Total knee replacement (TKR) design aims to restore normal kinematics with emphasis on flexion range. The survivorship of a TKR is dependent on the kinematics in six-degrees-of-freedom (6-DoF). Stepping up, such as stair ascent is a kinematically demanding activity after TKR. The debate about design choice has not yet been informed by 6-DoF in vivo kinematics. This prospective randomised controlled trial (RCT) compared kneeling kinematics in three TKR designs.

68 participants were randomised to receive either cruciate retaining (CR-FB), rotating platform (CR-RP) or posterior stabilised (PS-FB) prostheses. Image quality was sufficient for 49 of these patients to be included in the final analysis following a minimum 1-year follow-up. Patients completed a step-up task while being imaged using single-plane fluoroscopy. Femoral and tibial computer-aided design (CAD) models for each of the TKR designs were registered to the fluoroscopic images using bespoke software OrthoVis to generate six-degree-of-freedom kinematics. Differences in kinematics between designs were compared as a function of flexion.

There were no differences in terminal extension between the groups. The CR-FB was further posterior and the CR-RP was more externally rotated at terminal extension compared to the other designs. Furthermore, the CR-FB designs was more posteriorly positioned at each flexion angle compared to both other designs. Additionally, the CR-RP design had more external femoral rotation throughout flexion when compared with both fixed bearing designs. However, there were no differences in total rotation for either step-up or down. Visually, it appears there was substantial variability between participants in each group, indicating unique patient-specific movement patterns.

While use of a specific implant design does influence some kinematic parameters, the overall patterns are similar. Furthermore, there is high variability indicating patient-specific kinematic patterns. At a group level, none of these designs appear to provide markedly different step-up kinematic patterns. This is important for patient expectations following surgery. Future work should aim to better understand the unique patient variability.

Thoracic kyphosis increases with age. The resultant increase in compression forces on the anterior vertebral bodies leads to further kyphotic deformity and, an increased likelihood of vertebral collapse. This study aimed to determine the relative efficacy of two therapeutic strategies commonly used to treat hyperkyphosis.

69 subjects (26 male: 43 female) were randomised into 4 groups: strengthening, postural re-education (PEd), both and control. The strengthening group attended a gym 3 times a week for 12 weeks to perform seated extension exercises. The PEd group had 3 physiotherapy sessions within a 12 week period in which they received postural assessment and a home exercise programme. The combined group received both interventions while the control group received neither. Outcome measurements were assessed at baseline and 12 weeks. They included static (inclinometer) and 6-hour angular measurements (using flexible electrogoniometer (FEG)) and physical function tests.

There were no significant differences between the marginal means of the angular measurements for any of the intervention groups. However, the group which received both interventions demonstrated reduced kyphosis as measured by the FEG angles (apex of the curve between T3 and T11), while the strengthening group showed reduced inclinometer angles (between T1 and T12). The strengthening group showed improvement in back extensor strength (BES) (0.6 +/− 0.2 N/kg, p < 0.01), time to walk 10 metres (−0.3 +/− 0.6 s, p < 0.05), and time to stand and sit 5 times (−0.9 +/− 0.6 s, p < 0.05). However, there was no relationship between change in BES and change in kyphotic angle. The PEd group showed the greatest improvement in the timed up and go test but this was not significant. Improvement in inclinometer angle over the 12 weeks was associated with degree of kyphosis at baseline (upright inclinometer r = −0.47, p=0.0001) but this relationship was not apparent in the FEG measurements. Both the FEG and inclinometer angles showed a marked decrease in degree of improvement in subjects aged >70.

(50/50). A combination of strengthening and PEd was most effective at reducing hyperkyphosis. BES was improved with resisted strengthening but not with home-based postural exercises. However, increased BES was not associated with decreased kyphosis. Larger baseline kyphosis was associated with greater angular improvement. Subjects aged >70 were less likely to improve.

