Axial forces were measured during limb lengthening in a series of ten patients with varying pathologies in order to assess the mechanical characteristics of the distracted tissues and the levels of axial force to which soft tissues are subjected during leg lengthening. The pattern of force was found to vary according to the underlying pathology. For post-traumatic shortening in adults both the peak and the resting forces rose steadily during lengthening reaching maximum forces of the order of 300 N. Patients with congenitally short limbs developed very high peak forces (in some cases over 1000 N) and also showed large amounts of force relaxation (typically 400 to 500 N). When very high levels of force were recorded, there was a higher complication rate. In particular, there was a high instance of angular deformity. This occurred because the loads encountered resulted in failure of some of the external fixation frames

This review is aimed at clinicians appraising preclinical trauma studies and researchers investigating compromised bone healing or novel treatments for fractures. It categorises the clinical scenarios of poor healing of fractures and attempts to match them with the appropriate animal models in the literature.

We performed an extensive literature search of animal models of long bone fracture repair/nonunion and grouped the resulting studies according to the clinical scenario they were attempting to reflect; we then scrutinised them for their reliability and accuracy in reproducing that clinical scenario.

Models for normal fracture repair (primary and secondary), delayed union, nonunion (atrophic and hypertrophic), segmental defects and fractures at risk of impaired healing were identified. Their accuracy in reflecting the clinical scenario ranged greatly and the reliability of reproducing the scenario ranged from 100% to 40%.

It is vital to know the limitations and success of each model when considering its application.

Dislocation remains a major concern after total hip replacement, and is often attributed to malposition of the components. The optimum position for placement of the components remains uncertain. We have attempted to identify a relatively safe zone in which movement of the hip will occur without impingement, even if one component is positioned incorrectly. A three-dimensional computer model was designed to simulate impingement and used to examine 125 combinations of positioning of the components in order to allow maximum movement without impingement. Increase in acetabular and/or femoral anteversion allowed greater internal rotation before impingement occurred, but decreases the amount of external rotation. A decrease in abduction of the acetabular components increased internal rotation while decreasing external rotation. Although some correction for malposition was allowable on the opposite side of the joint, extreme degrees could not be corrected because of bony impingement.

We introduce the concept of combined component position, in which anteversion and abduction of the acetabular component, along with femoral anteversion, are all defined as critical elements for stability.