (Figs. 2A, B, 3A, B). Therefore, the most caudal cross-sectional data were directly adjacent to and within 3 mm of the acetabular implant. The CT numbers (Hounsfield units) corresponding to the calibration phantom samples were used to calculate a linear relationship between measured CT numbers and known calcium concentrations via linear regression. The resulting slope and intercept were applied to the CT numbers measured within the trabecular bone to
Fig. 3. Schematic (A) and cross-section axial view (B) demonstrating the calibration phantom and the 2 cylinders of bone where the BMD calculations were derived.
744 The Journal of Arthroplasty Vol. 25 No. 5 August 2010 Table 1. Mean Bone Mineral Density Difference Calculations: Separate Regions BMD Difference (mg/mL) Porous Tantalum
Titanium
Posterosuperior Zone 1 Zone 2 Zone 3 Zone 4 Zone 5
−0.3 −14.1 −3.7 10.8 38.4
−21.2 −43.6 −42.9 −45.5 −28.6
Anterosuperior Zone 1 Zone 2 Zone 3 Zone 4 Zone 5
16.0 −9.5 −11.9 12.9 29.9
−19.2 −67.4 −62.9 −27.2 9.0
determine BMD. Bone mineral density was quantified in the five 3-mm cross-sectional slices in the 2 regions (anterosuperior and posterosuperior) of interest adjacent to the acetabular component. The BMD was compared to each of the 5 identical cross-sectional slices in the contralateral native hip to assess interval change. The difference between the periacetabular BMD of the contralateral native hip and the THA hip was calculated for each of the 5 periacetabular slices (levels). The relative difference was calculated as a percent of change from the contralateral native hip BMD. The means of the above measured changes in BMD were calculated and compared for differences between the 2 treatment groups. A 2-sample t test was used to assess differences between the 2 treatment groups and statistical significance was concluded for P ≤ .05.
Results All patients were ambulatory at the final follow-up. The mean follow-up was 7.7 years (range, 6.1-8.8 years) with a mean of 7.4 years in the tantalum group and 7.9 years in the titanium group. Harris Hip Scores improved from a mean 54 (range, 45-62) preoperatively to a mean of 92 (range, 71-100) postoperatively. There was no difference in the preoperative or postoperative Harris Hip Scores between the 2 patient groups (P = .3 and P = .8, respectively) with the numbers available. All radiographs and CT scans revealed well-fixed acetabular and femoral components. There were no radiolucent lines around the acetabular components and no evidence of implant migration. There were no osteolytic areas noted around any of the porous tantalum or titanium acetabular components at final radiographic review. Absolute BMD Difference The mean difference in BMD between the control hip and the periacetabular bone of the implanted hip was decreased in 9 of 10 acetabular levels (5 slices in each of the 2 regions, anterosuperior and posterosuperior) around the titanium cup and ranged from 19.2 to 67.4 mg/mL. The only level that demonstrated a mean
Percent BMD Change (%) P
Porous Tantalum
Titanium
P
.60 .37 .02 .006 .20
6.1 −8.8 −1.5 5.3 26.1
−12.2 −44.7 −31.1 −31.2 −11.0
.22 .12 .014 .017 .38
.38 .13 .16 .19 .45
31.7 12.2 28.3 41.0 30.1
6.5 −27.3 −37.4 −19.0 10.0
.41 .12 .10 .18 .47
increase in the BMD from the contralateral control hip was the anterosuperior level 5 (9.0 mg/mL), the most peripheral (or cephalad) level away from the titanium implant (Table 1). All 5 levels in the posterosuperior region adjacent to the titanium implant demonstrated a decrease in BMD. In contrast, 5 of the 10 acetabular levels in the porous tantalum group demonstrated in increase in the mean BMD difference over the control hip (range, 10.838.4 mg/mL), with 4 of the 5 located in 2 most cephalad levels (levels 4 and 5) in both anterosuperior and posterosuperior regions (Table 1). Greater BMD of the periacetabular bone around the porous tantalum over titanium implant was observed to be statistically significant in 2 posterosuperior levels (Fig. 4A), level 3 (P = .023; α = .05, β = 85.0%), and level 4 (P = .006; α = .05, β = 96.1%). These levels equate to a distance 9 to 15 mm from the implant surface. Percent Change in BMD (Relative Change) The mean percent change, or relative change in BMD from the control hip (considered the preoperative baseline), in BMD of the periacetabular bone was decreased in 8 of 10 acetabular levels around the titanium implant and ranged from 11.0% to 44.7% (Table 1). All 5 levels in the posterosuperior region adjacent to the titanium implant demonstrated a relative decrease in BMD. In contrast, 8 of 10 acetabular levels in the porous tantalum group demonstrated an increase in the mean percent change in BMD over the control hip and ranged from an increase of 5.3% to 41.0% in BMD (Table 1, Fig. 4B). The only 2 levels around the porous tantalum acetabular component that demonstrated a mean percentage decrease in BMD were in levels 2 and 3 of the posterosuperior region and were a nominal 8.8% and 1.5%, respectively. The mean percent change in periacetabular BMD around the porous tantalum implant was less than around the titanium implant in all 10 acetabular levels, and statistical significance was observed in level 3 (P = .014; α = .05, β = 90.9%) and level 4 (P = .017; α = .05, β = 88.5%) of the posterosuperior region (Table 1, Fig. 4B).
