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Femoral Bone Remodeling in Revision Total Hip Arthroplasty with Use of Modular Compared with Monoblock Tapered Fluted Titanium Stems

The Role of Stem Length and Stiffness

Huang, Yong MD1; Shao, Hongyi MD1; Zhou, Yixin MD, PhD1; Gu, Jianming MD1; Tang, Hao MD1; Yang, Dejin MD1

Author Information
The Journal of Bone and Joint Surgery: March 20, 2019 - Volume 101 - Issue 6 - p 531-538
doi: 10.2106/JBJS.18.00442
  • Disclosures

Abstract

Extensively porous-coated cylindrical cobalt-chromium stems have been the predominant choice for femoral revision in North America and have a reported survivorship of 88.0% to 96.5% at 10 years postoperatively1-5. However, tapered fluted titanium stems have gained popularity in revision total hip arthroplasty (THA) because they achieve better axial and rotational stability compared with extensively porous-coated cylindrical stems in the presence of massive femoral bone defects6,7 and have demonstrated high survivorship and promising clinical results8-21. Furthermore, proximal femoral bone can regenerate both when modular20,22,23 and monoblock6,13,14,24 tapered fluted titanium stems are involved.

Regeneration of proximal femoral bone following revision THA is important because it can restore femoral bone stock, which may help in cases in which re-revision is necessary. Restoration of proximal femoral bone reportedly ranges from 17% to 56%18,20,23 with use of modular Link MP (Waldemar Link) and 47% to 88%6,13,14,24 with use of monoblock Wagner SL (Zimmer) tapered fluted titanium stems. Femoral stress-shielding is a concern with extensively porous-coated cylindrical stems5,25,26 because these stems may weaken the strength of the femur and predispose the patient to future periprosthetic fractures. Femoral stress-shielding has also been reported with the modular Link MP stem, with rates ranging from 23% to 30% depending on the classification used and the Gruen zone20,27,28. However, little has been reported about stress-shielding associated with monoblock Wagner stems. Given that both modular and monoblock stems have demonstrated similarly excellent mid to long-term clinical results8-10,14,16,18-20,22,24,29-31, knowledge regarding differences in bone remodeling, such as proximal bone restoration and stress-shielding, is important for assisting surgeons in selecting the most appropriate stem.

We previously compared midterm clinical outcomes, survivorship, and complications for modular Link MP versus monoblock Wagner tapered fluted titanium stems32. To our knowledge, no study has compared the results of bone remodeling with use of modular Link MP and monoblock Wagner tapered fluted titanium stems. We evaluated the hypothesis that there was (1) less proximal femoral osseous restoration in the residual osteolytic areas, (2) more severe femoral stress-shielding, (3) more stem tip spot-welds, (4) more proximal radiolucent lines, (5) a larger stem diameter, and (6) more medullary canal filling in the modular cohort than in the monoblock cohort.

Materials and Methods

We obtained approval from our institutional review board and written documentation of informed consent from all patients. This retrospective comparative study assessed the use of modular compared with monoblock tapered fluted titanium stems in revision THAs performed at our institution between 2002 and 2014.

Data were reviewed for 182 hips in 175 patients treated with modular stems (Link MP) and 146 hips in 145 patients treated with monoblock stems (Wagner SL). A minimum of 2 years of radiographic follow-up was mandatory for inclusion in the study. In the modular group, 11 (6.0%) of the 182 hips were lost to follow-up, and patients representing an additional 11 hips (6.0%) died as a result of causes unrelated to the revision THA. Twenty-one of the remaining 160 hips did not have the minimum 2-year radiographic follow-up. Thus, the final study cohort for modular stems consisted of 139 (76.4%) of the original 182 hips.

In the monoblock cohort, 8 (5.5%) of the 146 hips were lost to follow-up, and patients representing an additional 9 hips (6.2%) died as a result of unrelated causes. Fifteen of the remaining 129 hips did not have the minimum 2-year radiographic follow-up. Thus, the final study cohort for monoblock stems consisted of 114 (78.1%) of the 146 hips.

