The bone mineral density (BMD) around the femoral component changes after total hip arthroplasty (THA).1 The mechanical stress caused by the femoral component affects the bone remodeling around the component.2 This post-THA bone remodeling may affect the longevity of the femoral component and increase the risk of postoperative peri-prosthetic fracture.3,4 Previous studies have shown that the BMD change around the femoral component is influenced by factors such as stem length, stem shape, material used, use of a cemented or noncemented component, porous coating, and medication.5–10 However, only a few studies have investigated the influence of the morphology of the preoperative proximal femoral canal on the postoperative BMD change around the femoral component.
In 1 study, finite element analysis showed that a tapered wedge-type stem resulted in a significantly smaller mechanical load on the proximal medial part of the femur using finite element analyses and significantly more severe BMD loss in the same area at 1 year postoperatively in patients with stovepipe femurs than in patients with normal and champagne-flute femurs.11 The authors suggested that the morphology of the preoperative proximal femoral canal affected the postoperative BMD change around the femoral component when using a tapered wedge-type stem.11
The Zweymüller-type stem is a cementless, tapered, rectangular cross-sectioned titanium alloy stem with a grit-blasted surface across its entire length.12–14 Its rectangular cross section provides rotational stability and achieves excellent fixation. The Zweymüller-type stem has shown excellent long-term clinical results.15–17 However, no studies have been performed to investigate the relationship between the morphology of the preoperative femoral canal and postoperative BMD change around the Zweymüller-type stem. The current authors hypothesized that the morphology of the preoperative proximal femur affects the BMD change around the Zweymüller-type stem after THA, similar to the tapered wedge-type stem. The aim of this study was to investigate the influence of the morphology of the preoperative proximal femoral canal on the postoperative BMD change in the femur around the Zweymüller-type stem.
Materials and Methods
Between June 2005 and November 2013, a total of 226 hips of 203 patients underwent THA using the femoral head using Zweymüller-type stems in the current authors' institution. The current authors excluded patients who underwent previous femoral osteotomy before THA (12 hips), those with intraoperative fracture (3 hips), and those without the data of the periprosthetic BMD postoperatively (91 hips). This study included 120 hips of 112 patients. Mean patient age was 61.2±12.8 years (range, 20–87 years). The patients comprised 95 women (101 hips) and 17 men (19 hips). The Alloclassic Zweymüller stem (Zimmer Biomet, Warsaw, Indiana) was used in 57 hips of 55 patients and the Profemur Z Modular stem (MicroPort Orthopedics Inc, Arlington, Tennessee) was used in 63 hips of 57 patients. All operations were performed by 3 experienced surgeons. Operations were performed using the anterolateral approach in 83 hips and the posterior approach in 37 hips. This study was approved by the current authors' institutional review board, and all patients provided informed consent.
To assess the morphology of the preoperative proximal femoral canal, the canal flare index (CFI)18 was determined using the preoperative anteroposterior radiographs on the operative side. Femurs were classified into the 3 morphologic groups of the proximal femoral canal according to the CFI: CFI<3.0, stovepipe group; 3.0<CFI<4.7, normal group; and CFI>4.7, champagne-flute group.
The periprosthetic BMD was measured by dual-energy radiography absorptiometry (QDR 4500A; Hologic Inc, Waltham, Massachusetts). The current authors obtained these scans of the operated proximal femoral region at 3 weeks postoperatively as a baseline reference and at 6, 12, and 24 months postoperatively. The periprosthetic BMD was measured within each region of interest (ROI) described by Gruen (Figure 1).19 The relative changes in the BMD were calculated from the BMD at 3 weeks postoperatively and at each follow-up period. The current authors compared the average percentage changes in the BMD at each ROI and at each follow-up period among the 3 morphological types of the preoperative proximal femoral canal.
Regions of interest (ROI) used to measure bone mineral density around the femoral component according to Gruen's zones.
Statistical analyses were performed using computer software (GraphPad Prism V.7.0; GraphPad Software, La Jolla, California). The current authors used chi-square test to compare the proportions of categorical variables and one-way analysis of variance test with Tukey's post hoc analysis to compare the averages of continuous variables among the 3 groups. P<.05 was considered statistically significant.
