Malnutrition is considered a risk factor for postoperative complications in total hip and knee arthroplasty, though prospective studies investigating this assumption are lacking. The aim of this study was to prospectively analyse the 90-day postoperative complications, postoperative length of stay (LOS) and readmission rates of patients undergoing primary total hip and total kneearthroplasty using albumin, total lymphocyte count (TLC) and transferrin as serum markers of potential malnutrition.

603 primary hip and 823 primary knee arthroplasties over a 3-year period from a single centre wereprospectively analysed. BMI, demographic and comorbidity data were recorded. Complications werecategorised as surgical site infection, venous thromboembolism (deep vein thrombosis andpulmonary embolus), implant related (such as dislocation), and non-implant related (such aspneumonia). Outcomes were compared between groups, with malnutrition defined as serumalbumin <3.5g/dL, transferrin <200 mg/dL, or TLC <1,500 cells/mm³.

Potential malnutrition was present in 9.3% of the study population. This group experienced a longeraverage LOS at 6.5 days compared to the normal albumin group at 5.0 days (p=0.003). Surgical siteinfection rate was higher in the malnourished group (12.5 vs 7.8%, p=0.02). There was no differencebetween the two groups in implant related complications (0.8 vs 1.0%, p=0.95) medicalcomplications (7.8 vs 13.3%, p=0.17), rate of venous thromboembolism (2.3 vs 2.7%) or 90-dayreadmission rate (14.1 vs 17.0%, p=0.56). TLC and transferrin were not predictive of any of theprimary outcomes measured (p<0.05). Pacific Island (p<0.001), Indian (p=0.02) and Asian (p=0.02) patients had lower albumin than NZ European.

This study demonstrates an association between low albumin levels and increased postoperativeLOS and surgical site infection in total joint arthroplasty, providing rationale for consideration ofpreoperative nutritional screening and optimisation.

The objective of this study was to determine whether the bearing surface is a risk factor for revision after late dislocation in total hip arthroplasty (THA).

Data from primary THAs were extracted from the New Zealand Joint Registry over a 13-year period. The mean age of patients was 68.9 years; 53.2% were female. The surgical approach used was posterior in 66% of THAs, lateral in 29% and anterior in 5%. There were 53,331 (65.1%) metal-on-polyethylene THAs, 14,093 (17.2%) ceramic-on-polyethylene, 8,177 (10.0%) ceramic-on-ceramic, 461 (0.5%) ceramic-on-metal, 5910, and (7.2%) metal-on-metal.

The primary endpoint was late revision for dislocation, with ‘late’ defined as greater than one year post-operatively. 73,386 hips were available for analysis. The overall revision rate was 4.3% (3,130 THAs), 1.1% (836) were revised for dislocation. Only 0.65% (470) hips were revised for dislocation after the first post-operative year.

The unadjusted hazard ratios (HR) showed significantly higher rates of revision for dislocation in ceramic-on-polyethylene (HR 2.48; p=0.001) and metal-on-polyethylene (HR 2.00; 95% p =0.007) compared to ceramic-on-ceramic. However, when adjusted for head size, age and surgical approach, only ceramic-on-polyethylene (HR 2.10; p=0.021) maintained a significantly higher rate of revision, whereas metal-on-polyethylene approached significance (HR 1.76; 95% p = 0.075).

In New Zealand, dislocation is the most common reason for revision, ahead of aseptic loosening of the acetabular component. The relationships between bearing materials and risk of revision for late dislocation is controversial. However, in this study ceramic-on-ceramic shows lower risk rates for revision than other bearing surface combinations. Low wear and less debris, limited peri-articular inflammatory reaction and an healthy fibrotic pseudo-capsule are potential factors determining long-term stability of the hip joint.

Ceramic-on-ceramic bearings are considered in several European and Asian countries a reliable alternative to metal-on-polyethylene, ceramic-on-polyethylene, or metal-on-metal (with small diameter heads) for total hip arthroplasty (THA) management. Reduced joint wear and limited peri-prosthetic osteolytic changes are the main reasons supporting the use of ceramic. So far, the available observational data show a low rate of revision following the use of Ceramic-on-Ceramic bearings, but concern remains regarding the risk of fracture and the prevalence of squeaking noises from the joint.

