Direct anterior approach (DAA) arthroplasty has generated great interest because of its minimally invasive and muscle sparing nature. Obese patients are reported to be associated with greater incidence of complications in primary joint replacement. The purpose of this study was to compare patient outcomes and complication rates between obese and non-obese patients undergoing primary total hip arthroplasty (THA) through a Bikini direct anterior incision.

This retrospective, single surgeon study compared the outcome of 258 obese patients and 200 non-obese patients undergoing DAA THA using a Bikini incision, over a 7-year period. The average follow-up was 4.2 years (range 2.6-7.6 years).

There were no statistically significant differences in the complication rate between the two groups. The obese group recorded 2 major (venous thromboembolism and peri-prosthetic fracture) and 2 minor complications (superficial wound infection), compared with the non-obese group, which recorded 2 major (deep-wound infection and peri-prosthetic fracture) and 1 minor complication (superficial wound infection). Patient-reported outcomes (WOMAC and Harris Hip Scores) showed significant post-operative improvements (p < 0.001) and did not differ between the two groups.

Bikini DDA THA does not increase the complication rate in obese patients and offers similar clinical improvements compared to non-obese patients.

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Introduction

Joint kinematics following total knee replacement (TKR) is important as it affects joint loading, joint functionality, implant wear and ultimately patient comfort and satisfaction. It is believed that restoring the natural motion of the joint (such as the screw-home mechanism) with a medial pivot knee implant will improve clinical outcomes. Daily activities such as stair climbing and stair descent are among the most difficult tasks for these patients. This study analysed dynamic knee joint motion after implantation of a medial pivot knee implant using fluoroscopy during stair ascent and descent activity.

Squeaking ceramics bearing surfaces have been recently recognised as a problem in total hip arthroplasty. The position of the acetabular cup has been alluded to as a potential cause of the squeaking, along with particular combinations of primary stems and acetabular cups. This study has used the finite element method to investigate the propensity of a new large diameter preassembled ceramic acetabular cup to squeaking due to malpositioning.

A verified three-dimensional FE model of a cadaveric human pelvis was developed which had been CT scanned, and the geometry reconstructed; this was to be used to determine the behaviour of large diameter acetabular cup system with a thin delta ceramic liner in the acetabulum. The model was generated using ABAQUS CAE pre-processing software. The bone model incorporated both the geometry and the materials properties of the bone throughout based on the CT scan. Finite element analysis and bone material assignment was performed using ABAQUS software and a FORTRAN user subroutine. The loading applied simulated edge loading for rising from a chair, heel-strike, toe off and stumbling.

All results of the analysis were used to determine if the liner separated from the shell and if the liner was toggling out of the shell. The results were also examined to see if there was a propensity for the liner to demobilise and vibrate causing a squeaking sound under the prescribed loading regime.

This study indicates that there is a reduction in contact area between the ceramic liner and titanium shell if a patient happens to trip or stumble. However, since the contact between the liner and the shell is not completely lost the propensity for it to squeak is highly unlikely.

The Birmingham Hip Mid Head Resection (BMHR) was designed to accommodate patients with lower quality bone in the proximal half of the femoral head. It is a relatively new conservative hip implant with promising early results. Finite element modelling may provide an insight into mid-term results.

A cadaveric femur was CT scanned and 3D geometry of the intact femur constructed. The correctly sized BMHR implants (with and without visual stop) were positioned and these verified by a surgeon; hence constructing the post-operative models. Walking loads were applied and contact surfaces defined.

Stress analyses were performed using the finite element method and contact examined. Also, a strain-adaptive bone remodelling analysis was run using 45% gait hip loading data. Virtual DEXA images were computed and were analysed in seven regions of the bone surrounding the implants.

The BMHR was found to be mechanically stable with all surfaces indicating micromotion less than the critical 150 microns. Stress distribution was similar to the intact femur, with the exception of the head-neck region where some stress/strain shielding occurs. This is mirrored in the bone remodelling results, which show some bone resorption in this region. The visual stop, which is designed to ensure that the stem is not overdriven during implantation, did not affect the stress/strain results; only on a very local scale.

There is minimal data available in the literature regarding conservative hip implants and no data regarding the BMHR. This study is the first to look at the mechanical response of the bone to this implant.