The demographic data of the groups classified by the morphology of the proximal femoral canal are shown in Table 1. In total, 26 (21.7%) femurs of 24 patients were classified as stovepipe femurs, 72 (60.0%) femurs of 67 patients were classified as normal femurs, and 22 (18.3%) femurs of 21 patients were classified as champagne-flute femur. The differences in age, sex, height, weight, body mess index, preoperative BMD of the proximal femur neck, and the ratio of the 2 prostheses were not statistically significant among the 3 groups.
Demographic Data of the 3 Groups According to Canal Flare Index
The relative changes of the periprosthetic BMD in the Gruen zones at 3 weeks and 24 months postoperatively in each group are shown in Figure 2 and Table 2. In the stovepipe group, the relative change in the BMD at 3 weeks and 24 months postoperatively decreased in the proximal region (ROI 1 and 7) and middle region (ROI 2 and 6). In the normal group, the relative change in the BMD decreased in the proximal region (ROI 1 and 7) and middle region (ROI 2 and 6) and increased in the distal region (ROI 5). In the champagne-flute group, the relative change in the BMD decreased in the proximal region (ROI 1 and 7) and increased in the distal region (ROI 3). However, the differences in the relative change in the BMD among the 3 groups were not statistically significant in all ROIs at each follow-up period.
Relative change of bone mineral density (BMD) in region of interest (ROI) 1 to 7 at 3 weeks and 6, 12, and 24 months postoperatively among the 3 morphological types of the preoperative proximal femoral canal.
Relative Changes (%) of the Periprosthetic Bone Mineral Density Between 6 and 24 Months Postoperatively
During the study period, no hips required revision surgery for any reason, including infection or loosening, in the 3 groups. The differences in the relative change in the BMD between 2 types of stems were not statistically significant in all ROIs at 24 months postoperatively.
To the current authors' knowledge, this is the first study to investigate the relationship between the morphology of the preoperative femoral canal and postoperative BMD change around the Zweymüller-type stem. The most important finding of this study is the lack of significant differences in the relative change in the BMD among the different morphologies of the proximal femoral canal. The result was different from the current authors' hypothesis that the morphology of the preoperative proximal femur affects the BMD change around the Zweymüller-type stem after THA. However, this result suggests that the preoperative morphology of the femoral canal does not affect the postoperative BMD change around the Zweymüller-type stem at 2 years postoperatively.
The tapered wedge-type stem, which is type 1 in Khanuja's classification of the cementless stem, achieves fixation by the wedge shape to the cortical bone in the metaphysis.12 Oba et al11 reported that when the tapered wedge-type stem was implanted, stovepipe femurs showed significantly greater BMD loss in the proximal medial area of the femur at 1 year postoperatively than normal and champagne-flute femurs. The stovepipe femurs needed larger stems to obtain an optimal fit of the stem than did the other types of femurs. Finite element analysis showed that larger stem for stovepipe femurs shifted the mechanical load distribution more distally and decreased the mechanical load in the proximal area of the femur.11 Such change of mechanical load distribution causes the stress shielding and might become a risk factor for the periprosthetic facture.11 Their results suggested that the tapered wedge-type stem should be used in consideration of preoperative morphology of the proximal femoral canal.
The Zweymüller-type stem, which is type 3C in Khanuja's classification, achieves 3-point fixation in the metaphyseal-diaphyseal junction and proximal part of the diaphysis.12 Brodner et al1 reported that when the Zweymüller-type stem was implanted, the BMD of the femur increased in Gruen zones 2, 4, and 5 and significantly decreased in Gruen zones 1, 6, and 7 at 5 years postoperatively. Their result suggests that the postoperative BMD around the Zweymüller-type stem decreases in the proximal region and increases in the distal region and that the distal region bears a larger mechanical load than the proximal region. However, they did not analyze the effect of the preoperative morphology of the proximal femoral canal on the postoperative BMD change in the proximal femur.