2D/3D image registration techniques have supplanted RSA for kinematic analysis as they are faster, non-invasive and enable pre and post op studies. Improved algorithms have solved the problem of accuracy of out-of-plane translation [1,2]. The aim of this study is to apply these new algorithms to the post op case.

In this study, Computer-Aided Design (CAD) models of the femoral and tibial components were registered to fluoroscopic images. The prosthesis (RBK knee, Global Orthopaedic Technology), was implanted into a sawbones knee. A perspex cage held the knee static while simultaneous fluoroscopy and dual X-rays were taken from 0 and 90 degrees flexion. Translations orthogonal to the fluoroscope were simulated by sliding the cage at 5 mm intervals. The CAD models were then registered with the fluoroscopy frames. Registration information was used to perform kinematic analysis.

This study has demonstrated greater accuracy for the post operative than pre-operative registration applications. The standard deviation of error for flexion/extension was 0.23° with respect to RSA. The average standard deviation of error for out-of-plane rotations (i.e. abduction/adduction and internal/external rotation) was 0.46°. Translations such as anterior-posterior drawer, compression/distraction and medio-lateral shift had errors of 0.16 mm, 0.17 mm and 0.59 mm, respectively. Both the registration and kinematic analysis accuracies for prosthesis components were superior to those for registration of natural (e.g. cadaver) bones [1]. While rotation accuracies improved about 0.1°, improvement in translation was substantial. In particular, medio-lateral translation accuracy has improved from 1 mm (in our previous study) to 0.59 mm, which is promising. It is worth noting that the best reported accuracy for out-of-plane or medio-lateral translation has been 1.03 mm [2]. Hence, this technique is competitive with other 3D/2D registration methods reported in the literature.

Our experiments show that our 3D CAD to 2D fluoroscopy registration method is sufficiently accurate to produce confident and reliable analysis of prospective kinematics studies.

Greater trochanteric pain syndrome (GTPS) is common, yet the impact on function and quality of life has not been measured. The aim of this study was to quantify the impact on function and quality of life, comparing the results to people with severe hip osteoarthritis and an asymptomatic control group.

Forty two people with GTPS – including 11 not actively seeking treatment and 11 seeking surgical treatment, 20 with severe hip osteoarthritis (OA), and 23 age and sex matched asymptomatic participants (ASC) where recruited from public and private hospitals, and the community. Upon confirming meeting inclusion and exclusion criteria participants were interviewed. Exclusion criteria included lumbar nerve root signs; inflammatory, neoplastic and metabolic disorders. Measured used were the Harris hip score (HHS); the Oswestry disability index (ODI); the Australian quality of life instrument (AQoL); the Functional co-morbidity index (FCI); and fulltime work assessments.

No difference was found between the GTPS and the OA group on the HHS, ODI, AQoL or the FCI measures. Both symptomatic groups were significantly more disabled than the ASC group on the HHS and ODI (p<0.001). The GTPS and OA groups had lower AQoL than the ASC group (p<0.001); and higher FCI results than the ASC group (GTPS vs ASC, p=0.005; OA vs ASC, p=0.019). GTPS participants were least likely to be in full time work; full time work participation probability (95% C.I.): GTPS Prob=0.288 (0.160 to 0.463), OA Prob= 0.518 (0.273 to 0.753); ASC group of Prob=0.676 (0.439 to 0.847).

People with GTPS have similar levels of pain, disability and quality of life, but are less likely to be in full time employment than people with severe hip OA which puts them at risk of economic hardship. Research on conservative and surgical treatments should measure pain, disability and work participation.

The standard approach for kinematic analysis of knee joints has been roentgen stereophotogrammetry (RSA). This approach requires implanting tantalum beads during surgery so pre- and post-surgery comparisons have not been conducted. CT- fluoroscopy registration is a non-invasive alternative but has had accuracy and speed limitations. Our new algorithm addresses these limitations.

Our approach to the problem of registering CT data to single-plane fluoroscopy was to generate a digitally reconstructed radiograph (DRR) from the CT data and then filter this to produce an edge-enhanced image, which was then registered with an edge-enhanced version of the fluoroscopy frame. The algorithm includes a new multi-modal similarity measure and a novel technique for the calculation of the required gradients.

