The Journal of Bone & Joint Surgery

The Journal of Bone & Joint Surgery, Volume 89, Issue 8

Scientific Articles | August 01, 2007

Progression of Acetabular Periprosthetic Osteolytic Lesions Measured with Computed Tomography

Donald W. Howie, MBBS, PhD, FRACS¹; Susan D. Neale, MSc¹; Roumen Stamenkov, MD¹; Margaret A. McGee, BSc, MPH¹; David J. Taylor, MBBS²; David M. Findlay, MSc, PhD¹

¹ Department of Orthopaedics and Trauma, Level 4, Bice Building, Royal Adelaide Hospital, North Terrace, Adelaide, 5000, Australia. E-mail address for S. Neale: susan.neale@health.sa.gov.au
² Department of Radiology, Level 3, Royal Adelaide Hospital, North Terrace, Adelaide, 5000, Australia

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Disclosure: In support of their research for or preparation of this work, one or more of the authors received, in any one year, outside funding or grants in excess of $10,000 from the National Health and Medical Research Council, Novartis Pharmaceuticals, Royal Adelaide Hospital, and Bristol-Myers Squibb/Zimmer. 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. A commercial entity (Novartis Pharmaceuticals Australia Pty Ltd and Bristol-Myers Squibb/Zimmer) paid or directed in any one year, or agreed to pay or direct, benefits in excess of $10,000 to a research fund, foundation, division, center, clinical practice, or other charitable or nonprofit organization with which one or more of the authors, or a member of his or her immediate family, is affiliated or associated.

Investigation performed at the Department of Orthopaedics and Trauma and the Department of Radiology, Royal Adelaide Hospital, and the Discipline of Orthopaedics and Trauma, University of Adelaide, Adelaide, Australia

The Journal of Bone and Joint Surgery, Incorporated

J Bone Joint Surg Am, 2007 Aug 01;89(8):1818-1825. doi: 10.2106/JBJS.E.01305

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Abstract

Background: A better understanding of the factors associated with the size and/or progression of osteolytic lesions has been hampered by a lack of sensitivity of radiographic measurement techniques.

Methods: We retrospectively analyzed quantitative computed tomography scans that had been made with use of a high-resolution multi-slice scanner with a metal artifact-suppression protocol. The scans had been made to determine the volume of osteolytic lesions around thirty-five cementless Harris-Galante acetabular components that had been in situ for at least ten years. Repeat scans of thirty hips allowed for the measurement of progression in the size of osteolytic lesions over a one-year period. Associations between the volume of osteolytic lesions, progression in the size of the lesions, polyethylene wear since the time of implantation, change in component position, and patient-related variables (age, gender, body mass index, activity level, walking limitations, joint pain, and function) were determined.

Results: In sixteen of the thirty hips that had repeat computed tomography scans, the lesions progressed in size during the study period. The median size of the lesions in these sixteen hips was 10.3 cm³ at the time of the initial scan, compared with 13.3 cm³ at a median of fifteen months later (p = 0.001). Osteolytic lesions measuring >10 cm³ in volume on the initial scan were 2.5 times (95% confidence interval 1.3 to 4.8 times) more likely to progress in size over one year than smaller lesions were. Patients with greater polyethylene wear rates, higher activity levels, no walking limitations, and larger prosthetic femoral head dimensions (26 or 28 mm) had significantly larger osteolytic lesions (p < 0.0001, p = 0.009, p = 0.006, and p = 0.028, respectively). Progression in the size of the osteolytic lesions over one year was significantly associated with larger initial osteolytic lesions (p = 0.002), greater polyethylene wear rates (p = 0.009), and larger (26 or 28-mm) prosthetic femoral head dimensions (p = 0.019).

Conclusions: There is considerable variation in the rates of progression of the size of osteolytic lesions around stable acetabular components. Lesion size and the progression of lesion size are generally related to polyethylene wear rates, higher patient activity levels, and larger-diameter femoral heads. Osteolytic lesions measuring >10 cm³ in volume are associated with a high rate of progression.

Level of Evidence: Prognostic Level III. See Instructions to Authors for a complete description of levels of evidence.