The objective of this study was to use a national arthroplasty registry to assess whether the choice of bearings – metal-on-polyethylene (MoP), ceramic-on-polyethylene (CoP), ceramic-on-ceramic (CoC), or metal-on-metal (MoM) – is associated with differences in the risk of revision.

Data from primary THAs were extracted from the New Zealand Joint Registry over a 15-year period. 97,889 hips were available for analysis. The mean age of patients was 68 years (SD +/− 11 years), and 52% were women. The median followup period in this patient population was 9 years (range, 1 to 15 years).

The primary endpoint was revision for any reason. Inclusion criteria were degenerative joint disease (84,894), exclusion criteria were previous surgery, trauma, and any other diagnosis (12,566). We also excluded patients operated on with a Ceramic-on-Metal THA, because of the small recorded number (429).

There were 54,409 (64.1%) MoP, 16,503 (19.4%) CoP, 9,051 (10.7%) CoC and 4,931 (5.8%) MoM hip arthroplasties. 3,555 hips were revised during the 15-year observation period. A multivariate assessment was carried out including the following risks factors available for analysis: age, gender, surgeon experience, use of cement.

Analysis of bearing surface type and revision showed a statistically significant lower risk for CoC hips (265 THAs, p≤0.01) when compared with CoP (537 THAs, HR 1.07, CI 0,92–1,26), MoP (2186 THAs, HR 1.39, CI 1.19–1,62), and MoM (576 THAs, HR 2.15, CI 1.84–2.51). The 15-year follow-up Kaplan-Meier survival analysis shows a 92% revision-free rate for CoC THAs (Figure 1). In particular, CoC THAs showed the lowest rates of revision for dislocation and for deep infection, when compared with the other bearings.

This registry study showed that the bearing surface is associated with the risk of revision. MoM bearing surfaces showed a high rate of revisions, while CoC THAs showed the lowest rate of revision compared to other bearing surfaces. Low wear and less osteolysis are the possible reasons for reduced risk of revision for aseptic loosening. We postulate that the healthy, fibrotic synovial-like pseudocapsule found in CoC THAs preserves the long-term stability of the joint and reduces the bio-burden for late deep infection. Future studies with larger data sets and longer follow-up should continue to investigate this query.

For any figures or tables, please contact the authors directly.

Malnutrition has been identified as an independent risk factor for surgical site infection (SSI). Guidelines advise that malnourished patients should be counselled and strategies implemented to improve nutritional intake prior to surgery. However, the current total hip replacement (THR) literature lacks of prospective clinical trials.

The aim of this study was to investigate if the probability of acute SSI following THR is related to whether or not the patient had pre-operative low transferrin levels, low albumin levels and/or low total lymphocyte cell count (TLC).

All patients receiving THR in a single Institution during a 2-year period were assessed prospectively for presence of malnutrition and SSI within 3 months after unilateral THR. Malnutrition was defined as albumin <35 g/L, transferrin <2g/L or TLC < 1.5 E⁹/L. Patients with previous infection(s) were excluded. Multivariate analysis included gender, age, BMI, diabetes, and immunocompromised status.

478 patients with a complete dataset were included in the study; 137 (27.6%) had at least one low value of transferrin, TLC or albumin. SSI was recorded in 22 hips (4.6%) including 6 deep infections. Hypoalbuminaemia (RR, 1.28, p=0.5), low transferrin (RR, 1.39, p=0.4) or low TLC (RR, 1, p=0.1) were not associated with SSI. Multivariate analysis showed no association between abnormal blood tests suggesting malnutrition, BMI and SSI. Only BMI had a low p value for association with SSI (p=0.006).

This study shows that abnormal blood tests associated with malnutrition are not an independent risk factor for SSI in THR.

Six week old male Sprague-Dawley rats were administered intravenous clozapine, quetiapine, haloperidol or vehicle once daily for a period of 42 days with access to only high fat diet and their weight was monitored regularly. At the end of the study the rats were killed and the tibiae excised and bone mineral density (BMD) measured with dual X-ray absorptiometry and bone architecture assessed with micro-computed tomography (micro-CT) and associated software. Results were subjected to one-way ANOVA and post hoc Dunnetts multiple comparison test.

