Quantitative Computed Tomography-Assisted Osteodensitometry of the Pelvis After Press-Fit Cup Fixation: A Prospective Ten-Year Follow-up

Kress, Alexander M. MD; Schmidt, Rainer MD, PhD; Vogel, Tobias MD; Nowak, Tobias E. MD; Forst, Raimund MD, PhD; Mueller, Lutz A. MD, PhD

Journal of Bone & Joint Surgery - American Volume:
doi: 10.2106/JBJS.J.01097
Scientific Articles

Background: As a follow-up of a previously reported three-year study, we analyzed the periprosthetic acetabular cortical and cancellous bone density changes at ten years after implantation of a press-fit cup.

Methods: Prospective clinical, radiographic, and quantitative computed tomography examinations were performed within ten days and at mean periods of one, three, and ten years after total hip arthroplasty with a press-fit cup, a femoral stem with a tapered design, and alumina-alumina pairing. Periacetabular cortical and cancellous bone density (mg CaHA/mL) in the cranial, ventral, and dorsal regions about the cup were measured for twenty-four hips in vivo.

Results: All acetabular cups showed radiographic signs of stable ingrowth, and no acetabular component had to be revised. The loss of periacetabular cancellous bone density about the cup was as much as –37% cranially, –60% ventrally, and –71% dorsally; the decrease was progressive between the one-year and three-year examinations only. In contrast, cortical bone density above the dome of the acetabular cup remained constant throughout the ten-year follow-up. A moderate change in cortical bone density of –5% to –18% was seen at the level of the cup ten years postoperatively.

Conclusions: Both periacetabular cortical and cancellous bone density changes were nonprogressive between the three-year and ten-year examinations after press-fit cup fixation.

Clinical Relevance: Ten years postoperatively, the measured periacetabular cortical and cancellous bone density changes had no influence on the good clinical and radiographic performance of the cup.

Author Information

1Department of Orthopaedic Surgery, Friedrich-Alexander-University of Erlangen-Nuremberg, Rathsberger Strasse 57, D-91054 Erlangen, Germany. E-mail address for L.A. Mueller: ltzmll@aol.com

2Department of Trauma Surgery, Johannes-Gutenberg-University, Langenbeckstrasse 1, D-55131 Mainz, Germany

Article Outline

The remodeling pattern of periprosthetic bone after total hip arthroplasty is of major interest to basic-science researchers as reflected by numerous publications on finite element models1-5. The clinical implication of these results is unclear as the natural long-term cause of periacetabular cancellous and cortical bone density changes has never been investigated1-5. Dual x-ray absorptiometry studies6-8 have shown substantial loss of pelvic bone density during the first six and twelve months after total hip arthroplasty with use of press-fit cups6,8.

None of the above-mentioned studies or the numerous other dual x-ray absorptiometry9-13 and finite element studies1,2,14,15 on femoral bone density changes have been able to demonstrate any evidence-based clinical relevance of their results, and it thus remains unknown whether bone density loss compromises the longevity of a total hip replacement.

As the elastic modulus of metal-backed cups used for young and active patients is greater than the surrounding acetabular bone, stress-shielding and the periprosthetic bone resorption phenomena may, in theory, be clinically relevant for high-demand patients with long-term functional expectations6,16,17.

In previous studies, we reported one-year and three-year quantitative computed tomography (CT) data on periprosthetic bone-density changes after total hip arthroplasty. We found a marked progressive cortical and cancellous bone density decrease in all regions of interest except for cortical bone adjacent to the dome of the cup18,19.

On the basis of previous findings, we hypothesized there would be further substantial loss of periacetabular cancellous bone density between the three-year and ten-year follow-up evaluations and that cortical bone density changes cranial to the cup would be nonprogressive.