The advantages of unicompartmental knee arthroplasty (UKA) include its bone preserving nature, lower relative cost and superior functional results. Some temporary pain has been reported clinically following this procedure. Could this be related to bone remodeling? A validated bone remodeling algorithm may have the answers…

A 3D geometry of an intact human cadaveric tibia was generated using CT images. An all poly unicompartmental implant geometry was positioned in an inlay and onlay configuration on the tibia and the post-operative models created. An adaptive bone remodeling algorithm was used with finite element modeling to predict the bone remodeling behavior surrounding the implant in both scenarios. Virtual DEXA images were generated from the model and bone mineral density (BMD) was measured in regions of interest in the AP and ML planes. BMD results were compared to clinical results.

The bone remodelling algorithm predicted BMD growth in the proximal anterior regions of the tibia, with an inward tendency for both inlay and onlay models. Looking in the AP plane, a maximum of up to 7% BMD growth was predicted and in the ML plane this was as high as 16%. Minimal BMD loss was observed, which suggests minimal disturbance to the natural bone growth following UKA.

Positron emission tomography (PET) scans showed active hot spots in the antero- medial regions of the tibia. These results were consistent with the finite element modeling results.

Bone remodeling behavior was found to be sensitive to sizing and positioning of the implant.

The adaptive bone remodeling algorithm predicted minimal BMD loss and some BMD growth in the anterior region of the tibia following UKA. This is consistent with patient complaint and PET scans.

Introduction: Periprosthetic bone resorption following total knee arthroplasty (TKA) is becoming a clinical concern. Decrease in bone quality jeapordises implant fixation, consequently leading to revision surgery. It has been suggested that a reduction in the local stress distribution may cause a decrease in bone mineral density (BMD). Computational bone remodelling has been used previously to predict bone adaptation in total hips. However, little has been reported on its use in TKA remodelling simulations. The aim of this study was to simulate the bone remodelling response of the femur and tibia following TKA, using an adaptive bone remodelling algorithm combined with the finite element (FE) method.

Methods: 3D femur and tibia models were constructed from human cadaveric computed tomography images. Total knee implant geometries were used to reconstruct the knee joint.(RBK, Global Orthopaedic Technology, Australia). Both the femur and the tibia models were loaded at 45% gait cycle for normal walking gait using loads based on Taylor et al. A strain-adaptive remodelling algorithm was used to predict the remodelling behaviour of the femur and tibia following TKA. Analysis was performed using ABAQUS. Virtual DEXA images were generated from the FE models at predetermined time-points, BMD gain and loss were also assessed both quantitatively and qualitatively.

Results: There was an increase and decrease in BMD for the femur and tibia models. BMD loss in the femur was predominantly experienced around the pegs and the distal femoral regions. Femoral BMD gain was displayed around the edges of the bone-implant interface, with higher activity at the anterior-medial and posterior-lateral aspects. BMD gain in the tibia was predominantly at the inferior end of the tibial tray’s keel, with the bone mass tending towards the medial aspect. Some bone gain was displayed on the medial side, surrounding the pegs and at the cortex. There was BMD loss on the lateral aspect of the tibia.

Discussion: The adaptive bone remodelling algorithm has shown a good correlation with clinical findings. Reports of clinical and FE studies have shown that for cemented knees, most bone loss occurs at the distal femoral region, especially at the anterior aspect. It has been reported that in the tibia there is generally an over-all decrease in BMD in the proximal tibia and increase below the keel. This is in accordance with our predictions. BMD gain was found to be more predominant on the medial aspect. This may be due to the more medially inclined loading ratio, which affects the stress distribution within the bone. BMD gain in the tibia is shown to follow a path, which starts at the bottom of the keel and tends medially towards the tibial cortex. This illustrates the inherent tendency of load transfer to follow along the stiffest structural path.

Geometric and material changes in the femoral neck following hip resurfacing have been linked to femoral neck fractures.

This study developed a unique method to determine the level of influence of the implant stem on the structural changes in the femoral neck following surgery.

A 3D femur model was generated using CT-images. The finite-element model was meshed using 10-noded tetrahedral elements. An ASR hip-resurfacing component (Depuy International, Leeds) was implanted into the femur in load sensitive position. A strain-adaptive bone-remodelling algorithm was used to determine the bone-remodelling behaviour of the femur over a minimum of 2-year period.