None of the patients who died or were lost to follow-up had undergone stem removal at the time of the latest follow-up. Prior to 2008, only modular stems were utilized at our institution because monoblock stems were not available. The proximal body of the modular stem in the present study was available in only 1 diameter. Eight surgeons performed the revision THAs, and the distribution between the 2 groups varied significantly (p < 0.001, Table I).

TABLE I - Distribution of the 8 Surgeons in the Modular and Monoblock Groups
Treatment Group*
Surgeon Modular Stem (N = 139) Monoblock Stem (N = 114)
1 36 (25.9%) 4 (3.5%)
2 5 (3.6%) 24 (21.1%)
3 5 (3.6%) 29 (25.4%)
4 9 (6.5%) 36 (31.6%)
5 34 (24.5%) 4 (3.5%)
6 33 (23.7%) 4 (3.5%)
7 4 (2.9%) 10 (8.8%)
8 13 (9.4%) 3 (2.6%)
*The values are given as the number of hips treated by each surgeon, with the percentage of hips in that treatment group in parentheses.

Clinical Assessment

For all patients, clinical data, including the use of strut allograft and extended trochanteric osteotomy (ETO), were prospectively collected in our department registration center at the time of the revision. Patients were encouraged to visit our hospital for evaluation at 1, 3, 6, and 12 months after revision, and annually thereafter. The attending surgeon assessed Harris hip score33 (HHS), and anteroposterior radiographs of the pelvis and lateral radiographs of the hip were made at each visit. Patients who could not return for office visits were contacted via telephone and were asked to mail their latest postoperative radiographs to us for review.

Radiographic Assessment

All serial radiographs were reviewed by 2 surgeons not involved with the surgical and clinical care of the patients; these reviewers were blinded to the clinical information and outcomes of the patients. Radiographs were assessed for preoperative Paprosky femoral defect classification34,35, and the most recent radiographs were used mainly for comparison as a means of determining bone remodeling signs. Other available postoperative follow-up radiographs were also reviewed as a means of assessing bone remodeling.

Proximal bone restoration in residual osteolytic areas was subjectively classified as osseous restoration, constant defects, or increasing defects, according to the criteria of Böhm and Bischel8. Stability of the stem was determined according to the criteria set forth by Engh et al.36 and partially modified by Rodriguez et al.27. Femoral stress-shielding was graded per the criteria described by Engh et al.37. Spot-welds refer to new bone formation bridging the stem surface and the endosteal bone27. Any radiolucent lines around the femoral stem were noted. Discrepancies in findings between the 2 reviewers were resolved by consensus. Stem diameter and medullary canal filling (measured as the ratio of stem width to femoral canal width) were determined at 3 different levels according to the method of Gutiérrez Del Alamo et al.: level A at the proximal one-third of the stem, level B at the midpoint of the stem, and level C at the distal one-third of the stem13. Stem length was defined as the distance from the stem shoulder to the tip. We also measured the length of the spline part for both stems and the length of the distal section of the modular stem. Variations in magnification for all radiographs were calibrated with use of the known femoral head diameter. Interobserver reliability was calculated on the basis of the agreement between the 2 reviewers, and intraobserver reliability was calculated on the basis of the agreement of the findings of the first evaluator on separate occasions, 4 weeks apart. The kappa values for intraobserver and interobserver reliability in the stress-shielding evaluation were 0.815 and 0.862, respectively. The intraclass correlation coefficients for intraobserver and interobserver reliability in the measurement of stem canal filling at level B were 0.977 and 0.95, respectively.

Statistical Analysis

Continuous variables (HHS, follow-up time, etc.) and ordinal categorical variables (proximal femoral bone restoration, femoral stress-shielding, and preoperative femoral bone defect) were examined with use of the Wilcoxon rank-sum test. Unordered categorical variables (stem type, stem tip spot-welds, periprosthetic radiolucent lines, etc.) were analyzed with use of either the Pearson chi-square test or the Fisher exact test. The Spearman rank correlation coefficient analysis was utilized to determine the correlations among different ordinal categorical variables and/or continuous variables. Ordinal categorical variable logistic regression (for the outcomes of femoral bone restoration and femoral stress-shielding) and binary logistic regression analysis (for the outcomes of stem tip spot-welds and periprosthetic radiolucent lines) were utilized to analyze the confounding effect of follow-up time (for the independent variables of follow-up duration and stem type). Multiple linear regression (for the HHS outcome) was utilized to adjust for the confounding effect of follow-up time on the relation of proximal femoral bone restoration with HHS (for the independent variables of follow-up duration and femoral bone restoration) or femoral stress-shielding with HHS (for the independent variables of follow-up duration and femoral stress-shielding). Significance was set at p < 0.05. All statistical analyses were conducted with use of SPSS (version 17.0 for Windows; SPSS).