Three lower limb specimens were implanted with 1 mm tantalum beads to act as fiducial markers. Fluoroscopy data was captured for a knee flexion and femur and tibia CT data was registered to the fluoroscopy images.

A previous version of our algorithm (developed in 2008) showed good accuracy for in-plane translations and rotations of the knee bones. However, this algorithm did not have the ability to accurately determine out-of-plane translations. This lack of accuracy for out-of-plane translations has also been the major limitation of other single-plane 2D-3D registration algorithms. Fregly et. al. and Dennis et. al. reported standard deviations for this measurement of 5.6 and 3.03 mm respectively. The latest version of our algorithm achieves error standard deviations for out-of-plane translations of 0.65 mm. The algorithm includes a new similarity measure, which calculates the sum of the conditional variances (SCV) of the joint probability distributions of the images to be registered. This new similarity measure determines the true 3D position of the bones for a wider range of initial disparities and is also faster than the cross-cumulative residual entropy (CCRE) measure used in the 2008 version. For a set of initial 3D positions ranging from ± 5 pixels and ± 5 degrees the proposed approach successfully determined the correct 3D position for 96% of cases–whilst the approach using CCRE was successful for only 49% of cases. The algorithm also required 60% less iterations than the previous CCRE approach.

The new registration algorithm developed for the project provides a level of accuracy that is superior to other similar techniques. This new level of accuracy opens the way for a non-invasive mechanism for sophisticated kinematic analysis of knee joints. This will enable prospective, longitudinal and controlled studies of reconstruction surgery.

The increasing rate of fragility fractures in the developed world is now well- documented and presents a significant challenge to Orthopaedics. Although guidelines exist for the management of osteoporosis, both before and after fracture events, little work has yet been done to measure the effect of interventions on reducing the rate of second fractures in the at-risk population. The longitudinal study, begun at The Canberra Hospital, aims to compare rates of second fractures in two populations of patients over 40 years who have sustained a low-impact, minor trauma fracture.

A retrospective study of medical records provides baseline information on current intervention rates for osteoporosis. Secondly, a prospective study population is recruited from patients presenting to Fracture Clinic at The Canberra Hospital.

Patients presenting to clinic will, after consent, undergo a screening process including blood test and DEXA scans, to confirm or reject a diagnosis of osteoporosis. Those diagnosed will be referred for medical management as well as non-pharmacological interventions.

Follow-up will be conducted at 12 months with repeat testing for bone density to determine whether the interventions have produced measurable improvement and patients will be followed up for five years to establish the rate of re-fracture.

A progress report will be presented to the conference advising on findings from the retrospective arm, scheduled for completion in September 2009, and results to date of the prospective arm. The study is a current work-in-progress, and will provide a basis for future research in this area.

Particulate wear debris from the UHMWPE component of implant prostheses typically causes inflammatory cascades leading to bone resorption and prosthesis loosening. Aseptic loosening is the leading cause of joint replacement failure. Green et al. have shown that the most biologically active polyethylene wear particles are in size range 0.3–10 micrometer, determined by filtration and Scanning Electron Microscopy.

A new methodology based on radioisotope tracing is investigated which promises aseptic loosening is the leading cause of joint replacement failureto be more sensitive and may allow the characterization of wear debris shedding on the nanometer-scale. A constant force knee simulator has been designed and constructed at the University of New South Wales, to generate reproducible wear patterns. Atomic Force Microscopy is used to measure the wear particle dimensions.

The constant axial force can be adjusted over a range of 0–1000 N, and flexion angles of 24°, 38°, 51° and 66° can be set. The UHMWPE wear surface is articulated at a rate of 1 cycle per second. It has been found that the simulator operates reliably over up to 2×10^6 cycles at various loads and flexion angles, and that wear debris can successfully be removed from the lubricant. For a walking cycle simulation, a wear rate of the order of 86 mg/10^6 cycles was measured using distilled water as lubricant.