All treatment groups were compared to control. There were no significant differences in body weight between the different groups at completion of the study. Clozapine treated animals alone showed a significant reduction in bone mineral density (p<0.05) however no differences were seen with haloperidol and quetiapine. Both haloperidol and quetiapine, but not clozapine, treatment showed a significant reduction in the bone to tissue volume ratio (BV/TV) by approximately 23% (p<0.05) and an increase in trabecular number (TbN) by approximately 21% (p<0.05). Trabecular bone architecture parameters for haloperidol and quetiapine, but not clozapine, showed more rod like and disconnected structure as reflected in the increases in structure model index (SMI) of around 15% (p<0.05) and trabecular pattern factor (TbPf) by 22% (p<0.05).

This data demonstrates that in rats receiving a high fat diet, haloperidol and quetiapine have an adverse effect on bone micro-architecture without significant change in whole body bone mineral density.

Clozapine did not affect bony architecture in a significant manner as reported in our earlier study, though bone mineral density was reduced. Reasons for the different effect of clozapine in this study are still uncertain but may be related to the significant weight loss seen at the end point of the previous study. Causes for osteoporosis and increased fracture risk in schizophrenia may include smoking history, malnutrition, limited sun exposure and compliance.

Long term administration of both typical and atypical anti-psychotics may have a negative effect on bone and is a further factor that can influence this risk. An awareness of this relationship is useful in the orthopaedic management of schizophrenic patients.

Aseptic loosening is the leading cause for revision in total hip arthroplasty. Retro-acetabular lysis is often a silent process until severe bone loss causes catastrophic failure. This presents a technically difficult problem for the surgeon and a poorer result for the patient compared to primary arthroplasty. While the major cause of osteolysis is reaction to polyethylene particles, there is little data on the initiation and progression of such lesions. Further, alterations in the mechanical environment caused by such pathology is unclear. We present our use of 3D, finite element (FE) models of retro-acetabular pathology to investigate the biomechanical effects of osteolysis in total hip arthroplasty. Axial CT scan slices from a patient with cystic osteolysis were selected. Areas of cortical bone, cancellous bone, the cup and the cyst are accurately identified. The axial images are matched to a predetermined grid and used to build a complex finte element model. In this way complex anatomy can be built into the FE model and used to map cystic lesions. Force is then applied to the acetabulum.

Initial analysis shows similar stress transmission in cystic disease compared to the post operative pelvis. Pelvic bone still behaves as a sandwich construct with transmission from the acetabulum to the SI joints, pubic symphysis and medial wall. In the setting of pelvic medial wall deficiency, stress transmission is altered with areas of low stress around the defect.

The FE models containing pathology can be compared to models with generic bone density values immediately after total hip arthroplasty. The presence of a cyst in cancellous bone with intact cortical bone, demonstrates strain patterns similar to the post operative pelvis. Once cortical bone loss occurs strain patterns begin to change. This may mark a critical point in osteolytic progression. We present a developing new tool to be used in the assessment of a patient population with retroacetabular cystic disease.

Correct positioning of the femoral component in resurfacing hip arthroplasty (RHA) is an important factor in successful long-term patient outcomes. Computer-assisted navigation (CAS) shows potential to improve implant positioning and possibly prolong survivorship in total hip and knee arthroplasty. The purposes of CAS systems in resurfacing the femoral head are to insert the femoral head and neck guide wire with greater accuracy and to help in sizing the femoral component, thus reducing the risk of notching of the head and neck junction. Several recent studies reported satisfactory precision and accuracy of CAS in RHA. However, there is little evidence that computer navigation is useful in the presence of anatomical deformities of the proximal femur, which is frequently observed in young patients with secondary degenerative joint disease.

The purpose of this in-vitro study was to determine the accuracy of an image-free resurfacing hip arthroplasty navigation system in the presence of two femoral deformities: pistol grip deformity of the head and femoral neck junction and slipped upper femoral epiphysis deformity. An artificial phantom leg with a simulated hip and knee joint was constructed from machined aluminum. Implant-shaft angles for the guide wire of the femoral component reamer were calculated, in frontal and lateral planes, with both a computer navigation system and an electronic caliper combined with micro-CT.