Results

There were no significant differences between the modular and non-modular groups in demographics, the use of ETO and strut allograft, and the preoperative and latest postoperative HHS (Table II). All stems obtained bone ingrowth fixation. More patients in the monoblock group had proximal femoral osseous restoration in residual osteolytic areas than in the modular group (p = 0.009) (Table III, Fig. 1). Restoration was positively associated with the intraoperative use of strut allografts (Fisher test; p = 0.038). In the modular group, restoration was also associated with the reasons for revision (p = 0.035) and associated positively with ETO (p = 0.019). Proximal femoral bone restoration was not associated with age (p = 0.055; Spearman correlation coefficient = −0.13), sex (p = 0.396), body mass index (BMI [p = 0.804]), or preoperative femoral defect (p = 0.116). After adjusting for follow-up time, restoration had no influence on the latest postoperative HHS (p = 0.723).

TABLE II - Patient Demographics, Including Preoperative and Latest Postoperative HHS
Variable Modular Group (N = 139) Monoblock Group (N = 114) P Value
Age(yr) 61.2 ± 10.9 (63.0; 29-80) 59.8 ± 13.2 (61.0; 23-84) 0.444
Female 70 (50.4%) 63 (55.3%) 0.437
BMI(kg/m 2 ) 27.5 ± 5.1 (26.8; 18.0-42.1) 26.7 ± 4.4 (26.4; 16.8-39.3) 0.365
Follow-up duration(yr) 6.3 ± 3.0 (5.3; 2.1-14.6) 5.1 ± 1.6 (5.0; 2.0-8.1) 0.006
ETO utilized 20 (14.4%) 25 (21.9%) 0.119
Strut allograft 26 (18.7%) 28 (24.6%) 0.258
Preoperative Harris hip score 39.5 ± 13.2 (40; 10-83) 41.4 ± 14.0 (44; 10-74) 0.177
Reasons for revision 0.813
 Aseptic loosening 119 (85.6%) 96 (84.2%)
 Infection 16 (11.5%) 13 (11.4%)
 Periprosthetic fracture 4 (2.9%) 5 (4.4%)
Paprosky femoral defect 0.738
 I 2 (1.4%) 1 (0.9%)
 II 12 (8.6%) 12 (10.5%)
 IIIA 65 (46.8%) 60 (52.6%)
 IIIB 47 (33.8%) 34 (29.8%)
 IV 13 (9.4%) 7 (6.1%)
Charnley classification* 0.343
 A 101 (72.7%) 90 (78.9%)
 B 3 (2.2%) 2 (1.8%)
 BB 14 (10.1%) 13 (11.4%)
 C 21 (15.1%) 9 (7.9%)
Latest postoperative HHS 86.1 ± 8.1 (87; 60-100) 86.2 ± 10.24 (87; 40-100) 0.534
*Charnley classification38: A = unilateral THA, contralateral hip not diseased; B = unilateral THA, contralateral hip diseased; BB = bilateral THA; C = other factors contributing to failure to achieve normal locomotion.
The values are given as the mean ± standard deviation, with the median and range in parentheses.
The values are given as the number of hips, with the percentage of hips in that treatment group in parentheses.