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Materials and Methods

We recruited twenty-six consecutive patients (twenty-six hips) with degenerative hip disease who had been admitted for total hip arthroplasty. The one-year and three-year results of this group were reported previously18,19. For this prospective study, all patients had follow-up quantitative CT examinations. Using a two-sided 95% confidence interval, we found that a comparative study with a sample size of twenty patients would have an 80% power to detect a 5% difference between serial quantitative CT measurements.

Concerning our exclusion criteria, we refer to our published three-year follow-up results18. Of the twenty-six patients, twenty-four (twenty-four hips) had postoperative one-year, three-year, and ten-year follow-up quantitative CT data sets for densitometry analysis. Two patients were excluded from the study (one patient had incomplete quantitative CT data sets, and one patient was lost to follow-up). The average age of the patients at the index operation was fifty-nine years (range, thirty-nine to seventy-seven years). There were nine women and fifteen men. The study was approved by the local ethics committee. Informed consent was obtained from all patients.

The Cerafit cup (Cerafit Triradius; Ceraver Osteal, Paris, France) is a fiber-mesh, metal-backed press-fit cup (52 to 60 mm in diameter) made of titanium alloy (Ti6Al7Nb; 3-mm thickness) with a flattened dome and a narrow equatorial rim with a matte surface finish. The cup is coated with hydroxyapatite (Ca3[PO4]2). An alumina (Al2O3) ceramic liner and an uncemented, rough blasted, triple-taper hydroxyapatite (Ca5[PO4]3[OH])-coated titanium alloy (Ti6Al7Nb) femoral stem with a 32-mm-diameter alumina (Al2O3) ceramic head was used in all hips. For specifications of the surgical and postoperative procedures, we refer to our prior publication18.

We obtained Harris hip scores20 and administered the Oxford patient questionnaire21 (preoperatively and at one, three, and ten years postoperatively). Serial anteroposterior and lateral radiographs of the hip were made preoperatively and at the time of clinical follow-up. Definite radiographic loosening of the acetabular component was analyzed according to the method of Hodgkinson et al.22.

Computed tomography scans were made within the first ten days after total hip arthroplasty and subsequently at the one-year, three-year, and ten-year follow-up examinations. The protocol for CT scans has been described previously18.

Three of the six axial scans were performed above the cup; three scans were performed at the level of the cup (Fig. 1). An analysis of the ventral and dorsal regions of the retroacetabular area was performed by dividing the scan images through a coronal line passing across the center of the ceramic head of the stem (Fig. 2). The noninvolved, contralateral side was used as a control.

To more easily interpret our data, we created three regions of interest (ROI), ventral, dorsal, and cranial to the cup, by calculating cortical and cancellous bone density (BD) values of the area (in square millimeters) of each scan under consideration according to the following expression18:

where a = area (mm2) of the respective ROI for CT scans 1, 2, and 3, and BD = bone density (mg CaHA/mL) of the respective ROI for CT scans 1, 2, and 3.

An extended CT scale was used to reduce metal artifacts and to allow better delineation of the prosthesis-bone interface23. The evaluation of data was performed with a specialized software tool (CAPPA postOP; CAS Innovations, Erlangen, Germany) and a calibration phantom for both the involved and the noninvolved side18,24.

The relative change in periprosthetic bone density was measured as a function of time with the initial postoperative bone density value as the baseline value25.

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Statistical Analysis

We describe the quantitative bone density measurements as the mean values and standard deviations. The target measurement was the intra-individual difference between the ten-day postoperative measurement and the one-year, three-year, and ten-year evaluations. The comparison of paired data with non-normally distributed differences was made with use of the Wilcoxon signed-rank test. A p value of ≤0.05 was considered significant. All calculations were made with use of SPSS (version 10; SPSS, Chicago, Illinois).

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Source of Funding

There was no external funding for this study.

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The mean Harris hip score was 44 points (range, 23 to 55 points) before the index operation and 94 points (range, 32 to 98 points) at the ten-year follow-up. Twenty-two hips (92%) were rated as good or excellent, two (8%) were rated as fair, and none was rated as poor. All hips were radiographically stable at the ten-year follow-up; all acetabular cups showed radiographic signs of osteointegration20. No hip required revision surgery.