Bone-Remodeling Cementless Acetabular Cups Meneghini et al
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Fig. 4. Bar graph illustrating the mean absolute (A) and percent (B) change in BMD around the porous tantalum and titanium acetabular components in the posterosuperior region. Level 1 is closest to, and level 5, farthest from the implant.
Discussion Remodeling and stress-shielding of the proximal femur in response to implants has been well studied, yet the periacetabular bone response to cementless hemispherical cups is less well characterized. Finite element modeling of press-fit, uncemented acetabular components has predicted bone remodeling due to nonphysiologic transfer of loads to the peripheral cortical acetabular bone, which results in significant attenuation of bone density medial and inferior to the acetabular implant [10]. This stressshielding pattern has been corroborated in the short term via clinical DEXA studies of periacetabular bone after cementless total hip arthroplasty [13,15,18]. However, the use of DEXA methodology did not allow assessment of the retroacetabular bone density and remodeling located posterior to the implant. Furthermore, these studies failed to directly compare stress-shielding adjacent to acetabular components composed of materials with differing stiffness. The results in the current study support the development of decreased BMD around solid-metal osseointegrated cementless acetabular components. In a prospective analysis of bone density adjacent to cementless acetabular implants with quantitative CT, BMD values declined 20% to 33% from preoperative values at a minimum of 1-year follow-up [20]. The authors reported a mean interval reduction in BMD of 75 mg/mL between the preoperative and follow-up evaluations. The mean BMD at final 1-year follow up in this report of 26 THAs was 155 ± 60 μg/mL on the operated side and 205 ± 90 μg/mL on the untreated side at the second
level adjacent to the implant [20]. Furthermore, consistent with our data in this report, the authors demonstrated the magnitude of stress-shielding was greatest immediately adjacent to the implant. Although the authors reported less stress-shielding further from the implant, a reduction in BMD by 35 μg/mL was observed 10 mm from the implant [20]. Concerns exist that acetabular stress-shielding may facilitate or even exacerbate the osteolytic response to bearing surface particles over time. This decrease in bone density, with or without osteolysis, may compromise the subsequent success of revision acetabular reconstruction if required. It has been documented that improved success in revision acetabular surgery is obtained when a greater amount of viable bone stock is available for component fixation [21]. Therefore, it is logical that choosing an acetabular component that will minimize the decrease in BMD attenuation at the time of primary THA may ultimately provide a better outcome at the eventual revision surgery. Field et al [14] conducted a prospective study of bone density around a novel, flexible, horseshoe-shaped cementless acetabular component (Cambridge cup) comprising a 3-mm bearing surface of polyethylene molded to a 1.5-mm backing of carbon fiber-reinforced polybutyleneterephthalate. The authors demonstrated preservation of bone density and lack of stress-shielding in the superior and superomedial weight-bearing regions at 2 years after THA using the more flexible acetabular component [14]. The importance of preserving periprosthetic bone stock
746 The Journal of Arthroplasty Vol. 25 No. 5 August 2010 cannot be overstated in a time where patients are receiving hip arthroplasty at younger ages and life expectancy continues to increase, escalating the likelihood of an eventual revision operation. Porous tantalum has emerged as a biologically and mechanically viable biomaterial. In addition to rapid bone ingrowth and increased interface strength [22], porous tantalum provides increased material elasticity and a surface frictional coefficient greater than other implant surfaces. The modulus of elasticity of porous-tantalum is between cortical and cancellous bone, significantly less than titanium and chromium cobalt materials [23]. This elasticity of porous tantalum likely creates a morephysiologic transfer of stresses to the periacetabular bone, theoretically decreasing detrimental stress-shielding. In a finite element analysis of periacetabular stresses, Brown et al [19] demonstrated that acetabular monobloc implants