The biomechanical consequence of insufficient femoral component lateralization and exposed cancellous bone in hip resurfacing arthroplasty

Computer-assisted navigation in Birmingham hip resurfacing: A case report

Component malpositioning during Birmingham hip resurfacing increases the risk for component wear, metallosis, component loosening, and the likelihood of dislocation and revision surgery. Computer-assisted navigation can increase the accuracy to which components are placed, and the utilization of this technology in Birmingham hip resurfacing is increasing. The present report summarizes the accuracy of acetabular component positioning in a Birmingham hip resurfacing case utilizing navigation. Intraoperative C-arm fluoroscopy following the use of the navigation tool confirmed excellent seating, positioning, and stability of the acetabular component. In addition, post-operative antero-posterior radiographs confirmed device accuracy and revealed a stable joint with no evidence of acetabular loosening or femoral fracture. Computer-assisted navigation may therefore be an effective tool to improve the accuracy of component positioning during Birmingham hip resurfacing.

The impact of proximal femoral morphology on failure strength with a mid-head resection short-stem hip arthroplasty

Mid-head resection short-stem hip arthroplasty is a conservative alternative to conventional total hip replacement and addresses proximal fixation challenges in patients not suitable for hip resurfacing. It is unclear whether proximal femoral morphology impacts the ultimate failure load of mid-head resection implanted femurs, thus the aim of this study was to investigate the effect of native neck-shaft angle (NSA) and coronal implant alignment on proximal femoral strength. In total, 36 synthetic femurs with two different proximal femoral morphologies were utilized in this study. Of them, 18 femurs with a varus NSA of 120° and 18 femurs with a valgus NSA of 135° were each implanted with a mid-head resection prosthesis. Femurs within the two different femoral morphology groups were divided into three equal coronal implant alignment groups: 10° valgus, 10° varus or neutral alignment. Prepared femurs were tested for stiffness and to failure in axial compression. There was no significant difference in stiffness nor failure load between femurs implanted with valgus-, varus- or neutrally aligned implants in femurs with a NSA of 120° (p = 0.396, p = 0.111, respectively). Femurs implanted in valgus orientation were significantly stiffer and failed at significantly higher loads than those implanted in varus alignment in femurs with a NSA of 135° (p = 0.001, p = 0.007, respectively). A mid-head resection short-stem hip arthroplasty seems less sensitive to clinically relevant variations of coronal implant alignment and may be more forgiving upon implantation in some femoral morphologies, however, a relative valgus component alignment is recommended.

Does the extent of osteonecrosis affect the survival of hip resurfacing?

BACKGROUND

The effect of the extent of osteonecrosis on the survival of hip resurfacing for osteonecrosis of the femoral head (ONFH) has not been well documented, but is a potentially important variable in the decision to perform resurfacing.

QUESTIONS/PURPOSES

We examined (1) the relationship between the volume of osteonecrosis in the femoral head before surgery and the extent of the residual necrotic bone after femoral head machining, (2) how the extent of the residual necrotic bone relative to the resurfaced femoral head (after femoral head machining) affected the survival of total hip resurfacing for patients with ONFH, and (3) how the extent of the necrotic bone relative to the entire femoral head (before femoral head machining) affected the survival and clinical outcome scores of patients who underwent total hip resurfacing.

METHODS

Thirty-three patients (39 hips) who underwent hip resurfacing were reviewed after a mean followup of 8 years. The extent of osteonecrosis in the femoral head and residual osteonecrosis in the implant bony bed after femoral head machining were estimated using a three-dimensional MRI-based templating system.

RESULTS

There was a statistically significant difference in the extent of osteonecrosis before and after femoral head machining, although the two were well correlated (r = 0.97). The mean percentage of osteonecrosis in the implant bony bed after femoral head machining was 5% smaller than that relative to the entire femoral head (range, -9% to 15%). There were no significant differences in implant survival between groups with small and large osteonecrosis classified by either the total amount of osteonecrosis before surgery or residual osteonecrosis after femoral head machining.

CONCLUSION

The extent of osteonecrosis in the femoral head significantly decreased after femoral head machining. Neither the residual osteonecrosis volume in the implant bony bed after femoral head machining nor the total amount of osteonecrosis before femoral head machining had significant influence on the survival of hip resurfacing.