TABLE III - Comparison of Proximal Femoral Bone Restoration of Residual Osteolytic Areas in the Modular and Monoblock Groups
Modular Group* (N = 139) Monoblock Group* (N = 114) Mann-Whitney U Value P Value
Proximal Femoral Bone Restoration 9,332.0 0.009
 Increasing defects 42 (30.2%) 22 (19.3%)
 Constant defects 54 (38.8%) 40 (35.1%)
 Osseous restoration 43 (30.9%) 52 (45.6%)
*The values are given as the number of hips, with the percentage of hips in that cohort in parentheses.

fig1
Fig. 1:
Anteroposterior radiographs of a 65-year-old man. Fig. 1-A Radiograph made prior to revision THA showing stem loosening. Fig. 1-B Radiograph made immediately after revision THA with a monoblock tapered fluted titanium stem. Figs. 1-C, 1-D, and 1-E Postoperative radiographs showing proximal femoral bone restoration of residual osteolytic areas (solid white arrows) at 3 months (Fig. 1-C), 3 years (Fig. 1-D), and 6 years (Fig. 1-E) after revision THA.

Patients with modular stems had more severe femoral stress-shielding than did patients with monoblock stems (p < 0.001) (Table IV, Fig. 2). Stress-shielding was positively correlated with preoperative femoral defect (p < 0.001; Spearman correlation coefficient = 0.298); however, it was not associated with age (p = 0.835), sex (p = 0.229), BMI (p = 0.757), the reason for revision (p = 0.700), the use of strut allografts (p = 0.062), or the use of an ETO (p = 0.109). After adjusting for follow-up time, the degree of stress-shielding of the femur had no influence on the latest postoperative HHS (p = 0.522).

TABLE IV - Comparison of Degree of Femoral Stress-Shielding in the Modular and Monoblock Groups
Modular Group* (N = 139) Monoblock Group* (N = 114) Mann-Whitney U Value P Value
Degree of Femoral Stress-Shielding 6,093.5 <0.001
 None 68 (48.9%) 81 (71.1%)
 First degree 6 (4.3%) 7 (6.1%)
 Second degree 14 (10.1%) 3 (2.6%)
 Third degree 19 (13.7%) 9 (7.9%)
 Fourth degree 32 (23.0%) 14 (12.3%)
*The values are given as the number of hips, with the percentage of hips in that cohort in parentheses.

fig2
Fig. 2:
Anteroposterior radiographs of a 69-year-old woman. Fig. 2-A Radiograph made prior to revision THA showing stem loosening. Fig. 2-B Radiograph made immediately after revision THA with a modular tapered fluted titanium stem. Fig. 2-C Radiograph made 1.2 years after revision THA showing fourth-degree stress-shielding (solid white arrow) and spot-welds at the stem junction (dotted white arrow). Fig. 2-D Radiograph made 3.8 years after revision THA showing a more severe degree of stress-shielding (solid white arrow) and persistent spot-welds at the stem junction (dotted white arrow).

Stem tip spot-welds developed in 88.5% of the modular stems compared with 47.4% of the monoblock stems (p < 0.001). Spot-welds developed at the modular junction in 38.8% of the modular stems (Fig. 2). No radiolucent lines were found around the distal section of either the modular or monoblock stems. Nevertheless, partial or circumferential radiolucent lines were observed around the proximal segment of 30.9% of modular stems, compared with 14.0% of monoblock stems (p = 0.002).

The diameter and medullary canal filling of the modular stems were both larger than those of the monoblock stems at levels A, B, and C (Table V). Compared with monoblock stems, the stem and spline were longer and shorter, respectively, in the modular stems (Table V). The length of the distal section of the modular stem was also shorter than the entire length of the monoblock stem (Table V). Proximal femoral bone restoration was negatively correlated with stem diameter and positively correlated with stem spline length, whereas the degree of femoral stress-shielding was positively correlated with stem diameter at levels A, B, and C and negatively correlated with stem spline length (Table VI). Bone restoration and the degree of femoral stress-shielding were both unrelated to medullary canal filling (Table VI).

