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.
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).
Source of Funding
There was no external funding for this study.
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.
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.
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.Copyright 2011 by The Journal of Bone and Joint Surgery, Incorporated