Introduction

Cephalomedullary nails (CMN) are commonly used for the treatment of intertrochanteric (IT) hip fractures. Total hip arthroplasty (THA) is commonly used as a salvage procedure for failed IT hip fractures that progress to post-traumatic arthritis. This study analyzed the complications of THA following treatment of failed IT hip fractures with cephalomedullary nails.

Introduction/Aims

An increased mortality associated with hip fractures has been recognized, but the impact of vertebral osteoporotic compression fractures (VCF) is still underestimated. The aim of this study was to report on the difference in survival for VCF patients following non-operative and operative [Balloon Kyphoplasty (BKP) or Vertebroplasty (VP)] treatments.

Sectioned femoral components retrieved from failed hip resurfacing arthroplasties show resorption of proximal femoral bone or formation of a fibrous membrane at the bone cement interface. Our study uses Finite Element Analysis (FEA) to examine the effects of the implant orientation on bone remodelling following hip resurfacing arthroplasty. A radiographic analysis of the proximal femur following hip resurfacing was conducted in order to draw a comparison to the FEA findings.

A 3D FEA model of the Birmingham Hip Resurfacing (BHR) was created based on the geometry and material properties of a 45 year-old female donor hip. Hip joint and muscle loads were applied. Bone remodelling stimuli was determined using changes in strain energy. A range of implant orientations were compared to study the affect on bone remodelling. A retrospective radiological analysis was undertaken on 100 hips with a minimum of 5 years follow up. Femoral neck diameter was measured at post-op, 2 and 5 years, as well as neck and stem shaft angles.

FEA showed that valgus orientation was associated with increased resorption underneath the shell. Varus orientation showed increased bone formation at the stem tip. The radiological analysis identified 2 distinct patterns of neck thinning. Slow thinners (76%) had < 5% reduction in neck diameter at 2 years and < 10% at 5 years. Rapid thinner (24%) had > 5% thinning at 2 years and > 10% at 5 years. The mean percentage reduction in neck diameter was significantly different between the two groups at the two time points (p< 0.01). The rapid group had a higher proportion of valgus aligned implants (88%) and a significant decrease in reconstructed offset (p=0.0023).

The FEA results have shown that stem alignment can affect bone resorption resurfacing. FEA results were consistent with the radiological findings. Additional retrieval studies are necessary to help understand aetiology of implant failures.

Introduction: Sectioned femoral components retrieved from failed hip resurfacing arthroplasties show resorption of proximal femoral bone or formation of a fibrous membrane at the bone cement interface, in a proportion of cases. We hypothesize that both scenarios create a functional discontinuity zone (FDZ), which exacerbates offloading the proximal bone and promoting resorption. Our study uses finite element modeling to examine the effects of the presence of an FDZ on bone remodeling following hip resurfacing arthroplasty. A radiographic analysis of the proximal femur following hip resurfacing was also conducted in order to draw a comparison to clinical findings.

Methods: The hip resurfacing FE models were oriented in 3 distinct stem-shaft angles: 136 ‘neutral’, 120 ‘varus’ and 150 ‘valgus’. A low-modulus (2 MPa) FDZ (approx. 2 mm thick) was simulated beneath the implant head. Femoral joint and muscle loads were applied to simulate peak joint loading during gait. Interface stress was compared for the normal and simulated FDZ resurfaced femurs. Bone remodeling stimuli was determined using changes in strain energy. A retrospective radiological analysis was undertaken on 98 hips (74 males and 24 females) with a minimum of 5 years follow up. Measurements of the prosthesis-shaft angle, pre–and post-operative femoral head offset and femoral neck diameter at 2 and 5 years were undertaken.

Results: The presence of the simulated FDZ in the FE analysis resulted in increased proximal-medial bone resorption and slightly greater bone formation surrounding the stem. Correspondingly, device-bone interface stresses were found to decrease proximally under the loading platform and increase at the stem, particularly adjacent to the stem-head junction. The valgus BHR femur led to increased resorption, especially around the periphery of the neck and on the medial side. The radiological analysis identified 2 groups; 22 hips (Group 1) had a mean 5.61mm (sd 2.07) reduction in neck diameter over 5 years and 76 hips (Group 2) demonstrated slow reduction in neck diameter, mean 1.13mm (sd 0.97). Neck thinning at 2 and 5 years was significantly greater for Group 1 (p< 0.0001). Group 1 hips had significantly greater reduction in femoral offset (p=0.041), with greater valgus angle oriented components (p=0.09). Reduction in femoral offset was significantly associated with greater valgus orientations (p< 0.0001). The Group 1 revision rate was 36.4% compared to 2.6% in Group 2 (p< 0.0001).

Discussion: The FE results support the hypothesis that the presence of a FDZ decreases load transfer to the proximal bone, resulting in increased medial stress shielding and resorption. These results are consistent with the Group 1 clinical findings. In order to better understand the cause of implant failures in hip resurfacing arthroplasty, additional retrieval studies are necessary.

