We have reviewed, retrospectively, all children with a lower limb deformity who underwent an acute correction and lengthening with a monolateral fixator between 1987 and 1996. The patients were all under the age of 19 years and had a minimum follow-up of eight months after removal of the fixator. A total of 41 children had 57 corrections and lengthening. Their mean age was 11.3 years (3.2 to 18.7) and there were 23 girls and 18 boys.

The mean maximum correction in any one plane was 23° (7 to 45). In 41 bony segments (either femur or tibia) a uniplanar correction was made while various combinations were carried out in 16. The site of the osteotomy was predominantly diaphyseal, at a mean of 47% (17% to 73%) of the total bone length and the mean length gained was 6.4 cm (1.0 to 17.0).

Univariate analysis identified a moderately strong relationship between the bone healing index (BHI), length gained, maximum correction and grade-II to grade-III complications. For logistic regression analysis the patients were binary coded into two groups; those with a good outcome (BHI ≤ 45 days/cm) and those with a poor outcome (BHI > 45 days/cm). Various factors which may influence the outcome were then analysed by calculating odds ratios with 95% confidence intervals.

This analysis suggested a dose response between increasing angular correction and poor BHI which only reached statistical significance for corrections of larger magnitude. Longer lengthenings were associated with a better BHI while age and the actual bone lengthened had little effect.

Those patients with a maximum angulatory correction of less than 30° in any one plane had an acceptable consolidation time with few major complications. The technique is suitable for femoral deformity and shortening, but should be used with care in the tibia since the risk of a compartment syndrome or neurapraxia is much greater.

The validation of a new classification of the external fixator screw-bone interface.

Screw loosening significantly affects the stability of an external fixator, however radiographs are normally taken to assess bone healing and not screw loosening. This study was performed to assess the inter and intra-observer reliability of radiographic features of external fixator screw loosening. Eight observers were shown plain radiographs of 120 external fixator bone-screw interfaces on two occasions, and were asked to grade the screws according to the following features.

Solid screw.

The overall kappa value for this study was 0.29, with the component values ranging from 0.15 to 0.41. To determine if the reliability could be improved, two observers classified 192 digitised radiographs of external fixator screws. On the first occasion the radiographs were shown at a size, brightness and contrast equal to the original film. On the second occasion the radiographs were subjected to image enhancement and magnification. This showed improvement in all the kappa values, the overall value increasing to 0.39, with similar improvements in the component parts. Unfortunately no observations were made of loose screws, therefore, two observers were asked to classify 160 digitised images of screws which were selected with a bias in their outcome, to remain solid or become loose. The observers obtained a kappa value > 0.50 for loose screws.

A classification system for the bone-screw interface is of value both in research and clinical practice. Despite the fact that standard radiographic views were used the classification system described shows satisfactory inter and intra-observer reliability and this improved when digital enhancement was applied.