Figures in this Article

Although often asymptomatic^1,2, periprosthetic osteolysis following total hip arthroplasty can be a cause of component loosening and periprosthetic fracture^3-6. However, clinical decision-making regarding whether or when to intervene surgically to prevent such complications is hampered by a lack of knowledge about which lesions may progress.

While it has been proposed that the quantity of wear debris from a polyethylene acetabular liner correlates with the size of the osteolytic lesion⁷, this proposition lacks specificity because it is not based on accurate measurements of either polyethylene wear or osteolytic lesion size^7,8. It is now generally accepted that plain radiography is not sufficiently sensitive for the reliable detection of the presence or extent of periprosthetic osteolysis^9-13.

Recently, new-generation high-resolution multi-slice or helical computed tomography with metal artifact-reduction techniques has been found to provide a sensitive and accurate way to measure the volume of osteolytic lesions close to metal prostheses^13-19. Three previous studies involving the use of computed tomography and two-dimensional measures of polyethylene wear demonstrated either no correlation or a poor correlation between the size of osteolytic lesions and the amount of polyethylene wear^15-17. Importantly, various types of implants were studied, and no measure of acetabular component migration was made to exclude the possibility that migration of the acetabular component into an osteolytic lesion might cause underestimation of the true size of the osteolytic lesion.

In the present study, we used quantitative computed tomography analysis to determine the size and the progression in size of osteolytic lesions adjacent to acetabular components. We also used computer-assisted measurements of two and three-dimensional polyethylene wear to try to quantify a relationship between the size of acetabular osteolytic lesions, progression in the size of the lesions, and wear of the polyethylene component. We then examined the association between these findings and a number of clinical and implant-related variables.

Materials and Methods

Materials and Methods | Results | Discussion | Acknowledgements | References

The present study was approved by the institutional human ethics review committee, and all patients gave informed consent. At our institution, patients with total hip arthroplasty who are suspected of having periacetabular osteolysis around a cementless acetabular component on plain radiographs are referred for a computed tomography scan as part of their clinical management. In order to examine as homogenous a group of patients as possible, we established specific inclusion criteria for this retrospective study, including (1) a minimum duration of follow-up of ten years after primary total hip arthroplasty, (2) the use of a cementless Harris-Galante-1 acetabular and femoral component (Zimmer, Warsaw, Indiana), and (3) performance of the surgical procedure by the same surgeon. Twenty-eight patients (thirty-five hips) met the inclusion criteria.

All acetabular components were implanted after line-to-line reaming. Fixation of the metal shell was augmented with at least two screws. The median size of the metal shell was 52 mm (range, 48 to 62 mm), and all of the ultra-high molecular weight polyethylene liners were sterilized by means of gamma irradiation in air. The femoral heads were constructed of modular cobalt-chromium and had a diameter of 22 mm (thirteen hips), 26 mm (eighteen), or 28 mm (four). The median age of the patients at the time of the primary arthroplasty was sixty years (range, thirty-five to seventy years), and the mean time since the arthroplasty was fifteen years (range, ten to nineteen years). Two patients (two hips) underwent revision surgery during the study period, and one patient (one hip) died fifteen months afte r the initial computed tomography scan. Of the remaining thirty-two hips (twenty-five patients), thirty had a repeat computed tomography scan at a median of fifteen months (range, twelve to twenty-seven months) after the initial computed tomography scan.