TABLE V - Comparison of Stem Diameter, Length, and Medullary Canal Filling at Levels A, B, and C*
Modular Group Monoblock Group P Value
Diameter at level:
 A (mm) 17.9 ± 2.6 (19.1; 11.9-22.2) 14.8 ± 1.9 (14.6; 11.2-21.4) <0.001
 B (mm) 16.2 ± 2.5 (16.4; 10.4-22.2) 13.5 ± 1.8 (13.3; 10.5-20.5) <0.001
 C (mm) 12.9 ± 2.2 (12.8; 8.5-19.72) 12.4 ± 1.7 (12.3; 8.4-18.5) 0.046
Canal filling at level:
 A 0.86 ± 0.09 (0.88; 0.54-0.99) 0.81 ± 0.08 (0.81; 0.64-0.98) <0.001
 B 0.92 ± 0.05 (0.93; 0.70-0.99) 0.85 ± 0.08 (0.84; 0.60-1.00) <0.001
 C 0.86 ± 0.07 (0.87; 0.56-0.99) 0.84 ± 0.09 (0.84; 0.57-0.99) 0.036
Stem length (mm) 223.6 ± 25.8 (220.2; 188.3-306.7) 201.5 ± 20.3 (199.0; 157.4-268.7) <0.001
Stem spline length (mm) 105.3 ± 16.7 (99.5; 68.4-178.0) 164.3 ± 20.8 (162.4; 118.2-245.1) <0.001
Distal section of modular Link MP stem versus entire Wagner stem length (mm) 140.1 ± 23.6 (138.4; 100.4-219.0) 201.5 ± 20.3 (199.0; 157.4-268.7) <0.001
*The values are given as the mean ± standard deviation, with the median and range in parentheses.
Values refer to the length of distal section of modular megaprosthesis stem.
Values refer to Wagner stem length.

TABLE VI - Spearman Correlation Coefficient Between Proximal Femoral Bone Restoration of Residual Osteolytic Areas or Degree of Femoral Stress-Shielding and Stem Diameter, Length, or Medullary Canal Filling or Lengths
Proximal Femoral Bone Regeneration of Residual Osteolytic Areas Degree of Stress-Shielding of the Femur
Spearman Correlation Coefficient P Value Spearman Correlation Coefficient P Value
Diameter at level:
 A –0.259 <0.001 0.327 <0.001
 B –0.411 <0.001 0.492 <0.001
 C –0.427 <0.001 0.423 <0.001
Canal filling at level:
 A –0.047 0.454 0.063 0.316
 B –0.060 0.343 0.096 0.126
 C –0.017 0.790 0.024 0.704
Stem length –0.071 0.260 0.164 0.009
Stem spline length 0.151 0.016 –0.173 0.006

Follow-up time had no confounding effect on the proximal femoral bone restoration in the residual osteolytic area (p = 0.995), femoral stress-shielding (p = 0.544), stem tip spot-welds (p = 0.085), or radiolucent lines (p = 0.294).

Discussion

Previous studies of tapered fluted stems9,13,14,18,20,22-24 have been limited to radiographic evaluation of 1 particular stem design, either modular or monoblock, but to our knowledge no study has compared bone remodeling around these different stem designs. The present study is the first to compare stem types regarding restoration of proximal femoral bone in residual osteolytic areas, femoral stress-shielding, and spot-welds, as well as radiolucent lines, stem diameters and lengths, and intramedullary canal filling of the stems.

The present study found that the rate of osseous restoration in the monoblock group was better than that in the modular group. The main reason for this finding may be that the modular stem had a shorter distal section with curvature that allowed the surgeon to impact a thicker distal section, achieving a better fit and fill than the longer straight monoblock stem, the tip of which may be stuck at the anterior cortex because of femoral bowing; additionally, the monoblock Wagner stem can also be initially stabilized with “3-point fixation.” This difference in stem thickness was also apparent at levels A, B, and C, at which the diameters of the modular stems were greater than those of monoblock stems. Modular stems with larger diameters had greater stiffness, which would result in an overall reduction in strain on the femur, potentially explaining why osseous restoration in the modular group was less than in the monoblock group.

McInnis et al.23 reported that 56% of patients using the modular stem had proximal femoral osseous restoration, and Amanatullah et al.20 reported 50% restoration, whereas Weiss et al.18 reported only 17%, compared with the 30.9% in the present study. The restoration rate for patients with Wagner stems has ranged from 47% to 88%8,13,14,24; in the present study, it was 45.6%. Weiss et al.18 believed that varying restoration rates could be because of different classification systems, different prosthesis sizes, varying durations of follow-up, and subjective reviews of radiographs.

The results of the present study showed that proximal femoral osseous restoration was positively associated with the use of strut allografts and ETO in the modular group. Strut allografts may promote greater load transmission through the proximal aspect of the femur, and ETO facilitates bone-healing and revascularization similar to that of fractures8,27, which could promote osseous restoration.