The debris particulates generated from the simulation have been characterized with Atomic Force Microscopy. In the nanometer range two characteristic types, clumps and fibrils, may be distinguished.

A constant force knee simulator has been shown to operate reliably over up to 2×10^6 cycles at various loads and flexion angles, and that wear debris particulates can be obtained. It has also been shown that atomic force microscopy is well suited to characterize nanometre size UHMWPE particles. In parallel, the wear debris generated from the experiments is being tested for their bioirritant characteristics on osteoblast cells (in the TORU laboratory at the John Curtin School of Medical Research at ANU).

The Biometrics Flexible Electrogoniometer (FEG) is a lightweight device which when attached over joints can record angular displacements for periods of up to 10 hours.

The aim of this study was to examine the validity of the FEG for measuring the thoracic spine using the Cobb angle and functional activities.

12 subjects (6 F, 71±11years; 6 M, 65±11) were X-rayed in upright and “slumped” standing with the FEG attached to their thoracic spine. Three Cobb angles were obtained from the vertebrae underlying the FEG at the outer margins (OEB); the inner margins (IEB), and the mid points (MEB), of the FEG end blocks.

Comparisons of FEG and Cobb angles were derived with intraclass correlation coefficients (ICC).

In a separate experiment, 12 subjects (8F, 43±13; 4M, 35±17) performed 7 functional activities one week apart in order to assess day-to-day reliability of the FEG. Time 1 and time 2 angles were compared with an ICC.

The mean FEG angles for upright and “slumped standing” were 31±7° and 39±8° (mean ± SD) respectively. The corresponding mean OEB Cobb angles were 48±13° and 52±12°; the mean IEB Cobb angles were 24±11° and 29±10°; and the MEB Cobb angles were 36±13° and 41±10°.

The correlation between the FEG and OEB Cobb was ICC(2,1)=0.85; between FEG and IEB Cobb was ICC(2,1)=0.77 and between FEG and MEB Cobb was ICC(2,1)=0.87.

The second experiment compared day-to day reliability of the FEG when used over the thoracic spine. For the 7 activities performed: standing erect, standing slumped, reaching up, reaching down, and walking on-the-spot, the mean correlation between the measures on day 1 and day 2 was ICC(2,1)=0.96 (range 0.94 to 0.98).

This study has shown that the FEG demonstrates excellent internal and external validity in the thoracic spine. Further, it has demonstrated that the FEG measures the segment of the spine between the mid-endblocks. Future studies are planned which will use the FEG to evaluate interventions aimed at treating thoracic kyphosis

To better understand the functional effects of pathologies, a system to capture accurate real-time 3D imaging of functional activities, without the limitations of RSA, is desirable. To address this problem, a new registration algorithm was developed to automatically determine the 3D kinematics of the knee using commonly available imaging modalities.

To evaluate this new registration algorithm, three cadaveric knees were implanted with 1mm tantalum beads to act as gold standard fiducial markers. The knees were flexed between 0 and 90° and fluoroscopy data was captured at a rate of 25 frames/sec and a resolution of 0.5 mm/pixel (Axiom Artis MP). “Pin-cushion” distortion and beam spreading were accounted for. CT data was captured using a Toshiba Aquillon 16 using bone and soft tissue algorithms. For every frame of the fluoroscopy data, the 3D femur and tibia data was individually registered to the fluoroscopy images using the new algorithm. This position data was then used to generate a kinematic 3D model. Similar fluoroscopy-to-CT registration techniques have been proposed for stationary image-guided surgery applications. The majority of these techniques use fluoroscopy images projected onto at least two different planes (with some systems using as many as 18 planes). Other techniques have been proposed that use a single-plane but require stochastic optimization procedures that perform in the order of 500 iterations to find the optimal 3-D registration. The reported average target registration errors (TREs) of these systems range from 0.5–1.2 mm. The newly developed registration technique requires only a single-plane fluoroscopy image and uses a novel gradient-descent optimization strategy that converges to the optimal 3-D position within 20–30 iterations. Preliminary results demonstrate that the performance of the new registration algorithm is able to align the bones of the knee with an average TRE of 0.57 mm. Up to 7 degrees of concurrent axial rotation was observed during flexion of the knees to 90°. The new registration algorithm developed for the project is capable of automatically determining the 3D kinematics of a knee joint using only single-plane fluoroscopy data. The new algorithm requires approximately one-tenth the number of iterations to find the optimal registration position when compared with existing single-plane techniques. Once it is established in vivo that this image registration technique has the accuracy of RSA, this method will permit real-time kinematic studies without tantalum beads. This will enable prospective longitudinal and controlled studies of reconstruction surgery, and conservative management of joint pathologies.