High-resolution spiral multi-slice computed tomography scans (SOMATOM Volume Zoom; Siemens, Munich, Germany) were used to identify and measure the volume of acetabular osteolytic lesions with use of a technique that had been previously validated in vitro^18,20. An extended scale technique with the window level up to 30710 Hounsfield units was used to suppress the resulting metal artifact¹⁴. Scans of the hip were made according to specific parameters (140 kV, 200 mA, 0.75-second rotation speed, 3-mm slices, 1-mm feed) from the top of the sacroiliac joint to 2 cm distal to the end of the femoral prosthesis. The reconstruction interval on coronal and sagittal images was 0.8 mm, with a slice thickness of 1.25 mm. Osteolysis was defined as a demarcated nonlinear lytic lesion measuring >3 mm in diameter²¹. Cross sections of the defects were identified on consecutive slices, and the perimeter of each defect was determined by tracing the inner border of the defect with use of the computer mouse on a Picker PQ6000 workstation (Picker International, Cleveland, Ohio). The volumes of isolated osteolytic lesions were calculated using algorithms coded into the commerical image-analysis software (Picker International) and were combined to give the total volume of osteolytic lesions (Figs. 1-A and 1-B). All measurements were made by a single blinded observer (R.S.). To check reproducibility, a second blinded observer (S.D.N.) also measured the volume of the osteolytic lesions in five hips. The intraobserver and interobserver measurement errors of osteolysis volume were 2% and 3%, respectively. The change in the volume of osteolytic lesions between scans was expressed as the progression of osteolysis per year, with the assumption that osteolysis progression was linear over that time. Progressive osteolysis was defined as an increase in volume of >1 cm³/yr. In all hips with progressive osteolytic lesions, and in particular those with large osteolytic lesions on the initial computed tomography scan, the increase in the size of the osteolytic lesions was substantially greater than the error associated with the computed tomography measurements¹³.

For thirty-one of the thirty-five hips that underwent subsequent computed tomography scans, immediate postoperative anteroposterior pelvic radiographs (made eight to ten days after surgery) and serial follow-up anteroposterior and lateral radiographs of the affected hip were available for review. As migration of the acetabular component into the osteolytic lesion might cause underestimation of the true size of the osteolytic lesion, we measured the position of the acetabular component from the time of component implantation on these serial digitized plain radiographs with use of Ein Bild Roentgen Analyse software^22-24. The error of this technique has been reported to be ±1 mm²². We defined a migrating component as one with >3 mm of total migration since implantation²⁵.

The PolyWare software program (Draftware Developers, Vevay, Indiana)²⁶ was used to determine linear and volumetric polyethylene wear by comparing the immediate postoperative plain radiograph with radiographs made at the time of the computed tomography scan. The mean of triplicate wear measurements made on each hip was used. The PolyWare program has a reported accuracy of 0.38 mm for linear wear and 123 mm³ for volumetric wear²⁷. To improve the accuracy of the analysis in instances in which the edge of the head shadow was difficult to discern, automatic edge detection was disabled. Intraobserver error was 5% for linear wear measurements and 6% for volumetric wear measurements. Two of the thirty-one hips that were analyzed had extreme wear values (3173 and 4318 mm³) and were confirmed as having wear-through of the acetabular liner at the time of subsequent revision surgery. These two hips were excluded from the wear analyses only.

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Fig. 1-A: Three-dimensional reconstructed images of computed tomography scans of total hip replacements were generated with use of True Life Anatomy software (Adelaide, Australia). In this example, the total volume of the acetabular osteolytic lesions measured was 26 cm³ (yellow), which increased to 43 cm³ (red) two years later.

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Fig. 1-B: Selected image from the computed tomography scan, showing areas of osteolysis around the acetabular component (arrows).

To determine whether patient-related variables were associated with the volume of osteolytic lesions and/or progression in the size of osteolytic lesions, relevant values (patient age at the time of primary total hip arthroplasty, gender, body mass index²⁸, the patient-reported Harris hip pain score²⁹, and the Harris hip score²⁹) were retrospectively obtained from data that had been prospectively entered into a joint replacement outcomes database. The patient's self-assessment of walking limitations was used to generate a Charnley classification³⁰, and patient activity was assessed with use of a patient-reported 5-point activity scale³¹.

To avoid skewing of average data due to important outliers, we expressed the data in the present study as medians. Statistical analysis was performed with use of GraphPad Prism software (version 4.00 for Windows; GraphPad Software, San Diego, California), with the level of alpha set at 0.05. Correlation analysis was undertaken with use of the Spearman rank correlation coefficient. We determined that a minimum sample size of ten was required for correlation analyses with a power of 90% (PowerCalc). Associations between the size of the osteolytic lesions at the time of the initial computed tomography scan and the rate of osteolysis progression as well as patient-related, clinical, and implant-related variables were examined with use of the Mann-Whitney U, Kruskal-Wallis, Fisher exact, or chi-square tests.