Following the analysis, the material properties and stresses in the neck region were mapped onto a cubic mesh, which simulated a CT stack. Moments of inertia, bending moments and shear was calculated for each slice along the neck of femur. These were compared to the pre-operative model.

Bone mineral density changes in the neck region were observed following implantation due to the changes in moments of inertia, bending moments and shear loading.

A method to determine the effect of implantation on the geometric and densitychanges in the femoral neck following resurfacing was developed. This methodology has shown that implant stem geometry affects the load transfer to the femur and the adaptive behaviour of the femoral neck. This will influence the structural integrity of the femoral neck and the long-term clinical outcome of the hip resurfacing component.

A recurrent fracture rate after vertebroplasty and balloon kyphoplasty is as high as 20%. Biomechanically, it has not been proven that refracture rate is due to the cement stiffness alone. This finite-element study investigated effects of cement-stiffness, bone-quality, cement-volume and height-restoration in treatment of vertebral compression fractures using balloon kyphoplasty.

A finite-element model of the lumbar spine was generated from CT-scans. The model comprised of two functional spinal-units, consisting of L2-L4 vertebral bodies, intervertebral-discs, and spinal ligaments. Cement volumes modelled were in the order of 15% and 30% of total vertebral body (VB) volume. Spinal fracture was modelled as being reduced and height of VB was restored. Kyphoplasty was performed. Three different bone qualities were modelled: healthy, osteopenic, osteoporotic. A compressive load was applied to the proximal endplate of L2. An anterior shift of the centre-of-gravity of upper body was simulated by increasing the moment arm of the applied load.

All results of the analysis were compared back to an intact spinal model of the same region under the same loading regime. All parameters affected the mechanical behaviour of the spine model, although changing the bone quality from normal to osteoporotic resulted in the least change. The cement stiffness was initially modelled with an elastic modulus between 0.5GPa and 2GPa. The results showed small differences relative to intact case in the lower modulus cement. A much higher cement stiffness of 8GPa resulted in larger changes in the stresses. The most significant parameter in this study was found to be the changed load path as a result of partial height restoration. This induced a moment in the construct and increased the stresses and strains in the anterior compartments of each vertebra as well as marked in the adjacent (upper and lower) vertebrae. The factor of safety calculation showed the centre of the L3 vertebra to be the most failure prone in all cases, with the osteoporotic bone models showing higher fracture tendencies.

This study indicates that healthier bone has a better chance of survival. Cement properties with lower cement elastic moduli induce stresses/strains which are more similar to the intact model. The best way to reduce the likelihood of failure is to restore the vertebral height.

Negative ulnar variance, lunate shape and increased load transmission are associated with Kienbock’s disease. This may reflect trabecular alignment being more susceptible to shear forces along “fault planes” in Type 1 lunates, causing microfractures and avascular necrosis. The aim of this study was to assess the relationship between lunate bone structure, density and ulnar variance.

Standard 90/90 radiographs of 22 cadaveric wrists were taken for ulnar variance and lunate shape. The lunates were harvested and routine CT scans (1mm) were taken in 22/22 in the coronal, sagittal and transverse planes. DICOM files were analysed using Mimics (Materialise, Belgium) to measure Hounsfield units. MicroCT scans (SkyScan, Belgium) (40 μm) were taken in 10/22 in the coronal plane and measured for trabecular angle at the proximal and distal joint surfaces and the ‘tilting angle’ (between scaphoid and radius joint surfaces). Data was anlaysed using one-way ANOVA tests using SPSS for Windows.

Negative ulnar variance was noted in 7/22, neutral 10/22 and positive 5/22. Lunate shape according to Zapico was 0/22 Type 1, 18/22 Type 2 and 4/22 Type 3.

Lunate bone density was significantly lower in the ulnar positive specimens compared to ulnar negative and neutral (p< 0.001) (fig. 1).

The average trabecular angle measured 84.7° (+/− 4.5°) at the proximal and 90.3° (+/− 2.6°) at the distal joint surfaces and tilting angle was 115.7° (+/− 12.0°) (fig. 2). The 50% slice on the microCT correlated best with xray measurements of this angle.

This study quantifies the previous finding that load transmission through the lunate and hence lunate bone density is related to ulnar variance and that this is higher in ulnar negative wrists. MicroCT is a useful modality to assess trabecular structure and supports the ‘fault plane’ hypothesis of Kienbock’s Disease.