Severe and recalcitrant Greater Trochanteric Pain Syndrome (GTPS), previously known as Trochanteric bursitis, has been associated with torn gluteal tendons. The aim of this study was to assess the physical, functional and quality of life outcomes of combined bursectomy and gluteal tendon reconstructive surgery.

24 patients underwent combined bursectomy and gluteal tendon reconstruction under one surgeon. They were contacted by mail, email, and telephone. 16 were available for examination, two had revision surgery, one had interview only, one moved interstate, one declined and three were lost to follow up. An independent standardised assessment was undertaken. Hip muscle strength was measured by hand-held dynamometry. Trendelenburg sign was measured according to Hard-castle’s protocol and by observing gait. Functional and quality of life measures were assessed via the Harris Hip Score and the Oswestry Disability scale. Pain and satisfaction was measured via a 10cm visual analogue scale.

All patients were female. The mean time from surgery was 18.9 months +/− 8.50. 10 had right sided surgery. The two patients who had revision surgery are not included in this data. Strength of hip abduction was weaker on the ipsilateral side (p< =.05). External rotation appeared to be weaker, however this was not statistically significant. Hardcastle’s single leg standing Trendelenburg sign was shorter on the ipsilateral side (16.3secs +/− 12.3 vs 22.1secs +/− 10.1, p< =.05). Five patients had an ipsilateral Trendelenburg gait, two had a contralateral Trendelen-burg gait. The mean recalled preoperative pain score was 67.73 +/− 31.51 out of 100. The mean post operative score was 14.44 +/− 16.1 (p< =.0005). Patient satisfaction with the results of surgery was rated at 80.7 +/−17.69, out of 100. With regard to function, the mean post operative Harris hip score was 70.9 +/− 25.73 out of 91, and the Oswestry disability score was 15.5 +/− 11.39, out of 100 where a low score indicates better outcome.

Combined bursectomy and gluteal tendon reconstruction appears to be an effective procedure for the relief of pain in patients with recalcitrant GTPS in most patients. High patient satisfaction levels suggest that function and quality of life are improved following surgery. A prospective longitudinal study has commenced to verify these results.

Introduction and Aims: Late degeneration of the ACL injured knee may be in part due to repeat injury, but also due to aberrant kinematics altering the wear pattern at the chondral surface. The aim of this study was to use tibio-femoral contact mapping by MRI to examine kinematic changes due to chronic ACL deficiency.

Method: Twenty subjects with a recent unilateral ACL deficiency (mean 13 months since injury) and 23 subjects with a chronic ACL deficiency (mean 18 years since injury) were recruited. Passive ligament laxity was quantified using a KT1000® device. Subjects performed a closed-chain leg press, relaxed and against a 15 kg weight. MRI recorded the tibio-femoral contact position at 15-degree intervals from zero to 90 degrees of knee flexion. Tibio-femoral contact points were measured at each position. Damage to the knee was recorded for all subjects by MRI, and at arthroscopy.