With normal anatomy we found close agreement between the CAS system and our measurement system. However, there was a consistent disagreement in both the frontal and lateral planes for the pistol grip deformity. Close agreement was found only on the frontal plane angle calculation in the presence of the slipped upper femoral epiphysis deformity, but calculation of the femoral head size was inaccurate.

This is the first study designed to assess the accuracy of a femoral navigation system for resurfacing hip arthroplasty in the presence of severe anatomical deformity of the proximal femur. Our data suggests CAS technology should not be used to expand the range of utilisation of resurfacing surgery, but rather to improve the surgical outcome in those with suitable anatomy.

Mechanical prophylaxis with foot-pumps provides an interesting alternative to chemical agents in the prevention of thomboembolic disease following major orthopaedic surgery procedures. Recent studies have suggested that simultaneous use of graduated compression stockings (GCS) may hinder the pneumatic compression effect of foot pumps. The hypothesis of this prospective study was that the use of foot-pumps without GCS does not affect the efficacy of DVT prophylaxis, and improves patient compliance. 846 consecutive patients admitted at a single institution undergoing total hip (THR) or knee replacement (TKR) were included in the study. The A-V Impulse System^® foot-pump units (Orthofix Vascular Novamedix, Andover, UK) was used in all patients. Forty-six patients discontinued the use of foot-pumps, leaving 400 patients who received foot-pumps in combination with GCS and 400 patients with foot pumps alone. Eleven patients of the stocking group (2.7%) and 9 patients of the no-stockings group (2.3%) developed postoperative symptomatic DVT (p=0.07). DVT was more frequent in TKR (10 of 364; 2.7%) than in THR (10 of 436; 2.3%). Non-fatal pulmonary embolism occurred in 4 patients out of 20 with symptomatic DVT, 2 each of the stockings and no stockings groups. The foot pump discontinuation rate of patients treated with stockings was 7%, versus 4% of patients treated without stockings (p< 0.05). In conclusion, management of patients with foot-pumps without GCS does not reduce efficacy of DVT prophylaxis after THR and TKR, and improves patient compliance.

Patient- specific orthopaedic models are currently used in computer navigation. They provide realistic 3-D geometries for assessment of device placement (e.g. tibial trays, hip implants). Models are generated at time of operation by the surgeon. But patient-specific models have other uses. We envisage a future in which realistic 3-D patient models are routinely used for predicting the outcome of surgical procedures and new devices and for general patient health monitoring.

We are currently developing accurate 3-D models directly from CT scan post-operation. They are being used in investigations of the progress of bone remodeling. Such work can provide valuable feedback on the outcome of new procedures and how bone remodels under load. Such models would eventually include other tissue such as muscles and skin.

But there are a number of research and development challenges associated with the creation of patient-specific models. They include

minimal use of radiation for data collection;

We are working to meet these challenges. They include the use of generic data to supplement patient data, efficient ways of morphing models to fit the patient, and multi-scale modeling strategies. Work in progress at the Auckland Bio-engineering Institute will be presented in this talk.

The management of peri-prosthetic fracture following total hip arthroplasty is difficult, requiring expertise in both trauma and revision surgery. With rising numbers of patients in the population living with hip prostheses in situ, their frequency is increasing, and controversy remains over their ideal management. The objective of this study was to review all peri-prosthetic fractures at a single institution to identify injury and treatment patterns, and their associated clinical outcomes.

Fifty-four peri-prosthetic fractures in 50 patients were reviewed to determine the relative frequency of fracture types, complication rates, and clinical outcomes. Patient data were obtained through review of the clinical notes and individual patient follow up. Clinical outcomes were evaluated using the Oxford Hip Score (OHS) and Harris Hip Score (HHS).

The fractures were classified using the Vancouver system, the majority of which were type B 1 (20) or type B2 (10). The mean time to union for all fracture types was 4.6 months. A high non-union rate was seen amongst fractures fixed operatively. Fifteen percent of fractures went on to develop loosening following treatment, suggesting under-recognition at the time of injury. The average HHS was 73.1 and OHS 30.3 for all fracture types, at a mean follow up of 3.3 years. Of the 15 patients treated with revision surgery, the most common complication was dislocation (27%).