composed of porous tantalum produced a bone stress pattern closer to the predicted stress pattern induced by cemented all-polyethylene acetabular shells, rather then cementless titanium shells. The data obtained in our current study, obtained with QCT at a mean of 7.7 years, supports a more physiologic stress transfer to the underlying subchondral bone via the more elastic porous tantalum monobloc acetabular components. This study resulted from a unique opportunity to study a group of patients from a larger prospective, randomized cohort study comparing acetabular implants differing in core material of either porous tantalum or titanium at intermediate-term follow up. The results uniquely demonstrate that stress shielding continues to occur beyond the short term in the acetabular bone adjacent to well-fixed titanium components. There was a statistically significant difference between the change in measured BMD between the 2 patient groups of porous tantalum and titanium implants in multiple zones adjacent to the acetabular implant in the posterosuperior region (P b .02) as detailed in Table 1 (Fig. 4A and B). These in vivo results at intermediate follow-up corroborate the finite element analysis predictions by Brown et al [19] and support consideration of the elastic properties of the implant materials when designing cementless, pressfit acetabular components. However, it is plausible that the improved stress transfer to the periacetabular bone is a result of the improved ingrowth achieved with the optimized frictional coefficient and biologically conducive surface properties of porous tantalum, rather than the elastic characteristics alone. Finally, this study provided bone density measurements from the retroacetabular region posterior to the implant using QCT methodology. Retroacetabular bone density is technically difficult to obtain with DEXA, and our data demonstrates this region of interest is susceptible to significant stress-shielding, compared to the anterosuperior bone density (Table 1). This study does have limitations. Primarily, the lack of preoperative bone density measurements in the subjects necessitated the use of the contralateral native hip as a
control for an estimation of preoperative BMD. Although certainly not optimal, the data from the native contralateral hip serves as a valid comparison to derive the interval change in the bone remodeling around the acetabular components. Despite these limitations, the authors believe the scientific merit of this data is supported by the derivation of patients from a prospective, randomized study; the quality and anatomic reach of the QCT and BMD calculations; and the intermediate follow-up of 7.7 years. In summary, these data demonstrates stress-shielding continues beyond the short-term around standard solid titanium cementless acetabular components. The data also supports that stress-shielding likely occurs less around a highly porous metal implant of material with an elastic modulus similar to bone.
Acknowledgments The authors would like to thank Jon Camp and his team in the Biomedical Imaging Resource Lab at Mayi Clinic for converting the QCT data to calibrated bone density in the regions of interest. The authors would also like to thank Cynthia McCollough’s team in the CT Clinical Innovation Center at Mayo Clinic for their efforts and contributions in protocol development, as well as data acquisition.
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18. Sabo D, et al. Periprosthetic mineralization around cementless total hip endoprosthesis: longitudinal study and crosssectional study on titanium threaded acetabular cup and cementless Spotorno stem with DEXA. Calcif Tissue Int 1998;62:177. 19. Brown TD, Poggie RA, Pederson DR. Finite element analysis of peri-acetabular stress of cemented, metal-backed, and porous tantalum backed acetabular components. Trans Orthop Res Soc 1999;24:747. 20. Wright JM, et al. Bone density adjacent to press-fit acetabular components. A prospective analysis with quantitative computed tomography. J Bone Joint Surg Am 2001; 83-A:529. 21. Lewallen DG. Acetabular revision: technique and results. In: Morrey BF, editor. Joint Replacement Arthroplasty. 3rd ed. Philadelphia: Churchill Livingstone; 2003 2003: p. 824. 22. Bobyn JD, et al. Characteristics of bone ingrowth and interface mechanics of a new porous tantalum biomaterial. J Bone Joint Surg Br 1999;81:907. 23. Cohen R. A porous tantalum trabecular metal: basic science. Am J Orthop 2002;31:216.