The results of the present study also showed that modular stems create more severe femoral stress-shielding, possibly because of the secure distal fixation and the higher stiffness of the modular stems relative to that of the monoblock stems, which reduces the effectiveness of proximal stem-bone contact and strain transfer. The modular Link MP stem mainly relies on the relative shorter distal section for fixation, whereas the Wagner stem relies on the longer whole length for fixation. A short distance of fixation at the distal aspect of the femur resulted in stress concentration distally and stress-shielding proximally around the stem. Stress-shielding was positively correlated with the severity of the preoperative femoral defect. One might speculate that the surgeon should use a thicker stem to gain adequate stability in the setting of severe femoral bone defects; thicker stems are more rigid and more easily produce stress-shielding. Rodriguez et al.27 and Amanatullah et al.20 reported that 29.7% and 23% of femora, respectively, developed diaphyseal stress-shielding with modular stems; these rates are comparable with those reported in the present study. Although stress-shielding had no influence on postoperative HHS, severe femoral stress-shielding may increase the risk of periprosthetic fractures. Furthermore, young patients with severe femoral stress-shielding may have inadequate bone stock for future revision surgery.

We found partial or circumferential radiolucent lines in the proximal segment of 30.9% of modular stems, with previous studies reporting rates of 38% to 42%20,22. The cause of proximal radiolucent lines in this study could be that the available modular Link MP stems all had 1 diameter for the proximal body. The lack of multiple diameters made it difficult to fit the proximal body to the proximal femoral bone. The elevated stiffness and the secure distal fixation of the modular stems prevented load transfer and lessened bone strain proximally. All of these factors may play a role in compromising bone ingrowth, resulting in persistent radiolucent lines in the modular group. Gutiérrez Del Alamo et al.13 reported radiolucent lines in 13.9% of the monoblock tapered fluted titanium stems, compared with 14% in the present study. However, the longitudinal ridges of monoblock stems make it difficult to assess radiolucent lines around them13.

Rodriguez et al.27 reported the development of spot-welds around the tips of all modular stems, compared with 88.5% in our study. Spot-welds at the junction of the modular stem are a characteristic radiographic sign; 38.8% of modular stems in the present study developed spot-welds at the modular junction, compared with 79.6% reported by Rodriguez et al.27. The spot-welds at the junction between the proximal segment and the curved distal section would indicate that this is the location of the delayed transfer of shear stress across the implant-bone interface, further shortening the effective stem length.

This study had several limitations. Modular and monoblock stems were not used during the same time period, so the presence of a learning curve could be a confounder. However, the 2 stems differ in curvature, length, and modularity, and the experience with either stem may not be applicable to the other. Second, only 76.4% of the modular stems and 78.1% of the monoblock stems were included. Third, follow-up durations differed significantly between the 2 groups, ranging from 2 to 14 years. However, follow-up time had no effect on outcome variables, and bone remodeling signs changed little with time over 3 years, which is consistent with results reported by Rodriguez et al.27. Fourth, not every patient had complete postoperative radiographs at 1, 3, 6, and 12 months, or annually thereafter; as such, data could not be compared for each patient at the same follow-up points. Fifth, not all radiographs obtained had the same position and exposure, which can impact classification of bone remodeling. Sixth, differences among surgeons may confound the outcomes; however, all of the revisions were performed by experienced senior surgeons who had been trained in the same hospital. We believe that all of these surgeons were equally experienced with both stems.

In summary, the present study found that, compared with the longer and thinner monoblock stems, modular stems had less proximal osseous restoration in residual osteolytic areas and more severe femoral stress-shielding, stem tip spot-welds, and radiolucent lines around the stiffer modular stems with a shorter distal section. The stem diameter and stiffness (which were influenced by stem length, curvature, and modularity) determined bone remodeling patterns.

Note: The authors acknowledge the dedicated work of Dr. Shengjie Guo for proofreading the article, Mrs. Xueying Li for statistical help, and medical editor Katharine O’Moore Klopf, ELS (East Setauket, New York, USA) for providing professional English-language editing of this article.

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