Cross-shear has been shown to increase ultra-high molecular weight polyethylene (UHMWPE) wear in pin-on-disk, total knee, total hip, and spinal disc replacement testing. Computer modelling of implant wear holds promise for improving efficiency in the development of new implant designs, but it is desirable to accurately account for the effects of cross-shear in the computational simulation. Several studies have sought to propose a quantitative metric for cross-shear in multidirectional sliding and to correlate average cross-shear intensity with apparent wear rate measured in experiments. The apparent wear rate accounts for the total volume loss from all points on the UHMWPE surface. In principle, if the cross-shear metric correlates with experimental wear rates, it is then possible to predict an estimated wear rate for any arbitrary set of kinematic inputs. UHMWPE wear may then be simulated numerically with some form of Archard’s law.

One limitation of the above approach is that counter-face kinematics are homogenized by the use of a spatially and temporally averaged apparent wear rate. In a sliding contact interface of a joint implant in vivo, the intensity of cross-shear wear may vary with time and location on the surface. To address this variation we have proposed a novel cross-shear metric (x*) and developed a modified form of Archard’s law that is capable to differentiate between unidirectional and multidirectional sliding wear. The wear model and x* have been implemented in an explicit finite element framework (ABAQUS) that is capable of quantifying wear from any number of wear surfaces (e.g., front side, backside, post) with completely general geometry and loading conditions.

Preliminary validation of x* and the wear model have been performed by comparison with data from the open literature. Cross-shear metric x* is easy to compute, exhibits invariance to the choice of kinematic reference frame, and is able to reliably distinguish between similarly shaped sliding paths of different lengths – all improvements compared to cross-shear metrics described elsewhere. The wear model that incorporates x* has shown good agreement with pin-on-disk and cervical disc replacement wear results previously reported. Ongoing research focuses on demonstrating similar validity of the model for cross-shear wear in hip and knee replacements.

Introduction: Outcome data is essential for clinical governance and research purposes, and will inform decisions on resource re-distribution. The Spinal Assessment Clinic (SAC) treats patients with low back pain referred by their GPs.

Method: Low Back Outcome Score (LBOS) data was collected at presentation (Q1) for 691 patients and on review (Q2) for 98 patients. At presentation further administrative information is also collected. At review Q2 patient satisfaction is recorded as well as the patient’s perception of the status of their LBP. Results were compared between three clinic locations; inner city (CIT), urban (URB) and semi-rural (RUR).

Results: Significantly more patients at the inner city clinic cancelled and re-appointed, and significantly fewer could be discharged after the first consultation. Equal numbers were employed in the three locations.

Despite failure of improvement of perceived LBP, many patients reported an improvement of LBOS.

Conclusion: Social and environmental factors influence behaviour within a treatment program. Patients can appreciate the difference between a satisfactory treatment experience and an actual change in their low back pain. Function can increase even when reported pain does not.