Results

Materials and Methods | Results | Discussion | Acknowledgements | References

Periacetabular osteolysis was detectable with computed tomography in thirty-two of the thirty-five hips, although all patients were selected for computed tomography scans because of presumed detection of osteolysis on plain radiographs. The median volume of the lesions in hips with computed tomography-demonstrated osteolysis was 8.1 cm³ (mean, 12.6 cm³; range, 0.7 to 49 cm³). For the thirty-one hips for which initial postoperative plain radiographs were available for comparison with the plain radiographs made just before the initial computed tomography scan, we divided the magnitude of migration into two groups: acetabular components that had migrated >3 mm and those that had migrated =3 mm, according to the system described by Schyterz et al.²⁵. Five acetabular components had migrated >3 mm (median, 5.0 mm; range, 3.6 to 7.2 mm). The median volume of osteolytic lesions as determined with computed tomography in that group was 11.2 cm³ (range, 0.7 to 16 cm³). In contrast, twenty-six hips had migrated =3 mm (median, 2.0 mm; range, 0.3 to 3.0 mm); the median volume of the osteolytic lesions in that group was 5.9 cm³ (range, 0 to 49 cm³). The difference in size of the osteolytic lesions in the two groups was not significant (p = 0.436).

The results of tests of association between the size of the osteolytic lesions and patient-related, implant-related, and clinical variables are shown in Table I. Good correlation was found between the size of the osteolytic lesion and the total volume of polyethylene wear as well as the polyethylene wear rate since implantation (r = 0.780 and r = 0.760, respectively). Hips with higher volumes of polyethylene wear and higher polyethylene wear rates tended to have larger osteolytic lesions (p < 0.0001). The presence or absence of acetabular component migration did not significantly influence this correlation (p = 0.89). Of interest, outliers were also identified with high wear rates (>80 mm³/yr) but relatively small osteolytic lesions (Fig. 2).

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TABLE I Results of Tests of Association Between the Volume of Osteolytic Lesions and Patient, Implant, and Clinical Variables

	All Components (N = 35)
Variable	Descriptive Statistics*	Correlation Coefficient and P Value	Test of Association
Osteolysis (cm³)	7.5 (0 to 49)	—	—
Total volumetric polyethylene wear (mm³)	816 (170 to 2213)	r = 0.780, p < 0.0001	Linear regression analysis
Annual polyethylene wear rate (mm³/yr)	56 (11 to 140)	r = 0.760, p < 0.0001	Linear regression analysis
Age (yr)	60 (35 to 70)	r = 0.309, p = 0.07	Linear regression analysis
Male:female ratio (no. of hips)	21:14	p = 0.09	Mann-Whitney U test
Body mass index (<25:=25) (no. of hips)	15:20	p = 0.960	Mann-Whitney U test
Implant duration (mo)	189 (120 to 225)	r = 0.123, p = 0.482	Linear regression analysis
Activity† (1:2:3:4:5:6) (no. of hips)	0:7:13:10:5:0	p = 0.009	Kruskal-Wallis test
Charnley grade‡ (A:B:C) (no. of hips)	9:4:22	p = 0.006 (A compared with B and C)	Mann-Whitney U test
Harris pain score§ (<40:=40) (no. of hips)	15:20	p = 0.582	Mann-Whitney U test
Harris hip score#(points)	82 (34 to 100)	r = 0.130, p = 0.517	Linear regression analysis
Femoral head size (22:26:28) (no. of hips)	13:18:4	p = 0.028 (22 compared with 26 and 28)	Mann-Whitney U test

The values are given as the median (with the range in parentheses) or as the distribution at the time of the latest computed tomography scan. †1 = bedridden or confined to wheelchair, 2 = sedentary, 3 = semi-sedentary, 4 = light labor or sports, 5 = moderate manual labor or sports, and 6 = heavy manual labor or vigorous sports³¹. ‡A = only one hip involved and with no other condition that interferes with walking, B = both hips involved but the rest of the body normal, and C = some other factor that interferes with walking ability³⁰. §44 = no pain, or pain ignored; 40 = slight occasional pain, no compromise in activities; 30 = mild pain, no effect on average activities, rarely moderate pain with unusual activity; 20 = moderate pain, tolerable but with some limitations of ordinary work or activity; 10 = marked pain, severe at times, with serious limitations of activity; 0 = totally disabled, crippled, pain in bed, or bedridden²⁹. #90 to 100 = excellent, 80 to 89 = good, 70 to 79 = fair, and <70 = poor²⁹.