Unlike Oba et al,11 who used the tapered wedge-type stem, the current authors used the Zweymüller-type stem and showed that the morphology of the proximal femoral canal did not affect the BMD change around the stem. This result suggests that the influence of the preoperative morphology of the proximal femoral canal on the postoperative BMD change varies between the different stem designs. The Zweymüller-type stem provides a constant fixation pattern regardless of the preoperative morphology of the proximal femoral canal. Unlike the tapered wedge-type stem, which should be used in consideration of the preoperative femoral canal morphology, the Zweymüller-type stem can be used without the consideration of the preoperative femoral canal morphology from the point of view of postoperative BMD change and can simplify the preoperative planning and preparation.
This study has some limitations. First, the current authors used 2 Zweymüller-type stems (Alloclassic Zweymüller stem and Profemur Z Modular stem). The Alloclassic Zweymüller stem has a 4.6° taper in the medial–lateral plane and a 1.8° taper in the anteroposterior plane. The roughness of the grit-blasted surface is 4 to 6 µm. In contrast, the Profemur Z Modular stem has a 5.0° taper in the medial–lateral plane and a 3.2° taper in the anteroposterior plane. The roughness of the grit-blasted surface is 8 µm. These small differences might affect the postoperative BMD change around the stem. However, the differences in the BMD change were not statistically significant in any ROIs between the 2 Zweymüller-type stems and there were no statistically significant differences in the proportions of each stem type between the study groups. The current authors believe that the different features of the 2 stems did not influence the results of this study. Second, the follow-up period was the first 2 years postoperatively. A previous study showed that the BMD around the Zweymüller-type stem rapidly changed during the first 6 months but slowly changed or did not change after 1 year.1 The current authors believe that the 2-year follow-up period in the current study is appropriate to analyze the initial BMD change around the Zweymüller-type stem.
The preoperative morphology of the femoral canal did not affect the postoperative BMD change around the Zweymüller-type stem at 2 years postoperatively. Even for stovepipe femurs and champagne-flute femurs, the same fixation at the metaphyseal-diaphyseal junction and proximal part of the diaphysis is expected as for normal femurs.
- Brodner W, Bitzan P, Lomoschitz F, et al. Changes in bone mineral density in the proximal femur after cementless total hip arthroplasty: a five-year longitudinal study. J Bone Joint Surg Br. 2004;86(1):20–26. doi:10.1302/0301-620X.86B1.14637 [CrossRef]
- Bobyn JD, Mortimer ES, Glassman AH, Engh CA, Miller JE, Brooks CE. Producing and avoiding stress shielding: laboratory and clinical observations of noncemented total hip arthroplasty. Clin Orthop Relat Res. 1992;(274):79–96.
- Kobayashi S, Saito N, Horiuchi H, Iorio R, Takaoka K. Poor bone quality or hip structure as risk factors affecting survival of total-hip arthroplasty. Lancet. 2000;355(9214):1499–1504. doi:10.1016/S0140-6736(00)02164-4 [CrossRef]
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- Hirata Y, Inaba Y, Kobayashi N, Ike H, Fujimaki H, Saito T. Comparison of mechanical stress and change in bone mineral density between two types of femoral implant using finite element analysis. J Arthroplasty. 2013;28(10):1731–1735. doi:10.1016/j.arth.2013.04.034 [CrossRef]
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Demographic Data of the 3 Groups According to Canal Flare Index
|Canal flare index, mean±SD||2.8±0.2||3.8±0.4||5.2±0.6||<.001|
|Age, mean±SD, y||63.3±14.9||61.4±13.2||58.1±7.2||.36|
|Sex, female/male, No.||24/2||60/12||17/5||.35|
|Height, mean±SD, cm||153.0±7.1||153.4±8.0||154.0±3.9||.63|
|Weight, mean±SD, kg||55.4±9.8||55.2±10.8||54.0±9.2||.93|
|Body mass index, mean±SD, kg/m2||23.7±4.0||23.4±3.4||22.8±3.8||.61|
|Preoperative bone mineral density of proximal femoral neck, mean±SD, g/cm2||0.65±0.2||0.71±0.2||0.75±0.1||.06|
|Stem, Alloclassic/Profemur Z, No.||15/11||33/39||9/13||.41|
Relative Changes (%) of the Periprosthetic Bone Mineral Density Between 6 and 24 Months Postoperatively
|Region of Interest||Mean±SD|
|6 Months||12 Months||24 Months||6 Months||12 Months||24 Months||6 Months||12 Months||24 Months|