Treatment of patients with peri-prosthetic fractures requires recognition of the challenging nature of these injuries, their associated poor prognosis, and high complication rate.

Implant malposition remains one of the common causes of total knee replacement (TKR) failure and increased wear. Recent advances in computer technology have made available navigation systems for TKR and other orthopaedic procedures. The purpose of our study was:

to develop a method to assess the accuracy of an image-free TKR navigation system;

The system chosen was an image-free system based on electromagnetic technology, the MedTronic AxiEM TKR navigation system. To facilitate measurements, an artificial leg (phantom) was constructed from machined Plexiglas with simulated hip and knee joints. Additional joints located at the midshaft of the tibia and femur allowed deformation in the flexion/extension (y), varus/valgus (x) and rotational (z) planes. Using a highly accurate digital calliper unit (FaroARM Technologies, USA) to precisely measure co-ordinates with pre-machined points on the phantom, a software program was developed to convert these local co-ordinates into a determination of actual leg alignment. This technique was verified using repeated measurement with variable coordinates, giving accuracy to within 0.05 of a degree.

Simulated procedures were then performed with both normal and abnormal leg mechanical axis. At specific points in the procedure, information was compared between the FaroARM digital measurements and the CAS system. Repeated serial measurements were undertaken. In the setting of normal alignment, accuracy to within one degree was demonstrated. In the setting of abnormal x, y and z plane alignment in both femur and tibia, accuracy to within two degrees was demonstrated.

Several clinical studies have been performed to assess the precision of computer navigation in TKR. This study was designed to assess the accuracy of a clinically validated navigation system. The study demonstrates the high level of in-vitro accuracy of the MedTronic AxiEM navigation system in both normal and abnormal mechanical leg alignment settings.

A number of densitometry studies have reported dramatic density losses in the acetabular region after uncemented Total Hip Arthroplasty (THA)1,2. However the mechanical implication of such loss is not yet known. This study aims to perform a mechanical analysis with patient specific Finite Element (FE) models to find out how the stress distribution affects the Bone Mineral Density (BMD) changes after uncemented THA.

An existing patient CT dataset collected for a densitometry study was used to generate patient-specific FE models with a previously validated FE mesh generation method3. Boundary and loading conditions included the hip joint force and the forces of 21 muscles attached to the pelvic bone at eight characteristic phases of a gait cycle 4. Tensile and compressive components of principal stresses were calculated after each simulation.

In general, both compressive and tensile principal stresses decreased after uncemented THA but the magnitude of decrease for tensile stresses was much greater than compressive stresses. The changes in tensile stresses were matched with BMD loss patterns. In particular, the densitometry study revealed that areas dorsal to the prosthesis lost more bone density than areas ventral to the prosthesis1. The stress distribution pattern showed that such areas experienced high tensile stress initially and then a dramatic decrease in their magnitude while their compressive stresses remained relatively unchanged. On the other hand, the regions where BMD was maintained - the areas superior to the cup - experienced high compressive stresses initially, which remained relatively high three years after the surgery.

Although it is a result from one patient, results suggest that changes to tensile and compressive stresses might influence BMD differently after uncemented THA. Our hypothesis is that regions with high tensile stress experience bone loss while BMD of the regions with high compressive stress are maintained. More patient datasets are being processed to test this hypothesis. Findings from this study can explain the phenomena of retroacetabular osteolysis, late migration and implant failure of press-fit cups observed in long-term clinical studies.

Implant malposition is one of the most common causes of failure in resurfacing arthroplasty of the hip (RAH). Recent advances in computer technology have made available navigation systems for RAH and other orthopaedic procedures. The purpose of our study was:

to develop a method to assess the accuracy of an image-free RAH navigation system;

We used the Ci-CAS RAH navigation system (DePuy - BrainLab). To facilitate measurements, an artificial leg (phantom) was constructed from machined aluminium with simulated hip and knee joints. The hip and knee articulating surfaces were synthetic bone material (Sawbones – Pacific Laboratories). An additional joint located at the trochanteric region allowed deformation in varus/valgus and ante/retroversion of the head/neck segment. Using a highly accurate digital calliper unit (FaroARM Technologies, USA) to precisely measure co-ordinates with pre-machined points on the phantom, a software program was developed to convert these local co-ordinates into a determination of actual anatomy and leg alignment. This technique was verified using repeated measurement with variable co-ordinates, giving accuracy to within 0.05 of a degree.

