48 Original article Gap index: a good predictor of failure of plaster cast in distal third radius fractures Ajay Malviya, Dionysis Tsintzas, Kamal Mahawar, Christopher E. Bache and Philip R. Glithero The aim of this study was to assess the usefulness of the cast index and an indigenously developed gap index as measures of poor moulding of plaster. Twenty cases of re-manipulation of distal third radius fractures in children excluding growth plate injuries were compared with a control of 80 patients. A significant difference ( < 0.001) was observed in the cast index and the gap index of both the groups. The gap index was more sensitive than the cast index in predicting failure. At a level of cast index of more than 0.8 the relative risk of failure is 6.8 as compared with 35 when the sum of the gap index is more than 0.15. The gap index is a better predictor of failure than the cast index. A quick assessment of these indices is a good practice before accepting any plaster following a manipulation of distal radial fractures. J Pediatr Orthop B 16:48–52  c 2007 Lippincott Williams & Wilkins. Journal of Pediatric Orthopaedics B 2007, 16:48–52 Keywords: distal radius, fracture, gap index, prediction, re-manipulation Freeman Hospital, Newcastle upon Tyne, UK Correspondence and requests for reprints to Mr Ajay Malviya, MS, MRCS Ed, Free- man Hospital, Freeman Road, High Heaton, Newcastle upon Tyne NE7 7DN, UK Tel: + 441912859890; e-mail: malviya7@aol.com Introduction Distal radius fractures in children are well recognized for re-displacement after manipulation. Variable rates of re-displacement [1–5] have been reported ranging from 7 to 39%. Several studies have looked into the causes of this loss of position. Factors include a non-anatomical reduction [6,7], the position of the forearm after manipulation [8–10], the inclusion of the elbow in the plaster [11], the pre-manipulation displacement [3,6,12] and the seniority of the surgeon [1]. Poor moulding of the plaster and excessive padding is a recognized cause of a loose fitting plaster leading to re- displacement. The cast index [11] has been previously described as a radiological measure of moulding of the plaster. We propose another radiological index, the gap index, as a tool to identify a poorly applied plaster. The gap index is a measure of poor moulding and excessive padding applied before plaster application. The aim of our study was to assess the ability of the cast and the gap index to predict a failure of plaster for distal third radius fractures, excluding growth plate injuries, in children. Our hypothesis was that these objective measures of poor moulding might reflect the final outcome of plaster treatment. Materials and methods Between January 2002 and September 2004, 267 distal third radius fractures were treated in our unit either by manipulation under anaesthesia alone or with supple- mental K-wire fixation. Among those who had manipula- tion alone, 20 patients had a loss of position that required a re-manipulation under anaesthesia. We compared this group with a cohort of 80 consecutive similar fractures treated by manipulation under anaesthesia and plaster application over a period from April 2003 to July 2004. Only patients with fractures in the distal third radius in the diaphyseal or metaphyseal region were included while epiphyseal injuries were excluded. All patients had a displacement that was warranted to be significant enough to need manipulation in the morning trauma meeting. All had an above-elbow plaster of paris cast with elbow at 901 flexion following manipulation. Wool was used for padding. Failure of treatment was considered if the axial angulation of the fracture was more than 101 during follow-up even if re-manipulation was not an endpoint. This value was selected because of previous reports suggesting that an angulation of more than 101 may not remodel satisfacto- rily in patients above 6 years of age [13]. Five patients out of the cohort of 80 had an axial angulation of more than 101 during follow-up and were included in the failure group. We therefore had a total of 25 patients in the failure group and 75 in the control group. We compared the demographics, the displacement and angulation of the fracture, the cast index and the gap index of the two groups. The follow-up radiographs were evaluated until discharge. A two-tailed Student t test was used for statistical analysis. 1060-152X  c 2007 Lippincott Williams & Wilkins Copyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.