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TABLE I Results of Tests of Association Between the Volume of Osteolytic Lesions and Patient, Implant, and Clinical Variables

	All Components (N = 35)
Variable	Descriptive Statistics*	Correlation Coefficient and P Value	Test of Association
Osteolysis (cm³)	7.5 (0 to 49)	—	—
Total volumetric polyethylene wear (mm³)	816 (170 to 2213)	r = 0.780, p < 0.0001	Linear regression analysis
Annual polyethylene wear rate (mm³/yr)	56 (11 to 140)	r = 0.760, p < 0.0001	Linear regression analysis
Age (yr)	60 (35 to 70)	r = 0.309, p = 0.07	Linear regression analysis
Male:female ratio (no. of hips)	21:14	p = 0.09	Mann-Whitney U test
Body mass index (<25:=25) (no. of hips)	15:20	p = 0.960	Mann-Whitney U test
Implant duration (mo)	189 (120 to 225)	r = 0.123, p = 0.482	Linear regression analysis
Activity† (1:2:3:4:5:6) (no. of hips)	0:7:13:10:5:0	p = 0.009	Kruskal-Wallis test
Charnley grade‡ (A:B:C) (no. of hips)	9:4:22	p = 0.006 (A compared with B and C)	Mann-Whitney U test
Harris pain score§ (<40:=40) (no. of hips)	15:20	p = 0.582	Mann-Whitney U test
Harris hip score#(points)	82 (34 to 100)	r = 0.130, p = 0.517	Linear regression analysis
Femoral head size (22:26:28) (no. of hips)	13:18:4	p = 0.028 (22 compared with 26 and 28)	Mann-Whitney U test

Patients with higher activity scores, no walking limitations (Charnley grade A), or a larger femoral head size were also found to have significantly larger osteolytic lesions on the most recent computed tomography scan (Table I). The median volume of osteolytic lesions around acetabular components in patients with the activity categories sedentary, semi-sedentary, light labor or sports, and moderate manual labor and sports were 1.9, 7.4, 7.6, and 27 cm³, respectively (p = 0.009). The median volume of osteolytic lesions in patients with no walking limitations (Charnley grade A) was 16 cm³, compared with 5.4 cm³ in patients with walking limitations due to other joint or health problems (Charnley grade B or C) (p = 0.006). The median volume of osteolytic lesions around acetabular components with a 26 or 28-mm femoral head was 9.4 cm³, as compared with a median volume of 5.1 cm³ for lesions around acetabular components with a 22-mm femoral head (p = 0.028). In this cohort, the inclusion of hips with >3 mm of migration did not affect the median volumes of osteolytic lesions in any of the categories (Table I).

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Fig. 2: Relationship between the total volume of osteolytic lesions and the polyethylene wear rate. The polyethylene wear rate was calculated as the total wear volume divided by the time since prosthesis implantation. Components that had migrated >3 mm over the life of the implant were excluded from the regression analysis.

Of the thirty hips that underwent repeat computed tomography at a median interval of fifteen months (range, twelve to twenty-seven months), sixteen (53%) had lesions that progressed in size during the study period (Fig. 3). The median size of the lesions in these sixteen hips was 10.3 cm³ on the initial computed tomography scan, compared with 13.3 cm³ on the repeat computed tomography scan; this increase was significant (p = 0.001). The hips with osteolytic lesions that increased in size during the study period had significantly larger osteolytic lesions on the initial computed tomography scan (median, 10.3 cm³; mean, 17.2 cm³; range, 1.9 to 45.3 cm³) as compared with those with lesions that did not increase in size (median, 4.0 cm³; mean, 5.3 cm³; range, 0 to 28.2 cm³) (p = 0.002). Importantly, eight of the nine hips with osteolytic lesions measuring >10 cm³ on the initial computed tomography scan had an increase in lesion size. Osteolytic lesions measuring >10 cm³ in volume on the initial computed tomography scan were 2.5 times (95% confidence interval, 1.3 to 4.8 times) more likely to progress in size over one year than osteolytic lesions measuring =10 cm³ in volume.