Simulated procedures were performed with both normal and abnormal anatomy of the proximal femur. At specific points in the procedure, information was compared between the FaroARM digital measurements and the Ci-CAS system. Repeated serial measurements were undertaken. In the setting of normal alignment, accuracy to within 0.5 degrees was demonstrated. In the setting of abnormal alignment (varus/valgus and ante/retroversion) of the proximal femur, accuracy to within 2 degrees was demonstrated.

To our knowledge, this is the first study to assess accuracy of a RAH navigation system. The study demonstrates a satisfactory level of accuracy for the Ci-CAS in both normal and abnormal anatomical settings. Currently, no international standard or methodology exists against which these results can be compared. In the near future, introduction of new navigation technologies will make crucial the development of international standards for pre-clinical validation of computer-assisted navigation systems. The present study is a first attempt to address this issue.

The purpose of the current study was to compare the results of the P.F.C. Rotating Platform Knee System and the P.F.C. Knee System in a randomised prospective study.

151 patients were enrolled in the study, totalling 172 knees, with 84 receiving fixed bearing and 88 receiving rotating platform knees. Patients were enrolled prospectively with preoperative assessment of the defined outcome measures including Knee society score, Oxford Knee Score, SF-36 score, WOMAC score, VAS pain score and ROM. These outcome measures were repeated at 6 months, 1 year and 2 years

At the two year follow up patients were assessed and results analyzed to see if there were any significant differences with regard to outcome measures. There was no statistically significant difference for the SF12, Oxford knee score or the VAS pain score. In the Knee Society Score a trend effect at the one year follow up (almost significant at p = 0.07) was seen with the rotating group scoring better in overall function than the fixed group. At the two year follow-up this difference between groups became significant (p = 0.05) and the rotating group had significantly better overall function than the fixed group.

The early results show a better functional score with comparable subjective outcome measures with regard to pain and ROM. The long term purported benefits of the rotating platform versus fixed will be watched with interest in the medium to long term follow up.

Proximal tibial bone density (BD) is a key determinant in the migration of implants following total knee arthroplasty (TKA). CT-osteodensitometry offers three-dimensional, volumetric analysis of both cortical and cancellous regions and has been shown to be both accurate and reliable about the hip.

The primary objective of this study is to assess differences in the bone remodelling pattern (tibial segment) of patients operated on using fixed or rotating platform, using CT-osteodensitometry. The hypothesis is that the presence of a rotating platform optimises stress distribution along the proximal tibia, resulting in reduced proximal bone remodelling of tibia. The secondary objective is to correlate the osteodensitometry and clinical data at follow-up.

Following completion of the initial assessment and after obtaining written informed consent, the patients were randomly allocated to either a PFC Sigma fixed bearing or a PFC Sigma rotating platform (mobile bearing) total knee system (DePuy, Leeds, UK). Clinical follow-ups were performed at three months, one year, and two years after the index operation. CT scans were performed immediately post-operatively and at one and two years respectively. 52 knees (47 patients) were randomised for cemented TKR management using rotating or fixed tibia platform and followed up at one and two years.

At the two year follow-up, the knee function and patellar scores were comparable in both groups. There were no radiographic signs of loosening, and no revision surgery was performed. Sequential BD measurement from post-operative baseline to two year follow-up showed minimal differences between the fixed and rotating platform implants. Bone density assessment showed a 20% mean loss of cancellous BD and minimal changes of cortical BD, with little differences between the two groups. Loss of cancellous BD appears progressive.

In conclusion, this study shows that the mode of fixation of the TKR polyethylene liner does not influence bone remodelling of the proximal tibia after surgery. This is the first study performed to assess separately cortical and cancellous BD changes in THR. There are no data in the literature for comparison.

