- Research article
- Open Access
- Open Peer Review
Maintenance of bone mineral density after implantation of a femoral neck hip prosthesis
© Decking et al; licensee BioMed Central Ltd. 2008
- Received: 02 April 2007
- Accepted: 31 January 2008
- Published: 31 January 2008
Stress shielding of the proximal femur has been observed in a number of conventional cementless implants used in total hip arthroplasty. Short femoral-neck implants are claiming less interference with the biomechanics of the proximal femur. The goal of this study was to investigate the changes of bone-mineral density in the proximal femur and the clinical outcome after implantation of a short femoral-neck prosthesis.
We prospectively assessed the clinical outcome and the changes of bone mineral density of the proximal femur up to one year after implantation of a short femoral neck prosthesis in 20 patients with a mean age of 47 years (range 17 to 65). Clinical outcome was assessed using the Harris Hip Score. The WOMAC was used as a patient-relevant outcome-measure. The bone mineral density was determined using dual energy x-ray absorptiometry, performed 10 days, three months and 12 months after surgery.
The Harris Hip Score improved from an average preoperative score of 46 to a postoperative score at 12 months of 89 points, the global WOMAC index from 5,3 preoperatively to 0,8 at 12 months postoperatively. In contrast to conventional implants, the DEXA-scans overall revealed a slight increase of bone mineral density in the proximal femur in the 12 months following the implantation.
The short femoral neck stem lead to a distinct bone reaction. This was significantly different when compared to the changes in bone mineral density reported after implantation of conventional implants.
- Bone Mineral Density
- Proximal Femur
- Stem Design
- Conventional Implant
- Periprosthetic Bone Mineral Density
The remodelling of the proximal femur after total hip replacement depends on the implants size, geometry and stiffness. Considerable bone resorption in the proximal femur as an answer to stress shielding of the bone surrounding total hip implants has been demonstrated after total hip arthroplasty with a medullar fixation [1–3]. Conventional cementless implants have shown a constant decrease of periprosthetic bone mineral density (BMD) in the proximal femur, as demonstrated by dual-energy x-ray absorptiometry (DEXA) especially over the course of the first year following surgery. Short femoral-neck implants are claiming less interference with the biomechanics of the proximal femur. As such, they may be an alternative to conventional implants, especially for younger patients, where a higher revision rate has been reported. While the mean age of patients requiring a total hip replacement is constantly decreasing, the Swedish Hip Arthroplasty Registry  reports an implant survival of 74.9% at 14 years for male patients younger than 50 years, compared to a survival of 84.4% for male patients between the age of 60 and 75 years. Although the reason for failure in the group or young patients is multifactorial, short stemmed femoral shaft prostheses have the theoretical advantage to preserve bone at the initial implantation  and ideally maintain this amount of bone over time for upcoming revisions. While long-term results for this group of implants have not been reported, the concept has been supported by biomechanic in vitro research, using composite and cadaver femora models [6–8]. The primary goal of this study was to prospectively investigate the in-vivo changes of bone-mineral density as a parameter of bone remodelling around a short, femoral-neck prosthesis. The secondary goal was to report on its clinical outcome.
For statistical analysis JMP IN statistical software (SAS Institute Inc, NC, USA) for Macintosh was used in its version 5.1.2. At first the presuppositions for a normal distribution were tested. Since histograms and the Shapiro-Wilk-tests were not able to show a normal distribution in all cases, the Wilcoxon signed-ranks test was used to statistically compare the density changes over the 12 months following surgery. The level of significance was set at alpha = 5%.
HSS and WOMAC; preoperative and postoperative scores
Harris Hip Score preop
Harris Hip Score 12 months
WOMAC 12 months
Bone Mineral Density
Bone density and changes between measurements at 10 days, 3 months and 12 months postop
10 d-3 mo
10 d- 12 mo
mean bone density in g/cm2
mean bone density in g/cm2
mean change in %
mean bone density in g/cm2
mean change in %
Changes in mean bone mineral density in percent, compared with the first postoperative values
Yamaguchi et al.
Aldinger et al.
Venesmaa et al.
Leichtle et al.