Results: The tibio-femoral contact pattern of the ACL injured knee was different from the healthy contralateral knee (p = 0.001). The contact pattern of the recently injured knees was different to the chronic ACL deficient knees (p = 0.034). In the recently injured knees the lateral compartment of the knee showed a posterior pattern of femoral contact, and in the chronic ACL deficient knees the medial compartment showed a posterior femoral contact pattern, particularly at zero and 15 degrees of knee flexion (p < 0.01), with the femur two millimetres (mean, SD 3.2mm) posterior on the tibial plateau. There was no difference in passive laxity between the recent and chronic injured knees (side-to-side difference: 5.8mm±2.4 for the recently injured knees, and 4.6±2.8mm for the chronic ACL-deficient knees). Nine of 20 recently injured knees had associated joint damage: three medial and three lateral meniscal tears, two with medial femoral condyle and two with patello-femoral damage. Eleven of 23 chronic ACL deficient subjects had associated joint damage: 15 medial and 16 lateral meniscus tears, 16 with medial and 12 with lateral compartment chondral damage. Greater kinematic changes in the chronic ACL deficient knees were associated with more severe chondral damage in the medial compartment.

Conclusion: ACL injury shifts the axis of rotation of the knee medially. In chronic ACL deficiency the tibio-femoral contact pattern is altered in the medial compartment, where it is associated with joint damage. These findings describe the relationship between aberrant kinematics and wear in the ACL deficient knee.

Introduction and Aims: The chronic ACL deficient knee has a natural history of degeneration and deterioration in function. It is unclear whether reconstruction will prevent this sequela. Reconstruction using hamstrings graft techniques have not been yet been evaluated over the long term. This prospective study used MRI to measure tibio-femoral contact patterns pre-and post-reconstruction.

Method: There were 20 subjects with an ACL injury of three years standing. The diagnosis was clinical and confirmed at surgery. They performed a closed chain leg-press, relaxed and against a 150N load. MRI recorded the tibio-femoral contact position at 15-degree intervals from zero to 90 degrees of knee flexion. Passive laxity was measured with a KT1000, and knee outcomes recorded using a Cincinnati score. Testing was performed pre-operatively, at 12 weeks and two years post-operatively.

Results: KT1000 showed a side-to-side difference of 5.1 ± 2.6mm pre-operatively, 2.5 ± 2.2mm at 12 weeks and 2.1 ± 2.3 at two years. Using Cincinnati ratings five rated ‘fair’, five rated ‘good’, and eight rated ‘excellent’. Tibio-femoral contact patterns loaded were not different loaded or unloaded, but medial and lateral compartments of the knee were significantly different (p< 0.001), demonstrating the longitudinal rotation of the knee during flexion, for healthy and injured knees. Pre-operatively the tibio-femoral contact patterns for the ACL injured knee were different to the healthy knee (p=0.014). At 12 weeks post-operatively the tibio-femoral contact patterns were not significantly different (p=0.117), and at two years the contact patterns were restored to those of the healthy knee (p=0.909). However, there were changes to the lateral compartment contact pattern that affected both the ACL injured, reconstructed and healthy knees over the two-year time period. In the healthy knees and also the reconstructed knees the lateral compartment showed less tibio-femoral rollback at two years.

Conclusion: The knee reconstruction restored the tibio-femoral contact pattern to that of the healthy contralateral knee, but both the healthy and reconstructed knees showed changes over time independent of surgery. One or more of the listed authors are receiving or have received benefits or support from a recognised academic body for the pursuance of the study.

Introduction and aims: Osteoarthritis (OA) of the knee is a widespread problem, yet there is little known about the kinematics of the osteoarthritic knee, and nothing about the tibio-femoral contact pattern. This study aimed to describe the role of tibio-femoral interface events in articular surface wear and degenerative change.

Method: Fourteen subjects with symptomatic OA in one knee, and no pain or injury in the contralateral knee were recruited. The tibio-femoral contact pattern was recorded for both knees, while performing a supine leg-press from 0 to 90 degrees flexion against a 150N load. Severity of osteoarthritis was measured by Kellgren Lawrence grade, bone mineral density (BMD) using Dual Energy X-ray Absorptiometry close to the subchondral bone, diagnostic MRI, and joint damage recorded at knee arthroplasty. Pain and disability was recorded using a WOMAC questionnaire.