Long term clinical follow-up of total hip arthroplasty (THA) has identified problems associated with cyst formation. Such cysts are formed as a result of expansile osteolysis, which starts on a small area of the skeleton and spreads into the bone away from the surface of the prosthesis. Since large areas of the prosthesis are still in immediate contact with the skeleton the prosthesis is not loose and the patients are usually without pain. However this form of osteolysis may destroy large areas of the skeleton before it is detected and result in a sudden fracture due to a weakened skeleton. While there are some short term prospective trials that have shown changes in bone density in the periacetabular region, one needs a biomechanical model to understand factors that influence bone remodeling leading to cyst formation. This study aims to develop a mathematical model for studying the mechanical effects of bone cysts in the acetabulum of THA patients.

2D finite element (FE) models of patients with known restroacetabular cystic disease were generated using coronal CT images from the central region of the acetabulum. The boundary between bone and soft tissue was segmented and an FE model generated. Mesh convergence tests were performed to identify a suitable level of mesh refinement. Three material zones representing– cortical bone (E=17GPa), cancellous bone (E=1GPa) and a titanium cup (E=120GPa) – were included in the model. A series of simulations were run to investigate how cysts affect stress distribution as well as the mechanical consequence of medial wall deficiency.

The presence of a cyst did not alter the pattern of stress distribution in the lateral and medial wall. But the strain energy function increased significantly at the inferior margin of the cyst within its cancellous bone. This may encourage bone formation at the cyst margin and help to explain the sclerotic walls seen in some cysts. Models with absent medial walls showed that both compressive and tensile stresses lowered in the cortical wall and the strain energy function reduced almost to zero. This suggests that a medial wall defect has a high risk of progression.

The current 2D model cannot incorporate complex acetabular geometry or complex forces acting on the hip. Therefore the current model will be further developed into a 3D FE model of the whole pelvis that also represents the pelvic ring structure more adequately. Physiologically meaningful boundary conditions as well as patient specific geometry and material properties will be used to investigate mechanical effects of bone cysts realistically.

Introduction: This study was initiated to evaluate cortical and cancellous bone density (BD) changes of the acetabulum after cemented and uncemented total hip arthroplasty (THA) using computer tomography (CT)-assisted osteodensitometry in-vivo.

Materials and Methods: 15 cemented ZCA Pfannen cups (Zimmer, USA) (age 78 years) and 21 press-fit Trilogy cups (Zimmer, USA) (age 72 years) were implanted by one surgeon. All hips were investigated by a standardized CT-mode (slice thickness 2 mm, table feed 5 mm, extended CT-scale). 6 CT-scans at the level of the cup and 4 scans above the dome of the cup were analyzed 2 weeks and 2 years after surgery. Cancellous and cortical bone mineral density (BMD) (CaHA mg/ml) were elaborated with a special software tool (CAPPA postOP, CAS Innovations AG, Erlangen).

Results: 2 years after index operation cemented cups showed mild cancellous BD loss (−8% to −20%) and no significant cortical BD changes cranial, significant cortical (−4% to −20%) and cancellous (−16% to −44%) BD loss ventral and no significant BD changes dorsal to the cup.

For press-fit cups we observed highly significant (p< 0,01) cancellous BD loss in all sectors (−17% to −53%), cortical BD loss ventral and dorsal to the cup (−12% to −23%) and very limited BD loss cranial (−4% to −13%) to the cup.

Conclusions CT-assisted osteodensitometry allows a thorough assessment of the actabular bone in-vivo. Different patterns of stress shielding were observed for cemented and press-fit cups. For the press-fit cup high BD loss for both cortical and cancellous bone was observed in all areas adjacent to the pelvic implant, except for cortical BD at the acetabular dome, suggesting fixation of the cup in the cranial cortical bone. Comparetively less BD loss was seen for cemented cups in all sectors, especially for cancellous bone, suggesting a more physiological stress transfer to both cortical and cancellous pelvic bone.

Recently, the International Consensus Statement Group reported that mechanical prophylaxis with foot-pumps should be considered “Grade A” recommendation for prevention of deep-vein thrombosis after total hip replacement. Mechanical prophylaxis with foot-pumps provides an interesting alternative to chemical agents in the prevention of thomboembolic disease following major orthopaedic surgery procedures. Recent duplex ultrasound studies have suggested that simultaneous use of graduated compression stockings (GCS) may hinder the pneumatic compression effect of foot pumps. The hypothesis of this prospective study was that the use of foot-pumps without GCS does not affect the efficacy of DVT prophylaxis, and improves patient compliance.

