Biomed. Eng.-Biomed. Tech. 2015; 60(3): 263–267

Short communication Alastair Shipman, Joseph Alsousou, David J. Keene, Igor N. Dyson, Sarah E. Lamb, Keith M. Willett and Mark S. Thompson*

Quantitative biomechanical comparison of ankle fracture casting methods Abstract: The incidence of ankle fractures is increasing rapidly due to the ageing demographic. In older patients with compromised distal circulation, conservative treatment of fractures may be indicated. High rates of malunion and complications due to skin fragility motivate the design of novel casting systems, but biomechanical stability requirements are poorly defined. This article presents the first quantitative study of ankle cast stability and hypothesises that a newly proposed close contact cast (CCC) system provides similar biomechanical stability to standard casts (SC). Two adult mannequin legs transected at the malleoli, one incorporating an inflatable model of tissue swelling, were stabilised with casts applied by an experienced surgeon. They were cyclically loaded in torsion, measuring applied rotation angle and resulting torque. CCC stiffness was equal to or greater than that of SC in two measures of ankle cast resistance to torsion. The effect of swelling reduction at the ankle site was significantly greater on CCC than on SC. The data support the hypothesis that CCC provides similar biomechanical stability to SC and therefore also the clinical use of CCC. They suggest that more frequent re-application of CCC is likely required to maintain stability following resolution of swelling at the injury site. Keywords: ankle fracture; biomechanical testing; close contact casting. *Corresponding author: Mark S. Thompson, Botnar Research Centre, Nuffield Orthopaedic Centre, Institute of Biomedical Engineering, Department of Engineering Science, University of Oxford, Windmill Road, Oxford OX3 7LD, UK, Phone: +44 (0)1865 737845, Fax: +44 (0)1865 737, E-mail: [email protected] Alastair Shipman and Igor N. Dyson: Botnar Research Centre, Nuffield Orthopaedic Centre, Institute of Biomedical Engineering, Department of Engineering Science, University of Oxford, Windmill Road, Oxford OX3 7LD, UK Joseph Alsousou, David J. Keene, Sarah E. Lamb and Keith M. Willett: Kadoorie Centre for Critical Care Research and Education, Nuffield Department of Orthopaedics, Rheumatology and Musculoskeletal Sciences, Level 3, John Radcliffe Hospital, University of Oxford, Oxford OX3 9DU, UK

DOI 10.1515/bmt-2014-0069 Received July 16, 2014; accepted January 20, 2015; online first February 19, 2015

Introduction Ankle fractures have a significant and persistent impact on mobility, with older patients often having the worst outcomes [6, 8, 11–13]. Incidence in developed countries is increasing concurrently with demographic increases in the older population [3, 7, 14]. Ankle fracture in older people is usually a low-energy injury and has features of an osteoporotic fracture [10]. As with all fragility fractures, this is placing an increasing burden on society in terms of health and social care costs [2]. Surgical interventions for an ankle fracture are associated with inconsistent clinical outcomes in older patients [12]; however, traditional standard casting (SC) is also associated with higher rates of loss of reduction due to lower stability offered to the fracture site when compared to internal fixation [5]. Surgery and casting interventions are also complicated by frail skin and higher rates of comorbidities in older adults. These factors motivate the design of novel casting systems that provide (1) skin protection to reduce the risk of pressure sores on the skin, which is a complication associated with SC, and (2) sufficient biomechanical stability to maintain an accurate reduction of the fracture. The biomechanical requirements for cast stability are however poorly defined. A novel close contact casting (CCC) system has recently been proposed, adapting the concept of total contact casting already widely used in treatment of diabetic patients with skin ulcers. This system is at present the subject of a pragmatic, multi-centre randomised controlled equivalence study of CCC and open reduction and internal fixation (ORIF) in patients over the age of 60 (AIM Ankle Injury Management; Current Controlled Trials Register ISRCTN04180738) [16].

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264      A. Shipman et al.: Biomechanical comparison of ankle fracture casting methods The aim of CCC is to create a more intimate anatomic fit to the lower leg shape and distribute forces more evenly over the skin compared with SC, avoiding high local contact stresses thereby protecting the skin under the cast. This is achieved with a plaster of Paris cast applied over a minimal lining thickness compared to SC, which is moulded firmly to maintain reduction and reinforced with a fibreglass topcoat. A vascular laboratory investigation has confirmed the potential for improved skin viability outcomes with CCC [15]. CCC has a radically different mechanical structure and fit to the patient to SC raising questions of the biomechanical stability afforded and the maintenance of this stability following the resolution of oedema at the fracture site. The aim of this study was to design a novel biomechanical test method to assess the stability of conservative treatments for ankle fractures and to assess the relative stability provided by CCC and SC systems. This was achieved through the construction of a mechanical test rig simulating severe torsional loading on cast-treated transected mannequin legs that incorporated a model of ankle swelling. The hypothesis was that CCC would provide at least equivalent resistance to torsion at the ankle as SC, both at the low load range required to maintain reduction and at maximal loads. The torque-rotation characteristics of six CCC and six SC applied to the legs by an experienced surgeon trained in use of both SC and CCC were measured for comparison.

Materials and methods Two right-side adult male mannequin legs were transected perpendicular to their long axis at the malleoli (Figure 1A). The resulting foot and shank sections were then filled with acrylic cement in which a steel dowel and copper socket were embedded, permitting low friction rotation. The axis of this rotation was parallel with the long axis of the shank and passed through the midpoint of the line connecting the extremities of the malleoli and the centroid of the proximal shank cross section. One leg incorporated an inflatable bladder that simulated tissue oedema, increasing the diameter of the ankle by one third, and its subsequent resolution. The below-knee casts, all consisting of a stockinette lining, plaster of Paris and a fibreglass topcoat, were applied by a surgeon experienced in both SC and CCC techniques to the legs (Figure 1B). Between the lining and the plaster of Paris, SC included overlapping layers of wool roll and felt. In contrast, CCC had only one thin layer of non-overlapping wool roll and small felt pads placed over moulding points and over areas of bony prominence (malleoli, metatarsal heads, anterior tibia, calcaneum and fibular head). For the swelling model, the bladder was inflated with air during casting to obtain a controlled increase of the minor axis dimension

by 33% as measured with callipers. Casts were allowed to dry for 45 min before testing. Custom-designed fixtures enabled non-slip clamping of both the foot and the proximal end of the shank segment and accurate alignment of the axis defined by the dowel-socket connection with the test machine axis (Tecquipment Ltd, Nottingham, UK; Figure 1C). Rotation was applied to the foot segment, and simultaneous measurements of the foot rotation angle (shaft encoder from Avago Technologies, San Jose, CA; precision 0.03°) and resulting torque (torque cell from Norbar Torque Tools Ltd, Banbury, UK; precision 0.05 N m) were captured at 4  Hz in LabView (National Instruments, Austin, TX). Ten cycles of internal and external rotation from -40° to 40° were applied under rotation control to each cast at a mean rate of 5 degrees/s, producing a set of torque-rotation curves characteristics for each cast. Torque rotation data were collected, throughout all 10 loading cycles, for three SC and three CCC on the unmodified transected mannequin leg and for three SC and three CCC on the leg swelling model, first in the swollen and then unswollen state. Statistical analysis (Mann-Whitney U-test) was carried out with significance set to p 

Quantitative biomechanical comparison of ankle fracture casting methods.

The incidence of ankle fractures is increasing rapidly due to the ageing demographic. In older patients with compromised distal circulation, conservat...
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