Results: Severity of OA in the knees ranged from grade two to four (mode=4) in the symptomatic knee, and from zero to three (mode=0) in the contralateral knee. Contact in the lateral compartment of the knee was more anterior on the tibial plateau than healthy knees (p≤ 0.01), and this was associated with severity of OA (p≤ 0.01). Contact in the medial compartment was also more anterior on the tibial plateau, and this was associated with severity of OA. Abnormality in tibio-femoral contact patterns was associated with disability reported by the WOMAC score (r= 0.54). There was no significant difference in BMD between the OA and contralateral knees. However, the BMD was correlated with pain and physical function of the WOMAC score, that is, as function decreased, bone density increased in the arthritic compartment (r = 0.49 to 0.63; p≤ 0.01).

Conclusion: Severity of osteoarthritis was associated with loss of rollback normally coupled with flexion, especially in the lateral compartment. Consequently longitudinal rotation was lost. In severe osteoarthritis, ACL integrity did not affect the contact pattern. Kinematic abnormalities may explain loss of range of motion, and patterns of wear in osteoarthritic knees.

Aim: To determine the cost of medical treatment of infection following total joint replacement (TJR) of the hip or knee. With this information, and obtaining the current costs of antibiotics, antibiotic loaded cement and laminar flow theatres, we aimed to calculate the relative cost- benefit of these prophylactic strategies to prevent infection

Method: Fifty two patients who were admitted to The Canberra Hospital (TCH) for treatment of infection at following total joint arthroplasty between January 1996 and January 2001. A detailed cost analysis of treatment costs following infection was performed. All ward, theatre, prosthesis, investigation, pharmaceutical, allied health and medical costs were collated to produce a total cost of treatment. Current costs of prophylactic antibiotics, antibiotic cement and laminar flow theatres were obtained from suppliers. Costs were calculated for different combinations of prophylactic measures using the rates of deep periprosthetic infection reported through the Swedish Arthroplasty Registry.

Results: There were 41 deep infections and 13 superficial. The average cost for the 54 patients for the in hospital treatment of infection was $41,215. The cost of treating a superficial infection with antibiotics alone averaged $17,663. The average cost of a two stage revision procedure for deep periprosthetic infection was $79,623. Assuming a hospital volume of 150 cases per year, the use of prophylactic intravenous antibiotics, the use of laminar flow and the combined use of antibiotics and laminar flow were significantly cost effective. The addition of antibiotic loaded cement was marginally cost ineffective in combination with either or both of intravenous antibiotics or laminar flow.

Conclusion: The in hospital costs for the treatment of infection after TJR in the Australian setting have been addressed for the first time. Past studies have underestimated the cost of treatment. With this information, we have shown that the combinations of laminar flow and intravenous antibiotic for prophylaxis against infection in TJR are justified on a purely financial cost benefit basis.

Introduction Late degeneration of the ACL injured knee may be in part due to repeat injury, but also due to aberrant kinematics altering the wear pattern at the chondral surface. The aim of this study was to use tibio-femoral contact mapping by MRI to examine kinematic changes due to chronic ACL deficiency.

Methods Twenty-three subjects with a history of chronic ACL deficiency (mean 18 years since injury) performed a closed chain leg press, relaxed and against a 15 kilogram weight. MRI recorded the tibio-femoral contact position at 15° intervals from 0° to 90° of knee flexion. Intra-articular pathology was assessed for all subjects by MRI, and at arthroscopy for 10 subjects.

Results The tibio-femoral contact pattern of the ACL injured knee differed from the healthy contralateral knee (p=0.003). This difference was greatest in the medial compartment, particularly at 0° and 15° of knee flexion (p< 0.01), with the femur two millimetres (mean, SD 3.2 mm) posterior on the tibial plateau. Damage to the chondral surface was seen in the medial compartment in 16 subjects and lateral compartment in 12; medial meniscus damage was present in 16 subjects and lateral meniscus in 15. Chondral surface damage correlated with the difference in the tibio-femoral contact pattern between the healthy and injured knee in the medial compartment of the knee. Joint damage was not related significantly to time since injury, or Cincinnati knee score. Joint damage was related to level of sports participation, but probably indicates that as the joint failed, subjects curtailed their activity.