In the sixteen hips with an increase in the size of osteolytic lesions between the time of the initial computed tomography scan and the time of the repeat computed tomography scan, the median volume of polyethylene wear and the median polyethylene wear rate were significantly higher than those in the fourteen hips in which the osteolytic lesions did not progress in size (p = 0.013 and 0.009, respectively) (Table II). Good correlation (r = 0.690) was found between the polyethylene wear rate at the time of the initial computed tomography scan and progression in the size of the osteolytic lesions (Fig. 4). In hips with a volumetric polyethylene wear rate of <40 mm³/year, the progression in the size of acetabular osteolytic lesions was small. The presence or absence of acetabular component migration did not significantly influence this correlation (p = 0.854).

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TABLE II Results of Tests of Association Between the Progression in Size of Osteolytic Lesions and Patient, Implant, and Clinical Variables

	Descriptive Statistics*
Variable	Nonprogressive Osteolytic Lesions (N = 14)	Progressive Osteolytic Lesions (N = 16)	P Value	Tests of Association
Osteolysis (cm³)	4.0 (0 to 28.2)	10.3 (1.9 to 45.3)	0.002	Mann-Whitney U test
Total volumetric polyethylene wear (mm³)	662 (175 to 1326)	1339 (395 to 2128)	0.013	Mann-Whitney U test
Annual polyethylene wear rate (mm³/yr)	42 (12 to 91)	93 (28 to 151)	0.009	Mann-Whitney U test
Age (yr)	60 (49 to 69)	58 (43 to 67)	0.279	Mann-Whitney U test
Male:female ratio (no. of hips)	8:6	11:5	0.707	Fisher exact test
Body mass index (<25:=25) (no. of hips)	4:10	8:8	0.284	Fisher exact test
Implant duration (mo)	177 (134 to 212)	166 (149 to 186)	0.088	Mann-Whitney U test
Activity† (1:2:3:4:5:6) (no. of hips)	0:3:7:3:1:0	0:1:7:5:3:0	0.498	Chi-square test
Charnley grade (A:B:C)‡(no. of hips)	2:1:11	7:2:7	0.103 (A compared with C)	Fisher exact test
Harris pain score§ (<40:=40) (no. of hips)	6:8	7:9	1.00, 0.933	Fisher exact test, Mann-Whitney U test
Harris hip score#(points)	80 (47 to 97)	88 (34 to 97)	0.506	Mann-Whitney U test
Femoral head size (22:26:28) (no. of hips)	8:6:0	2:11:3	0.019 (22 compared with 26 and 28)	Fisher exact test

The values are given as the median (with the range in parentheses) or as the distribution at the time of the initial computed tomography scan. †1 = bedridden or confined to wheelchair, 2 = sedentary, 3 = semi-sedentary, 4 = light labor or sports, 5 = moderate manual labor or sports, and 6 = heavy manual labor or vigorous sports³¹. ‡A = only one hip involved and with no other condition that interferes with walking, B = both hips involved but the rest of the body normal, and C = some other factor that interferes with walking ability³⁰. §44= no pain, or pain ignored; 40 = slight occasional pain, no compromise in activities; 30 = mild pain, no effect on average activities, rarely moderate pain with unusual activity; 20 = moderate pain, tolerable but with some limitations of ordinary work or activity, 10 = marked pain, severe at times, with serious limitations of activity; and 0 = totally disabled, crippled, pain in bed, or bedridden²⁹. #90 to 100 = excellent, 80 to 89 = good, 70 to 79 = fair, and <70 = poor²⁹.