In vitro strain measurements after implantation of the Cut prosthesis have shown less change in the proximal femur when compared to conventional implants. In a experimental setting using strain gauges in cadaver femora , strains increased at the strain gauges referring to the ROIs 2, 6 and 7. Since strain gauges only record local strain information on the outer cortical bone surface, they do not fully reflect the in-vivo loading, especially in the cancellous bone of the femur. As such, when comparing the data of the in vivo- and in vitro-results, it is evident that these experimental data have to be regarded with some scepticism. However, even though the distribution of the changes did not perfectly fit the distribution of the BMD-measurements, strain-changes recorded for the short stem in the in vitro study were far smaller than those seen in conventional implants. Steinhauser et al  evaluated three different short femoral neck implants in composite femora using photoelastic coating techniques and compared changes in hoop-strains with a conventional implant. In this setting, the strains recorded with the Cut-implant mostly stayed within the 95% confidence intervals of the native composite femora. Nevertheless, there was a clear reduction of stresses medially at the height of the lesser trochanter, where a small, but significant reduction of BMD is seen in the in vitro-data after one year (ROI 6). They also recorded an increase in strains laterally where the lateral tip of the implant pushes against the cortical bone, corresponding to the significant increase in ROI 3 in the in vivo BMD-changes. Munting and Verhelpen showed a similar pattern of strain distribution in a (quite different) experimental stemless prosthesis , which used varying trans-trochanteric screw-fixations on the lateral cortex. When a successor of this implant was followed in vivo, the authors also reported on a maintenance of BMD  in the proximal femur over the course of up to 6 postoperative years. And Joshi and co-workers  used a FEA-model to predict the stresses around a short implant with cables attachments around the greater trochanter. This was able to reduce the stress peaks at the lateral side of the femur. However, the authors concluded, that in vivo and in vitro testing of the prosthesis were still pending.
As a number of clinical studies suggest, the largest part of bone remodelling following total hip arthroplasty ceases within the first postoperative year [20, 22, 23]. As such, the follow-up in this study should be long enough to show the specific reaction to the short stem observed. Nevertheless, it cannot be ruled out that the changes around this rather untypical implant might have a different course over time. We used a longitudinal study design, as only prospective analysis with the baseline taken after the surgery can provide reliable information about the actual loss of bone density [14, 22]. The methods used in our follow-up were standardized and the rotation of the leg was strictly controlled as suggested by studies on the precision of measurement of periprosthetic bone mineral density [24, 25].
Conservation of bone stock is an essentially important principle especially in young patients, where the chances for revisions during the patient's lifetime are high. As the data presented here show, an alternative prosthesis design is able to reduce stress-shielding-related bone resorption in the proximal femur. Nevertheless, the authors are aware that other factors influence survival and the clinical results of total joint implants. When Ender and co-workers  followed the Cut-stem clinically, they reported on an unacceptable high rate of revisions after a midterm follow- up averaging 5.1 years. This might be due to a difference in the indications for choosing the implant. For our study, the patients were carefully selected, and only young patients with an anatomy believed to be suitable for this special implant were chosen. The implant was not used in patients with coxa vara or valga, nor with an increased anteversion of the femoral neck. All procedures were carefully planned, with the templated stems accurately fitted within the femoral neck and the distal lateral part of the prosthesis firmly against the subtrochanteric cortical bone, and this position was than achieved at the operation. Radiographic and clinical results after the short follow-up of 12 months showed no signs of early failure in this small patient group, and the postoperative HSS and WOMAC-scores match the results seen the follow-up of conventional stems. Other groups using small-sized implants with an intertrochanteric fixation like the Mayo Conservative Hip stem (Zimmer, Inc., Warsaw, IN, USA) reported superior functional results in short-term follow-up when compared to a standard cementless stem , as well as an excellent survival of 98% at 10 years .
However, the data presented here mainly focus on the bone mineral density changes of a specific stem in a selected group of patients, and does not include any information on the long-term survival of the implant.
The implantation of a short femoral neck stem lead to a distinctive bone reaction, which differed to the changes seen after implantation of conventional implants. Only further analysis with a longer follow-up of larger patient collectives will be able to show if this is leading to an acceptable survival as well as proven advantages in the case of revision, and subsequently to a clearer view which patients might profit from the use of alternative stem-designs.
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