Conclusions The kinematic consequences of chronic ACL injury may in part be responsible for the pattern of degenerative change, especially in the medial compartment of the knee.

In relation to the conduct of this study, one or more of the authors is in receipt of a research grant from a non-commercial source.

Introduction In-vivo study by MRI has contributed to the understanding of knee kinematics for prosthetic design and the impact of knee pathology. The aim of this study was to compare the characteristics of knee motion exhibited by the tibio-femoral contact footprint and centre of the femoral posterior condyles over the tibial plateau during the flexion arc.

Methods Twelve subjects (five males, ages 19 to 42 years) with a unilateral anterior cruciate ligament (ACL) injury performed a supine leg press against a 15 kg load. Sagittal MR images recorded the motion from 0° to 90° flexion of both knees. The tibio-femoral contact points and the position of the posterior condylar centres were measured against the reference of the tibial plateau, at 15° intervals, for both the healthy knee and the ACL injured knee.

Results The tibio-femoral contact points in the healthy knee began anteriorly on the tibial plateau, and progressed posteriorly during the flexion arc. From 0° to 30° the footprints of the medial and lateral compartments are almost parallel. Onward from 30°, the lateral condyle moved further posteriorly than the medial condyle. In the ACL injured knee the footprint was more posterior on the tibial plateau, particularly on the lateral condyle, suggesting the axis of rotation of the knee had shifted medially due to ACL loss. The posterior condylar centres in the healthy knee were positioned over the centre of the tibial plateau in knee extension. The medial condylar centre remained central during the flexion arc, whereas the lateral condylar centre moved back over the tibial plateau. In the ACL injured knee the posterior condylar centres were more posterior than the healthy knees from 0° to 15°, but from 30° to 90° are not significantly different to the healthy knee. The difference in the position of the posterior condylar centres in the ACL injured knee between 0° and 15° indicates the effect of ACL deficiency on the governing axis of the knee in extension, as evidenced clinically by the pivot shift.

Conclusions The tibio-femoral contact patterns describe events at the articular surface whereas posterior condylar centres reflect the movement of the axis of rotation at the knee; two characteristics of knee kinematics. The altered action of the ACL injured knee at the tibio-femoral interface has implications for the development of wear changes in the chronic ACL injured knee.

In relation to the conduct of this study, one or more of the authors is in receipt of a research grant from a non-commercial source.

Aim: A method of analysing the kinematics of the knee has been developed using magnetic resonance imaging (MRI) at regular intervals of knee motion. This method was tested for reliability and applied to normal and injured knees.

Method: MRI scans were used to study knees of 11 healthy subjects and knees of eight patients ACL with injuries. Scans were taken at 15 degrees intervals from 0 degrees to 90 degrees flexion, using a positioning jig to enable scanning with the knees either loaded or unloaded. The tibiofemoral contact points were mapped for each knee position, loaded and unloaded, in the medial and lateral compartments. The data were analysed for repeatability by interclass correlation and compared with known data.

Results: High reliability was achieved using T1 weighted fast spin echo images, scanned into Adobe Photoshop, or gradient echo sequences, downloaded as Dicom files and analysed using Osiris. Gradient echo sequences have advantages in efficiency without loss of reliability. Analysis of ACL-deficient knees confirmed aberrant contact-point behaviour compared with the non-injured side. The results showed that right and left normal knees and loaded and unloaded uninjured knees did not demonstrate significant differences but that medial and lateral condyles did demonstrate significant differences reflecting the longitudinal rotation of the knee during flexion.

Conclusions: The technique of MR imaging of the knee for kinematic analysis has been demonstrated to be robust and reliable. This technique may be applied to assessments of the effect of knee injury on the biomechanics of the knee or the results of surgical and physiotherapy interventions.