Starting in the 1970s, long-term survivorship of total hip and knee arthroplasty has been under investigation for the Scandinavian population with the aid of implants registries. In the United States, no national arthroplasty registry currently exists. Nationwide inpatient discharge databases in the United States have proven useful when comparing the revision burden in the United States and Scandinavia. For this study, we compared the implant survivorship in the Medicare population with contemporaneous registry-based data from well-established and validated Scandinavian arthroplasty registries. The 5% systematic sample of Medicare claims from 1997 to 2004 were examined for primary and revision THA and TKA claims. The Medicare beneficiary ID was used to follow patients longitudinally between primary and revision surgery. De-identified data from the Norwegian and Swedish national hip and knee registry were also obtained for the same time period. During the 8-year study period, 30,583 and 62,878 elective primary total hip or knee replacements, respectively, were extracted from the Medicare data. In the same time period, 41,823 and 15,927 primary total hips or knees replacements were identified from the Norwegian registry. 82,037 primary total hips were identified from the Swedish registry. Survivorship was assessed by the Kaplan-Meier method, and Cox regression was used to evaluate the effect of patient attributes and cross-country comparisons. The K-M estimate showed that 8 years post-primary surgery, 93.6% of THA and 96.2% of TKA remained revision-free among the elderly Medicare population. By comparison, among Norwegians aged 65 and above, 96.0% of THA and 93.6% of TKA remained revision-free. In the US, men had a significantly higher risk of knee revision than women, but no significant gender difference among hip revision. In Norway, men had a significantly higher risk of hip revision, but no differences in knee revisions. In Sweden, men had a significantly higher risk of hip revision (5.4% vs. 3.3%). Older patients had a reduced risk of revision as compared with younger patients, in both the US and in Norway. The survival of THA is significantly better in Norway than in the US with a hazard ratio of 1.64 (p< 0.05). On the other hand, TKA had a better survival experience in the US than in Norway, with a hazard ratio of 0.55 (p< 0.05). This is the first study to evaluate the utility of Medicare as a source of THA and TKA survivorship data and to compare prosthesis outcomes in Medicare with those from Scandinavian arthroplasty registries. Unlike the Norwegian and Swedish registry data, the reasons for revision (e.g., femoral loosening) are not captured and thus greatly limited the value of the Medicare data as a tool to understand the need for revision, thereby helping to improve implant longevity and to reduce the associated cost and burden to the patient and care provider.

There has been renewed interest in metal-on-metal bearings as hip resurfacing components for treatment in young, active patients. This study examines the effects of fixation (cemented or uncemented heads) and bone-implant interface conditions (stem-bone and head-bone) on the biomechanics of the Birmingham hip resurfacing (BHR) arthroplasty, using high resolution, 3-d computational models of the bilateral pelvis from a 45-year-old donor. Femoral bone stress and strain in the natural and BHR hips were compared. Bone remodelling stimuli were also determined for the BHR hips using changes in strain energy. Proximal femoral bone stress and strain were non-physiological when the BHR femoral component was fixed to bone. The reduction of strain energy within the femoral head was of sufficient magnitude to invoke early bone resorption. Less reduction of stress was demonstrated when the BHR femoral component was completely debonded from bone. Bone apposition around the distal stem was predicted based on the stress and strain transfer through the stem. Femoral stress or strain patterns were not affected by the type of fixation medium used (cemented vs. Uncemented). Analysis of proximal stress and strain shielding in the BHR arthroplasty provides a plausible mechanism for overall structural weakening due to loss of bony support. It is postulated that the proximal bone resorption and distal bone formation may progress to neck thinning as increasing stress and strain transfer occurs through the stem. This may be further exacerbated by additional proximal bone loss through avascular necrosis. Medium term retrieval specimens have shown bone remodelling that is consistent with our results. It is unclear if the clinical consequences of neck thinning will become more evident in longer-term follow-ups of the BHR.

One potential limitation with uncemented, hemispherical metal-backed acetabular components is stress shielding of bony structures due to the mismatch in elastic modulus between the metal backing and the peri-prosthetic bone. A proposed substitute is a horseshoe-shaped acetabular component, which replicates the bony anatomy. One such device, the Cambridge cup, has shown successful clinical and radiological outcomes at five years follow-up (Brooks 2004, Field 2005). We conducted a study of the Cambridge cup from a biomechanical perspective, using validated, high-resolution computational models of the bilateral hip. Peri-prosthetic stress and strain fields associated with the Cambridge cup were compared to those for the natural hip and a reconstructed hip with a conventional metal-backed hemispherical cup during peak gait loading. We found that the hemispherical cup caused an unphysiologic distribution of bone stresses in the superior roof and unphysiologic strain transfer around the acetabular fossa. These stress distributions are consistent with bone remodelling. In contrast, the peri-acetabular stresses and strains produced by the Cambridge cup differed from the natural hip but were more physiologic than the conventional hemispherical design. With the Cambridge cup, stresses in the superior acetabular roof, directly underneath the central bearing region, were greater than with the conventional design. Despite the thin bearing, the peak liner stresses in the Cambridge cup (max. tensile stress: 1.2 MPa; yield stress: 4.5 MPa) were much lower than the reported material strengths. Fossa loading by the hemispherical cup has been suggested as a possible mechanism for decreased implant stability (Widmer 2002). Conversely, the Cambridge cup produced semi-lunar peri-prosthetic stress fields, consistent with contact regions measured in natural hips (Widmer 2002). These analyses provide a better understanding of the biomechanics of the reconstructed acetabulum and suggest that a change in component geometry may promote long-term fixation in the pelvis.