THREE–DIMENSIONAL AUTO COMPUTER AIDED DESIGNING (AUTO-CAD): A NEW TOOL FOR ORTHOPAEDIC SURGEONS

Abstract

A novel study is done to show use of Auto CAD in orthopaedics at various stages. Auto-CAD is a Three-Dimensional (3-D) software used for drafting and designing and it requires training and practice. Orthopaedic surgeons are usually not trained to use this software but the author an orthopaedic surgeon was trained and certified by City and Guilds to use this software.

Since 1998 the CAD knowledge was used in orthopaedics by using solid model designing. A digital picture of radiograph or bone was taken through the camera and was imported in CAD drawing. The picture was scaled to the actual dimensions of the patient and analysed. Auto-CAD was used for radiographic evaluation, identifying the percentage of magnification of the radiographs, and to measure angles and dimensions of the bones. On the radiographs a new ‘Tuning Fork Lines’ (TFL) were drawn to assess the talar shift in ankle fractures. Functional Acetabular Index (FAI) again a new measure was used to calculate the change of angle of the new cup position in total hip replacements due to pelvic tilt and leg length discrepancy (LLD) on weight bearing. CAD was also used for joint replacement pre-operative planning and patient follow-up. Interhip, inter-knee and inter-malleolar distance was measured to identify femoral valgus angle to guide distal femoral cut in total knee replacements (TKR). Measurements of apparent leg length and angle of fixed deformities of the joints were also studied. Femoral head neck ratio was studied in relation to range of movements (ROM), impingement and stability of the hip joint. Some specific designing work was also under taken. The results showed that the percentage of magnification of the radiographs varied from 9% to 23% (mean 16 %). TFL on antero-posterior view within 30° of rotations of ankle joint confirmed anatomical talar positions and abnormal shifts. One centimetre of LLD changed FAI by 2.5° and 10° of abduction deformity resulted in apparent lengthening of 3.87 cm. Increased femoral head neck ratio for joint bearing improved ROM, increased stability of the hip joint and reduced impingement. Clinical measure was more important than anatomical angles for TKR.

The author found 3-D Auto-CAD to be very useful in his clinical practice. It is very reliable in analysing radiographs and the pictures with great accuracy without any need for markers in vivo. Hopefully it will open up more horizons and will be used widely by orthopaedic surgeons in the future.