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TABLE II Results of Tests of Association Between the Progression in Size of Osteolytic Lesions and Patient, Implant, and Clinical Variables

	Descriptive Statistics*
Variable	Nonprogressive Osteolytic Lesions (N = 14)	Progressive Osteolytic Lesions (N = 16)	P Value	Tests of Association
Osteolysis (cm³)	4.0 (0 to 28.2)	10.3 (1.9 to 45.3)	0.002	Mann-Whitney U test
Total volumetric polyethylene wear (mm³)	662 (175 to 1326)	1339 (395 to 2128)	0.013	Mann-Whitney U test
Annual polyethylene wear rate (mm³/yr)	42 (12 to 91)	93 (28 to 151)	0.009	Mann-Whitney U test
Age (yr)	60 (49 to 69)	58 (43 to 67)	0.279	Mann-Whitney U test
Male:female ratio (no. of hips)	8:6	11:5	0.707	Fisher exact test
Body mass index (<25:=25) (no. of hips)	4:10	8:8	0.284	Fisher exact test
Implant duration (mo)	177 (134 to 212)	166 (149 to 186)	0.088	Mann-Whitney U test
Activity† (1:2:3:4:5:6) (no. of hips)	0:3:7:3:1:0	0:1:7:5:3:0	0.498	Chi-square test
Charnley grade (A:B:C)‡(no. of hips)	2:1:11	7:2:7	0.103 (A compared with C)	Fisher exact test
Harris pain score§ (<40:=40) (no. of hips)	6:8	7:9	1.00, 0.933	Fisher exact test, Mann-Whitney U test
Harris hip score#(points)	80 (47 to 97)	88 (34 to 97)	0.506	Mann-Whitney U test
Femoral head size (22:26:28) (no. of hips)	8:6:0	2:11:3	0.019 (22 compared with 26 and 28)	Fisher exact test

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Fig. 3: Progression in the volume of osteolytic lesions adjacent to cementless acetabular components, expressed as the increase in the volume of osteolytic lesions per year to correct for different time intervals between scans. The graph shows the progression of osteolysis measured in thirty hips. The diagonal line indicates no change in volume. Data points above the line indicate progression in the size of the osteolytic lesions. CT = computed tomography.

A significant association was also found between progression in the size of the osteolytic lesions and femoral head size (Table II). There were significantly more hips with osteolytic lesions that progressed in size around acetabular components with a 26 or 28-mm femoral head than around acetabular components with a smaller (22-mm) femoral head (p = 0.019). No other variables examined were significantly associated with the progression in the size of acetabular lesions (Table II).

Discussion

Materials and Methods | Results | Discussion | Acknowledgements | References

We found a considerable variation in the size and progression of osteolytic lesions around cementless acetabular components of the same design. Most hips had relatively small osteolytic lesions adjacent to the acetabular component, even after long implantation times, and the osteolytic lesions in many of these hips were relatively quiescent. Hips with larger osteolytic lesions on the initial computed tomography scan were most likely to show progression. Lesions with a total size of >10 cm³ were 2.5 times more likely to progress over one year when compared with smaller lesions. These results agree with those of Schwarz et al.¹⁹, who reported progression of osteolysis over one year in a mixed population of twenty patients with established periacetabular osteolysis, also with use of computed tomography imaging but with no measure of migration.

We also found a direct correlation between the volume of polyethylene wear and the size of the osteolytic lesion: the greater the volume of polyethylene wear, the larger the size of the osteolytic lesion. The wear rates that we found were slightly higher than, but consistent with, previously reported rates⁸ and reflected the duration of implantation and the fact that we chose to selectively make computed tomography scans for hips with signs of osteolysis on plain radiographs. Importantly, there were some hips with large amounts of polyethylene wear but only small osteolytic lesions (Fig. 2). In those instances, the acetabular component was stable and so the osteolytic lesions were in fact small and not just apparently so because of migration of the acetabular component into the osteolytic region. We found no evidence that migration of the acetabular component increased the size of osteolytic lesions or that migration of the acetabular component into the osteolytic lesion caused us to underestimate osteolysis.