For all twenty-four acetabular components inserted without cement, fixation was good without a change in position on radiographs at a mean of 10.5 years (range, 10.1 to 11.4 years) of follow-up. Mean lateral inclination was 35° (range, 25° to 48°) at two weeks postoperatively and 35° (range, 25° to 47°) at the time of ten-year follow-up. Three cups had zone-II gaps with initial radiolucencies at two weeks postoperatively; two had closed the gap at the ten-year follow-up, and one had a nonprogressive zone-II radiolucency. No osteolysis was detected on radiographs.

We observed a substantial decrease of cancellous bone density in all regions of interest adjacent to the cup, whereas cortical bone density changes were minimal cranial to the cup. The mean change in cancellous bone density about the cup was −33% (range, −28% to −37%) cranially, −38% (range, −2% to −60%) ventrally, and −32% (range, −16% to −71%) dorsally at the time of the ten-year follow-up. The mean cortical bone density showed little change cranial to the cup (−2%; range, 0% to −15%), while the mean change was −16% (range, −15% to −18%) ventral and −13% (range, −5% to −18%) dorsal to the cup at the time of the ten-year follow-up (Fig. 3; see Appendix).

Bone density changes were highest between the ten-day and one-year control, moderate between the one-year and three-year follow-up examination, and minimal between the three-year and ten-year evaluation: cancellous bone density loss was progressive between the one-year and three-year postoperative evaluation cranial and dorsal to the cup. Between the three-year and ten-year evaluation, cancellous bone density values remained constant cranial and ventral to the cup and even slightly increased dorsal to the cup. Cortical bone density cranial and ventral to the cup slightly decreased between the three-year and ten-year follow-up, while little change was observed dorsal to the cup (Fig. 3; see Appendix).

On the noninvolved side, the mean change in cortical bone density was −3% (range, −1% to −6%) and the mean change in cancellous bone density was −6% (range, −4% to −12%) at the time of the ten-year follow-up. The mean changes of cortical and cancellous bone density of the noninvolved side were not significant.

As no cup showed radiographic signs of loosening and the clinical outcome was rated as good or excellent for all patients, a statistical analysis of the effect that bone density changes might have on the clinical or radiographic outcome was not done.

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Little is known about the in vivo load transfer of periprosthetic cortical and cancellous bone after press-fit acetabular cup insertion18,19,26. In vitro analysis suggests a correlation between bone density changes and mechanical loading of the bone27-29. Studying the current literature, we expected the insertion of a press-fit cup to alter the physiological stress transfer at the ilium, thus leading to marked bone density changes of certain anatomic areas and structures of the pelvis18,19,26,29,30. Specifically, we expected to find substantial loss of periacetabular cancellous bone density and an increase in cortical bone density cranial to the cup. We anticipated the cancellous bone density loss to be progressive after the third postoperative year8,18,31 and to perhaps compromise the longevity of the implant.

Our results confirmed some, but not all, of our hypotheses. First, we found no marked decrease in cancellous bone density in any areas surrounding the cup between the three-year and ten-year follow-up. The clinical and radiographic results and the longevity of the implant were not compromised. Second, although cortical bone density slightly decreased cranial to the cup between the three-year and the ten-year analysis, a homeostatic strain configuration was seen in this area as no significant cortical bone density changes were observed in that region at the ten-year follow-up.

The low bone density decrease on the noninvolved side might be attributed to the high activity level of the study group (the patients had an average age of sixty-nine years and an average Harris hip score of 94 points at the time of the ten-year follow-up) as well as to the fact that we investigated the bone density changes of fifteen men and only nine women.