The biomechanical effect of notch size, notch location, and femur orientation on hip resurfacing failure

For hip resurfacing, this is the first biomechanical study to assess anterior and posterior femoral neck notching and femur flexion and extension. Forty-seven artificial femurs were implanted with the Birmingham hip resurfacing (BHR) using a range of notch sizes (0, 2, and 5 mm), notch locations (superior, anterior, and posterior), and femur orientations (neutral stance, flexion, and extension). Implant preparation was done using imageless computer navigation, and mechanical tests measured stiffness and strength. For notch size and location, in neutral stance the unnotched group had 1.9 times greater strength than the 5-mm superior notch group (4539 N versus 2423 N, p=0.047), and the 5-mm anterior notch group had 1.6 times greater strength than the 5-mm superior notch group, yielding a borderline statistical difference (3988 N versus 2423 N, p = 0.056). For femur orientation, in the presence of a 5-mm anterior notch, femurs in neutral stance had 2.2 times greater stiffness than femurs in 25° flexion (1542 N/mm versus 696 N/mm, p = 0.000). Similarly, in the presence of a 5-mm posterior notch, femurs in neutral stance had 2.8 times greater stiffness than femurs in 25° extension (1637 N/mm versus 575 N/mm, p = 0.000). No other statistical differences were noted. All femurs failed through the neck. The results have implications for BHR surgical techniques and recommended patient activities.

Computer navigation experience in hip resurfacing improves femoral component alignment using a conventional jig

BACKGROUND

The use of computer navigation has been shown to improve the accuracy of femoral component placement compared to conventional instrumentation in hip resurfacing. Whether exposure to computer navigation improves accuracy when the procedure is subsequently performed with conventional instrumentation without navigation has not been explored. We examined whether femoral component alignment utilizing a conventional jig improves following experience with the use of imageless computer navigation for hip resurfacing.

MATERIALS AND METHODS

Between December 2004 and December 2008, 213 consecutive hip resurfacings were performed by a single surgeon. The first 17 (Cohort 1) and the last 9 (Cohort 2) hip resurfacings were performed using a conventional guidewire alignment jig. In 187 cases, the femoral component was implanted using the imageless computer navigation. Cohorts 1 and 2 were compared for femoral component alignment accuracy.

RESULTS

All components in Cohort 2 achieved the position determined by the preoperative plan. The mean deviation of the stem-shaft angle (SSA) from the preoperatively planned target position was 2.2° in Cohort 2 and 5.6° in Cohort 1 (P = 0.01). Four implants in Cohort 1 were positioned at least 10° varus compared to the target SSA position and another four were retroverted.

CONCLUSIONS

Femoral component placement utilizing conventional instrumentation may be more accurate following experience using imageless computer navigation.

Predictors of femoral neck fracture following hip resurfacing: a cadaveric study

We aimed to establish if radiological parameters, dual energy x-ray absorptiometry (DEXA) and quantitative CT (qCT) could predict the risk of sustaining a femoral neck fracture following hip resurfacing. Twenty-one unilateral fresh frozen femurs were used. Each femur had a plain digital anteroposterior radiograph, DEXA scan and qCT scan. Femurs were then prepared for a Birmingham Hip Resurfacing femoral component and loaded to failure. Results demonstrated that gender and qCT measurements showed strong correlation with failure load. QCT could be used as an individual measure to predict risk of post-operative femoral neck fracture. However, when qCT is unavailable; gender, pre-operative DEXA scan and Neck Width measurements can be used together to assess risk of post-operative femoral neck fracture in patients due to undergo hip resurfacing.

A biomechanical comparison of epiphyseal versus metaphyseal fixed bone-conserving hip arthroplasty

BACKGROUND

The Birmingham Mid-Head Resection (BMHR) is a bone-conserving, short-stem alternative to hip resurfacing for patients with abnormal femoral head anatomy.

METHODS

The current study examines whether a bone-preserving femoral component that is fixed into the femoral neck metaphysis provides a mechanical advantage in terms of resisting femoral neck fracture in comparison with a conventional hip resurfacing implant in a human cadaveric femoral model.

RESULTS

Femora with a BMHR femoral component failed at an average of 23% less load than those prepared with a conventional hip resurfacing component (mean and standard deviation, 5434 ± 2297 compared with 7012 ± 2619 N; p < 0.001).

CONCLUSIONS

An uncemented, metaphyseal fixed, bone-conserving femoral implant does not provide superior mechanical strength or increased resistance to femoral neck fracture in comparison with a conventional hip resurfacing arthroplasty.