The presence of outliers (i.e., hips with large volumes of polyethylene wear but without large osteolytic lesions) suggests that, despite controlling for many variables such as implant type and surgeon, factors other than wear and migration may determine the extent and progression of periprosthetic osteolysis. It is well known that, in order for wear-particle-induced periprosthetic osteolysis to occur, access of wear particles to periprosthetic bone is required. Lesions were commonly found in close association with screws and screw holes, and further studies to determine the importance of screw holes may point to the need to provide better sealing of these holes. Our definition of osteolysis progression may also have influenced the results. Progression of >1 cm³ per year was the amount chosen as being clearly greater than the error of measurement and an amount considered to be a clinically important increase in osteolysis.

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Fig. 4: Relationship between progression in the size of osteolytic lesions and the polyethylene wear rate. The polyethylene wear rate was calculated as the total wear volume divided by the time since prosthesis implantation. Components that had migrated >3 mm over the life of the implant were excluded from the regression analysis.

In the present study, patient activity, walking limitations, and femoral head size were all found to be positively associated with the size of the osteolytic lesions around a stable acetabular component. This is not surprising, given that positive associations between polyethylene wear and patient activity and femoral head size have been reported previously^32-34. Interestingly, other than polyethylene wear, femoral head size was the only patient or implant-related variable that was found to be positively associated with the progression of osteolysis. However, any associations that may exist between these variables and the progression of osteolysis in a one-year period may be difficult to establish because of the relatively short monitoring period. With longer periods of monitoring of osteolysis progression in more patients, the ability to detect associations will strengthen.

Importantly, the polyethylene wear rate was highly correlated with the progression of osteolysis, suggesting that, in the majority of hips, the polyethylene wear rate is a good indicator of osteolysis progression. At this time, we cannot make recommendations about when or how frequently to undertake quantitative computed tomography analyses of osteolysis as we recognize that the wear rate may not be linear over time. One approach may be that, because patients with an overall wear rate of <40 mm³ per year tended to have little or no increase in the size of the lesion, patients with small osteolytic lesions and a low polyethylene wear rate on plain radiographs could be followed with radiographic analyses of wear alone. An increase in the wear rate may then be an indication for additional computed tomography analysis. In contrast, patients with large osteolytic lesions and a polyethylene wear rate of 40 mm³ per year may warrant closer monitoring with computed tomography. Further studies over longer time-periods at a number of orthopaedic centers will be required to better define the place for screening plain radiographs and computed tomography in the evaluation of osteolysis.

The present study focused on the evaluation and management of patients who had periacetabular osteolysis without clinically important pain or other symptoms. While findings may be different for symptomatic patients, it is more likely that symptomatic patients warrant imminent revision surgery and are less likely to be candidates for prospective monitoring. Importantly, newer computed tomography technology, such as that used in the present study, may provide a means of evaluating the outcomes of medical treatments such as the use of antiresorptive agents to treat periprosthetic osteolysis, particularly bisphosphonates, which have emerged as potential pharmacological treatments^35,36.

In summary, the present study demonstrates that sensitive, validated computed tomography measurement of osteolysis combined with accurate measurement of component migration and wear demonstrates considerable variation in the progression of osteolysis around stable acetabular components. While the polyethylene wear rate was a relatively good indicator of osteolysis progression, there was substantial variation among individuals, which requires additional investigation. Importantly, while small lesions may demonstrate some progression, osteolytic lesions measuring >10 cm³ in size were associated with a high risk of progression. ?

Acknowledgements

Materials and Methods | Results | Discussion | Acknowledgements | References

Note: The authors thank B. Cornish, Orthopaedic Surgeon, G. Kourlis, Department of Radiology, Royal Adelaide Hospital; and Perrett's Medical Imaging at the Memorial Hospital, Adelaide, Australia, for their contributions to this study.

References

Materials and Methods | Results | Discussion | Acknowledgements | References

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Topics

hip region ; osteolysis ; polyethylene ; osteolytic lesion

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Donald W. Howie, Susan D. Neale, Roumen Stamenkov, Margaret A. McGee, David J. Taylor, David M. Findlay; Progression of Acetabular Periprosthetic Osteolytic Lesions Measured with Computed Tomography. The Journal of Bone & Joint Surgery. 2007 Aug;89(8):1818-1825.

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