846 consecutive patients admitted at a single institution undergoing total hip (THR) or knee replacement (TKR) were included in the study. The A-V Impulse System® foot-pump units (Orthofix Vascular Novame-dix, Andover, UK) was used in all patients. Forty-six patients discontinued the use of foot-pumps, leaving 400 patients who received foot-pumps in combination with GCS and 400 patients with foot pumps alone.

Eleven patients of the stocking group (2.7%) and 9 patients of the no-stockings group (2.3%) developed postoperative symptomatic DVT (p=0.07). DVT was more frequent in TKR (10 of 364; 2.7%) than in THR (10 of 436; 2.3%). Non-fatal pulmonary embolism occurred in 4 patients out of 20 with symptomatic DVT, 2 each of the stockings and no stockings groups. The foot pump discontinuation rate of patients treated with stockings was 7%, versus 4% of patients treated without stockings (p< 0.05).

In conclusion, management of patients with foot-pumps without GCS does not reduce efficacy of DVT prophylaxis after THR and TKR, and improves patient compliance.

The aim of this prospective study was to assess safety and effectiveness of Spacer-K, a pre-formed articulating spacer made of gentamicin-impregnated acrylic bone cement. used in two-stage exchange technique for the infected total knee arthroplasty.

32 consecutive patients infected total knee arthroplasty were operated on in two centres. Infection was caused by Coagulase neg. Staphylococcus (21), Staphylococcus aureus (4), and others (4). In 3 case microorganisms were not identified. The mean implantation time of the spacer was 11 weeks (range, 7 to 28 weeks). The rehabilitation program between stages consisted in early range of motion exercises and partial weight-bearing. In one patient removal of the spacer and insertion of a molded cement block with vancomycin was necessary because of insurgence of methicillin-resistant Staphylococcus aureus.

The mean follow-up of 32 knees after removal of the spacer and insertion of the new prosthesis was 40 months (range, 24 to 75 months. The mean Knee Society functional score during spacer management was rated 75 points (range, 41 to 91 points) and was rated 87 points (range, 77 to 97 points) at latest follow-up. Preoperative range of motion improved markedly after removal of the infected prosthesis and insertion of the spacer. Neither breakage nor clinically relevant surface wear were detected. No implant-related complications were observed. Bone stock remained unchanged during the whole management process.

The Spacer-K is safe and effective for the management of the infected total knee arthroplasty.

Introduction: Management of periprosthetic fractures following total hip arthroplasty (THA) represents a difficult clinical problem, requiring expertise in both trauma and revision surgery. Estimates of the prevalence of postoperative fracture range from 0.1% to 2.1%, and with rising numbers of patients in the population living with hip prostheses in situ there is evidence that their frequency is increasing. There remains a paucity of data on the functional outcome of these injuries, and the objective of this study was to analyse outcomes for revision THA following periprosthetic fracture, and compare these to elective revision surgery.

Methods: 233 patients (234 hips) undergoing revision THA for femoral fracture were identified from the New Zealand National Registry, and clinical outcomes were measured using Oxford Hip Scores (OHS) completed six months post operatively. A control group of 234 patients undergoing elective revision THA was selected and matched for age, sex, and time since index operation.

Results: Comparative analysis of the registry patients showed clinical outcomes were significantly worse following revision THA for fractured femur than in controls (mean OHS 28.6 vs 23.6, p=0.006), though this difference was not apparent in patients under the age of 65 years (mean OHS 26.1 vs 23.8, p =0.6). A higher mortality rate was found among fracture patients (17.1% versus 10.7 %, p=0.05), and a statistically significant higher number of periprosthetic fracture patients died within 6 months of their surgery in comparison to controls (7.3% versus 0.9%, p=0.003). A higher rate of re-revision was observed in the fracture group (7.7% versus 2.6%, p=0.02).

Conclusions: To our knowledge this study represents the largest comparative series of periprosthetic fractures in THA with functional outcome data yet reported. Management of patients with periprosthetic fractures requires recognition of the challenging nature of these injuries, their associated poor prognosis, and high complication rate.