High radiation exposure and time-consuming manual segmentation of cortical and cancellous bone are the major limitations of quantitative CT osteodensitometry. Nonetheless, quantitative CT is suitable for repeated longitudinal measurements in clinical research studies. The effective dose of radiation received during one periacetabular quantitative CT examination is 0.8 to 1.6 mSv, approximately 30% to 60% of the natural yearly radiation exposure in Central Europe23,32. Exposure to serial quantitative CT with twelve-month intervals has been considered acceptable by our local ethics committee who approved osteodensitometry studies26.

Quantitative CT-assisted osteodensitometry and dual x-ray absorptiometry are sensitive and precise tools for detecting changes in periprosthetic bone density33-36. Measurement of the changes in bone density after the surgical procedure provides a good indication of the pattern of load transfer between implant and host bone4. However, in the pelvic region, dual x-ray absorptiometry is limited by low resolution and the lack of three-dimensional information7. Computed tomography is a standardized imaging method for assessment of bone structures with high validity and resolution, which also allows separation of cortical and cancellous bone structures19,36,37. Thus far, both dual x-ray absorptiometry and quantitative CT-assisted osteodensitometry studies have lacked clinical relevance, as they have been unable to predict early failure of the prostheses before it is imminent clinically or radiographically8,19,26,31.

Some finite element analyses have suggested major changes in acetabular load transmission pattern after uncemented total hip arthroplasty3,4, while others have noted little change in bone density of the ilium after simulated press-fit cup fixation5. Recent finite element studies have pointed out that the simulation results are highly dependent on the chosen boundary conditions and on a differentiation between the elastic properties for cancellous and cortical bone1,2,14. Nevertheless, to date, computational bone-remodeling simulations have primarily been compared with dual x-ray absorptiometry results, a method that lacks the ability to differentiate between cortical and cancellous bone1,2,4,14,38. Long-term quantitative CT-assisted osteodensitometry studies are necessary to calibrate the bone adaptation laws for cortical and cancellous bone density changes and thus validate the finite element calculations, which may then generate accurate patient-specific meshes for simulation of acetabular components and their effect on pelvic bone remodeling39,40. This technology would be useful for a radiation-free prediction of bone remodeling and quality of implant fixation with use of prostheses with different designs and biomaterials. In the future, this tool could be applied for preclinical validation of new implants before their widespread use35. As the Cerafit cup is a made of a 3-mm-thick metal-backed titanium alloy, we believe that the measured bone density changes are independent of the used pairing.

In all of our patients, the cups had radiographic signs of osseous ingrowth at the time of the ten-year follow-up, no osteolysis or progressive radiolucent lines were detected, and the clinical results were good or excellent. Our data present the natural course of periacetabular cortical and cancellous bone density changes over a ten-year follow-up period for well-fitting hydroxyapatite-coated, fiber-mesh press-fit components. Even cancellous bone density loss of as much as –71% and a cortical bone density loss of as much as –18% at the time of the ten-year follow-up appear not to be concerning in relation to the survival of the implants, and this finding needs to be considered for all bone density studies suggesting the use of bone-modulating drugs to enhance the longevity of total hip replacement8.

To our knowledge, this study is the first prospective long-term observation of periacetabular cortical and cancellous bone density changes after fixation of a press-fit acetabular cup. Nevertheless, the number of patients included in this study is too small to allow evidence-based conclusions as to whether the described bone density losses play or will play any clinically relevant role. Only future studies with much larger patient cohorts might clarify whether cortical and cancellous periacetabular bone density loss will compromise implant longevity and especially whether the observed substantial loss of cancellous bone density is one of several concurrent factors predisposing to osteolysis adjacent to the implants.

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Appendix Cited Here...

A table showing postoperative and follow-up mean cancellous and cortical bone density measurements and pairwise difference is available with the online version of this article at jbjs.org.

Investigation performed at the Department of Orthopaedic Surgery, Friedrich-Alexander-University of Erlangen-Nuremberg, Erlangen, Germany

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Disclosure: The authors did not receive any outside funding or grants in support of their research for or preparation of this work. Neither they nor a member of their immediate families received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity.

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